Ross Sea Ice: A Climate Anomaly Explained

Regional climate responds to global temperature trends, but is also influenced by regional-specific and/or hemispheric processes.  For example, as a result of global warming and higher polar temperatures some regions of Antarctica are impacted by higher surface temperatures, reduced sea ice extent, and the collapse of large ice shelves.  In contrast, the Ross Sea in the Southern Ocean is experiencing a significant positive trend in sea ice extent (Comiso et al., 2011). These changes, including increased surface extent, a longer season of sea ice production (Stammerjohn et al., 2012), and enhanced sea ice export from the shelf are strongly explained by changes in the Southern Annular Mode (SAM).

SAM is the dominant mode of variability of atmospheric circulation in the high southern latitudes and in recent years has shifted towards a positive polarity (Thompson & Solomon, 2002) that involves a strengthening of the circumpolar vortex and intensification of the westerlies that encircle Antarctica (Marshall, 2003).  Variability in this mode can arise naturally, but some evidence suggests that modern trends are forced by the Southern Hemisphere ozone hole and increased greenhouse gases (Shindell & Schmidt, 2004). Some modeling studies suggest that the positive polarity trend will continue in the near future (Miller et al., 2006).

For most of the year, much of the Ross Sea is capped by ice, except for five regions known as polynyas (areas of reduced ice concentration surrounded by heavy pack ice) (Jacobs & Comiso, 1989).  Polynyas are regions of active sea ice formation. Atmospheric cooling sustains sea ice formation, increasing sea surface salinity due to brine rejection and the production of dense Shelf Water that fill most of the continental shelf bottom layer (Smith et al., 2012). Dense waters move down the shelf and expand throughout the southern Pacific sector as Antarctic Bottom and Intermediate waters.

The Ross Sea polynya, the largest polynya in the region, occurs north of the Ross Ice Shelf on the southwestern Ross Sea, and forms between the end of October and late November as a result of katabatic wind intensification (Jacobs & Comiso, 1989) and synoptic winds (Zwally et al., 1985). Interannual variability in the timing (Arrigo et al., 1998) and size (Bromwich et al., 1998) of the Ross Sea polynya are controlled by winter temperatures from the surface, as well as, by heat entrainment to the ocean surface layer from the underlying warm Circumpolar Deep Water (CDW) (Jacobs & Comiso, 1989).  

At the same time that the sea-ice season has been lengthening and the sea ice extent has been growing, the Ross Sea polynyas are becoming increasingly persistent and larger in extent (Parkinson, 2002).  Notably, polynyas are an important component of the Ross Sea ecosystem, as they facilitate biogeochemical processes that favor extremely high primary production. They are also involved in the life cycle of important middle-trophic level species.  Stronger winds tend to benefit the development of phytoplankton blooms in polynyas that have more stratified waters. As a consequence, phytoplankton blooms in coastal polynyas are also responding to large climate variability in the Southern Ocean (Montes-Hugo & Yuan, 2012).

Given its biogeochemical significance, the Ross Sea is considered to play a substantial role in the Southern Ocean carbon cycle by serving as a major regional oceanic CO2 sink.  The regional phytoplankton production draws down pCO2 in surface waters and drives the export productivity of organic carbon, a mechanism that has positive mitigation effect on the high atmospheric levels of CO2. Indeed, approximately 27% of the total Southern Ocean CO2 sink is specific to the Ross Sea (Arrigo et al., 2008).

The Ross Sea is rapidly changing towards a colder system that is favorable to sea ice formation. Under current projections, an ensemble of climate models indicates a continuation of the poleward shift of the westerlies due to continued warming of the lower atmosphere and no recovery of the ozone hole (Russell et al., 2006; Ainley et al., 2010).  Despite these assessments, the International Panel on Climate Change (IPCC) predicts that the Ross Sea will be the last portion of the Southern Ocean with sea-ice year round.  Projected sea ice coverage for the Ross Sea is expected to remain the same until 2020-2030 (Ainley et al., 2010). However, if the climate continues to warm by the end of the 21st century (Fig. 8 - Ainley et al., 2010), sea ice extent in the Ross Sea will be highly reduced, with major implications for the rich and diverse Ross Sea ecosystem.

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