India Meteorological Department southwest monsoon 2005 end-of-season report

Report
from Government of India
Published on 05 Oct 2005


HIGHLIGHTS

  • For the country as a whole, the seasonal rainfall from 1 June to 30 September was 99% of its long period average (LPA)

  • Among the four homogeneous regions, southwest monsoon seasonal (June to September) rainfall over northeast India was deficient by 20%. Southwest monsoon seasonal rainfall over Central India, Northwest India and South Peninsula was 110%, 90% and 112% of LPA respectively.

  • During the season, rainfall was not well distributed in time. Rainfall over the country was below normal (12% below LPA) in June. However, monsoon was active in July with excess rainfall (14% above LPA). Monsoon was subdued in August with a large deficiency of 28% of LPA. In September (rainfall +17% above LPA), monsoon became active again helping a timely revival and improving the seasonal rainfall situation over the country.

  • Southwest monsoon seasonal (June-September) rainfall was excess/normal in 32 out of 36 meteorological sub-divisions and the remaining 4 subdivisions registered deficient rainfall.

  • 72 % of the meteorological districts received excess/normal rainfall whereas 26 % received deficient and only 2% received scanty rainfall .

  • Only one sub division (Jharkhand) experienced moderate drought conditions (rainfall deficiency of 35%) at the end of the season.

  • Out of 525 meteorological districts, 88 districts (17%) experienced moderate drought and 17 districts (3%) experienced severe drought conditions at the end of the season.

  • Out of the 168 meteorological districts which suffered from drought conditions last year, as many as 113 districts received normal/excess rainfall in this monsoon season, but drought continued in 55 districts.

  • IMD's long range forecasts for the monsoon onset over Kerala and for the 2005 southwest monsoon season rainfall over the country as a whole, central India and NW India proved to be correct.

1. ONSET PHASE

An important aspect of the onset phase was late arrival of monsoon over Peninsula and eastern parts of India, but early advance over northwest parts of India.

Southwest monsoon advanced over south Andaman Sea and parts of southeast Bay on 26th May, about one week behind the normal date. It advanced over some parts of southeast and southwest Bay, and entire Andaman Sea on 28th. On 31 May, it advanced over east central Bay and some more parts of south Bay.

Southwest monsoon advanced over Kerala on 5 June as a weak current, about 4 days later than its normal date. Monsoon covered some more parts of peninsular India on 8 June. However, there was an extended hiatus in advancement of monsoon from 9 to 15 June. During this period, due to the prevalence of anomalous anticyclone over central India, severe heat wave conditions prevailed over many parts of the country, especially over Orissa and coastal Andhra Pradesh. Monsoon advanced over northeast India only on 16 June, almost two weeks behind its normal date. From 18 June, monsoon advanced along the west coast and by 21 June it covered the Konkan and parts of Gujarat state. On 24 June, monsoon progressed rapidly into remaining parts of Maharashtra, Chattisgarh, Jharkhand and parts of Madhya Pradesh. Further advance of southwest monsoon was swift and it covered the entire country on 30 June about 2 weeks ahead of its normal date. During the period 1901-2004, only on 7 occasions (1960, 1961, 1975, 1980, 1994, 1996 and 1998) monsoon covered the entire country in June itself.

2. SYNOPTIC FEATURES AND ASSOCIATED RAINFALL ACTIVITY

During the season, as many as 11 low pressure areas formed over the Indian region, 8 over the Bay of Bengal, 2 over the Arabian sea and 1 over land. The systems that formed over the Bay of Bengal followed almost a normal track along the monsoon trough across central India and adjoining Gangetic Plains. These systems resulted in persistent rainfall activity leading to flooding over Gujarat, Madhya Pradesh, Orissa, Maharashtra and Interior parts of Karnataka. Out of these low pressure areas, five developed into monsoon depressions and one into a cyclonic storm. This is for the first time after 1997, that a low pressure system intensified into a cyclonic storm during September.

In June, one depression formed over northeast Arabian Sea and adjoining Saurashtra and Kutch, which moved west-northwestwards and was short-lived. Another depression formed over land (Gangetic west Bengal) in June, which moved west-northwestwards and dissipated over Uttar Pradesh. On 29 July, another depression formed over northwest Bay of Bengal which moved in a west-northwest direction all the way to east Rajasthan and neighbourhood and dissipated as a low pressure area. In the second week of September, one depression formed over northwest Bay, which moved west-northwestwards across central India. A depression, which formed over Northeast Arabian sea (14-17 September) moved inland and caused heavy rainfall in Gujarat State. A low pressure area formed over Northeast Bay on 17 September drifted south-westwards and intensified into a cyclonic storm. This system crossed the north Andhra coast near Kalingapatnam resulting in heavy rainfall over Andhra Pradesh and parts of Maharashtra, Gujarat and subsequently over northwest India.

