Record-Warm North Atlantic: Cause & Weather Impacts

Post by
James Peacock MSC
Record-Warm North Atlantic: Cause & Weather Impacts

As the year began, the North Atlantic was already near record-warm for the time of year, second only to last year, during which an absolute record high was set 1st-7th September.

This year, the basin mean temperature stayed within the 2nd-4th place range until a week into March. At that point, the situation escalated, with 2023 reaching a record high and then staying there for weeks, then months. Not only that, but by a wide margin, well distanced from the next-warmest years.

Line graph showing the North Atlantic average sea surface temperatures (NOAA OISST v2.1 data) for each year 1981-2023.

While this is against a series reaching back to ‘only’ 1981, it’s still a truly extreme event with serious implications for weather patterns in Europe and northern Africa.

The Leading Culprit: Strong Cyclonic Anomalies

Map showing the annual surface vector wind climatology (overall mean across 30 years) for the North Atlantic.

‘Cyclonic anomalies’ refers to an unusual strength and/or persistence of low pressure systems driving winds in a cyclonic fashion. In the northern hemisphere, that’s anticlockwise (it’s the opposite in the southern hemisphere).

When it comes to ocean temperatures, what’s important is how such anomalies modify the usual climatology of wind flow across the basin. For the North Atlantic (see left), that’s eastward flow in the north, south-westward in the southeast, and westward in the southwest.

Now let’s look at the cyclonic anomalies. These initially became prominent during Sep-Dec 2022 across a region spanning from to the south of Greenland to far-western Europe (see below-left).

Maps of the North Atlantic and western Europe plus north-western Africa,  showing anomalies of mean surface vector wind for Sep-Dec 2022 (left) and Mar-May 2023 (right).

Then, they shifted westward and intensified, resulting in an impressive cyclonic anomaly pattern during Mar-May 2023 (above-right). By plotting wind speed anomalies regardless of direction (see below-left), we can see how this modified the climatology across the basin.

Maps of the North Atlantic and western Europe plus north-western Africa,  showing anomalies of mean scalar wind speed (left) during Mar-May 2023 and sea surface temperature on 18 Jun 2023 (right).

Evidently, wind speeds were below normal across much of a wide arc spanning from the southwest to the northeast of the basin via the southeast.

Corresponding Arcs

Now look at the sea surface temperature (SST) anomaly map displayed above-right. The same arc can be seen but with opposing anomalies; above normal SSTs where the wind speed was below normal.

This is no coincidence. Reduced wind speed means less air moving against the ocean surface, which reduces how much heat can be transported away. In a sense, it’s like blowing on a hot drink less – except that during spring and summer, the North Atlantic heats up over time due to sunlight.

The main deviation we can see on the map is to the south of Greenland, where abnormally high wind speeds exist alongside well above normal SSTs. This can be explained by the wind having been on average westward in that region during Mar-May 2023; unusually warm waters have been transported there from the vicinity of north-western Europe.

A Concerning Contributor:
Reduced Particulate Emissions from Shipping

This past decade, great strides have been made improving air quality via reduced particulate emissions from shipping. This is beneficial for wildlife and human health but has a substantial unwanted side effect: The lowered particulate concentration means sunlight isn’t being scattered or reflected as much as it used to be.

More of it’s reaching the surface and when that surface is the dark, transparent ocean, a lot of it is absorbed. It follows that long-term oceanic warming may well be getting a boost from the cleaner air - although to what extent is a matter of ongoing research.

So, now we’ve covered how the record-warm North Atlantic came to be, but what of its impact on weather patterns? Read on for the insight.

More Heat & More Moisture

The most obvious implication of all this additional oceanic warmth is an increased amount of heat in the overlying atmosphere. Typically, air over the North Atlantic frequently finds its way eastward to northern Europe before either heading south or spreading out generally across the continent.

So, the record oceanic warmth puts the baseline, around which day-to-day temperatures fluctuate based on weather conditions, well above normal for most of Europe.

This has contributed to an impressively warm first half to June 2023 across most of north-western Europe, as displayed below-left. Notice, however, the stark contrast for the northeast and southeast.

Maps of Europe showing mean 2 m temperature anomalies (left) and sea-level pressure anomalies (right) for 1st-17th June 2023.

When it comes down to it, weather patterns have the greater impact because they can either bring air across from the North Atlantic or keep it away. As shown above-right, across 1st-12th June, high pressure has been stronger and more persistent than usual from to the south of Iceland to central Scandinavia.

Map of Europe showing the mean surface precipitation rate anomalies for 1st-17th June 2023.

On the eastern flank, this has driven more frequent southward winds than usual, bringing cool air down from the Arctic. Meanwhile, unusually low pressure to the west of Iberia (south-western Europe) has transported more air than usual from the eastern North Atlantic to north-western Europe.

Along the way it crossed Iberia, yet temperatures have been near average there. What gives? Well, that’s where the added moisture comes into play.

As you can see to the right, it’s been far wetter than usual in Iberia. That isn’t just down to the low pressure; unusually warm ocean temperatures mean a faster evaporation rate from the surface. So, the supply of moisture has been greater than usual, resulting in extreme rainfall events.

A Complex Response

When it comes to the impact of a record-warm North Atlantic, this strong role of weather patterns makes it difficult to ascertain what the result will be for any given week or even month. What we can say is that overall, across many months, there will be a tendency for both temperatures and atmospheric moisture content to be above normal across Europe.

Whenever weather patterns bring air in off the North Atlantic coupled with abundant sunshine, temperatures will have a good shot at approaching or exceeding record highs for the time of year. When they instead support heavy rain events, they will have an increased propensity to reach extreme proportions.

What About Africa?

Despite being adjacent to the record-warm seas, northern Africa away from the immediate western coast will probably see little direct impact on weather conditions. That’s because the wind typically blows southward or south-westward there; toward the North Atlantic rather than from it.

In fact, it may have been contributing to the oceanic warmth, as there have been some reports that dust transport from the Sahara has been unusually low in recent weeks. Dust blocks sunlight, so when there’s less than usual, the ocean receives more energy.

Will it Cause a Hyperactive Atlantic Hurricane Season?

In short, not necessarily. Reason being, alongside the extreme North Atlantic warmth, there’s also a large excess of warmth in the tropical Pacific, especially in the east.

Map showing sea surface temperature anomalies (NOAA OISST v2.1) worldwide on 18th June 2023.

This is associated with what’s known as an El Niño event. This type of event typically causes anomalous wind patterns across the western and central tropical Atlantic that interfere with the development of tropical cyclones.

So, while there will be far more heat and moisture for tropical disturbances to work with, those that advance beyond the eastern tropical Atlantic will likely have a hard time organising into tropical cyclones.

Odds are, the two driving forces will cancel out, resulting in a near normal Atlantic Hurricane season. The eastern tropics are a wildcard, though – it’s plausible that this region sees so many more tropical cyclones than usual that even if unusually few occur near the Americas, the basin sees an above average season.

Already there are signs of this, with the formation of tropical storm Bret to the east of the Caribbean on 19th June. It's the furthest east that one has ever been recorded in June. What's more, there's another tropical disturbance to its east which also has a good shot at becoming a tropical cyclone. Two in quick succession, in that region, would be well outside of historical precedent for June.

Of course, being in uncharted territory, we can’t reasonably rule out a busier season near the Americas. Here’s hoping that doesn’t come to pass – after several busy seasons in recent years, those areas could really do with a quiet one.

James Peacock MSc

Head Meteorologist at MetSwift