Evidence for a Wind-Driven Intensification of the Kuroshio Current Extension from the 1970s to the 1980s

Abstract

The spatial and temporal characteristics of oceanic thermal variations in the mixed layer and main thermocline of the midlatitude North Pacific are distinctive, suggesting different physical origins. Within the main thermocline (400-m depth), the variability is dominated by a westward-intensified pattern of decadal scale, indicative of enhanced eastward geostrophic flow along the southern flank of the Kuroshio Current extension during the 1980s relative to the 1970s. The authors argue that the decadal-scale change in the strength of the Kuroshio extension was a result of the dynamical adjustment of the oceanic circulation to a decadal variation in wind stress curl according to Sverdrup theory. Four-times daily wind stress fields from the National Center for Atmospheric Research?National Centers for Environmental Prediction reanalysis project are used to compute the decadal change in Sverdrup transport associated with the 1976/77 climate transition. It is shown that the decadal changes in Sverdrup transport inferred from the wind stress curl field and in observed geostrophic flow inferred from the upper-ocean thermal field are consistent both in terms of spatial pattern and magnitude. The decadal change in depth-averaged geostrophic transport along the Kuroshio extension (referenced to 1 km) is 11.6 Sv, similar to the Sverdrup transport change (11.5?13.9 Sv). The decadal-scale thermocline variation along the western boundary between 30° and 40°N exhibits a lag of approximately 4?5 yr relative to the decadal variation in the basin-wide wind stress curl pattern. This delay may be indicative of the transient adjustment of the gyre-scale circulation to a change in wind stress curl via long baroclinic Rossby waves.