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Motoki Nagura and Yukio Masumoto

Abstract

A wake due to islands in background zonal flow has been observed in the equatorial Pacific Ocean. This study detects and examines a wake due to the Maldives in the eastward Wyrtki jet in the Indian Ocean. Observations by acoustic Doppler current profilers deployed east of the Maldives show semiannual variability in cross-equatorial currents, which cannot be explained by annual monsoonal wind forcing. Output from a high-resolution ocean general circulation model (OGCM) shows that the semiannual current variability is a part of a stationary wavelike pattern of meridional currents, which appears east of the Maldives concurrently with the eastward Wyrtki jet. Idealized numerical experiments are conducted using a 1.5-layer model, in which an equatorial jet driven by wind forcing or steady inflow impinges islands that are similar to the Maldives in shape. The results show the meandering of the equatorial eastward jet east of the model islands, and the resulting cross-equatorial currents have a similar pattern compared to those in the OGCM simulation. The momentum budget analysis obtained from the OGCM simulation and the layer model experiments shows a significant contribution of momentum advection to the generation of the wake. Also, the layer model experiments exhibit that the wake is essentially stationary; its zonal wavelength becomes larger when the eastward jet is stronger, and the wake is absent when the equatorial jet is westward. The similarity of the wake in the equatorial jet to stationary damped Rossby waves in the quasigeostrophic barotropic ocean model is discussed.

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TAKURO MATSUTA and YUKIO MASUMOTO

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The non-locality of eddy-mean flow interactions, which appears explicitly in the modified Lorentz diagram as a form of the interaction energy, and its link to other estimation methods are revisited, and a new formulation for the potential enstrophy is proposed. The application of these methods to the Kuroshio extension region suggests that the combined use of energy analysis with other methods, including the potential enstrophy diagram, provides more comprehensive understandings for the eddy-mean flow interactions in the limited region. It is shown that the interaction energy is transported from the nearshore and upstream regions to the downstream region in the form of the interaction energy flux, causing acceleration of the Kuroshio extension jet in the downstream region. The potential enstrophy diagram indicates that the eddy field decelerates (accelerates) the jet in the nearshore (downstream) region, which is a consistent result with the energy analysis. It turns out that the interaction potential enstrophy flux is radiated from a region of the eddy kinetic energy maximum towards the upstream region, which is the opposite direction from the interaction energy flux. The interaction potential enstrophy flux originated from this eddy kinetic energy maximum region also convergences near the center of the northern recirculation gyre of the Kuroshio extension region and tends to stabilize the structures of the recirculation gyre. Together with the energy analysis that indicates the eddy field accelerates the northeastern part of the recirculation gyre through the local interactions, the present analyses support the arguments on the eddy-driven northern recirculation gyre.

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Motoki Nagura, Yukio Masumoto, and Takanori Horii

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This study examines heat advection due to mixed Rossby gravity waves in the equatorial Indian Ocean using moored buoy observations at (0°, 80.5°E) and (0°, 90°E) and an ocean general circulation model (OGCM) output. Variability associated with mixed Rossby gravity waves is defined as that at periods of 10–30 days, where both observations and the OGCM results show high energy in meridional velocity and meridional gradient of temperature. The 10–30-day variability in meridional velocity causes convergence of heat flux onto the equator, the net effect of which amounts to 2.5°C month−1 warming at the depth of the thermocline. Detailed analysis shows that the wave structure manifested in temperature and velocity is tilted in the xz plane, which causes the phase lag between meridional velocity and meridional temperature gradient to be a half cycle on the equator and results in sizable thermocline warming. An experiment with a linear continuously stratified model shows that the contributions of many baroclinic modes, and the right zonal wavelength of wind forcing, are essential in generating the correct wave structure. It is also shown that contributions of mixed Rossby gravity waves to cross-equatorial heat transport are negligible, as temperature variability associated with this wave mode has a node on the equator.

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Tomomichi Ogata, Motoki Nagura, and Yukio Masumoto

Abstract

A possible formation mechanism of mean subsurface upwelling along the equator in the Indian Ocean is investigated using a series of hierarchical ocean general circulation model (OGCM) integrations and analytical considerations. In an eddy-resolving OGCM with realistic forcing, mean vertical velocity in the tropical Indian Ocean shows rather strong upwelling, with its maximum on the equator in the subsurface layer below the thermocline. Heat budget analysis exhibits that horizontal and vertical heat advection by deviations (i.e., due to deviations of velocity and temperature from the mean) balances with vertical advection caused by mean equatorial upwelling. Horizontal heat advection is mostly associated with intraseasonal variability with periods of 3–91 days, while contributions from longer periods (>91 days) are small. Sensitivity experiments with a coarse-resolution OGCM further demonstrate that such mean equatorial upwelling cannot be reproduced by seasonal forcing only. Adding the intraseasonal wind forcing, especially meridional wind variability with a period of 15 days, generates significant mean subsurface upwelling on the equator. Further experiments with idealized settings confirm the importance of intraseasonal mixed Rossby–gravity (MRG) waves to generate mean upwelling, which appears along the energy “beam” of the MRG wave. An analytical solution of the MRG waves indicates that wave-induced temperature advection caused by the MRG waves with upward (downward) phase propagation results in warming (cooling) on the equator. This wave-induced warming (cooling) is shown to balance with the mean equatorial upwelling (downwelling), which is consistent with simulated characteristics in the OGCM experiments.

