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Coastal upwelling along the southwestern coast of Sumatra is a seasonal upwelling that occurs in areas of high sea surface temperature and abundant precipitation at the southeastern edge of the Ind...

To understand the coastal upwelling system along the southern coast of Java, we investigated ocean temperature and salinity obtained from an Argo float. In 2008, a positive Indian Ocean Dipole (IOD) event began to develop in early May and anomalously cool SST developed around south of Java from May to September. During the peak of the IOD, an Argo float successfully observed vertical structure of temperature and salinity within 90 km from Java. The float observed two intraseasonal-scale temperature cooling events in July and August, with significant upward movements of the thermocline more than 90 m. Concurrent with the signals, anomalous southeasterly alongshore winds, lowering of local SST and sea level, and upward expansion of high-salinity water were also observed. During the event in August, vertical velocity estimated by the anomalous wind stress agreed well with the observations. These results indicate that the Argo float observed the coastal upwelling, which was enhanced by the 2008 positive IOD, along the southern coast of Java.

The Triangle Trans‐Ocean Buoy Network (TRITON) project by the Japan Agency for Marine-Earth Science and Technology began with deployment in the western tropical Pacific Ocean in 1998 and has shifted to steady, long-term observations since 1999. After on-site inter-comparison with the Autonomous Temperature Line Acquisition System mooring system of the Tropical Atmosphere and Ocean (TAO) array by the National Oceanic and Atmospheric Administration, the TRITON array became the international TAO/TRITON array in 2000 as a key component of the Global Ocean and Climate Observing Systems. The TAO/TRITON array took over from the TAO array, which was developed during the Tropical Ocean and Global Atmosphere program (1985–1994), and replaced the western part of TAO with new additional real-time measurements of salinity and ocean currents. In 2001, two TRITON moorings were deployed in the eastern Indian Ocean for capturing the eastern pole of the Indian Ocean Dipole. From this initiative, the Indian Ocean Observing System (IndOOS) was designed, and the Indian Ocean mooring array (Research Moored Array for Africa–Asian–Australian Monsoon Analysis and Prediction) was developed as a key component of IndOOS. In this paper, 15 years of progress in the TRITON project in the western Pacific and eastern Indian Oceans is reviewed with regards to scientific outcomes, technological development, and collaborations with international and domestic partners. Future directions for sustainable observation in the Pacific and Indian Oceans are also discussed.

The observed variability of zonal currents (ZC) at the Equator, 90°E shows a strong seasonal cycle in the near-surface 40–350 m water column with periodic east–west reversals most pronounced at semiannual frequency. Superposed on this, a strong intraseasonal variability of 30–90 day periodicity is also prominently seen in the near-surface layer (40–80 m) almost throughout the year with the only exception of February–March. An eastward flowing equatorial undercurrent (EUC) is present in the depth range of 80–160 m during March–April and October–November. The observed intraseasonal variability in the near-surface layer is primarily determined by the equatorial zonal westerly wind bursts (WWBs) through local frictional coupling between the zonal flow in the surface layer and surface zonal winds and shows large interannual variability. The eastward flowing EUC maintained by the ZPG set up by the east–west slope of the thermocline remotely controlled by the zonal wind (ZW) and zonally propagating wave fields also shows significant interannual variability. This observed variability on interannual time scales appears to be controlled by the corresponding variability in the alongshore winds off the Somalia coast during the preceding boreal winter, the ZW field along the equator, and the associated zonally propagating Kelvin and Rossby waves. The salinity induced vertical stratification observed in the near-surface layer through barrier layer thickness (BLT) effects also shows a significant influence on the ZC field on intraseasonal time scale. Interestingly, among all the 8 years (2001–2008), relatively weaker annual cycle is seen in both ZC in the 40–350 m water column and boreal spring sea surface temperature (SST) only during 2001 and 2008 along the equator caused through propagating wave dynamics.

Sea level variations along the coasts of Sumatra and Java were investigated to determine the coastal upwelling signal that is linked to local sea surface temperature (SST) variability. We used Indonesian tidal station data together with satellite SST data and atmospheric reanalysis data. The sea level variations along the southern coast of Java have a significant coherence with remote wind, local wind, and local SST variations, with an intraseasonal time scale of 20–50 days. Assuming that a coastal upwelling signal would appear as a sea level drop (SLD), we focused on intraseasonal-scale SLD events in the data. Significant upwelling signals are frequently observed during both the boreal summer and winter. To evaluate the impact of the coastal upwelling on local SST, we examined statistical relationships between sea level and SST variations. The results demonstrated that events that occurred during April–August were associated with local SST cooling. The horizontal distribution of the SST cooling was analogous with annual mean SST, suggesting the importance of intraseasonal-scale coastal upwelling in forming the climatic conditions of the southeastern tropical Indian Ocean.

