An oceanic heat transport pathway to the Amundsen Sea Embayment

PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE 10.1002/2015JC011402 Key Points: The divergence of heat advection leads to change in oceanic heat content. Cross-shelf heat input dominates the ASE heat content at depth. Vorticity input from the local wind- stress curl sustains cross-shelf transport. Correspondence to: A. R. Rodriguez, arodrigu@ucsd.edu Citation: Rodriguez, A. R., M. R. Mazloff, and S. T. Gille (2016), An oceanic heat transport pathway to the Amundsen Sea Embayment, J. Geophys. Res. Oceans, 121, doi:10.1002/ 2015JC011402. An oceanic heat transport pathway to the Amundsen Sea Embayment Angelica R. Rodriguez 1 , Matthew R. Mazloff 1 , and Sarah T. Gille 1 Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA Abstract The Amundsen Sea Embayment (ASE) on the West Antarctic coastline has been identiﬁed as a region of accelerated glacial melting. A Southern Ocean State Estimate (SOSE) is analyzed over the 2005–2010 time period in the Amundsen Sea region. The SOSE oceanic heat budget reveals that the contribution of parameterized small-scale mixing to the heat content of the ASE waters is small compared to advection and local air-sea heat ﬂux, both of which contribute signiﬁcantly to the heat content of the ASE waters. Above the permanent pycnocline, the local air-sea ﬂux dominates the heat budget and is controlled by seasonal changes in sea ice coverage. Overall, between 2005 and 2010, the model shows a net heating in the surface above the pycnocline within the ASE. Sea water below the permanent pycnocline is isolated from the inﬂu- ence of air-sea heat ﬂuxes, and thus, the divergence of heat advection is the major contributor to increased oceanic heat content of these waters. Oceanic transport of mass and heat into the ASE is dominated by the cross-shelf input and is primarily geostrophic below the permanent pycnocline. Diagnosis of the time-mean SOSE vorticity budget along the continental shelf slope indicates that the cross-shelf transport is sustained by vorticity input from the localized wind-stress curl over the shelf break. Received 18 NOV 2015 Accepted 18 MAR 2016 Accepted article online 23 MAR 2016 1. Introduction The West Antarctic Ice Sheet (WAIS) contains 3.8 million km 3 of ice [Oppenheimer, 1998] and has the poten- tial to cause global eustatic sea level rise of more than 3 m [Bamber et al., 2009]. Thus, the stability of WAIS has been the focus of much current research [Joughin and Alley, 2011; Joughin et al., 2014; Rignot et al., 2014]. Satellite observations have shown that the most signiﬁcant ice mass loss occurs along the Amundsen Sea coastline where several outlet glaciers drain into the embayment [Rignot et al., 2008, 2013; Pritchard et al., 2012]. Ice ﬂow in this region has increased by 77% since the 1970s [Mouginot et al., 2014]. Thinning of the ﬂoating ice-shelves and glacial grounding line retreat have been attributed to basal melting forced by heat input from inﬂowing Circumpolar Deep Water (CDW) [Payne et al., 2004; Walker et al., 2007; Wahlin et al., 2010]. CDW in the Amundsen Sea Embayment (ASE) is typically 3–48C warmer than the in situ freezing point and can be found as far south as the grounding zone of the Pine Island Glacier (PIG) [Jenkins et al., 2010; Jacobs et al., 2011]. On monthly timescales, basal melt is thought to be driven largely by processes outside the subice-shelf cavity [Schodlok et al., 2012; Heimbach and Losch, 2012]. Hence, knowledge of the ocean state at the continental shelf slope is crucial to modeling studies of subice-shelf processes. An understanding of the delivery mechanisms of CDW to the ASE is essential to predicting further inﬂuence of ocean forcing on the outlet glaciers of the ASE. A modeling study by Thoma et al. [2008] linked cross- continental-shelf CDW transport variability to zonal wind variability along the continental shelf edge due to migration of the Amundsen Sea Low. Building upon these results, Steig et al. [2012] proposed that tropical Paciﬁc warming brought about the modern period of PIG retreat through a shift in the ASE zonal wind regime. C 2016. American Geophysical Union. V All Rights Reserved. RODRIGUEZ ET AL. This study aims to investigate the large-scale pathways of CDW delivery to the ASE. Our primary tool is a Southern Ocean State Estimate (SOSE) [Mazloff et al., 2010], which provides a realistic model of the Southern Ocean dynamics and physical properties, as well as atmospheric forcing, through assimilation of all available observations. It uses the OCean Comprehensible Atlas (OCCA) global state estimate [Forget, 2010] to con- strain the initial and northern boundary conditions and the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA)-Interim (http://www.ecmwf.int/) to constrain the meteorological bound- ary conditions at the ocean-atmosphere interface. Because the SOSE solution includes the entire Southern ASE HEAT TRANSPORT