Takashi Kagimoto

Seasonal Transport Variations of the Kuroshio: An OGCM Simulation

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 s21), 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.

Simulated Multiscale Variations in the Western Tropical Pacific: The Mindanao Dome Revisited

Abstract Using a high-resolution OGCM result, multiscale variations of the Mindanao Dome (MD) are discussed. The MD is generated by local Ekman upwelling as the positive curl of the Asian winter monsoon increases over the western tropical Pacific, as discussed in the literature. It is shown, however, that the MD decays owing to the Pacific basinwide annual cycle; the warm anomaly that propagates from the eastern tropical Pacific plays an important role in the attenuation of the MD. Also, the negative wind stress curl associated with the northward shift of the ITCZ in the eastern Pacific and the easterly component of the trade winds in the central Pacific strengthens the warm anomaly. The interannual variation of the MD is governed by variations in both the Ekman upwelling forced locally and the downwelling forced remotely in the east. At its northern fringe, the MD is attenuated by the westward propagating warm eddies on the North Equatorial Current, which are triggered by a negative wind stress curl and ...

Decadal Sea Level Variability in the South Pacific in a Global Eddy-Resolving Ocean Model Hindcast

Sea level variability and related oceanic changes in the South Pacific from 1970 to 2003 are investigated using a hindcast simulation of an eddy-resolving ocean general circulation model (OGCM) for the Earth Simulator (OFES), along with sea level data from tide gauges since 1970 and a satellite altimeter since 1992. The first empirical orthogonal function mode of sea level anomalies (SLAs) of OFES exhibits broad positive SLAs over the central and western South Pacific. The corresponding principal component indicates roughly stable high, low, and high SLAs, separated by a rapid sea level fall in the late 1970s and sea level rise in the late 1990s, consistent with tide gauge and satellite observations. These decadal changes are accompanied by circulation changes of the subtropical gyre at 1000-m depth, and changes of upper-ocean zonal current and eddy activity around the Tasman Front. In general agreement with previous related studies, it is found that sea level variations in the Tasman Sea can be explained by propagation of long baroclinic Rossby waves forced by wind stress curl anomalies, if the impact of New Zealand is taken into account. The corresponding atmospheric variations are associated with decadal variability of El Nino– Southern Oscillation (ENSO). Thus, decadal sea level variability in the western and central South Pacific in the past three and half decades and decadal ENSO variability are likely to be connected. The sea level rise in the 1990s, which attracted much attention in relation to the global warming, is likely associated with the decadal cooling in the tropical Pacific.

A subsurface countercurrent along the east coast of Luzon

A hydrographic section in the region east of Luzon was repeated 14 times during the period from 1986 to 1991. The data revealed the existence of a subsurface countercurrent located on the shoreward side of the Kuroshio with its upper boundary at about 500 m. The countercurrent, which should be called the Luzon Undercurrent (LUG), was only about 50 km wide, which is comparable to the baroclinic radius of deformation. Despite considerable variabilities both in velocity profile and intensity, the LUC appears to be a permanent feature. Over the period of observations, the maximum speed in the LUC calculated from the mean temperature and salinity by assuming geostrophy (relative to 2500 db) was 7 cm s(-1) at about 700 m and its mean geostrophic volume transport was 3.6 Sv (1 Sv = 10(6) m(3) s(-1)). About 28% of this transport was composed of the low-salinity North Pacific Intermediate Water (NPIW) advected to the south along the coast of Luzon

Interannual-to-Decadal Variability in the Oyashio and Its Influence on Temperature in the Subarctic Frontal Zone: An Eddy-Resolving OGCM Simulation

Abstract Output of an eddy-resolving OGCM simulation is used to investigate mechanisms for interannual-to-decadal variability in the Oyashio and its influence on the subarctic frontal zone in the western North Pacific. Lag correlation analysis reveals that positive anomalies both in basin-scale wind stress curl and in local Ekman pumping can intensify the southward Oyashio almost simultaneously via barotropic and baroclinic Rossby wave propagations, respectively. The Oyashio strength can also be influenced by anomalous Ekman pumping that is exerted in the western portion of the basin through the baroclinic wave propagation with the lag of 3 yr, which appears to arise from a periodicity in the wind field. The intensification of the Oyashio is accompanied by negative anomalies both in the sea surface temperature and height off of Hokkaido Island of Japan and is followed by their eastward development along the southern branch of the Oyashio Extension and associated subarctic frontal zone in association with ...

