On the relationship between Indian Ocean Dipole-Indian Summer Monsoon and Summer North Atlantic Oscillation

Using 60 years of observed and re-analysis data sets a link between positive Indian Ocean dipole (PIOD), Indian Summer Monsoon (ISM) and Summer North Atlantic Oscillation (SNAO) is proposed. When positive IOD years occur independently without Elnino the presence of positive phase of SNAO is noticed. In such cases over the Atlantic region the centers of action associated with SNAO is shifted northward leading to the change in the mean position of Atlantic jet and storm tracks. Persistence of storm tracks at the new location amounts to a stationary waves, which have a barotropic structure in the vertical. Therefore, the temperature anomalies associated with the stationary wave influences the Tropospheric Temperature (TT), which is clearly seen in the average TT anomaly, showing warming over southern Eurasia and India. This affects the north-south TT gradient in the monsoon region and influences the ISM (Goswami et al., 2006). Concurrently the local Hadley cell associated with PIOD strengthens and enhances the anomalous subsidence over the Mediterranean/Sahara region which in turn strengthens the Azores high and the hence the SNAO itself. Thus the combined influence of PIOD and SNAO produces a positive feed back mechanism for ISM. On the other hand most of the PIOD years co-occurring with Elnino years are accompanied by the negative phase of SNAO. The analysis of the atmospheric circulation clearly indicates the Pacific North American (PNA) type pattern, which negatively influence the SNAO (Huang et al., 1998). This is consistent with the observation that ISM is not enhanced in all PIOD years. The analysis of 60 years historic sea surface temperature runs with the high resolution atmospheric model shows that the model is able to reproduce these teleconnections.