In August, rainfall activity was subdued over the country. The monsoon trough remained close to the foothills during the second and fourth week. The offshore trough along the west coast was also absent during the month.

On 27 July, an unprecedented heavy rainfall of 94.4 cm was recorded at Santacruz (Mumbai). It was a devastating rainstorm that crippled the lifeline infrastructure at Mumbai for days together. Presence of a well marked low pressure area over Madhya Pradesh, marked offshore trough at the surface along the west coast and a well marked east-west oriented shear line in the lower troposphere must have contributed to a favourable setting for the enhanced meso-scale convection around Mumbai on that day.

3. RAINFALL DISTRIBUTION DURING THE MONSOON SEASON

During the 2005 monsoon season, rainfall activity was not well distributed in time. Due to the late onset and advancement of monsoon, rainfall activity during the first three weeks of June was subdued. However, monsoon became active by third week of June and continued to be so till the end of July. However, in August, monsoon was subdued over the country with a large deficiency of 28%. Towards the end of first week of September, monsoon revived again, improving the rainfall scenario in most parts of the country. Month-wise distribution of rainfall departures is given in Fig. I.

In 2005, the southwest monsoon seasonal (June to September) rainfall over the country as a whole was 99% of its LPA. The cumulative rainfall from 1 June to 30 September 2005 was excess in 9 meteorological sub-divisions, normal in 23 meteorological sub-divisions and deficient in 4 sub-divisions. The sub-division wise cumulative rainfall distribution is shown in Fig.2. Only Jharkhand experienced moderate drought conditions (rainfall deficiency of 35%) at the end of monsoon season.

By 22 June, cumulative rainfall over the country as a whole was deficient by 49%. However, good rainfall activity in the crucial month of July helped the rainfall situation to improve to 5% above LPA by 3 August. Due to the subdued rainfall activity in August, cumulative rainfall over the country decreased to 8% below LPA by 7 September. The revival of monsoon during the second week of September improved the rainfall situation.

Figures 3. and 4. depict week by week and the cumulative rainfall over the season. Large rainfall deficiency was observed during the first three weeks of June, whole of August and the first week of September. Rainfall over the country as a whole was excess during the last week of June, most of July, and second and third week of September.

The southwest monsoon rainfall (June to September) for the period 1 June to 30 September 2005 for the country as a whole and the four broad homogeneous regions are as follows:

Region
Actual (mm)
Normal(mm)
% Departure
All-India
879.3
892.5
-1 %
Northwest India
552.1
611.6
-10%
Central India
1094.9
993.2
+10%
South peninsula
808.9
722.6
+12%
North east India
1140.9
1430.7
-20%

Out of 525 meteorological districts, (21%) districts received excess rainfall, (51%) normal, (26%) deficient and (2%) scanty rainfall. Data from 22 districts were not available.

4. WITHDRAWAL OF SOUTHWEST MONSOON

This year, monsoon withdrew from extreme west Rajasthan on 2nd September at its near normal date. However, its further withdrawal was delayed due to the prevalence of circulation patterns causing rainfall over northwest India till the last week of September. On 28 September, the monsoon withdrew from entire Punjab, west Rajasthan, parts of east Rajasthan, Gujarat state, Haryana and Jammu and Kashmir. It withdrew from entire east Rajasthan, Haryana including Delhi, parts of Uttaranchal, Himachal Pradesh west Uttar Pradesh and west Madhya Pradesh on 29.

5. LONG RANGE FORECAST OF MONSOON RAINFALL

In May 2005, IMD using an indigenously developed statistical model, predicted that the monsoon onset over Kerala would take place between 4 and 10 June. The actual onset over Kerala was on 5 June, within the limits of IMD's prediction.

As per the long range forecast for the 2005 South-west monsoon seasonal rainfall issued by IMD in April and updated in Ist week of July, the seasonal rainfall for the country as a whole was expected to be near normal and quantitatively 98% of LPA with a model error of ±4%. The season ended with the area-weighted rainfall for the country as a whole as 99% of the LPA, very close to the prediction of IMD. Over the 4 broad homogenous regions of India, the rainfall was expected to be 97% of its LPA over Northwest India, 102% of LPA over Central India, 95% of LPA over NE India and 97% of LPA over South Peninsula with a model error of ±8%. The actual rainfall over these 4 regions was 90%, 110%, 80 % and 112% of the LPA respectively. IMD also issued the long range forecast for July rainfall for the country as a whole as 97% of its LPA. However, the actual rainfall in July 2005 (114% of LPA) was more than the IMD's prediction.

This year, the long range forecasts for the country as a whole, Central India and NW India proved to be correct. IMD's long range forecast overestimated rainfall activity over NE India, while it underestimated over South Peninsula.


Fig. 2 Subdivision wise Cumulative Rainfall

Fig. 3 : Week - by - Week Progress of the Monsoon Rainfa

Fig. 4  :  Week - by - Week Progress of the Monsoon Rainfall - 2005 ( Cumulative)