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Review of Southwest Monsoon 2008

New Delhi : October 1,2008

        Director General, India Meteorological Department, Dr. Ajit Tyagi today reviewed the Southwest Monsoon 2008 at a press conference in New Delhi. 
 

       Following are the salient features of Southwest Monsoon 2008: 
 

  • The cumulative seasonal rainfall for the country as a whole was near normal. Rainfall for the season (1st June to 30th September, 2008) was 98% of its long period average (LPA).

  • The spatial distribution of seasonal monsoon rainfall during 2008 was largely uniform with 30 meteorological subdivisions recording normal rainfall. Only 2 (Punjab and Orissa) and 4 (Nagaland, Manipur, Mizoram & Tripura, West Madhya Pradesh, Vidarbha and Kerala) subdivisions recorded excess and deficient rainfall respectively.

  • Monsoon rainfall was marked by large temporal variation for the country as a whole, as rainfall was 27% above LPA in June and that in July was 17% below LPA. The rainfall was near normal during August and September with departure from LPA of - 3% and -1% respectively.

  • Considering the spatial distribution of rainfall, there was excess rainfall in June over northwest India in association with mid latitude westerly systems and over northeast India due to monsoon depressions.  The rainfall in July was excess along the foothills of the Himalayas leading to flood over the Bihar and northeastern states and was deficient over central India. The deficient rainfall over peninsular India during June and July was compensated by the excess rainfall during August and September.

  • While, there was rapid progress of monsoon over most parts of the country after the onset over Kerala on 31st May, there was delay in withdrawal of monsoon from northwest India. Monsoon covered the entire country on 10th July against its normal date of 15th July.  The withdrawal of monsoon from west Rajasthan commenced on 29th September 2008 against normal date of 1st September.

  • Compared to last two years, the frequency on monsoon depressions has been less with development of only four depressions during this monsoon season. However, seven low pressure areas developed during the season and contributed to the seasonal rainfall.

  • IMDs long range forecast for the seasonal rainfall over the country as a whole and over different homogeneous regions except northwest India have been accurate. However, the seasonal rainfall over northwest India and rainfall during July for the country as a whole could not be predicted. While the prediction overestimated the rainfall during July for the country as a whole, it underestimated the seasonal rainfall over northwest India.

 
ONSET OF SOUTHWEST MONSOON 
 

      Southwest monsoon advanced over parts of southeast Bay, most parts of Andaman Sea and Bay Islands on 10 May, 2008, about 5 days ahead of its normal date (Fig. 1). The monsoon set in over Kerala on 31 May, one day prior to the normal date. Further, advance took place quite rapidly mainly due to a depression (5 6 June) over the east central Arabian Sea and a well marked low pressure area (9 11 June) over Saurashtra & Kutch and neighbourhood.   By 16 June, southwest monsoon covered most parts of the country outside some parts of Rajasthan. The rapid advance of monsoon could be attributed to the interaction of the monsoon circulation with mid-latitude westerly system.  Subsequently, there was a hiatus in the further advance due to the weakening of the monsoon current. The monsoon covered the entire country by 10 July, against normal date of 15 July.  
 

RAINFALL DISTRIBUTION 
 

      For the country as a whole, the seasonal rainfall from 1st June to 30th September was 98% of its LPA. Seasonal rainfall over NW India, Central India, NE India and South Peninsula was 107%, 96%, 94% and 96% of the LPA respectively. The spatial distribution of seasonal monsoon rainfall during 2008 was almost uniform with 30 meteorological subdivisions recording normal rainfall. Only 2 (Punjab and Orissa) and 4 (Nagaland, Manipur, Mizoram and Tripura, West Madhya Pradesh, Vidarbha and Kerala) subdivisions recorded excess and deficient rainfall respectively. Out of 516 meteorological districts for which data are available, 392 districts (76) % of the meteorological districts received excess/normal rainfall and the remaining 124 districts (24%) received deficient/scanty rainfall during the season. 65 districts (13%) experienced moderate drought and 14 districts (3%) experienced severe drought at the end of the season. Thus during the 2008 monsoon season, rainfall activity was well distributed in space with most parts of the country received near normal seasonal rainfall. 
 

   Monsoon rainfall was marked by large temporal variation. Monsoon rainfall over the country as a whole was 27% above LPA during June and 17% below LPA in July. It was near normal during August (97% of LPA) and September (99% of the LPA).

