Relationships between interannual anomalies of sea surface temperature in the Indian Oc ean and tropical boreal summer intraseasonal oscillations: Observations and simu lations

Relationships between the interannual anomaly of sea surface temperature (SST) in the tropical Indian Ocean and the different propagation modes of tropical boreal summer intraseasonal oscillations (BSISO) as well as their physical processesare analyzed by using the observational data and long-term (70-year) simulation outputs based on the SINTEX-F oceanatmosphere coupled general circulation model. The results show that interannual SST anomaly over the Indian Ocean may significantly influence both the northward and eastward propagation of BSISO. The northward propagating BSISO over the central and eastern Indian Ocean weakens (strengthens) during the positive (negative) phase of the Indian Ocean dipole. The eastward propagating BSISO over the equatorial Indian Ocean and western Pacific (40°E-180°) strenthens (weakens) during the positive (negative) basin wide anomaly (BWA) condition. The interannual variations of SST in the tropical Indian Ocean affect the variation of the intensity of the different ropagating BSISO modes by modulating the atmospheric background circulation and structure of BSISO. During negative (positive) dipole summers, the increasing (decreasing) of easterly vertical shear in atmospheric troposphere reinforces (reduces) the barotropic vorticity and moisture convergence in the boundary layer at the northern side of the convection distrubance, resulting in a strong (week) meridional asymmetry of BSISO and thus in a stronger (weeker) northwardpropagating BSISO. The different BWA modes in the Indian Ocean impact the background of equatorial westerlies and thermodynamic exchanges across the airsea interface, causing changes in the intensity of equatorial eastwardpropagating BSISO. During the positive BWA summer, equatorial westerlies are stronger than the climatological mean. Under this stronger westerlies background, the weakening (strengthening) of wind speed east (west) of the disturbance center is distinct, and so is the evaporation, resulting in larger amplitude of SST increasing (decreasing) to the east (west). The larger amplitude of SST increasing (decreasing) leads to the stronger convergence (divergence) and much more (little) water vapor to the east (west). As a result, the equatorial eastward propagating BSISO is stronger during the positive BWA condition. During the negative BWA summer, equatorial westerlies are weaker than the climatological mean since the above physical process is weakened with the equatorial eastward propagating BSISO weaker.