Investigation of the Southern Annular Mode and the El Nin?o-Southern Oscillation Interactions

by 1978- Fogt, Ryan Lee

The dominant mode of climate variability across the Southern Hemisphere is the Southern Hemisphere Annular Mode (SAM); however other modes associated with El Niño- Southern Oscillation (ENSO) variability also play a significant role. Notably, recent studies demonstrate significant correlation between ENSO and the SAM, primarily during austral summer in the 1980s and 1990s, and austral spring in the 1990s. The analysis presented here expands upon these studies to examine historical evidence of their coupling and the mechanisms that generate the shared variability. Using the methodology similar to previous studies, reconstructed indices of the SAM are calculated for each season and for the annual mean spanning nearly 150 years. Using these new SAM reconstructions, it was determined that the relationship between SAM and ENSO is quite variable in the historical record, and that periods similar to the 1990s when the two were strongly correlated in austral summer have occurred in the past. Using wavelet and spectral analyses, it was shown that the positive correlation between the SAM and ENSO is generated by interactions with 3-7 year periods. Stationary and transient eddies of heat and momentum are examined and reveal that during periods when the two climate modes are positively correlated, a band of E-P flux convergence during El Niño events (divergence during La Niña), associated with ii changes in the storm track and southern branch of the ENSO horseshoe pattern, extend from Indonesia to the high latitude south Pacific where the maximum teleconnection occurs. When the modes are positively correlated, the changes to the aforementioned storm track exist in a continuous fashion from the Rossby source region to the South Pacific where they interact with similar storm track anomalies forced by the SAM. The interaction of the storm tracks allows the tropical signal to penetrate deeper into the high southern latitudes and maintain a stronger anomaly, compared to periods when the two modes are anticorrelated and opposing storm track anomalies block the propagation of the tropical signal into the high latitudes. iii In honor of the love of my life, my inspiration, and my best friend, Andrea iv