The Antarctic Half of the Global Thermohaline Circulation is Collapsing
Edge of the Filchner-Ronne Ice Shelf in the Weddell Sea, Photo: Ralph Timmermann, Alfred Wegener Institute. Freshening of water on the top layer of the ocean, caused by increasing glacial melting and increasing precipitation has stopped the formation of a large area of open water in this region. This area of open water the size of New Zealand produced a huge volume of cold salty Antarctic bottom water in the 1970s. This source of the coldest water in the ocean has shut down since then because the fresh water layer is preventing warmer, saltier water from below from coming to the surface.
That increase in freshwater is coming from two sources: Climate change has amplified the global water cycle, increasing both evaporation and precipitation. And Antarctic glaciers have been calving and melting at a greater rate. Both of these sources end up contributing more freshwater to the Weddell Sea than what the area experienced in the past, the researchers note.
To look at what the future might hold for this system, de Lavergne and colleagues turned to a set of 36 climate models. Those models, which predict that dry places of the world generally get drier and wet places get wetter, show that this area of the Southern Ocean should see even more precipitation in the future. The models don’t include melting glaciers, but those are expected to add more freshwater, which could make the lid on the system even stronger, according to the researchers.
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In 1974, newly available satellite observations unveiled the presence of a giant ice-free area, or polynya, within the Antarctic ice pack of the Weddell Sea, which persisted during the two following winters. Subsequent research showed that deep convective overturning had opened a conduit between the surface and the abyssal ocean, and had maintained the polynya through the massive release of heat from the deep sea. Although the polynya has aroused continued interest the presence of a fresh surface layer has prevented the recurrence of deep convection there since 1976, and it is now largely viewed as a naturally rare event.
Here, we present a new analysis of historical observations and model simulations that suggest deep convection in the Weddell Sea was more active in the past, and has been weakened by anthropogenic forcing. The observations show that surface freshening of the southern polar ocean since the 1950s has considerably enhanced the salinity stratification. Meanwhile, among the present generation of global climate models, deep convection is common in the Southern Ocean under pre-industrial conditions, but weakens and ceases under a climate change scenario owing to surface freshening. A decline of open-ocean convection would reduce the production rate of Antarctic Bottom Waters, with important implications for ocean heat and carbon storage, and may have played a role in recent Antarctic climate change.