Nobuo Suginohara

Deep Pacific Circulation Controlled by Vertical Diffusivity at the Lower Thermocline Depths

Abstract Deep Pacific circulation is investigated by using a World Ocean model with depth-dependent vertical diffusivity. Vertical diffusivity estimated from observations, 0.1 × 10−4 m2 s−1 for the upper layer and 3.0 × 10−4 m2 s−1 for the bottom layer, is adopted. Comparison is made between cases with different vertical diffusivity at middepths. With larger vertical diffusivity at middepths, the deep Pacific circulation becomes stronger. This is due to enhanced heat exchange between the thermocline water and the deep water through more intense diffusion at middepths. The water below the thermocline is warmed and that at the thermocline is cooled for the whole basin. The warmed deep water leads to larger heat loss through the sea surface, causing the enhanced deep-water formation in the deep-water formation region. On the other hand, the cooled thermocline water leads to larger heat gain through the sea surface where the thermocline water outcrops, counterbalancing the larger heat loss in the deep-water f...

An Ecological-Physical Coupled Model Applied to Station Papa

A vertical one-dimensional ecosystem model was constructed and applied to Station Papa. The model has seven compartments (phytoplankton, nitrate, ammonium, zooplankton, particulate organic matters, dissolved organic matters, dissolved oxygen) and was coupled with a mixed layer model for calculating diffusion coefficient which appears in the governing equations. The mixed layer model was driven by SST, SSS data observed at Station Papa in 1980 and ECMWF wind data for 1980, and the ecosystem model was driven by fixing nitrate concentration in deep layer to an observational value. The phytoplankton maximum in March was reproduced by the model although the maximum in fall-winter could not be reproduced. The model also suggests the importance of studying nitrification. As a whole, the model could reproduce characteristic features at Station Papa such as the summer ammonium maximum at 50 m depth, the summer dissolved oxygen maximum at 70 m depth and the absence of remarkable phytoplankton bloom.

Effects of locally enhanced vertical diffusivity over rough bathymetry on the world ocean circulation

Observations indicate that vertical diffusivity in the deep ocean is considerably enhanced over rough bathymetry and is very small elsewhere. Here we investigate effects of locally enhanced vertical diffusivity over rough bottom topography on the world ocean circulation by use of a coarse resolution ocean general circulation model. Vertical diffusivity is enhanced in the model, taking into account effects of internal tide breaking. For comparison, two cases of an experiment are carried out, where vertical diffusivity increases with depth without horizontal inhomogeneity. Horizontal distribution of deep upwelling is qualitatively different, depending on whether or not vertical diffusivity is horizontally inhomogeneous. Upwelling of Circumpolar Deep Water in the Pacific and Antarctic Bottom Water in the Atlantic is confined where vertical diffusivity is enhanced in the case of horizontally inhomogeneous vertical diffusivity, whereas it is horizontally uniform in the other cases. This difference leads to different three-dimensional structure of the deep circulation.

Coastal Upwelling: Onshore–Offshore Circulation, Equatorward Coastal Jet and Poleward Undercurrent over a Continental Shelf-Slope

Abstract The onshore-offshore circulation, equatorward coastal jet and poleward undercurrent associated with coastal upwelling are studied with numerical models. The model ocean has a continental shelf-slope uniform in the longshort direction and is forced by the wind stress with a limited longshore extent. The thermocline intersects the shelf-slope and the internal radius of deformation is smaller than the width of the shelf-slope. This may be a typical situation for coastal upwelling regions such as those off Oregon and northwest Africa. As the initial response to the onset of the winds, the Ekman offshore flow and the compensating onshore flow are induced and the equatorward flow develops over the shelf-slope. When the first mode coastal-trapped wave from the equatorward edge of the forcing region arrives, the onshore compensating flow offshore of the coastal area begins to decrease in strength and eventually offshore flow accompanied by downwelling dominates. Thus, the upwelling tends to be confined t...

Roles of the Okhotsk Sea and Gulf of Alaska in forming the North Pacific Intermediate Water

Recently obtained World Ocean Circulation Experiment (WOCE) sections and pre-WOCE hydrography are used to study the water-mass structure and formation and transformation of North Pacific Intermediate Water (NPIW). Five neutral density surfaces are selected and mapped, encompassing NPIW from 400 to 900 m in the subtropical latitudes with a distance of ∼100 m between a pair of surfaces. NPIW is defined as a subtropical gyre salinity minimum which is well followed by a neutral density surface σN = 26.9. Formation and transformation of NPIW is examined by the mapped Turner angle on neutral density surfaces. Apparent diffusive double diffusion is found in the Alaskan gyre on σN = 26.5 neutral surface, in the northwest subpolar gyre and the Okhotsk Sea on σN = 26.9 neutral surface, and mainly in the Okhotsk Sea on the two deep neutral surfaces σN =27.2 and σN = 27.4. These diffusive regions indicate transformation sources for NPIW. Along with additional information of potential vorticity and stream function, it is found that there are two different NPIW formation sources: one in the Gulf of Alaska characterized by high potential vorticity and the other in the Okhotsk Sea characterized by low potential vorticity. The former lies shallower at σN =26.2–26.5, but its effect deepens to NPIW core density level at σN = 26.8 on the basis of potential vorticity distribution. The latter includes the influence of the northwest subpolar gyre and extends much deeper to σN = 27.4. We call them Gulf of Alaska Intermediate Water (GAIW) and Okhotsk Intermediate Water (OIW), respectively. GAIW contributes to NPIW in the eastern part of the subtropical gyre east of date line, whilst OIW dominates in the west and entire lower part of NPIW. Seasonal flow stream function mapped on neutral surfaces shows that the contribution of GAIW to NPIW occurs mainly in the wintertime, because in winter a significant northward shift of zero wind stress curl makes the Gulf of Alaska an additional source for NPIW.

