Variabilities of Strait Throughflow in the Northwest Pacific Based on HYCOM and PHY Data

In this paper, the volume transport and flow structure through key straits of the Northwest Pacific is quantitatively diagnosed using assimilated data from HYCOM and PHY from 1993 to 2020. The vertical flow structures illustrated by the two kinds of data are largely similar in terms of the distribution of flow core and magnitude of velocity, which are consistent with previous observations. The average volume transport of Kuroshio is approximately 31.3 Sv, which is significantly larger compared with observations along the PN-Line, suggesting the Kuroshio has been overestimated by the assimilated data from HYCOM and PHY. Further, the seasonal to interannual variations of shelf current transport, including the Kuroshio, Taiwan Warm Current, Jeju Warm Current and Japan Sea throughflow, are checked and compared between two models, the profiles of which are selected according to the climatological current structure from a combination of model reanalysis and observation data products. Results show that the seasonal variations are generally consistent between two kinds of data at each of the eight profiles, with all of the volume transports maximum in summer and minimum in winter. In the interannual scale, the long-term intensification of throughflow in the Tsushima Strait, Soya Strait and Tsugaru Strait, together with the synchronous weakening of Kuroshio, has been detected by both datasets. This suggests that the weakening Kuroshio tend to branching more water towards the Japan Sea while a slightly smaller amount of warm water will enter the open sea through the Tokara Strait.