Publications in Refereed Journals

• An evaluation of the E3SMv1 Arctic ocean and sea-ice regionally refined model

  • Veneziani, M., W. Maslowski, Y. J. Lee, G. D’Angelo, R. Osinski, M. R. Petersen, W. Weijer, A. P. Craig, J. D. Wolfe, D. Comeau, and A. K. Turner, 2022.
  • Geoscientific Model Development, 15, 3133-3160, doi: 10.5194/gmd-15-3133-2022.
  • View abstract (inline)

    The Energy Exascale Earth System Model (E3SM) is a state-of-the-science Earth system model (ESM) with the ability to focus horizontal resolution of its multiple components in specific areas. Regionally refined global ESMs are motivated by the need to explicitly resolve, rather than parameterize, relevant physics within the regions of refined resolution, while offering significant computational cost savings relative to the respective cost of configurations with high-resolution (HR) everywhere on the globe. In this paper, we document results from the first Arctic regionally refined E3SM configuration for the ocean and sea-ice components (E3SM-Arctic-OSI), while employing data-based atmosphere, land, and hydrology components. Our aim is an improved representation of the Arctic coupled ocean and sea-ice state, its variability and trends, and the exchanges of mass and property fluxes between the Arctic and the sub-Arctic. We find that E3SM-Arctic-OSI increases the realism of simulated Arctic ocean and sea-ice conditions compared to a similar low-resolution E3SM simulation without the Arctic regional refinement in ocean and sea-ice components (E3SM-LR-OSI). In particular, exchanges through the main Arctic gateways are greatly improved with respect to E3SM-LR-OSI. Other aspects, such as the Arctic freshwater content variability and sea-ice trends, are also satisfactorily simulated. Yet, other features, such as the upper-ocean stratification and the sea-ice thickness distribution, need further improvements, involving either more advanced parameterizations, model tuning, or additional grid refinements. Overall, E3SM-Arctic-OSI offers an improved representation of the Arctic system relative to E3SM-LR-OSI, at a fraction (15 %) of the computational cost of comparable global high-resolution configurations, while permitting exchanges with the lower-latitude oceans that cannot be directly accounted for in Arctic regional models.

• Labrador Sea freshening linked to Beaufort Gyre freshwater release

  • Zhang, J., W. Weijer, M. Steele, W. Cheng, T. Verma, and M. Veneziani, 2021.
  • Nature Communications, 12, 1-8, doi: 10.1038/s41467-021-21470-3.
  • View abstract (inline)

    The Beaufort Gyre (BG), the largest Arctic Ocean freshwater reservoir, has drastically increased its liquid freshwater content by 40% in the past two decades. If released within a short period, the excess freshwater could potentially impact the large-scale ocean circulation by freshening the upper subpolar North Atlantic. Here, we track BG-sourced freshwater using passive tracers in a global ocean sea-ice model and show that this freshwater exited the Arctic mostly through the Canadian Arctic Archipelago, rather than Fram Strait, during an historical release event in 1983–1995. The Labrador Sea is the most affected region in the subpolar North Atlantic, with a freshening of 0.2 psu on the western shelves and 0.4 psu in the Labrador Current. Given that the present BG freshwater content anomaly is twice the historical analog studied here, the impact of a future rapid release on Labrador Sea salinity could be significant, easily exceeding similar fluxes from Greenland meltwater.

• On the generation of Weddell Sea Polynyas in a high-resolution Earth system model

  • Kurtakoti, P. K., M. Veneziani, A. Stössel, W. Weijer, and M. Maltrud, 2021.
  • Journal of Climate, 34, 2491-2510, doi: 10.1175/JCLI-D-20-0229.1.
  • View abstract (inline)

    Larger Weddell Sea polynyas (WSPs), differentiated in this study from the smaller Maud Rise Polynyas (MRPs) that form to the east of the prime meridian in the proximity of the Maud Rise seamount, have last been observed in the 1970s. We investigate WSPs that grow realistically out of MRPs in a high-resolution preindustrial simulation with the Energy Exascale Earth System Model, version 0.1. The formation of MRPs requires high resolution to simulate the detailed flow around Maud Rise, whereas the realistic formation of WSPs requires a model to produce MRPs. Furthermore, WSPs tend to follow periods of a prolonged buildup of a heat reservoir at depth and weakly negative wind stress curl in association with the core of the Southern Hemisphere westerlies at an anomalously northern position. While this scenario also leads to drier conditions over the central Weddell Sea, which some literature claims to be a necessary condition for the formation of WSPs, our model results indicate that open-ocean polynyas do not occur during periods of weakly negative wind stress curl despite drier atmospheric conditions. Our study supports the hypothesis noted in earlier studies that a shift from a weakly negative to a strongly negative wind stress curl over the Weddell Sea is a prerequisite for WSPs to form, together with a large heat reservoir at depth. However, the ultimate trigger is a pronounced MRP, whose associated convection creates high surface salinity anomalies that propagate westward with the flow of the Weddell Gyre. If large enough, these anomalies trigger the formation of a WSP and a pulse of newly formed Antarctic Bottom Water.

• The Zapiola Anticyclone: A lagrangian study of its kinematics in an eddy-permitting ocean model

  • W. Weijer, A. Barthel, M. Veneziani, H. Steiner, 2020.
  • Deep-Sea Research Part I: Oceanographic Research Papers, 164, 103308, doi: 10.1016/j.dsr.2020.103308.
  • View abstract (inline) Full text (pdf)

    The Zapiola Anticyclone (ZA) is a strong, O(100 Sv), barotropic vortex in the center of the Argentine Basin that is tied to a bathymetric feature called the Zapiola Rise. It is regionally significant for two reasons: first, the strong vortex is a dynamical barrier that inhibits the lateral exchange of water, and hence has the ability to trap water for a long period of time. Second, its dynamics is governed by a balance between eddy-driven mass convergence and divergent Ekman transport, which gives rise to strong downwelling and Ekman pumping into the bottom boundary layer. This study investigates the kinematics of the ZA by studying the fate of the water parcels that are trapped by the ZA. We use output from a five-year simulation with an eddy-permitting ocean model, and we use a Lagrangian approach to track water parcels originating from within the ZA. We determine basic statistics of the parcel trajectories, including retention time, number of revolutions, vertical displacement, and temperature and salinity changes. The picture that emerges is one of water parcels spiraling downward through the water column, undergoing downwelling while they revolve anticyclonically around the center of the ZA. In our experiment, water parcels spend on average 451 days within the ZA, and make 2.6 revolutions around its center, with each revolution taking somewhere between 100 and 200 days. On average, parcels undergo a 94 m descent, 0.03C cooling and 0.0042 psu freshening. But individual parcels can undergo more than 800 m of downwelling, 0.2C of cooling, and 0.02 psu of salinity change. We believe that vertical motions of this order of magnitude, and the associated water mass transformations, are unique in the abyssal mid-latitude oceans.

• CMIP6 models predict significant 21st century decline of the Atlantic Meridional Overturning Circulation

  • W. Weijer, W. Cheng, O. A. Garuba, A. Hu, and B. T. Nadiga, 2020.
  • Geophysical Research Letters, 47, e2019GL086075, doi: 10.1029/2019GL086075.
  • View abstract (inline) Full text (pdf)

    We explore the representation of the Atlantic Meridional Overturning Circulation (AMOC) in 27 models from the CMIP6 multi-model ensemble. Comparison with RAPID and SAMBA observations suggests that the ensemble mean represents the AMOC strength and vertical profile reasonably well. Linear trends over the entire historical period (1850-2014) are generally neutral, but many models exhibit an AMOC peak around the 1980's. Ensemble mean AMOC decline in future (SSP) scenarios is stronger in CMIP6 than CMIP5 models. In fact, AMOC decline in CMIP6 is surprisingly insensitive to the scenario at least up to 2060. We find an emergent relationship among a majority of models between AMOC strength and 21st century AMOC decline. Constraining this relationship with RAPID observations suggests that the AMOC might decline between 6 and 8 Sv (34-45%) by 2100. A smaller group of models projects much less AMOC weakening of only up to 30%.

