Deep water overflow through the southern gap of the Oki Spur in the Japan Sea

Abstract

Water exchange processes between abyssal basins are a problem of universal interest because they control deep water climate. In this study, structure and variability of deep currents running through the southern gap of the Oki Spur from the Tsushima to Yamato Basins in the southern Japan Sea were investigated using the moored current meters and closely spaced hydrographic observations. Our observations revealed that the deep currents are cold, dense water overflows from the Tsushima Basin. Cold water spilling from the southern gap flows southward on the eastern flank of the spur as cold bottom water. Water mass analyses revealed that cold bottom water mixed with upper warmer water decreased its density during the early movement stages and then increased its density by mixing with lateral saline water. A streamtube model assuming a steady state roughly reproduced the depth, density, and velocity of cold bottom water, suggesting that it was in a near geostrophic balance. A strong southward current event accompanying the cold bottom water was observed from late December 1999 to early February 2000. An ocean reanalysis dataset showed that the event was associated with the Tsushima Current in the upper layer, suggesting that surface current variations promoted abyssal inter-basin water mass exchanges. In addition, current fluctuations of ∼10-day periods prevailed in the overflows, suggesting baroclinic instability.