El Nino effect pushes global monthly mean ocean temp to all-time high

Around 27% of the global ocean was experiencing a marine heatwave as of August 15 according to an assessment by ocean scientists associated with World Meteorological Organisation’s World Climate Research Programme

New Delhi El Nino has put oceans on an uncharted path since April with global monthly mean ocean temperatures currently at an all-time high relative to temperatures since records started to be kept, and potentially extending even further into the past.

A family cools off in the ocean in Kihei, Hawaii (AP)

Around 27% of the global ocean was experiencing a marine heatwave as of August 15 according to an assessment by ocean scientists associated with World Meteorological Organisation’s (WMO) World Climate Research Programme.

Sea surface temperatures (SSTs) observed since April show a far sharper spike than the warming over land especially when compared with the long-term mean for the 1982 to 2011 period as tracked by University of Maine’s Climate Reanalyzer. The global average SST on Sunday (September 10) was 21.1°C compared to 20.7°C on the same day last year. The average land air temperature on September 11 was 16.65°C compared to 16.11°C on t he same day last year.

Marine heatwaves (MHWS) are currently being recorded over large parts of the Indian Ocean including the Kerala basin, parts of Arabian Sea and the Bay of Bengal closer to Sunderbans and Odisha as per data from Indian National Center for Ocean Information Services referred to by the research group.

Some of these heatwave regions are in “strong” category while others are in “moderate“ category.

These categories are defined by the intensity of the MHW scaled by the threshold temperature anomaly exceeding the climatology. For example, the event having a severity of less than 2 units is indexed as “moderate”, 2-3 units as “strong”, 3-4 is “severe” and more than 4 units as “extreme” as per methodology adopted.

“Like terrestrial heatwaves kill people, marine heatwaves adversely impact marine life. They kill coral reefs and fish. Marine heatwaves can also impact local atmosphere through air-sea interaction. But these effects could be short lived as marine heatwaves are,” said M Rajeevan, former secretary, ministry of earth sciences.

Marine heatwaves occur when ocean temperatures in a given region are well above average for an extended period of time.

Long-range seasonal forecasts and experimental marine heatwave forecasts by National Oceanic and Atmospheric Administration have predicted continuing widespread warming over the coming months. Marine heatwaves in the tropical Atlantic have a 50-80% chance of persisting into the northern winter; heatwave conditions are also forecast to persist in the western Indian Ocean through September (70-90% chance), with a likelihood of a heatwave continuing between the Southern India and the Horn of Africa continuing through the end of the year (50-90%). Models also predict an elevated risk of marine heatwave conditions along the US west coast in spring next year as the current El Nino continues to strengthen, the research group said.

Warmer ocean temperatures impact the marine environment and associated ecosystems, and ocean heat can also fuel developing tropical cyclones, the group said in its analysis released last week.

“Marine heatwaves have increased by up to four-fold in the tropical Indian Ocean, aided by rapid warming in the Indian Ocean and strong El Ninos. Most of today’s marine heatwaves, about 90% of them, are attributable to global warming. The impact of these extreme ocean temperatures is yet to be clearly understood. Preliminary research shows that these marine heatwaves can energise tropical cyclones, alter monsoon patterns, and drive heatwaves on to the land. Marine heatwaves precede intense cyclones such as Amphan, which undergo rapid intensification from a weak to extremely severe cyclone in less than 24 hours,” said Roxy Mathew Koll, climate scientist at Indian Institute of Tropical Meteorology and member of the WMO research group.

These events cause habitat destruction due to coral bleaching, seagrass destruction, and loss of kelp forests, affecting the fisheries sector adversely. An underwater survey showed that 85% of the corals in the Gulf of Mannar near the Tamil Nadu coast got bleached after the marine heatwave in May 2020. Marine heatwaves can result in mass fish mortality and impact marine aquaculture also.

Apart from ongoing El Nino conditions, other factors can also explain recent warming in the oceans, scientists said explaining that the North Atlantic has experienced a sequence of marine heatwaves extending from the tropics to mid-latitudes. This warming pattern is consistent with the negative phase of the North Atlantic Oscillation, a natural pattern of atmospheric variability that can impact surface winds and influence ocean temperatures. The North Atlantic Oscillation was strongly negative from mid-April to mid-May and most of July, consistent with the observed Atlantic warming pattern.

“In addition to mechanisms related to natural variability of the climate system, we know that about 90% of the excess heat associated with global warming has been absorbed by the ocean, causing the global ocean surface temperature to increase by about 0.9°C since pre-industrial times. It is very likely that climate change has contributed to the intensity and widespread coverage of current marine heatwaves,” the research group said.

Climate model projections suggest further warming of the Indian Ocean in the future, which will intensify the marine heatwaves and their impact on the monsoon rainfall. “We need to enhance our ocean observational arrays to monitor these events accurately and update our weather models to skilfully predict the challenges presented by a warming world. The Covid pandemic during 2020-22 resulted in critical gaps in our ocean monitoring systems, due to lack of deployment and maintenance cruises. As a result, we are unable to closely monitor these marine heatwaves or utilise the data for improving weather forecasts,” Koll added.

El Nino conditions have developed for the first time in seven years. El Nino has a strong influence on the southwest monsoon in India. El Nino years are characterised by an unusual warming of waters in the eastern equatorial Pacific, which has a high correlation with warmer summers and weaker monsoon rains in India.