This study will combine observational analyses and three-dimensional global climate model (GCM) experiments to gain insight to the tropical teleconnections and decadal variability of the Southern Hemisphere Annular Mode (SAM). Observations show an increasing positive polarity to the SAM in recent decades denoted by an increase in intensity of the circumpolar vortex. The study will test two hypotheses: that the teleconnection of the El Nino-Southern Oscillation (ENSO) to the Antarctic region is modulated by the zonal wind structure over the Southern Hemisphere extratropics, or alternatively, that different tropical forcing leads to decadal variability in the teleconnection. As a corollary, the SAM and ENSO in turn influence precipitable water transport over the Antarctic continent, and hence the ice mass balance. A new level of understanding of the SAM and ENSO linkages to global teleconnections and global climate change will be achieved by use of both observations and GCM simulations.
The observational research includes a dynamical study of the role of momentum, moisture and heat transports during the modern satellite era, and the modeling study includes GCM simulations with the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 4 (CAM4).
Broader impacts of this study include contributions to the goals of the International Polar Year, as well as career development for a graduate student and a research scientist. Material for university lectures will be developed, and lectures and field trips for students from the Columbus Public Schools will be facilitated. The project will produce a webpage developed by high school students, and will archive publicly-available results at the Byrd Polar Research Center (BPRC) and the National Snow and Ice Data Center. Collaboration with BPRC's outreach coordinator will yield data and visualization activities for middle and high school students to be used by a local science-oriented public high school and by summer programs at Ohio State University.
The El Niño-Southern Oscillation (ENSO) is a widely recognized contributor to global atmospheric-ocean variability, and is a highly recognized climate phenomenon known to affect weather patterns across the globe. In the middle and high-latitudes of the Southern Hemisphere, the leading driver of climate variability is the Southern Annual Mode (SAM), also known as the high-latitude mode or Antarctic Oscillation. It represents the approximately zonally-symmetric variations in the westerly flow and can occur without external forcing. The results from this project represent advancement in our understanding of the complex system of climate variability across the Southern Hemisphere involving these two climate modes, providing an important foundation for studies in all disciplines related to this topic. Through observations and modeling, we have examined dynamics associated with recent decadal interactions between ENSO and SAM in the Southern Hemisphere as well as identified physical mechanisms by which SAM alters the ENSO variability of Southern Hemisphere climate. Through collaboration with a former Ph.D. student at The Ohio State University Dr. Ryan Fogt (Assistant Professor of Meteorology and Director of the Scalia Laboratory for Atmospheric Analysis in the Department of Geography at Ohio University), our observational study (Fogt et al. 2011, Climate Dynamics, doi: 10.1007/s00382-010-0905-0) demonstrates non-linear relationships between ENSO and SAM. Their interaction varies on seasonal and decadal time scales with strong relationships verified during austral summer as well as during the 1970s and 1990s. Anomalous circulation in the southern high-latitudes is enhanced when ENSO phases with SAM, that is when La Niña events occur during the positive phases of SAM (strong westerly flow) or El Niño events occur during negative SAM phases (weakened westerly flow). Transient eddy momentum fluxes in the Pacific Ocean are responsible for the anomalous flow, and those associated with individual ENSO and SAM events are shown to create zonal wind anomalies that are intensified during in phase events. Additional partnerships with other researchers have led to reconstructions of SAM variability back to 1865 for austral summer and autumn and back to 1905 for winter and spring (Jones et al. 2009, J. Climate, 22, doi: 10.1175/2009JCLI2785.1 and Fogt, et al. 2009, J. Climate, doi: 10.1175/2009JCLI2786.1.). A comparison between models included in the Intergovernmental Panel on Climate Change Fourth Assessment Report and our reconstructions show the models do not fully simulate the natural variability at particular times during the reconstructions. However, the models do capture the most recent (1957-2005) positive SAM trends during austral summer which is the strongest in the last 150 years and a clear indication of anthropogenic impacts on Southern Hemisphere climate. Stratospheric ozone depletion is shown to be the dominant force of seasonal SAM variability with greenhouse gases playing a less significant role. These observational studies provided the motivation for our numerical modeling study. Dr. Keith Hines (Research Scientist) and Aaron B. Wilson (graduating Doctoral student) from The Ohio State University have used a well-respected global atmospheric climate model, the National Center for Atmospheric Research Community Atmosphere Model (CAM) version 4, to confirm the observed findings of significant ENSO-SAM relationships on atmospheric circulation variability in the Southern Hemisphere. Multi-year climate simulations display realistic features of ENSO and SAM including SAM’s enhancement of the anomalous circulation in high-latitudes during in phase events. CAM simulations verify observed ENSO variability related to "ENSO flavors", that is the effects on atmospheric circulation by ENSO events vary depending on whether the strongest tropical SST signature is located in the eastern or central tropical Pacific Ocean. These changes to circulation include increased blocking over Australia and a wintertime southward shift in the sub-tropical jet stream across South America. In higher latitudes, changes to the large scale wave patterns shown in reanalysis data are well matched by CAM, indicating an overall westward shift in the large scale circulation structure. This leads to a weakening of the Antarctic Dipole shown by changes to meridional heat and momentum fluxes across the Antarctic Peninsula. Publications concerning the numerical modeling study are forth-coming. Finally, this study impacts other climate-related topics of the Southern Hemisphere including sea-ice studies, ice core paleoclimatology, and warming of West Antarctica, all of which have been shown to be highly influenced by ENSO-SAM interactions. This research significantly contributes to our knowledge of Southern Hemisphere climate and to modeling this environment, which have important implications on climate change impacts throughout the world and our ability to project future climate change. For instance, our top performing global climate models do not accurately project changes to sea ice observed in the Southern Hemisphere over recent decades. ENSO flavors are also likely to garner more attention in the coming decades as well, and this study is one of the first to comprehensively evaluate their potential impacts on the high-latitudes of the Southern Hemisphere.
