Spring asymmetric mode in the tropical Indian Ocean: role of El Nino and IOD

The spring asymmetric mode over the Tropical Indian Ocean (TIO) is characterized by contrasting patterns of rainfall and surface wind anomalies north and south of Equator. The asymmetric pattern in rainfall has evolved as a leading mode of variability in the TIO and is strongly correlated with El Nio-Southern Oscillation (ENSO) and positive Indian Ocean Dipole (IOD). The evolution of the asymmetric pattern in rainfall and surface wind during pure El Nio/IOD and co-occurrence years are examined in the twentieth century reanalysis for the period of 1871-2008 and atmospheric general circulation model (AGCM) simulations. The study revealed that spring asymmetric mode is well developed when El Nio co-occurred with IOD (positive) and is driven by the associated meridional gradients in sea surface temperature (SST) and sea level pressure (SLP). The pure El Nio composites are characterized by homogeneous (spatially) SST anomalies (positive) and weaker SLP gradients and convection, leading to weak asymmetric mode. The asymmetric mode is absent in the pure IOD (positive) composites due to the persistence of east west SST gradient for a longer duration than the co-occurrence years. The meridional gradient in SST anomalies over the TIO associated with the ENSO-IOD forcing is therefore crucial in developing/strengthening the spring asymmetric mode. The northwest Pacific anticyclonic circulation further strengthen the asymmetric mode in surface winds by inducing northeasterlies in the north Indian Ocean during pure El Nio and co-occurrence years. The simulations based on AGCM, forced by observed SSTs during the period of 1871-2000 supported the findings. The analysis of available station and ship track data further strengthens our results.