• Relative impact of sea ice and temperature changes on Arctic marine production

  • G. A. Gibson, W. Weijer, N. Jeffery and S. Wang, 2020.
  • Journal of Geophysical Research, Biogeochemistry, 125, e2019JG005343, doi: 10.1029/2019JG005343.
  • View abstract (inline)

    We use a modern Earth system model to approximate the relative importance of ice vs. temperature on Arctic marine ecosystem dynamics. We show that while the model underestimates upper water column nitrate across the region, this nitrate bias is likely responsible for the apparent underestimation of ice algae production. Despite this shortcoming, the model appears to be a useful tool for exploring the impacts of environmental change on phytoplankton production and carbon dynamics over the Arctic Ocean. Our experiments indicate that under a warmer scenario, the percentage of ocean warming that could be apportioned to a reduction in ice area ranged from 11 to 100%, while decreasing ice area could account for 22‐100% of the increase in annual ocean primary production. The change to CO2 air‐sea flux in response to ice and temperature changes averaged an Arctic‐wide 5.5 Tg C /yr (3.5%) increase, into the ocean. This increased carbon sink may be short‐lived, as ice cover continues to decrease and the ocean warms. The change in carbon fixation from phytoplankton in response to increased temperatures and reduced ice was generally more than a magnitude larger than the changes to CO2 flux, highlighting the importance of fully considering changes to the marine ecosystem when assessing Arctic carbon cycle dynamics. Our work demonstrates the importance of ice dynamics in controlling ocean warming and production, and thus the need for well‐behaved ice and BGC models within Earth system models if we hope to accurately predict Arctic changes.

• The Atlantic meridional overturning circulation in high resolution models

  • Hirschi, J. J.-M., B. Barnier, C. Böning, A. Biastoch, A. T. Blaker, A. Coward, S. Danilov, S. Drijfhout, K. Getzlaff, S. M. Griffies, H. Hasumi, H. Hewitt, D. Iovino, T. Kawasaki, A. E. Kiss, N. Koldunov, A. Marzocchi, J. V. Mecking, B. Moat, J.-M. Molines, P. G. Myers, T. Penduff, M. Roberts, A.-M. Treguier, D. V. Sein, D. Sidorenko, J. Small, P. Spence, L. Thompson, W. Weijer, X. Xu, 2020.
  • Journal of Geophysical Research-Oceans, 125, e2019JC015522, doi: 10.1029/2019JC015522.
  • View abstract (inline)

    The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N —a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high‐resolution models. Furthermore, we will describe how high‐resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.

• Causal interactions between Southern Ocean polynyas and high-latitude atmosphere-ocean variability

  • Z. S. Kaufman, N. Feldl, W. Weijer, M. Veneziani, 2020.
  • Journal of Climate, 33, 4891-4905, doi: 10.1175/JCLI-D-19-0525.1.
  • View abstract (inline)

    Weddell Sea open-ocean polynyas have been observed to occasionally release heat from the deep ocean to the atmosphere, indicating that their sporadic appearances may be an important feature of high-latitude atmosphere–ocean variability. Yet, observations of the phenomenon are sparse and many standard resolution models represent these features poorly, if at all. We use a fully coupled, synoptic-scale preindustrial control simulation of the Energy Exascale Earth System Model (E3SMv0-HR) to effectively simulate open-ocean polynyas and investigate their role in the climate system. Our approach employs statistical tests of Granger causality to diagnose local and remote drivers of, and responses to, polynya heat loss on interannual to decadal time scales. First, we find that polynya heat loss Granger causes a persistent increase in surface air temperature over the Weddell Sea, strengthening the local cyclonic wind circulation. Along with responding to polynyas, atmospheric conditions also facilitate their development. When the Southern Ocean experiences a rapid poleward shift in the circumpolar westerlies following a prolonged negative phase of the southern annular mode (SAM), Weddell Sea salinity increases, promoting density destratification and convection in the water column. Finally, we find that the reduction of surface heat fluxes during periods of full ice cover is not fully compensated by ocean heat transport into the high latitudes. This imbalance leads to a buildup of ocean heat content that supplies polynya heat loss. These results disentangle the complex, coupled climate processes that both enable the polynya’s existence and respond to it, providing insights to improve the representation of these highly episodic sea ice features in climate models.

• Role of AMOC in transient climate response to greenhouse gas forcing in two coupled models

  • A. Hu, L. Van Roekel, W. Weijer, O. A. Garuba, W. Cheng, B. T. Nadiga, 2020.
  • Journal of Climate, 33, 5845-5859, doi: 10.1175/JCLI-D-19-1027.1.
  • View abstract (inline)

    As the greenhouse gas concentrations increase, a warmer climate is expected. However, numerous internal climate processes can modulate the primary radiative warming response of the climate system to rising greenhouse gas forcing. Here the particular internal climate process that we focus on is the Atlantic meridional overturning circulation (AMOC), an important global-scale feature of ocean circulation that serves to transport heat and other scalars, and we address the question of how the mean strength of AMOC can modulate the transient climate response. While the Community Earth System Model version 2 (CESM2) and the Energy Exascale Earth System Model version 1 (E3SM1) have very similar equilibrium/effective climate sensitivity, our analysis suggests that a weaker AMOC contributes in part to the higher transient climate response to a rising greenhouse gas forcing seen in E3SM1 by permitting a faster warming of the upper ocean and a concomitant slower warming of the subsurface ocean. Likewise the stronger AMOC in CESM2 by permitting a slower warming of the upper ocean leads in part to a smaller transient climate response. Thus, while the mean strength of AMOC does not affect the equilibrium/effective climate sensitivity, it is likely to play an important role in determining the transient climate response on the centennial time scale.

• E3SMv0‐HiLAT: A modified climate system model targeted for the study of high‐latitude processes

  • M. Hecht, M. Veneziani, W. Weijer, B. Kravitz, S. Burrows, D. Comeau, E. Hunke, N. Jeffery, J. Urrego‐Blanco, H. Wang, and S. Wang, 2019.
  • Journal of Advances in Modeling Earth Systems, 11, 2814-2843, doi: 10.1029/2018MS001524.
  • View abstract (inline)

    We document the configuration, tuning, and evaluation of a modified version of the Community Earth System Model version 1 (Hurrell et al., 2013, https://doi.org/10.1175/BAMS-D-12 -00121.1), introduced here as E3SMv0-HiLAT, intended for study of high-latitude processes. E3SMv0-HiLAT incorporates changes to the atmospheric model affecting aerosol transport to high northern latitudes and to reduce shortwave cloud bias over the Southern Ocean. An updated sea ice model includes biogeochemistry that is coupled to an extended version of the ocean model's biogechemistry. This enables cloud nucleation to depend on the changing marine emissions of aerosol precursors, which may be expected in scenarios with strongly changing sea ice extent, oceanic stratification and associated nutrient availability, and atmospheric state. An evaluation of the basic preindustrial state of E3SMv0-HiLAT is presented in order to ensure that its climate is adequate to support future experimentation. Additional capability is not achieved without some cost, relative to the extraordinarily well-tuned model from which it was derived. In particular, a reduction of bias in cloud forcing achieved over the Southern Hemisphere also allows for greater Southern Ocean sea ice extent, a tendency that has been partially but not fully alleviated through experimentation and tuning. The most interesting change in the behavior of the model may be its response to greenhouse gas forcing: While the climate sensitivity is found to be essentially unchanged from that of Community Earth System Model version 1, the adjusted radiative forcing has increased from within one standard deviation above that of Coupled Model Intercomparison Project Phase 5 models to nearly two standard deviations.

• Enhancing skill of initialized decadal predictions using a dynamic model of drift

  • B. T. Nadiga, T. Verma, W. Weijer, and N. M. Urban, 2019.
  • Geophysical Research Letters, 46, 9991-9999, doi: 10.1029/2019GL084223.
  • View abstract (inline)

    Since near-term predictions of present-day climate are controlled by both initial condition predictability and boundary condition predictability, initialized prediction experiments aim to augment the external-forcing-related predictability realized in uninitialized projections with initial-condition-related predictability by appropriate observation-based initialization. However, and notwithstanding the considerable effort expended in finding such “good” initial states, a striking feature of current, state-of-the-art, initialized decadal hindcasts is their tendency to quickly drift away from the initialized state, with attendant loss of skill. We derive a dynamical model for such drift, and after validating it we show that including a recalibrated version of the model in a postprocessing framework is able to significantly augment the skill of initialized predictions beyond that achieved by a use of current techniques of postprocessing alone. We also show that the new methodology provides further crucial insights into issues related to initialized predictions.

