The strong positive phase of the Indian Ocean Dipole (IOD) - thought to be at least partially responsible for the extended easterly wind regime over Peninsular India and the raging fires over Eastern Australia - may be on the mend at last. The positive phase of the IOD is marked by abnormal warming of the West Indian Ocean (near Somalia and the Horn of Africa and adjoining South-West Arabian Sea), which causes the horizontal winds to converge over an area of lower pressure. The ensuing ascending motion of air sets up clouds, storms or heavy rain. 

Easterly winds converging into this boiling pot have been interacting at times with the westerly wind regime over Peninsular India. The ensuing chaotic weather has produced locally heavy rain over parts of Andhra Pradesh, Tamil Nadu and Kerala over the past three or  four days.

In this manner, the rains have spilled beyond the normal period of closure of the North-East monsoon around December-end. Extended forecasts by the India Meteorological Department (IMD) show the seas around Sri Lanka and South Tamil Nadu will continue to see some activity for another week, by when global weather models expect the positive IOD to dissipate. 

Successful post-monsoon season

Jatin Singh, Managing Director of private forecaster Skymet Weather, said that both the North-East monsoon (over South Peninsula) and the pan-India post-monsoon season concluded  on December 31, 2019. The post-monsoon country-wide cumulative surplus stood at 29 per cent. All the five meteorological subdivisions under the footprint of the North-East monsoon recorded normal or above normal rains.

In the second half of the previous week, a prolonged cold day and cold wave conditions that had gripped the hills of North-West India had abated with the arrival of a western disturbance and its associated warmth. Unseasonal rains accompanied by hailstorms had lashed a few places in Madhya Pradesh and Maharashtra. Uttar Pradesh, Bihar, Jharkhand, Chhattisgarh, Odisha, West Bengal and the entirety of North-East India experienced wet spells between January 2 and 4. Telangana recorded some untimely rains, ranging from moderate to heavy, in the past week. In fact, the rains in Hyderabad broke a 15-year-old record for this time of the year.

Part of Andhra Pradesh, Tamil Nadu and Kerala, too, have been experiencing hit-or-miss thunderstorms as easterly winds meandered into the region and ran into opposing westerly winds. Chennai is forecast to witness partly to mostly cloudy conditions for another day today (Monday) with an estimated 30-40 per cent chance of rain peaking to 60 per cent into the evening.

Almost similar conditions are forecast for Puducherry while it will be partly cloudy over Salem, Coimbatore, Tiruchirappalli, Madurai and Thoothukudi. The Chennai Met Office under the IMD has forecast the possibility of light to moderate rain over isolated parts of Tamil Nadu and Puducherry for at least three more days. The significant rains recorded yesterday (Sunday) were mostly from the South, topped by Sivagiri (Tirunelveli) at 5 cm; Watrap (4 cm); Rajapalayam (3 cm); and Ambasamudram and Peraiyur (2 cm). Chennai city recorded  light to moderate rain even this (Monday) morning, as the local bloggers tweeted.    

IOD fuelling Australian fires

Meanwhile, the convergence of winds over the West Indian Ocean is matched only by their divergence (this is accepted global wind behaviour) to the farther East of the Indian Ocean, especially the Australian latitudes, marked by a downward motion of air (subsidence). This increases pressure on the ground heats up the atmosphere, and doesn't allow the formation of clouds. And this is what has contributed to conducive conditions for fires in Australia, which has so far consumed a wide swathe of geography roughly the size of Kerala.

The Climate Prediction Centre of the US National Weather Service says the strong subsidence over Australia has led to significant heat lately, which has in turn led to a number of wildfires. Unfortunately, this heat may continue at least for another week, especially in the eastern part of the continent, the agency said.

The Australian Bureau of Meteorology said that the IOD value has fallen from a peak of +2.2 degree Celsius in mid-October, to the latest weekly value of +0.6 degree Celsius. Warmer than average waters persist near the Horn of Africa, but waters in the East Indian Ocean are now near-average. While the index is still above the positive IOD threshold of +0.4 degree Celsius, most international climate models indicate the positive IOD will dissipate in January. The rate of current weakening would suggest a return to neutral is likely any time soon.

Positive IOD events in spring (as happened this year) are often associated with a more severe fire season for South-East Australia in the summer months.

Outlook for Pacific weather 

The Australian monsoon, which should have started in December, has been delayed after its Indian counterpart overstayed its welcome and started withdrawing from only October 9, making it amongst the most delayed in history. The likelihood of average to above-average rain in early 2020 in Australia is expected to increase as the influence of the IOD declines. The positive IOD, which has been the dominant climate influence for North Australia in recent months, has weakened further in the past week. The retreat of the IOD's drying influence is reflected in the rainfall outlook for January to March 2020, which indicates roughly equal chances of above- or below-average rainfall across most of North Australia. 

In the tropical Pacific Ocean, most indicators suggest continued neutral conditions (neither El Nino nor La Nina) and warmer-than-average sea-surface temperatures in the far West tropical Pacific (early signs of a La Nina?) may be contributing to some changes in local weather patterns over the region. International climate models though forecast neutral conditions to remain during the March-to-May period.

Meanwhile, Chennai bloggers and their followers were appreciative of the extended stay of the easterly winds over Tamil Nadu and Puducherry and recounted how they have altered the weather conditions lately.

 

Related Topics
comment COMMENT NOW