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Iwao Ueki, Nobuhiro Fujii, Yukio Masumoto, and Keisuke Mizuno

Abstract

For the purpose of climate research and forecasting the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) in the Indian Ocean has been planned. Development of RAMA has been gradually accelerated in recent years as a multinational effort. To promote RAMA the authors have developed a small size buoy system, which uses the slack-line mooring method, intended for the easy handling of maintenance on a relatively small vessel. The authors have also conducted a field experiment of the simultaneous deployment of new slack-line mooring and conventional taut-line mooring in the eastern Indian Ocean. This paper describes the performance of the newly developed buoy system, especially the data consistency against the taut-line mooring system, which is usually used for a tropical moored buoy array. Although the slack-line mooring method has the advantage of downsizing the total mooring system, it also has the disadvantage of having relatively large vertical shifts of installed sensors produced by a large migration of the surface buoy. To offset this disadvantage to a certain extent, a data reconstruction method has been developed and evaluated. Through the data comparison between both mooring systems, it is confirmed that the reconstructed data of the newly developed buoy can basically capture the same features as that observed with a conventional taut-line mooring system. The maximum mean difference of −0.16°C and the maximum root-mean-square (RMS) difference of 0.58°C for temperature appeared within the thermocline layer, whereas the maximum mean difference of 0.02 and the maximum RMS difference of 0.09 for salinity appeared within the mixed layer. Considering a distance of 8 n mi between the two moorings, these values are acceptable for regarding that the two moorings can observe same feature. Results of this study support the introduction of various types of mooring systems for a multinational approach of RAMA and contribute to the further progress of RAMA, climate research, and forecasting.

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Takeshi Doi, Tomoki Tozuka, Hideharu Sasaki, Yukio Masumoto, and Toshio Yamagata

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Using outputs from a high-resolution OGCM, seasonal and interannual variations of the Angola Dome (AD) are revisited. Although the AD was previously considered to be one large cold tongue extending from the West African coast, it is shown that two cold domes exist. These two domes have remarkably different mechanisms for their seasonal variation. The weak dome, whose center is located at 6°S, 1°E, develops from May to September owing to the divergence of heat transport associated with upwelling. The strong dome, on the other hand, extends from the west coast of Africa between 20° and 15°S, and develops from April to August by the surface heat flux. The interannual variation of the weak dome is strongly influenced by the Atlantic Niño. An unusual relaxation of easterly wind stress in the central equatorial Atlantic Ocean associated with the Atlantic Niño triggers second baroclinic downwelling equatorial Kelvin waves, which propagate eastward along the equator and poleward along the coast after reaching the African coast as coastal Kelvin waves. Then, downwelling Rossby waves radiate away from the coast and cause significant warming in the weak dome region. The interannual variation of the South Equatorial Undercurrent may be associated with that of the AD; its transport decreases by 0.6 Sv, and its core shifts equatorward by 0.2° when the AD is anomalously weak.

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Jing-Jia Luo, Swadhin K. Behera, Yukio Masumoto, and Toshio Yamagata

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Surface air temperature (SAT) over the globe, particularly the Northern Hemisphere continents, has rapidly risen over the last 2–3 decades, leading to an abrupt shift toward a warmer climate state after 1997/98. Whether the terrestrial warming might be caused by local response to increasing greenhouse gas (GHG) concentrations or by sea surface temperature (SST) rise is recently in dispute. The SST warming itself may be driven by both the increasing GHG forcing and slowly varying natural processes. Besides, whether the recent global warming might affect seasonal-to-interannual climate predictability is an important issue to be explored. Based on the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) climate prediction system in which only observed SSTs are assimilated for coupled model initialization, the present study shows that the historical SST rise plays a key role in driving the intensified terrestrial warming over the globe. The SST warming trend, while negligible for short lead predictions, has substantial impact on the climate predictability at long lead times (>1 yr) particularly in the extratropics. The tropical climate predictability, however, is little influenced by global warming. Given a perfect warming trend and/or a perfect model, global SAT and precipitation could be predicted beyond two years in advance with an anomaly correlation skill above ∼0.6.

