GOVERNMENT OF INDIA

INDIA METEOROLOGICAL DEPARTMENT

 

                                                                                                                             

 

SOUTHWEST MONSOON 2007

END OF SEASON REPORT

 

 

HIGHLIGHTS

 

  • For the country as a whole, the seasonal rainfall from 1st June to 30th September was 105% of its long period average (LPA)

 

  • Seasonal rainfall was excess by 26% over South Peninsula. It was deficient (15% below LPA) over Northwest (NW) India, 8% above LPA over Central India and 4% above LPA over Northeast (NE) India.

 

  • Out of the 36 meteorological sub-divisions, the seasonal (June-September) rainfall was excess in 13 and normal in 17 sub-divisions. However, it was deficient in 6 sub-divisions.

 

  • Out of 513 meteorological districts for which data were available, 72% of the meteorological districts received excess/normal rainfall and the remaining 28% received
    deficient/scanty rainfall during the season.
    77 districts (15%) experienced moderate drought and 30 districts (6%) experienced severe drought at the end of the season.

 

  • Five sub-divisions (viz. West Uttar Pradesh, Haryana, Chandigarh and Delhi, Punjab, Himachal Pradesh and east Madhya Pradesh) experienced moderate drought conditions (rainfall deficiency of 26% to 50%) at the end of the season.

 

  • IMD’s long range forecasts for July rainfall over the country as a whole and the 2007 seasonal rainfall over NW India and NE India were proved to be accurate. However, the 2007 monsoon seasonal rainfall over the country as a whole was more than the predicted value.

 


1.       ONSET OF SOUTHWEST MONSOON

Southwest monsoon advanced over the south Andaman Sea, Nicobar Islands and parts of southeast Bay of Bengal on 10 May about 5 days ahead of its normal date. This was associated with the formation of a Depression over the north Andaman Sea (3rd – 5th May) and the strengthening of the cross equatorial flow. However, the subsequent advance, was delayed by the formation of the cyclonic storm ‘Akash’ (13th  – 15th May) over the east central Bay which had an unconventional origin in the mid-latitude westerlies. The system moved northeastward and crossed Bangladesh coast. It disrupted the monsoon flow by prolonging the mid-latitude westerly intrusion over the region. The monsoon revived gradually and arrived over Kerala on 28th May, four days prior to the normal date. Once again, the monsoon flow pattern was disrupted due to the formation of the Super Cyclonic Storm ‘Gonu’ over the east central Arabian Sea (1st – 7th June) which crossed Oman coast and subsequently the Makaran coast. Further advance of monsoon took place on 8th June, after a hiatus of 9 days. It covered the north-eastern states by 10th June, Peninsular and Central India by 25th June and subsequently the entire country on 4th July, nearly 11 days ahead of normal date.

Fig. 1 gives the isochrones of advance of southwest monsoon 2007.

 

2.           SYNOPTIC FEATURES

Formation of two intense low pressure systems over the Arabian Sea in the month of June is a unique feature of the southwest monsoon 2007, barring the years 1948, 1930, 1925 & 1907. Gonu is the first ever Super Cyclone formed over the Arabian Sea. The Cyclonic Storm, ‘Yemyin’ (25-26 June) formed from the remnants of a Deep Depression which formed over the Bay of Bengal and emerged into the Arabian Sea as a low pressure area after traversing the peninsula. This system moved away north-westwards and crossed Pakistan coast, without affecting the weather over the country. Apart from the above two Cyclonic Storms, 11 more low pressure systems including 4 Deep Depressions, 1 Depression, 4 well marked low pressure areas and 2 low pressure areas formed during the season. Most of these systems formed over the Bay of Bengal except a well marked low pressure area (23rd – 25th September)  which formed over the Arabian Sea in September. All the systems over the Bay of Bengal moved generally in a west-northwesterly to northwesterly direction, giving rise to extremely heavy rainfall (25 cm. or more) many a times over Orissa, Gangetic West Bengal, Bihar, Andhra Pradesh, Chhattisgarh, Madhya Pradesh, Rajasthan, Gujarat and also in Maharashtra and Karnataka.

Tracks of the Cyclonic Storms and Depressions during the season are shown in Fig. 2.

 

3.  RAINFALL DISTRIBUTION DURING MONSOON SEASON

The southwest monsoon rainfall (June to September) for the period 1 June to 30 September 2007 for the country as a whole and the four broad homogeneous regions are as follows:

 

Region

Actual (mm)

Normal (mm)

Percentage Departure

All-India

936.9

892.2

5%

Northwest (NW) India

520.8

611.6

-15%

Central India

1073.8

993.9

8%

South peninsula

907.3

722.6

26%

Northeast (NE) India

1485.9

1427.3

4%

 

 

In 2007, the southwest monsoon seasonal (June to September) rainfall over the country as a whole was 105% of its LPA.  Seasonal rainfall over NW India was below its LPA by 15%. However, over south Peninsula, seasonal rainfall was above its LPA by 26%. Similarly, Central India and NE India also experienced above average seasonal rainfall (8% and 4% above LPA respectively). The above average performance of the monsoon rainfall over the country was mainly due to the excess rainfall observed over South Peninsula and Central India.

The cumulative rainfall from 1 June to 30 September 2007 was excess in 13, normal in 17 and deficient in 6 meteorological sub-divisions. The sub-divisionwise cumulative rainfall distribution is shown in Fig.3. Five sub-divisions (West Uttar Pradesh, Haryana, Chandigarh and Delhi, Punjab, Himachal Pradesh and east Madhya Pradesh) experienced moderate drought conditions (rainfall deficiency of 26% to 50%) at the end of the season. Arunachal Pradesh received deficient rainfall (20% below its LPA).

