4 October 2002

BHARAT MAUSAM VIGYAN VIBHAG

INDIA METEOROLOGICAL DEPARTMENT

SOUTHWEST MONSOON 2002 END-OF-SEASON REPORT

Back to home                     Fig 1            Fig 2                           Fig3                                    

Summary: The year 2002 was the first all-India drought year after a continuous spell of 14 good monsoons that followed the previous all-India drought year of 1987. The seasonal rainfall (June-September) for the country as a whole was 19% below normal. 29% area of the country experienced drought conditions with 19% area under moderate category and 10% under severe category. July had the worst rainfall deficiency of 49%.

An assessment of the performance of the southwest monsoon during the period 1 June to 30 September 2002 is given in this report.

1. Season’s Rainfall

When the rainfall for the monsoon season of June to September for the country as a whole is within 10% of its long period average, it is categorised as a normal monsoon. When the monsoon rainfall deficiency exceeds 10%, it is categorised as an all-India drought year.

This year, the southwest monsoon rainfall for the country as a whole was 19% below normal, making 2002 an all-India drought year.

The performance of monsoon rainfall over smaller areas of the country is monitored by evaluating the departures from the normal for each meteorological sub-division and district. The rainfall is classified as excess, normal, deficient or scanty as per the following criteria :

Excess : +20% of normal or more

Normal : +19 % to –19% of normal

Deficient : –20% to –59% of normal

Scanty : –60% of normal or less

At the end of the monsoon season, 15 out of 36 meteorological sub-divisions, covering 44% area of the country, received normal to excess rainfall (Fig.1). Out of the remaining 21 sub-divisions, only two viz., East and West Rajasthan, are in the scanty category and 19 sub-divisions are in the deficient category.

Out of the 523 meteorological districts, 272 (53%) are in deficient category, and 40 (8%) are in scanty category. 172 (33%) of the districts have normal rainfall and 33 (6%) have excess rainfall.

Table 1 gives the rainfall situation in different states, including the number of meteorological districts in each category in every state.

2. Vagaries of 2002 Monsoon

The behaviour of the 2002 monsoon was intriguing. Its onset over Kerala was on time but it reached northwest India only by mid-August. June rainfall was above normal but July recorded the lowest rainfall in history. There was a remarkable recovery in August, which saved the situation from worsening. The week-by-week variation of cumulative rainfall for the country as a whole is shown in Fig. 2.

2.1 Onset and Advance of Southwest Monsoon

The onset of the southwest monsoon over Kerala was on 29 May, 3 days earlier than its normal date of 1 June. By 12 June, the southwest monsoon covered peninsular India, northeastern region and some parts of east central India as per its normal pattern. Thereafter, the progress was halted for about a week. Monsoon strengthened along the west coast after the first fortnight of June in association with an off-shore trough.

A well marked low pressure area formed over northwest Bay of Bengal on 20 June. It moved across the central parts of the country and brought good rainfall over the entire central belt from Orissa / Gangetic West Bengal to Maharashtra / Gujarat region across Jharkhand, Chhattishgarh and Madhya Pradesh. The interior parts of Andhra Pradesh and Karnataka also received good rains. The low pressure weakened on 28 June over southeast Rajasthan. It caused exceptionally heavy rains giving rise to floods disrupting rail and road traffic in parts of Maharashtra and Gujarat for days together. As per media reports, 130 deaths occurred in Maharashtra and 75 in Gujarat. The cumulative rainfall for the country as a whole towards the end of June was 4% above normal.

In the absence of any synoptic system in the first fortnight of July, no further advance of monsoon took place during this period. Monsoon advanced upto Delhi and neighbourhood as a weak current on 19 July. However, there was another long delay in the subsequent advance of monsoon and it covered the entire country only by 15 August.

 

2.2 Hiatus in Progression of Monsoon

The first half of July was characterised by a dry spell, which resulted in prolonged summer conditions over north and northwest India. This pronounced ‘Break’ in the southwest monsoon season did not spare even the northeastern region where rainfall activity was also subdued. A feeble low pressure area formed in the head Bay of Bengal on 15 July. In association with this low pressure area there was a temporary respite from the dry spell when many parts of the country received scattered monsoon rains. The low pressure weakened on 19 July and the monsoon trough again shifted to the foothills. In association with the break monsoon phase, Bihar and Assam rivers were in floods.

The monthly rainfall distribution is shown in Fig. 3. The monthly rainfall deficiency exceeded 75% during July in central India. Only the northeastern parts and neighbouring Bihar and adjacent areas recorded normal rainfall in July. For the country as a whole, the rainfall deficiency was 49%, which was the worst in the history of recorded observations. The combined June-July deficiency was 30% which also had few parallels.

 

2.3 Revival of Monsoon in August

A redeeming feature of this year’s monsoon was that unlike previous years of major droughts, the pronounced lack of rainfall activity in July did not continue into August and September. The monsoon revived in the first week of August and was maintained by a series of low pressure areas that formed in the northwest Bay of Bengal and moved across the central parts of the country. This resulted in wiping out a large deficiency of rainfall particularly from the central parts of the country. Barring the last low pressure area that moved right upto Gujarat, the earlier lows were short-lived having average longevity of about 4 days. They weakened before reaching East Madhya Pradesh and merged with the seasonal trough. Their remnant circulations kept the monsoon trough alive and it improved the rainfall distribution particularly in the central parts of the country.

