Short circuiting the thermohaline circulation

Ladies and Gentlemen,

For this post, I will look at how the Younger Dryas (YD) the most-well researched of the Late Glacial abrupt climate shifts, supposedly cut-off the thermohaline circulation (THC) causing an abrupt cooling event. The THC, also known as the Atlantic Meridional Overturning Circulation (AMOC), is the method by which the ocean regulates global energy budgets by transporting heat and water across the globe and through the water column. By transporting heat from the equator polewards, together atmospheric circulation, heat is transported to mid- and high-latitude areas. The THC is driven by ocean currents which travel as a factor of sea-water density, affected by temperature and salinity. 

Cold, saline water at high latitudes is transported to low latitudes via the oceans' deep currents, warmer water is then transported from low-latitudes to replace this deficit. Water from the Northern Atlantic sinks and flows to the Southern Hemisphere and eventually to the conveyors circulating the Antarctic continent. Here more cold, saline water joins and is transported to the Indian Ocean before interactions with the Pacific basin. In areas of upwelling, especially in the Pacific, cold deep water rises to the surface and is heated and evaporated leaving saltier water behind. Such water flows North to join up with the Gulf Stream which travels from the Gulf of Mexico along the North American Eastern Seaboard and eventually towards NW Europe. The evaporated heat from this maintains the relatively mild British climate for its latitude. Evaporation, sea-ice formation and cooling within this process leaves very cool, saline water behind which sinks to the deep to re-start the process.  Since the THC relies on the sinking of cold, saline water in the polar regions, if a large volume of freshwater was dumped into the system, the water would become too light to sink. In this case, no warmer water would replace the regular sinking cold water and so the heat transfer to the polar regions would cease from the THC causing a rapid return to glaciation. Please see Figure 1 for a visual representation of the Earth's ocean currents.

Figure 1: Thermohaline circulation (Source: 1. TSC)

Interposed between the start of the Holocene and the Allerod/Bolling warming stages, the YD cooled the Earth from c.12,800 cal yr BP before coming to an abrupt stop c.11,500 cal yr BP. As temperatures rose through the Allerod and Bolling warm stages, the Laurentide ice sheet over North America retreated creating the largest North American lake by volume, Lake Agassiz. In these warm stages, the Lake periodically released water to the North (Arctic Ocean), South (Gulf of Mexico) and East (North Atlantic Ocean) as shown by Figure 2. 

Figure 2: Suggested overflow routes from Lake Aggasiz causing the Younger Dryas (2. Broecker, 2006). Note: the axes show latitude (y) and longitude (x)

However, it is widely believed that a large outburst of freshwater from Aggasiz into the North Atlantic caused the YD. This disrupted the THC plunging the Northern Hemisphere, and especially Europe into a period of cooling once more. The evidence for such large outbursts affecting the THC and causing the YD is well summarised by Teller et al. (2002). Teller et al suggest that freshwater inputs to the THC as low as a 0.1 Sv  flux (where 1 Sverdrup = 1 x 10⁶ m³s^-1) may interrupt the formation of North Atlantic Deep Water (NADW) which drives the cool, saline water in the deep THC of the North Atlantic. Data from Lake Agassiz outbursts suggest that a 0.3 Sv flood flux of 9500km³, the second largest recorded outburst, occurred 12.9 ka cal yr BP in line with the beginning of the Younger Dryas. Its route was through the Great-Lakes to the East and the St. Lawrence River flowing NW into the North Atlantic. If this flux was seen over a period of 1 year, it would be at least 6 times higher than the regular flow into the St. Lawrence from Agassiz.

Other authors have proposed different reasons for the inception of the YDs which will be explored next time. Following this, I will look at evidence for the YD's abrupt termination.

1. TSC: thermohaline circulation
2. Broecker (2006) doi:10.1126/science.1123253