Department of Atmospheric and Oceanic Sciences, UW-Madison
Assessing Extratropical Influence On Tropical Climatology And Variability With Regional Coupled Data Assimilation
Room 811 AOSS, May 3, 2017, 1:45 PM
Tropical variability (e.g. ENSO) and climatology (e.g. ITCZ) were initially thought to be determined mostly by local forcing and ocean-atmosphere interaction in the tropics. Since late 20th century, numerous studies have showed that extratropical forcing could affect, or even largely determine some aspects of the tropical climate. Due to the coupled nature of the climate system, the challenge of determining and further quantifying the causality of extratropical forcing on the tropical climate remains. This dissertation studies the extratropical influence on both tropical variability and climatology using the Regional Coupled Data Assimilation (RCDA) method in a coupled general circulation model (CGCM).
Following the previous perfect-model RCDA experiments that demonstrated strong extratropical control on ENSO variability, RCDA experiments with real world reanalysis data show significant extratropical impact on both tropical climatology and variability in the CGCM. For climatology, the modelís double-ITCZ bias is improved systematically when SST, air temperature and wind are corrected toward real world data from the extratropics into the tropics progressively. Coupled dynamics, as well as atmospheric and oceanic processes, play important roles in this extratropical-to-tropical teleconnection. Meanwhile for variability, certain historical ENSO events are reproduced by assimilating extratropical atmospheric observations, which indicates the extratropical impact found in the perfect-model experiments to be somewhat applicable to the real world. The varying impacts of the precursors on different historical ENSO events are explored by case studies and additional sensitivity experiments.