The Once and Future Battles of Thor and the Midgard Serpent, or: The Westerlies and the Antarctic Circumpolar Current in Global Climate

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Summer School on Global Sustainability

Dominated by the Antarctic Circumpolar Current (ACC), the vast Southern Ocean can influence large-scale surface climate features on various time scales. Its climatic relevance stems in part from it being the region where most of the transformation of the World Ocean’s water masses occurs. In climate change experiments that simulate greenhouse gas–induced warming and ozone depletion, the response of the Southern Ocean circulation patterns to the change in the Westerlies make it a region where much of the future oceanic heat storage takes place, though the magnitude of that heat storage is one of the larger sources of uncertainty associated with the transient climate response in such model projections. These links are explored here in a climate model context by analyzing a suite of experiments produced in support of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report. The influence of the predicted change in Southern Ocean circulation over the rate of global atmospheric warming will be examined, as well as potential impacts on polar and global marine ecosystems.

Talk Summary (from student group)

This talk made the case for the fundamental importance of the Antarctic Circumpolar Current (ACC) as a major driver of the global climate. Powered by the Southern Hemisphere Westerly winds, this current circumnavigates the globe from west to east uninterrupted by land across the range of latitudes that pass through Drake’s Passage (the narrowest part of the southern ocean). The current is so strong that it reaches the ocean floor at depths of up to 5000 meters and sustains a flow rate of 145 million m3/second. The depth and flow are unprecedented elsewhere on earth. In addition to this direct forcing, the winds (deflected by the coriolis “force”) also drive a steady flow of surface water northward away from the pole in all directions around Antarctica. Deep ocean water is drawn to the surface from as much as 2,000m below the surface to replace the northward flowing water (often surfacing for the first time after as long as 800 years). Deep ocean water is rich in carbon of biological origin, but current and future atmospheric concentrations of CO2 create a gradient sufficient to drive further absorption. The cold deep water also absorbs large amounts of heat from the atmosphere after it surfaces. After a fairly brief appearance, the heat and CO2 laden water circulates back into the deep ocean, effectively sequestering atmospheric CO2 and heat in the ocean.

These dynamics are strongly suggested by observational data and are present in the best of the most recent generation of climate models, but there is limited agreement among models on the exact scale of these effects, largely due to model sensitivities to the predicted position of the Westerlies. Because the dynamics in the southern ocean have the potential to place a substantial drag on the climate system, thus decreasing the climate sensitivity to GHG forcing, developing a clear understanding of the dynamics related to the ACC is an important research goal for climate modelers. In the meantime, the interaction between the Southern Hemisphere Westerly winds and the Antarctic Circumpolar Current has already been demonstrated as a likely explanation of the coupling of CO2 and temperature in the paleo-climate record. The system passes three distinct tests that the paleo record presents: coincident CO2 and temperature changes, Antarctic temperatures that rise several hundred years before CO2 at the end of glacial periods, and the deep Atlantic carrying a large burden of biotic CO2.