• Stability of the Atlantic Meridional Overturning Circulation: A review and synthesis

  • W. Weijer, W. Cheng, S. S. Drijfhout, A. V. Fedorov, A. Hu, L. C. Jackson, W. Liu, E. L. McDonagh, J. V. Mecking, and J. Zhang, 2019.
  • Journal of Geophysical Research: Oceans, 124, 5336-5375, doi: 10.1029/2019JC015083.
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    The notion that the Atlantic Meridional Overturning Circulation (AMOC) can have more than one stable equilibrium emerged in the 1980s as a powerful hypothesis to explain rapid climate variability during the Pleistocene. Ever since, the idea that a temporary perturbation of the AMOC —or a permanent change in its forcing— could trigger an irreversible collapse has remained a reason for concern. Here we review literature on the equilibrium stability of the AMOC and present a synthesis that puts our understanding of past and future AMOC behavior in a unifying framework. This framework is based on concepts from Dynamical Systems Theory, which has proven to be an important tool in interpreting a wide range of model behavior. We conclude that it cannot be ruled out that the AMOC in our current climate is in, or close to, a regime of multiple equilibria. But there is considerable uncertainty in the location of stability thresholds with respect to our current climate state, so we have no credible indications of where our present-day AMOC is located with respect to thresholds. We conclude by identifying gaps in our knowledge and proposing possible ways forward to address these gaps.

• Preconditioning and formation of Maud Rise Polynyas in a high-resolution Earth system model

  • P. K. Kurtakoti, M. Veneziani, A. Stössel, W. Weijer, 2018.
  • Journal of Climate, 31, 9659-9678, doi: 10.1175/JCLI-D-18-0392.1.
  • View abstract (inline)

    Open-ocean polynyas (OOPs) in the Southern Ocean are ice-free areas within the winter ice pack that are associated with deep convection, potentially contributing to the formation of Antarctic Bottom Water. To enhance the credibility of Earth system models (ESMs), their ability to simulate OOPs realistically is thus crucial. Here we investigate OOPs that emerge intermittently in a high-resolution (HR) preindustrial simulation with the Energy Exascale Earth System Model, version 0.1 (E3SMv0), an offspring of the Community Earth System Model (CESM). While low-resolution (LR) simulations with E3SMv0 show no signs of OOP formation, the preindustrial E3SMv0- HR simulation produces both large Weddell Sea polynyas (WSPs) as well as small Maud Rise polynyas (MRPs). The latter are associated with a prominent seamount in the eastern Weddell Sea, and their preconditioning and formation is the focus of this study. The steep flanks of the rugged topography in this region are in E3SMv0-HR sufficiently well resolved for the impinging flow to produce pronounced Taylor caps that precondition the region for convection. Aided by an accumulation of heat in the Weddell Deep Water layer, the ultimate trigger of convection that leads to MRPs is the advection of anomalously high upper-ocean-layer salinity. The crucial difference to WSP producing LR ESM simulations is that in E3SMv0-HR, WSPs are realistically preceded by MRPs, which in turn are a result of the flow around bathymetry being represented with unprecedented detail.

• Can the salt-advection feedback be detected in internal variability of the Atlantic Meridional Overturning Circulation?

  • W. Cheng, W. Weijer, W. M. Kim, G. Danabasoglu, S. G. Yeager, P. R. Gent, D. Zhang, J. C. H. Chiang, and J. Zhang, 2018.
  • Journal of Climate, 31, 6649-6667, doi: 10.1175/JCLI-D-17-0825.1.
  • View abstract (inline)

    Evidence for the assumptions of the salt-advection feedback in box models is sought by studying the Atlantic meridional overturning circulation (AMOC) internal variability in the long pre-industrial control runs of two Earth System Models. The first assumption is that AMOC strength is proportional to the meridional density difference between the North Atlantic and the Southern Oceans. The model simulations support this assumption, with the caveat that nearly all the long time-scale variability occurs in the North Atlantic density. The second assumption is that the freshwater transport variability by the overturning at the Atlantic southern boundary is controlled by the strength of AMOC. Only one of the models shows some evidence that AMOC variability at 45°N leads variability in the overturning freshwater transport at the southern boundary by about 30 years, but the other model shows no such coherence. In contrast, in both models this freshwater transport variability is dominated by local salinity variations. The third assumption is that changes in the overturning freshwater transport at the Atlantic southern boundary perturb the north-south density difference, and thus feed back on AMOC strength in the north. No evidence for this assumption is found in either model at any time scale, although this does not rule out that the salt advection feedback may be excited by a strong enough freshwater perturbation.

• Lagrangian timescales of Southern Ocean upwelling in a hierarchy of model resolutions.

  • Drake, H. F., A. K. Morrison, S. M. Griffies, J. L. Sarmiento, W. Weijer, and A. R. Gray, 2018.
  • Geophysical Research Letters, 45, 891-898, doi: 10.1002/2017GL076045.
  • View abstract (inline)

    In this paper we study upwelling pathways and timescales of Circumpolar Deep Water (CDW) in a hierarchy of models using a Lagrangian particle tracking method. Lagrangian timescales of CDW upwelling decrease from 87 years to 31 years to 17 years as the ocean resolution is refined from 1° to 0.25° to 0.1°. We attribute some of the differences in timescale to the strength of the eddy fields, as demonstrated by temporally degrading high‐resolution model velocity fields. Consistent with the timescale dependence, we find that an average Lagrangian particle completes 3.2 circumpolar loops in the 1° model in comparison to 0.9 loops in the 0.1° model. These differences suggest that advective timescales and thus interbasin merging of upwelling CDW may be overestimated by coarse‐resolution models, potentially affecting the skill of centennial scale climate change projections.

• Spiraling pathways of global deep waters to the surface of the Southern Ocean.

  • Tamsitt, V., H. F. Drake, A. K. Morrison, L. D. Talley, C. O. Dufour, A. R. Gray, S. M. Griffies, M. R. Mazloff, J. L. Sarmiento, J. Wang and W. Weijer, 2017.
  • Nature Communications, 8, 172, doi: 10.1038/s41467-017-00197-0.
  • View abstract (inline)

    Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years.

• Local atmospheric response to an open-ocean polynya in a high-resolution climate model.

  • W. Weijer, M. Veneziani, A. Stössel, M. W. Hecht, N. Jeffery, A. Jonko, T. Hodos, and H. Wang, 2017.
  • Journal of Climate, 30, 1629-1641, doi: 10.1175/jcli-d-16-0120.1.
  • View abstract (inline) Full text (pdf)

    In this paper the atmospheric response to an open-ocean polynya in the Southern Ocean is studied by analyzing the results from an atmospheric and oceanic synoptic-scale resolving Community Earth System Model (CESM) simulation.While coarser-resolution versions of CESMgenerally do not produce open-ocean polynyas in the SouthernOcean, they do emerge and disappear on interannual time scales in the synoptic-scale simulation. This provides an ideal opportunity to study the polynya’s impact on the overlying and surrounding atmosphere. This has been pursued here by investigating the seasonal cycle of differences of surface and air-column variables between polynya and nonpolynya years. The results indicate significant local impacts on turbulent heat fluxes, precipitation, cloud characteristics, and radiative fluxes. In particular, it is found that clouds over polynyas are optically thicker and higher than clouds over sea ice during nonpolynya years. Although the lower albedo of polynyas significantly increases the net shortwave absorption, the enhanced cloud brightness tempers this increase by almost 50%. Also, in this model, enhanced longwave radiation emitted from the warmer surface of polynyas is balanced by stronger downwelling fluxes from the thicker cloud deck. Impacts are found to be sensitive to the synopticwind direction. The strongest regional impacts are foundwhen northeasterlywinds cross the polynya and interact with katabatic winds. Surface air pressure anomalies over the polynya are only found to be significant when cold, dry air masses strike over the polynya (i.e., in the case of southerly winds).

• Atlantic multi-decadal oscillation covaries with Agulhas leakage.

  • A. Biastoch, J. Durgadoo, A. Morrison, E. van Sebille, W. Weijer, and S. Griffies, 2015.
  • Nature Communications, 6, 10082, doi: 10.1038/ncomms10082.
  • View abstract (inline)

    The interoceanic transfer of seawater between the Indian Ocean and the Atlantic, ‘Agulhas leakage’, forms a choke point for the overturning circulation in the global ocean. Here, by combining output from a series of high-resolution ocean and climate models with in situ and satellite observations, we construct a time series of Agulhas leakage for the period 1870–2014. The time series demonstrates the impact of Southern Hemisphere westerlies on decadal timescales. Agulhas leakage shows a correlation with the Atlantic Multi-decadal Oscillation on multi-decadal timescales; the former leading by 15 years. This is relevant for climate in the North Atlantic.

• Eddy-driven sediment transport in the Argentine Basin: is the height of the Zapiola Rise hydrodynamically controlled?

