10 May 2012

Sea Ice-Ocean-Atmosphere Interaction in the North Atlantic Marginal Ice Zone

Science

The marginal ice zone (MIZ) is a key area of polar ice and is the first area impacted by changes in the ocean and atmosphere. The observed first mode of sea ice concentration variability is characterized by dipole pattern ice concentration anomalies, which are found to be coherent with the North Atlantic Oscillation (NAO)—the dominant mode of atmospheric variability in this area. The largest variability of arctic sea ice concentration is in the North Atlantic (NA) sector of the MIZ. Dr. Detelina Ivanova, a scientist at Lawrence Livermore National Laboratory (LLNL), together with Drs. Julie McClean from Scripps Institution of Oceanography (SIO) and Elizabeth Hunke from Los Alamos National Laboratory (LANL), studied the role of the surface heating changes by the atmosphere. They also studied advected poleward heat by the ocean for the marginal sea ice variability in the NA marginal ice cover. This research is published in the Journal of Geophysical Research - Oceans.

Approach

In their study, researchers used a moderately fine-resolution (0.4 degree horizontal, 40 vertical) coupled ocean-sea ice model, using the Parallel Ocean Program (POP) and sea ice model components (CICE), developed at LANL. They forced these with high-frequency atmospheric reanalysis fluxes from the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The fidelity of the simulated mean climatological state as well as the variability of ice concentration, total ice area, ice thickness, and drift were compared to observational datasets from satellite and ice drift buoy measurements. Basin-scale changes were examined using winter composite analyses over the period 1990–1999 in the lower atmosphere/surface ocean/sea-ice in simulated Arctic and Nordic Seas before and after the North Atlantic Oscillation (NAO) phase-switch in 1995.

The model results were consistent with observations in that reduced concentrations were found in the Nordic and Barents Seas and increased values occurred in the Labrador Sea. The authors regionally analyzed the upper-ocean mixed-layer heat budget in the Barents Sea, Nordic Seas, and the Irminger Sea to determine the winter-to-winter changes in the ocean heat advection and mixed-layer net fluxes, relating them to the ice melt and ice area changes.

Impact

This work shows bottom ice melt is dominating top ice melt, signifying the role of the ice-ocean exchange for ice thermodynamics in the MIZ. Results indicate ocean advection anomalies are closely related to anomalous bottom ice melt rates. An estimate of a mixed-layer ocean heat budget in key regions in NA reveal the NAO-related changes in the upper-ocean-layer temperature below the sea ice in MIZ are mostly caused by anomalous surface heat fluxes as well as advected temperature anomalies within the same order of magnitude, but always smaller. The entrainment of heat from the deeper ocean may also play a key role. The study provides improved understanding of factors controlling ice edge variability, which is valuable for further ice-ocean model development. In addition, it provides a better understanding of observed ice area changes.

Summary

The paper is focused on the sea-ice/ocean/atmosphere interaction in the North Atlantic (NA) Marginal Ice Zone (MIZ), where the largest variability in the Arctic sea ice concentration is observed. The goal of the study is to assess the relative roles of changes in surface heating by the atmosphere and poleward heat advection by the ocean for the marginal ice cover interannual (NAO-related) variability. The results are based on coupled sea-ice ocean simulation (LANL POPCICE) forced with NCEP/NCAR atmospheric reanalysis. Some of the most important findings are that: bottom ice melt is dominating top ice melt, signifying the role of the ice-ocean heat exchange for the ice thermodynamics in this area; ocean advection anomalies are closely related to anomalous bottom ice melt rates. An estimate of ocean mixed-layer heat budget in key regions in the NA show NAO-related changes in the upper ocean layer temperature below the sea ice in MIZ are mostly caused by anomalous surface heat fluxes as well as advected temperature anomalies within the same order of magnitude, but always smaller. In addition, entrainment of heat from the deeper ocean may play a key role. The study provides improved understanding of factors controlling ice edge variability, which is valuable for further ice-ocean model development as well as a better understanding of observed ice area changes.

Contact
Chun Zhao
Acknowledgments

This research was supported by the Office of Science (Biological and Environmental Research), U.S. Department of Energy grant DE-FG02-05ER64119, the Office of Naval Research, and the Lawrence Livermore National Laboratory (LLNL) postdoctoral program. Additional support was provided under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory contract DE-AC52-07NA27344. E. Hunke is supported by the Earth System Modeling Program within the Biological and Environmental Research division of the Department of Energy Office of Science. The model run was conducted as part of a Department of Defense High Performance Computing Center Grand Challenge Grant at the Naval Oceanographic Office (NAVO), the Maui High Performance Computing Center (MHPCC) and the Army Research Laboratory. We thank Karl Taylor (LLNL), David Bader (LLNL), Prasad Thoppil (NRLSSC), and Don Stark (NCAR) for their useful comments and advice about the manuscript. We also thank the anonymous reviewers who helped us to improve the manuscript.