Guest essay by Eric Worrall
According to climate scientists, less dense meltwater on the surface of the Antarctic ocean reduced convection between the surface and ocean depths, leaving heat trapped in the depths.
One small area of ocean not changed by global warming
Date:May 6, 2020
Climate and marine scientists are observing pervasive warming of the ocean and the land surfaces across the globe. Since the middle of the 19th century, the average global temperature recorded on the land surface has risen by around one degree centigrade, and by 0.6 degrees across the ocean surface. Global warming has been most pronounced in the alpine regions and the Arctic.
A few years ago, Haumann and Gruber and various colleagues already discovered the reason for this expansion of sea ice in the Southern Ocean. They noticed that stronger southerly winds over this period propelled more of the sea ice that is being formed along the coast out into the open sea, enhancing the melting there. The resulting stronger conveyor belt enhanced the transport of freshwater from near the continent out into the open ocean. This is because when sea ice is being formed from seawater, the salt is left behind, whereas when the sea ice melts in the summer well away from the coast, the freshwater is released into the surface, reducing the salinity of the seawater there.
This reduction in surface salinity strengthened the vertical stratification of the seawater: the fresher, and in this part of the ocean lighter water stays in the upper 100 m, while the denser saltier water remains below. In general, the saltier and colder the water, the greater its density and the greater its depth in the ocean.
Smaller heat exchange between the water layers
The stronger stratification reduced the exchange of heat between the deeper layers and the surface water, causing the heat to remain trapped at depth. In addition, the air above the Southern Ocean during winter is generally colder than the temperature of the seawater. Combined with the reduction of the vertical exchange of heat in the ocean, this ultimately created the observed situation where the surface water cooled and the subsurface warmed.
In addition, the current study went only up to 2011. “We have observed a trend reversal since 2015. The sea ice around the Antarctic is now starting to recede at a rapid rate,” says the ETH Professor. “And this is very much in line with the overall trend of continuing global warming.“
The abstract of the study;
Sea‐Ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate
Much of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or freshwater fluxes. However, these mechanisms so far failed to fully explain the surface trends and the concurrent subsurface warming (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind‐driven northward sea‐ice transport, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward sea‐ice transport, a strengthened salinity‐dominated stratification cools the open‐ocean surface waters between the Subantarctic Front and the sea‐ice edge. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea‐ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice‐shelf melting. Moreover, the subsurface warming could account for about 8 ± 2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea‐ice changes thereby may have contributed to the slowdown of global surface warming over this period. Our conclusions are robust across all considered sensitivity cases, although the trend magnitude is sensitive to forcing uncertainties and the model’s mean state. It remains unclear whether these sea‐ice induced changes are associated with natural variability or reflect a response to anthropogenic forcing.
Don’t you love climate science? When sea ice accumulates, scientists adjust the models until they get the right result. When the sea ice melts, well that is what you would expect from global warming.