Ocean Circulation in a Warming Climate

According to Toggweiler and Russell (2008), “[climate] models predict that the ocean's circulation will weaken in response to global warming, but the warming at the end of the last ice age suggests a different outcome”.

The authors highlight the ‘conundrum’ that ocean circulation, mixing, and ventilation during the Last Glacial Maximum (“LGM”) was weaker than present, despite (1) the fact that “the polar oceans were colder than the tropic ocean” compared to today, (2) the common perception is that “it is windier in the winter than in the summer”, and (3) the scientific consensus that “winds in the atmosphere drive most of the circulation in the ocean” (Toggweiler and Russell, 2008).

In fact, overturning circulation or thermohaline circulation of deep-ocean waters were observed to have “strengthened as Earth began to warm about 18,000 years ago” (Toggweiler and Russell, 2008).

The issue which presented itself, then, was whether researchers’ general understanding of ocean circulation was incomplete, or the flaw resided in “the old truism about the strength of the winds during the cold glacial period” (Toggweiler and Russell, 2008).

The authors found the answer in the joint mechanics of wind-driven currents on the surface and overturning or thermohaline circulation within the interior, “because the circulation in the interior is now increasingly seen as being driven by turbulent mixing from the winds and tides and directly by the winds themselves”. In other words, the effect of winds is not confined to drag and surface currents, but also in deeper layers of water within the interior.

The ozone hole over Antarctica also has a direct influence on ocean temperatures.

“[The] depletion of stratospheric ozone of Antarctica” had the effect of cooling the air above Antarctica (Toggweiler and Russell, 2008), creating a gap in the roof of the “greenhouse”, dampening the enhanced greenhouse effect. This cooling leads to a greater temperature difference between Equatorial and polar regions, which has “increased the thermal contrast in the south and helped to make the southern westerlies stronger”. The Southern Ocean westerly winds play an integral part in overturning circulation within the ocean interior (Toggweiler and Russell, 2008).

For example, westerly winds above the Antarctic Circumpolar Current (“ACC”) “push cold, fresh surface waters away from Antarctica across the ACC… and draw slightly warmer and salty water that is low in oxygen up from the interior to the surface... As a result, the water just below the surface around Antarctica is now warmer, saltier and lower in oxygen” (Toggweiler and Russell, 2008).

Reference:

Toggweiler, J.R., and Russell, J. (2008). ‘Ocean circulation in a warming climate’. Nature 451(7176) 2008, pp.286–288.