Hiroshi Ishizaki

Decadal variability of the Subtropical Front of the western North Pacific in an eddy‐resolving ocean general circulation model

[1]xa0We examined decadal variability of the Subtropical Front (STF) of the western North Pacific by using a North Pacific ocean general circulation model (OGCM), comparing the results of three simulations with different horizontal resolutions. In the long-term mean fields, the eddy-resolving model (10 km) was able to simulate the distributions of the STF and the associated Subtropical Countercurrent (STCC) between 20°N and 30°N better than either the non-eddy-resolving model (100 km) or the eddy-permitting model (20 km) because it simulated the Kuroshio recirculation gyre more realistically. The simulated STF intensity exhibited significant decadal-scale variations: it was stronger in the late 1970s and weaker in the early 1990s. During these two periods, the simulated mode waters showed corresponding differences in their potential vorticity minima density and paths. We also investigated the relationship between the decadal-scale STF variability and atmospheric forcing. The results suggest that the decadal-scale STF variability can be largely explained by changes in the mode waters formed in the western North Pacific and advected to north of the STF by the subtropical gyre in response to a change in surface westerly winds that occurred in the mid-1970s.

Mid‐depth freshening in the North Pacific subtropical gyre observed along the JMA repeat and WOCE hydrographic sections

[1] We report a freshening at mid-depth in the North Pacific subtropical gyre by using long-term repeat hydrographic data along the 137°E section and one-time hydrographic data along the World Ocean Circulation Experiment Hydrographic Program (WHP) P2 and P3 sections. North of 15°N along the 137°E section, we estimated a linear freshening trend of 0.0015/yr between the main thermocline and the salinity minimum layer of the North Pacific Intermediate Water, mainly caused by isopycnal surface deepening due to warming, and by westward shifts of the salinity-minimum tongue due to strengthening of the subtropical gyre. Furthermore, along the WHP-P2 section, the linear freshening trend could be classified into several groups according to longitude. Such spatial differences in the freshening trend seem to reflect differences in the formation processes and mid-depth pathways of the salinity minimum waters.

Effects of the Westerly Wind Stress over the Southern Ocean on the Meridional Overturning

Abstract Numerical experiments were conducted to clarify the processes through which the Southern Ocean wind affects the meridional overturning (NA cell) associated with North Atlantic Deep Water production. These were based on idealized single- and twin-basin (idealized Atlantic and Pacific Ocean) models with a periodically connected passage under various forcings at the surface. Relationships among the wind stresses, the NA cell, and the buoyancy fluxes were investigated. Increased westerly wind stresses increase the surface buoyancy gains in the Southern Ocean under the density-restoring boundary condition. The buoyancy anomalies excited in the Southern Ocean propagate as baroclinic waves into the northern North Atlantic, modify the density field, and enhance the NA cell, which increases buoyancy losses there until the global buoyancy flux budget balances. The results from experiments using a realistically configured global ocean model confirm that the Southern Ocean wind effects on the NA cell can be ...

Impact of Solar Radiation Data and Its Absorption Schemes on Ocean Model Simulations

Since absorption of solar radiation plays a major role in heating the upper ocean layers, it is essential for modeling physical, chemical and biological processes (e.g., ocean general circulation or marine carbon cycle). In order to simulate the upper ocean thermal structures as realistically as possible, an ocean general circulation model (OGCM) requires accurate solar radiation data, used as the surface boundary condition. In this sense, it is important to recognize the quality of the solar radiation data being expected or suitable for OGCMs beforehand. The appropriate choice of absorption schemes of solar radiation is also important for ocean modeling in the upper ocean. The absorption of solar radiation is greatly affected by many factors, such as the wavelength of sunlight, the zenith angle and ocean optical properties in the ocean interior. Many absorption schemes have attempted to mimic these processes, but the impact of those schemes on the upper ocean thermal structures is not yet fully understood.

Direct measurements of deep current at 162°E south of the equator in the Melanesian Basin: a trial to detect a cross-equatorial deep western boundary current

We conducted 1-year-long mooring observations four times below 2000xa0m, slightly south of the equator (2°39′ to 4°35′S) at 162°E in the Melanesian Basin in order to detect the southward deep western boundary return current crossing the equator. Contrary to our initial expectation of the deep flow scheme in the equatorial western boundary region, the observed results indicated a fairly complicated flow configuration. We analyzed the results with the help of a high-resolution model simulation. The ensemble average of the horizontal flow at each level near the deep western boundary indicates a significant westward flow at 2000 and 2250xa0m, with an insignificant southward component at 2500 and 2750xa0m. The annual mean meridional transports are very small (>1xa0Sv) and insignificant, with an ensemble-averaged value of 0.3xa0Sv (southward) ±0.4xa0Sv at most. Combining this with high-resolution model results, it is deduced that the southward transport of the deep western boundary current (DWBC) leaving the equator may be smaller than those obtained by low-resolution models, because of trapping of its fairly large fraction in the equatorial zone. Annual-scale flow patterns are classified into several categories, mainly based on the meridional-flow dominating or the zonal-flow dominating pattern. A case of the meridional-flow dominating patterns may possibly capture an annual-scale variability of DWBC, because its meridional transport variation, though somewhat weak, is consistent with that simulated. The zonal-flow dominating regime includes two types: long-lasting, almost steady westward flows and long-term zonal flow oscillations. The former seems to comprise well-known zonally elongated and meridionally narrow structures of the zonal flow beneath the thermocline in the equatorial region. The ensemble-averaged flow mentioned above is dominated by this type at the upper two levels 2000 and 2250xa0m, with total westward transport of 1.6xa0±xa00.7xa0Sv. The latter type seems to be a manifestation of the vertically propagating equatorial annual Rossby waves.

Evidence of equatorial Rossby wave propagation obtained by deep mooring observations in the western Pacific Ocean

We investigated long-term zonal flow (U) oscillations in the deep layer in the western Pacific Ocean by comparing measurements obtained from deep mooring observations near the equator with a high-resolution model. For the comparison, we used mooring data at 2,000xa0m and below north and south of the equator obtained from preceding studies. In our model results, vertically propagating equatorial annual Rossby waves were detected in the velocity and pressure anomaly (Pa) fields. Pa had a multi-cell meridional structure symmetric about the equator. The corresponding meridional distribution of U, averaged along the wave propagation, was the same as that previously obtained from Argo float data, suggesting the existence of the multi-cell structure in Pa in the real ocean. A harmonic analysis of the simulated U indicated that the equatorial annual Rossby waves excited over the central and eastern equatorial Pacific propagated down to below 2,000xa0m in the western equatorial Pacific. Decomposition of the annual U harmonics indicated that the multi-cell structure in pressure was not a manifestation of a single meridional mode but a composite of several lowest meridional modes, suggesting that the cell widths may change according to relative mode intensities. The mooring data north and south of the equator was consistent with the simulated multi-cell structure in Pa of the equatorial Rossby waves, although some discrepancies remained. We consider that the observed variation in U is a manifestation of the Rossby wave propagation in the real ocean. The present study is the first to recognize the existence of equatorial annual Rossby waves below 2,000xa0m in the western Pacific Ocean from direct current observations.