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Biogeochemistry

Impact of Climate Change on Antarctic Waters

Source: Bronselaer, B., J. L. Russell, M. Winton, N. L. Williams, R. M. Key, J. P. Dunne, R. A. Feely, K. S. Johnson, and J. L. Sarmiento (2020), Importance of wind and meltwater for observed chemical and physical changes in the Southern Ocean. Nature Geoscience, 13, 35-42 doi:10.1038/s41561-019-0502-8

The Southern Ocean, which surrounds Antarctica, regulates the global climate system by absorbing a large amount of heat and carbon dioxide (CO2) from the atmosphere. Recent decades have seen rapid changes to the physical and biogeochemical properties in the Southern Ocean, including ocean warming and acidification. Determining the mechanisms driving these changes is necessary to improve future climate projections.

Caption: Autonomous floats, like this one, drift around the ocean, measuring properties of seawater (SOCCOM)

A recent study published in Nature Geoscience characterizes the changes to Southern Ocean waters since 1990 using observational data. Then the researchers investigated how climate model simulations can be improved to better capture these observed changes, and thus help improve future climate projections.

Making observations in the Antarctic is difficult due to rough seas and harsh weather conditions, so this study uses data collected by more than 100 autonomous robotic floats that drift with the ocean currents and take measurements every 10 days. Data from the floats, which were collected between 2014 and 2019, were compared to shipboard data from 1990 to 2004. This comparison showed that Southern Ocean waters have gotten warmer, more acidic, and lower in oxygen during the past several decades. The magnitude of these changes is not captured in most climate model simulations, and the researchers leading this study wanted to figure out why.

 

Freshwater discharge from melting ice sheets has increased in recent years. Westerly winds over the Southern Ocean have also gotten stronger and shifted closer to the Antarctic continent. By running new climate model simulations that account for meltwater and altering wind patterns, the researchers were able to reproduce the observed physical and biogeochemical changes. Therefore, accurate representation of Antarctic meltwater and winds is likely necessary to improve future climate projections.

Caption: Satellite image of the West Antarctic Ice Sheet. This study finds that freshwater discharge from ice sheets is key to understanding observed changes in Southern Ocean waters. (NASA Earth Observatory via Wikimedia Commons)

The researchers assessed the future impact of wind and meltwater by running their model up to the year 2100. They found that accounting for changing winds and meltwater caused the Southern Ocean to absorb less CO2 from the atmosphere than is predicted by other climate models. The results also showed nutrients accumulating near the Antarctic coasts rather than being exported out into the open ocean, which could impact the distribution of marine organisms that need those nutrients to grow.

This study demonstrate the critical importance of Antarctic winds and meltwater to modeling recent changes in Southern Ocean physical and biogeochemical properties, as well as predicting future climate. Trying to understand what controls these changes, through studies such as this, is necessary given the key role of the polar regions in the global climate system. Integration of observations with models will also be key to improving climate projections.

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