      

FLOOD SITUATIONS 
 

      The uneven temporal distribution caused flood situations on different occasions in many states viz., Assam, Meghalaya, Arunachal Pradesh, Nagaland, Manipur, Mizoram, Tripura, West Bengal, Orissa, Bihar, Uttar Pradesh, Haryana, Delhi, Punjab, Himachal Pradesh, Rajasthan, Gujarat, Maharashtra and Andhra Pradesh. 
 

 CHIEF SYNOPTIC FEATURES  
 

      The excess rainfall in June over northwest India was mainly associated with mid latitude westerly systems and that over northeast India due to monsoon depression.  The rainfall in July was excess along the foothills of the Himalayas leading to flood over the Bihar and northeastern states and was deficient over central India. The deficient rainfall over peninsular India during June and July was compensated by the excess rainfall during August and September.   
 

      Compared to last two years, the frequency of monsoon depressions has been less with development of only four depressions during this monsoon season. These included one depression over the Arabian Sea and another over Bay of Bengal during June, one land depression over coastal Orissa during August and one deep depression over the Bay of Bengal during September. The tracks of these systems are shown in Fig. 2.  The month of July was devoid of any monsoon depression like the previous July of 1995, 1998, 2000, 2001, 2002 and 2004. However, seven low pressure areas developed during the season and contributed to the seasonal rainfall.  
 

      From Fig.2, the depression over the Arabian Sea during 5th to 6th June moved away westwards and weakened over the Ocean. The second depression  over the Bay of Bengal  during 16th to 18th June crossed Bangladesh coast and moved across Gangetic West Bengal and Jharkhand. It then moved as a low pressure area upto east Uttar Pradesh and adjoining east Madhya Pradesh. The system caused heavy to extremely heavy rainfall over Gangetic West Bengal, north Orissa and Jharkhand leading to flood over these regions.   The third system was a land depression (9 10 August) over coastal Orissa and was short lived with the life period of less than 12 hours.  The fourth system was a deep depression (1519 September) over the northwest Bay of Bengal which crossed Orissa coast near Chandbali and moved across north Orissa, north Chhattisgarh, northeast Madhya Pradesh and central Uttar Pradesh. The remnant low pressure area moved upto northwest Uttar Pradesh.  This system caused heavy to extremely heavy rainfall over Orissa and Chhattisgarh leading to severe flood over Orissa. This system also interacted with mid-latitude westerly systems and caused good rainfall over northwest India and led to flood over Haryana and Himachal Pradesh.  
 

      Apart from the above systems, 7 low pressure areas formed during the season.  
 

  WITHDRAWAL OF SOUTHWEST MONSOON. 
 

    There was a delay in the commencement of withdrawal of southwest monsoon from extreme west Rajasthan. The southwest monsoon withdrew this year from entire Jammu & Kashmir, Himachal Pradesh, Punjab, Haryana, Chandigarh & Delhi, west Rajasthan, most parts of Uttarakhand, west Uttar Pradesh and east Rajasthan, some parts of north Gujarat State and north Arabian Sea on 29th September. The normal date of withdrawal of southwest monsoon from west Rajasthan is 1 September. The delay was mainly due to the presence of systems in westerlies over northwest India interacting with the monsoon circulation.  Comparing with recent years (1990-2007), the latest withdrawal in recent years from west Rajasthan took place on 30 September during 2007.  
 

PERFORMANCE OF LONG RANGE FORECAST ISSUED BY IMD  
 

      Onset of monsoon over Kerala:  
 

      Using an indigenously developed statistical model, IMD predicted that monsoon onset over Kerala would take place on 29th May with a model error of 4days. This year, the monsoon onset over Kerala was on 31st May, just one day earlier than its normal date and hence within the forecast range. 
 

      Long range forecast of rainfall: 

 

      The details of the long range forecasts issued by IMD and the actual rainfall are shown in the Table below:  
 

REGION

PERIOD

ISSUED ON

FORECAST

ACTUAL

 
All India

June to September

16th April, 2008

99% of LPA 5%

 
98% of LPA

30th June, 2008

100% of LPA 4%

All India

July

30th June, 2008

98% of LPA 9%

83% of LPA

Northwest India

 
June to September

 
 
30th June, 2008

96% of LPA 8%

107% of LPA

Northeast India

101% of LPA 8%

94% of LPA

Central India

101% of LPA 8%

96% of LPA

South India

98% of LPA 8%

96% of LPA

 

       IMDs long range forecast for the seasonal rainfall over the country as a whole and over different homogeneous regions except northwest India have been accurate. However, the seasonal rainfall over northwest India and rainfall during July for the country as a whole could not be predicted. While the prediction overestimated the rainfall during July for the country as a whole, it underestimated the seasonal rainfall over northwest India.   

 

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