We used more than two decades (1997–2019) of satellite-estimated phytoplankton chlorophyll-a concentration (Chl-a) during September–December time intervals to discern the spatial and temporal changes of phytoplankton biomass in the Indonesian Maritime Continent (IMC) associated with different climatic events. In the IMC coastal areas (except for the northern Malacca Strait) climate changes influenced Chl-a by varying rainfall/river discharge. Chl-a declined most notably during concurrent positive Indian Ocean Dipole (IOD) and El Nino events, whereas they increased most notably during concurrent negative IOD and La Nina events. In the open ocean areas of the IMC, climate changes influenced Chl-a by varying the occurrence of upwelling and downwelling. The dominant climate mode determining Chl-a shifted from the IOD in the eastern Indian Ocean to the El Nino/Southern Oscillation (ENSO) in the western Pacific Ocean. In the eastern Indian Ocean, the impact of the IOD on Chl-a was more than one order of magnitude greater than the impact of ENSO. The results implied that low-trophic-level marine organisms such as phytoplankton, high-trophic-level marine organisms such as fish, and biogeochemical cycles in the eastern Indian Ocean of the IMC may all be more affected by the IOD than by the ENSO.

A systematic salinity variation in the upper ocean may have an impact on air–sea interactions through a change in ocean stratification and hence on the oceanic response to atmospheric forcing. In this study, we evaluate the possible role of salinity variation in the oceanic response to intraseasonal atmospheric forcing, by investigating the ocean temperature and salinity variation in the eastern Indian Ocean. We primarily used data from three moored buoys located in an area with a large salinity gradient in the eastern equatorial Indian Ocean. Observed upper-layer salinity variation shows significant spectral peaks at intraseasonal time scales. Analysis indicates that surface zonal currents mainly produce the intraseasonal salinity variation through zonal advection with these currents induced by the Madden–Julian Oscillation (MJO). Composite analyses focusing on 35 significant MJO events during 2002–2012 confirmed that intraseasonal atmospheric forcing resulted in variations of net surface heat flux, mixed layer temperature and salinity, and mixed layer depth. We also found that a large salinity change could increase the amplitude of mixed layer temperature variation by changing the mixed layer depth. A possible process by which intraseasonal salinity variation could affect sea surface temperature is discussed.

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 x–z 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.

[1] Observations from moored subsurface acoustic Doppler current profilers (ADCPs) have revealed the structure of cross-equatorial meridional currents in the eastern equatorial Indian Ocean. Using observational data longer than 8 years, we present observations of the seasonal and interannual variation in the cross-equatorial currents in the 2000s. In line with earlier observations and numerical simulations, there were significant subsurface currents that displayed seasonal variation. The annual mean condition was dominated by currents during the boreal summer monsoon in which the Ekman transports are southward on both sides of the equator. The vertical structure of the mean meridional current displayed evidence of a shallow “equatorial roll,” consisting of northward surface flow and southward subsurface flow above the base of the isothermal mixed layer. The seasonal variation in upper 120 m meridional transport can be explained by wind-driven cross-equatorial Ekman/Sverdrup transport. Interannual variability was higher in the winter monsoon period than in the summer monsoon. The interannual variation could be related to the Pacific El Nino/Southern Oscillation (ENSO). ENSO-induced variation in the southeasterly trade wind could modulate meridional Sverdrup transport. Implications of these observational results for meridional heat transport are discussed.

[1] The cooperative Indian Ocean experiment on intraseasonal variability in the Year 2011 (CINDY2011) was conducted to capture atmospheric and oceanic characteristics of the Madden-Julian Oscillation (MJO) in the central Indian Ocean from late 2011 to early 2012. During CINDY2011, the research vessel (R/V) MIRAI stayed at 8°S, 80.5°E for two months during the special observing period (SOP). Intraseasonal convection associated with the MJO was organized in the central Indian Ocean in late October and late November during the SOP. In the middle of November, both sea surface temperature (SST) and mixed layer temperature decreased suddenly when cold low salinity water intruded into the upper layer around the R/V MIRAI. This intrusion was accompanied by a surface current change from southwestward to westward/west-northwestward associated with the passage of the annual oceanic downwelling Rossby wave. The mixed layer heat budget analysis shows that horizontal advection plays an important role in the abrupt cooling whereas the net surface heat flux cannot account for the cooling. This is an interesting result because the associated downwelling Rossby wave is usually considered to increase SST through a reduction of entrainment cooling. In addition, for the second MJO event convection was activated around 20 November over the central north and equatorial Indian Ocean but not in the south. It is suggested that the cooler surface waters (as seen at the location of the R/V MIRAI) tended to suppress the initial atmospheric convection, resulting in the lagged convective onset in the end of November over the central south Indian Ocean.

