Meridional Variation of the Mascarene High and Atmospheric Transient Eddy Dynamical Forcing over the Southern Indian Ocean in Austral Winter

Abstract

Abstract This study reveals a close relation between the Mascarene high (MH), atmospheric transient eddies (hereafter transient eddies or eddies for short), and the sea surface temperature (SST) front over the southern Indian Ocean in austral winter. Climatologically, the subpolar westerly jet couples well with transient eddies via eddy–mean flow interaction and the anticyclonic vorticity to its north helps with anchoring the MH. On the interannual time scale, the MH exhibits a dominant meridional variation accompanied by intensity variability. When the MH moves poleward and intensifies, positive quasi-barotropic geopotential height anomalies associated with a warm temperature feature the southern flank of MH. As a result of the modified mean temperature gradient, the subpolar jet and transient eddies’ activity are enhanced near the jet exit; in contrast, the subtropical westerly jet and eddies are weakened over the jet entrance, mainly via the baroclinic energy conversion. As feedback, the anomalous transient eddies can trigger the poleward shift of MH by diverging the extended Eliassen–Palm (E-P) flux from subpolar to subtropical region and thus the intensification of subpolar jet and weakness of subtropical jet. Such positive feedback between the meridional variation of MH and transient eddies could be attributed to the underlying SST anomalies. Early SST warming appears over the southwestern Indian Ocean 3 months prior and shifts the Agulhas SST front poleward. The poleward Agulhas SST front could further induce a southward displacement of the activity of transient eddies by changing the low-level atmospheric baroclinicity. Hence, the SST anomalies over the southern Indian Ocean may trigger the meridional variation of MH via the positive eddies–mean flow feedback.