Changing patterns

The cumulative rainfall during the monsoon in northeastern India has been significantly in deficit.

R. RAMACHANDRAN in New Delhi

DROUGHT in the northeast of India is a contradiction in terms. But that is exactly what one has witnessed this year as the south-west monsoon, usually associated with heavy rainfall and floods in the region, draws to a close. Even though the monsoon is well on its way to being a "normal" one for the country as a whole, until the week ending September 13 the cumulative rainfall in each of the four meteorological subdivisions of the northeastern region has been significantly in deficit. The overall deficiency for the entire region is about 26 per cent.

Of the northeastern States, Arunachal Pradesh is in deficit by 27 per cent, Assam by 21 per cent, Meghalaya by 68 per cent, Nagaland by 50 per cent, Manipur by 18 per cent and Tripura by 17 per cent; only Mizoram has had surplus rainfall, of 12 per cent. Data of the India Meteorological Department (IMD) show that 26 of the 57 districts of the region are facing "drought" - drought being defined by the IMD as a rainfall deficit of 25 per cent or more from the district-level long-period average (LPA). These include seven out of 13 districts in Arunachal Pradesh, seven out of 22 in Assam, two out of three in Meghalaya, all four in Nagaland, one of three in Manipur, two out of three in Tripura and three out of six in (sub-Himalayan) West Bengal.

Though the week ending September 13 itself received above average rainfall, since this average itself is less than 10 per cent of the total average for the season, it could hardly be expected to offset the cumulative deficit since June 1.

In fact, in the following week (beginning September 14) the rainfall again fell below normal while there was above normal rainfall in the southern peninsula, as it has been for most of the season this year. In fact, this year's rainfall distribution has been so skewed that there has been significant shortfall in the entire North.

Meteorologically speaking, though the situation is quite unusual and even rare, it is not unique. What could, however, be of concern is the fact that such conditions have arisen more frequently in recent years. The 2005 monsoon too resulted in significant rainfall deficit in all the four meteorological subdivisions of the northeastern region, though only two came to be categorised as "deficient" as per the IMD's terminology which defines it so only when deficiency is 20 per cent or more. Like this year, last year too the shortfall extended to Bihar, Jharkhand and eastern Uttar Pradesh. In fact, with a shortfall of 35 per cent, Jharkhand experienced a severe drought.

District-wise distribution in the region also shows that the number of districts that were seriously affected were fewer. But some districts that were hit badly, such as Jorhat, Morigaon, Nowgong and Sonitpur in Assam, Changlang, Lower Subansiri and Tawang in Arunachal Pradesh, Jaintia Hills in Meghalaya and nearly all the districts of Nagaland, are facing drought conditions for the second year in succession.

(A caveat about rainfall data of the region is, however, in order here. Rain gauge stations in the region, because of the terrain and attendant network management problems, are sparse.

As a result, when one station reports good rainfall, this may not extend to all areas around the station. Conversely, when a station reports shortfall, this may not extend to the surrounding regions. Though the data give a picture of the gross monsoon situation in the region, these are not accurate indicators of the precise district-level conditions.

The real situation is best judged from socio-economic conditions prevalent in the region. The apparent changing rainfall profile in the region really calls for strengthening the rain gauge and communication network in the region.)

According to M. Rajeevan of the National Climate Centre (NCC) of the IMD, Pune, though the meteorological conditions that have led to rainfall deficit in the northeastern region are somewhat similar to the situation in 2005, the conditions seem to be closer to what was witnessed in 1994.

In fact, like this year, in 1994 too rainfall was in excess (+20 per cent or more) in the central and west Indian meteorological subdivisions of west Rajasthan, east Rajasthan, west Madhya Pradesh, east Madhya Pradesh, the Gujarat Region, Saurashtra and Kutch and central Maharashtra. These too were "normal" monsoon years for the country as a whole, as 2006 is likely to be. Total rainfall in 2005 was 99 per cent of the LPA, while 1994 was 110 per cent.

In 1994 too, all four subdivisions of the North-east had deficient rainfall. Arunachal Pradesh had a deficit of 28 per cent, Assam and Meghalaya 22 per cent, Nagaland, Manipur, Mizoram and Tripura 28 per cent and sub-Himalayan West Bengal and Sikkim 34 per cent. The rainfall deficit did not, however, extend westwards into the Gangetic plain of the north except the Bihar plains which had a deficit of 20 per cent.

The atmospheric conditions during the south-west monsoon that generally bring copious rainfall to the northeastern and Himalayan regions are what are collectively termed in meteorological parlance as "monsoon break conditions" when the monsoon retreats from the central and southern peninsula to the northern and northeastern regions. This period can last from five days to a week.

During the south-west monsoon, an elongated low-pressure area is established over central India. Meteorologists refer to this as the `monsoon trough'.

The trough axis, which is roughly parallel to the southern periphery of the Himalayan range, is oriented from the Bay of Bengal in a north-western direction into the mainland. It is a quasi-permanent feature of the summer monsoon, but the position of the axis shows variations during different phases of the monsoon when it is anchored differently.

When it is anchored south of its normal position, we have the active phase of the monsoon. The rear-end extends into the ocean, enabling depressions and other rain-bearing systems that form in the Bay of Bengal to move along the trough and cause rainfall over the peninsular region.

On the other hand, when the axis moves north and is located close to the Himalayan foothills, there is a temporary break in the monsoon in the peninsular region.

The northward movement leads to a concentration of the rainfall over the region. The normal duration of the monsoon-break period is five to seven days, but prolonged breaks of up to even a couple of weeks have been observed on a few occasions. It has been observed that the longer breaks tend to occur during the second half of the monsoon in the months of August and September.

The significant aspect of this year, as well as that of 2005 and 1994, is that typical break conditions did not arise at all. According to Rajeevan, the monsoon trough was in fact small and was out of its normal position.

It was generally located over central India, bringing rains over the southern peninsula in August and September. The number of depressions over the Bay of Bengal was, in fact, more than usual, according to Rajeevan.

As many as six active systems had formed - a situation normally associated with winter - and four of them did move into the northeastern region. But in the absence of break conditions, the genesis could not be sustained and they weakened by the time they arrived over the Himalayan and northeastern regions. As a result, the precipitation that they did lead to was not widespread. They extended only over a region of 100-200 km radius, pointed out Rajeevan.

Is this part of the changing trends in the monsoon rainfall pattern? "Very hard to say," says Rajeevan.