Interannual-to-Decadal Variability in the Oyashio and Its Influence on Temperature in the Subarctic Frontal Zone: An Eddy-Resolving OGCM Simulation

Abstract

Output of an eddy-resolving OGCM simulation is used to investigate mechanisms for interannual-to-decadal variability in the Oyashio and its influence on the subarctic frontal zone in the western North Pacific. Lag correlation analysis reveals that positive anomalies both in basin-scale wind stress curl and in local Ekman pumping can intensify the southward Oyashio almost simultaneously via barotropic and baroclinic Rossby wave propagations, respectively. The Oyashio strength can also be influenced by anomalous Ekman pumping that is exerted in the western portion of the basin through the baroclinic wave propagation with the lag of 3 yr, which appears to arise from a periodicity in the wind field. The intensification of the Oyashio is accompanied by negative anomalies both in the sea surface temperature and height off of Hokkaido Island of Japan and is followed by their eastward development along the southern branch of the Oyashio Extension and associated subarctic frontal zone in association with a southward displacement of their axes. These changes are associated with cool sea surface temperature anomalies and low potential vorticity anomalies at the thermocline level in the frontal zone. The surface cooling, thus induced in the frontal zone by those oceanic processes, accompanies anomalous downward surface heat fluxes, indicative of ocean-to-atmosphere feedback forcing associated with the Oyashio variations.

Role of the Gulf Stream and Kuroshio–Oyashio Systems in Large-Scale Atmosphere–Ocean Interaction: A Review

“…Resolution of conventional gridded temperature data is not quite sufficient for distinguishing the two fronts. A 50-yr hindcast integration using an eddy-resolving OGCM [the OGCM for the Earth Simulator (OFES)] with observed daily atmospheric forcing (Masumoto et al 2004;Ohfuchi et al 2007) reveals distinctive characteristics of temperature variability associated with the KE and OE fronts that arise from differences in their vertical structure (Nonaka et al 2006). The OE front, located at ;418N, is strongest at the surface, whereas the KE front, located at ;358N, is strongest between 200 and 600 m below the surface (Fig.…”

“…When displaced meridionally, the OE and KE fronts create strong thermal anomalies mainly in the mixed layer (ML) and at the thermocline level, respectively. Given its deep structure, the KE front is strongly influenced by large-scale changes in the subtropical gyre driven by wind stress changes across the North Pacific (Qiu 2000(Qiu , 2003Qiu and Chen 2005;Taguchi et al 2005;Nonaka et al 2006), as discussed in section 2f.…”

“…However, the southward shift happened slightly earlier for the OE front, indicating that their axial migration may not be synchronized. In the OFES hindcast integration, the intensity and latitude of the OE front are closely related to the strength of the Oyashio, which tends to be strongly influenced by the wind forcing exerted on the western portion of the subpolar gyre but not on the central or eastern portion (Nonaka et al 2008). The SAFZs are also characterized by meridional salinity gradients with less saline water to their north, and thus their axial displacement yields salinity anomalies that are collocated with SSTAs.…”

“…Furthermore, the OFES hindcast integration suggests that the intensification of the Oyashio and the southward displacement of its extension generated negative SSTAs along the OE front in the mid-1980s (Nonaka et al 2006(Nonaka et al , 2008. During the same period, cooling along the KE front was caused by the southward shift of the KE current rather than its intensification (Deser et al 1999).…”

“…Anomalous geostrophic heat advection and/or the axial migration of the WBCs and associated frontal zones can yield subsurface temperature anomalies that persist over multiple years ( Fig. 3), which can be entrained into the ML in fall and winter (Nonaka et al 2006). In summer, the shallow ML shields the subsurface water from thermal damping by Q net .…”