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The western Indian Ocean has been warming far more than researchers believed earlier, according to a joint study by Indian and French scientists. They ascribe this to human interference in the atmosphere and to the El Niño effect, though other researchers are not sure if El Niño is responsible

The western Indian Ocean has been warming at a faster rate than any other tropical ocean in the world, according to an Indo-French meteorological study. (Image by Connie Ma)

The western Indian Ocean has been warming at a faster rate than any other tropical ocean in the world, according to an Indo-French meteorological study. (Image by Connie Ma)

Rising ocean temperatures have been one of the reasons for the rise in the number of cyclones in the Arabian Sea, says M.R. Ramesh Kumar from the National Institute of Oceanography in Goa.

The western Indian ocean and north-western Arabian Sea have warmed by more than 1-1.5 degrees Celsius in the month ending October 15. However, there are factors other than sea surface temperatures (SSTs) which give rise to cyclones in the Arabian Sea.

The western Indian Ocean, previously believed to be cool, has been warming for over a century, according to a recent Indo-French meteorological study. This has the potential to weaken the monsoon.

Researchers from the Indian Institute of Tropical Meteorology (IITM) in Pune, Sorbonne University in Paris and Ferguson College in Pune have analysed data from 1900 to 2012 and published a study titled The Curious Case of Indian Ocean Warming in the current issue of The Journal of Climate, brought out by the American Meteorological Society.

They assert that SSTs in the western Indian Ocean were previously thought to be cool, unlike its eastern counterpart, where such temperatures can exceed 28 degrees Celsius in summer.

However, the western Indian Ocean has now also reached this high temperature. What is more, it has been warming at a faster rate than any other tropical ocean in the world.

“Anthropogenic (man-made) global warming is noticeably strong since the 1950s,” Mathew Koll Roxy, the lead author of the paper, from the Centre for Climate Research at the IITM, told, “which is why scientists have paid less attention to regional trends over longer periods.”

While the eastern “warm pool” of this ocean has warmed by 0.7 degrees Celsius in this 112-year period, the western basin has become hotter by 1.2 degrees, or almost twice as much, which the researchers believe is a curious anomaly.

The western basin has been warming since 1900, while the rest of this ocean has started heating only since the 1950s. The former’s mean SST in summer at the beginning of the twentieth century was 26.5 degrees Celsius, while the rest of the ocean recorded 27.2 degrees.

Like other regions across the world, the oceans may be warming due to man-made factors.

The warming of this hitherto cool region can affect the circulation and rainfall of the Asian monsoon and also impact marine life in this biologically productive region.

The study was the outcome of Indo-French collaboration carried out under the National Monsoon Mission set up by India’s Ministry of Earth Sciences.

The researchers believe that the long-term warming of the Indian Ocean is caused by El Niño events – the global phenomenon associated with a band of warm ocean water temperatures that periodically develops off the Pacific coast of South America.

“The warming over the western Indian Ocean is due to the periodic El Niños,” said Roxy. “At first impression, this seems to imply that there is no anthropogenic role in it. There is a twist here, though. The Indian Ocean warming has accelerated in the last half of the twentieth century, which is due to the increase in the frequency and magnitude of the El Niños. It is highly likely that this increase in El Niño events is in turn due to the increase in greenhouse gases, which are caused by man-made interventions.”

While the Pacific Ocean faces both El Niño and its cooling counterpart La Niña, the Indian Ocean does not. The periodical El Niño events over the east Pacific alter atmospheric circulation across the globe and weaken the winds over the western Indian Ocean.

This is responsible for warming this region of the ocean, while the lack of La Niña events mean there is no cooling of Indian Ocean waters.

According to other researchers, including H. Annamalai at the International Pacific Research Centre, School of Ocean and Earth Science & Technology at the University of Hawaii in Honolulu, a warm Indian Ocean has the potential to weaken the El Niño during its developing and terminating phases.

The frequency of El Niño events has also been increasing, causing heat to “pile up” over the Indian Ocean and resulting in excessive warming.

The Indo-French researchers find that since 1950, a few warm events have approximated the El Niño “anomalous” sea level temperature rise which exceeds 0.77 degrees Celsius.

“This places these warm events almost on par with the El Niños in magnitude,” their study concludes. “Considering the long-term persistence of these events, the Indian Ocean warming scenario and related climate dynamics are factors to be vigilant of, while assessing long-term climate change and variability.”

The Intergovernmental Panel on Climate Change (IPCC) in its most recent fifth assessment report notes how 90% of the heat due to global warming in the last four decades has been accumulated in the oceans.

Asked whether this rise in temperature in the western Indian Ocean affects the south-west and north-east monsoon in the same, or different, ways, Koll said, “The warming trend is maximum for the summer (June-September) temperatures. As a result, we expect the monsoon also to respond to this warming. However, we have only hypothesized the potential effects of the warming on the south-west monsoon in this study. A companion study on warming vs monsoon is under review. The role of a warming Indian Ocean on the north-east monsoon would be different. We haven’t done any analysis on this aspect.”

Some doubts

Raghu Murtugudde of the Earth System Science Interdisciplinary Centre of the University of Maryland questioned how the paper stated that the Indian Ocean had more data than the eastern Pacific. He told, “Once you go beyond 1970, there is less and less data to have confidence in either region. A hypothesis was proposed in another paper that the eastern Pacific will warm less in a global warming scenario because of the way the ocean responds to warming. One can argue that the cold SSTs in the eastern Pacific since 1998 are consistent with that hypothesis. But we cannot confirm past trends in either the east Pacific or the western Indian Ocean.”

“El Niños appear only every few years but the Indian Ocean is strongly forced by the two monsoons every year. So the warming anomaly introduced by the El Niño must survive several years of strong monsoon variability and it is not clear how this can happen. This paper does not touch that issue at all.”

“If the warming of the western Indian Ocean was to affect the monsoon, then why didn’t the authors say how it has been affected over the period of analysis? Monsoon rainfall is the most reliable data we have, so why not test if there is a consistent trend in the monsoon or the null hypothesis that the monsoons have weakened, the winds over the Somali coast have weakened and thus this region has warmed because the Somali upwelling is weakened?”

“So the paper raises an interesting possibility of an El Niño contribution to the western Indian Ocean warming but only points to the need for much more research to understand the remaining issues – especially the fate of El Niño and monsoon in a warming world.”

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