Mechanisms behind Sub-seasonal Rainfall variabilities of Indian Monsoon

The summer monsoon season over India consists of various Intra-seasonal features like the onset, Intra-Seasonal Oscillation, synoptic variabilities and extreme events. Each of these intra-seasonal modes has inter-annual variations associated with it. Without any pre-assumption of the afore-mentioned intra-seasonal variabilities, for the first time, the connection between inter-annual and monthly variations of monsoon rainfall are studied using variations to the mean seasonal cycle. We explain the governing mechanisms behind those variations of the seasonal cycle. Analysis shows that, June rainfall variability, which is related to onset, can be associated with Madden Julian Oscillation (MJO) along with mean background precipitable water. The positive (negative) phase of MJO is found to be responsible for fast (slow) advancement of monsoons. It is observed that high mean background precipitable water in the delayed onset years, can trigger onset rainfall even when there is a negative phase of MJO. The July-September rainfall, on the other hand, is related to evolving El Nino Southern Oscillation (ENSO). ENSO related net moisture flux over India is found to be very small in June, increases in July and becomes maximum in September. To have an accurate forecast of the seasonal cycle of Indian Monsoon rainfall (IMR), models must capture these mechanisms with greater accuracy.

Keywords: seasonal cycle, intra-seasonal variability, Madden Julian Oscillation, El Nino Southern Oscillation, Indian Monsoon Rainfall