Search Results

You are looking at 1 - 10 of 11 items for

  • Author or Editor: Nobuo Suginohara x
  • Refine by Access: All Content x
Clear All Modify Search
Nobuo Suginohara

Abstract

Quasi-geostrophic waves in a two-layer ocean with bottom topography on a β-plane are examined in detail. The bottom slopes in an arbitrary direction but gently and uniformly. A clear understanding of vertical normal modes is obtained from the use of the “upper layer component” and the “lower layer component” as a basic concept, where the upper layer component (ULC) and the lower layer component (LLC) are motions confined to the upper and the lower layers, respectively, and are independent of each other. Modification of the normal modes from ULC and LLC is measured by the effect of divergence which couples upper and lower layer motions. The effect of bottom topography tends to suppress the coupling. The extent to which the effect of topography suppresses the coupling depends on the thickness ratio and relative effects of the planetary and topographies β as well as the horizontal scale of waves relative to the internal radius of deformation. In some realistic circumstances, the upper layer mode is isolated completely from the bottom and the lower layer mode is trapped completely by the bottom even for waves whose wavelength is much longer than the internal deformation radius. When the phase speeds of the ULC and LLC are identical, resonance takes place and induces an interchange of their properties. Occurrence of resonance is typical for a general orientation of bottom topography.

Full access
Nobuo Suginohara

Abstract

Poleward propagation of coastal-trapped waves induced by a baroclinic equatorial Kelvin wave incident on the eastern boundary is studied in numerical models. When the thermocline is shallower than shelf depth and so intersects a vertical coastal wall, a coastal-trapped, internal Kelvin-type wave keeps propagating poleward. The only change in its structure is that its trapping width decreases in accordance with the decrease in the local deformation radius. On the other hand, when the thermocline intersects a continental slope, which represents a typical situation for the eastern tropical Pacific, baroclinic disturbances decrease in amplitude as they propagate poleward, and eventually disappear at middle latitudes. Transformation of the baroclinic disturbances to quasi-barotropic shelf waves takes place. Part of the barotropic energy leaks away from the coastal region in the form of barotropic Rossby waves. As the period (wavelength) of an incident equatorial Kelvin wave increases, baroclinic disturbances propagate farther poleward.

Full access
Nobuo Suginohara

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 to the coastal area. For the alongshore flow, the equatorward flow tends to be confined to the coastal area, and the poloward undercurrent develops below the thermocline over the slope. When the second mode wave arrives, the upwelling is further confined to the coastal area and the equatorward coastal jet and poleward undercurrent cease to develop. Thus, coastal trapped waves, which are neither internal Kelvin nor barotropic shelf waves, play essential roles in determining the upwelling circulation.

Full access
Nobuo Suginohara
and
Yoshiteru Kitamura

Abstract

Long-term coastal upwelling over a continental shelf-slope with emphasis on the planetary dispersion of Rossby waves is studied with numerical models. The ocean is forced by a 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. Two case studies with and without the β effect are studied. lie early-stage response within a week or so is similar between the two. For an f-plane case, the whole-stage response is accounted for by coastal-trapped wave dynamics, and only equatorward flow exists over the shelf–slope at an advanced stage. However, inclusion of the β effect leads to a significant change in the alongshore flow pattern at the advanced stage. Due to the westward Rossby dispersion of the equatorward flow associated with the first and second coastal-trapped mode responses, the poleward undercurrent develops in the thermocline layer over the shelf. The time for the undercurrent to appear depends on the speed of the westward dispersion of the second-mode response. The undercurrent obtained in the present model may represent the one observed off Oregon and California.

Full access
Hiroyasu Hasumi
and
Nobuo Suginohara

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 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 reasonable intensity. Another experiment with UTOPIA for the vertical diffusivity of 5 × 10−5 m2 s−1 shows that the circulation associated with NADW is relatively insensitive to vertical diffusivity, compared with the circulation associated with AABW and CDW.

