Scientists say that Cyclone Ockhi, which ravaged large parts of India’s west coast in December, indicates that such severe storms are increasing in the Arabian Sea due to climate change
“We predicted that intense cyclones will increase during the late monsoon season in the Arabian Sea due to anthropogenic (manmade) forcing,” says Hiroyuki Murakami, a Princeton University scientist who is probably the foremost authority on the likelihood of cyclones rising in the Arabian Sea.
“Cyclone Ockhi is in line with my recent paper published in Nature Climate Change. According to the India Meteorological Department (IMD), it was a Very Severe Cyclonic Storm, but very close to an Extremely Severe Cyclonic Storm (ESCS),” Murakami told indiaclimatedialogue.net.
Murakami and his co-authors noted that in 2014 and 2015, tropical storms — defined by the World Meteorological Organisation as gaining maximum speeds exceeding 46 metres per second, or 165.6 km per hour — were first observed over the Arabian Sea and caused extensive damage.
In 2014, there was Cyclone Nilofar, the first ESCS in the post-monsoon season from October to December. It caused flash floods in northeast Oman, killing four people. In the following year, two more storms of the same category — Cyclone Chapala and Cyclone Megh — were observed back-to-back in the same season.
With wind speeds approximating those of a hurricane, both made landfall in Yemen, killing 26 people and displacing thousands. “This was the first instance that more than one ESCS was observed within one year in the Arabian Sea,” the authors noted in the paper published in the Nature Climate Change journal. It was this anomaly that prompted them to conduct their study.
Using a high-resolution global model that accurately simulates the climatological distribution of ESCSs, the authors show that manmade forcing “has likely increased the probability of late-season ECSCs occurring in the Arabian Sea since the preindustrial era.”
However, the timing of such late-season storms in 2014 and 2015 was random and not predictable. Continued manmade forcing will further amplify the risk of cyclones in the Arabian Sea, with corresponding socio-economic implications, the researchers said. They cited another recent study, which showed that manmade black carbon particles and sulphate emissions might have increased the intensity of storms over this sea.
Among other causes, one possibility is that the Arabian Sea surface becomes warmer than other seas during this period. Second, due to the interplay of global warming, climate variability and weather changes, the winter northeast monsoon circulation has been weakening over the years.
“Murakami’s study does not give us a clue on how and why the cyclones are increasing. Is it due to increasing ocean temperatures, which can influence the potential intensity of a cyclone? However, Arabian Sea surface temperatures during November-December are generally conducive for cyclones, and increasing temperatures need not necessarily trigger cyclones,” Roxy Mathew Koll of Pune-based Indian Institute of Tropical Meteorology (IITM) and lead author of a 2015 study of Indian Ocean warming, told indiaclimatedialogue.net. “This means there are other important factors leading to the genesis of cyclones.”
“One important factor is the wind shear, or the change in direction and speed of the winds from bottom to the top of the atmosphere. Generally, this wind shear is relatively strong in the Arabian Sea compared to the Bay of Bengal. That is, while the winds are generally in the same direction throughout the atmospheric column for Bay of Bengal, they are in the opposite direction for the Arabian Sea,” Koll said. “Opposing winds prevent cyclones from developing vertically, which is why the number of cyclones are fewer in the Arabian Sea.”
“This is what Murakami says is changing. They show that the wind shear is weakening in the Arabian Sea with increased carbon dioxide in the atmosphere. They link the change in wind shear to a weakening of the north-easterly monsoon winds,” said Koll. “However, we have not found any evidence for a weakening north-easterly monsoon. Hence, we need more research to connect all the missing links to explain the mechanism behind increasing Arabian Sea cyclones.”
“Regardless of the results, we can say that Arabian Sea is quickly responding to climate change signals — warming rapidly and driving more and more cyclones and extreme rainfall events,” he said. “We still do not know how much of a role climate change has played on Ockhi. It is important to note that its genesis was in the Bay of Bengal as a depression, and not in the Arabian Sea. It was the upper level easterly winds which steered the depression from the Bay of Bengal to the Arabian Sea.”
“Meanwhile, the sea surface temperatures and the wind shear in the Arabian Sea were conducive for the depression to intensify as a moderate cyclone. It is also to be noted that a La Niña (a weather pattern in the Pacific Ocean that is less damaging than El Niño) event is unfolding over the Pacific. Studies have shown that La Niña conditions change the atmospheric circulation over the north Indian Ocean and make them favourable for cyclogenesis.”