[1] Warm sea surface temperature (SST) anomalies of more than 1 °C occurred in the southeastern tropical Indian Ocean and peaked during August to October 2010. The anomalous SST warming was associated with the negative phase of the Indian Ocean dipole (IOD) phenomenon. In this study, observational data from a moored buoy were used together with satellite and atmospheric reanalysis data sets to clarify the processes that produced the anomalously warm SST in 2010. We focused on the location 5°S, 95°E where in situ measurements of more than 10 years by a moored buoy were available. The buoy observations captured the oceanic conditions related to the anomalous warming event of 2010. Heat balance analysis demonstrated that air-sea heat fluxes and horizontal heat advections mainly account for the mixed layer temperature variation. Reduced latent heat loss had a major role in producing the warm SST anomalies. Meridional heat advection also contributed to the warm SST anomalies where the southeastward surface current brought warmer water to the southeastern tropical Indian Ocean. The present results from the observations suggest that air-sea heat exchanges play an active role in the SST anomalies in the southeastern tropical Indian Ocean during the negative IOD. In contrast, in the case of cold SST anomalies in the eastern Indian Ocean during the positive IOD, ocean heat advections mainly control the mixed layer temperature variation. These results suggest that the IOD includes different feedback mechanisms in its positive and negative phases.

The Mirai Indian Ocean cruise for the Study of the Madden-Julian oscillation (MJO)-convection Onset (MISMO) was a field experiment that took place in the central equatorial Indian Ocean during October–December 2006, using the research vessel Mirai, a moored buoy array, and landbased sites at the Maldive Islands. The aim of MISMO was to capture atmospheric and oceanic features in the equatorial Indian Ocean when convection in the MJO was initiated. This article describes details of the experiment as well as some selected early results. Intensive observations using Doppler radar, radiosonde, surface meteorological measurements, and other instruments were conducted at 0°, 80.5°E, after deploying an array of surface and subsurface moorings around this site. The Mirai stayed within this buoy array area from 24 October through 25 November. After a period of stationary observations, underway meteorological measurements were continued from the Maldives to the eastern Indian Ocean in early December. All observatio...

The high-frequency times-series data collected during 28 th October to 21 st November 2006 onboard R/V Mirai provided the first evidence of short-term bio-physical coupling in the central equatorial Indian Ocean (EIO). The predominant feature of the thermal structure was the down-sloping of 28 o C-isotherm and up-sloping of 14 o C-isotherm indicating deepening of surface-layer and shoaling of lowerthermocline respectively. Concomitant with the deepening of surface-layer, nitracline and subsurface chlorophyll maximum (SCM) also deepened. From the prominent bi-weekly periodicity in the 14 o Cisotherm and the upward phase propagation in the meridional current velocity, we infer that shoaling was the response of mixed Rossby-Gravity (MRG) waves. Based on temperature and currents we propose that deepening of surface-layer was caused by the eastward advection of warmer waters associated with the weakening of Indian Ocean Dipole (IOD). This made the upper ocean more nutrientdepleted and biologically less-productive, an indication that the central EIO is returning to its pre-IOD condition.

Intraseasonal vertical velocity variation in the central equatorial Indian Ocean was investigated using observations from the field experiment "Mirai Indian Ocean Cruise for the Study of the MJO-convection Onset" (MISMO) and from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA), in October-November 2006. Using an array of four subsurface moored acoustic Doppler current profilers (ADCPs), we estimated vertical velocity by applying the continuity equation. Results indicated alternating downwelling and upwelling episodes at around thermocline depth, with maximum amplitudes larger than 8.0×10 -5 m s -1 , or about 7 m per day. The vertical velocity variation was mainly produced by the divergence/convergence of meridional currents, with a quasi-biweekly period of 11-16 days. The temporal changes in temperature around thermocline depth were consistent with variations in vertical velocity, whereas upwelling had less impact on the surface layer temperature. Intraseasonal variations in the ocean may be a part of biweekly fluctuations by remotely forced mixed Rossby-gravity waves, which have a meridional current maximum at the equator, accompanied by divergence/convergence in the surface layer a few degrees from the equator.

