Sensitivity of Water Masses in the Ross Sea to Changes in the Winds and Atmospheric Temperatures

The Ross Sea is one of the few locations along the Antarctic margins where Antarctic Bottom Water (AABW) is exported to the World Ocean and thus the formation on the Ross Sea continental shelf of cold and saline Shelf Water (SW), a precursor to AABW, has consequences for the Global Thermohaline Circulation. Meanwhile, transport of relatively warm, nutrient-rich Circumpolar Deep Water (CDW) onto the continental shelf has important consequences for physical and biological processes in the Ross Sea. Strengthening of the cold southerly winds over the Ross Sea is thought to be one of the causes for the observed increases in sea-ice extent in this area and may have significant effects on other aspects of the circulation. A high resolution (5 km) regional ocean/sea-ice/ice shelf model of the Ross Sea is used to examine the effects of changes in the winds on the formation of SW, transport of CDW onto the shelf, vertical mixing of CDW and its transformation into Modified CDW (MCDW) and basal melt of the Ross Ice Shelf (RIS). Simple increases in the wind speed with no other atmospheric changes actually reduced the sea-ice, opposite of what has been observed in the past three decades. Increases in the winds combined with spatially uniform decreases in the air temperatures led to realistic increases in sea-ice concentrations. Stronger winds and cooler air temperatures both led to increases in the quantity of CDW advected onto the continental shelf and increases in the vertical mixing of MCDW into the upper water column, possibly increasing nutrient transport into the euphotic zone. The increased winds worked against the cooler air temperatures in changing the basal melt rate of the RIS and the slight change (a 4% increase) in the basal melt makes it difficult to distinguish the dominant forcing factor.

AR4 future scenario simulations typically show atmospheric warming and changes in wind speed (increases and decreases) and direction over the Ross Sea. One would expect from the idealized forcing simulations that warmer temperatures would reduce the MCDW that gets to the upper shelf waters, although this could be balanced out by changes in the winds. Results from simulations forced with winds and air temperatures from the SRES A1B scenario simulations from the MPI ECHAM5 model show lower transport of CDW onto the continental shelf and decreased mixing of MCDW into the upper waters for 2046-2050 when compared to the end of the 20th century. The MCDW concentrations on the shelf are about the same for 2096-2100 compared to the end of the 20th century, although many other aspects of the circulation are different. The basal melt rate of the RIS increased slightly by 2046-2050 (6% increase) and 2096-2100 (9% increase).

There has also been an observed freshening of the Ross Sea over the last 50 years and it has been proposed that this is a signature of increased meltwater advected from the Amundsen Sea. A simplistic freshening of the water advected into the model domain for the 2046-2050 and 2096-2100 simulations did not have a significant effect on the modeled sea-ice extent or CDW transport onto the shelf. However, the freshening reduces the MCDW that is mixed into the upper waters over the shelf, increases the basal melt rate of the RIS for 2046-2050 (10% increase compared to the 2046-2050 simulation with no freshening, 17% increase compared to the end of the 20th century) and 2096-2100 (12%/22% increase) and leads to a major reduction in the volume of SW produced.