Full access
Hideyuki Nakano
and
Nobuo Suginohara

Abstract

A series of middepth zonal flows observed in the Pacific is produced in a World Ocean model with the horizontal resolution of 1° × 1° and 40 vertical levels. It is demonstrated that the middepth zonal flows are driven by the wind and reach down to several thousand meters. The surface wind gyres appear to shift poleward with depth, leaving behind the gyres that originate from the equatorial response. In meridional sections, the pattern of the zonal flows slants poleward with increasing depth. The formation mechanism for the middepth zonal flows is clarified using an idealized basin model and a semianalytical model of vertical normal mode decomposition. In the models, the inclusion of vertical diffusion is essential. The zonal flows at low latitudes are formed as the equatorial response to uniform zonal winds. The response at middle and high latitudes is accounted for as follows. Quasigeostrophic (QG) dynamics with vertical diffusion reproduces the reversal of the zonal flows with depth. The slanting pattern of the zonal flows is due to the non-QG effect for the response of the vertical higher modes. Then the inclusion of horizontal diffusion reduces the response of the gravest modes. The third to fifth vertical modes are very important for forming the middepth wind-driven circulation. The wind-driven circulation at middepths is very weak compared to that in the surface layer, but this is sufficient to overcome the weak thermohaline circulation in the middepth Pacific.

Full access
Hideyuki Nakano
and
Nobuo Suginohara

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 abyssal water. Thus, the incorporation of the BBL model can reduce the warming bias for the abyssal water in a coarse resolution model without modifying surface boundary conditions. In addition, it can also alleviate the model bias of too shallow extension of North Atlantic Deep Water.

Without the BBL model, the dense water from the Nordic seas does not flow southward and remains south of Iceland, forming a strong front there. The effect of Gent and McWilliams parameterization for mesoscale eddies to flatten isopycnal surfaces leads to unrealistic cooling south of Iceland. Unrealistic cooling also takes place around Antarctica. The incorporation of the BBL model drastically reduces this cooling by preventing the formation of the artificial front.

Full access
Hiroyuki Tsujino
and
Nobuo Suginohara

Abstract

A thermohaline circulation enhanced by wind forcing is demonstrated in an idealized basin model, and a mechanism that provides a connection between wind forcing and a thermohaline circulation is clarified. A rectangular ocean that extends over the Northern and Southern Hemispheres is driven by differential heating and wind stress at the sea surface. The differential heating is so distributed that the deep water is formed at the southern end of the model ocean. The wind stress is so distributed that there are three wind-driven gyres in the Northern Hemisphere, and it is not imposed in the Southern Hemisphere. Comparison is made between the cases with and without the wind stress. When the wind forcing is imposed, the basin-scale meridional circulation increases in intensity. This is due to the enhanced surface heating in the cyclonic wind-driven gyre with the Ekman upwelling and the accompanying enhanced surface cooling in the deep-water formation region. In the cyclonic wind-driven gyre, the Ekman upwelling brings up the thermocline to the subsurface depths to enhance the surface heating and also the downward heat conduction from the sea surface to the deep layer, which leads to warming of the deep water. Thus, the enhanced surface heating in the cyclonic gyre is balanced with the enhanced surface cooling in the deep-water formation region due to the warmed deep water. In this way, the wind forcing enhances a thermohaline circulation that connects the deep-water formation region to the cyclonic wind-driven gyre with the Ekman upwelling.

Full access
Hiroyuki Tsujino
,
Hiroyasu Hasumi
, and
Nobuo Suginohara

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 formation region. The deep water brought up to the middepths does not further upwell to the sea surface due to the small upper-layer vertical diffusivity, but it flows back to the deep-water formation region, slowly upwelling within the middepths. In this way, the enhanced meridional overturning forms in the deep Pacific. The layered deep Pacific meridional circulation is realistically reproduced when vertical diffusivity is larger at middepths. This circulation yields tracer distributions that compare well with observations. Such a strong deep Pacific circulation does not occur when vertical diffusivity is taken larger at middepths but is held constant below the middepths. For realistic reproduction of the deep Pacific circulation, vertical diffusivity needs to keep increasing with depth beginning at the lower thermocline depths.

Full access
Nobuo Suginohara
,
Shigeaki Aoki
, and
Masao Fukasawa

Abstract

No abstract available.

Full access