Enhanced 2-h–8-day Oscillations Associated with Tropical Instability Waves

Abstract

Abstract The westward South Equatorial Current (SEC) and eastward Equatorial Undercurrent (EUC) is a marginally stable current system due to the strong vertical shear. The existence of wavelike motions may locally reduce the Richardson number enough to trigger instabilities. Here, velocity measurements from the Tropical Atmosphere Ocean (TAO) array are used to examine the variability of oscillations within 0.125–12 cycles per day (cpd). It is found that the 0.125–12-cpd oscillations become more energetic in the presence of strong tropical instability waves (TIWs). The enhancement of shear variance is most pronounced around the EUC core (115 m), while prominent elevation of kinetic energy occurs around 85 m, where the EUC shear is strongest. Particularly, the energetic 0.125–12-cpd oscillations during strong TIW seasons do not cycle on a daily basis and are more evident during the southward phase of TIWs. The enhanced 0.125–12-cpd oscillations during strong TIW seasons can be ascribed neither to the changing background stratification nor to the vertical migration of EUC core at the corresponding time scales. Its strength is tightly correlated with the EUC shear and, to a lesser extent, the TIW shear. A partial correlation analysis suggests that the correlation to the TIW shear is mainly due to the association between EUC and TIW shear. The strength of the 0.125–12-cpd oscillations does not follow the variation of surface wind speed and wind curl, implying that they are not directly generated by surface wind forcing.