Seasonal Variability of Subthermocline Eddy Kinetic Energy East of the Philippines

Seasonal modulation of subthermocline eddy kinetic energy (EKE) east of the Philippines and its associated dynamics are studied, using mooring measurements and outputs from an eddy-resolving ocean general circulation model for the period from 2000 to 2017. Significantly high EKE appears below the thermocline in the latitude band between 5° and 14°N east of the Philippines. Separated by 10°N, the EKE in the northern and southern parts of the region shows nearly opposite seasonal cycles, with its magnitude reaching a maximum in early spring and minimum in summer in the northern part and reaching a maximum in summer and minimum in winter in the southern part of the region. Further investigation indicates that both baroclinic and barotropic instabilities are essential in generating the subthermocline eddies, but the seasonal variation of subthermocline EKE is mainly caused by the seasonal modulation of barotropic instability. The seasonal modulation of barotropic instability in the northern and southern part of the region is associated with the seasonal evolution of North Equatorial Undercurrent and Halmahera Eddy, respectively.