[1] An anomalous climate mode, the positive Indian Ocean dipole (IOD), occurred in 2006 with the anomalous sea surface temperature (SST) distribution in the tropical Indian Ocean. Using various types of observational data, we investigated the temperature variation in the surface mixed layer in the eastern Indian Ocean to clarify the processes that produced the anomalous SST variation in 2006. Analysis was conducted at an intraseasonal time scale and focused on a location (5°S, 95°E) where in situ measurements by the Triangle Trans-Ocean Buoy Network were available. Temporal changes in the mixed layer temperature were obtained from the buoy data. Air-sea heat fluxes and horizontal heat advection were estimated from the buoy data, satellite-based data, and reanalysis products. Heat balance analysis demonstrated that air-sea heat fluxes and horizontal heat advection mainly accounted for the mixed layer temperature variation. The results indicate that the relative importance of the heat fluxes and horizontal heat advections changed remarkably with the onset of the IOD. During January to mid-May 2006, before the onset of the IOD, the temperature variation was mainly explained by the net surface heat flux at an intraseasonal time scale. During the IOD in late August to November 2006, the northwestward horizontal temperature gradient and the surface current produced large horizontal heat advection that exceeded the contribution of surface heat fluxes. These results confirm the importance of oceanic processes in the evolution of the IOD, and the heat balance analysis would be a fundamental example in validating model outputs for the Indian Ocean.

[1] Intensive observations, using an array of surface and subsurface moored buoys, are conducted around 80.5°E in the equatorial Indian Ocean during October/November 2006. An intriguing data set of atmospheric and oceanic variables during a peak phase of a positive Indian Ocean Dipole is obtained. The ocean observation data shows relatively shallow thermocline, which intensifies with time during the one-month period, and eastward subsurface zonal flow under westward flowing surface current, generating unusually strong vertical shear above the thermocline. Intraseasonal meridional current variability is also observed. Upper-ocean volume budget analysis indicates that a strong upwelling event, larger than 10 m/day, and associated upward movement of the isotherms below the thermocline occur for a few days in early November. These observed data demonstrates unusual conditions of the upper ocean during boreal autumn in 2006.

In order to understand the variation from intraseasonal to interannual time scale in the Indian Ocean, we have conducted observation by mooring buoy array mainly in the eastern tropical Indian Ocean. Here, some results of observational studies in terms of intraseasonal to semiannual change in the thermal field and recent observed Indian Ocean Dipole events are shown. Also future mooring buoy observation plan in relation to international Indian Ocean buoy network are introduced.

[1] Details of subsurface ocean conditions associated with the Indian Ocean Dipole event (IOD) were observed for the first time by mooring buoys in the eastern equatorial Indian Ocean. Large-scale sea surface signals in the tropical Indian Ocean associated with the positive IOD started in August 2006, and the anomalous conditions continued until December 2006. Data from the mooring buoys, however, captured the first appearance of the negative temperature anomaly at the thermocline depth with strong westward current anomalies in May 2006, about three months earlier than the development of the surface signatures. These subsurface evolutions within the ocean would be a key factor for better understanding of IOD mechanisms and its predictability, and are providing a fundamental dataset for validation of modeling outputs.

[1] A relationship between the timing of El Nino onset and the subsequent evolution is examined, using 130-year long time series of Nino-3.4 index from 1871 to 2000. It is found that El Nino events can be classified into two major types: one is the onset of which is from April to June (spring type) and the other is from July to October (summer-fall type). Here, the duration of El Nino is defined as the period when the 5-month running mean anomaly of Nino-3.4 index is exceeding 0.5°C. As a result, 24 El Nino events are identified, and classified into 10 spring type events and 14 summer-fall type events. In general, spring type events grow greater in magnitude, and take the mature phase around a boreal winter and the evolution is relatively regular. On the contrary, summer-fall type events are relatively weaker in magnitude, and have rather irregular aspects.

(1) Oceanic and atmospheric anomaly fields in the tropical Pacific are investigated to extract some characteristics from twelve El Nino events in the past 49 years. The results show that entire equatorial upper ocean heat content (Teq) is discharged after the mature phase of seven El Nino events (A-type events), while Teq is not well discharged in other five events (B-type events). Furthermore, A-type events generally have larger amplitudes of tropical oceanic and atmospheric anomalies than B-type events. In addition, the durations of A-type events are longer than those of B-type events. It is also found that A-type events accompany large positive wind stress curl anomaly and resultant poleward Sverdrup transport in the tropical South Pacific after the mature phase of El Nino events, while B-type events do not. This appearance of large positive wind stress anomaly should be one of the reasons why Teq is well discharged in A-type events. Citation: Hasegawa, T., T. Horii, and K. Hanawa (2006), Two different features of discharge of equatorial upper ocean heat content related to El Nino events, Geophys. Res. Lett., 33, L02609, doi:10.1029/2005GL024832.