[A-CG38] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG38-15] Extratropical forcing of intraseasonal equatorial Kelvin waves over the Indian and Pacific Oceans

*Fukutomi Yoshiki¹, Hidenori Aiki¹ (1.Institute for Space-Earth Environmental Reasearch, Nagoya University)

Keywords:Equatorial Kelvin waves, Intraseasonal variability, Extratropical-tropical interaction

Equatorial Kelvin waves (KWs) are major convective disturbances accompanying tropical super cloud clusters (SCCs) which propagate eastward along the equatorial belt. KWs generally span the broad range from synoptic to intraseasonal time scales and they have horizontal scales of about zonal wavenumber 1-10. This study examines characteristics and origins of intraseasonal equatorial KWs which have periods of about 10-20 days in the Indian Ocean-western Pacific region during Austral winter. They are distinguished from synoptic-scale (about 2-8 days) KWs. An extended empirical orthogonal function (EEOF)-based composite analysis is performed on 8-25-day filtered outgoing longwave radiation and atmospheric circulation fields during June-August 1979-2018. The analysis results reveal the structure and evolution of the KWs and associated extratropical-tropical interactions. The composite KW patterns exhibit a zonal wave number 3 structure with eastward phase speed of around 12-16m/s. The KWs initially develop over the Indian Ocean, then they propagate eastward into the eastern Pacific after passing through the Maritime Continent (MC). Initiation mechanisms of KWs due to Southern Hemisphere extratropical wave activity is explored by identifying precursor wave signals in the extratropics. Southerly surge-like flows in the lower troposphere are identified over the western Indian Ocean and the western Pacific close to the MC associated with the KW development. These two southerly flows are linked to initiation of KWs at each location. They are originated in the extratropical wave train which progress eastward into the central-eastern South Pacific. There is strong evidence that equatorward advection of westerly momentum is enhanced by the southerly surge-like flow. This equatorward transport of westerly momentum plays a vital role in initiating KWs at these two locations.