Anticyclones don’t let other weather systems pass by and create conditions for extreme weather on their peripheries as well
A Zoom Earth map of the Arabian Sea littoral shared by Jason Nicholls
@jnmet on X (formerly Twitter) Mumbai and Dubai, cosmopolitan cities separated by 2,000 km of the Arabian Sea and Persian Gulf, suffered from two different extreme weather events on April 16, 2024. While torrents of rain along with lightning lashed Dubai flooding large parts of the city, Mumbai sweltered under an intense humid heat wave.
Both these events were precipitated by a single weather system, a massive anticyclone, along with some localised weather phenomena, and further fuelled by general warming and increased moisture levels in the atmosphere.
The Konkan coast of India, especially the city of Mumbai, has been undergoing a humid heatwave for the past two days. On April 15, Santa Cruz weather station in the city recorded a maximum temperature of 37.9 degree Celsius (°C) and had a high relative humidity of 55 per cent at 5:30 pm, according to Rushikesh Agre, a Harvard University certified independent weather forecaster based in Pune.
The maximum temperature is usually measured at mid-day. Humidity levels would have been similar at the time. These conditions would translate to a wet bulb temperature of 29.3°C. This would have meant that the temperature felt by the citizens of Mumbai would have been much higher.
“There was a steep jump of 4.7°C in the maximum temperature in 24 hours as the peak temperature measured on April 14 was 33.2°C”, Agre told Down To Earth (DTE).
“Further, there was no respite from the heat even at night. The minimum temperature recorded on April 15 was 27.5°C. But it felt close to 30°C,” informed Agre. Santa Cruz station recorded a maximum of 39°C on April 16, its highest in the last 15 years.
On the other side of the Arabian Sea in Dubai more than a year and a half’s worth of rainfall (127 mm) fell in a single day on April 16. The city’s airport and major freeways got flooded, with many people taking to X (formerly Twitter) to share videos of the devastation caused by the event.
Other Emirati cities like Sharjah and Abu Dhabi also received excess rainfall on April 16, with the United Arab Emirates (UAE) receiving its highest rainfall in 75 years, when records began. Other countries in the region like Oman and parts of Saudi Arabia and Bahrain also received excessive rainfall, with 18 people killed in Oman due to flash flooding.
The extreme rainfall and flooding in Dubai and the unbearable heat of Mumbai were linked by a huge anticyclonic system. “The mid-tropospheric anticyclone covers the entire northern Indian Ocean from West Asia to the Himalayan foothills,” Raghu Murtugudde, a professor of climate studies at the Indian Institute of Technology, Bombay and emeritus professor at University of Maryland told DTE.
Anticyclones are areas of high atmospheric pressure where winds blow in a downward sinking motion and in the process, compress and heat up. They are generally related to large-scale and elongated heatwaves. But they also form a blocking pattern which doesn’t let other weather systems pass by and create conditions for extreme weather on their peripheries as well.
“The net result of the anticyclone is a large scale heat dome with air sinking, compressing and warming. The problem is you are sweeping in warm air from West Asia and the Arabian Sea and sinking and heating it further. Humidity around Mumbai is naturally high because this is the pre-monsoon moisture loading with added air sweeping in from the warm northern Arabian Sea,” explained Murtugudde.
Usually, Mumbai is cooled down by sea breezes at night. But the anticyclone and its winds may have blocked the flow of these sea breezes. This meant that the night temperatures were also higher than normal, according to Agre.
The anticyclone blocked the passing of a western disturbance over West Asia, which may have induced the storm system that caused the rainfall and lightning. “There was a western disturbance. But it also interacted with the western edge of the anticyclone. So it is partly related to the large anticyclone,” said Murtugudde.
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The storm system’s rainfall may have also been accentuated by cloud seeding operations conducted by the National Centre of Meteorology of the UAE, according to some media reports, and also due to excessive dust in the region which can also act as natural cloud seeders. During cloud seeding operations, chemicals such as silver iodide are used to form ice crystals which become nuclei for water vapour to settle around and form rain bearing clouds. The UAE has been known to have used such operations to enhance rainfall in the desert region in the past as well. These have been controversial elsewhere.
“The massive rain bomb in #Dubai - 6”- almost 2 years of rain in a day, was due to a very blocked pattern and slow moving southern jet stream with embedded disturbances. That is the primary cause. Could cloud “seeding” have enhanced it?” Jeff Berardelli, chief meteorologist and climate specialist with WFLA-TV based in Tampa Bay, Florida, wrote on X.
“Dubai is no stranger to dust but the storm had engulfed a huge amount of dust right over the area. Dust is also a cloud seeder, we call it condensation nuclei, so how can one be certain the man-made seeding was responsible when a desert’s worth of dust was suspended overhead?” Berardelli added.
There may have been other factors adding to these concoctions for both regions. “The transition from El Nino to La Nina is underway. So, a shift in the upper level jet stream is expected,” said Murtugudde. The upper level jet stream brings the western disturbances to India, causing rainfall and snowfall in the winter and spring months for India.
“But the record heat of 2023 into 2024 so far, has combined with West Asia and the Arabian Sea warming to form a fairly large cauldron of heat that is quite unusual. Increased humidity means the thermal energy doesn’t escape to space. So it can add to the misery,” he added.
For Mumbai, the main problem was that the people were not informed at the right time or properly about the humid heat wave.
“Wet bulb temperature is getting a lot of attention as some human limit. But it is better to use the heat index for now to avoid creating scary messages. People in the tropics are used to such heat and humidity and early warnings have to be put out with information on nearby hydration centers, shelters, free public transportation and so on. Strict orders can issued to avoid exposing kids and elders to hot temperatures outside or inside,” concluded Murtugudde.
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