  • W. Weijer, M. E. Maltrud, W. B. Homoky, K. L. Polzin, and L. R. M. Maas, 2015.
  • Journal of Geophysical Research, 120; doi: 10.1002/2014JC010573.
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    In this study, we address the question whether eddy-driven transports in the Argentine Basin can be held responsible for enhanced sediment accumulation over the Zapiola Rise, hence accounting for the existence and growth of this sediment drift. To address this question, we perform a 6 year simulation with a strongly eddying ocean model. We release two passive tracers, with settling velocities that are consistent with silt and clay size particles. Our experiments show contrasting behavior between the silt fraction and the lighter clay. Due to its larger settling velocity, the silt fraction reaches a quasisteady state within a few years, with abyssal sedimentation rates that match net input. In contrast, clay settles only slowly, and its distribution is heavily stratified, being transported mainly along isopycnals. Yet, both size classes display a significant and persistent concentration minimum over the Zapiola Rise. We show that the Zapiola Anticyclone, a strong eddy-driven vortex that circulates around the Zapiola Rise, is a barrier to sediment transport, and hence prevents significant accumulation of sediments on the Rise. We conclude that sediment transport by the turbulent circulation in the Argentine Basin alone cannot account for the preferred sediment accumulation over the Rise. We speculate that resuspension is a critical process in the formation and maintenance of the Zapiola Rise.

• Southern Ocean dynamics and biogeochemistry in a changing climate: Introduction and overview.

  • S. M. Downes, W. Weijer, N. Jeffery, M. Mazloff, and J. Russell, 2015.
  • Deep Sea Research II, 114, 1-2; doi: 10.1016/j.dsr2.2015.02.013.

• Ocean currents generate large footprints in marine palaeoclimate proxies.

  • E. van Sebille, P. Scussolini, J. Durgadoo, F. Peeters, A. Biastoch, W. Weijer, C. Turney, C. Paris, and R. Zahn, 2015.
  • Nature Communications, 6, 6521; doi: 10.1038/ncomms7521.
  • View abstract (inline)

    Fossils of marine microorganisms such as planktic foraminifera are among the cornerstones of palaeoclimatological studies. It is often assumed that the proxies derived from their shells represent ocean conditions above the location where they were deposited. Planktic foraminifera, however, are carried by ocean currents and, depending on the life traits of the species, potentially incorporate distant ocean conditions. Here we use high-resolution ocean models to assess the footprint of planktic foraminifera and validate our method with proxy analyses from two locations. Results show that foraminifera, and thus recorded palaeoclimatic conditions, may originate from areas up to several thousands of kilometres away, reflecting an ocean state significantly different from the core site. In the eastern equatorial regions and the western boundary current extensions, the offset may reach 1.5°C for species living for a month and 3.0°C for longer-living species. Oceanic transport hence appears to be a crucial aspect in the interpretation of proxy signals.

• Modal variability in the southeast Pacific Basin: energetics of the 2009 event.

  • W. Weijer, 2015.
  • Deep-Sea Research II, 114, 3-11; doi: 10.1016/j.dsr2.2012.10.002.
  • View abstract (inline) Full text (pdf)

    We study the barotropic variability in the Southeast Pacific Basin, in particular focusing on the extreme event during the fourth quarter of 2009. A 3-year integration of a barotropic shallow-water model forced with wind stress anomalies generates localized variability that is similar in spatial extent and amplitude as the observed anomalous event. An eigenmode analysis of the same model shows the presence of several free modes in the Southeast Pacific, but projection of the modal patterns on the model output shows that their amplitudes are low. Instead, the mode is interpreted as an almost-free mode. The modal excitation accounts for a considerable fraction (23% on average) of the kinetic energy input by the wind stress in the Southeast Pacific Basin, increasing to 38% for the anomalous event in 2009. Surprisingly, a similar but weaker event during the third quarter of 2008 appears to have been more significant from an energetics point of view, with almost 50% of the energy being input into the mode. Key areas of energetic dissipation appear to be the Eltanin Fracture Zone, the crest of the East Pacific Rise, and the Chile Rise/East Pacific Rise intersection.

• Impact of Agulhas Leakage on the Atlantic overturning circulation in the CCSM4.

  • W. Weijer and E. van Sebille, 2014.
  • Journal of Climate, 27, 101-11, doi: 10.1175/jcli-d-12-00714.1.
  • View abstract (inline) Full text (pdf)

    The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30–40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed.

• Sensitivity of a strongly eddying global ocean to North Atlantic freshwater perturbations.

  • M. den Toom, H. A. Dijkstra, W. Weijer, M. W. Hecht, M. E. Maltrud, and E. van Sebille, 2014.
  • Journal of Physical Oceanography, 44, 464-481, doi: 10.1175/jpo-d-12-0155.1.
  • View abstract (inline)

    The strongly eddying version of the Parallel Ocean Program (POP) is used in two 45-yr simulations to investigate the response of the Atlantic meridional overturning circulation (AMOC) to strongly enhanced freshwater input due to Greenland melting, with an integrated flux of 0.5 Sverdrups (Sv; 1 Sv = 10⁶ m³ s^-1). For comparison, a similar set of experiments is performed using a noneddying version of POP. The aim is to identify the signature of the salt advection feedback in the two configurations. For this reason, surface salinity is not restored in these experiments. The freshwater input leads to a quantitatively comparable reduction of the overturning strength in the two models. To examine the importance of transient effects in the relation between AMOC strength and density distribution, the results of the eddy-resolving model are related to water mass transformation theory. The freshwater forcing leads to a reduction of the rate of light to dense water conversion in the North Atlantic, but there is no change in dense to light transformation elsewhere, implying that high density layers are continuously deflating. The main focus of the paper is on the effect of the AMOC reduction on the basinwide advection of freshwater. The low-resolution model results show a change of the net freshwater advection that is consistent with the salt advection feedback. However, for the eddy-resolving model, the net freshwater advection into the Atlantic basin appears to be unaffected, despite the significant change in the large-scale velocity structure.

• Agulhas ring formation as a barotropic instability of the retroflection.

  • W. Weijer, V. V. Zharkov, D. Nof, H. A. Dijkstra, W. P. M. de Ruijter, A. Terwisscha van Scheltinga, and F. Wubs, 2013.
  • Geophysical Research Letters, 40, 5435-5438; doi: 10.1002/2013GL057751.
  • View abstract (inline) Full text (pdf)

    Agulhas Leakage is an important link in the global ocean circulation, as it transfers a significant volume of relatively warm and salty water from the Indian Ocean to the Atlantic Ocean. The main route of this transfer is through the shedding of large Agulhas rings from the Agulhas retroflection. In this paper we study the dynamics of the ring formation process by analyzing the stability of the Indian/Atlantic supergyre in a reduced gravity model. We show that the ring‒shedding process results from a barotropic instability of the steady circulation in the Agulhas retroflection region. The destabilizing mode appears to be linked to a Rossby basin mode of the combined South Indian/Atlantic basin, which is localized in the retroflection region by the background flow.

• Pacific Decadal Variability: paced by Rossby basin modes?

  • W. Weijer, E. Munoz, N. Schneider, and F. Primeau, 2013.
  • Journal of Climate, 26, 1445-1456, doi: 10.1175/jcli-d-12-00316.1.
  • View abstract (inline) Full text (pdf)

    A systematic study is presented of decadal climate variability in the North Pacific. In particular, the hypothesis is addressed that oceanic Rossby basin modes are responsible for enhanced energy at decadal and bidecadal time scales. To this end, a series of statistical analyses are performed on a 500-yr control integration of the Community Climate System Model, version 3 (CCSM3). In particular, a principal oscillation pattern (POP) analysis is performed to identify modal behavior in the subsurface pressure field.

    It is found that the dominant energy of sea surface temperature (SST) variability at 25 yr (the model equivalent of the Pacific decadal oscillation) cannot be explained by the resonant excitation of an oceanic basin mode. However, significant energy in the subsurface pressure field at time scales of 17 and 10 yr appears to be related to internal ocean oscillations. However, these oscillations lack the characteristics of the classical basin modes, and must either be deformed beyond recognition by the background circulation and inhomogeneous stratification or have another dynamical origin altogether. The 17-yr oscillation projects onto the Pacific decadal oscillation and, if present in the real ocean, has the potential to enhance the predictability of low-frequency climate variability in the North Pacific.

• Response of the Atlantic Ocean circulation to Greenland ice sheet melting in a strongly-eddying ocean model.

