Compared to the dynamics of the predominantly geostrophic along-shelf current, our understanding of the cross-shelf dynamics in the Sea of Okhotsk is inadequate despite their importance in water ...mixing and nutrient entrainment. We investigated the cross-shelf overturning circulation along the East Sakhalin Current, which is a source of nutrients such as iron for the western North Pacific. Here, we reveal that the cross-shelf circulation during winter is characterised by a nearshore upwelling and a shelf-break downwelling under a downwelling-favourable monsoon wind, contrary to a classical Ekman overturning (EOT). This reverse EOT is driven by the internal water stress, which is caused by intensive vertical mixing and geostrophic vertical shear in the shelf-break front produced by riverine discharges from the far-eastern Eurasian Continent. The EOT blocks the Ekman onshore transport from the open ocean, thereby producing a deep mixed layer at the shelf break. Scaling analyses indicate the applicability of this mechanism to various other shelf-break fronts.
Previous studies have suggested that the quasi-stationary jets, sometimes called the “Isoguchi jets” in the western North Pacific Ocean, regulate the warm and salty conditions in the transition ...domain between the Subarctic Boundary and the Subarctic Front. Here, we show that mesoscale eddies and interannual/decadal modulations are responsible for the Kuroshio water intrusion into the transition domain. A case study using the Lagrangian coherent structure suggests that the northward shift of the Kuroshio Extension forms a favorable velocity field for the Kuroshio water intrusion around the Subarctic Boundary, while the geometric structure inside the Isoguchi jet is quasi-permanent.
In this research, we studied the upwelling in the northwestern Gulf of Alaska using the climatological January mean and data from the output of the Ocean General Circulation Model for Earth Simulator ...(OFES2). Specifically, we analyzed the upwelling in the regions where the Alaska Coastal Current (ACC) flows out of the Shelikof Strait (especially the part to the west of Kodiak Island) and where the ACC and the Alaskan Stream (AS) are confluent. In both regions, strong geostrophic currents and downwelling-favorable wind predominate in winter. Furthermore, there are freshwater discharges along the Alaskan coast and an observed mean current vertical shear in the ACC. We revealed that when the internal water stress is larger than the wind stress inside the study regions, this could be decisive in terms of the local horizontal velocity divergence and further upwelling, even if the region is away from the coast and lacks upwelling-favorable wind conditions. Geostrophic stress is part of the internal water stress and is a product of the geostrophic current shear (due to the thermal wind relation) and the vertical viscosity coefficient. The analysis indicated that a front with a large geostrophic stress may act as a “virtual wall” and contribute to local upwelling within a depth of approximately 100 m in the study regions. This process could provide a heuristic for understanding the distribution of pollock in the areas during February and March, which corresponds to the simulated upwelling region.
A compilation of surface water nutrient (phosphate, nitrate, and silicate) and partial pressure of CO
2
(
p
CO
2
) observations from 1961 to 2016 reveals seasonal and interannual variability in the ...North Pacific. Nutrients and calculated dissolved inorganic carbon (DIC) reach maximum concentrations in March and minimum in August. Nutrient and DIC variability is in-phase (anti-phase) with changes in the mixed layer depth (sea surface temperature) north of 30 °N, and it is anti-phase (in-phase) with changes in Chl-a north of 40 °N (in 30 °N–40 °N). Seasonal drawdown of nutrients and DIC is larger toward the northwest and shows a local maximum in the boundary region between the subarctic and subtropics. Stoichiometric ratios of seasonal drawdown show that, compared to nitrate, silicate drawdown is large in the northwestern subarctic including the Bering and Okhotsk seas, and drawdown of carbon is larger toward the south. Net community production in mixed layer from March to July is estimated to be more than 6 gC/m
2
/mo in the boundary region between the subarctic and subtropics, the western subarctic, the Gulf of Alaska, and the Bering Sea. Nutrient and DIC concentrations vary with the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation which cause changes in horizontal advection and vertical mixing. The DIC trend is positive in all analysis area and large in the western subtropics (> 1.0 μmol/l/yr). Averaged over the analysis area, it is increasing by 0.77 ± 0.03 μmol/l/yr (0.75 ± 0.02 μmol/kg/yr).
