Variability in the Indian Ocean

with emphasis on ENSO-associated variations since 1970.

Jean-Luc LeBlanc

Abstract: In this present study, the author uses Sea Level Pressure (SLP) data, FIorida State University (FSU) pseudo-wind stress data (since 1970) and Comprehensive Ocean and Atmosphere Data Set (COADS)Sea Surface Temperature (SST) data to analyse the hydro-climatic variahility in the Indian Ocean for the 25 years period 1970-1994 and to describe the characteristics of El-Nino-Southern Oscillation (ENSO) phenomenom in the Indian Ocean. This helps us to confirm and complete our understanding of the coupling, through atmospheric circulation and ocean-atmosphere interactions, between the Indian and Pacific oceans.

Pressure gradient generates wind which is the motor of the ocean upper-layer circulation. Therefore the author starts its reasoning by analysing SLP anomallies in tbe Seychelles and in Darwin. This analysis leads him to the definition of a so-called "Indian Ocean Index", similar to the Southern Oscilation Index (SOI)for the Pacific, and which validity will be discussed.

Zonal winds, SST and Ekman pumping interannual variations are studied in relation with ENSO so to depict their patterns in the Indian Ocean during these events. The signaI of these three parameters are decomposed in space and time EOF. The analysis in different regions of the Indian Ocean show a strong interannual coupling with the Pacific Ocean, and that during ENSO years, winds are directed to the west with stronger intensity (weaker during La Nina years) and SST rises up as a consequence of the deepening of the upper-layer by negative Ekman pumping (i.e. downwelling). These results are in accordance with previous work on SLP, wind and SST variability, but Ekman pumping had not yet been studied in relation with ENSO.

Comparisons with the SOI shows that some ENSO events could have been generated in the south Indian Ocean as a consequence of monsoon anomalies. A quasi-quadrennial (QQ) period is identified in zonal wind and Ekman pumping signals through frequency analysis. QQ variability in SST and zonal wind has also been found in the equatorial Pacific and is thought to be the fundamental period for ENSO. Thus, Ekman pumping is identified as the major air-sea mechanism that forces this QQ oscillation which has the form of a standing wave with slight eastward propagation.