Southern hemisphere thermohaline circulation stability and effect on global climate: results from coupled modeling

Abstract

This thesis investigates the stability of the Southern Hemisphere ocean thermohaline circulation, particularly the deepest branch, known as Antarctic Bottom Water (AABW), to Antarctic ice melt, and its role in the global thermohaline circulation (THC) in both present day and glacial climate states using a coupled climate model of intermediate complexity. The thesis is in two parts: in the first, present day experiments show AABW is stable to injections of freshwater (FW) from the Antarctic ice sheet equivalent to over 10m global sea level rise on centennial to millenial timescales, and does not have a stable ’off’ state like the corresponding Northern Hemisphere THC - North Atlantic Deep Water (NADW). This is due to vastly different geographies, where zonally unrestricted Southern Hemisphere westerly winds drive Ekman pumping of salty Circumpolar Deep Water (CDW) largely independently of reduced thermohaline feedbacks during FW forcing, thus acting as a mechanism to resalinise surface oceans and reinvigorate AABW once FW forcing ceases. During times of FWintrusion however, AABWdoes temporarily ’switch off’, causing local surface cooling, deep warming, decreases in oceanic northward heat transport and reductions in sea surface and surface air temperatures over the Southern Hemisphere mid-latitudes and tropics. The second part of the thesis aims to gain insight into the sensitivity of these results to base climate. A glacial simulation with ice sheets, atmospheric CO2 concentrations and orbital configurations like those of the Last Glacial Maximum, 21,000 years ago, is constructed, and similar experiments to Part 1 show the stability of the global THC has changed markedly. This is consistent with the colder and less stratified ocean, leading to a more fragile global THC where density differences between Southern and Northern Hemisphere overturning water-masses are smaller. Though AABW remains stable, FW anomalies propagating to the North Atlantic from the Antarctic can now dominate the bipolar density seesaw, switching the Northern Hemisphere THC into a different state where NADW reduces to zero and North Pacific Deep Water (NPDW) becomes dominant. This leads to extensive cooling over the North Atlantic and warming over the North Pacific and, to lesser extent, the Southern Ocean.