Influence of local dynamical air-sea feedback process on the Hawaiian lee countercurrent

Local air-sea interactions over the high sea surface temperature (SST) band along the Hawaiian Lee Countercurrent (HLCC) are examined with a focus on dynamical feedback of SST-induced wind stress to the ocean using the atmosphere-ocean coupled general circulation model (CGCM). A pair of ensemble CGCM simulations are compared to extract the air-sea interactions associated with HLCC: the control simulations and other simulations, the latter purposely eliminating influences of the high SST band on the sea surface flux computations in the CGCM. The comparison reveals that oceanic response to surface wind convergence and positive wind stress curl induced by the high SST band increases (decreases) the HLCC speed in the southern (northern) flank of the HLCC. The HLCC speed changes are driven by the Ekman suction associated with positive wind stress curl over the warm HLCC via the thermal wind balance. The HLCC speed increase is more significant than its decrease. This dynamical feedback is likely to be important to sustain the extension of the HLCC far to the west. The heat budget analysis confirms that advection of warm water from the west associated with this significant current speed increase plays a role in the southward shift of theHLCCaxis. The dynamical feedback with the HLCC speed increase can potentially amplify the seasonal and interannual variations of HLCC.