Southwest monsoon has made a resounding start promising decent rainfall and equitable distribution. It is a 4 month-long journey amidst dynamic atmospheric and oceanic conditions. Oceanic factors like ENSO (El Nino Southern Oscillation), IOD (Indian Ocean Dipole), and transiting MJO (Madden Julian Oscillation) have a strong impact on the performance of monsoon in the Indian subcontinent. They rarely resonate altogether and therefore hold premium stakes for the seasonal rainfall.
ENSO phenomenon contributes significantly to seasonal climate fluctuations in many regions of the globe and the southwest monsoon is the largest of all in size and duration. During the last 4 weeks, SST’s (Sea Surface Temperature) returned to near average across most parts of the equatorial Pacific Ocean.
The Oceanic Nino Index (ONI) is based on SST departure from average in the Nino 3.4 region, and is the principal measure for monitoring, assessing, and predicting ENSO. There is a wide range of numerical models being used, both statistical and dynamical to study and understand the complete spectrum of Pacific Ocean parameters. The average of these models still projects a neutral but marginally -ve Nino 3.4 through the monsoon season. It means Nino conditions are not going to corrupt the ISMR (Indian Summer Monsoon Rainfall).
Indian Ocean Dipole is defined by the difference in SST’s between the eastern and western tropical Indian Ocean. The IOD is neutral now. The latest weekly value of the IOD index to 06 June was -0.65°C, the 3rd consecutive week below the threshold of -0.4°C.
While a -ve IOD event is not declared until the index remains below the threshold for at least 8 weeks. It is to be kept in mind that -ve IOD does not offer support to the monsoon performance.
It may be noted that model accuracy during early June is still relatively low for IOD forecast. Model accuracy will improve significantly by end of the month.
Intraseasonal variability in the atmosphere, which is often related to the MJO, can significantly impact surface and sub-surface conditions across the Pacific Ocean. It also means that passage of MJO can bring about changes in the temperature profile of ocean surface and in turn the Nino indices.
Next 2 weeks, the MJO will meander in Phase 1, 2 and 3 over the Indian Ocean, albeit with marginal amplitude, but still good enough for accentuating convection over the Bay of Bengal.