Adam Sobel, a Columbia University meteorologist and author of Storm Surge: Hurricane Sandy, Our Changing Climate, and Extreme Weather of the Past and Future , who is studying the likelihood of a cyclone hitting Mumbai, said: “We have made good progress on the assessment of the probability that Mumbai could be hit by a cyclone. The big picture is what we already knew — the probability of a severe cyclone landfall here is small, but definitely not zero. We just have done a little more work to quantify that. And hopefully Ockhi will help people to understand that conclusion a bit better.”
Mumbai had a close shave with Cyclone Ockhi. In a knee-jerk reaction, the state government declared a school holiday on December 6, although better forecasting would have informed it that the storm had bypassed Mumbai and was weakening even when it landed in Gujarat.
However, Sobel explained the nuances of such a situation. “Weather forecasters feel a great sense of responsibility. They are afraid to cry wolf,” he told indiaclimatedialogue.net. “The science is always ahead of forecasters; they can do what couldn’t be done five years ago. The computers are more powerful. However, the forecasters say, we have to study this for a while.”
“Ockhi was exactly the worst case because it started very close to land. This is unusual. I happened to be in the Maldives in 2011 when this happened. Due to the lack of a forecast, many Sri Lankan fishermen were killed,” said Sobel. “The models are the same. Regarding Ockhi, the IITM had the forecast five days earlier. The IMD probably knew too; there may have been something in their weather forecast. When there’s a cyclone, forecasters in the US, as indeed the IMD, put out a whole lot of special products: there are track forecasts, warnings. Even in the US, the cyclone warnings may be in the fine print, whereas with a hurricane warning, they’re ringing the bell.”
Asked whether earlier warnings, given the improved technology nowadays, might lose fisher folk their livelihoods, but not their lives as with Ockhi, Sobel replied that it was something of a toss-up. If such early warnings didn’t materialise, it would cost the Met Department its credibility, and in the future, people might not heed its warnings seriously.
Because Cyclone Ockhi formed so close to the land, one couldn’t track it so far in advance. If one had a forecast a day ahead, it would at least save the fisher folk who were venturing out that day, though not the typical fleet in southern states which spends around ten days at sea.
However, the cyclone showed that Mumbai is indeed vulnerable to such storms. Gujarat has been hit by cyclones four times since the 1970s. This time, the track of Ockhi could have turned right and hit Mumbai. For some reason, they tend to go north. In New York, the turn that Hurricane Sandy took had never happened before, so it just goes to show that the unexpected can always occur.
Link with climate change
“When Murakami was in Japan four years ago, he was studying how Arabian Sea cyclones could be caused by climate change, and now he’s moved to Princeton,” Sobel noted. “They’ve got the world’s best model for predicting such cyclones. This shows not only that the risk is rising in the Arabian Sea but it already has increased due to climate change.”
“In the last decade or so, the activity in the Arabian Sea has been due to natural variability, but these researchers say that it has already increased since the early 20th century till now due to climate,” he told indiaclimatedialogue.net. “Furthermore, the big increase is specifically in this post-monsoon season. Most storms here go to the west and hit countries like Yemen; some go up and hit Pakistan.”
“If you see the four cyclones that hit Gujarat in the last 40 years, three of them occurred during the May-June monsoon, but one occurred in November. The big increase is in the post-monsoon now. Ockhi looks suspiciously consistent with their findings. It remains a rare event, but one which is possible.”
“Theirs is a high-resolution climate model,” he said. “Most models don’t represent cyclones very well because the pixels aren’t sharp enough. Princeton probably has the best model in the world for predicting climate and cyclones. Here the laws of physics are solved on the computer as accurately as you can. It’s the best we can do.”
Asked about the seeming mismatch between the amount of science that is being done on cyclones and the warnings, Sobel replied: “The mismatch is there, but most of the times it doesn’t matter as much as it does in this case. It originated so close to land that they couldn’t have really done a forecast. However, this artificial barrier — between predicting a cyclone and issuing a warning — doesn’t need to be there any more.”
The National Oceanic and Atmospheric Administration (NOAA) in the US observes that the Arabian Sea is at particular risk because the countries that surround it are heavily populated and cyclones there have nowhere to go except land.