Start Date/Time: Thursday, September 29, 2011, 3:30 PM
Ending Date/Time: Thursday, September 29, 2011, 4:20 PM
Location: ATG 310 C
Atmos & Climate Dynamics Seminar Series (ATM S 521)
Coordinator: Prof. John M. Wallace
Speaker: Dr. Qianghua Ding, Earth & Space Sciences, UW
Title: Influence of the Tropics on the Southern Annular Mode
Perturbations in the Southern Annular Mode (SAM) are shown to be significantly correlated with SST anomalies in the tropical central Pacific during austral winter, and SST anomalies in the tropical eastern Pacific during austral summer. The SAM signature in the Pacific sector resembles a tropically-forced Rossby wave train, the so-called Pacific-South American pattern, while the signature in the Indian Ocean sector is a zonally elongated meridional dipole. Thus, the SAM tends to behave differently in the Eastern and Western Hemispheres, with internal dynamics prevailing in the Indian Ocean sector and the forced response to tropical SST anomalies exerting a strong influence in the Pacific sector. The tropically-forced component of the SAM in the Pacific Sector is related to a geographically fixed active Rossby wave source to the east of Australia within the core of the subtropical jet. In addition to the well-documented positive trend in summer, the SAM also exhibits a significant negative wintertime trend since 1979, characterized by prominent geopotential height increases over the high latitudes. In both seasons, SAM trends are closely linked to long term trends in tropical Pacific SST. Although the SAM is an intrinsic feature of high-latitude variability, maintained by wave mean flow interactions, the SAM index reflects the superposition of both high latitude and tropically forced variability.