The North Atlantic Ocean has been unusually warm since early March. appeared on Twitter this week delivered messages about. Meteorological institutes such as the British MetOffice Hadley Center have since dived into it. What is going on?
Most tweets referenced the website Climate Reanalyzer, which combines and visualizes all kinds of international datasets. By selecting on ‘SST’ (sea surface temperature) you can call up maps or charts of the North Atlantic. On one of those graphs you can see that from the beginning of March the sea water temperature far exceeds all measurements from previous years (from 1981). On the maps for April and May, a large area off the coast from Portugal to West Africa appears to have warmed up relatively strongly.
“It is still too early for a conclusive explanation,” says oceanographer Erwin Lambert of KNMI. He distinguishes factors that play a role in the longer and shorter term. At the root, he says, is global warming. “In the past century, the North Atlantic has already warmed three-quarters to one degree. And that warming will continue for the time being.”
In recent years in particular, he has seen temperatures in the North Atlantic rise relatively quickly. This may have to do with the phase the North Atlantic is in. Temperatures on the surface periodically alternate between relatively warm and cold – the so-called Atlantic Multidecadal Oscillation. A period usually lasts thirty to forty years. “Since 2020, that AMO has been in a positive, warm phase,” says Lambert.
What may also play a role are the stricter environmental requirements that have applied to shipping since 2020. Marine fuel may contain only a small amount of sulphur. Since then, ships have emitted much less polluting sulfur compounds. The air is getting cleaner. But that also means that more sunlight reaches the surface of the water. That then heats up more. “What is the dominant factor in the rapid warming we have seen in recent years is impossible to say.”
A factor that plays a role in the shorter term, says Lamber, is the persistent low-pressure area that overlies much of the ocean. Usually there is a high pressure area above the North Atlantic Ocean in the period February-June at the height of the Tropic of Cancer. This creates strong trade winds blowing from the east. Those strong winds cause the surface ocean water warmed by the sun to mix with colder, deeper water. But the low pressure area that has been there since the beginning of March is disrupting those trade winds. As a result, there is less mixing of superficial and deep water. “The water surface therefore remains relatively warm,” says Lambert. The website of ‘Severe weather Europe’, which reports on weather extremes in Europe, said explained this clearly at the end of May.
Another thing that is striking is the amount of dust that blows over the ocean from the Sahara. It has fallen sharply since mid-April and is well below average. Less dust in the air also means more sunlight reaching the surface of the water. “That can also contribute to the high water temperatures,” says Lambert.
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On Twitter, people are concerned about whether the major current in the Atlantic Ocean, the so-called AMOC, can now come to a standstill. This current brings, among other things, warm water from the Gulf of Mexico to Western Europe, and ensures a relatively mild climate there. Will that process stop? And can we expect a strong cooling of Western Europe? “The consensus among experts,” says Lambert, “is that the flow will weaken somewhat in the coming decades, but that will not have major effects on the climate.” According to Lambert, the recent temperature records of the ocean water have little or no effect on the AMOC anyway. “This outlier is too short for that.”
Read also: Scientists predict new El Niño and temporary additional warming
Twitter users also wondered if there is a link with El Niño, but Lambert does not see that either. This climate phenomenon is playing out in the Pacific Ocean and is only just getting started. “It cannot have an effect on the Atlantic right now.”
However, El Niño could affect the hurricane season in the Atlantic Ocean later this year, which started in early June. With the current, extremely high water temperatures, there is extra energy in the water, which could lead to more hurricanes. But an El Niño usually has a depressing effect on the formation of hurricanes in the Atlantic Ocean. The question is how this works out. Colorado State University, which releases a fairly reliable hurricane forecast every year, expected this season fifteen storms and seven hurricanes, three of them major. She classifies this as near average; close to average.
A version of this article also appeared in the June 17, 2023 newspaper.