Effects of Bottom Boundary Layer Parameterization on Reproducing Deep and Bottom Waters in a World Ocean Model

Abstract A simple bottom boundary layer (BBL) model is developed to be incorporated into a z-coordinate medium resolution ocean general circulation model. In the BBL model, velocity is calculated from the pressure gradient, which is also calculated within the BBL. Preliminary experiments using an idealized basin model clearly document that, for reproducing realistic overflow/downslope flow, it is essential to adopt the horizontally distributed Rayleigh drag coefficient in the BBL model and also that, for avoiding warming of the abyssal ocean owing to unphysical strong flows created by the pressure gradient error along the steep slope, it is necessary to limit the area of the BBL. This BBL model is successfully incorporated into a World Ocean model with 1° × 1° resolution, producing the overflow/downslope flow in the northern North Atlantic and around Antarctica. The dense overflow/downslope flow water provides the nucleus of the abyssal water in the World Ocean, leading to the better representation of the...

Dynamics of the Kuroshio large meander. Barotropic model

The bimodality of the Kuroshio path is studied numerically with a barotropic inflow-outflow model. The dynamics that determines the path depends on the Rossby number,Ro (proportional to inlet velocity) and the Reynolds number (representing effects of viscosity). At lowRo (<Ro1) only a meander path occurs, while at highRo(Ro2) only a straight path is developed. Between these critical values (Ro1≦Ro≦Ro2) either of the two paths can occur (multiple states), and the choice of path is determined by its history. Increase (decrease) inRo acrossRo2 (Ro1) leads to catastrophic transition from one path to the other. In the intermediate range (Ro1≦Ro≦Ro2), the straight path is conditionally unstable to finite amplitude disturbances, and abrupt changes to the meander path take place. Absolute vorticity is almost conserved along the meander path, while along the straight path it is dissipated in large amount near the coast. At low Re, the flow tends to a viscous flow, and steady states are obtained. At highRe, time variations with different periods for the meander and straight paths become dominant. Intermittent transitions from one state to the other without any changes of external parameters are found at intermediateRo and at highRe.

Sensitivity of a Global Ocean General Circulation Model to Tracer Advection Schemes

Abstract Vertical diffusivity at the thermocline depths is now believed to be as small as 1 × 10−5 m2 s−1. In order to accomplish a reliable simulation of the World Ocean for the vertical diffusivity of 1 × 10−5 m2 s−1, two advective tracer transport schemes, the Uniformly Third-Order Polynominal Interpolation Algorithm (UTOPIA) of Leonard et al and the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) of Smolarkiewicz, are incorporated into an ocean general circulation model. Intercomparison is made among simulations using UTOPIA, MPDATA, and the centered differencing scheme. When UTOPIA or MPDATA is adopted, features at the thermocline depths are realistically simulated. Increase in computational cost is moderate. Circulations associated with Antarctic Bottom Water (AABW) in the Atlantic and Circumpolar Deep Water (CDW) in the Pacific are not reproduced at all for such small vertical diffusivity, although the circulation associated with North Atlantic Deep Water (NADW) has reason...

Atlantic deep circulation controlled by heating in the Southern Ocean

Thermohaline circulation has been considered to be driven by localized buoyancy loss through the sea surface at high latitudes and broadly distributed buoyancy gain elsewhere. Our numerical modeling study, however, shows that buoyancy gain for the Atlantic deep circulation is localized in the Southern Ocean. Wind-induced upwelling there causes efficient heat transfer to the deep ocean, and controls intensity of the Atlantic deep circulation as thermohaline circulation.

An ecosystem model for the North Pacific embedded in a general circulation model: Part I: Model description and characteristics of spatial distributions of biological variables

Abstract An ecosystem model is embedded in an ocean general circulation model (OGCM) and the ecosystem–physical combined model is applied to the North Pacific. The OGCM yields realistic physical environments concerning the mixed layer depth (MLD) and vertical flow except for MLD off Sanriku coast, Japan, and on both sides of the equator. The modeled nitrate, chlorophyll zooplankton, net primary production, and total organic nitrogen are within the order of magnitude of the observations. The biological activities are low in the subtropical region and high in the subpolar and the equatorial region as observations show. The modeled primary production rate shows a local maximum along the subpolar front, which seems to have a counterpart in the observation. There, the temperature and the nutrient condition are best combined for photosynthesis. The modeled concentrations differ from the observations in some respects: chlorophyll concentration is low in the subpolar region and high in the subtropical and the equatorial region compared with the Coastal Zone Color Scanner observation; the equatorial net primary production rate is higher than the observed. Part of the disagreement may be ascribed to the photoadaptation process, which is not included in the model and/or the simple extrapolation of the temperature dependence of photosynthesis.