Atsushi Kubokawa

Interdecadal Thermocline Variability in the North Pacific for 1958–97: A GCM Simulation*

An ocean general circulation model is forced with the NCEP reanalysis wind stress for 1958‐97 to understand mechanisms of ocean subsurface variability. With relatively high horizontal (1 83 18) and vertical (41 levels) resolutions, the model produces mode waters on a range of density surfaces in the western, central, and eastern North Pacific, in qualitative agreement with observations. These mode waters appear as a thermostad or a region of weak stratification in the upper thermocline as they flow southward from their formation regions in the Kuroshio and its extension. In the model, subsurface temperature variability in the central subtropical gyre reaches a maximum within the thermostad, in contrast to what might be expected from the linear baroclinic Rossby wave theory. This variance maximum is associated with the longitudinal shift in the path of mode waters. In particular, deepened mixed layer and accelerated eastward currents in the Kuroshio Extension by wind changes in the mid-1970s act cooperatively to move the central mode waters toward the east, causing large subsurface temperature anomalies. Besides the local maximum in the central North Pacific subtropical gyre, two additional maxima of the subsurface anomaly are identified in the northwestern and southern parts of the gyre, respectively. Among these subsurface anomaly centers, the one in the northwestern North Pacific has a strong effect on the model sea surface temperature, suggesting that the Kuroshio and its extension are a key region to decadal/interdecadal ocean‐atmosphere interaction. Finally, possible effects of atmospheric thermal forcing are discussed.

Ventilated Thermocline Strongly Affected by a Deep Mixed Layer: A Theory for Subtropical Countercurrent

Abstract The mixed layer thickness generally increases in the northward direction, but its change seems to occur rather sharply in a narrow transition zone, referred to as a mixed layer front here. The author investigates the effects of the mixed layer front on the ventilated thermocline structure, especially focusing on the generation mechanism of the subtropical countercurrent. If the mixed layer front is not parallel to the surface density contour, the fluid with minimum isopycnal potential vorticity is formed around the intersection of the mixed layer front and outcrop line. If the mixed layer front slants northeastward and surface density is zonally uniform, as seen in a numerical experiment carried out by Kubokawa and Inui in which the subtropical countercurrent was reproduced, the minimum potential vorticity on a high-density isopycnal occurs in the east, while that in a low-density isopycnal occurs in the west. In the present study, analytic solutions for a simple three-layer model are presented f...

Subtropical Countercurrent in an Idealized Ocean GCM

Abstract In the North Pacific, there is a shallow eastward current called the subtropical countercurrent, which flows across the central subtropical gyre. The present article studies the generation mechanism of the subtropical countercurrent reproduced in an ocean general circulation model (GCM) with a simple geometry, driven by surface wind stress and surface buoyancy forcing. In the ocean GCM, the deep mixed layer occurs in the northern part of subtropical gyre and shoals abruptly in the central subtropical gyre. The mixed layer front, the narrow transition zone of the mixed layer depth, slants from the western central subtropical gyre to the northeast, and the low potential vorticity fluid is formed at the intersection of the mixed layer front and the outcrop line. Since the surface density is almost zonally uniform and the mixed layer front slants northeastward, the minimum potential vorticity fluids on denser isopycnals are formed in the northeastern region, while those on lighter isopycnals are form...

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.

Oceanic fronts and jets around Japan: a review

This article reviews progress in our understanding of oceanic fronts around Japan and their roles in air–sea interaction. Fronts associated with the Kuroshio and its extension, fronts within the area of the Kuroshio-Oyashio confluence, and the subtropical fronts are described with particular emphasis on their structure, variability, and role in air–sea interaction. The discussion also extends to the fronts in the coastal and marginal seas, the Seto Inland Sea and Japan Sea. Studies on oceanic fronts have progressed significantly during the past decade, but many of these studies focus on processes at individual fronts and do not provide a comprehensive view. Hence, one of the goals of this article is to review the oceanic fronts around Japan by describing the processes based on common metrics. These metrics focus primarily on surface properties to obtain insights into air–sea interactions that occur along oceanic fronts. The basic characteristics derived for each front (i.e., metrics) are then presented as a table. We envision that many of the coupled ocean-atmosphere global circulation models in the coming decade will represent oceanic fronts reasonably well, and it is hoped that this review along with the table of metrics will provide a useful benchmark for evaluating these models.

