Long-term variation in volume transport of the Tsushima warm current estimated from ADCP current measurement and sea level differences in the Korea/Tsushima Strait

Abstract

ADCP current data and sea level data from tide gauges and satellite altimetry were used in order to analyze long-term variation of the Tsushima Warm Current (TWC) volume transport passing through the Korea/Tsushima Strait. The annual mean transport of the TWC estimated from ADCP current measurement for about 17 years (1997–2013) is 2.64 ± 0.41 Sv (Sv = 10⁶ m³s⁻¹) in the Korea/Tsushima Strait, 1.53 ± 0.36 Sv (about 58% of the total transport) in the western channel, and 1.11 ± 0.14 Sv (about 42%) in the eastern channel. The TWC shows the minimum transport (1.74 Sv) in January and the maximum transport (3.10 Sv) in October, with seasonal variation of up to 1.36 Sv (51.5% of the annual mean transport). The seasonal variation of the total TWC transport in the Korea/Tsushima Strait is mainly similar to that in the western channel. The annual mean transport of the TWC and its seasonal variation in the Korea/Tsushima Strait, the western and eastern channels are all similar in three datasets (ADCP, tide gauge, satellite altimetry). The annual mean volume transport of the TWC estimated from long-term tide gauge data was 2.57 ± 0.37 Sv (total volume transport) for 44 years (1975–2018) in the Korea/Tsushima Strait, 1.51 ± 0.32 Sv in the western channel and 1.06 ± 0.14 Sv in the eastern channel. Long-term variation of the TWC transport shows a decreasing trend for 1975 to 1988, and an increasing trend from 1989 to 2018. For the latter period (1989 to 2018), monthly transport tends to increase in all months throughout the years, showing a greater increase, especially in spring and summer months. The increase of the transport in the eastern channel was about 2.8 times larger than that in the western channel. It is suggested that the increasing trend of the TWC transport since 1989 is related to the Pacific Decadal Oscillation (PDO), which has been changed to negative direction since mid-1980s. During the period when the negative PDO index is strong, the negative wind stress curl weakens in the mid-latitudes of the North Pacific, and the Kuroshio in the East China Sea also weakens. As a result, the flow is strengthened toward the Korea/Tsushima Strait, increasing the TWC transport.