El Nino, La Nina and the Indian sub-continent

By M R Ramesh Kumar
Published: Wednesday 23 April 2014

Monsoon in India will be governed by where and when El Nino is formed and how strong or weak it would be

The two important meteorological events as far Indian sub continent is concerned are monsoons (southwest monsoon and northeast monsoon) and cyclones, which generally form during the pre-monsoon (March-May) and the post-monsoon season (October–December). And the two important phenomena which influence the Indian Ocean are the El Nino and Southern Oscillation (popularly known as ENSO) and the recently discovered Indian Ocean Dipole that refers to the difference in temperatures between the surface waters in the west and east of the ocean. 

El Nino can further be classified into two types— traditional El Nino which is characterized by strong anomalous warming in the eastern equatorial Pacific and the El Nino Modoki that is associated with strong anomalous warming in the central tropical Pacific and cooling in the eastern and western tropical Pacific. 

Monsoon in India will be governed by where and when this El Nino is formed and how strong or weak it would be. Here, I talk about three things that would be influenced by El Nino or El Modoki. 

If El Nino over the Pacific Ocean increases in intensity this year, monsoon onset over Kerala (MOK) would be severely impacted and El Nino and Indian Ocean Dipole events will be important factors. This is based on a research by me and my students Syam Sankar and Sumesh K G and Chris Reason. The research has already been published in various national and international journals after studying inter-annual variations using data from over 60 years (1948–2009). 

The most significant features during early MOK years or the years of early monsoon is the abnormal persistence of westerlies and enhanced convection over the south east Arabian Sea and the southern Bay of Bengal.  In the delayed MOK years or late monsoon the reasons are persistence of easterlies and suppressed convections in the same area. 

Further the sea surface temperature anomalies in the western Pacific are negative during delayed MOK and positive during early MOK. The second observation is the impact of El Niño Modoki events on the formation of tropical cyclones over north Indian Ocean. The observation is based on study published in Natural Hazards in 2013. It has been noticed that the frequency of tropical cyclones are more over Arabian Sea and less over Bay of Bengal during the El Niño Modoki years as compared to the El-Niño years. Our present study also suggests that concurrent by occurring Positive Indian Ocean Dipole (PIOD) with El Nino Modoki events can significantly alter the factors which influence the development or strengthening of cyclonic circulation in the atmosphere (also known as cyclogenesis) over the Arabian Sea as compared to a pure El Nino Modoki year. This cyclonic circulation is influenced by several coupled ocean-atmospheric phenomena such as El Nino, El Nino Modki, Indian Ocean Dipole and Madden Julian Oscillation. 

The third observation is about tropical cyclones over north Indian Ocean during La-Niña Modoki years. La Nina Modoki is the counter part of El Nino Modoki and is characterised by colder central Pacific being flanked by warmer eastern and western Pacific. According to our study published in Indian Journal of Geo Marine Sciences in 2010, the frequencies of cyclonic storms in Bay of Bengal is more during La-Niña Modoki years than traditional La-Niña years. And the frequencies of severe cyclonic storms are more during canonical La-Niña years than La-Niña Modoki years. The study further shows that the La-Niña modoki events are not conducive for the formation of cyclones over Arabian Sea. Over Bay of Bengal, there is a reduction in the magnitude of low level convergence, during La-Niña Modoki years compared to the canonical La-Niña years; this suppresses the formation of severe cyclonic storm. Results of our study are for a limited period only (1979 – 2010) and cannot be generalised.  


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