1 1 REGIONAL CHARACTERISTICS OF THE 2019 SOUTHWEST MONSOON Sunitha Devi S., Krishna Mishra, S.P.Singh, Naresh Kumar, A. Kashyapi & K. Sathi Devi This chapter discusses the observed features of southwest monsoon 2019 depicting various synoptic situations during the advance, mature and withdrawal phases and the salient features of the synoptic scale systems. 1.1 Onset and Advance of Southwest Monsoon 2019 1.1.1 Arrival of Southwest Monsoon Current over the Andaman Sea During mid-May, gradual development of southerly to southwesterly flow occurred over southern parts of north Indian Ocean. Within a couple of days, the cross-equatorial flow strengthened and deepened over the Andaman Sea. This feature along with enhanced cloudiness and rainfall in association with a cyclonic circulation at mid-tropospheric levels over Andaman Sea, led to the arrival of southwest monsoon (SWM) over south Andaman Sea, some parts of South Bay of Bengal and Nicobar Islands on 18th May. Sustained rainfall activity over Andaman & Nicobar Islands and prevalence of southwesterly winds led to further advance of SWM into some more parts of southeast Bay of Bengal and north Andaman Sea, remaining parts of Nicobar Islands and southern parts of Andaman Islands on 25 th May. In association with further deepening of south westerlies in the near equatorial belt and over south & adjoining east central Bay of Bengal, the SWM further advanced into southernmost parts of Maldives-Comorin area, some more parts of southwest and southeast
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Heat waves during the southwest monsoon season (June- September) along with the dates.
Fig.1.6 also indicates areas that experienced isolated extremely heavy rainfall (Rainfall
amount ≥20 cm reported during the 24 hours ending at 0300 UTC) events during the
season.
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Fig.1.6: High Impact Weather Events pertaining to events & dates as per legends
The season witnessed a very large number of ‘High Impact weather events, of which
‘floods’ remained to be the most frequent and widespread phenomenon. It may be noted
that, almost all meteorological sub-divisions experienced one or other category of severe
weather event during the season. Incessant rainfall associated with the formation and
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movement of the monsoon low pressure systems in the presence of strong cross equatorial
flow often caused flood situations over various areas during different parts of the season
Maharashtra experienced one of the worst flood situations during past 50 years, due to
frequent heavy rain spells and several extremely heavy rain events, especially during August
& September.
Several record-breaking extreme rainfall and resultant Flood events caused human
casualty and property damage in states including Maharashtra, Karnataka, Kerala, West
Bengal, Odisha, Uttar Pradesh and Madhya Pradesh during later part of July to September.
Localised and brief flood events occurred due to record breaking rain events in Jammu &
Kashmir on 30th& 31st July and in Gujarat on 1st August.
1.6. Low Pressure Systems over West Pacific Ocean/ South China Sea/South Indian
Ocean
1.6.1 West Pacific Ocean/South China Sea
There were, in all, 19 low pressure systems (reaching the intensity of Tropical
depression and above) in the northwest Pacific Ocean / South China Sea during June –
September 2019. On an average, about 2 systems form during June, 4 in July and 5 each in
August & September. The month wise break-up of this year is given below:
Low Pressure Systems June July August September TOTAL
Tropical Depression (T.D.) 01 01 00 01 03
Tropical Storm (T.S.) 01 03 03 02 09
Typhoon/Super Typhoon 00 00 03 04 07
TOTAL 02 04 06 07 19
1.6.2 South Indian Ocean
No low pressure system (having intensity more than D) was reported in south Indian
Ocean during June- September 2019.
1.7 Withdrawal of Southwest Monsoon
The isochrones of withdrawal of monsoon are given in Fig.1.7. Due to the prevalence
of an active Inter Tropical Convergence Zone, across central India, north Indian Ocean,
extending upto western north Pacific Ocean, the withdrawal of southwest monsoon was
delayed upto the 1st week of October. Anti cyclonic circulation in lower levels over Rajasthan
could only be established after 05th October. This led to the most delayed commencement of
withdrawal since 1961. Against normal date of withdrawal of SWM 01st September, the
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Monsoon Withdrawal commenced only on 09th October.In 1961 it was 01st October followed
by 30th September in 2007.
Fig.1.7: Isochrones of withdrawal of South West Monsoon 2019.
It withdrew from some parts of Haryana, Punjab and north Rajasthan on 09th
October. With the increase in dominance of mid-latitude circulation regime over the northern
half of India and consequent reduction in moisture led to further withdrawal of southwest
monsoon from entire Northwest India, some parts of West Bengal, Bihar, Jharkhand,
Chhattisgarh, Madhya Pradesh, Gujarat and north Arabian Sea during 10th -12th October;
from entire north Bay of Bengal, some parts of central Bay of Bengal, entire Odisha,
Chhattisgarh, some parts of Coastal Andhra Pradesh, some parts of Telangana, most parts
of Maharashtra, some parts of North Interior Karnataka, entire north Arabian Sea and some
parts of central Arabian Sea during 13th-15th October. Thus in a rapid phase, the Southwest
Monsoon withdrew from the entire country), giving way to simultaneous commencement of
northeast monsoon rains on 16th October, 2019.
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1.8 Concluding remarks.
The cumulative seasonal rainfall over the country as a whole was 110% of its Long
Period Average (LPA).
Southwest monsoon set in over Kerala on 8th June, with a delay of 7 days. Further
progress over south peninsular & central India was also on a sluggish phase mainly
due to the formation of very severe cyclonic storm ‘Vayu’ over the Arabian Sea
during 10th -17th June. It covered the entire country on 19th July, with a delay of 4
days from normal.
Delay in the advance of monsoon over major parts of central and adjoining south
Peninsular India resulted in large rainfall deficiency in the month of June, which was
33 % less than its LPA.
This year, June & September witnessed the formation of one VSCS each, while 1 DD
formed during August and 1 D during September. Though the frequency of cyclonic
storms is less during the southwest monsoon period, there had been 3 more such
years in the recent 30 years, viz., 1996 (2 in June, one each over the Bay of Bengal
& the Arabian Sea), 2007 (2 in June over the Arabian Sea) and 2015 (1 in June over
the Arabian Sea and the other in July, over the Bay of Bengal).
The reduced frequency of monsoon depressions as noticed in recent years (with
exception of years like 2007) are consistent with some of the latest findings that there
is a long term decrease in the intense monsoon low pressure systems and a
corresponding increase in the frequency of weaker low pressure systems.
Due to the prevalence of an active Inter Tropical Convergence Zone, across central
India, north Indian Ocean, extending upto western north Pacific Ocean, the
withdrawal of southwest monsoon was delayed upto the 1st week of October. Though
the withdrawal commenced from northwest India on 9th October as against the
normal date of 1st September, it withdrew on a rapid phase from the entire country by
16th October (against the normal date of 15th October), giving way to simultaneous
commencement of northeast monsoon rains.
Several record breaking extreme rainfall and resultant Flood events caused human
casualty and property damage in states including Maharashtra, Karnataka, Kerala,
West Bengal, Odisha, Uttar Pradesh and Madhya Pradesh during later part of July to
September.
References :
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