Tropical Instability Wave Variability in the Pacific and Its Relation to Large-Scale Currents

Abstract

Abstract Shipboard acoustic Doppler current profiler (ADCP)-derived zonal currents from 170° to 110°W are assembled into composite seasonal and ENSO cycles to produce detailed representations of large-scale ocean flow regimes that favor tropical instability waves (TIWs). The instability-favorable portion of these cycles, namely, the August– October period of the seasonal cycle and the pre-December period of the ENSO cold phase, both have intense westward flow in the South Equatorial Current, most particularly the branch north of the equator (SECN), and strengthened eastward flows in the North Equatorial Countercurrent (NECC) and the Equatorial Undercurrent (EUC). Taken together these flows enhance current shear in the two regions generally associated with TIW activity, namely, the cyclonic and anticyclonic shear regions located to the south and north of the SECN, respectively. Direct correlation of ADCP currents and CTD densities to an instability index derived from equatorial 13–30-day meridional velocit...