11:30 〜 11:45
[ACG38-04] Generalization about the Barotropic-to-Baroclinic Evolution of the Soya Warm Current: a Buoyancy Shutdown Theory Consistent with Mechanical Mixing
★Invited Papers
The Soya Warm Current (SWC) is a coastal boundary current of the Sea of Okhotsk that enters from the Soya Strait and flows along the northeastern part of Hokkaido, Japan. The SWC acquires the maximum volume transport during summer and then maintains a surface jet-stream structure exceeding 1 m s-1 from the Soya Strait to the tip of Shiretoko peninsula. However, this flow experiences a drastic change in the internal structures. When the summer SWC flows through the Soya Strait it is a vertically uniform barotropic current with a flat seasonal pycnocline. In contrast, at the downstream region, the summer SWC becomes a surface-intensified baroclinic current with a tilting density front, thereby suppressing the frictional spindown process through bottom friction.
In this study, we tried to generalize the downstream evolution of the SWC from the barotropic current structure to the baroclinic one. The buoyancy shutdown theories consider a downwelling-favorable current with ambient continuous stratification over sloping frictional bottom that is similar to the SWC and emphasize the importance of the bottom Ekman transport and the thermal convection for the development of the bottom boundary layer (BBL) with the baroclinic current structure. The bottom Ekman transport causes subduction of lighter upper-layer waters below denser lower-layer waters along the slope, so that the density inversion occurs between the BBL and the overlying interior. The thermal convection immediately resolves the static instability, so that the BBL entrains the base of the interior and thickens while forming the cross-isobath density gradient. However, we had a question as to whether the thermal convection truly develops the BBL since it essentially causes a diabatic change in the density. For example, the density of a subducted lighter water within the BBL non-adiabatically increases because of the vertical mixing with the overlying denser water, resulting in a density gap zone between the BBL and the interior. However, it does not appear in the SWC region since the density front of the SWC is continuously connected with the seasonal pycnocline at the subsurface. Additionally, the descent of the seasonal pycnocline near the Soya Strait corresponds with the offshore Ekman transport as if it is an adiabatic displacement. Consistently, a recent study with large eddy simulation showed that during the BBL growth the contribution of buoyant production is negligible compared with the shear production, indicating that it may not be a buoyant convective process but a mechanical mixing process.
Therefore, we considered the BBL growth consistent with a mechanical mixing process. Generally, the descent of turbulent flow adjacent to bottom slope accompanies a shear-driven entrainment and increases its thickness because of a velocity jump at the base of the interior. Applied this mechanism to the buoyancy shutdown theory, we could formulate that when a water column in the BBL descends it mechanically entrains the same density water in the interior simultaneously. So, in the Lagrangian view, the density of a water column in the BBL conserves along the flow within the BBL. Then, the bottom Ekman transport moves isopycnals above the bottom slope in the downslope direction and, simultaneously, among the vertically displaced distance the mechanical mixing generates to inclines the isopycnals in the interior. Our theory was validated by an idealized numerical model experiment where the downslope trajectory of the seasonal thermocline in the SWC region was modelled. Furthermore, in terms of the adiabatic change in the density, we discussed that solar energy storing in the seasonal pycnocline is efficiently converted into the available potential energy within the BBL as a result of the barotropic-to-baroclinic evolution of the SWC.
In this study, we tried to generalize the downstream evolution of the SWC from the barotropic current structure to the baroclinic one. The buoyancy shutdown theories consider a downwelling-favorable current with ambient continuous stratification over sloping frictional bottom that is similar to the SWC and emphasize the importance of the bottom Ekman transport and the thermal convection for the development of the bottom boundary layer (BBL) with the baroclinic current structure. The bottom Ekman transport causes subduction of lighter upper-layer waters below denser lower-layer waters along the slope, so that the density inversion occurs between the BBL and the overlying interior. The thermal convection immediately resolves the static instability, so that the BBL entrains the base of the interior and thickens while forming the cross-isobath density gradient. However, we had a question as to whether the thermal convection truly develops the BBL since it essentially causes a diabatic change in the density. For example, the density of a subducted lighter water within the BBL non-adiabatically increases because of the vertical mixing with the overlying denser water, resulting in a density gap zone between the BBL and the interior. However, it does not appear in the SWC region since the density front of the SWC is continuously connected with the seasonal pycnocline at the subsurface. Additionally, the descent of the seasonal pycnocline near the Soya Strait corresponds with the offshore Ekman transport as if it is an adiabatic displacement. Consistently, a recent study with large eddy simulation showed that during the BBL growth the contribution of buoyant production is negligible compared with the shear production, indicating that it may not be a buoyant convective process but a mechanical mixing process.
Therefore, we considered the BBL growth consistent with a mechanical mixing process. Generally, the descent of turbulent flow adjacent to bottom slope accompanies a shear-driven entrainment and increases its thickness because of a velocity jump at the base of the interior. Applied this mechanism to the buoyancy shutdown theory, we could formulate that when a water column in the BBL descends it mechanically entrains the same density water in the interior simultaneously. So, in the Lagrangian view, the density of a water column in the BBL conserves along the flow within the BBL. Then, the bottom Ekman transport moves isopycnals above the bottom slope in the downslope direction and, simultaneously, among the vertically displaced distance the mechanical mixing generates to inclines the isopycnals in the interior. Our theory was validated by an idealized numerical model experiment where the downslope trajectory of the seasonal thermocline in the SWC region was modelled. Furthermore, in terms of the adiabatic change in the density, we discussed that solar energy storing in the seasonal pycnocline is efficiently converted into the available potential energy within the BBL as a result of the barotropic-to-baroclinic evolution of the SWC.