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Melting glaciers cool Southern Ocean

Date: 15 June 2020 Tags: Climate Change

Issue

Research suggests glacial melting might explain the recent decadal cooling and sea ice expansion across Antarctica’s Southern Ocean.

 

Background

The Southern Ocean near Antarctica has never been easy to study. Its challenging conditions have placed it out of reach to all but the most intrepid explorers.

 

Details

  • For climate modellers, the surface waters of the Southern Ocean provide a different kind of challenge: It doesn’t behave the way they predict it would.

  • In recent decades, as the world warms, the Southern Ocean’s surface temperature has cooled, allowing the amount of ice that crystallizes on the surface each winter to grow. 

  • There are 30 or so climate models used to foresee what the world might look like as the climate changes. These models don’t match the recent observations of surface temperature in the Southern Ocean, leaving scientists with lots of question.

  • Earlier researchers had explored two possible influences driving the observed ocean trends: greenhouse gas emissions, and westerly winds — strengthened by expansion of the Antarctic ozone hole — blowing cold water northward from the continent. 

  • The researchers carried out the experiments with the GISS global climate model where they abruptly introduce a fixed increase in melt water around Antarctica and then record how the model responds.

  • The researchers then apply the model’s response to a previous climate state to estimate how the climate should react to the observed forcing. The results are then compared to the observational record, to see if a factor is missing. This method is called hindcasting.

  • In their hindcasting, they estimate what would have happened to a pre-industrial Southern Ocean if up to 750 gigatons of meltwater were added each year.

  • When this increase in glacial melt was added to the model, it led to sea surface cooling, decreases in salinity, and expansion of sea ice coverage that are consistent with observed trends in the Southern Ocean during the last few decades.

  • Their model results suggest that meltwater may account for the majority of previously misunderstood Southern Ocean cooling.

  • The model shows that a warming climate may be driving, in a counterintuitive way, more sea ice by increasing the rate of melting of Antarctica’s glaciers. The paper may solve the disconnect between what was expected and what was observed in the Southern Ocean.

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