The Arabian Sea faces an increased frequency and duration of marine heatwave days, finds study

The Arabian Sea is experiencing a rapid increase in heatwave days with sea surface temperature extremes in the last decade sometimes stretching out through a season, according to a new study. In the last four decades, between 1982 and 2019, every decade has seen roughly 20 more marine heatwave days, says the study, that used satellite data to highlight the trends of these marine temperature extremes on the west coast of India.

The increasing trend of marine heatwave (MHW) days is more noticeable after the year 2000, and is clearly visible after 2015. The accelerated trend of the heatwave days is found to be driven by the rapid rise in the mean sea surface temperature (SST) of the Arabian Sea in the recent decade, the paper highlights.

Longer heatwave days are also associated with climate patterns such as El Niño, which is the warm phase of the El Niño Southern Oscillation (ENSO), a naturally occurring climate phenomenon that involves fluctuating ocean temperatures in the tropical Pacific, coupled with changes in the atmosphere. La Nina is the cooling phase of ENSO. According to the World Meteorological Organisation, all naturally occurring climate events now take place against a background of human-induced climate change which is worsening extreme weather and affecting the water cycle.

In the Indian Ocean basin, El Niño influences more than 70 - 80 percent of observed heatwave days through the Indian Ocean Basin Mode, a delayed warming response within the Indian Ocean in response to El Niño, said physical oceanographer Abhisek Chatterjee.

“Our analysis shows that the frequency and duration of marine heatwaves exhibit a rapidly increasing trend of roughly 20 days per decade (1.5-2 events per decade) in the northern Arabian Sea and the southeastern Arabian Sea close to the west coast of India. This increase is a multifold increase in MHW days from the 1980s,” study lead author Chatterjee at Indian National Centre for Ocean Information Services (INCOIS), Ministry of Earth Sciences, Hyderabad, India, told Mongabay-India.

“We need to predict marine heatwaves and unpack their impacts in the Arabian Sea region including understanding their influence on the local marine ecosystem, the migration of species, and the associated fishery-dependent economy,” said Chatterjee, adding that while the understanding of the origin of MHWs across the globe has advanced rapidly over the last decade, there are not many studies to document these events in the northern Indian Ocean.

The study relied on sea surface temperature data from satellite sources to analyse three primary metrics of heatwaves: duration, frequency, and intensity for the period of 1982–2019 (1982 onwards is referred to as the era of satellite observations). After 2000, the number of heatwave days increased significantly, with an average of 10-20 percent days experiencing heatwaves in almost all years. A rapid increase in heatwave days is observed from 2015 onwards with at least 25-50 percent days experiencing heatwaves.

The increase in frequency and duration of marine heatwaves is also emphasised in the Intergovernmental Panel on Climate Change’s Special Report on the Oceans and Cryosphere in a Changing Climate which states that marine heat-related events have increased globally and over the period 1982-2016; they have doubled in frequency and have become longer-lasting, more intense and more extensive. The panel iterates that it is very likely that between 84-90 percent of marine heatwaves that occurred between 2006 and 2015 are attributable to the human-caused temperature increase.

Global interest in marine heatwave research

Australia-based scientist and lecturer, Alex Sen Gupta, highlights the 2010-2011 Ningaloo Niño off Western Australia that spurred research into marine heatwaves (MHW). Ningaloo Niño led to extensive and sustained loss of kelp habitat, fisheries closures and the invasion of tropical species into temperate waters. Sen Gupta was not associated with the Arabian Sea study.

“It was an incredible, massive event and it had really big ecological impacts that were very long-lasting as well. So even though the event occurred in 2010, we’re still seeing its impacts. And that sort of drove this global interest in marine heatwaves. So it’s been rising exponentially since then,” said Sen Gupta at the Climate Change Research Centre, University of New South Wales, Sydney, Australia.

Another iconic heatwave (the most intense and the highest category MHW in the satellite era) event was in 2015. It encapsulated most of the Indian Ocean and coincided with the strong El Niño that year, causing widespread coral bleaching in the Lakshadweep archipelago.

“We did a study a few years ago that tells us that over the last century, globally we’ve had about a 54 percent increase in the amount of time that we would consider as a marine heatwave. So if before we had six days a year that was considered a marine heatwave, now we have nine or 10 days a year. And that number is going up rapidly as the mean sea surface temperature is going up rapidly,” said Sen Gupta.

In India, researchers also point to a link between recent cyclones in the Arabian Sea and the prolonged marine heatwaves. “We also see that a prolonged marine heatwave in the Arabian Sea is associated with a cyclone. And the cyclone helps the heatwave to decay. Bigger cyclones such as in 2010, in 2020 (cyclone Nisarga) and in 2021, were associated with marine heatwaves that were prolonged,” said Chatterjee.

“There is a clear signal between cyclones and marine heatwaves and we need to extract the information and incorporate them in modeling,” corroborates Roxy Mathew Koll at the Indian Institute of Tropical Meteorology (IITM), Pune, who was not involved in the present study.

Koll and his colleagues showed in a recent study that an increase in marine heatwave events in the northern Indian Ocean affects monsoon rainfall in India. Marine heatwaves over the western Indian Ocean and the Bay of Bengal are causing drying conditions over the central Indian subcontinent. At the same time, the marine heatwaves over the north Bay of Bengal are causing a “significant increase” in the rainfall over south peninsular India.

Like Chatterjee at INCOIS, Koll’s research group also used satellite sources between 1982 and 2019. They showed that the western Indian Ocean region experienced the largest increase in marine heatwaves at a rate of about 1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.5 events per decade. The western Indian Ocean experienced 66 events while the Bay of Bengal had 94 events over the last four decades, between 1982 and 2018.

