Eco. Env. & Cons. 19 (2) : 2013; pp. (595-599) Copyright@ EM International ISSN 0971–765X Indian summer monsoon onset variations and consecutive rainfall over India M.V. Subrahmanyam 1 , B. Pushpanjali 2 and K.P.R. Vittal Murthy 2,3 1 Marine Science Department, Zhejiang Ocean University, Zhoushan, Zhejiang, China 2 Department of Environmental Science, Acharya Nagarjuna University, Guntur, AP, India 3 Department of Meteorology and Oceanography, Andhra University, Visakhapatnam, AP, India (Received 2 May, 2013; Accepted 28 May, 2012) ABSTRACT For an Indian agriculture and economy, summer monsoon plays a vital role. The date of monsoon onset changes in every year, the climatological date of onset of monsoon is on 1st June. In this article, we ana- lyzed the rainfall variations in relation to monsoon onset dates for the period of 1970-2010. With respect to the onset date there is a variation of monsoon rainfall in the monsoons months (JJAS) and also with the average seasonal rainfall. In the study period, we found that monsoon onset delayed for 17 days in 1972 and there is a deficit in monsoon rainfall. On the contrary when the monsoon onset is early for 13 days in 1990, the rainfall is above normal. As it is well known, the global phenomena like El Nino/La Nina also affecting the delay or advance in the monsoon onset and also in subsequent rainfall. Detailed discussions about the onset dates and the consequences rainfall during the monsoon months are given. Key words : Onset of monsoon, Seasonal rainfall, El Nino/La Nina. Introduction Monsoon onset over Kerala (MOK) has been consid- ered as the beginning of the Indian principal rainy season, i.e. during the southwest monsoon season or simply the summer monsoon season. Over 70% of the annual rainfall in India occurs during the sum- mer monsoon season from June to September (Parthasarathy et al. 1994). The date of MOK, which marks the beginning of the monsoon rains over the southern Indian peninsula, is an important event in the socioeconomic life of the people of India, par- ticularly in agricultural planning that affects food production and the living of over one billion people. The MOK is associated with a large area of orga- nized rainfall caused by deep convection extending eastward a few thousand kilometers from the low- latitude regions of the Arabian Sea in the north In- dian Ocean (Joseph et al. 1994). The moisture re- quired for this large area of rainfall is mainly pro- duced in the south Indian Ocean and carried to the convective heat source associated with MOK by a strong cross-equatorial low-level jet stream (Findlater 1969; Joseph and Sijikumar 2004). The MOK that marks the beginning of the LLJ is impor- tant to a large part of the globe, namely, the tropical Indian Ocean, South Asia, and the western North Pacific Ocean. Although the onset of monsoon is as- sociated with changes in the atmospheric circulation features in the lower and upper troposphere, a sus- tained increase in the rainfall at the observatory sta- tions of Kerala and the island stations over the south-east Arabian Sea is an essential feature of the monsoon onset. It is difficult to quantify these pre- cisely and so the experience of the forecaster plays a key role in declaring the date of monsoon onset in individual years Ananthakrishnan and Soman (1988). The long-term mean date of (MOK), the southernmost state of India, is close to 1 June and it has a standard deviation of about 8 days. The long