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

Abstract

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 1^st 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., 1^st September). Monsoon retreats from most parts of the country except south Peninsula and some parts of neighboring central India by 15^th October coinciding with the existing normal and subsequently northeast monsoon gets established over south Peninsula.