Ice shelf basal melt rates around Antarctica from simulations and observations

PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE 10.1002/2015JC011117 Key Points: Explicit representation of ice-shelf- ocean interaction in an ocean reanalysis Compare modeled basal melt rates and pattern to estimates derived from satellite data Evaluate sources of error in explicit representation of ice shelf ocean interaction Correspondence to: M. P. Schodlok, schodlok@jpl.nasa.gov Citation: Schodlok, M. P., D. Menemenlis, and E. J. Rignot (2016), Ice shelf basal melt rates around Antarctica from simulations and observations, J. Geophys. Res. Oceans, 121, 1085– 1109, doi:10.1002/2015JC011117. Received 8 JUL 2015 Accepted 21 DEC 2015 Accepted article online 26 DEC 2015 Published online 3 FEB 2016 Ice shelf basal melt rates around Antarctica from simulations and observations M. P. Schodlok 1,2 , D. Menemenlis 2 , and E. J. Rignot 2,3 Joint Institute for Regional Earth System Science and Engineering, University of California Los Angeles, Los Angeles, California, USA, 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, 3 Department of Earth System Science, University of California Irvine, Irvine, California, USA Abstract We introduce an explicit representation of Antarctic ice shelf cavities in the Estimating the Cir- culation and Climate of the Ocean, Phase II (ECCO2) ocean retrospective analysis; and compare resulting basal melt rates and patterns to independent estimates from satellite observations. Two simulations are car- ried out: the ﬁrst is based on the original ECCO2 vertical discretization; the second has higher vertical resolu- tion particularly at the depth range of ice shelf cavities. The original ECCO2 vertical discretization produces higher than observed melt rates and leads to a misrepresentation of Southern Ocean water mass properties and transports. In general, thicker levels at the base of the ice shelves lead to increased melting because of their larger heat capacity. This strengthens horizontal gradients and circulation within and outside the cav- ities and, in turn, warm water transports from the shelf break to the ice shelves. The simulation with more vertical levels produces basal melt rates (1735 6 164 Gt/a) and patterns that are in better agreement with observations. Thinner levels in the sub-ice-shelf cavities improve the representation of a fresh/cold layer at the ice shelf base and of warm/salty water near the bottom, leading to a sharper pycnocline and reduced vertical mixing underneath the ice shelf. Improved water column properties lead to more accurate melt rates and patterns, especially for melt/freeze patterns under large cold-water ice shelves. At the 18 km grid spacing of the ECCO2 model conﬁguration, the smaller, warm-water ice shelves cannot be properly repre- sented, with higher than observed melt rates in both simulations. 1. Introduction The Antarctic Ice Sheet loses mass through iceberg calving at the ice shelf front and basal melt at the base of the ice shelves. The latter is a larger contribution by 20% [Rignot et al., 2013] and has a larger impact on local water mass formation. On the one hand, ice shelves, which comprise about 45% of the Antarctic coast- line, provide heat and freshwater ﬂuxes through basal melt at the ice-shelf-ocean interface, transforming water masses in the sub-ice-shelf cavity and on the continental shelf. These water masses subsequently con- stitute an important part of deep and bottom water mass formation on the southern continental shelf and beyond [Foldvik et al., 1985; Hellmer et al., 2005; Schodlok et al., 2008]. Icebergs, on the other hand, ﬂoat away from their region of calving and provide heat and freshwater ﬂuxes farther away from the ice shelf, constituting local sources for, e.g., bio-productivity [Smith et al., 2007], but not contributing to water mass formation near the ice-shelf-ocean interface. One exception is when icebergs become grounded, in which case they can impact continental shelf circulation with consequences to exchanges between continental shelf and ice-shelf cavity water masses [Grosfeld et al., 2001], hence changing ice-shelf-ocean heat and fresh- water ﬂuxes and the ice-shelf mass balance. C 2015. American Geophysical Union. V All Rights Reserved. SCHODLOK ET AL. Previous studies [e.g., Gille, 2002; Jacobs, 2006; Schmidtko et al., 2014] have reported signiﬁcant recent changes in Southern Ocean water masses. The general trends are toward increased temperature and decreased salinity, which might cause or result from increasing Antarctic Ice Sheet mass loss, changes in precipitation, sea ice formation, wind regime and associated changes in ocean and atmosphere circulation. To quantify the ocean contribution to the mass loss of the Antarctic ice sheet, we need to investigate and understand the mechanisms of heat and freshwater ﬂuxes between the open ocean and the ice shelf cav- ities and the impact of these ﬂuxes on basal melt patterns and rates. Improved quantiﬁcation and under- standing of ice-shelf-ocean interaction will enable better description and prediction of the impact of ice shelves on ice sheet mass balance and global sea level change. Changes in ice shelf thickness and ICE SHELF BASAL MELT AROUND ANTARCTICA