Vorticity Balance off the South Shore of Oahu, Hawaii, Derived by High-Frequency Radio Doppler Current Observations

High-frequency radio Doppler surface current observations off the south shore of Oahu, Hawaii, are used to calculate the vorticity equation at a 2-km spatial resolution in terms of a time-dependent and time-mean surface balance. First-order terms are mean advection of mean vorticity, vortex stretching, and a residual, which is treated as unquantified terms such as wind stress curl, bottom pressure torque, and noise. The most striking feature in the 2-yr time-mean vorticity balance is the anticorrelation between advection of vorticity and vortex stretching implying that potential vorticity (PV) advection is the most dominant mechanism in the area. Several terms in the depth-integrated vorticity balance were also estimated. The bottom pressure torque acts as a first-order term only in areas of shallow topography. A PV analysis resulted in the 50-m Penguin Bank steering the westward Hawaiian Lee Current.