Response of the Ryukyu Current to Climate Change During 1993–2018: Is There a Robust Trend?

Linear trends in the Ryukyu Current, a part of the western boundary current in the western North Pacific flowing on the seaside of the Ryukyu Island chain, were investigated using reanalysis data during 1993-2018. The subsurface Ryukyu Current has weakened along its path during the recent decades. Two determinant factors for the weakened subsurface Ryukyu Current are as follows. First, the first baroclinic topographic Rossby wave propagated signals emanating from the Tokara Strait (TK) and the Kerama Gap (KG) southward along the eastern slope of the Ryukyu Island chain, which depressed the onshore side of the isopycnal. The weakened Kuroshio in the TK and weakened overflow through the KG during this period led to isopycnal shoaling along the onshore side east of the Ryukyu Island chain, slowing down the Ryukyu Current. Second, anticyclonic eddies emanated from the interior region near 30 degrees N and 170 degrees W increased during this period. These anticyclonic eddies, translating southwestward, reached east of the Ryukyu Island chain, and finally deepened the offshore side of the isopycnal depth of the Ryukyu Current. The isopycnal across the Ryukyu Current velocity core became less steep, thereby weakening the subsurface Ryukyu Current. Moreover, a positive trend of sea surface height anomaly southeast of Miyakojima, driven by wind stress changes in high latitudes (near the Kuroshio Extension band), strengthened the northward current in the upper layer southeast of Miyakojima. Plain Language Summary The Ryukyu Current is part of the western boundary current of the North Pacific subtropical gyre on the eastern slope of the Ryukyu Island chain. This study revealed that the Ryukyu Current subsurface velocity core underwent a noticeable weakening trend during 1993-2018. The weakened Ryukyu Current was instigated by the combined effect of the topographic Rossby wave originating in the straits (e.g., the Tokara Strait and Kerama Gap) of the Ryukyu Island chain, and the increasing anticyclonic eddy emanating from the interior region east of the Kuroshio Extension. The former processes led the onshore side of the isopycnal to shoal, while the latter process induces the isopycnal to deepen on the offshore side. The isopycnal near the Ryukyu Current velocity core becomes less steep, thereby weakening the subsurface Ryukyu Current.