Climate Change

Extraordinary marine heatwave in North Atlantic Ocean may continue throughout 2023

The month of May this year witnessed the highest sea surface temperature since 1850

By Akshit Sangomla
Published: Wednesday 05 July 2023
This is happening at a time when global ocean heat content and sea surface temperatures have been at a record high for the past two months. Photo: iStock

An extremely unusual marine heatwave is occurring in the North Atlantic Ocean, especially around the United Kingdom (UK) and Ireland. It has lasted for more than two months and may continue through the rest of the year as well. 

The occurrence is part of a worldwide pattern driven by global warming and the development of El Nino conditions in the equatorial Pacific Ocean. 

A marine heat wave is an extended period of warmer-than-normal sea surface temperatures (SST).

Such warming has never been observed in the region before and could lead to long-lasting impacts on marine flora and fauna, on livelihoods and local weather patterns, even heatwaves on the land. 

“The current marine heatwave is classified as an Extreme Category IV / V Marine Heatwave — extremely unusual for this time of year,” said Craig Donlon, head of Earth Surfaces and Interior Section at European Space Agency (ESA), in a statement. 

On June 18, temperatures in the North Sea were more than five degrees Celsius above normal, while they were 8°C above normal in the Baltic Sea, according to the ESA satellite data. 

Source: European Space Agency

The month of May witnessed the highest SSTs since 1850, according to the UK Meteorological Office. 

“May 2023 was nominally the warmest May in terms of sea-surface temperature around the near-coastal waters of the UK since the late 19th century,” said Stephen Belcher, chief scientist, UK Met Office. 

“The value is about 1.60°C above the average for May over the period 1961-1990,” he added. 

This is happening at a time when global ocean heat content and SSTs have been at a record high for the past two months and many of the world’s oceans have been undergoing marine heat waves that may prevail the entire year.

On June 28, 2023, the National Oceanic and Atmospheric Administration (NOAA) of the United States said around 40 per cent of global oceans were already experiencing marine heatwave conditions. 

This includes the “equatorial Pacific, the Northeast Pacific, the Northwest Pacific in the Kuroshio extension region and the Sea of Japan, the tropical North Atlantic, the Northeast Atlantic along the Iberian coast as far northward as Ireland and the UK, the Southwest Pacific just southeast of New Zealand, and the Western Indian Ocean southeast of Madagascar”, according to NOAA. 

This is the highest percentage among all months since 1991. NOAA forecasts that this figure would increase to 50 percent by September. 

NOAA predicted that there was a 50-60 per cent chance that marine heatwave conditions will persist in the Atlantic Ocean around Ireland and the UK till August, though the intensities will decrease over time. 

For the North Atlantic Ocean in general, there was a 90-100 per cent chance of marine heat wave conditions continuing till August and an 80-90 per cent probability of SSTs being warmer than normal till end of the year. 

The conditions are going to persist until March 2024 for the Pacific Northwest with a 70-80 per cent probability, according to NOAA. 

El Nino, weaker high pressure system

Multiple reasons have been cited for the North Atlantic Ocean marine heat waves, some of which could also be true for marine heat waves occurring across the planet’s oceans. 

“Marine heatwaves occur due to a combination of atmospheric and oceanographic processes,”  the ESA noted.

“What is important to realise is that significant warming is also evident over the Tropical Pacific as part of the current El Nino system, accompanied by widespread surface ocean warming in the Pacific and Atlantic oceans,” said Donlon. 

El Nino is the warmer-than-normal phase of the El Nino Southern Oscillation phenomenon in the equatorial Pacific Ocean and its onset was declared on July 4 by the World Meteorological Organization. 

Many scientists have also cited a weaker-than-normal high pressure system over the North Atlantic Ocean as a reason for the warming in the region, which has already led the UK to record its hottest June since 1850, according to the Met Office. 

The high pressure system over the North Atlantic is responsible for driving winds over the region. Therefore, when it is weaker than normal, temperatures rise.

"Marine heatwaves can disrupt ocean ecosystems and the coastal communities that rely upon them,” said NOAA in a press release. It added:

Weeks, months or years of unusually warm waters can cause mass die-offs of fish, marine mammals and seabirds, disrupt food webs and fisheries, bleach corals, spur harmful algal blooms and wipe out seaweeds. Billions of dollars are lost in such events around the world each year.

“This is a really startling global situation because the additional surface heating we see at this time will eventually be mixed into the ocean water column,” said Donlon. 

Some of this excess heat will find its way into the Arctic Ocean via ocean currents through the Fram Strait and Norwegian Sea further exacerbating the demise of Arctic sea ice, he added. “We will be monitoring in detail to see how all these aspects evolve with great interest.”

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