The dynamics of a warming Southern Ocean and its interactions with Antarctic continental shelf circulation patterns brought about by the intensification of the Southern Hemisphere westerlies, circumpolar winds which prevail between 30 and 60 0 S, remains uncertain in several respects. Current global climate models (e.g. IPCC/AR4) attribute intensification of the westerlies to anthropogenic increases in greenhouse gases in the troposphere along with decreased levels of stratospheric ozone, seasonally observed as the polar ozone hole. Other dynamical oceanic processes may also be involved.
This numerical and analysis project seeks to further evaluate the contribution of oceanic heat content to the accelerating warming and sub-glacial melting currently being observed in the terminal ice streams in the Amundsen and Bellingshausen Seas, and the adjacent rapidly warming Western Antarctic Peninsula (WAP). Use of the Palmer Station LTER data set, which extends back to 1993, provides a observational series which in part covers the time period where changes in atmospheric circulation and air temperatures are most likely to have taken place. Use of a regional oceanic numerical model (3-D, fully dynamic) under contrasting wind regimes corresponding to observed (more positive) trends in the Southern Annular Mode will shed light on the dynamics of climate change in these sensitive continental shelf regions of the continent.
The impact of melting glaciers and ice sheets on global sea level rise is an acknowledged area of uncertainty in the most recent IPCC assessment, and would be a direct impact of climate change on society in this century.
PROJECT OUTCOME (FOR FULL DETAILS, SEE PROJECT FINAL REPORT): 1. Fundamental objective of project has been answered ( "better understand the source/mechanisms responsible for this increased ocean heat …"). a. We found no clear signal suggesting relationship between depth of the UCDW core (Tmax) and strength or sign of SAM index. b. We found a strong correlation between expanded UCDW decadal average heat content (QUCDW) data in grid box outside of Amundsen Sea Embayment (ASE) and Southern Ocean 0-2000 m decadal average heat content (OHC0-2km) of Levitus et al., 2013. c. There is strong covariability between QUCDW increase with Southern Ocean deep water T increase of Gille, 2002. d. Results suggest that the source of the increased heat in UCDW is due to warming water, not from tilt of isopycnals. 2. Showed that the mechanism for delivery of UCDW to the WAP continental shelf is via UCDW-core eddies shed from the ACC embedded in advective intrusions onto the shelf. a. UCDW entry onto the WAP continental shelf is via eddies embedded in advective intrusions, consistent with the idealized model results of St.-Laurent et al., 2013. b. Most recently we found that a time series of on-shore flow from our mooring array shows that as the westerlies intensify (with stronger +SAM), there is more onshore flow delivering more heat. 3. Found that the ODU west Antarctic Peninsula model shows extremely impressive fidelity to the data. a. In addition to support for model in 2.a above, we obtained additional field data showing further match of the model to the data. Specifically, we were given operational control of the LTER SLOCUM ocean gliders on the continental shelf in the 2013 annual LTER field season in an effort to track the eddies delivering UCDW from the ACC. We (particularly, Ph.D. student Darren McKee) established a box that could be surveyed in a manner that essentially guaranteed encountering an eddy based on our mooring data and the Dinniman model runs. Indeed, not only did the glider encounter the eddies as predicted, but at time intervals estimated from the dynamics of our published generation mechanism (Martinson and McKee, 2012). Most impressively, after passing through the eddy, the model results were used to predict where the glider should go to intercept it again. Several eddies were encountered and via the model average currents were tracked successfully until they dissipated near a seamount (showing low Richardson numbers consistent the dissipation). The eddies followed bathymetry closely. This glider work was presented as McKee's Masters project and is currently being revised for publication. The primary objectives of this data component have been satisfied. All results have been, or are in the process of being, written up for publication. References: Gille, S.T., 2002. Warming of the Southern Ocean Since the 1950s, Science, 295, 1275-1277. Martinson, D.G., and D.C. McKee (2012). Transport of Warm Upper Circumpolar Deep water onto the Western Antarctic Peninsula Continental Shelf., Ocean Science, 8, 433–442. St-Laurent, Pierre, John M. Klinck, Michael S. Dinniman, 2013: On the Role of Coastal Troughs in the Circulation of Warm Circumpolar Deep Water on Antarctic Shelves. J. Phys. Oceanogr., 43, 51–64. Intellectual Merit Information from this study provides better understanding of how the various processes contribute to the import of ocean heat to the WAP. This information should prove invaluable for assessing future climate change which is of societal relevance. Of particular value is the estimate linking these results to the SAM index, representing the large scale state of the Southern Hemisphere atmosphere (Section 2b. above). This should help researchers project these results both into the future and into the past where observations do not exist. Relationship of the heat delivery by eddies, the increase in this delivery with increasing westerlies and method of eddy dissipation, currently being written up by Ph.D. student McKee, will provide invaluable information to the glacial community assessing potential sea level rise. Broader Implications The approach, analyses, and results of this study are currently serving as excellent real-world examples for Martinson's class on Quantitative Methods of Data Analysis. Students already showing appreciation of real world examples demonstrating the analysis methods being taught in class. All results are currently be widely disseminated (papers published or being completed, oral presentations, and introduction to our various web sites). This study has become the primary focus of a Ph.D. level candidate under Martinson's supervision. Society will directly benefit from application of these results to better assessment of potential sea level rise.