In light of the recent evidence, experts have called for “prioritisation” of the prediction of marine heatwaves and monitoring subsurface ocean waters.

“We also need to strengthen our in-situ ocean understanding through enhanced ocean observations because while satellite sources capture temperature data, the data captured is an underestimate. Even a two-degree Celsius difference between satellite data and in-situ observations is huge,” Koll said.

Marine heatwave predictions and forecasts can come in handy for policymakers, fishery managers and resource managers. “They may be able to do things such as switch to different species, or change when they’re going to harvest their products, or they might – if it’s an oyster farm– take the oysters and put them in different regions. So, in short timescales, we can think of certain management solutions once our predictions become good enough,” added Alex Sen Gupta.

“And that’s one of the things that science is trying very hard at moment to do is to try and make our models better. So we can actually either predict individual marine heatwaves or try to give statistical forecasts to say that a marine heatwave is more likely to occur in this season. The other timescale, of course, is the general global warming.”

“We need to slow down global warming and reduce greenhouse gas emissions,” Sen Gupta said.

Read more: Broaden understanding of marine heatwaves in the Indian Ocean: Experts

Knock-on effects of marine heatwaves can last for decades

Based on the coverage of satellite data, the years 2010 and 2016 show the longest heatwave days in the Arabian Sea, which were due to the overlap between strong El Niño and the average ocean warming trend, causing high and persistently warm sea surface temperature (SST) across the Arabian Sea. As a case study, the researchers unravelled the year 2010 – the longest observed event since the availability of satellite observations; the heatwave lasted for about 60-70 days.

The heatwave started brewing sometime in the last week of March, 2010, in the northern and northeastern parts of the Arabian Sea when the sea surface temperature was hovering around 27-29 degrees Celsius i.e. about 0.5 degrees Celsius more than the threshold. It spread by the third week of April, 2010 and covered the entire northern part of the Arabian Sea. The heatwave intensified by May as the sea warms further. By mid-June, the heatwave waned with the summer monsoon clouds setting in across the Western Ghats of India. The onset of summer monsoon clouds reduced the shortwave radiation, eventually contributing to the decreasing temperature and termination of the heatwave.

INCOIS’s Abhisek Chatterjee says longer marine heatwaves could also worsen impacts on corals because of water’s high specific heat capacity. “Once it gets warm, it will not cool fast.”

The increased frequency of these events does not allow ecosystems, such as coral reefs, enough time to bounce back. Scientists studying the mass coral bleaching in India’s Lakshadweep archipelago in 2015-2016 El Niño-associated bleaching event, called the damage “heartwrenching.” The El Niño hit the reefs in Lakshadweep before they could completely recover from 2010’s catastrophic El Niño.

The cascading impacts of MHWs can last for decades – the loss of fisheries, destruction of kelp forests or seagrass habitats and their natural capacity to store carbon, mass mortalities of wildlife and knock-on economic consequences that already run into billions of U.S. dollars.

“For example, the Ningaloo event had a massive effect on the fishery in that year, but it completely devastated the habitat. The kelp habitat up and down the coastline for hundreds of kilometers was destroyed and they haven’t come back since then,” tells Sen Gupta.

“You’ve got losses in relatively small regions from tens to up to hundreds of millions of dollars related to a single marine heatwave event,” he said, referring to the research on the global perspective of the socioeconomic impacts of marine heatwaves.

In 2015, Kripa Vasant, who contributed to the recent IPCC report on adaptation, was shocked to see a massive drop in phytoplankton density in southeastern Arabian Sea waters off the coast of Kerala along India’s southwest coast. This stretch is home to the Malabar upwelling zone, one of the most important upwelling zones in the world. Upwelling occurs when winds push surface water away from the shore and deeper nutrient-rich water rises to fill the gap.

Upwelling encourages the growth of plant life, including phytoplankton. These phytoplankton provide food for fish, marine mammals, seabirds, and other critters. These upwelling zones are good fishing grounds. The Malabar upwelling zone, with its phytoplankton-rich waters, supports a major habitat of the Indian oil sardine, nicknamed ‘family provider’ because of its contribution to Kerala’s coastal economy.

“In 2015, there was almost no phytoplankton. This glut was linked to the extreme El Nino event which caused high sea surface temperatures. The combination of low food availability (phytoplankton) with the high sea surface temperatures in the sardine habitat in 2015 was like a ‘drought in the ocean’. This ‘oceanic drought’ contributed to the drop in sardine fishery that year (estimated to be to the tune of  150 crore rupees), which was already reeling from a decline in the past few years due to over-exploitation. The fishing community was hit very hard,” added Vasant, who was with Central Marine Fisheries Research Institute, Kochi, Kerala and is now Member Secretary, Coastal. Aquaculture Authority, Chennai.

Chatterjee says it is important to look at marine heatwaves in the context of mitigation actions associated with impacts of extreme events such as El Niño on land. “During El Niño, you always prepare for a weaker monsoon (because El Niño is known to suppress monsoon rainfall) because the El Niño forecast is usually given a season in advance. So, we actually have a mitigation policy on how to handle the farming situation, but we don’t consider it in terms of the impacts on marine food sources.”

“The marine food system is also linked to India’s economy and we need to look at marine heat extremes also in that context,” he added.

Vasant agrees that the prediction of marine heatwaves will help the fisher community prepare for the worst and such extreme events at the sea need to recognised as disasters. “When fishery is hit due to ocean extremes, the community doesn’t receive relief as ‘drought in the sea’ is not recognised,” she added.

Banner image: Sunset at a Goa beach, Arabian Sea. The Arabian Sea is experiencing a rapid increase in heatwave days with sea surface temperature extremes in the last decade. Photo by Kai Müller/Pixabay.