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Richard Greatbatch and Toshio Yamagata

Abstract

We examine the steady state response in the equatorial regions of a nonlinear, one-active-layer, reduced-gravity shallow water ocean model with meridional boundaries that is driven by uniform wind forcing. When the wind forcing is westward, the final steady state is one of no motion with a zonal pressure gradient balancing the wind forcing, as implied by linear theory. However, when the wind forcing is eastward, a steady state is reached in which a narrow eastward jet along the equator is balanced by a broader westward return flow, the solution resembling Fofonoff's free inertial mode. The entire circulation is confined within equatorial regions. It is shown that Rossby wave energy propagating eastwards from the western boundary is crucial for setting up the mode. This steady solution with motion was found even in a case with weak wind forcing. However, it is easily “wiped out” by friction (esqecially lateral mixing) in this case. In general, the solutions are found to be sensitive to the lateral mixing which can prevent the equatorial jet from extending all the way to the eastern boundary.

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Tomoki Tozuka and Toshio Yamagata

Abstract

Using various observational data, the seasonal cycle of the tropical Pacific is investigated, suggesting the existence of an “annual El Niño–Southern Oscillation (ENSO).” A positive sea surface temperature anomaly (SSTA) appearing off Peru in boreal winter triggers a series of air–sea interactions that consist of westward propagations of positive SSTA, westerly wind anomalies, and negative outgoing longwave radiation anomalies. At the same time, the westerly wind anomaly generates cold temperature anomalies in the off-equatorial region, and they propagate westward as a “cold” Rossby wave, reaching the western tropical Pacific in boreal summer to autumn. A semiresonant condition between the westward propagating component of winds and the first-meridional-mode Rossby wave plays an important role in the amplification. The evolution of cold phase in the latter half of the year is almost a mirror image of the warm phase. From a new viewpoint of the annual ENSO, the ENSO is interpreted as the interaction between two distinct modes of air–sea interaction: the annual ENSO mode and an “interannual ENSO” mode. The eastward-propagating interannual ENSO mode is an air–sea coupled mode, which is triggered by the westerly wind stress anomaly in the western equatorial Pacific and leads to the deepening of the thermocline and the warming of SST in the central and eastern equatorial Pacific. This results in a modulation of the annual ENSO mode with a weaker cold season and stronger warm season owing to less effective upwelling of the cold subsurface water. The decadal variation of ENSO is explained by changes in the relative phase and amplitude of these two modes. The increase in the amplitude of the interannual ENSO mode after the late 1970s favors the appearance of the eastward propagation of ENSO signals.

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Toshio Yamagata and Satoshi Iizuka

Abstract

A general ocean circulation model is used to analyze seasonal variations in the Guinea Dome and Angola Dome regions. The cold Guinea Dome develops from June through September due to active divergence of heat transport. The cooling is related to upwelling generated by the local wind stress curl associated with the northeast trade winds. The Guinea Dome thus provides an active mechanism of absorbing heat from the atmosphere. The coastal Guinea region experiences semiannual warming in April–July and November–December due to intrusion of coastal downwelling Kelvin waves from the equatorial region.

The Angola Dome in the Southern Hemisphere is found to be cooled from March through August. The surface flux plays a major role in the seasonal variation of the heat budget in contrast to the situation of the Guinea Dome in the Northern Hemisphere. The Angola front located at the northern border of the cold Angola Dome becomes distinguishable particularly during the boreal fall because of the intrusion of warm water from the equatorial region.

The coastal warming occurs twice a year in both hemispheres mostly due to intrusion of warm water accumulated in the eastern border of the Gulf of Guinea. The semiannual relaxation of the trade winds east of 30°W as well as the semiannual intensification of the eastward component of the West African monsoon is responsible for this remarkable oceanic phenomenon.

