On multiple equilibria of the global ocean circulation and the preference for North Atlantic sinking

Abstract

In the ocean circulation there is the peculiar feature that heat transport is northwards throughout the entire Atlantic ocean. This means that the Atlantic heat transport in the southern hemisphere is towards the equator. Also, the heat transport in the Atlantic is much larger that in the Pacific. This northward heat transport is mainly due to the meridional overturning circulation in the Atlantic, where surface waters flow northward, release their heat to the atmosphere (thus becoming colder and gaining density), sink to depth at high northern latitudes and flow southwards as so-called North Atlantic Deep Water. The question why there is deep water formation in the Atlantic ocean and not in the Pacific ocean is central to this thesis. Deep water formation, or the sinking of water, is possible, because the water in the North Atlantic has high density. This high density is due to cold temperatures and high salt content, which is transported there by the surface currents. If the sinking is disrupted for some reason -- for example, melting of the Greenland ice sheet is very strong for a certain period, making the water of the North Atlantic too fresh to sink -- then the supply of salty water ceases too. This means the sinking of the water cannot just begin again; the water is too buoyant. The boundary conditions have not changed (the melt has stabilized at the former rate, there are no changes in precipitation and atmospheric temperature), but still the ocean circulation is fundamentally different from before. This notion, that there are multiple possible circulation patterns under equal forcing, is called the concept of multiple equilibria. There is more and more paleoclimatic evidence that such a collapse of the meridional overturning circulation has happened in the past sometimes, for example at the end of the last glacial. In this thesis I argue that these multiple equilibria are important to explain the salinity contrast and the difference in meridional flow between the Atlantic and Pacific. The main argument is that there is a certain selection process, that starts deep water formation at a certain location. Then the salt transport by the currents will help to sustain the meridional overturning circulation that arises. I also turn to the question how we could measure whether or not the ocean circulation is close to a collapse. The idea is that if the overturning circulation imports freshwater (at the boundary at 35 Southern Latitude, and it slows down for some reason, then the import of freshwater slows down too, thereby making the North Atlantic saltier again and increasing the sinking motion. However, when the meridional overturning circulation exports freshwater and it slows down for some reason, then there will be accumulation of freshwater, decreasing the density and slowing the sinking motion even more. So, if we can determine whether the overturning circulation imports or exports freshwater, we also have determined if it can potentially collapse.