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Julia Hazel (UCLA) - Exploring the Wind-Driven Near-Antarctic Circulation

March 6, 2019 3:30pm
MSA 7124

The circulation at the margins of Antarctica closes the meridional overturning circulation, ventilating the abyssal ocean with 02 and modulating deep CO2 storage on millennial timescales. This circulation also mediates the melt rates of Antarctica’s floating ice shelves, and thereby exerts a strong influence on future global sea level rise. The near-Antarctic circulation has been hypothesized to respond sensitively to changes in the easterly winds that encircle the Antarctic coastline. In particular, the easterlies may be expected to weaken in response to the ongoing strengthening and poleward shifting of the mid-latitude westerlies, associated with a trend toward the positive index of the Southern Annular Model (SAM). However, multi-decadal changes in the easterlies have not been systematically quantified, and previous studies have yielded limited insight into the oceanic response to such changes.

 

In this work we first quantify multi-decadal changes in wind forcing of the near-Antarctic ocean using a suite of reanalysis products that compare favorably with local meteorological measurements. Contrary to expectations, we find that the circumpolar-averaged easterly wind stress has not weakened over the past 3-4 decades, and if anything has slightly strengthened.  However, there has been a substantial increase in the seasonality of the easterlies: our results suggest that in austral summer the intensification of the SAM has weakened the easterly winds, while during austral winter an intensification of Antarctica’s katabatic winds has strengthened the easterlies. These trends have wide-ranging implications for oceanic transport of heat to Antarctica’s floating glaciers, formation of dense waters on the continental shelf, and sea ice production and export.

 

To explore the impacts of Antarctica’ changing winds, we develop a comprehensive model of the southern Weddell Sea, including the Filchner-Ronne Ice Shelf (FRIS), Antarctica’s largest floating glacier.  We investigate the wind-driven sensitivity of the circulation, dense water production, and glacial melt to idealized climate changes. We find that the circulation is relatively insensitive to changes in the zonal winds and atmospheric temperature, but strongly sensitive to changes in the meridional winds. Varying the strength of the meridional winds by a few tens of percent is sufficient to switch the FRIS cavity circulation between bi-stable ``warm’’ and ``cold’ states, accompanied by an order of magnitude change in the glacier’s melt rate. These findings imply that Antarctica’s major ice shelves may experience rapid changes in melt if certain climatic thresholds are exceeded, and that such changes would strongly inhibit a return to present-day melt rates. Alternatively, existing cold, dense water masses within the cavities might buffer against future intrusions of warm water and acceleration of Antarctic mass loss.