The monsoon's onset over the Kerala (MOK) coast has been the subject of many studies by meteorologists and ocean-atmosphere scientists. Now the relative roles of El Nino and the Indian Ocean Dipole (IOD) events in influencing the monsoon onset date are analysed in a recent paper in the journal Theoretical and Applied Climatology by Dr. M.R. Ramesh Kumar.
Dr. Ramesh Kumar is Scientist G, Deputy Director, Physical Oceanography Division of National Institute of Oceanography (NIO), Goa and the lead author.
After considering various parameters such as sea surface temperatures, winds, convection etc., the researchers concluded after statistical analyses that The El Nino Southern Oscillation (ENSO), La Nina, Positive Indian Ocean Dipole (PIOD) and Negative Indian Ocean Dipole (NIOD) play an important role in determining the MOK date.
El Nino is an ocean-atmosphere phenomenon originating just south of the equator in the Pacific Ocean and responsible for warm, wet conditions in Northern Peru and Ecuador in December-January accompanied by dry, drought like conditions in Australia and Indonesia.
La Nina is also an ocean-atmosphere phenomenon, at the same regions, but producing opposite effects to that of El Nino. The atmospheric component of El Nino and La Nina is called the Southern Oscillation.
The recently discovered IOD is effective all over the Indian Ocean and is responsible for sea surface temperature and wind anomalies occurring on inter-annual time scales.
One phase of the IOD (also called Positive IOD or PIOD) causes a cooling of sea surface temperatures in the eastern tropical Indian Ocean (off Java Island), bringing droughts in the Indonesian and Australian region, while the western tropical Indian Ocean (off the east coast of Africa) tends to experience a warming of sea surface temperatures, which activates atmospheric convection and brings increased rainfall.
The other phase
The other phase (negative phase of IOD — NIOD) in contrast, involves high sea surface temperatures in the eastern Indian Ocean and low in the west, bringing more rain to the Indonesian and Australian region and less to the East African countries.
IOD exerts a major impact on seasonal climatic conditions in the countries of the Indian Ocean region.
The tropical Indian Ocean gradually warms during an ENSO year reaching a maximum during January–March, about one season after the sea surface temperatures in the central Pacific Ocean have peaked.
The region of maximum convection, the tropical convective maximum over Indonesia and the western Pacific during the spring season (March to May) plays an important role in the monsoon onset over India.
The monsoon onset over Kerala is associated with a gradual moisture build-up over the Arabian Sea followed by a rapid intensification of the Arabian Sea winds. The whole process of monsoon onset thus requires about 2 to 3 weeks of pre-conditioning by the atmosphere over the Arabian Sea.
Using statistical models based on those years which could be classified to a high degree of certainty as El Nino, La Nina and positive or negative IOD the researchers determined that in EL Nino and PIOD and concurrent EL Nino-PIOD years, the sea surface temperatures in the pre-monsoon season (March to May) were cooler leading to a delayed monsoon onset. On the other hand, in La Nina, NIOD and concurrent La Nina-NIOD years, the sea surface temperatures were warmer, leading to an early monsoon onset over Kerala.
If the SST is high in the North Indian Ocean and Arabian Sea, in particular during the pre- monsoon season, then these conditions are conducive for developing low pressure over the area, thus creating a greater inter-hemispheric pressure gradient and strong cross-equatorial flow, all of which are conducive for early monsoon onset over Kerala.
An analysis
An analysis of convective activity similar to that described earlier for sea surface temperatures based on the same years and pre-monsoon season (March, April, May months) shows that there is very little convection during the pre-monsoon months in the Arabian Sea in the El Nino, PIOD and concurrent El Nino-PIOD years causing a delay in monsoon onset whereas the convection is much greater and predominant in La Nina, NIOD and concurrent La Nina-NIOD years, resulting in an early onset highlighting the role of these phenomena in Arabian Sea convection and monsoon onset date over Kerala.
