Oceanography (Mar 2013)
Extent and Energetics of the Hawaiian Lee Countercurrent
Abstract
Direct velocity observations from drogued drifters in the Hawaiian Island region are used to map the time-mean and seasonal variability of the Hawaiian Lee Countercurrent (HLCC). The density of these data has more than doubled since the initial discovery of the HLCC. They provide valuable absolute estimates of HLCC velocity structure and variability, complementing data derived from geostrophy and numerical simulations. The data demonstrate that the HLCC has a peak annual mean velocity > 9 cm s–1, with the strongest velocities along 19.75°N and eastward speeds in the longitudinal range 170°W to 157°W. The HLCC is relatively weak from March to May compared to its strength in other months. In the longitude band 160°–168°W, an eddy-to-mean energy flux of 3.3 ± 1.2 µW m–3 is found in the annual mean associated with the Reynolds shear stress, maintaining the shear between the HLCC and the North Equatorial Current to its south. This shear stress is associated with energetic anticyclonic eddies that are shed from the Big Island of Hawaii and propagate west-southwest. This energy flux is nearly twice as large during the peak HLCC months of August to January. It is sufficient to spin up the HLCC in O(10 days), and it is associated with an eddy spin-down time of O(100 days).
