Across-shelf scale-selective separation of a quasi-geostrophic current and internal gravity waves off southeastern Hokkaido, Japan

Abstract

Interactions between oceanic mesoscale currents and inertia-gravity waves (IGWs) are thought to facilitate the energy transition toward submesoscale regimes. Such mesoscale–submesoscale transition processes involve unbalanced motions, such as the IGWs, making it difficult to distinguish between them in observational data. Using across-shelf scale-selective filters, we separated the IGWs from a quasi-geostrophic (QG) current, the Coastal Oyashio (CO), based on a CTD-measured hydrographic section with ∼2-km resolution across the Pacific shelf off southeast Hokkaido, Japan, and clarified the mesoscale–submesoscale transition processes in this IGW–QG coupled system. To determine the optimum smoothing scale ${\Delta Y}_o$, we formulated indices based on non-dimensional numbers such as the Rossby number $R_o$ and the Richardson number $R_i$, calculated from the CTD-derived along-shelf baroclinic velocity. Applying a low-pass filter with ${\Delta Y}_o$ to the in-situ density section, where $R_o$ and $R_i^{-1}$ were of magnitude O(1), resulted in a smoothed section where both $R_o$ and $R_i^{-1}$ were reduced to ∼O(0.1), with $R_o\approx R_i^{-1}$. The smoothed isopycnals were sloped over the Rossby radius of deformation $L_d$, consistent with the geostrophic adjustment theory. More quantitatively, the diagnosis yielded an optimal smoothing scale of ${\Delta Y}_o$ = 6 km. The IGWs propagating along the subsurface pycnocline, characterized by $L_d\approx$ 1 km (i.e., a wavelength around 6 km, $2\pi L_d$), were successfully separated from the baroclinic jet structure of the CO, which had $L_d\approx$ 10 km. As a result, our observations revealed a hybrid of symmetric and gravitational instability associated with IGW-breaking near the core of the CO baroclinic jet.