September has been a very different month compared to recent months. Following the hottest and possibly driest summer on record for the continent, September has turned out cooler and wetter. The most likely cause of the change is down to the tropics!

Sept anomaly

Year-to-date

Current SST’s

Despite the UK overall winding up slightly warmer than average (due to very warm opening week), the September temperature trend has been heading downwards and rainfall upwards as the tropical Atlantic has finally begun to waken from it’s summer slumber.

Thank the eventual uptick in tropical cyclone activity and it’s interaction with the jet stream and mid to high latitude pattern.

As heat has lifted north, so Greenland has dramatically warmed (following a cool summer) but in turn this has cooled both Europe and to an extent, eastern North America (-NAO).

The below graphic shows the low latitude cyclones and their turn north, northeast.

Note, after NAO being near neutral July, positive August it swings negative in September as pressure pattern between low and mid latitudes flips!

Indeed the lack of ‘weather’ for much of the past summer has allowed the Atlantic to really heat up. This in turn has led to development, strengthening and longer lasting ‘tropical systems’ further north than usual.

I believe the strong heating over the Northern Hemisphere land and ocean prevented the tropics from producing cyclones during July and August (no need for nature to transfer heat towards the pole when it’s already warm in northern lats). Once the pattern changed across the mid latitudes with temperature and pressure decreasing, so the tropics turned on and this has allowed a significant shift. Question is how permanent is this shift?

The abnormal warmth over the North Atlantic could give rise to an increase in rainfall for Ireland, UK and much of Western Europe if the October pattern becomes a locked in ‘Atlantic driven’ pattern. We also could see cool outbreaks into Europe, especially later autumn with weakening of the polar vortex caused by both tropical activity and northward heat transfer, even due to unusually late heat waves over Asia (China, Koreas, Japan).

Last week’s spell of ‘north-northeast’ flow can be attributed to the northward push from Hurricane/Ex-Hurricane Fiona which triggered the NAO to go negative for the 2nd time in a week with strong warming over Greenland and downstream cool for Europe.

The now ‘wet and windy’ spell was triggered or at least aided by Hurricane Ian merging with a cold trough over the SE United States which enhanced jet stream winds exiting E North America which in turned led to the nearby passage of a deep low just off Scotland. (see video below)

When's the last time we saw this widespread intensity of rainfall over the UK? Possibly have to go all the way back to February! pic.twitter.com/5a9upFk8WC

— MarkVoganWeather.com (@MarkVogan) September 30, 2022

Further deep lows will pass just to the NW of the UK in the coming days then over following few days.

An Atlantic driven October would likely bring a wet but likely warmer-than-normal month due to how warm the Atlantic is. A year-long build-up of ocean warming will likely keep WESTERN Europe warm through the rest of autumn 2022. I suspect central and eastern Europe is most likely to turn colder, quickest. However, it could well turn pretty wet and eventually cooler down the road.

October looks very changeable. Very zonal with west-east high under trough setup.

Plenty of rain takes us through at least mid-Oct.

Note W, N areas wettest, S driest.

CFSv2 for Oct.

 

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