Tomoki Tozuka

On the role of the Agulhas system in ocean circulation and climate

The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.

Can Luzon Strait transport play a role in conveying the impact of ENSO to the South China sea

The Luzon Strait transport (LST) from the Pacific into the South China Sea (SCS) is examined using results from a high-resolution ocean general circulation model. The LST from the model has a mean value of 2.4 Sv (Sv ( 106 m3 s 21) and reaches its seasonal maximum (6.1 Sv westward) in winter and seasonal minimum (0.9 Sv eastward) in summer. Both the annual mean and seasonal variation of LST compare favorably with earlier observations. On an interannual time scale, LST tends to be higher during El Nino years and lower during La Nina years, with its maximum (minimum) leading the mature phase of El Nino (La Nina) by 1 month. The interannual variation of LST appears to be oppositely phased with the Kuroshio transport east of Luzon, indicating a possible nonlinear hysteresis of the Kuroshio as a driving mechanism of LST. For the annual average, water leaving the SCS in the south is of higher temperature than that with LST, thus producing a cooling advection in the upper 405 m equivalent to a surface heat flux of 2 19 Wm 22. Most of this cooling advection is balanced by the atmospheric heating (17 W m22). From late spring to early fall, surface heat flux is the primary heating process; only a small part of the heat content change can be explained by heat advection. But, in winter, heat advection seems to be the only important process responsible for the cooling in the upper layer of the SCS. The interannual variation of the upper-layer heat content has a strong signature of ENSO, cooling in the development of El Nino and warming in the development of La Nina. An oceanic connection is revealed, in which LST seems to be a key process conveying the impact of the Pacific ENSO into the SCS.

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 Modulations of the Indian Ocean Dipole in the SINTEX-F1 Coupled GCM

The decadal variation in the tropical Indian Ocean is investigated using outputs from a 200-yr integration of the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) ocean– atmosphere coupled model. The first EOF mode of the decadal bandpass- (9–35 yr) filtered sea surface temperature anomaly (SSTA) represents a basinwide mode and is closely related with the Pacific ENSOlike decadal variability. The second EOF mode shows a clear east–west SSTA dipole pattern similar to that of the interannual Indian Ocean dipole (IOD) and may be termed the decadal IOD. However, it is demonstrated that the decadal air–sea interaction in the Tropics can be a statistical artifact; it should be interpreted more correctly as decadal modulation of interannual IOD events (i.e., asymmetric or skewed occurrence of positive and negative events). Heat budget analysis has revealed that the occurrence of IOD events is governed by variations in the southward Ekman heat transport across 15°S and variations in the Indonesian Throughflow associated with the ENSO. The variations in the southward Ekman heat transport are related to the Mascarene high activities.

Seasonal Variation of the Seychelles Dome

Abstract Using an ocean general circulation model (OGCM), seasonal variation of the Seychelles Dome (SD) is investigated for the first time. The SD is an oceanic thermal dome located in the southwestern Indian Ocean, and its influence on sea surface temperature is known to play an important role in the Indian monsoon system. Its seasonal variation is dominated by a remarkable semiannual cycle resulting from local Ekman upwelling. This semiannual nature is explained by different contributions of the following two components of the Ekman pumping: one term that is proportional to the planetary beta and the zonal wind stress and the other term that is proportional to the wind stress curl. The former is determined by the seasonal change in the zonal component of the wind stress vector above the SD; it is associated with the Indian monsoon and causes downwelling (upwelling) during boreal summer (boreal winter). The latter, whose major contribution comes from the meridional gradient of the zonal wind stress, als...

