Interannual variability of eddy kinetic energy in the South China Sea related to two types of winter circulation events

Interannual variations of the eddy kinetic energy (EKE) related to two types of winter circulation events ("O" and "U") were investigated based on the outputs of the Ocean General Circulation Model (OGCM) for the Earth Simulator (OFES) and the corresponding energetic analyses. Results show that the EKE is strong and extends eastward to offshore the Vietnam coast about 2 degrees, associated with the weaker South China Sea western boundary current (SCSwbc) in "O" type years, while the EKE is weak and high value that can be attained is narrowed along the coast, associated with the stronger SCSwbc in "U" type years. The energy budget shows that the wind stress and barotropic/baroclinic instability are important factors to regulate the EKE in "U" and "O" years. For "U" years, under a strong winter monsoon forcing, the SCSwbc strengthen, the directly wind work and barotropic conversion from the mean kinetic energy (MKE) to EKE are weak, thus the EKE decrease corresponding to the baroclinic conversion from the kinetic energy to potential energy. However, the situation is reversed in "O" years. Under the influence of El Nino events, wind stress forces can weaken SCSwbc and enhance EKE in pattern "O", whereas La Nina events have relatively weaker influences. The barotropic conversion rate in "O" type is nearly eight times of the "U" type. The pressure work and advection term are the main sources to greatly suppress EKE in the SCSwbc region.