In the last week of August, a low pressure developed over northwest Bay on 22 August. It moved westwards across central India and weakened over Gujarat and neighbourhood on 26 August. The monsoon trough during these developments was in the normal position or to its south. Therefore, the rainfall activity was enhanced over Orissa, Telangana, Interior Maharashtra and Gujarat during the week that ended on 28 August. The western end of monsoon trough again shifted towards foothills on 27 August. This increased the rainfall activity over western Himalayas and Haryana. Another feeble low pressure area formed over northwest Bay and adjoining coastal Orissa on 29 August. It moved across Orissa and Chhattisgarh and weakened over East Madhya Pradesh on 2 September. However, upper air circulation persisted till 4 September. A western disturbance also moved across western Himalayas. These conditions again led to enhancement of rainfall activity over most parts of central and northwest India.

 

2.4 Withdrawal Phase of Monsoon

September is a month of monsoon withdrawal. However, the monsoon withdrawal from West Rajasthan started this year on 16 September, a little later than its normal date. A low pressure area that formed over northwest Bay on 8 September, moved in a northwesterly direction and weakened over West Uttar Pradesh on 14 September. Its remnant circulation continued to affect the northern parts right upto 17 September. This brought a good surge in monsoon rainfall activity first over central parts of the country and then over the northern parts. There was a heavy rainstorm on 12 September when Delhi recorded 15 cm rain.

The last week of September witnessed another low pressure development in the northwest Bay that affected the eastern States. There was normal to excess rainfall activity over northeastern states, West Bengal, Orissa, Bihar, Jharkhand and East Uttar Pradesh.

By 30 September, the southwest monsoon had withdrawn from northwest India and parts of western and central India. Further withdrawal of monsoon is likely to take place during the first week of October. It does not however signal the end of rains in the peninsula, where the southwest monsoon continues upto mid-October. October to December is the main season of the northeast monsoon rainfall over Tamil Nadu.

3. Drought

3.1 Definitions of Drought

There is no universally accepted definition of drought. However, drought is broadly perceived in three different ways :

    1. Meteorological drought: When actual rainfall over an area is significantly less than the climatological mean.
    2. Hydrological drought: When there is marked depletion of surface water causing very low stream flow and drying of lakes, reservoirs and rivers.
    3. Agricultural drought: When inadequate soil moisture produces acute crop stress and affects productivity.

Meteorological drought need not necessarily result in agricultural drought, since timely rainfall during critical crop phases may save the crop, or irrigation water may be available. However, rainfall being the ultimate source of water, in the event of an extreme rainfall deficiency, its agricultural and hydrological impacts are inevitable.

It is common for drought conditions to exist on a sub-division or district level even when the monsoon is normal for the country as a whole. For such small areas, IMD defines a moderate meteorological drought when the rainfall is 26-50% below normal and a severe meteorological drought when it is more than 50% below normal.

When the rainfall deficiency for the country as a whole is more than 10% of normal and more than 20% of the country’s area is affected by drought conditions, the situation is defined as an all-India drought year.

 

 3.2 Drought of 2002

By the above definition, 2002 becomes an all-India drought year and the following sub-divisions have severe / moderate drought :

Severe Drought

Met. sub-division % departure of rainfall

West Rajasthan -71%

East Rajasthan -60%

Moderate Drought

Met. sub-division % departure of rainfall

Haryana, Chandigarh & Delhi -36%

Punjab -36%

Coastal Andhra Pradesh -26%

Rayalaseema -33%

North Interior Karnataka -31%

South Interiror Karnataka -44%

Coastal Karnataka -30%

Tamil Nadu -45%

Kerala -35%

Lakshadweep -45%

29% area of the country (total area of the above listed sub-divisions) is under drought, 10% area under severe drought and 19% under moderate drought. 37% districts of the country have moderate drought and 15% have severe drought.

Rainfall deficiency of 49% in the month of July, on all-India basis, is a case of the worst meteorological drought. Only on two occasions in the past (1911 and 1918) it was over 45%, and both the years had major droughts.

July rainfall is critical to agriculture. July is the rainiest month of the monsoon season that registers one-third of the seasonal rainfall. Although long range prediction of monthwise rainfall distribution at the beginning of the monsoon season is beyond the current state-of-art, efforts will have to be made to develop a model for prediction of July rainfall using meteorological data upto June end, so as to enable a mid-season correction.

Table 2 shows a comparison of the 2002 monsoon with three major droughts of recent times (1972, 1979 and 1987).

 

 

Table 2

Monthwise Rainfall Distribution

(Percentage Departure for the country as a whole

in recent major drought years)

YEAR

JUNE

JULY

AUG

SEPT

JUNE-SEPT

1972

-27

-31

-14

-24

-24

1979

-15

-16

-19

-28

-19

1987

-22

-29

-4

-25

-19

2002

+4

-49

-4

-10

-19

 

The rainfall during June, August and September 2002 was better than in 3 other drought years, but the July rainfall was worst. The seasonal rainfall deficiency in 2002 is comparable with that of 1987 and 1979.