Abstract
The spatial distribution and seasonality of halocline structures in the subarctic North Pacific (SNP) were investigated using Argo profiling float data and various surface flux data ...collected in 2003–17. The permanent halocline (PH) showed zonal patterns in the spatial distributions of its depth and intensity and tended to be shallow and strong in the eastern SNP but deep and weak in the west. Mean distributions of PH depth and intensity corresponded to the winter mixed layer depth and sea surface salinity, respectively, indicating that it forms in association with the development of the winter mixed layer. In the Western Subarctic Gyre and Alaskan Gyre, where a relatively strong PH formed, PH intensity and depth showed clear seasonal variations, and deepening of the mixed layer compressed the underlying PH during the cooling period, resulting in intensification and development of the PH in late winter. In both regions, upwelling of high-salinity water also contributed to PH intensification. The summer seasonal halocline (SH) showed distinct zonal differences in frequency and intensity, which were opposite to the PH distribution. While an SH formed in the western and central SNP and coastal regions, it was seldom present in the eastern area. This zonal contrast of SH corresponded to freshening of the mixed layer during the warming period, primarily reflecting freshwater flux. Geostrophic and Ekman advection play important roles in spatial differences in SH intensity and depth. SH development contributed to PH intensification in the following winter, by decreasing salinity above the PH through entrainment.
Abstract
The interbasin exchange between the Sea of Okhotsk and the North Pacific governs the intermediate water ventilation and fertilization of the nutrient-rich subpolar Pacific, and thus has an ...enormous influence on the North Pacific. However, the mechanism of this exchange is puzzling; current studies have not explained how the western boundary current (WBC) of the subarctic North Pacific intrudes only partially into the Sea of Okhotsk. High-resolution models often exhibit unrealistically small exchanges, as the WBC overshoots passing by deep straits and does not induce exchange flows. Therefore, partial intrusion cannot be solely explained by large-scale, wind-driven circulation. Here, we demonstrate that tidal forcing is the missing mechanism that drives the exchange by steering the WBC pathway. Upstream of the deep straits, tidally-generated topographically trapped waves over a bank lead to cross-slope upwelling. This upwelling enhances bottom pressure, thereby steering the WBC pathway toward the deep straits. The upwelling is identified as the source of joint-effect-of-baroclinicity-and-relief (JEBAR) in the potential vorticity equation, which is caused by tidal oscillation instead of tidally-enhanced vertical mixing. The WBC then hits the island chain and induces exchange flows. This tidal control of WBC pathways is applicable on subpolar and polar regions globally.
ABSTRACT
The dynamics of a quasi-stationary jet along the Subarctic Front in the North Pacific Ocean (the Western Isoguchi Jet) were investigated using an idealized two-layer model. The experiments ...suggested that a seafloor topography, which is 500 m high, produces a jet along its eastern flank. The formation mechanism of the jet can be explained via baroclinic Rossby wave characteristics. Baroclinic Rossby waves propagate along characteristic curves, which are significantly distorted by anticyclonic barotropic flow on the seafloor topography. A baroclinic surface jet is formed where a characteristic curve originating in the subtropical gyre and one originating in the subpolar gyre meet because the pycnocline depth varies discontinuously at this location. The barotropic flow on the seafloor topography is induced by eddies.
On the Kamchatka Peninsula in the Russian Far East, 405 glaciers with an estimated total mass of 49 Gt were reported in the 1970s. These have been retreating at an accelerated rate since the start of ...the 21st century. Because glacier studies in this region are scarce, ice loss and its influence on sea level rise and regional environments is poorly understood. In this study, we analyzed satellite data to quantify glacier mass change from 2000 to 2016 in six major glacier-covered regions on the peninsula. The mean rate of the glacier mass change over the study period was −0.46 ± 0.01 m w.e. a−1 (total mass change was −4.9 ± 0.1 Gt, −304.2 ± 9.1 Mt a−1), which is slightly lower than other regions in mid-latitude and subarctic zones. The mass loss accelerated from >−0.33 ± 0.02 m w.e. a−1 in the period 2000–2006/2010 to <−1.65 ± 0.12 m w.e. a−1 in 2006/2010–2015/16. The increase in mass loss is attributed to a rise in average decadal summer temperatures observed in the region (+0.68°C from 1987–99 to 2000–13). Moreover, a recent trend in Pacific decadal oscillation suggests future acceleration of mass loss due to a decline in winter precipitation.