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Tomoki Tozuka and Toshio Yamagata

Abstract

Using various observational data, the seasonal cycle of the tropical Pacific is investigated, suggesting the existence of an “annual El Niño–Southern Oscillation (ENSO).” A positive sea surface temperature anomaly (SSTA) appearing off Peru in boreal winter triggers a series of air–sea interactions that consist of westward propagations of positive SSTA, westerly wind anomalies, and negative outgoing longwave radiation anomalies. At the same time, the westerly wind anomaly generates cold temperature anomalies in the off-equatorial region, and they propagate westward as a “cold” Rossby wave, reaching the western tropical Pacific in boreal summer to autumn. A semiresonant condition between the westward propagating component of winds and the first-meridional-mode Rossby wave plays an important role in the amplification. The evolution of cold phase in the latter half of the year is almost a mirror image of the warm phase. From a new viewpoint of the annual ENSO, the ENSO is interpreted as the interaction between two distinct modes of air–sea interaction: the annual ENSO mode and an “interannual ENSO” mode. The eastward-propagating interannual ENSO mode is an air–sea coupled mode, which is triggered by the westerly wind stress anomaly in the western equatorial Pacific and leads to the deepening of the thermocline and the warming of SST in the central and eastern equatorial Pacific. This results in a modulation of the annual ENSO mode with a weaker cold season and stronger warm season owing to less effective upwelling of the cold subsurface water. The decadal variation of ENSO is explained by changes in the relative phase and amplitude of these two modes. The increase in the amplitude of the interannual ENSO mode after the late 1970s favors the appearance of the eastward propagation of ENSO signals.

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Tomonori Matsuura and Toshio Yamagata

Abstract

The properties of a new equation governing the evolution of planetary eddies larger than the radius of deformation are numerically investigated. Two types of dynamical balances showing remarkable solitary behavior are found. The first is the balance between the weak dispersion due to the planetary beta-effect and the weak nonlinearity due to the continuity equation. Only anticyclonic eddies are extremely long-lived due to this balance. The second is the balance between weak lateral advection due to a particular westward flow and weak planetary dispersion. The collision experiment shows robustness of the two-dimensional solitary eddy, suggesting the existence of a two-dimensional soliton of the latter type.

Also discussed is the relevance of our results to the evolution of the anticyclonic eddies off the Pacific coast of Central America reported by Stumpf and Legeckis (1977).

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Takashi Kagimoto and Toshio Yamagata

Abstract

Numerical simulation is performed using a high-resolution ocean general circulation model to investigate seasonal variations of the Kuroshio transport. The simulated velocity profiles of the Kuroshio agree surprisingly well with ADCP observations and dynamic calculations. The annual mean of the model Kuroshio transport relative to 700 m across the PN line near the Nansei (Ryukyu) Islands is about 25 Sv (Sv ≡ 106 m3 s−1), which is almost the same with the estimate based on the long-term hydrographic observations. The model transport variations across the PN line are also almost the same as the observation; the transport shows a weak maximum in summer and a weak minimum in winter. Although the Sverdrup balance is valid in the broad interior of the basin, it fails to predict the variations as well as the transport of the Kuroshio south of Japan due to existence of the Kuroshio recirculation.

The above discrepancy between the Sverdrup theory and the model (observations, as well) is studied in detail by analyzing the torque balance. In winter the Kuroshio transport across the PN line is much smaller than expected from the Sverdrup theory because the topographic control prevents the western boundary current from intruding west of the continental slope near the Nansei Islands. The current over the slope region changes its direction from winter to summer due to anticyclonic eddy activity related to the joint effect of baroclinicity and bottom topography. The deep northeastward current over the slope in winter is canceled in summer by the eddy activity so that the interaction between the continental slope and the current is much reduced. Since the same eddy activity intensifies the Kuroshio recirculation, the Kuroshio transport across the PN line in the East China Sea is increased in summer.

The present study demonstrates that comparing model results with observations requires a model resolution suitable enough to resolve locations of observations as well as essential dynamics related to the interaction between baroclinic ocean currents and bottom topography.

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Jing-Jia Luo and Toshio Yamagata

