[A-CG38] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG38-04] Impacts of bottom topography on the formation of the North Pacific subtropical-subarctic frontal zone

*Humio Mitsudera¹, Hayato Ashida², Toru Miyama³ (1.Institute of Low Temperature Science, Hokkaido University, 2.Graduate School of Environmental Science, Hokkaido University, 3.JAMSTEC)

Keywords:subtropical-subarctic frontal zone, Emperor Seamounts, transition domain, quasi-stationary jets

The subtropical-subarctic frontal zone in the North Pacific exhibits a complex water mass structure because the water of Kuroshio origin and the water of Oyashio origin are confluent, stirred there and mixed. Despite this complexity, two quasi-stationary fronts form in this region that characterize the frontal zone, which are the subarctic front (SAF), recognized by potential temperature of 4 ℃ contour at a depth of 100 m, and the subarctic boundary (SAB), recognized by salinity of 34 contour at a depth of 100 m; the so-called transition domain forms in between. This frontal zone is generally considered as a wind-driven gyre boundary between the subtropical and subarctic gyres. However, the frontal zone is located around 40 °N, which is further south than the location of the Sverdrup gyre boundary that forms at approximately ~45 °N. Here, we study the effects of bottom topography on the formation of the subtropical-subarctic frontal zone in the western North Pacific, with a particular attention to the Emperor Seamounts. We conducted an eddy-resolving two-layer modeling with a realistic topography of the ETOPO1, forced by a zonally-averaged wind stress derived from the JRA-55 do. The model exhibits a realistic frontal zone structure of the North Pacific comprising the SAF and SAB, as well as the transition domain. Eddy driven bottom currents are generated over topography, which in turn causes substantial impacts on the formation of upper layer frontal-zone structures. By conducting topography on/off experiments, we found that the Emperor Seamounts bring the subarctic thin layer southward across the Sverdrup gyre boundary. The thinner layer of the subarctic origin and thicker layer of the subtropical origin meet and mix there, leading to the formation of the SAB and the transition domain.