Upwelling of cold subsurface water can occur when ocean currents pass through straits, significantly affecting biological productivity and the transport of heat and materials. In the Soya Strait, for example, Mitsudera et al. (2011) demonstrated through idealized numerical experiments that upwelling occurs when a stratified barotropic flow climbs over shallow topography and satisfies a critical condition, in which the flow speed matches the phase speed of an internal Kelvin wave, thereby inducing hydraulic control. However, because bottom friction was included in their experiments, upwelling caused by shoreward bottom Ekman transport converging at the coast may have affected the interpretation of the results. The purpose of this study is therefore to re-examine the upwelling mechanism due to hydraulic control using numerical experiments similar to those of the previous study, while also formulating upwelling due to Ekman transport and clarifying the parameter dependence of both mechanisms.
First, a comparison of numerical experiments with and without bottom friction showed that upwelling driven by Ekman transport had a non-negligible influence on the results of the previous study. We then re-examined upwelling due to hydraulic control using experiments without bottom or wall friction, and found that the internal interface displacement changes little downstream of the location where the critical condition is satisfied. We therefore focused on the internal interface displacement theoretically predicted from the critical condition as an index of upwelling due to hydraulic control, and investigated its dependence on flow speed and stratification structure.
Next, we formulated upwelling (vertical displacement) driven by Ekman transport and showed, through comparison with numerical experiments in which flow speed and stratification were varied, that the theory successfully reproduces the vertical displacement obtained in the experiments. The theory further indicates that the magnitude of the vertical displacement due to Ekman transport depends mainly on the Ekman layer thickness, pycnocline strength, total water depth, and the distance over which upwelling accumulates, and we investigated the parameter dependence on these quantities.
In this study, we quantitatively evaluated upwelling due to hydraulic control and bottom Ekman transport, and clarified the conditions under which each type of upwelling is likely to occur on the basis of parameter dependence. These results suggest that similar upwelling may occur not only in straits but also in regions with bottom topography where, in the Northern (Southern) Hemisphere, the flow has the coast on its left (right). As specific examples, upwelling due to hydraulic control may occur in the Tsugaru Strait, whereas upwelling due to Ekman transport may occur in the Anadyr Strait.

Upwelling,Hydraulic control,Bottom Ekman transport