  • W. Weijer, M. E. Maltrud, M. W. Hecht, H. A. Dijkstra, and M. Kliphuis, 2012.
  • Geophysical Research Letters, 39, L09606; doi: 10.1029/2012GL051611.
  • View abstract (inline) Full text (pdf)

    The sensitivity of the Atlantic Meridional Overturning Circulation (AMOC) to high-latitude freshwater input is one of the key uncertainties in the climate system. Considering the importance of the AMOC for global heat transports, and the vulnerability of the Greenland Ice Sheet (GrIS) to global warming, assessing this sensitivity is critical for climate change projections. Here we present a unique set of computational experiments to investigate the adjustment of the AMOC to enhanced melt water from the GrIS under present-day conditions. For the first time, the response in a global, strongly-eddying ocean model is systematically compared to that of an ocean model typical of IPCC-class climate models. We find that the overall decline of the AMOC on decadal time scales is quantitatively similar (<10%) in the two configurations. Nonetheless, the transient response is significantly different, as the AMOC decline and reduction in wintertime convection is markedly more gradual and persistent in the strongly-eddying configuration.

• Mean and variability of the tropical Atlantic Ocean in the CCSM4.

  • E. Munoz, W. Weijer, S. Grodsky, S. C. Bates, and I. Wainer, 2012.
  • Journal of Climate, 25, 4860-4882, doi: 10.1175/jcli-d-11-00294.1.
  • View abstract (inline)

    This study analyzes important aspects of the tropical Atlantic Ocean from simulations of the fourth version of the Community Climate System Model (CCSM4): the mean sea surface temperature (SST) and wind stress, the Atlantic warm pools, the principal modes of SST variability, and the heat budget in the Benguela region. The main goal was to assess the similarities and differences between the CCSM4 simulations and observations. The results indicate that the tropical Atlantic overall is realistic in CCSM4. However, there are still significant biases in the CCSM4 Atlantic SSTs, with a colder tropical North Atlantic and a hotter tropical South Atlantic, that are related to biases in the wind stress. These are also reflected in the Atlantic warm pools in April and September, with its volume greater than in observations in April and smaller than in observations in September. The variability of SSTs in the tropical Atlantic is well represented in CCSM4. However, in the equatorial and tropical SouthAtlantic regions, CCSM4 has two distinct modes of variability, in contrast to observed behavior. A model heat budget analysis of the Benguela region indicates that the variability of the upper-ocean temperature is dominated by vertical advection, followed by meridional advection.

• The Southern Ocean and its climate in CCSM4.

  • W. Weijer, B. M. Sloyan, M. E. Maltrud, N. Jeffery, M. W. Hecht, C. A. Hartin, E. van Sebille, I. Wainer, and L. Landrum, 2012.
  • Journal of Climate, 25, 2652-2675, doi: 10.1175/jcli-d-11-00302.1.
  • View abstract (inline) Full text (pdf)

    The new Community Climate System Model, version 4 (CCSM4), provides a powerful tool to understand and predict the earth’s climate system. Several aspects of the Southern Ocean in the CCSM4 are explored, including the surface climatology and interannual variability, simulation of key climate water masses (Antarctic Bottom Water, Subantarctic Mode Water, and Antarctic Intermediate Water), the transport and structure of the Antarctic Circumpolar Current, and interbasin exchange via the Agulhas and Tasman leakages and at the Brazil–Malvinas Confluence. It is found that the CCSM4 has varying degrees of accuracy in the simulation of the climate of the Southern Ocean when compared with observations. This study has identified aspects of the model that warrant further analysis that will result in a more comprehensive understanding of ocean–atmosphere–ice dynamics and interactions that control the earth’s climate and its variability.

• Varied representation of the Atlantic Meridional overturning circulation across multidecadal ocean reanalyses.

  • E. Munoz, B. Kirtman, and W. Weijer, 2011.
  • Deep Sea Research, 58, 1848-1857, doi: 10.1016/j.dsr2.2010.10.064.
  • View abstract (inline)

    The Atlantic Meridional Overturning Circulation (AMOC) is an integral part of the circulation in the Atlantic Ocean. The AMOC is typically characterized by the meridional streamfunction, which is also an indicator of the thermohaline circulation. This paper explores the mean state and long-term variability of the Atlantic meridional streamfunction and Atlantic meridional heat transport from six multidecadal ocean reanalysis. Statistics based on the meridional overturning streamfunction and the meridional heat transport are computed in the same manner across the products, and analyzed. The maximum streamfunction values in the Atlantic are located between 35N and 50N for the various ocean reanalyses. The Atlantic heat transport is greater between the Greater Antilles (at about 20N) and 25N, similar to the observations. The streamfunction and heat transport have stronger seasonality in the deep tropics. In the North Atlantic the annual harmonic of the streamfunction has greater amplitude in July between 40N and 50N. Even though there are differences in the mean strength of the streamfunction and the heat transport, there are robust similarities in their seasonal cycle across ocean reanalyses.

    Furthermore, although the year-to-year variability of the meridional heat transport is not consistent across products, there are other aspects that are consistent. The relationship between streamfunction and meridional heat transport at 30N (i.e., in the northern hemisphere) shows greater consistency than that between streamfunction and meridional heat transport at 30S (i.e., in the southern hemisphere). Also, even though the North Atlantic sea surface temperature (SST) anomalies are greatly consistent, their relationship with the meridional streamfunction is disparate. However, the relationship between North Atlantic SST anomalies and meridional heat transport shows some consistency with respect to the meridional profile. The greater numbers of temperature data assimilated may help in constraining the meridional heat transport more than the streamfunction, thereby indicating the meridional heat transport to be an important field to analyze with respect to observed changes in the AMOC.

• Retroflection from a double slanted coastline: a model for the Agulhas leakage variability.

  • V. Zharkov, D. Nof and W. Weijer, 2010.
  • Ocean Science, 6, 997-1011, doi: 10.5194/os-6-997-2010.
  • View abstract (inline)

    The Agulhas leakage to the South Atlantic exhibits a strong anti-correlation with the mass flux of the Agulhas Current. When the Agulhas retroflection is in its normal position near Cape Agulhas, leakage is relatively high and the nearby South African coastal slant (angle of derivation from zonal) is very small and relatively invariant alongshore. During periods of strong incoming flux (low leakage), the retroflection shifts upstream to Port Elizabeth or East London, where the coastline shape has a "kink", i.e., the slant changes abruptly from small on the west side, to large (about 55 deg) on the east side. Here, we show that the variability of rings shedding and anti-correlation between Agulhas mass flux and leakage to the South Atlantic may be attributed to this kink.

    To do so, we develop a nonlinear analytical model for retroflection near a coastline that consists of two sections, a zonal western section and a strongly slanted eastern section. The principal difference between this and the model of a straight slanted coast (discussed in our earlier papers) is that, here, free purely westward propagation of eddies along the zonal coastline section is allowed. This introduces an interesting situation in which strong slant of the coast east of the kink prohibits the formation and shedding of rings, while the almost zonal coastal orientation west of the kink encourages shedding. Therefore, the kink "locks" the position of the retroflection, forcing it to occur just downstream of the kink. Rings are necessarily shed from the retroflection area in our kinked model, regardless of the degree of eastern coast slant. In contrast, a no-kink model with a coastline of intermediate slant indicates that shedding is almost completely arrested by that slant.

    We suggest that the observed difference in ring-shedding intensity during times of normal retroflection position and times when the retroflection is shifted eastward is due to the change in the retroflection location with respect to the kink. When the incoming flux detaches from the coast north of the kink, ring transport is small; when the flux detaches south of the kink, transport is large. Simple process-oriented numerical simulations are in fair agreement with our analytical results.

• An almost-free barotropic mode in the Australian-Antarctic Basin.

  • W. Weijer, 2010.
  • Geophysical Research Letters, 37, L10602; doi: 10.1029/2010GL042657.
  • View abstract (inline) Full text (pdf)

    The Australian-Antarctic Basin (AAB) is known for its high levels of intraseasonal variability; sea-surface height variability exceeds background values by factors of 2 over thousands of kilometers. This paper addresses the hypothesis that this variability is caused by trapping of barotropic energy by the basin geometry. Analysis of a multi-year integration of a shallow-water model shows that the variability is dominated by a single, large-scale statistical mode that is highly coherent over the entire AAB. The flow associated with this mode is northwestward along the Southeast Indian Ridge, southward in the Kerguelen Abyssal Plain, and eastward in the southern AAB. The mode is interpreted as an almost-free topographically trapped mode, as it is confined by contours of potential vorticity that almost entirely enclose the AAB. The apex of the Wilkes Abyssal Plain represents the strongest barrier to the modal circulation: here velocities are strongest, making it a key area for dissipation of kinetic energy through bottom friction and eddy viscosity.

• Modal decay in the Australia-Antarctic Basin.