4. Causes of 2002 Monsoon Failure - A Preliminary Analysis

As per the long range forecast for the 2002 southwest monsoon seasonal rainfall issued by IMD on 25 May, the rainfall for the country as a whole was likely to be normal, 101% of its long period average (LPA) with a model error of ± 4%. Over the 3 broad homogeneous regions of India, the rainfall was likely to be 104% of its LPA over Northwest India, 99% of the LPA over the Peninsula and 100% of the LPA over Northeast India with a model error of ± 8%. The actual rainfall over these 3 regions was 74, 78 and 89% of the LPA respectively.

It may be mentioned here that long range forecasts of the Indian southwest monsoon rainfall are generated by many other national and global centres which employ different prediction techniques. None of these forecasts indicated a large deficiency of the monsoon rainfall that was actually observed. This calls for a thorough scientific investigation. In 2002, not a single monsoon depression formed over the Bay of Bengal mainly because of the poor strength of low-level southwesterly flow which was accompanied by an anomalous anticyclone over western India. In fact the strength of positive pressure anomaly rose to 3.0 hPa over Saurashtra coast during July and got diluted to 1.0 hPa only in August. This weakened the meridional pressure gradient and therefore the monsoon flow. The cross equatorial flow was extremely weak. The strength of upper air easterlies was also below normal. The low pressure areas could not be steered all the way upto northwest India. The upper air Tibetan anticyclone had shifted south of its normal position.

The El Nino signals were mixed. The El Nino abated in Nino 1+2 region (eastern equatorial Pacific) from June onwards; but it built up in the Nino 3 and 4 (west and central Pacific) regions. IMD, while issuing the long range forecast for 2002 monsoon, had indicated the existence of the El Nino and its possible adverse influence on the monsoon performance.

In order to unravel the causes of the anomalous behaviour of the 2002 monsoon, Indian meteorologists are planning to meet at a brainstorming session in November 2002. Preliminary results of the Arabian Sea Monsoon Experiment (ARMEX) organised from 15 June to 15 August this year, in which a large number of national organisations participated to gather ocean and atmospheric observations, are also likely to become available by that time. These observations may throw some light on the behaviour of the Arabian Sea component of the monsoon circulation. The national consensus emerging from this session will provide guidance for further developments of long range monsoon forecasting techniques in IMD and other organisations.

 

 Table 1

Statewise Rainfall Distribution

S.No.

STATE/UNION TERRITORY [UT]

Rainfall departure from normal (%)

Number of Districts in Different Categories

E

N

D

S

ND

Total

1

Andaman & Nicobar Islands [UT]

-24%

0

0

2

0

0

2

2

Arunachal Pradesh

-22%

3

3

5

1

1

13

3

Assam

-12%

4

10

7

1

0

22

4

Meghalaya

-36%

1

1

0

1

0

3

5

Nagaland

-30%

0

2

2

0

0

4

6

Manipur

-10%

1

1

0

1

0

3

7

Mizoram

-30%

0

1

1

0

0

2

8

Tripura

1%

0

3

0

0

0

3

9

Sikkim

15%

0

1

0

0

0

1

10

West Bengal

0%

3

12

2

0

0

17

11

Orissa

-19%

0

15

13

0

0

28

12

Jharkhand

3%

2

6

4

0

0

12

13

Bihar

1%

2

17

7

0

3

29

14

Uttar Pradesh

-23%

2

25

36

0

1

64

15

Uttaranchal

-2%

4

3

5

0

0

12

16

Haryana

-38%

0

3

11

5

0

19

17

Chandigarh [UT]

-4%

0

1

0

0

0

1

18

Delhi [UT]

-39%

0

0

1

0

0

1

19

Punjab

-36%

0

5

11

0

0

16

20

Himachal Pradesh

-20%

1

2

9

0

0

12

21

Jammu & Kashmir

22%

4

3

3

1

0

11

22

Rajasthan

-64%

0

0

12

20

0

32

23

Madhya Pradesh

-18%

1

15

27

2

0

45

24

Chhattisgarh

-16%

0

7

8

0

0

15

25

Gujarat

-25%

2

5

16

2

0

25

26

Daman, Dadra & Nagar Haveli [UT]

-17%

0

1

0

0

0

1

27

Diu [UT]

 

0

0

0

0

1

1

28

Goa

-23%

0

0

1

0

0

1

29

Maharashtra

-12%

3

19

11

0

0

33

30

Andhra Pradesh

-25%

0

7

16

0

0

23

31

Tamil Nadu

-45%

0

1

23

5

0

29

32

Pondicherry [UT]

-40%

0

0

1

0

0

1

33

Karnataka

-37%

0

2

25

0

0

27

34

Kerala

-35%

0

1

12

1

0

14

35

Lakshadweep [UT]

-45%

0

0

1

0

0

1

 

TOTAL

 

33

172

272

40

6

523

Legend:

E: Excess; N: Normal; D: Deficient; S: Scanty; ND: No Data;