Southern Hemisphere extra-tropical forcing: a new paradigm for ENSO Pascal Terray, 2010 R 陳漢卿

Numerous studies have documented the significant influence of ENSO on climate variability in the Southern hemisphere. (Karoly 1989; Kidson and Renwick 2002; Trenberth et al. 2002; Carleton 2003) Some indicate that ENSO are influenced by Antarctic Circumpolar Wave(ACW) or the Southern Annular Mode (SAM). (Van Loon and Shea 1987; Peterson and White 1998; White et al. 2002; White and Annis 2004)

The associated EOF modes can be regarded as the regional expression of a global-scale Southern Hemisphere mode of variability.

Southern Hemisphere Subtropical air-sea feedbacks SSTa propagate to tropics Weakening of ITCZ forces Cold Kelvin wave response in upper troposphere Western wind & convective anomalies over the western Pacific ENSO onset

Indian OceanAtlantic Ocean Surface latent heat, Cloudiness, wind-induced mixing

Southern Hemisphere Subtropical air-sea feedbacks SSTa propagate to tropics Weakening of ITCZ forces Cold Kelvin wave response in upper troposphere Western wind & convective anomalies over the western Pacific ENSO onset

Kelvin wave

Southern Hemisphere Subtropical air-sea feedbacks SSTa propagate to tropics Weakening of ITCZ forces Cold Kelvin wave response in upper troposphere Western wind & convective anomalies over the western Pacific ENSO onset

Sets the stage for deep convection by destabilizing the vertical structure of the atmosphere (e.g. due to the increased vertical temperature gradient) Decreasing the value of surface boundary layer moisture required for the convection to occur over the western equatorial Pacific

Westerly wind Downwelling Kelvin waves along the thermocline Warming in eastern & central Pacific Bjerkness feedback

Southern Hemisphere Subtropical air-sea feedbacks SSTa propagate to tropics Weakening of ITCZ forces Cold Kelvin wave response in upper troposphere Western wind & convective anomalies over the western Pacific ENSO onset

Data source Hadley Center SST & SLP NOAA Climate Prediction Center OLR OAFlux surface heat flux