  • W. Weijer, S. T. Gille and F. Vivier, 2009.
  • Journal of Physical Oceanography, 39, 2893-2909, doi: 10.1175/2009jpo4209.1.
  • View abstract (inline) Full text (pdf)

    The barotropic intraseasonal variability in the Australia–Antarctic Basin (AAB) is studied in terms of the excitation and decay of topographically trapped barotropic modes. The main objective is to reconcile two widely differing estimates of the decay rate of sea surface height (SSH) anomalies in the AAB that are assumed to be related to barotropic modes. First, an empirical orthogonal function (EOF) analysis is applied to almost 15 years of altimeter data. The analysis suggests that several modes are involved in the variability of the AAB, each related to distinct areas with (almost) closed contours of potential vorticity. Second, the dominant normal modes of the AAB are determined in a barotropic shallow-water (SW) model. These stationary modes are confined by the closed contours of potential vorticity that surround the eastern AAB, and the crest of the Southeast Indian Ridge. For reasonable values of horizontal eddy viscosity and bottom friction, their decay time scale is on the order of several weeks. Third, the SW model is forced with realistic winds and integrated for several years. Projection of the modal velocity patterns onto the output fields shows that the barotropic modes are indeed excited in the model, and that they decay slowly on the frictional O(3 weeks) time scale. However, the SSH anomalies in the modal areas display rapid O(4 days) decay. Additional analysis shows that this rapid decay reflects the adjustment of unbalanced flow components through the emission of Rossby waves. Resonant excitation of the dominant free modes accounts for about 20% of the SSH variability in the forced-model run. Other mechanisms are suggested to explain the region of high SSH variability in the AAB.

• A scalable and adaptable solution framework within components of the the Community Climate System Model.

  • K. J. Evans, D. W. I. Rouson, A. G. Salinger, M. A. Taylor, W. Weijer and J. B. White III, 2009.
  • Lecture Notes in Computer Science, 5545, 332-341,  doi: 10.1007/978-3-642-01973-9_37.
  • View abstract (inline)

    A framework for a fully implicit solution method is implemented into (1) the High Order Methods Modeling Environment (HOMME), which is a spectral element dynamical core option in the Community Atmosphere Model (CAM), and (2) the Parallel Ocean Program (POP) model of the global ocean. Both of these models are components of the Community Climate System Model (CCSM). HOMME is a development version of CAM and provides a scalable alternative when run with an explicit time integrator. However, it suffers the typical time step size limit to maintain stability. POP uses a time-split semi-implicit time integrator that allows larger time steps but less accuracy when used with scale interacting physics. A fully implicit solution framework allows larger time step sizes and additional climate analysis capability such as model steady state and spin-up efficiency gains without a loss in scalability. This framework is implemented into HOMME and POP using a new Fortran interface to the Trilinos solver library, ForTrilinos, which leverages several new capabilities in the current Fortran standard to maximize robustness and speed. The ForTrilinos solution template was also designed for interchangeability; other solution methods and capability improvements can be more easily implemented into the models as they are developed without severely interacting with the code structure. The utility of this approach is illustrated with a test case for each of the climate component models.

• Normal modes of the Mascarene Basin.

  • W. Weijer, 2008.
  • Deep-Sea Research I, 55, 128-136, doi: 10.1016/j.dsr.2007.10.005.
  • View abstract (inline) Full text (pdf)

    In this paper the origin of the bi-monthly variability in the Mascarene Basin is reconsidered. Free oscillatory modes of the Mascarene Basin are determined by performing normal mode analysis on the motionless solution in a barotropic shallow-water model with realistic bathymetry. Several modes are identified with monthly to bi-monthly time scales. The mode that agrees best with recent current meter observations can be interpreted as a barotropic Rossby basin mode, confined to the tilted geometry of the Mascarene Basin.

• Multiple oscillatory modes of the Argentine Basin. Part I: Statistical analysis.

  • W. Weijer, F. Vivier, S. T. Gille, and H. A. Dijkstra, 2007.
  • Journal of Physical Oceanography, 37, 2855-2868, doi: 10.1175/2007jpo3527.1.
  • View abstract (inline); Full text (pdf)

    Observations of the sea-surface height in the Argentine Basin indicate that strong variability occurs on a time scale of 20-30 days. The aim of this study is to determine the physical processes responsible for this variability.

    First, results are presented from two statistical techniques applied to a decade of altimetric data. A Complex Empirical Orthogonal Function (CEOF) analysis identifies the recently discovered dipole mode as the dominant mode of variability. A Principal Oscillation Pattern (POP) analysis confirms the existence of this mode, which has a period of 25 days. The second CEOF displays a propagating pattern in the northern Argentine Basin, plus a rotating dipole in the southwest corner. The POP analysis identifies both patterns as individual modes, with periods of 30 and 20 days, respectively.

    Second, the barotropic normal modes of the Argentine Basin are studied, using a shallow-water model capturing the full bathymetry of the basin. Coherences between the spatial patterns of these modes and altimeter data suggest that several of the basin modes are involved in the observed variability.

    This analysis implies that the 20-day mode detected by recent bottom pressure measurements is a true barotropic mode. However, the 25-day variability, as found in altimeter data, cannot be directly attributed to the excitation of a free Rossby basin mode.

    This study indicates that the results of several apparently conflicting observations of the flow variability in the Argentine Basin can be reconciled by assuming that multiple basin modes are involved.

• Multiple oscillatory modes of the Argentine Basin. Part II: The spectral origin of basin modes.

  • W. Weijer, F. Vivier, S. T. Gille, and H. A. Dijkstra, 2007.
  • Journal of Physical Oceanography, 37, 2869-2881, doi: 10.1175/2007jpo3688.1.
  • View abstract (inline); Full text (pdf)

    In this paper the spectrum of barotropic basin modes of the Argentine Basin is shown to be connected to the classical Rossby basin modes of a flat-bottom (constant depth), rectangular basin.

    First, the spectrum of basin modes is calculated for the Argentine Basin, by performing a normal mode analysis of the barotropic shallow-water equations. Then a homotopy transformation is performed that gradually morphs the full-bathymetry geometry through a flat-bottom configuration into a rectangular basin. Following the eigenmodes through this transition establishes a connection between most of the basin modes and the classical Rossby basin modes of a rectangular geometry. In particular, the 20-day mode of the Argentine Basin is identified with the lowest order mode of classical theory.

    Sensitivity studies show that the decay rate of each mode is controlled by bottom friction, but that it is insensitive to lateral friction; lateral friction strongly impacts the oscillation frequency. In addition, the modes are found to be only slightly sensitive to the presence of a background flow.

• Energetics of wind-driven barotropic variability in the Southern Ocean.

  • W. Weijer, and S. T. Gille, 2005.
  • Journal of Marine Research, 63, 1101-1125, doi: 10.1357/002224005775247562.
  • View abstract (inline); Full text (pdf)

    This study addresses the energetics of the Southern Ocean, in response to high-frequency wind forcing. A constant-density, multi-layer model is forced with a band of stochastically varying wind stress. The focus is on the interplay between the surface layer and the interior circulation.

    In line with previous examinations, it is concluded that the interior ocean is not directly energized by the wind work, but rather through the work done by the pressure field. The spatial and temporal characteristics of these terms differ substantially. Although the wind work may be negative in extensive regions of the World Ocean, the pressure work energizes the interior circulation almost everywhere.

    For low-frequency variability, the total work done by the wind and pressure on the barotropic flow is comparable, but discrepancies may arise for high-frequency variability. A mechanism is identified through which kinetic energy can leak from the wind-driven surface layer to the barotropic flow.

• Adjustment of the Southern Ocean to wind forcing on synoptic time scales.

  • W. Weijer, and S. T. Gille, 2005.
  • Journal of Physical Oceanography, 35, 2076-2089, doi: 10.1175/jpo2801.1.
  • View abstract (inline); Full text (pdf)

    This study addresses the response of the Southern Ocean to high-frequency wind forcing, focusing on the impact of several barotropic modes on the circumpolar transport. A suite of experiments is performed with an unstratified model of the Southern Ocean, forced with a stochastic wind stress that contains a large range of frequencies with synoptic time scales.

    The Southern Ocean adjustment displays a different character for frequencies below and above 0.2 cpd. The low-frequency range is dominated by an 'almost-free-mode' response, in the region where contours of $f/H$ are obstructed by only a few bathymetric features; the truly free-mode only plays a minor role. Topographic form stress, rather than friction, is the dominant decay mechanism of the Southern Mode. It leads to a spin-down time scale of the order of 3 days.

    For the high-frequency range, the circumpolar transport is dominated by the resonant excitation of oscillatory modes. The 'active' response of the ocean leads to strong changes and even discontinuities in the phase relation between transport and wind stress.

• High-frequency wind forcing of a channel model of the ACC: Normal mode excitation.

