Semiannual variability in temperature and salinity observed by Triangle Trans-Ocean Buoy Network (TRITON) buoys in the eastern tropical Indian Ocean

The time series of temperature and salinity obtained by Triangle Trans-Ocean Buoy Network buoys in the eastern Indian Ocean was used to investigate the characteristics of their variations and possible mechanisms of the semiannual ocean variability. Spectral analysis indicates that the dominant temperature signals in the surface layer and below the thermocline have intraseasonal and semiannual timescales, respectively. The semiannual isotherm deepening in the thermocline appears clearly during the monsoon transition periods, and indicates the vertical phase shift below the thermocline. The characteristic speed of the wave estimated from the vertical phase shift corresponds to a value between the characteristic speeds of the second and third baroclinic modes. The correlation between the zonal surface wind stress on the equator and the depth of 20 degrees C isotherm variation shows that the forcing region of the semiannual isotherm variation is located around 68 degrees E to 78 degrees E longitude. The estimation of characteristic speed from the lag between these variations also shows a value between the second and third vertical-mode gravity waves. The salinity variations also indicate that the intraseasonal and semiannual signals are dominant in the surface layer and the thermocline, respectively. The appearance of salty water around the upper thermocline lags the subsurface zonal current variation by about 45 days on the semiannual timescale, suggesting the advection of salty water by the subsurface currents along the equator. There is a possibility that the contact between the subsurface salty water and surface fresh water results in the formation of a barrier layer in the eastern basin.