Abstract
The Soya Warm Current (SWC), which is the coastal current along the northeastern part of Hokkaido, Japan, has a notable baroclinic jet structure during summer. This study addresses the formation mechanism of the baroclinic jet by analyzing a realistic numerical model and conducting its sensitivity experiment. The key process is the interaction between the seasonal thermocline and the bottom Ekman layer on the slope off the northeastern coast of Hokkaido; the bottom Ekman transport causes subduction of the warm seasonal thermocline water below the cold lower-layer water, so the bottom mixed layer develops with a remarkable cross-isobath density gradient. Consequently, the buoyancy transport vanishes as a result of the thermal wind balance in the mixed layer. The SWC area is divided into two regions during summer: upstream, the adjustment toward the buoyancy shutdown is in progress; downstream, the buoyancy shutdown occurs. The buoyancy shutdown theory assesses the bottom-mixed-layer thickness to be 50 m, consistent with observations and our numerical results. The seasonal thermocline from June to September is strong enough to establish the dominance of the buoyancy shutdown process over the frictional spindown.
Highlights
The Soya Warm Current (SWC) is the coastal current flowing southeastward along the northeastern part of Hokkaido, Japan (Fig. 1a)
Hokkaido side of the Soya Strait (Fig. 3) is highly stratified because of the seasonal thermocline (Aota et al 1988; Nakata et al 1999; Nakanowatari et al 2017), we consider that the seasonal thermocline in the strait tilts as the summer SWC flows downstream, so that a baroclinic jet structure with a marked horizontal temperature gradient develops along the Hokkaido coast (Matsuyama et al 2006; Ishizu et al 2006)
We found that a key process is the buoyancy shutdown process that occurs owing to the interaction between the seasonal thermocline and the bottom Ekman layer on the coastal slope off northeastern Hokkaido; the bottom Ekman transport causes the subduction of warm seasonal thermocline water below the cold lower-layer water, thereby causing development of the bottom mixed layer and tilting the seasonal thermocline vertically
Summary
The Soya Warm Current (SWC) is the coastal current flowing southeastward along the northeastern part of Hokkaido, Japan (Fig. 1a). Hokkaido side of the Soya Strait (Fig. 3) is highly stratified because of the seasonal thermocline (Aota et al 1988; Nakata et al 1999; Nakanowatari et al 2017), we consider that the seasonal thermocline in the strait tilts as the summer SWC flows downstream, so that a baroclinic jet structure with a marked horizontal temperature gradient develops along the Hokkaido coast (Matsuyama et al 2006; Ishizu et al 2006). The theory implies that the seasonal thermocline inclines towards the sea surface so steeply that the cold water below the thermocline outcrops near Cape Krilon in Sakhalin Island (e.g., Fig. 3) In their idealized model, further, since the upwelling over the shallow sill induces long baroclinic Rossby waves along the current downstream, a cold water belt develops subsequently as a result of adjustment through a frontal wave propagation (namely, a baroclinic adjustment).
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