Without assimilating ocean subsurface observations, model initial conditions show a strong spurious cooling drift of subsurface temperature; this is caused by large negative surface heat flux damping arising from the SST-nudging initialization. The spurious subsurface cooling drift acts to weaken the initial SST warming trend during model forecasts, leading to even negative trends of global SAT and precipitation at long lead times and hence deteriorating the global climate predictability. Concerning the important influence of the subsurface temperature on the global SAT trend, future efforts are required to develop a good scheme for assimilating subsurface information particularly in the extratropical oceans.

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Yushi Morioka, J. V. Ratnam, Wataru Sasaki, and Yukio Masumoto

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Distinct pattern of interannual variability in sea surface temperature (SST) in the South Pacific [i.e., the South Pacific subtropical dipole (SPSD)] is examined using outputs from a coupled general circulation model. The SPSD appears as the second empirical orthogonal function (EOF) mode of the SST anomalies in the South Pacific and is associated with a northeast–southwest-oriented dipole of positive and negative SST anomalies in the central basin. The positive and negative SST anomaly poles start to develop during austral spring, reach their peak during austral summer, and gradually decay afterward. Close examination of mixed-layer heat balance yields that the SST anomaly poles develop mainly because warming of the mixed layer by shortwave radiation is modulated by the anomalous mixed-layer thickness. Over the positive (negative) pole, the mixed layer becomes thinner (thicker) than normal and acts to enhance (reduce) the warming of the mixed layer by climatological shortwave radiation. This thinner (thicker) mixed layer may be related to the suppressed (enhanced) evaporation associated with the overlying sea level pressure (SLP) anomalies. Weaker-than-normal surface wind also contributes to the thinner mixed layer in the case of the positive pole. Furthermore, the SLP anomalies are linked with the geopotential height anomalies in the upper troposphere and are associated with a stationary Rossby wave pattern along the westerly jet in the midlatitudes. This suggests that the SLP anomalies that generate the SPSD are not locally excited but remotely induced signals.

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Yushi Morioka, Koutarou Takaya, Swadhin K. Behera, and Yukio Masumoto

Abstract

The interannual variations in the summertime Mascarene high have great impacts on the southern African climate as well as the sea surface temperature (SST) in the southern Indian Ocean. A set of coupled general circulation model (CGCM) experiments are performed to examine a role of the interannual SST variability in the southern Indian Ocean on the summertime Mascarene high variability. The dominant interannual variability in the summertime Mascarene high shows the strengthening (weakening) in its southern part throughout the austral summer (December–February). However, in the experiment where the interannual SST variability in the southern Indian Ocean is suppressed, the strengthening (weakening) of the Mascarene high in its southern part does not persist until February. Also, the Mascarene high variability and its associated SST anomalies in December and January are found to increase (decrease) the southern African rainfall via more (less) moisture supply from the southern Indian Ocean. The Mascarene high variability is actually associated with a meridional dipole of positive and negative SST anomalies, which in turn produces that of the meridional SST gradient anomaly. This causes a southward (northward) shift of the storm tracks and hence the westerly jet, favoring the strengthening (weakening) of the Mascarene high in its southern part. This local ocean–atmosphere feedback effectively operates in February, when the meridional dipole of the SST anomalies reaches the maximum. These results provide new insight into the important role of the local SST variability in the summertime Mascarene high variability and hence the southern African climate.

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Hideharu Sasaki, Bunmei Taguchi, Nobumasa Komori, and Yukio Masumoto

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Local air–sea interactions over the high sea surface temperature (SST) band along the Hawaiian Lee Countercurrent (HLCC) are examined with a focus on dynamical feedback of SST-induced wind stress to the ocean using the atmosphere–ocean coupled general circulation model (CGCM). A pair of ensemble CGCM simulations are compared to extract the air–sea interactions associated with HLCC: the control simulations and other simulations, the latter purposely eliminating influences of the high SST band on the sea surface flux computations in the CGCM. The comparison reveals that oceanic response to surface wind convergence and positive wind stress curl induced by the high SST band increases (decreases) the HLCC speed in the southern (northern) flank of the HLCC. The HLCC speed changes are driven by the Ekman suction associated with positive wind stress curl over the warm HLCC via the thermal wind balance. The HLCC speed increase is more significant than its decrease. This dynamical feedback is likely to be important to sustain the extension of the HLCC far to the west. The heat budget analysis confirms that advection of warm water from the west associated with this significant current speed increase plays a role in the southward shift of the HLCC axis. The dynamical feedback with the HLCC speed increase can potentially amplify the seasonal and interannual variations of HLCC.

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