Takanori Horii

Mismo field experiment in the equatorial Indian Ocean

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...

Meridional Heat Advection due to Mixed Rossby Gravity Waves in the Equatorial Indian Ocean

AbstractThis 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 bar...

Intraseasonal coastal upwelling signal along the southern coast of Java observed using Indonesian tidal station data

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.

Impact of intraseasonal salinity variations on sea surface temperature in the eastern equatorial Indian Ocean

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.

Bio‐physical coupling and ocean dynamics in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole

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.

Development of buoy array in the Eastern tropical Indian Ocean and observed variability

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.