Bifurcation of the East India Coastal Current east of Sri Lanka

The East India Coastal Current (EICC) flows equatorward during October–December carrying low salinity water from the Bay of Bengal en route. Using results from a high resolution ocean general circulation model, satellite altimeter data, Argo float profiles and ocean color images we show that the EICC bifurcates east of Sri Lanka. One part continues along the coast of Sri Lanka but the major part of the EICC, called here as the East Sri Lanka Jet (ESLJ) flows eastward into the Bay of Bengal. As a result of this bifurcation, there is offshore transport of chlorophyll a rich low salinity water from the coast of Sri Lanka. Altimeter data from 1993–2004 show that the bifurcation occurred every year except during the Indian Ocean Dipole (IOD) years of 1994 and 1997. The bifurcation occurs when an anticyclonic eddy that propagates westward ahead of a downwelling Rossby wave front impinges on the Sri Lanka coast. This new finding suggests that the main route of the low salinity water from the Bay of Bengal into the southeastern Arabian Sea may not be along the coast around Sri Lanka but through the Winter Monsoon Current.

Intraseasonal eddies in the Sulawesi Sea simulated in an ocean general circulation model

The intraseasonal variability associated with mesoscale eddies in the Sulawesi Sea simulated in a high resolution ocean general circulation model is described in detail. The cyclonic eddies, with a diameter of about 400 km, are generated at the entrance of the Sulawesi Sea between the Mindanao and the Halmahera Islands with 40 days interval. They are associated with a high speed (> 20 cm/s) down to 1000 m level. The anticlockwise circulation in the Sulawesi Sea, reported so far in both models and observations, may be a long time-averaged image of the above energetic eddies. The intraseasonal eddies significantly affect the volume transport through passages in the northern part of the Indonesian archipelago. The intraseasonal transport variation, however, is highly damped within the Indonesian seas in the present model.

Parallel Architecture and its Performance of Oceanic Global Circulation Model Based on MOM3 to be Run on the Earth Simulator

Abstract. In this study, we will present latest results from evaluation of our computational optimized code OFES based on MOM3 to run on the Earth Simulator. O ( 10 ) years integration with 0.1 degree for horizontal will be one of the first attempts to solve the largest scale scientific simulations. In order to keep the flexibility of MOM3 from points ofscientific view, we consider two types of parallel architectures due to the difference from resolution to represent physical performance in oceanic phenomena. One is, for the relative lower resolved phenomena with longer integration time, characterized by using shared memory system for improvement parallel performance within a single node composed of 8PEs. To achieve the most efficiency parallel computation inside of a node, we modified MPI library into assembly coded library. Another parallel computational improvement, for case of ultra high resolution of 0.1 degree for horizontal, employed by only communication with MPI library, which is not distinct from inside or outside of node. In this case, we took into account a mount of computation in halo region to attain to huge parallelized performance. As the results, the computational efficiency has been achieved high computational speed with more about 500 times performance comparing CPU time on a single node. The load imbalance was not recognized. In this paper, we will indicate optimization strategy for both two cases to attain target performance and results from measurement on the Earth Simulator. Experiments for ultra high resolution case carried out by using 188 nodes, which is composed of 1500 PEs.

Simulated Circulation in the Indonesian Archipelago from a High Resolution Global Ocean General Circulation Model on the Numerical Wind Tunnel

To represent both the basin-scale circulation in the ocean and the local small-scale variations at the same time, a high resolution global ocean general circulation model (OGCM) is essential. Recent progress in parallel computing techniques together with the high ability of computer itself makes it possible to use such a high resolution OGCM for climate variability issues. We have developed and are using a global OGCM based on the Princeton Ocean Model, with 1/6 degrees horizontal grid resolutions. This model has been running on the Numerical Wind Tunnel at National Aerospace Laboratory in Japan, using 64 PEs with 256 MB memory for each PE, and is successful in reproducing very realistic variations of the currents and tracer fields. In the present study, details of the model and some examples of those variations from 16-year calculation with climatological forcing fields are introduced.