Humio Mitsudera

Iron supply to the western subarctic Pacific: Importance of iron export from the Sea of Okhotsk

[1]xa0Iron is an essential nutrient and plays an important role in the control of phytoplankton growth (Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991; Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface-mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron-rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea.

Observations of the Subtropical Mode Water Evolution from the Kuroshio Extension System Study

Abstract Properties and seasonal evolution of North Pacific Ocean subtropical mode water (STMW) within and south of the Kuroshio Extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard ADCP measurements taken during 2004. The presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of STMW. Within the recirculation gyre, STMW formed from late-winter convection that reached depths greater than 450 m near the center of the gyre. The lower boundary of STMW, corresponding to σθ ≃ 25.5 kg m−3, was set by the maximum depth of the late-winter mixed layer. Properties within the deep portions of the STMW layer remained largely unchanged as the season progressed. In contrast, the upper boundary of the STMW layer eroded steadily as the seasonal thermocline deepened from late April to August. Vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of ...

Response of the Kuroshio Extension to Rossby Waves Associated with the 1970s Climate Regime Shift in a High-Resolution Ocean Model*

The response of the Kuroshio Extension (KE) to large-scale Rossby waves remotely excited by wind stress changes associated with the 1970s climate regime shift is studied using a high-resolution regional ocean model. Two ensemble simulations are conducted: The control run uses monthly climatological forcing while, in the second ensemble, anomalous forcing is imposed at the model eastern boundary around 165°E derived from a hindcast of decadal changes in subsurface temperature and salinity using a coarser-resolution model of the Pacific basin. Near the KE, ocean adjustment deviates strongly from the linear Rossby wave dynamics. Most notably, the eastward acceleration of the KE is much narrower in meridional extent than that associated with the incoming Rossby waves imposed on the eastern boundary. This KE acceleration is associated with an enhanced potential vorticity (PV) gradient across the front that is consistent with the inertial western boundary layer theory: the arrival of the Rossby waves at the western boundary causes the eastward current to accelerate, leading to enhanced advection of low (high) PV water of subtropical (subarctic) origin along the western boundary layer. The meridional dipole of PV anomalies results in a pair of anomalous recirculations with a narrow eastward jet in between. A three-layer quasigeostrophic model is used to demonstrate this inertial adjustment mechanism. Finally, transient eddy activity increases significantly and the eddy momentum transport acts to strengthen the mean flow response. The result that ocean physical response to broad-scale atmospheric forcing is large near the KE front has important implications for fisheries research.

Formation regions of Subantarctic Mode Water detected by OFES and Argo profiling floats

[1]xa0We investigated the distribution of the deep mixed layer in the Southern Ocean, where Subantarctic Mode Water (SAMW) forms, by using Argo profiling floats and the ocean general circulation model for the Earth simulator (OFES). The mixed-layer depth (MLD) generally increased eastward from 50°–180°E and from 180°–80°W, with distinct local maxima near major bathymetric features. A sudden transition from deep to shallow mixed layer was found along the equatorward limit of the deep mixed layer. This diagnosed MLD front coincides with the line of ug · ∇σs = 0, indicating the distributions of the general deepening and local maxima of MLD were affected by near-surface geostrophic flow. The flow is steered by the major bottom topographies, and the local MLD maxima occurred where the flow direction was deflected southward. This implies a link between the bathymetric features and formation of thick SAMW.

A series of cyclonic eddies in the Antarctic Divergence off Adélie Coast

[1]xa0The spatial and temporal characteristics of cyclonic eddies in the Antarctic Divergence off Adelie Coast are described using repeated in situ observations between 2001 and 2003, satellite observations, and results of a high-resolution ocean general circulation model. Satellite ocean color images and altimetry clearly revealed a series of cyclonic eddies, with diameters of about 100–150 km that were separated by about 150 km in the zonal direction. The eddies are found at almost the same locations and can be traced for at least two months in the spring-summer period. Cold and fresh anomalies were found throughout the water column in the cores of the cyclonic eddies. The water properties in the core of the eddies are similar to water found over the continental slope, 150–200 km to the south. The geopotential anomaly of the cold features was smaller than the sea level anomaly detected with satellite altimetry, indicating that significant barotropic flow is associated with the eddies. The Ocean general circulation model for the Earth Simulator (OFES) reproduces a series of cyclonic eddies similar to those observed, including a significant barotropic component. In the model, the series of eddies appear to originate further west over the continental slope region with no consistent phase propagation. The combination of repeat in situ observations, remote sensing and high-resolution model results confirms the existence of persistent cyclonic eddies near 140°E and suggests the eddies play an important role in the exchange of water across the Antarctic Divergence.

