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

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 velocities confirms the importance of the strengths of the SECN and NECC in determining the timing of TIW events. Very little correlation was found in the EUC, implying that its strength is not a determining factor in such timing. Reynolds stress and density flux calculations indicate that in a time-averaged sense TIWs derive energy from both the cyclonic and anticyclonic flanks of the SECN, and from both sides of the equatorial cold tongue. During low-instability periods these Reynolds stresses and fluxes substantially vanish, indicating that eddy energy production ceases. This is in marked contrast to Baturin and Niller's study, which indicated that eddy energy production was relatively continuous at 110°W. The current structures of individual months associated with TIW activity show substantial variability among themselves. Combined with previous findings of multiple modes of instabilities, this indicates that caution is required when attempting to model instabilities from averages of observed background flows such as those presented here.