Effects of air–sea coupling on the boreal summer intraseasonal oscillations over the tropical Indian Ocean

Abstract

The effects of air–sea coupling over the tropical Indian Ocean (TIO) on the eastward- and northward-propagating boreal summer intraseasonal oscillation (BSISO) are investigated by comparing a fully coupled (CTL) and a partially decoupled Indian Ocean (pdIO) experiment using SINTEX-F coupled GCM. Air–sea coupling over the TIO significantly enhances the intensity of both the eastward and northward propagations of the BSISO. The maximum spectrum differences of the northward- (eastward-) propagating BSISO between the CTL and pdIO reach 30% (25%) of their respective climatological values. The enhanced eastward (northward) propagation is related to the zonal (meridional) asymmetry of sea surface temperature anomaly (SSTA). A positive SSTA appears to the east (north) of the BSISO convection, which may positively feed back to the BSISO convection. In addition, air–sea coupling may enhance the northward propagation through the changes of the mean vertical wind shear and low-level specific humidity. The interannual variations of the TIO regulate the air–sea interaction effect. Air–sea coupling enhances (reduces) the eastward-propagating spectrum during the negative Indian Ocean dipole (IOD) mode, positive Indian Ocean basin (IOB) mode and normal years (during positive IOD and negative IOB years). Such phase dependence is attributed to the role of the background mean westerly in affecting the wind-evaporation-SST feedback. A climatological weak westerly in the equatorial Indian Ocean can be readily reversed by anomalous zonal SST gradients during the positive IOD and negative IOB events. Although the SSTA is always positive to the northeast of the BSISO convection for all interannual modes, air–sea coupling reduces the zonal asymmetry of the low-level specific humidity and thus the eastward propagation spectrum during the positive IOD and negative IOB modes, while strengthening them during the other modes. Air–sea coupling enhances the northward propagation under all interannual modes due to the persistent westerly monsoon flow over the northern Indian Ocean.