Abstract
The Kuroshio takes a greatly southward displaced path called a large-meander (LM) path off the southern coast of Japan on interannual to decadal time scales. The transition of the current path from a non-large-meander path to an LM path is the most salient ocean current variation south of Japan. The change in pressure on the seafloor due to the formation of the LM path in September 2017 is of critical importance to understand the dynamics of the LM path and to distinguish the change due to the Kuroshio path variation from changes due to crustal deformation. Hence, we examined the seafloor pressure across the continental slope off the eastern coast of Kyushu for the period March 2014 to April 2019. The pressure and its cross-slope gradient over the continental slope shallower than 3,000 m beneath near the Kuroshio are invariable. As a mesoscale current path disturbance, called a small meander, passed over the observation stations, the pressure decreased by approximately 0.1 dbar on the continental slope deeper than 3000 m and was kept low until the end of the observation period (April 2019). The pressure decrease is consistent with the changes in sea surface height and subsurface water density and is caused by the baroclinic enhancement of the Shikoku Basin local recirculation. This seafloor pressure change implies a strengthening of the deep southwestward current, possibly as a part of a deep cyclonic circulation in the Shikoku Basin. The present study demonstrated that, in addition to altimetric sea surface height data, hydrographic data are useful to distinguish the ocean variation in seafloor pressure from the variation due to crustal deformation, and vice versa.
Highlights
Winds blowing over the whole subtropical North Pacific drive a basin-scale anticyclonic circulation, which is referred as the North Pacific subtropical gyre
AN estimated sensor drifts of the ocean bottom pressure gauges (OBPs) data and corrected the data, analyzed the OBP, hydrographic, altimetric SSH data, and wrote the manuscript
All coauthors collaborated with the corresponding author (AN) in the construction of the manuscript and approved the final manuscript
Summary
Winds blowing over the whole subtropical North Pacific drive a basin-scale anticyclonic circulation, which is referred as the North Pacific subtropical gyre. Nagano et al (2018) examined the seafloor pressure change in the region south of Shikoku associated with the 2004–2005 LM path of the Kuroshio, a stable-type LM path They demonstrated that after the eastward propagation of a deep cyclonic eddy leading to the small meander propagation, the deep Kuroshio current was enhanced in the LM period. The LM path is active at present as of November 2020 the cyclonic LM eddy has been separated from the Kuroshio It is unknown whether or not the unstable-type LM path of the Kuroshio is associated with the substantial deep current following the isobaths of the continental slope and is influenced by the topographic steering. By using altimetric SSH and hydrographic data collected during the period 2017–2019, we revealed that the seafloor pressure change is attributable to the change of the vertical water density distribution associated with the formation of the LM path and demonstrated that hydrographic data in combination with SSH data are useful to identify the seafloor pressure variation due to the Kuroshio variation and to distinguish it from the variation due to crustal deformation
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