Buoyancy shutdown process for the development of the baroclinic jet structure of the Soya Warm Current during summer
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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.
KeywordsSoya Warm Current Baroclinic jet Buoyancy shutdown Frictional spindown Seasonal thermocline Bottom Ekman transport Realistic numerical model
Concerning dynamics of the SWC, Mitsudera et al. (2011) showed that a barotropic current with a subsurface thermocline obtains a baroclinic jet structure as it passes over a shallow sill in a strait as a result of hydraulic control associated with internal Kelvin waves. 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). This is consistent with observational results such that the outcrop of cold water is frequently seen offshore of the summer SWC (e.g., Ishizu et al. 2006).
Mitsudera et al. (2011) conjectured that the baroclinic adjustment on a barotropic current downstream of the strait causes both the cold water belt and the formation of the baroclinic jet structure of the current. However, on a closer examination of their idealized numerical results, a baroclinic structure in the current was established first, and the cold water belt developed afterwards through the frontal wave propagation. Therefore, the formation of the baroclinic jet structure in the SWC should be discussed separately from the cold belt formation. In particular, effects of the bottom friction were overlooked in their theoretical argument.
In this article the formation of the baroclinic jet structure in the SWC is studied from the viewpoint of the buoyancy shutdown theory (e.g., Garrett et al. 1993; Chapman and Lentz 1997; Chapman 2002). The theory focuses on the interaction between the stratification and the bottom Ekman layer and explains the adjustment process of a stratified along-isobath flow over a sloping bottom. The steady state has been referred to as the buoyancy shutdown (e.g., Benthuysen et al. 2015) or the buoyancy arrest (e.g., Brink and Lents 2010). This study uses the former. Note that we do not discuss the formation of the cold water belt but focus on the baroclinic jet formation associated with the buoyancy shutdown.
We hypothesized that the buoyancy shutdown theory is a reasonable application to the summer SWC. Since the SWC is a geostrophic current flowing on the right-hand side of the northeastern coast of Hokkaido (Ohshima 1994), the bottom Ekman transport likely occurs in the downslope direction and can tilt the seasonal thermocline in the same way as the theory. The slope along the northeastern coast of Hokkaido is nearly uniform (Fig. 1b), which is also suited for the theory. According to Matsuyama et al. (2006) and Ebuchi et al. (2009), in addition, the temperature-front structure and the maximum speed of the summer SWC do not vary significantly in the downstream region. This suggests that the dissipation due to the bottom friction acting on the SWC is weak, consistent with the theory.
The goal of this study is to clarify the formation mechanism of the baroclinic jet structure of the summer SWC. We analyze a realistic numerical model (Kuroda et al. 2014) and conduct its sensitivity experiment. This article is organized as follows. The outline of the model and its reproducibility of the SWC are shown in Sect. 2. The buoyancy shutdown process of the SWC is investigated in Sect. 3. In Sect. 4, the sensitivity of the SWC to the bottom friction and the seasonal thermocline is examined. Section 5 presents the summary and discussion of this study.
2 Model overview
2.1 Model configuration
We used a realistic numerical ocean model (referred to as the Hokkaido model hereafter) constructed by Kuroda et al. (2014). The Hokkaido model is a triply nested model based on the Regional Ocean Modeling System (ROMS) (e.g., Shchepetkin and McWilliams 2005). Three models with resolutions of 1/2°, 1/10° and 1/50° are connected by one-way nesting. The realistic topography (Fig. 1a) is incorporated into the Hokkaido model. The horizontal resolution of 1/50° is approximately 2 km. As shown by Sakamoto et al. (2010), grid sizes less than at least 2 km are essential to simulate the buoyancy shutdown process sufficiently. The Hokkaido model employs a terrain-following S coordinate with 21 vertical layers. The vertical resolutions around the SWC region (depth < 100 m) are within 5 m, which is a typical scale for the Ekman-layer thickness. Therefore, we expect that the Hokkaido model can represent a bottom flow field of the SWC. The bottom stress is modeled by the quadratic drag law with a coefficient of 0.003. Instead of K-profile parameterization (KPP) in the original model configuration (Kuroda et al. 2014), the Mellor-Yamada level 2.5 scheme (Allen et al. 1995) is used to examine the mixing process adjacent to the bottom. The Hokkaido model is driven by climatologic fluxes. At the sea surface, we sequentially estimate momentum, heat and freshwater fluxes using CORE normal-year forcing (Large and Yeager 2004) during the model run. On the lateral boundaries, the 3.8-day mean output derived from the 1/10° parent model (Kuroda et al. 2013) is imposed. The parent model also provides the initial condition with the Hokkaido model. The total integrated period of the three models is 20 years. The Hokkaido model is integrated only for the last 5 years. We analyzed the monthly mean outputs in the last year of the integration. Although the Hokkaido model does not contain the sea ice, this would not be a serious problem for reproducing the summer SWC. Further, we do not consider tides as we focus on the basic mechanism of the buoyancy shutdown processes that cause baroclinicity in the SWC. However, tides potentially affect the buoyancy shutdown process, since the tidal forcing enhances the bottom drag dissipation (e.g., Lee et al. 2000).