Breaking of unsteady lee waves generated by diurnal tides

[1]xa0Diapycnal mixing caused through breaking of large-amplitude internal lee waves generated by sub-inertial diurnal tides, which are modulated with a 18.6-year period, is hypothesized to be fundamental to both the intermediate-layer ventilation and the bi-decadal oscillation around the North Pacific Ocean. The first observational evidence of such wave breaking is presented here. The breaking wave observed had ∼200 m height and ∼1 km width, and its associated diapycnal mixing was estimated to be ∼1.5 m2 s−1, with a temporal average ∼104 times larger than typical values in the open oceans. Our estimate suggests that a similar mixing process occurs globally, particularly around the Pacific and Antarctic Oceans.

Simulations of chlorofluorocarbons in and around the Sea of Okhotsk : Effects of tidal mixing and brine rejection on the ventilation

[1]xa0Ventilation of waters in and around the Sea of Okhotsk was investigated using simulations of chlorofluorocarbons (CFCs) in the northwestern North Pacific. We used an ocean general circulation model coupled with a sea ice model. The model reproduces the distributions of CFCs similar to observed values and indicates the importance of tidal mixing along the Kuril Islands and brine rejection to ventilation of waters in and around the Sea of Okhotsk. To clarify the role of each process, numerical experiments excluding one of the two processes were carried out. Results show that brine rejection transports CFCs into the intermediate layer as deep as 200–400 m along the path of dense shelf water in the western Sea of Okhotsk, but hardly to other areas and layers. On the other hand, tidal mixing transports CFCs into the intermediate and deeper layers throughout the Sea of Okhotsk. We conclude that the tidal mixing has a greater influence than brine rejection on the ventilation of layers below the winter mixed layer.

Effects of along‐shore wind on DSW formation beneath coastal polynyas: Application to the Sea of Okhotsk

[1]xa0It is known that salinity anomaly under a polynya reaches and remains an equilibrium value S* after termination of its initial increase associated with brine rejection at the surface. In this paper, we investigate effect of along-shore (downwelling-favorable) wind on the equilibrium salinity anomaly by idealized numerical calculations and scale-based estimates. Numerical calculations showed that high saline water beneath polynya is advected downstream by wind-driven circulations over the shelf besides baroclinically developed eddies, which consequently induces a decrease in S* beneath polynya. The downwelling-favorable wind generates an offshore overturning flow through lower layers, referred to as Ekman Compensation Flow (ECF), which causes a great offshore salinity flux, as well as an along-shore current. We also constructed an equation for estimation of S* from the viewpoint of salinity budget over the polynya region, in which lateral salinity fluxes caused by ECF, along-shore wind-driven current and baroclinic eddies, are scaled. The solution S* was also verified by a series of numerical calculations. Furthermore, we investigated the effects of along-shore wind on dense water generation beneath the Okhotsk coastal polynyas. We conducted simplified numerical experiments assuming the Okhotsk situation, in which Fs and offshore width b of polynya are predicted by a thermodynamic polynya model with ECMWF meteorological variables. The simulated salinity shows a good agreement with the direct measurements. The theoretical estimates for S* was also applied to two Okhotsk polynyas, northwestern polynya (NWP) and northern polynya (NP). In conclusion, we found that the along-shore wind causes greater salinity decrease in NP than in NWP, whose variations substantially depend on the Aleutian Low activity.

Blocking of the Kuroshio Large Meander by Baroclinic Interaction with the Izu Ridge

This paper discusses the role of the Izu Ridge in blocking the Kuroshio large meander from propagating eastward across the ridge. It is shown that a combination of the sloping bottom with baroclinicity in the Kuroshio flow is important for blocking of the large meander. It produces a cyclonic torque over the western slope of the ridge when the large meander impinges upon it. That is, the cyclonic torque is formed ahead of the large meander, which results in blocking and amplification of the meander upstream of the Izu Ridge. The baroclinicity of the Kuroshio over the ridge is caused by baroclinic topographic Rossby waves generated when the large meander encounters the ridge.

Formation Mechanism of Huge Coastal Polynyas and Its Application to Okhotsk Northwestern Polynya

Abstract This paper investigates the formation mechanism of broad coastal polynyas beyond 100 km in offshore width. It is known that two regimes for wind-driven polynya opening exist: one is a convergent regime at the polynya edge in which inner frazil ice catches up with outer consolidated ice, whereas the other is a divergent regime in which the consolidated ice drifts offshore faster than the frazil ice at the edge. In this study, the authors focus on the latter, divergent polynya-edge regime. Because in the divergent regime the polynya possibly evolves without bound, they consider a thermal growth for inner frazil ice to find a finite solution of offshore width. Then, the authors investigate responses of the polynya opening for various wind angles ϕ from the offshore direction from the viewpoint of the polynya-edge regimes. At first, the authors estimate the deviation angle and wind factor for the frazil and consolidated ice based on each momentum balance, because sea ice motion driven by wind varies ...