Cross-equatorial influences of submonthly scale southerly surges overthe eastern Indian Ocean during Southern Hemisphere winter

摘要

Lower-tropospheric southerly surges occurring at submonthly timescales overthe eastern Indian Ocean (EIO) during Southern Hemisphere (SH) winter areextratropical-tropical interaction phenomena involving various processes. This studyexamined the physical mechanism driving equatorial wave disturbances associated withsurge-induced cross-equatorial flow over the EIO. Japanese 25-Year Reanalysis (JRA25)and Tropical Rainfall Measuring Mission Satellite (TRMM) 3B42 data products from Juneto August for the 7 years from 1998 to 2004 were analyzed. Composite analyses oflow-level wind and vorticity fields reveal that an equatorial clockwise gyre develops justafter cross-equatorial penetration of southerly surge winds. This gyre is interpreted asan equatorially trapped Rossby wave that develops asymmetrically around the equator inresponse to surge-induced cross-equatorial flow. Eddy vorticity budget analysisdemonstrates the dynamic role of the southerly surge in the genesis of the equatorialclockwise gyre. The result indicates that an injection of negative absolute vorticity into theequatorial tropics due to both advective and convergent processes associated with thesurge winds is primarily responsible for the gyre genesis. Cross-equatorial advection ofbasic state absolute vorticity by the surge (eddy) winds and vortex stretching by surgewind convergence are most effective in spinning up the equatorial clockwise gyre. Waveactivity flux analysis further emphasizes that Rossby wave energy propagation from theSH extratropics into the equatorial EIO region is caused by the southerly surge. Thesedynamic diagnostics suggest that the southerly surge plays a lateral forcing role in excitingthe equatorial wave disturbance over the EIO. Composite analyses of moisture flux andprecipitation anomalies characterize the equatorial clockwise gyre as a convectivelycoupled equatorial Rossby wave. Enhanced cross-equatorial moisture transport due to thesoutherly surge appears to be linked to precipitation along the west coast of the southernIndo-China Peninsula. A case study of the southerly surge event during August 2000illustrates the formation of the equatorial clockwise gyre and its convectively coupledequatorial Rossby wave signature.