Heat balance and regime shifts of the mixed layer in the Kuroshio Extension

Abstract We investigated the seasonal variations of the mixed layer depth and temperature in the Kuroshio Extension region (145–180°E, 30–36°N), and studied the causes of the mixed layer depth and temperature ‘regime shifts’ which occurred in the late 1980s, using upper ocean thermal and heat flux datasets incorporated with a bulk mixed layer model. The mixed layer from fall to winter is cooled by the net surface heat flux, the Ekman transport and the entrainment, and warmed by the horizontal heat convergence resulting from the Kuroshio heat advection. The mixed layer depth is controlled by the entrainment and the horizontal transport divergence/convergence which act to slow the mixed layer deepening from fall to early winter and then in winter deepen the mixed layer. The entrainment velocity is significantly influenced by the temperature difference between the mixed layer and the layer below. The mixed layer shift from its deep to shallow phase occurred in 1985. This shift is preceded the SST-shift in 1988 by three years, and was caused by the horizontal transport divergence anomaly. The horizontal heat convergence of the Kuroshio Extension caused the SST-shift in 1988, whereas its anomaly had been already positive since 1983. The delay from 1983 to 1987 can be attributed to the effects of the negative fall–SST anomaly, stronger surface heat flux and Ekman cooling and the shoaling of the mixed layer depth. It is suggested that the Kuroshio current system plays a major role in forcing the SST-shift and thus the subsequent climate regime shift.

On the Growth and Decay of the Subtropical Dipole Mode in the South Atlantic

AbstractUsing observational data and outputs from an ocean general circulation model, the growth and decay of the South Atlantic subtropical dipole (SASD) are studied. The SASD is the most dominant mode of interannual variability in the South Atlantic Ocean, and its sea surface temperature (SST) anomaly shows a dipole pattern that is oriented in the northeast–southwest direction. The positive (negative) pole develops because the warming of the mixed layer by the contribution from the climatological shortwave radiation is enhanced (suppressed) when the mixed layer is thinner (thicker) than normal. The mixed layer depth anomaly over the positive (negative) pole is due to the suppressed (enhanced) latent heat flux loss associated with the southward migration and strengthening of the subtropical high. During the decay phase, since the temperature difference between the mixed layer and the entrained water becomes anomalously large (small) as a result of the positive (negative) mixed layer temperature anomaly, ...

On the Ningaloo Niño/Niña

Abstract Using both observational and reanalysis data, evolution processes of a regional climate phenomenon off Western Australia named recently “Ningaloo Niño (Niña)” are studied in detail. It is also shown that the Ningaloo Niño (Niña) has significant impacts on the precipitation over Australia. The Ningaloo Niño (Niña), which is associated with positive (negative) sea surface temperature (SST) anomalies and atmospheric anomalies off the western coast of Australia, peaks during austral summer and is classified into two types based on the difference in the evolution process. The first type called a locally amplified mode develops through an intrinsic unstable air–sea interaction off the western coast of Australia; an anomalous cyclone (anticyclone) generated by positive (negative) SST anomalies forces northerly (southerly) alongshore wind anomalies, which induce coastal downwelling (upwelling) anomalies, and enhance the positive (negative) SST anomalies further. The second type called a non-locally amplified mode is associated with coastally trapped waves originating in either the western tropical Pacific, mostly related to El Niño/Southern Oscillation, or the northern coast of Australia. Positive (negative) SST anomalies in both modes are associated with an anomalous low (high) off the western coast of Australia. The sea level pressure (SLP) anomalies in the locally amplified mode are regionally confined with a cell-like pattern and produce a sharp offshore pressure gradient along the western coast of Australia, whereas those in the non-locally amplified mode tend to show a zonally elongated pattern. The difference is found to be related to conditions of the continental SLP modulated by the Australian summer monsoon and/or the Southern Annular Mode.

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.

The Atlantic Meridional Mode and Its Coupled Variability with the Guinea Dome

Abstract Using an ocean–atmosphere coupled general circulation model, air–sea interaction processes associated with the Atlantic meridional mode are investigated from a new viewpoint of its link with the Guinea Dome in the northern tropical Atlantic. The subsurface thermal oceanic dome develops off Dakar from late spring to late fall owing to wind-induced Ekman upwelling. Its seasonal evolution is due to surface wind variations associated with the northward migration of the intertropical convergence zone (ITCZ). Since the upwelling cools the mixed layer in the Guinea Dome region during summer, it is very important to reproduce its variability in order to simulate the sea surface temperature (SST) there. During the preconditioning phase of the positive (negative) Atlantic meridional mode, the dome is anomalously weak (strong) and the mixed layer is anomalously deep (shallow) there in late fall. This condition reduces (enhances) the sensitivity of the mixed layer temperature to the climatological atmospheri...