  • W. Weijer, 2005.
  • Ocean Modelling, 9, 31-50, doi: 10.1016/j.ocemod.2004.04.001.
  • View abstract (inline); Full text (pdf)

    The response of the Antarctic circumpolar circulation to wind stress variability is studied in a simple model of the Southern Ocean. The model consists of a zonally reentrant channel with mid-ocean barrier, forced by surface heat flux and stochastic wind stress. The MITgcm code is used to solve for temperature, sea level, and the velocity field.

    The channel transport responds actively to the stochastic wind forcing through the excitation of several eigenmodes. The fundamental basin mode is excited, which has a period of about 20 days. However, the response is dominated by a topographic mode with a period of 3 days. This barotropic mode is related primarily to topographic Rossby waves propagating on the submarine sill, and Kelvin waves propagating westward on the southern boundary. The mode shares characteristics with numerical predictions of normal modes in the Southern Ocean, as well as with recently observed modes of variability around Antarctica, with periods between 1 and 2 days. These results show that both topographic normal modes and Rossby basin modes can play a role in the adjustment of the ACC to high-frequency wind stress variability.

    The presence of the resonance in the system leads to interesting phase behavior between the wind stress forcing and the ocean transport: for a considerable frequency range, the transport seems to lead the wind stress variability. This shows that the classical notion of transport passively lagging wind forcing is inadequate when eigenmodes are excited.

• Stability of the global ocean circulation: basic bifurcation diagrams.

  • H. A. Dijkstra and W. Weijer, 2005.
  • Journal of Physical Oceanography, 35, 933-948, doi: 10.1175/jpo2726.1.
  • View abstract (inline); Full text (pdf)

    A study of the stability of the global ocean circulation is performed within a coarse-resolution general circulation model. Using techniques of numerical bifurcation theory, steady states of the global ocean circulation are explicitly calculated as parameters are varied. Under a freshwater flux forcing that is diagnosed from a reference circulation with Levitus surface salinity fields, the global ocean circulation has no multiple equilibria. It is shown how this unique-state regime transforms into a regime with multiple equilibria as the pattern of the freshwater flux is changed in the northern North Atlantic Ocean. In the multiple-equilibria regime, there are two branches of stable steady solutions: one with a strong northern overturning in the Atlantic and one with hardly any northern overturning. Along the unstable branch that connects both stable solution branches (here for the first time computed for a global ocean model), the strength of the southern sinking in the South Atlantic changes substantially. The existence of the multiple-equilibria regime critically depends on the spatial pattern of the freshwater flux field and explains the hysteresis behavior as found in many previous modeling studies.

• A systematic approach to determine thresholds of the ocean's thermohaline circulation.

  • H. A. Dijkstra, L. te Raa and W. Weijer, 2004.
  • Tellus, 56A, 362-370, doi: 10.3402/tellusa.v56i4.14417.
  • View abstract (inline); Full text (pdf)

    A systematic approach is proposed to determine thresholds in freshwater flux perturbations related to abrupt changes in the ocean's thermohaline circulation. The typical problem considered is the response of the thermohaline driven flow to a localized change, of specified strength and duration, in the surface freshwater flux. The initial response due to the freshwater anomaly is considered as a finite amplitude perturbation. An estimate of this response can be obtained by using ideas from dynamical systems theory. Central quantity to determine whether such a perturbation leads to instability (i.e. a `collapsed' state) is the sign of the tendency of a specific energy functional. The approach is first illustrated with a simple box model and then shown to give good results in a global ocean general circulation model.

• Stability of the global ocean circulation: the connection of equilibria in a hierarchy of models.

  • H. A. Dijkstra and W. Weijer, 2003.
  • Journal of Marine Research, 61, 725-743, doi: 10.1357/002224003322981129.
  • View abstract (inline); Full text (pdf)

    We address the problem of the multiple equilibria of the thermohaline circulation in a hierarchy of models. The understanding of the relation between bifurcation diagrams of box models, two-dimensional models and those of a global ocean general circulation model, is facilitated through analysis of the equilibrium solutions of a three-dimensional Atlantic-like sector model with an open southern channel. Using this configuration, the subtle effects of the wind-stress field, the effects of continental asymmetry and the asymmetry in the surface freshwater flux can be systematically studied. The results clarify why there is an asymmetric Atlantic circulation under a near equatorially-symmetric buoyancy forcing. They also lead to an explanation of the hysteresis regime that is found in models of the global ocean circulation. Both explanations are crucial elements to understanding the role of the ocean in past and future climate changes.

• Multiple oscillatory modes of the global ocean circulation.

  • W. Weijer and H. A. Dijkstra, 2003.
  • Journal of Physical Oceanography, 33, 2197-2213, doi: 10.1175/1520-0485(2003)033<2197:momotg>2.0.co;2.
  • View abstract (inline); Full text (pdf)

    For the first time, the (linear) stability of the global ocean circulation has been determined explicitly. In a low-resolution general circulation model, a steady state is computed directly by solving the elliptic boundary value problem. The stability of this solution is determined by solving the generalized eigenvalue problem. Although the steady global circulation is (linearly) stable, there are two interesting oscillatory modes among the least stable ones, with periods of about 3800 and 2300 yr. These modes are characterized by buoyancy anomalies that propagate through the ocean basins as they are advected by the global overturning circulation. The millennial time scale is set by the time it takes for anomalies to travel, at depth, from the North Atlantic to the North Pacific. Further analyses confirm that the advective feedback between the steady flow and buoyancy anomalies is an essential process in the propagation mechanism. The growth rate of the millennial modes is controlled by vertical mixing. It is argued that these internal ocean modes may be a relevant mechanism for global climate variability on millennial time scales.

• A fully-implicit model of the global ocean circulation.

  • W. Weijer, H. A. Dijkstra, H. Oksuzoglu, F. W. Wubs and A. C. de Niet, 2003.
  • Journal of Computational Physics, 192, 452-470, doi: 10.1016/j.jcp.2003.07.017.
  • View abstract (inline); Full text (pdf)

    With the recent developments in the solution methods for large-dimensional nonlinear algebraic systems, fullyimplicit ocean circulation models are now becoming feasible. In this paper, the formulation of such a three-dimensional global ocean model is presented. With this implicit model, the sensitivity of steady states to parameters can be investigated efficiently using continuation methods. In addition, the implicit formulation allows for much larger time steps than can be used with explicit models. To demonstrate current capabilities of the implicit global ocean model, we use a relatively low-resolution (43 horizontally and 12 levels vertically) version. For this configuration, we present: (i) an explicit calculation of the bifurcation diagram associated with hysteresis behavior of the ocean circulation and (ii) the scaling behavior of the Atlantic meridional overturning versus the magnitude of the vertical mixing coefficient of heat and salt.

• Imperfections of the three-dimensional thermohaline circulation: Hysteresis and unique-state regimes.

  • H. A. Dijkstra, W. Weijer and J. D. Neelin, 2003.
  • Journal of Physical Oceanography, 33, 2796-2814, doi: 10.1175/1520-0485(2003)033<2796:iotttc>2.0.co;2.
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    Different equilibria of oceanic thermohaline circulation may exist under the same forcing conditions. One of the reasons for the existence of these multiple equilibria is a feedback between the overturning circulation and the advective transport of salt and heat. In an equatorially symmetric configuration, the multiple equilibria arise through symmetry-breaking pitchfork bifurcations when the strength of the freshwater forcing is increased. Here, continuation methods are used to track the fate of the different equilibria under equatorially asymmetric conditions in a three-dimensional, low-resolution ocean general circulation model in an Atlantic-like basin coupled to an energy-balance atmosphere model. The effect of the continental geometry, the presence of the Antarctic Circumpolar Current (ACC), and asymmetric air-sea interaction on the preference of equilibria are considered. Although all asymmetry-inducing mechanisms favor northern Atlantic sinking states, the open Southern Ocean and ACC are shown to be substantial contributors. The origin of the hysteresis behavior between strong and weak overturning states is clarified in terms of the overall bifurcation picture. The disappearance of a class of southern sinking equilibria because of the combined effects of all asymmetry mechanisms leads to a substantial regime with a unique steady state. The relationship between the hysteresis regime and the unique-state regime provides a larger context for quantitative determination of the relevance of each to climate.

• Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy.

  • W. Weijer, W. P. M. De Ruijter, A. Sterl and S. S. Drijfhout, 2002.
  • Global and Planetary Change, 34, 293-311,  doi: 10.1016/s0921-8181(02)00121-2.
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    The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage is found to influence the Atlantic overturning circulation in a low-resolution Ocean General Circulation Model. The model used is the Hamburg Large-Scale Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas Leakage is parameterized by sources of heat and salt in the upper South Atlantic Ocean, that extend well into the intermediate layers.

