ENTER EL NI O
A SEE-SAW OF `ACTIVE' AND `BREAK' PHASES
The discovery of itcz cracked open a window into global teleconnections, which dictate the flow of monsoonal winds. One such connection is with El Niño Southern Oscillation (enso). It denotes an interdecadal, alternating pattern of heating and cooling in the Pacific Ocean. During the heating phase,El Niño is characterised by the heating of the Eastern Pacific that extends as a tongue towards the Central Pacific region. “The heating up of the Eastern Pacific causes a deflection in the path of the South-Eastern trade winds and suppresses the Indian monsoon,” explains Rajeevan. An example of this was the drought year of 2009. “The intense oceanic heating to the east of the Indian subcontinent caused winds to gush eastward towards the Pacific Ocean instead of South Asia.The monsoon winds completely missed the Indian subcontinent and a convection zone was created over the ocean instead.This is what caused the severe deficit in 2009,” says R Krishnan,programme manager at the Centre for Climate Change Research Centre at iitm.
Climatologists believe there is a strong correlation between enso and the occurrence of drought years in India (see 'El Niño effect' on p28). A brief analysis of rainfall records reveals that eight of the 15 drought years experienced between 1951 and 2014 coincided with enso (see ‘Off to another bad start’, DownTo Earth, 16-30 June,2015).“This year, enso signals have been the dominant signals and that’s why they have featured in all the forecasting models across the world,”says Suryachandra A Rao, associate director of the National Monsoon Mission.
Certain observations also show monsoons that are normal or even marginally above normal during El Niño years.This has fuelled speculation in recent years that the link between enso and the Indian monsoon is weakening.But such speculation may be premature as enso is hardly the only factor in determining the strength of the monsoon.Another factor is differential heating between the Arabian Sea and the Bay of Bengal in the Indian Ocean, known as the Indian Ocean Dipole (iod).It has been observed to occur in alternate phases in the Arabian Sea and the Bay of Bengal.
iod and its associated atmospheric component known as the Equatorial Indian Ocean Oscillation (equinoo) must be given part of the credit for offsetting the impact of El Niño in certain years,according to Rajeevan. “We have seen that during El Niño years, if the Arabian Sea exhibits higher temperatures than the Bay of Bengal, it can offset the monsoon-suppressing effect of enso to a certain degree. But if the Bay of Bengal heats up more than the Arabian Sea during an El Niño event,it can further reduce monsoon rains in India,” says Rajeevan. Gadgil believes it is equinoo that neutralises the effect of El Niño in certain years.The monsoon of 1997-98 is a classic example of this,she says.(See ‘Unmasking the monsoon’on p18.) While the convergence zone is critical to rainfall in India,it is also responsible for the intraseasonal variability. Since the monsoon was first studied, cyclical patterns of alternating monsoonal strengths within a season have been observed.The most critical of these are the three-nine days’cycles of “active” and “break”phases observed as a kind of regulating mechanism. These phases are caused due to the movement of the convective system arising out of the convergence zone over the Indian Ocean, particularly along the Bay of Bengal.
While active phases are characterised by deep depressions and cyclonic storms, especially over the Bay of Bengal, no such systems develop during break phases.Break phases occur due to a northward propagation of the convective cloud zones due to movements of the itcz. It is characterised by rainfall over Himalayan foothills and in the southeastern part of the peninsula, while the rest of central India and the core monsoon zone in the subcontinent experience drier conditions. “Breaks occur when air in the upper troposphere descends to disturb monsoon circulation.The upper troposphere is driven by global connections, although the mechanisms are unclear which makes conclusive predictability impossible,” explains J Srinivasan, chairperson of the Divecha Centre for Climate Change at the Indian Institute of Sciences in Bengaluru.
Unfortunately, the predictability of these short duration synoptic cycles is most crucial for farmers.