[A-CG29] Extratropical oceans and atmosphere

[ACG29-08] On seasonality of the quasi-stationary jet (Western Isoguchi Jet) in the subtropical-subarctic frontal zone in the North Pacific

*Humio Mitsudera¹, Masato Miyake² (1.Institute of Low Temperature Science, Hokkaido University, 2.Graduate School of Environmental Science, Hokkaido University)

Keywords:subtropical-subarctic frontal zone, quasi-stationary jet, seasonal variations

The subtropical-subarctic frontal zone exhibits complex water mass structures because of the confluence of Kuroshio-origin water and Oyashio-origin water. Despite this complexity, there are quasi-stationary jets, such as the Western Isoguchi Jet (WIJ), which regulate the frontal zone structures linked with the low-rise bottom topographic features in the western North Pacific. The WIJ carries warm Kuroshio-origin water to the subarctic gyre, and thus plays an important role in the formation of sea surface temperature variations in the subtropic-subarctic frontal zone, which is likely correlated with mid-latitude atmospheric circulation variations of the northern hemisphere. It is also known that the WIJ has a seasonality in its strength, in which the WIJ is strong in summer and fall, whereas weak in spring. In this study we investigated mechanisms of the seasonality of the WIJ. We examined two hypotheses, which are (1) barotropic response of the WIJ to the seasonal variations of the wind forcing, and (2) seasonal changes of the Kuroshio water that enters the WIJ. As for the first hypothesis, simple numerical experiments with a rectangular low-rise topography were conducted to examine ocean responses to the wind forcing. When an annual barotropic Rossby wave is incident from the east to a low topographic feature, a back scattering of short Rossby waves occurs from the topography eastward, forming annual velocity variations in a manner consistent with the WIJ’s variations. To investigate the second hypothesis, we utilized a Lagrangian approach. The Kuroshio origin water in the WIJ was tracked backward by particles that are advected by the surface flow field retrieved from satellites. The Kuroshio origin particles tend to enter the WIJ via the Oyashio Second Branch during summer predominantly, which is consistent with the summer intensification of the WIJ. The seasonality of the Kuroshio water intrusion is found to be characterized by the closeness of the southernmost latitude of a referenced sea surface height (SSH) contour, which is detemined by the SSH along the southern flank of the WIJ, to the Kuroshio Extension. The seasonality of the Kuroshio water intrusion into the WIJ exhibits decadal time scale modulations, which correlated with the northward/southward migration of the Kuroshio Extension.