Extratropical Forcing of Convectively Coupled Kelvin Waves during Austral Winter

Abstract

Observations are presented that link extratropical Rossby wave disturbances excited in the Southern Hemisphere subtropical jet to the initiation of convectively coupled Kelvin waves in the Pacific intertropical convergence zone (ITCZ) during austral winter. A baroclinic, zonal wavenumber 6, eastward-propagating Rossby wave train in the subtropical jet turns northeastward in the vicinity of Australia, inducing upper tropospheric divergence and vertical motion fields that spread equatorward and induce cloudiness anomalies in the Tropics. Lower tropospheric pressure surges excited from the extratropics also induce Kelvin wave?like geopotential height and temperature anomalies at the surface, providing additional lower tropospheric convergence and vertical motion forcing. The tropical outgoing longwave radiation (OLR) and circulation fields propagate eastward in tandem with the extratropical Rossby wave train at approximately 17 m s?1. Kelvin wave activity in the central Pacific ITCZ thus appears to be associated with eastward-propagating Rossby wave activity in the extratropics, which is traveling at phase speeds similar to those observed in developed convectively coupled Kelvin waves (15?20 m s?1). The longer timescale relationship between subtropical jet activity and Kelvin wave variability in the Tropics is determined through the calculation of monthly averaged composite fields. When Kelvin wave OLR activity is enhanced (suppressed) in the tropical Pacific, eastward-propagating Rossby wave activity in the Kelvin wave phase speed band (8?30 m s?1) is anomalously strong (weak) in the subtropical jet. A case study is presented that suggests that enhanced Kelvin wave activity in the central Pacific ITCZ is associated more strongly with enhanced eastward-propagating Rossby wave activity in the subtropics than with the local thermal and moisture boundary conditions in the tropical Pacific.