2.2 Reproducibility of the SWC
In this subsection we present simulated features of the summer SWC in the Hokkaido model to evaluate both the reproducibility and the hypothesis mentioned in Sect. 1, especially from a point of view of the relations between the SWC and the seasonal thermocline.
Corresponding to the SSH distribution in Fig. 5a, the model reproduced a southeastward jet with a speed of 0.6–0.8 m s−1 from the southwestern coast of the Sakhalin up to the eastern end of the northeastern coast of Hokkaido (Fig. 5b). This jet speed is scaled by the geostrophic balance. The simulated jet axis is located 10–20 km from the coast in a similar manner to the observations, although the reproduced speed is slower than the result observed by ocean radars, which is greater than 1 m s−1 (Ebuchi et al. 2009). The simulated SST off the northeastern coast of Hokkaido ranges between 18 and 20 °C, which is similar to the SST observed by satellite (e.g., Ishizu et al. 2006). The SST is colder around the southwestern tips of the Sakhalin than its surroundings as observed. This implies that the hydraulic control, shown theoretically by Mitsudera et al. (2011), occurs in the model, and the relatively cold water is apparent offshore of the SWC in the monthly mean field.
To sum up, the Hokkaido model reproduces the observed features well, especially the relations between the SWC and the seasonal thermocline, although the simulated speed and volume transport tend to be slightly weaker than those observed. Thus, the analysis using the Hokkaido model is appropriate for studying the formation mechanism of the baroclinic jet.
3 Buoyancy shutdown process of the SWC
Figure 9 shows that the cross-isobath bottom flow is almost zero in the coastal area shallower than 80 m between approximately 142.5°E and 144.0°E where the SWC flows. In this region, the isopycnals on the bottom (black contours in Fig. 9) are parallel to the water depth contours (white contours). This indicates that the baroclinic jet is arrested on isobaths eastward of 142.5°E away from the Soya Strait. Thus, the buoyancy shutdown occurs in the downstream region of the SWC where the baroclinic jet exits. On the other hand, the cross-isobath bottom flow between the Soya Strait and 142.5°E is not zero, being accompanied by the offshore movement of the density contours. Therefore, the adjustment toward the buoyancy shutdown occurs immediately downstream from the Soya Strait.
Figure 10 shows that the bottom mixed layer thickens gradually from less than 20 m at the Soya Strait to approximately 50 m at approximately 142.5°E as the water mass of 26.0σ θ migrates offshore from the water depth of 30 m to 60 m. The mixed-layer development ceases farther downstream as vertical mixing weakens by one order of magnitude. The evolution of the bottom mixed layer corresponds to the spatial variation in the cross-isobath bottom flow (see Fig. 9). That is, it is in the upstream region of the SWC where the bottom Ekman transport causes subduction of the warm seasonal thermocline water below the cold lower-layer water, resulting in the growth of the bottom mixed layer. Subsequently, the bottom mixed layer obtains a uniform thickness of approximately 50 m between approximately 142.5°E and 144.0°E. Therefore, there is a clear separation between the up- and downstream regions of the SWC, where in the former the adjustment toward the buoyancy shutdown is in progress, while in the latter the buoyancy shutdown occurs.
To conclude, the baroclinic jet structure of the SWC is well explained by the buoyancy shutdown theory. The adjustment process of the SWC is also seen in a similar way in July and September (not shown), when the subsurface pycnocline and the surface-intensified vertical shear are enhanced in the Hokkaido model (Fig. 8b).
4 Sensitivity of the SWC
In this section, the importance of the buoyancy shutdown process on the SWC’s current structure is examined by the two kinds of methods as follows: a sensitivity experiment to the bottom friction; a comparison between the SWC in April and that in August by focusing on the presence or absence of the seasonal thermocline.
4.1 Bottom friction
We examined the sensitivity of the baroclinic jet structure to the bottom friction on the northeastern Hokkaido coastal slope. We turned off the bottom stress at the dotted region in Fig. 1b and ran this “no-bottom-stress” model over the same period (referred to as the sensitivity experiment) as that using the previously described Hokkaido model (referred to as the control run in this section). The most important point of the sensitivity experiment is that the interaction between the seasonal thermocline and the bottom Ekman layer on the northeastern coastal slope of Hokkaido does not occur because of the absence of bottom friction. That is, the buoyancy shutdown does not occur.
On closer examination, we noticed that the SWC in the sensitivity experiment exhibits a bottom-intensified velocity structure (> 0.7 m s−1) approximately 10 km offshore from the coast. This may be a consequence of the geostrophic adjustment by shelf waves in a stratified fluid (Rhines 1970; Pedlosky 1987). The adjustment is consistent with Ohshima (1994), although in the presence of the stratification, the vortex stretching by the bottom slope occurs effectively near the bottom.
