Multidecadal Variability in the Climate System over the Indian Ocean Region during the Austral Summer

Several independent historical studies of global atmospheric and oceanic parameters have identified low-frequency fluctuations in the global climate system. Much of this research has focused on Europe, the Atlantic Ocean, and North America. However, recent interest has begun to encompass decadal to multidecadal variability across the Indo-Pacific region. Such variability has been detected in sea surface temperature (SST), mean sea level pressure (MSLP), and surface wind fields over both the landmasses and the oceans. Around the Indian Ocean basin, the broad periods before and after the 1940s show important differences in features such as Indian southwest monsoonal rainfall and circulation patterns, relationships between austral summer rainfall in southern Africa and the El Ni˜o–Southern Oscillation phenomenon, and Australasian MSLP. Very little is known about this variability, particularly during the austral summer. In an effort to isolate such fluctuations and work toward understanding the physical dynamics operating on such timescales, SST, MSLP, atmospheric circulation, vertical motion, and cloudiness anomalies are constructed and analyzed for austral summer (JFM) conditions over the Indian Ocean region during four 21-yr epochs since 1900. The results of this research suggest that SSTs were cooler at midlatitudes and warmer in the subtropical southern Indian Ocean in the periods 1900–20 and 1921–41, compared with the 1942–62 and 1963–83 epochs. The most pronounced changes are found along the Agulhas Current outflow zone across the midlatitudes of the southwest Indian Ocean, with indications of coherent SST fluctuations in the northwest regions of the basin and in the northwest Pacific Ocean. Changes in surface wind anomalies are also observed. During 1900–20 and 1921–41, an anomalous atmospheric cyclonic feature is seen over the southern Indian Ocean, while in the later 1963–83 period, a distinct anticyclonic anomaly is evident in this region. This change suggests that the semi-permanent anticyclone in the mean flow field of the atmosphere over the southern Indian Ocean in JFM was weaker in the first 42 yr of this century. Concurrent variations are found in the trade wind regime over the western equatorial Pacific. Velocity potential field anomalin derived from the surface winds, show a strengthening of tropical-subtropical convergence over time. These observations, together with those of cloudiness and MSLP and a brief examination of near-global MSLP correlations and SST data back to 1879, point to a consistent fluctuation in ocean–atmosphere forcings during this century. Independent ocean general circulation model simulations involving modulations to global wind stresses or the Indonesian throughflow suggest that the sub-tropical/midlatitude southern Indian Ocean, and particularly the Agulhas outflow zone, is sensitive to low-frequency changes in wind and/or thermohaline forcing. Such long-term fluctuations in the mean state of the climate system may have ramifications for interannual variability and features such as the El Niño–Southem Oscillation phenomenon.