Air–sea interaction mechanisms and low-frequency climate variability in the South Indian Ocean region

Abstract

Long-term observations indicate that the Indian Ocean displays significant low-frequency variability in mean sea-level pressure, near-surface wind, cloud and sea-surface temperature (SST). A general circulation model is used to study the response of the atmosphere to an idealized SST anomaly pattern (warm in southern mid-latitudes, cool in southern tropics) that captures the essence of observed multidecadal SST variability as well as that associated with ENSO in the South Indian Ocean. The major objectives are to investigate air–sea interaction mechanisms potentially associated with the variability and whether the atmospheric response to the SST is likely to lead to maintenance or damping of the original SST anomaly pattern, and on what time scale. Two types of experiment are performed to tackle these objectives. An ensemble of roughly 1-year-long integrations suggests that the seasonal-scale response of the atmosphere to the imposed SST anomaly includes reduced genesis and density of cyclones in the mid- to higher latitudes, and an indication of a shift in their tracks relative to climatology. It is argued that these changes together with those to the near-surface winds could be expected to lead to variations in surface fluxes that would tend to reinforce the original SST anomaly pattern on seasonal scales. A 21 year integration of the model with the SST anomaly pattern imposed throughout indicates that a low is generated near, and downstream of, the warm mid-latitude anomaly. On decadal/multidecadal scales, the associated changes to the surface winds are argued as being likely to lead to changes in surface fluxes and in the strength of the South Indian subtropical gyre that would oppose the original anomaly. The current and previous model results together with the observations then support the idea that the observed multidecadal variability in atmospheric circulation and SST of the South Indian Ocean during the past century may have arisen through a combination of basin scale atmosphere–ocean interaction and a remotely forced component. © 1998 Royal Meteorological Society