Impact of Low-Level Southerly Surges on Mixed Rossby Gravity Waves over the Central Pacific.

Abstract:

This study examines dynamical impacts of lower-tropospheric southerly wind surges originating in midlatitudes of the Southern Hemisphere (SH) on the development of mixed Rossby gravity (MRG) waves over the central Pacific during June-August 1979-2012, through the statistical analysis of the JRA-55 products and NOAA outgoing long wave radiation data. The central Pacific MRG waves are identified by an extended EOF (EEOF) analysis on 2-8-day filtered daily 850-hPa meridional wind anomalies during June-August 1979-2012. Composite analysis based on the leading EEOF time coefficients is able to capture the development of the MRG waves associated with a southerly surge originating in the SH extratropics. As a weak clockwise gyre as a part of an off-equatorial easterly wavetrain moves eastward and southeastward from the off-equatorial eastern Pacific into the equatorial central Pacific, the southerly surge penetrates into the equatorial tropics at around 150W. Then, the clockwise gyre develops into a MRG-type gyre over the central Pacific. A transition from an easterly wave-type gyre into a MRG-type gyre occurs associated with the southerly surge. The southerly surge forms a cross-equatorial flow on the western flank of the MRG-type gyre. The gyre is amplified when the southerly surge reaches the equatorial tropics. At the same time, convection coupled with the MRG-type gyre is enhanced. The southerly surges are originated in the midlatitude South Pacific, and they are induced by synoptic-scale baroclinic disturbances propagating along the SH midlatitude westerly jet. An eddy vorticity budget analysis indicates that the southerly surge plays an important role in spinning up the MRG-type gyre through transient advection of absolute vorticiy. A case study of a MRG-wave event in mid-July 2006 also illustrates development of a MRG wave associated with the southerly surge and an easterly wave-to-MRG wave transition.