Review on North Pacific Subtropical Countercurrents and Subtropical Fronts: role of mode waters in ocean circulation and climate

A Subtropical Countercurrent (STCC) is a narrow eastward jet on the equator side of a subtropical gyre, flowing against the broad westward Sverdrup flow. Together with theories, recent enhanced observations and model simulations have revealed the importance of mode waters in the formation and variability of North Pacific STCCs. There are three distinct STCCs in the North Pacific, maintained by low potential vorticity (PV) that mode waters carry from the north. Model simulations show that changes in mode water ventilation result in interannual to interdecadal variations and long-term changes of STCCs. STCCs affect the atmosphere through their surface thermal effects, inducing anomalous cyclonic wind curl and precipitation along them. Thus, mode waters are not merely passive water masses but have dynamical and climatic effects. For temporal variability, atmospheric forcings are also suggested to be important in addition to the variability of mode waters. STCCs exist in other oceans and they are also flanked by mode waters on their poleward sides, suggesting that they are maintained by similar dynamics.

A theory of semigeostrophic gravity waves and its application to the intrusion of a density current along a coast: Part 2. Intrusion of a density current along a coast in a rotating fluid

The intrusion of a density current along a coast in a rotating fluid is investigated both theoretically and experimentally. The theoretical model is a shock wave solution of a semigeostrophic gravity wave which was investigated in Part 1 (Kubokawa and Hanawa, 1984). In the experimental results, the propagation speed of the leading edge of light fluid along a vertical boundary and the current width and depth are nearly equal to those estimated by the shock wave theory. The generation of a frontal wave at the leading edge of the density current is observed. The propagation velocity of these frontal waves agrees well with that of theoretically-predicted semigeostrophic gravity waves.

Instability of a geostrophic front and its energetics

Abstract The instability of a current with a geostrophic surface density front is investigated by means of a reduced gravity model having a velocity profile with nearly uniform potential vorticity. It is shown that currents are unstable when the mean potential vorticity decreases toward the surface front at the critical point of the frontal trapped waves investigated by Paldor (1983). This instability is identical with that demonstrated by Killworth (1983) in the longwave limit. The cross-stream component of mass flux and the rates of energy conversions among the five energy forms defined by Orlanski (1968) are also calculated. The main results are as follows, (a) The mass flux toward the surface front is positive near the front and negative around the critical point. The positive mass flux near the front does not vanish at the position of the undisturbed surface front, so that the mean position of the front moves outward and the region of the strong current spreads. (b) The potential energy of the mean f...

Evaporation-Wind Feedback and the Organizing of Tropical Convection on the Planetary Scale. Part I: Quasi-Linear Instability

Abstract Recent GCM experiments have suggested the existence of a zonal wavenumber one convective mode in the aqua-planet atmosphere. This paper reports that a planetary-scale mode can be generated in a very simple reduced gravity model that is linear except for two nonlinearities in its cumulus parameterization: conditional heating and wind speed-dependent surface evaporation. The behavior of the model solution is shown to be independent of the perturbation amplitude so that a constant growth rate can be defined. This amplitude-independent nonlinear system is here called the quasi-linear (QL) system. An instability is found in a moist stable atmosphere at rest, which is stable in existing theories. A global integral theorem confirms the existence of the QL instability. The instability has an equatorially trapped, zonal wavenumber one structure, growing exponentially and propagating eastward at a speed close to that of the neutral, linear moist Kelvin wave. A new type of evaporation-wind feedback (EWFB) i...

Oscillations with Two Feedback Processes in a Coupled Ocean–Atmosphere Model

Abstract A one-layer reduced-gravity ocean model and a linear single-mode baroclinic atmosphere model are coupled to investigate the ocean-atmosphere interaction in the tropics. The original Kraus-Turner formula is adopted to parameterize the entrainment in the ocean mixed layer and an external wind is imposed to sustain a basic state of the ocean. By examining the linear stability of the model, we show that the existence of ocean upwelling is one of the necessary conditions for eastward propagating instability. According to the upwellings strength, the tropical ocean is separated into two regions, i.e., a stable region in the west and an unstable one in the cast. In the fully nonlinear experiment, oscillation with a period of several years appears, which is asymmetric between the warm and cold phases because of the existence of two different coupled instabilities. The eastward unstable mode is responsible for the warm phase, while the cold phase is attributed to the unstable Rossby mode, which advects w...