Abstract

Using outputs of a high-resolution ocean general circulation model, upper-ocean heat content budget and mixed layer heat budget are analyzed to investigate the reason for the 1988–89 decadal warming event in the northern North Pacific. The model reproduces realistic upper-ocean temperature changes in comparison with observational data. This analysis suggests that the horizontal mean geostrophic advection of anomalous temperature is the main contributor to the heat content increase around 1988–89, and surface heat flux forcing is the main contributor to increasing mixed layer temperature. The anomalous geostrophic advection of mean temperature plays a negative role in the increase of both the upper-ocean heat content and mixed layer temperature in midlatitudes around 1988–89. Another negative contribution to the mixed layer temperature increase is provided by the Ekman advection. In the Kuroshio Extension region, the warm upper-ocean heat content anomaly appears in 1987–88, in which the mean geostrophic advection also plays a dominant role. South of Japan the decadal warming appears even earlier, around 1985–86. The anomalous Kuroshio transport shows a decadal decreasing trend since the early 1980s and therefore cannot explain the late 1980s warming event in midlatitudes. The 1988–89 event is found to be closely linked with the decadal change of the Kuroshio path south of Japan. It is found that subtropical Rossby waves may influence the decadal temperature changes south of Japan.

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Jing-Jia Luo and Toshio Yamagata

Abstract

Using outputs of a high-resolution ocean general circulation model, upper-ocean heat content budget and mixed layer heat budget are analyzed to investigate the reason for the 1988–89 decadal warming event in the northern North Pacific. The model reproduces realistic upper-ocean temperature changes in comparison with observational data. This analysis suggests that the horizontal mean geostrophic advection of anomalous temperature is the main contributor to the heat content increase around 1988–89, and surface heat flux forcing is the main contributor to increasing mixed layer temperature. The anomalous geostrophic advection of mean temperature plays a negative role in the increase of both the upper-ocean heat content and mixed layer temperature in midlatitudes around 1988–89. Another negative contribution to the mixed layer temperature increase is provided by the Ekman advection. In the Kuroshio Extension region, the warm upper-ocean heat content anomaly appears in 1987–88, in which the mean geostrophic advection also plays a dominant role. South of Japan the decadal warming appears even earlier, around 1985–86. The anomalous Kuroshio transport shows a decadal decreasing trend since the early 1980s and therefore cannot explain the late 1980s warming event in midlatitudes. The 1988–89 event is found to be closely linked with the decadal change of the Kuroshio path south of Japan. It is found that subtropical Rossby waves may influence the decadal temperature changes south of Japan.

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Gareth P. Williams and Toshio Yamagata

Abstract

We examine the relevance to Jupiter's atmosphere of the solitary vortices favored at scales intermediate to those of the quasi-geostrophic (QG) and planetary-geostrophic motions. Horizontal divergence plays a crucial role in the intermediate-geostrophic (IG) dynamics and leads to asymmetries in vortex behavior; in partcular, anticyclonic vortices are generally more stable than cyclonic vortices when the mean flow is weak or westerly. The IG vortices always propagate westward at close to the planetary long-wave speed, regardless of the mean zonal flow. Meridional shear influences only secondary aspects of vortex behavior. Although governed by a form of the Korteweg-deVries (KdV) equation, vortex encounters produce coalescence not soliton behavior.

Jupiter's Great Red Spot and Large Ovals appear to be in, or close to, an IG balance while the Small Ovals lie in a QG balance. The stability of anticyclonic IG vortices may explain why most of Jupiter's super-eddies prefer anticyclonic spin. Solutions to the shallow water (SW) equations, using Jovian parameters, show that an IG vortex with the scale and environment of the Great Red Spot has great longevity and that such a vortex may originate in a weak barotropic instability of the zonal currents. Strong barotropic instability on the IG scale differs from its counterpart on the QG scale and produces multiple, steep, isolated vortices resembling the Large Ovals.

Equations are derived for all forms of geostrophic balance (three basic classes, ten subsets) to investigate the uniqueness of the IG system. Numerical studies use the IG β-plane equation to examine basic modal properties and the full SW equations to examine the Jovian eddies.

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Toshio Yamagata and Shin-Ichiro Umatani

Abstract

The bimodality of the Kuroshio path south of Japan is presented from a new viewpoint of direct interaction of current with local coastal geometry. By solving the barotropic quasi-geostrophic equation in a channel with steplike coastal geometry, we demonstrate that the model Kuroshio can actually show the localized, bimodal behavior for a reasonable range of inlet current speed. The amplitude of the large meander is approximately given by 2U max/β. In contrast to all “nonlocal” model results, our local coherent structures have nothing to do with the basin-size geometry such as Kyushu and the Izu-Ogasawara Ridge. In general, the present study suggests that even a small feature of coastline geometry may trigger a big change in a nearshore current.

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