The southern oscillation over the indian ocean

Abstract The Southern Oscillation (SO) signal at the surface level over the Indian Ocean is studied during the period 1954‐1976‐ The data set consists of ship observations analysed on a grid mesh of 2 degrees to determine monthly mean fields of wind, pressure, SST. air temperature and cloud cover. Time series of the parameters over different areas show the evidence of an SO signal over the Indian Ocean. Warm and high pressure episodes occur during El Nino event years. High correlation exists between pressure, SST, air temperature fields and the SO index (SOI) north of 10cS. Significant correlation between SOI and cloudiness and zonal wind is mainly found in the eastern Indian Ocean. Composite analysis of he pressure, SST, cloud cover and zonal component of the wind suggests that minimum cloudiness occurs slightly ahead of maximum pressure whereas weaker westerlies over northern Indian Ocean are associated with higher SST a few months later. Power spectral analysis demonstrate the existence of a major periodicity at 40 months, Fields of percentage of variance in the spectral range between 30 and 50 months show a stronger SO signal in the eastern than in the western part of the Indian Ocean, From bandpass filtering and phase spectral analysis of the time series, the phase relationships between the different parameters are determined. Minimum cloudiness occurs with maximum pressure during a phase of decreasing westerlies which reduce the cooling of the ocean and induces an above normal SST and air temperature. The results also suggest the eastward propagation of zonal wind anomalies over the equatorial Indian Ocean. Warmings over the eastern Indian Ocean are occurring simultaneously with the warmings over the Central Pacific Ocean.