Annual versus semi-annual eddy kinetic energy variability in the Celebes Sea

Eddy kinetic energy (EKE) in the Celebes Sea (CS) is investigated using a global, eddy-resolving, ocean general circulation model (OGCM) output. The OGCM simulation shows that a strong EKE is confined in the upper 300-m layer. The period of EKE variability varies with depth. In the 0–100-m layer, EKE has a distinct annual cycle that is strong in winter (December–February) and weak in summer (June–August). However, in the 100–300-m layer, semi-annual variation is dominant, which shows stronger EKE in spring and fall and weaker EKE in summer and winter. An eddy energy budget analysis reveals that the barotropic eddy energy conversion rate has a vertical structure similar to that of EKE. Compared to the barotropic eddy energy conversion rate, the baroclinic eddy energy conversion rate is much smaller and does not match the EKE vertical pattern. The budget analysis indicates that the variation in EKE in the CS is governed by barotropic instability of the background circulation. Further analysis reveals that the variation in the regional background circulation with depth is due to a combination of the forcing of the local monsoon and the Mindanao Current (MC) in the western tropical Pacific.