Cross‐equatorial influences of submonthly scale southerly surges over the eastern Indian Ocean during Southern Hemisphere winter

Abstract

Lower‐tropospheric southerly surges occurring at submonthly timescales over the eastern Indian Ocean (EIO) during Southern Hemisphere (SH) winter are extratropical‐tropical interaction phenomena involving various processes. This study examined the physical mechanism driving equatorial wave disturbances associated with surge‐induced cross‐equatorial flow over the EIO. Japanese 25‐Year Reanalysis (JRA25) and Tropical Rainfall Measuring Mission Satellite (TRMM) 3B42 data products from June to August for the 7 years from 1998 to 2004 were analyzed. Composite analyses of low‐level wind and vorticity fields reveal that an equatorial clockwise gyre develops just after cross‐equatorial penetration of southerly surge winds. This gyre is interpreted as an equatorially trapped Rossby wave that develops asymmetrically around the equator in response to surge‐induced cross‐equatorial flow. Eddy vorticity budget analysis demonstrates the dynamic role of the southerly surge in the genesis of the equatorial clockwise gyre. The result indicates that an injection of negative absolute vorticity into the equatorial tropics due to both advective and convergent processes associated with the surge winds is primarily responsible for the gyre genesis. Cross‐equatorial advection of basic state absolute vorticity by the surge (eddy) winds and vortex stretching by surge wind convergence are most effective in spinning up the equatorial clockwise gyre. Wave activity flux analysis further emphasizes that Rossby wave energy propagation from the SH extratropics into the equatorial EIO region is caused by the southerly surge. These dynamic diagnostics suggest that the southerly surge plays a lateral forcing role in exciting the equatorial wave disturbance over the EIO. Composite analyses of moisture flux and precipitation anomalies characterize the equatorial clockwise gyre as a convectively coupled equatorial Rossby wave. Enhanced cross‐equatorial moisture transport due to the southerly surge appears to be linked to precipitation along the west coast of the southern Indo‐China Peninsula. A case study of the southerly surge event during August 2000 illustrates the formation of the equatorial clockwise gyre and its convectively coupled equatorial Rossby wave signature.