•Simple biogeochemical model reproduces phosphate variability in Oyashio region.•Coastally trapped current system is a major factor of phosphate transport in winter.•Winter monsoon strength is an ...indicator for coastally trapped current.•Ekman upwelling in the Sea of Okhotsk is driving force of decadal variability.•West Pacific teleconnection pattern is key factor for decadal variability of phosphate.
In the Oyashio region, remarkable climatic signals are observed in biogeochemical parameters such as phosphate (PO4) concentration and debate continues regarding possible causes. Using a regional ice–ocean coupled model with a simple biogeochemical cycle, this study investigated the mechanisms controlling interannual–decadal variations in surface PO4 in the Oyashio region and their relationships to climate change. Hindcast experiments forced with atmospheric reanalysis data for 1980–2010 and 18.6-year tidal mixing strength in the Kuril Straits qualitatively simulated interannual–decadal variations of surface PO4, including a realistic seasonal cycle. Interannual fluctuations of simulated PO4 in the Oyashio region are prominent in winter and characterized by year-to-year variability. Budget analysis of PO4 in the mixed layer showed that the wintertime increase in PO4 is caused by lateral advection as well as by local vertical convection. The geostrophic current variability responsible for lateral advection of PO4 is related primarily to the barotropic response of arrested topographic waves in the Sea of Okhotsk as well as the wind-driven gyre in the North Pacific, both of which are regulated by the strength of the wintertime monsoon atmospheric pattern. On a decadal timescale (>7 years), temporal variations of surface PO4 in the Oyashio region are characterized by decadal-scale fluctuation with positive (negative) peaks around 1985, 1995, and 2005 (1990 and 2000). A series of sensitivity experiments demonstrated that the decadal variability of PO4 is largely explained by atmospheric wind conditions; however, modulation by 18.6-year tidal mixing is not negligible. Diagnostic analysis of wind-forced-experiment data revealed that the decadal PO4 signal is advected from the Sea of Okhotsk, where 8-year leading wintertime Ekman upwelling supplies PO4-rich water in the northern shelf region, and that the responsible atmospheric circulation is related to the West Pacific pattern. Our model simulation suggests that the wintertime wind-driven current system in the Sea of Okhotsk is important to the system feeding surface nutrients into the Oyashio region on an interannual–decadal timescale.
Ice bands are frequently observed over marginal ice zones in polar seas. A typical ice-band pattern has a regular spacing of about 10 km and extends over 100 km in the marginal ice zone. Further, the ...long axis of an ice band lies to the left (right) with respect to the wind direction in the Northern (Southern) Hemisphere. Here, the study shows that the resonance between ice-band pattern propagation and internal inertia-gravity waves below the sea ice well explains the ice-band pattern formation. Internal waves are generated by the difference between the stress on the open water and the stress on ice-covered water. This in turn reinforces the formation of an ice-band pattern with a regular band spacing. Specifically, the authors have found the following: 1) A band spacing on the order of 10 km is selected by the resonance condition in which the ice-band pattern propagation speed coincides with the phase speed of internal inertia-gravity waves. 2) The ice bands tend to develop favorably when the wind direction and the band propagation direction are nearly parallel. The velocity acceleration caused by the periodic differential stress associated with the ice bands, driven by the wind parallel to the band propagation direction, is important. The wind direction may turn to the left (right) slightly in the Northern (Southern) Hemisphere as a result of the Coriolis force acting on ice. Satellite images confirmed that the band spacing of the ice-band pattern in the polar seas is consistent with this theory.
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