# Overturning Circulation

The Meridional Overturning Circulation in the Atlantic Ocean (AMOC) is an important component of the climate system. Knowledge of its stability and variability is essential to understand past and future climate fluctuations. We recently compared the sensitivity of the AMOC with respect to Greenland freshwater forcing in ocean models with different resolutions (Weijer et al. 2012). We found that in a strongly-eddying ocean model the AMOC response is stronger than in a non-eddying, IPCC-class ocean model, and that the qualitative response is different as well. Explicitly resolving eddy transports is hence crucial for a correct representation of AMOC sensitivity.

This study was followed up by den Toom et al. (2014), where we addressed the question whether the salt advection feedback is still active in a strongly eddying overturning circulation. No evidence for this feedback was found, putting into question whether concepts of AMOC stability learned from low-resolution models carry over to the turbulent regime.

However, conventional ocean models are not well-suited to study the stability and sensitivity of the AMOC. In collaboration with Henk Dijkstra, I was involved in the development of a fully-implicit climate model, THCM (Weijer et al. 2003). This model uses concepts from dynamical systems theory and advanced numerical methods to solve steady patterns of the ocean circulation and their linear stability. For a 5 degree global configuration, we determined the least stable modes that threaten the stability of a decent global overturing circulation. Among the three least stable modes, two oscillatory modes were found, and one real mode. The oscillatory modes could be identified as overturning oscillations, for which temperature and salinity anomalies are advected by the global overturning circulation (Weijer and Dijkstra 2003).

For an even higher resolution, we explicitly computed the classical hysteresis loop of the global overturning circulation with respect to North Atlantic freshwater input; not by time-integration and monitoring quasi-equilibrium behaviour, but through parameter continuation. This allowed us to identify even the unstable branch connecting the conveyor belt circulation and the collapsed state (Dijkstra and Weijer 2003; Dijkstra and Weijer 2005).

Before turning to the global problems, I studied the thermohaline circulation in an idealized double hemispheric configuration, with equatorially symmetric surface forcing. In this context, equatorially asymmetric ("pole-to-pole") circulation patterns arise due to non-linear dynamics of the flow, despite symmetric forcing conditions. The question is whether the mechanism of symmetry breaking, as found in two-dimensional models, survives when three-dimensional effects (particularly rotation) are taken into account. An analysis of the energy transfer between the basic state and the most unstable perturbation showed that the mechanism underlying the symmetry breaking pitchfork bifurcation is essentially the same for two- and three-dimensional models of the thermohaline circulation (Weijer and Dijkstra 2001).

For my PhD research I studied the stability of the AMOC with respect to Southern (specifically Agulhas Leakage) sources of heat and salt. We found that these sources could significantly impact the stability, as in particular the salt import could strengthen the AMOC and enhance its stability (Weijer et al. 1999; Weijer 2000; Weijer et al. 2001; Weijer et al. 2002).