The Southern Oscillation (SO) signal at the surface level over the Indian Ocean is studied during the period . 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 10"s. Significant correlation between SO1 and cloudiness and zonal wind is mainly found in the eastern Indian Ocean. Composite analysis of the 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.

SUMMARYThe interannual variability of surface observations in the equatorial Indian Ocean is investigated from 23 years of ship reports . In October and Novcmbcr, during certain years, the monthly analyses show strong wind anomalies in the eastern and ccntral equatorial Indian Ocean. Simultaneously, cloud cover anomalies of opposite signs are observed in the eastern Indian Ocean and in thc central Indian Ocean. A linear analysis of heat-induced circulation suggests that the surface wind anomalies are forced by anomalieb of rainfall. This circulation is probably dissipative with time scales in the range two to five days.The reasons for the maintenance of anomalous circulations are not wcll known. It is noted that during most El Nino events in the Pacific Ocean, anomalies are observed in the Indian Ocean. However, there arc other years with the same anomalous pattern, 1961 for example. During the years with anomalously little cloudines5 in the eastern Indian Ocean, sea surface temperatures lower than normal are found in thc castcrn Indian Ocean. This suggests that coupled air-sea dynamics must be taken into account to understand the interannual variability.

The interannual variability of surface observations in the equatorial Indian Ocean is investigated from 23 years of ship reports (1954‐1976). In October and November, during certain years, the monthly analyses show strong wind anomalies in the eastern and central equatorial Indian Ocean. Simultaneously, cloud cover anomalies of opposite signs are observed in the eastern Indian Ocean and in the central Indian Ocean. A linear analysis of heat‐induced circulation suggests that the surface wind anomalies are forced by anomalies of rainfall. This circulation is probably dissipative with time scales in the range two to five days. The reasons for the maintenance of anomalous circulations are not well known. It is noted that during most El Niño events in the Pacific Ocean, anomalies are observed in the Indian Ocean. However, there are other years with the same anomalous pattern, 1961 for example. During the years with anomalously little cloudiness in the eastern Indian Ocean, sea surface temperatures lower than normal are found in the eastern Indian Ocean. This suggests that coupled air‐sea dynamics must be taken into account to understand the interannual variability.

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