The oceans and the monsoon

N. Gopal Raj

Keralites call it “ Idavapathy,” the rains that come in the middle of the Malayalam month of “ Idavam.” It marks the arrival of the South-West monsoon in India, the end of a long, hot summer and the start of a four-month season that provides well over three-quarters of the rain that this country receives each year. Once the monsoon sets in, its changing moods, be it the torrential downpours or maddening lulls, are closely watched, endlessly discussed and fretted over.

The monsoon is a hugely complex and chaotic phenomenon that involves an intricate interplay among the oceans, land and the atmosphere. Scientific understanding of the subtle choreography that decides how the the rainy season unfolds has grown by leaps and bounds. There is, nevertheless, much that must be deciphered. One part of that puzzle is the role that the oceans around India play in bringing and sustaining the monsoon.

‘An ocean drama'

“There is a drama going on in the Indian Ocean,” remarked P.V. Joseph, a veteran meteorologist who has continued to pursue his research interests after retiring from the India Meteorological Department (IMD). Dr. Joseph was speaking at a meeting on the monsoon organised by the Thiruvananthapuram Met. Office in May this year.

About a month and a half before the monsoon arrives in Kerala, the central Bay of Bengal warms. An extensive cloud band then forms in the southern Bay, which produces pre-monsoon rains in Kerala and Tamil Nadu, and moves north-east, taking the monsoon to Myanmar and South-East Asia.

The Bay cools as a result while a large stretch of the central Arabian Sea grows steadily warmer. Then, to the south of this warm patch of ocean, rain clouds begin building up near the equator. “It is those clouds that develop, move north and bring the monsoon to Kerala,” he said.

Onset over Kerala

The timing of the onset over Kerala is strongly influenced by temperatures on either side of the equator in the Indian Ocean and the Pacific, Dr. Joseph told this correspondent. If the southern side near the equator is unusually warm while the northern part is cold, the monsoon will be delayed.

The sea surface temperature has a critical bearing on cloud formation. In 1984, Sulochana Gadgil, an honorary professor at the Indian Institute of Science (IISc), Dr. Joseph, and N.V. Joshi, also on the faculty of the IISc, published a paper in the journal Nature that showed that over tropical oceans, the propensity for occurrence of deep cloud systems, such as depressions, becomes high once the sea surface temperature crosses 28° Celsius.

Warm oceans are a necessary, but not a sufficient, condition for such cloud formation (a process scientists term as convection). When ocean temperatures are maintained at above the threshold of 28° degrees Celsius, as is the case with the Bay of Bengal much of the time, it ceases to be the limiting factor for convection, noted Prof. Gadgil when she spoke to The Hindu.

Atmospheric conditions then influence the extent of cloud formation. Water that evaporates from the ocean surface must be able to rise high up into the atmosphere and form deep clouds. Gradients in temperature across the ocean surface can, it appears, play a significant role in creating such conditions in the atmosphere above them.

When the rains failed

Consequently, these temperature gradients can strongly influence the course of the monsoon. In 2009, for instance, the rains failed and the country suffered one of the worst droughts in over a century. That year, the rains had been especially poor in June and the shortfall in nationwide rainfall that resulted was not made up in the rest of the season.

During a monsoon's onset and subsequently, a cloud band develops near the equatorial Indian Ocean from time to time and then migrates northward across India. This vast cloud belt, which will typically stretch from the Arabian Sea to the Bay of Bengal and beyond, is what provides large-scale rain.

As the cloud band moves over the country, weather systems, such as lows and depressions, form in the Bay and move westwards across the land, bringing much rain. Ample convection over the surrounding oceans is therefore essential for the northward propogation of the cloud band as well as for generating weather systems over the Bay and sustaining the rain they produce.

Generally, in May and June during the monsoon's onset, the Bay is warmer than the eastern equatorial Indian Ocean and convection is favoured at the former location. However, in June 2009, the surface waters of the Bay were colder than usual while a large stretch of the equatorial Indian Ocean was exceptionally warm.

Consequently, convection over the Bay appears to have been suppressed, resulting in the massive deficit in all-India rainfall that was observed, pointed out P.A. Francis of the Indian National Centre for Ocean Information Services (INCOIS) at Hyderabad (an autonomous body under the Ministry of Earth Sciences) and Prof. Gadgil in a paper published later that year. Such an adverse gradient in sea surface temperature was a rare event, they noted.

During the course of a monsoon, weather systems spawned over the Bay deliver a great deal of rain over large parts of the country. Researchers have found that a gradient in the surface temperature between the northern and southern part of the Bay can be a crucial trigger for convection and rainfall over the central Bay.

The temperature in the northern Bay can rise and fall by as much as 1° to 2° Celsius, sometimes in just a few days. Such oscillations in temperature are possible because the top layer of water in this part of the ocean has low salinity. This low-salinity layer acts like a thin plate that can heat up and cool rapidly, explained D. Shankar of the National Institute of Oceanography (NIO).

Temperature changes

In a paper that appeared in the Journal of Earth System Science in 2007, Dr. Shankar, along with S.R. Shetye, the NIO director, and Dr. Joseph, looked at temperature changes during the monsoon in the northern and southern Bay and correlated that with rainfall events in the central Bay for seven years, from 1998. They reported that when the northern Bay was warmer than the southern part by more than 0.75°, it began to rain in the central Bay shortly afterwards (the lag was seven days or less in most cases).

The north-south gradient in the ocean surface temperatures led to an atmospheric pressure gradient, which, in turn, drew in winds from the west. The result was a swirling cyclonic circulation in the atmosphere that enhanced convection and produced rain, the scientists explained in their paper.

But to better understand how the oceans around India are influencing the monsoon, scientists need know more about what's happening below the waves as well, not just the temperature at the surface.

The sea surface temperature is only an expression of what happens to the top layer of the ocean, observed P.N. Vinayachandran of the IISc. This upper strata, which varies in depth, is known as the “mixed layer” because it is stirred and mixed well by the winds. As a result of such stirring, temperature and salinity are uniform in this layer. Warmed by the sun's rays, it also has the highest temperature. Below it, the temperature of the ocean falls sharply with depth.

Rain, river discharges

As the Bay of Bengal receives more rain as well as greater river discharges than the Arabian Sea, its surface waters are much less salty.

With the less salty and therefore less dense water floating on top, the stirring created by winds produces a much shallower mixed layer in the Bay than in the Arabian Sea.

“That helps the Bay remain warm,” he said. During the monsoon, once a weather system develops and then moves over to the land, it cools the Bay only a little. The Bay also has the advantage that, unlike in the Arabian Sea, upwelling, that brings up colder water from deeper down in the ocean, is very inefficient. Since the mixed layer in the Bay is shallow, it heats quickly with solar radiation and is soon ready to support the next weather system.

Consequently, the Bay is able to sustain weather systems right through the monsoon and beyond, remarked Dr. Vinayachandran.

Given the importance of the mixed layer, the temperature and salinity profile in the oceans around India need to be monitored at least up to a depth of 100 metres, according to Dr. Joseph. This was vital for understanding the monsoon and thereby to increase the capability to forecast its course.