Wind driven currents south of Oahu

Abstract

Two years of HFDR currents south of Oahu were analyzed to understand the main modes of circulation. A relatively slow ( 20 cm/s) westward flow was observed with eastward reversals in winter and spring. To understand these reversals, their dependence on local wind forcing was investigated using buoy measurements and a 1.5-km resolution operational atmospheric model. Southerly winds were typically found when the reversals occurred. The dominant EOF mode accounts for 58% of the variance, and is significantly correlated with the meridional wind. To further assess the effect of the winds, the vorticity balance was estimated. The advective non-linear term is mostly balanced by vertical stretching of the water column and a residual term derived from the sum of those two terms. This residual term, is significantly correlated with the wind stress curl term derived from an atmospheric model, indicating that it is the main external forcing in the vorticity generation. An operational ROMS nested model for the Hawaiian Archipelago is used to understand the vertical behavior of the wind driven currents. Vertical modes computed from model currents are well correlated with both the wind and wind stress curl during trade wind conditions and suggest a rapid decay of wind effects below the Mixed Layer Depth.