 Out of 513 meteorological districts for which data were available, 144 districts (28%) received deficient rainfall (rainfall deficiency more than 19%) during the season, out of which 77 districts (15%) experienced moderate drought conditions (rainfall deficiency 26% to 50%) and 30 districts (6%) experienced severe drought conditions (rainfall deficiency 51% and more). The rainfall was excess ( actual rainfall higher than LPA by 20% or more ) over 164 districts (32%) during the season.

 

Month-wise distribution of rainfall departure over the country as a whole is given below:

 

June:    19% above LPA,                  July:            3% below LPA,

August: 1% below LPA                              September:  18% above LPA

 

The spatial distribution of monthly rainfall is shown in Fig.4.

 

Figures 5 and 6 depict the monsoon rainfall as received week by week and the cumulative rainfall during the season respectively. In June, the weekly rainfall was below normal during the first two weeks and above normal during the remaining weeks.  In July, the rainfall was above normal during the first two weeks and below normal during the remaining weeks. In August, the rainfall was below normal during all weeks except the first week. In September, the rainfall was below normal during the 3rd week and above normal during all other weeks. Large rainfall deficiency was observed during the 1st week of June, 3rd and 4th weeks of July and 3rd week of August.  During the season, cumulative seasonal rainfall over the country as a whole remained always above normal since last week of June (Fig.6). By this week, the cumulative seasonal rainfall was above normal by 7%. At the end of subsequent week (1st week of July) the cumulative seasonal rainfall increased and became above normal by 20%.  However, by the end of July, the cumulative seasonal rainfall decreased and became 3% above normal. At the end of August, the cumulative seasonal rainfall was 2% above normal, and it became 5% above normal by end of the season.

 

4.         Flood situations

            The uneven distribution of rainfall in space and time caused flood situations in many states viz. Assam, Meghalaya, Arunachal Pradesh, Manipur, Tripura, Andhra Pradesh, Kerala, Karnataka, Maharashtra, Orissa, Chattisgarh, Gujarat, Rajasthan, Madhya Pradesh, West Bengal, Jharkhand, Bihar, Uttar Pradesh, Himachal Pradesh, Uttarakhand, Jammu & Kashmir, Punjab and Haryana during various parts of the season.

 

5.         Withdrawal of southwest monsoon.

            This year, there was an unusual delay in the withdrawal of monsoon from extreme west Rajasthan, due to the prevalence of cyclonic circulations, availability of moisture and sporadic rainfall over the region. However, the southwest monsoon withdrew from western parts of Rajasthan and some parts of Punjab and Haryana on 30th September. The normal date of withdrawal from west Rajasthan is 15th September. During the period 1960-2006, the most delayed date of monsoon withdrawal from extreme west Rajasthan was 28th September, which occurred in the years 1964 & 1970. In the year 1990 also the withdrawal started as late as 27th September.

 

6.       LONG RANGE FORECAST OF MONSOON RAINFALL

In May 2007, using an indigenously developed statistical model, IMD predicted that monsoon onset over Kerala would take place on 24th May with a model error of ±3 days. This year, the monsoon onset over Kerala was on 28th May, four days earlier than its normal date of 1st June.

As per the long range forecast for the 2007 southwest monsoon seasonal rainfall issued in April, the seasonal rainfall for the country as a whole was expected to be 95% of LPA with a model error of ± 5%. In the updated forecast issued on 29 June, the forecast for the country as a whole was revised as 93% of LPA with a model error of ± 4%.  The season ended with the area-weighted rainfall for the country as a whole at 105% of the LPA, more than the error limit of the IMD’s long range forecast. Considering 4 broad homogenous regions of India, rainfall was expected to be 90% of its LPA over NW India, 96% of LPA over Central India, 98% of LPA over NE India and 94% of LPA over South Peninsula with a model error of ±8%. The actual rainfall over these 4 regions was 85%, 108%, 104% and 126% of the LPA respectively. Thus, the seasonal rainfall over NW and NE India was well-predicted, whereas the rainfall over both Central India and South Peninsula was higher than predicted.

 IMD also issued the long range forecast for rainfall over the country as a whole in July 2007 as 95% of its LPA. The actual rainfall in July 2007 was 97% of LPA, very close to the predicted value. The Table below gives the summary of the verification of the long range forecasts issued for the 2007 southwest monsoon.

 

Table

Details of long range forecasts and actual rainfall.

Region

Period

Issued on

Forecast

Actual

All India

June  to September

19  April, 2007

95% of LPA ± 5%

105% of LPA

29 June, 2007

93% of LPA ± 4%

All India

July

29 June, 2007

95% of LPA ± 9%

97% of LPA

Northwest India

June  to September

29  June, 2007

90% of LPA ± 8%

85% of LPA

Northeast India

98% of LPA ± 8%

104% of LPA

Central India

96% of LPA ± 8%

108% of LPA

South Peninsula

94% of LPA ± 8%

126% of LPA

 


 

 

 

 

 

Fig.1. Progress of Southwest Monsoon – 2007.

 

 


 

 

 

 

 

 

 

 

 

Fig.2. Tracks of the low pressure systems over Indian seas during the Southwest Monsoon Season– 2007.

 

 

 


Fig.3. Sub-divisionwise rainfall distribution over India during southwest monsoon

  season (June to September) – 2007


 

 

Fig.4. Sub-divisionwise monthly rainfall distribution over India

          during southwest monsoon season – 2007


 

 

Fig.5. Progress of the weekly monsoon rainfall - 2007

 

Fig.6. Progress of the weekly cumulative monsoon rainfall - 2007