Large-scale circulation changes in the Kuroshio Extension region of the western North Pacific Ocean are investigated using the sea surface height (SSH) data measured by the TOPEX/Poseidon satellite. Over the last decade, the low-frequency signal of the Kuroshio Extension was characterized by a modulation in intensity of its zonal mean flow: the zonal mean Kuroshio Extension jet weakened progressively from 1993 to 1996 and this trend reversed after 1997. The modulation of the mean flow affects the regional kinetic energy field, with a more intense mean zonal jet inducing a more zonally elongated high kinetic energy band.
By examining baroclinic adjustment processes under linear vorticitydynamics, we find that this modulation in the zonal mean flow is remotely forced by wind-stress curl anomalies in the eastern North Pacific Ocean related to the Pacific decadal oscillations (PDOs). Specifically, the weakening trend in the mean Kuroshio Extension jet in 1993--96 was caused by westward expansions of negative SSH anomalies south of the Kuroshio Extension and positive SSH anomalies north of the Kuroshio Extension. In contrast, westward expansions of positive (negative) SSH anomalies to the south (north) of the Kuroshio Extension jet following 1997 resulted in a strengthening trend in the zonal Kuroshio Extension jet. Emergence of oppositely-signed SSH anomalies on the two sides of the Kuroshio Extension jet is due to the latitude-dependent speeds of the baroclinic Rossby waves, which carry wind-induced SSH anomalies that are generated in the eastern North Pacific at different phases of the PDOs. With the geographic locations of the Kuroshio Extension and the atmospheric wind forcing, the maximum modulation of the Kuroshio Extension jet is found to occur when the surface forcing in the eastern Nrth Pacific has a period of about 10 yrs. It is hypothesized that the baroclinically modulating Kuroshio Extension contributes to set the time scales of the North Pacific midlatitude climate system. |
|
|