An analysis of monthly rainfall and the meteorological conditions associated with the deficient rainfall towards the end of 2017 southwest monsoon season

This paper studies the summer monsoon 2017 and examines the number of parameters which we believe were important in understanding why monsoon failed in second half over India. The list of parameters includes monthly mean or anomalies of the following fields : sea surface temperature, outgoing longwave radiation, stream function of lower and upper atmosphere, velocity potential and monthly and seasonal precipitation. ENSO conditions were mainly neutral with warm ENSO neutral conditions observed in the first half and cool ENSO neutral conditions observed in the second half. As a result, influence on the monsoon from the large scale SST forcing from Pacific Ocean was nearly absent during the season. However, Positive IOD conditions over the Indian Ocean during the monsoon season, particularly during first half of the monsoon were prominent. The transition of warmer than normal SSTs (June and July) to normal SSTs (August) and then becoming cooler than normal SSTs (September) in the equatorial Indian Ocean had a significant influence which lead monsoon to fail in second half. &#x0D;  </jats:p

Asymmetry in the Indian Summer Monsoon Rainfall response to two types of La Niña evolution

This study attempts to understand the asymmetry in the Indian Summer Monsoon Rainfall (ISMR) response to two types of La Niña whether they evolved from El Niño or La Niña in the previous boreal winter season. It was seen that nine La Niña years during the monsoon season were preceded by El Niño (hereafter ELLA) whereas eight were preceded by La Niña (hereafter LALA) during the period 1961-2021. India received more rainfall during the ELLA years as compared to the LALA years, linked to the difference in the Sea Surface Temperature (SST) and large-scale atmospheric circulation anomalies over the tropical Pacific and Indian oceans. Based on the strength and patterns of the cold SST anomaly and shift in the Walker circulation over the equatorial Pacific Ocean, the enhanced (weakened) convection over the Indian landmass during the ELLA (LALA) years, contributed to more (less) rainfall over India.</jats:p

Normal dates of onset/progress and withdrawal of southwest monsoon over India

The new normal dates of onset/progress and withdrawal of southwest monsoon over the country are computed based on the operationally declared dates of these events by India Meteorological Department (IMD) during recent years. The normal onset/progress dates were calculated based on 1961-2019 data and that of withdrawal were calculated based on 1971-2019 data. This study also suggests new objective rainfall criteria to define the monsoon onset/ progress over various parts of the country. The new objective criteria are based on the IMD daily rainfall data at 1° × 1° (latitude × longitude) spatial grids over the country. The new criteria were designed so as to closely simulate IMD’s operational declaration of onset/progress of monsoon. However, no new criteria are suggested for defining withdrawal dates. The dates of monsoon onset/progress derived in each of these 1° × 1° grids based on the new criteria for the period 1961-2019 were also used to compute normal onset/progress dates of monsoon over the country. The climatological normal dates of onset and withdrawal of monsoon over India currently used by IMD for operational services are based on old data period (1901-1940) obtained from the rainfall analysis of 149 stations. This study proposes the new normal dates of monsoon onset/progress (based on the new rainfall criteria) and withdrawal (based on the operational data) to replace the existing normal. On comparing the new normal dates of monsoon onset and withdrawal with the existing normal dates of these events, interesting difference were observed. The monsoon onset/progress is relatively delayed over most parts of the Indian monsoon region in the new normal compared to the existing normal. However, monsoon onset/ progress is relatively faster in the new normal than the existing normal over Lakshadweep Islands, a few grids from western part of northeast India and western parts of south Peninsula and some areas of north and extreme northwest India. As per the new normal, the monsoon sets over Kerala around 1st June, nearly same as the existing normal date and covers the entire country one week before the existing normal. However, monsoon withdrawal from northwest India is delayed by more than 2 weeks compared to the existing normal date (i.e., 1st September). Monsoon retreats from most parts of the country except south Peninsula and some parts of neighboring central India by 15th October coinciding with the existing normal and subsequently northeast monsoon gets established over south Peninsula.</jats:p