Southern Ocean

The Southern Ocean is a region of extremes: it is exposed to the most severe winds, it touches on the largest ice shelves, has the most extensive seasonal sea ice cover, and houses one of the most productive (albeit iron-starved) ecosystems on our planet. The interactions between the ocean, atmosphere, cryosphere and marine ecosystems are intense, but greatly influence the global climate system. Our HiLAT project aims to understand these interactions and feedbacks, and the associated impacts on the climate system.

Southern Annular mode in CCSM4

A faithful representation of these processes in coupled climate models is therefore essential for reliable climate projections. In Weijer et al. (2012) we studied several aspects of the Southern Ocean in the Community Climate System Model version 4 (CCSM4), addressing topics like the surface climatology and its interannual variability, the representation of water masses, the structure of the Antarctic Circumpolar Current, and interocean exchanges. I co-edited a special issue of Deep Sea Research II focusing on Southern Ocean dynamics and biogeochemistry in a changing climate (Downes et al. 2015).

Dynamically, the Southern Ocean is unique in the fact that it lacks continental boundaries at a range of latitudes. This has important implications for the dynamics that govern the ocean circulation there. In collaboration with Sarah Gille at Scripps Institution of Oceanography, I studied the response of the Southern Ocean to high-frequency (order days to weeks) variability of Southern Ocean winds. I forced the MITgcm with stochastic winds, and found that the response was dominated by barotropic modes (Weijer 2005; Weijer and Gille 2005a). The so-called Southern Mode (Hughes et al., 1999) was found to account for a "reddening" of the spectrum (since it is a non-oscillatory mode), while several topographic modes gave rise to resonant peaks. The resonance yielded an interesting coherence between zonal transport and wind stress for periods below about a week.

The decay of these modes appeared to be controlled by circulation/bathymetry interaction, independent of frictional parameters. However, friction obviously plays a crucial role in the energy balance. In Weijer and Gille (2005b) we explored the energy flow from power input by the wind stress to the dissipation by frictional effects in more detail.