Climate and weather researchers at the National Centre for Atmospheric Science set out the likely drivers of unusually warm oceans in the North Atlantic, Pacific, and Antarctic regions.

The North Atlantic’s sea surface temperature is currently 1.1°C warmer than average, compared to the last 40 years.

“Before 2023, the warmest period of the past 40 years was 0.55°C above average for the North Atlantic region. So far this year we are observing ocean temperatures twice as high as before in this part of the world, and this is well outside what is expected,” explains Dr Till Kuhlbrodt from the National Centre for Atmospheric Science and University of Reading.

The sea surface temperature of the Pacific Ocean is warming too, signalling the onset of El Niño, and the current temperature of the Weddell Sea off the coast of Antarctica is also exceptionally high.

Dr Kuhlbrodt describes what the implications of these unusually warm oceans are:

“As winter begins in the Southern hemisphere, the warmer than usual Weddell Sea means that ice cover is exceptionally small. The extent of sea ice is currently so small that it is well outside of what has been observed over the past 40 years, at this time of year. The global extent of sea ice is also well outside the range of variation seen in the last 40 years.

“I find the simultaneous occurrence of extreme warming in different ocean regions worrying. I don’t think we understand at this time why this is happening. There are different environmental processes and drivers that could be contributing and interacting to cause the sea surface temperature rise. But we need to keep monitoring data sent from autonomous floats in the ocean, and satellites, which are tracking sea surface temperatures. This data will begin to indicate how deep, strong, and sudden this warming has been,” raises Dr Kuhlbrodt.

The warming of sea surface temperatures is an anticipated effect of climate change.

Climate scientist Jon Robson from the National Centre for Atmospheric Science and the University of Reading explains:

“There is an established long-term trend of increased sea surface temperatures associated with climate change. However, the current high temperatures within the North Atlantic, Pacific and Antarctic oceans are likely being exacerbated by natural events.”

Some actions against air pollution may also be connected to high temperatures in the North Atlantic, Pacific and Antarctic oceans. In 2020 the International Maritime Organisation brought in regulations to reduce the sulphur content of fuel used by ships, reducing the sulphate aerosols released from ship emissions.

Sulphate aerosols can help reflect heat away from the sea surface. Reducing the amount of sulphate aerosols could contribute to warmer sea surface temperatures, however the strength of this connection is very uncertain.

“We know that El Niño can warm the planet significantly, up to about 0.2°C averaged over a given year,” says Kieran Hunt, a climate scientist at the National Centre for Atmospheric Science and University of Reading.

Scientists have confirmed that El Niño – which changes global atmospheric circulation patterns – has begun this year. The effects of El Niño are usually felt the strongest in the wider Pacific region, which can explain the warmer-than-average tropical Pacific sea surface that has kicked in over the past month.

But the effects of El Niño on other regions remains unclear. Dr Kuhlbrodt adds: “While El Niño can affect atmospheric circulation and temperature in the Southern Ocean in general, we do not know at the moment whether it can explain the very warm Weddell Sea.”

Tropical easterly winds, known as the trade winds, transport desert dust from the Sahara to the Atlantic Ocean. “Dust particles have a cooling effect in the lower air layers,” explains Dr Kuhlbrodt.

Currently, the trade winds are weaker than usual – possibly associated with the onset of El Niño – and so is the cooling effect of the Saharan dust particles on that region.

The reduced supply of cooling dust particles could be one explanation for the sudden rise in North Atlantic sea surface temperatures. However, changes in the trade winds themselves will also impact the sea surface temperatures in the North Atlantic due to the wind’s role in cooling the ocean. Weaker trade winds will lead to warmer temperatures through reduced cooling of the North Atlantic Ocean.

Driven by the unusually high sea surface temperatures in the North Atlantic, Pacific and Antarctic regions, the global extent of sea ice is well outside the range of variation seen in the last 40 years. Sea ice loss has many knock-on effects, including but not limited to: sea level rise, habitat loss, altered weather and circulation patterns due to changes in surface winds, and further global warming due to less sunlight being reflected from the ice surface.

Rising temperatures of the North Atlantic ocean may also have an impact on extreme weather. Higher sea surface temperatures in the North Atlantic are often associated with an increase in tropical storms, like hurricanes. However, during El Niño the development of hurricanes is usually suppressed. Therefore, it is uncertain whether the rising sea surface temperatures in the North Atlantic will cause more tropical storms.