4.2 Seasonal thermocline
5 Summary and discussion
We studied key processes that create the baroclinic jet structure of the summer SWC with a realistic numerical ocean model (Kuroda et al. 2014). 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. The formation of the baroclinic jet of the summer SWC is associated with the tilting seasonal thermocline in the bottom mixed layer. Finally, offshore buoyancy advection by the bottom Ekman transport vanishes as a result of the thermal wind balance in the mixed layer; hence, the jet development ceases. The bottom mixed layer development, and hence the baroclinic jet development, occurs in the upstream region of the SWC, whereas the baroclinic jet structure is fully developed in the downstream region where the buoyancy shutdown occurs. The buoyancy shutdown theory assesses the SWC mixed-layer thickness to be 50 m, consistent with both the observed (Fig. 2) and numerical (e.g., Fig. 7b) results. The seasonal thermocline from June to September is strong enough to establish the dominance of the buoyancy shutdown process over the frictional spindown.
We noted that the baroclinic jet does not form along the boundary between the salty SWC water and the relatively fresh water originating in the Sea of Okhotsk, because the water mass boundary is not a density front. Further, since the SWC structure is essentially barotropic if bottom friction is absent, the upwelling near the Soya Strait does not cause the formation of the baroclinic jet structure of the SWC far downstream from the strait but the buoyancy shutdown does.
As shown in the numerical (Figs. 7b, 8b, 11b) and observed (Aota 1975; Matsuyama et al. 2006; Fukamachi et al. 2008) results, the SWC is not at rest near the bottom, but substantial bottom flows are maintained even in the downstream region. Hence, the conventional theory (e.g., Chapman 2002) is not directly applicable to the SWC in this respect. Since the along-isobath SSH gradient almost vanishes eastward from 142.5°E (Fig. 16), the surface pressure gradient cannot account for the bottom flow formation. One of possible effects that produces bottom flows may be the form stress on density interfaces associated with baroclinic instability (Vallis 2006), which transfers upper-layer momentum down to the bottom. Indeed, the contour line of σ θ = 0.1 in Fig. 10 indicates that the bottom mixed layer is restratified at approximately 143°E where the mixing strength is minimal, suggesting the occurrence of baroclinic instability. It is intriguing that mixing is then revitalized farther downstream around 143.5°E. This may be attributed to the bottom flow interacting with the topographic ridge around 143.5°E (Fig. 1b). The formation of bottom flows is thus important for the SWC dynamics and needs further study.
In previous studies, the sea level difference between Wakkanai and Abashiri (e.g., Aota 1975; Shimada et al. 2012) predicts the SWC flow. However, SSH along the baroclinic jet axis away from the Soya Strait is almost uniform in the along-isobath direction (Fig. 16). This implies that the SWC speed in the downstream region does not depend on the along-isobath pressure gradient. Further, Ohshima et al. (2017) showed that the conventional relation exhibits a low correlation coefficient between spring and late autumn and suggested that different mechanisms from the alongshore pressure gradient exist during the period. This may be related to the buoyancy shutdown process discussed in our study.
The simulated turbulent kinetic energy field (Fig. 10) is consistent with the turbulence measurement of Ishizu et al. (2013). The latter showed that the observed vertical mixing was stronger in the upstream area of the SWC (between off Sarufutu and Esashi in Fig. 1b) than that in the downstream (off Ohmu in Fig. 1b). However, the dissipation of the observed temperature variances was not large enough to identify the density inversion. If turbulence measurements and bottom-boundary-layer measurements are carried out in longitudinal section between the Soya Strait and Sarufutu during summer, we expect that stronger vertical mixing caused by offshore buoyancy transport and the downstream evolution of the bottom mixed layer would be observed. This could give us an opportunity to obtain better understanding of the mechanism of the SWC.
We acknowledge Dr. S. Kida (Kyushu University) and Dr. S. J. Lentz (Woods Hole Oceanic Institution) for their valuable comments. Thanks are extended to Dr. A. Kubokawa, Dr. G. Mizuta, Dr. Y. Isoda and the membership of the EOAS Seminar at Hokkaido University for helpful discussions. We are indebted to Dr. N. Ebuchi for providing the workshop on the Soya Warm Current at the Institute of Low Temperature Science (ILTS), Hokkaido University. Comments from reviewers and the editor improved this paper greatly. Numerical calculations and analyses were conducted with the Pan-Okhotsk Information System at ILTS Hokkaido University, and the figures were prepared by GrADS and gnuplot. This study was partly supported by KAKENHI (26247076, JP16H01585, 17H01156) and the Fisheries Agency project (Shigen Hendo Yoin Bunseki Chosa), but the study contents do not necessarily reflect the views of the Fisheries Agency.
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