[A-CG38] 海洋と大気の波動・渦・循環の力学

[ACG38-05] A study of the simulated climatological January mean upwelling in the northwestern Gulf of Alaska

*ユアン ナン¹、三寺 史夫²、佐々木 英治³ (1.北海道大学、2.北海道大学低温科学研究所、3.国立研究開発法人海洋研究開発機構)

In this research, we studied the upwelling in the northwestern Gulf of Alaska using the climatological January mean. The data are from the output of the Ocean General Circulation Model for Earth Simulator (OFES2). Specifically, we analyzed the upwelling in the regions where the Alaska Coastal Current (ACC) flows out of the Shelikof Strait and where the ACC and the Alaskan Stream are confluent. In both regions, strong geostrophic currents and downwelling-favorable wind predominate in winter. Meanwhile, the ACC is enriched with freshwater discharged from the continent, forming freshwater fronts. We found that when the internal water stress is larger than the wind stress inside the study regions, it may be decisive in terms of the local horizontal velocity divergence and further upwelling even without a lateral boundary and upwelling-favorable wind. The internal water stress is modified by the geostrophic stress, which is a product of the geostrophic current shear and a high vertical viscosity coefficient, where the former is due to the thermal wind relation. The analysis indicated that a front with a large geostrophic stress may act as a ‘virtual wall’ and contributes to local upwelling within a depth of approximately 100m in the study regions. The revealed process could provide a heuristic for understanding the pollock distribution in the areas in February and March, which corresponds to the simulated upwelling region.