Quasi‐stationary jets transporting surface warm waters across the transition zone between the subtropical and the subarctic gyres in the North Pacific

Abstract

Surface flow jets and associated sea surface temperature (SST) distribution are investigated in the northwestern North Pacific, using satellite‐derived surface currents and SST data that can resolve fine spatial scale structure. The combined use of these data reveals warm tongue phenomena driven by surface geostrophic jets, which extend northeastward in the Kuroshio‐Oyashio transition area. They roughly coincide with the Subarctic Front (SAF) defined as the 4°C isotherm at 100 m depth. The phenomena appear throughout the year with seasonal cycles, which do not correspond with that of the subarctic North Pacific. Their positions are affected by bottom topography so that their quasi‐stationary and consistent features are suggested. Thus it is implied that these jets play an important role in transporting warm waters toward the subarctic region. The time series of SST and current fields describe the jets' year‐to‐year variability involving some changes in strength and connections with the Kuroshio Extension (KE), and demonstrate the effects of advection by the jets on SST fields. When KE extends northward at its crests for 1999–2002, one of the jets along SAF simultaneously strengthens, resulting in high SST region directly over and along the south side of the jet. Hydrographic data show warm, saline water intrusions along the jets. Their mean fields also reveal that these jets are vertically well‐developed, forming a boundary between the subtropical and subarctic gyres in the North Pacific.