Fast-changing weather patterns in the Indian Ocean, the Arabian Sea, and the Equatorial Pacific have combined to throw up contrasting signals with respect to the onset of the monsoon over Kerala. Forecasters blame contrasting signals from the Indian Ocean for the uncertainty, even as the Arabian Sea and the Equatorial Pacific to the Far-East are found to be obliging.
The Australian Bureau of Meteorology (BoM) said on Tuesday that an incoming, monsoon-driving Madden-Julian Oscillation (MJO) wave with moderate strength has lost pace near Africa.
Crucial factor
The MJO wave is a crucial determinant of the onset dynamics, and in the normal course, would pass over the West Indian Ocean and adjoining South Arabian Sea to precipitate the onset. Climate models predict this MJO wave would track towards the West Indian Ocean (South Arabian Sea) next week, but disagree somewhat on its strength. Some see the MJO signal becoming weak and indiscernible over the Indian Ocean. The band of lower pressure in the MJO wave sets up storms/depressions and underwrites monsoon onsets as it travels East.
“The India Met Department (IMD) is analysing the monsoon as being well South of its average position for this time of the year,” the BoM said.
Typically, its northern limit would be passing over Sri Lanka by now, and by June 1, should reach Kerala. “But, as of Tuesday (June 28), the monsoon line remains South of Sri Lanka. If the MJO stalls or completely dissipates prior to reaching the Indian longitudes, this could further delay the monsoon’s progress,” it said.
The BoM also said that the Indian Ocean could later compensate by hosting a positive phase of the Indian Ocean Dipole (IOD), the equivalent of the El Nino-La Nina phenomena in the Equatorial Pacific. A positive phase of the IOD represents anomalous warming of the West Indian Ocean (South Arabian Sea) relative to the East, and is considered a booster for a concurrent Indian monsoon.
“Warming off the Horn of Africa (Somali coast) has meant that the IOD index exceeded the positive threshold value this week,” the BoM said, adding, “all but one of the models surveyed suggest positive IOD will be maintained during June to August.” This corresponds with the most productive phase of the Indian monsoon, and could change its fortunes for the better.