The influence of the Hawaiian Archipelago upon the wind‐driven subtropical gyre in the western North Pacific

Abstract

In the middle of the Subtropical Gyre in the western North Pacific is the Subtropical Counter‐current, comprised of at least three narrow bands of eastward geostrophic mean flow near 20°N, 24°N, and 26°N, each extending from the sea surface through the main pycrocline. The origin of these narrow bands of eastward mean flow has not previously been explained. In this study, these bands are shown in the mean to extend zonally from the Philippine Sea (i.e., 140°E) to the vicinity of the Hawaiian Archipelago (i.e., 160°W). East of there, they cannot be found. Therefore a hypothesis is presented wherein the Hawaiian Archipelago (extending from 19°N to 26°N near 165°W) is seen to act as an effective barrier, modifying the response of the western subtropical North Pacific to wind‐driven effects in the eastern subtropical North Pacific and, in so doing, giving rise to these bands of eastward mean flow in the middle of the western Subtropical Gyre. A test of this hypothesis utilizes a Sverdrup [1947] model, driven by the annual, long‐term, mean wind stress curl, constructed on a 2° latitude/longitude grid from synoptic (6 hourly) surface wind observations for the 13‐year period 1968–1980. Two model simulations are conducted, one in the absence of the Hawaiian Archipelago and one in the presence of it. In the absence of the archipelago, none of the bands of eastward mean flow are simulated. In the presence of the archipelago, the narrow bands of eastward mean flow at 20°N, 26°N, and 24°N are simulated, although the latter band is very weak. These narrow bands of eastward flow are shown to occur in response to the deflection northward of the wind‐driven geostrophic streamlines in the central subtropical North Pacific by the Hawaiian Archipelago.