Search

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.

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.

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.