    It is shown that the model's overturning circulation is sensitive to the applied sources of heat and salt. The response of the overturning strength to changes in the source amplitudes is mainly linear, interrupted once by a stepwise change. The South Atlantic buoyancy sources influence the Atlantic overturning strength by modifying the basin-scale meridional density and pressure gradients. The non-linear, stepwise response is caused by abrupt changes in the convective activity in the northern North Atlantic.

    Two additional experiments illustrate the adjustment of the overturning circulation upon sudden introduction of heat and salt sources in the South Atlantic. The North Atlantic overturning circulation responds within a few years after the sources are switched on. This is the time it takes for barotropic and baroclinic Kelvin waves to reach the northern North Atlantic. The advection of the anomalies takes 3 decades to reach the northern North Atlantic.

    The model results give support to the hypothesis that the re-opening of the Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological data, may have stimulated the coincident strengthening of the Atlantic overturning circulation.

• A bifurcation study of the three-dimensional thermohaline ocean circulation: the double-hemispheric case.

  • W. Weijer and H. A. Dijkstra, 2001.
  • Journal of Marine Research, 59, 599-631, doi: 10.1357/002224001762842208.
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    Within a low-resolution primitive-equation model of the three-dimensional ocean circulation, a bifurcation analysis is performed of double-hemispheric basin flows. Main focus is on the connection between results for steady two-dimensional flows in a non-rotating basin and those for three-dimensional flows in a rotating basin. With the use of continuation methods, branches of steady states are followed in parameter space and their linear stability is monitored. There is a close qualitative similarity between the bifurcation structure of steady-state solutions of the two- and three dimensional flows. In both cases, symmetry-breaking pitchfork bifurcations are central in generating a multiple equilibria structure. The locations of these pitchfork bifurcations in parameter space can be characterized through a zero of the tendency of a particular energy functional. Although balances controlling the steady-state flows are quantitatively very different, the zonally averaged patterns of the perturbations associated with symmetry-breaking are remarkably similar for two-dimensional and three-dimensional flows, and the energetics of the symmetry-breaking mechanism is in essence the same.

• Stability of the Atlantic overturning circulation: Competition between Bering Strait freshwater flux and Agulhas heat and salt sources.

  • W. Weijer, W. P. M. De Ruijter and H. A. Dijkstra, 2001.
  • Journal of Physical Oceanography, 31, 2385-2402, doi: 10.1175/1520-0485(2001)031<2385:sotaoc>2.0.co;2.
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    The role played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean is examined. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, the Drake Passage and via Agulhas Leakage) are represented by sources and sinks. The profiles and amplitudes of these sources are based mainly on the heat and salt fluxes in a high-resolution ocean model (OCCAM).

    When applying realistic sources and sinks, a circulation is favored that is characterized by major downwelling in the northern hemisphere (NPP circulation), and resembles the present-day Atlantic overturning circulation. The Southern Ocean sources appear to stabilize this circulation, whereas Bering Strait freshwater input tends to destabilize it. Already a small buoyancy input at southerly latitudes is enough to prohibit the existence of a southern sinking circulation (SPP), leaving the NPP circulation as a unique and stable solution. A large, factor three increase in Bering Strait freshwater import would be necessary to bring the SPP circulation back into existence.

    Especially the Indian-Atlantic transfer of heat and salt, brought about by Agulhas Leakage, contributes considerably to the strength and, in particular, the stability of the northern sinking circulation. According to this model, shutting off Agulhas Leakage, and consequently the so-called warm water route for North Atlantic Deep Water (NADW) compensation, leads to a reduction of the overturning strength by 10% at most. These results imply that the way in which the NADW renewal takes place has implications for both the strength and stability of the Atlantic overturning circulation, giving the discussion about the warm vs. cold water route for NADW compensation dynamical significance.

    Moreover, when the stabilizing effect of Agulhas Leakage on the overturning disappears, the destabilizing influence of the Bering Strait freshwater input becomes more effective. The system is then close to a regime where the northern and southern overturning circulations coexist as stable solutions. Perturbations in Bering Strait inflow may then easily lead to switches between the two circulation states. These results suggest that the absence of Agulhas Leakage during the last ice-age may have contributed to weakening the glacial overturning circulation in the Atlantic. It may have made the thermohaline circulation vulnerable to variability, caused either by regime switches, or by the excitation of oscillatory modes. The sudden restart of the Atlantic overturning circulation at the beginning of the Holocene may well have been stimulated by the coincident reopening of the Agulhas gap.

    The presence of Agulhas Leakage may contribute to the relative stability of Holocene climate. Present-day climate may thus be more stable than previously thought.

• Impact of interbasin exchange on the Atlantic overturning circulation.

  • W. Weijer, W. P. M. De Ruijter, H. A. Dijkstra and P. J. van Leeuwen, 1999.
  • Journal of Physical Oceanography, 29, 2266-2284, 3184, doi: 10.1175/1520-0485(1999)029<2266:ioieot>2.0.co;2.
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    The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a data set based on WOCE hydrographic data. It is shown that the salt and heat transports brought about by the South Atlantic subtropical gyre play an essential role in the Atlantic heat and salt budgets. It is found that on average the exported North Atlantic Deep Water (NADW) is fresher than the return flows (basically composed of thermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater from the Atlantic.

    The sensitivity of the OC to interbasin fluxes of heat and salt is studied in a 2D model, representing the Atlantic between 60°N and 30°S. The model is forced by mixed boundary conditions at the surface, and by realistic fluxes of heat and salt at its 30°S boundary. The model circulation turns out to be very sensitive to net buoyancy fluxes through the surface. Both net surface cooling and net surface saltening are sources of potential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the stratification, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition of the NADW return transports, whether by a change in the ratio thermocline/intermediate water, or by a change in their thermohaline characteristics, might influence the Atlantic OC considerably.

    It is also shown that the circulation is much more sensitive to changes in the shape of the lateral buoyancy flux than to changes in the shape of the surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes of heat and salt are important for the strength and operation of the Atlantic thermohaline circulation, and should be correctly represented in models that are used for climate sensitivity studies.

• Indian-Atlantic interocean exchange: Dynamics, estimation and impact.

  • W. P. M. de Ruijter, A. Biastoch, S. S. Drijfhout, J. R. E. Lutjeharms, R. P.Matano, T. Pichevin, P. J. van Leeuwen and W. Weijer, 1999.
  • Journal of Geophysical Research, 104, 20885-20910, doi: 10.1029/1998jc900099.
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    Interocean exchange of heat and salt around South Africa is thought to be a key link in the maintenance of the global overturning circulation of the ocean. It takes place at the Agulhas Retroflection, largely by the intermittent shedding of enormous rings that penetrate into the South Atlantic Ocean. This makes it extremely hard to estimate the interocean fluxes. Estimates of direct Agulhas Leakage from hydrographic and tracer data range between 2 and 10 Sv (1 Sv = 10⁶ m³/s). The average ring shedding frequency, determined from satellite information, is approximately six rings per year. Their associated interocean volume transport is between 0.5 and 1.5 Sv per ring.

    A number of Agulhas rings have been observed to cross the South Atlantic. They decay exponentially to less than half their initial size (measured by their available potential energy) within 1000 km from the shedding region. Consequently, most of their properties mix into the surroundings of the Benguela region, probably feeding directly into the upper (warm) limb of the global thermohaline circulation. The most recent observations suggest that in the present situation Agulhas water and Antarctic Intermediate Water are about equally important sources for the Benguela Current. Variations in the strength of these may lead to anomalous stratification and stability of the Atlantic at decadal and longer timescales.

    Modeling studies suggest that the Indian-Atlantic interocean exchange is strongly related to the structure of the wind field over the South Indian Ocean. This leads in the mean to a subtropical supergyre wrapping around the subtropical gyres of the South Indian and Atlantic Oceans. However, local dynamical processes in the highly nonlinear regime around South Africa play a crucial role in inhibiting the connection between the two oceans. The regional bottom topography also seems to play an important role in locking the Agulhas Current's retroflection. State-of-the-art global and regional "eddy-permitting" models show a reasonably realistic representation of the mean Agulhas system, but the mesoscale variability and the local geometrical and topographic features that determine largely the interocean fluxes still need considerable improvement.

    In this article we present a review of the above mentioned aspects of the interocean exchange around South Africa: the estimation of the fluxes into the South Atlantic from different types of observations, our present level of understanding of the exchange dynamics and forcing, its representation in state-of-the-art models, and, finally, the impact of the Indian-Atlantic fluxes on regional and global scale, both within the Atlantic Ocean and in interaction with the overlying atmosphere.