Impact of atmospheric and oceanic feedbacks on the stability of the meridional overturning circulation

Abstract

The fundamental climatic importance of the Atlantic Meridional Overturning Circulation (AMOC) lies in the northward heat transport in the Atlantic Ocean associated with this component of the circulation. Despite its importance, the understanding of the AMOC dynamics, and in particular of its stability properties, is fragmentary at best. Paleoclimatic data and numerical models suggest that the AMOC may undergo abrupt, irreversible collapses if appropriately perturbed, in particular by freshwater anomalies in the northern North Atlantic. However, it is unclear whether such an abrupt transition is possible in the real ocean, and some state of the art coupled climate models show in fact no such collapse. This different behaviour has generally been attributed to deficiencies of simpler numerical models, but it has also been suggested that biases of freshwater transport in coupled climate models may prevent any irreversible collapse of the AMOC therein. In this thesis, a minimal atmospheric model is derived from a coarse resolution numerical model using linear regressions of surface fluxes. Combining an ocean general circulation model and this minimal atmospheric model, a Hybrid Coupled Model (HCM) was implemented and tested. The HCM was then used for studying the sensitivty of the AMOC stability to changes in the freshwater budget of the Atlantic Ocean. The numerical simulations performed indicate that the zonal salinity gradient at the southern end of the Atlantic Ocean plays a key role in controlling the sensitivity of the AMOC to freshwater perturbations. These results show that the AMOC response to external perturbations is strongly affected by the freshwater budget of the Atlantic Ocean, and by the biases that affect its representation in climate models. These results also stress the importance of the freshwater transport by the overturning circulation for the stability of the overturning circulation itself. These findings were confirmed and extended in a different numerical model, and further studied in the highly idealised framework of a box model. This simple model includes a basic representation of the Atlantic basin with a periodic channel at its southern end. In particular, it is suggested that an overturning rate scaling with the meridional density gradient is an essential element. Furthermore, it is shown under which conditions the freshwater transport by the overturning circulation can measure the AMOC stability in the model. The relationship between the meridional density gradient and the overturning circulation rate is further studied in an idealised numerical model which includes a basin spanning two hemispheres and a periodic channel at its southern end. It is shown that, even if the AMOC is in geostrophic balance, the meridional density gradient is highly correlated with the overturning rate. This is connected to the stratification induced by the presence of a periodic channel in the south. The AMOC strength is determined not only by the dense water formation in the north, but also by the water properties at the southern end of the Atlantic Ocean