Not So Flaming Junes (Except 2023)

We could be seeing an unusual event in the UK this year. June could be the warmest month of the year in the Central England Temperature (CET) record for the first time since 1970. That’s actually the longest gap between years when June has been the hottest or equal hottest month in the entire series, back to 1659, although June was only 0.1C cooler than July in 1993.

I recently discussed the quite warm June 2023 in the UK and presented a graph of June CET temperatures:

Most of the very hottest Junes – 1846, 1676, 1826, and 1976 – have been the opening act of long, hot summers, so June 2022 isn’t particularly notable. Rather, it simply reflects the fact that the average June is warmer than it was.

Back in 2016 I produced charts like the above for every month in the year. That for June, I recollect, was odd in showing much less of a warming signal than those for all the other months. Only in the last few years are the running means for June temperatures now exceeding those in the pre-global warming era.

Here, for example, is the chart for July:

21-year running mean July CET temperatures are over 0.5C or more greater than during any hot period we have seen before. July 2023, which seemed so cool, was in fact about average for the entire period from 1659 to 1980!

And here’s August:

The 21-year running mean is again 0.5C greater than during any previous period.

Note that, even though August tends to be a tad cooler on average than July, this August needs to be cooler than the mean for the last 5 years for June to be the hottest month of the year. Though, looking out the window, that seems to be a distinct possibility.

The million-dollar question, though, is why has June not warmed as much as other months?

The obvious explanation is that the temperature in June is more influenced by sea surface temperatures (SSTs) than other months – the return of the westerlies, the “European Monsoon” and all that. Unusually hot Junes would then occur when the “monsoon” is delayed or fails, I guess.

And elevated sea surface temperatures have certainly been the climate story of the year.

But the corollary is that this implies that either the relevant (North Atlantic) SSTs have only recently (say this century) broken out of their normal range or June weather patterns have changed. Perhaps the westerlies were returning a little earlier (on average) because continental Europe was warming up quicker.

Either way, the warmer SSTs and increasing June temperatures suggest – assuming it’s not all just random noise – that this pattern is breaking down. The sea is also warming.

Why, though? Unfortunately, there are many possible explanations, not mutually exclusive. It’s been suggested “that international rules aimed at reducing air pollution from maritime shipping could have inadvertently increased ocean warming”.

Hurricanes disperse heat from surface waters. Could fewer hurricanes (and typhoons) be part of the problem?

These mechanisms are plausible, but SSTs are controlled by, first of all, global circulation patterns. Could it be that the Atlantic Meridional Overturning Circulation (AMOC) process whereby warm water is transported to the Arctic has reached some kind of capacity limit, or, worse, I suppose, weakened due to the long-anticipated influx of fresh meltwater, principally from Greenland? This would be a big claim, and it should be noted that the warm SSTs, at least this year, are a global phenomenon (though some areas of warmth are clearly caused by the relatively well-understood El Nino phenomenon).

But one reason why SSTs may be “catching up” with land temperatures is often overlooked. We have not had a climatically significant volcanic eruption since Pinatubo in 1991. Signals of volcanism are very clear in the June CET graph (above) and in SSTs, as in this graph from the New York Times article already mentioned:

Krakatoa (1883), Agung (1963) and Pinatubo (1991) stand out quite well. Such events clearly depress SSTs below trend for a decade or so (but affect land temperatures for a shorter period). In the absence of comparable eruptions, you’d expect SSTs to warm faster, exactly as we see. I don’t know what the models are doing specifically, but if each individual model isn’t averaging out the cooling effect of volcanic sulphur dioxide aerosols over time, then the average of models (and model runs) certainly will be.

Sooner or later we’ll see an eruption sufficiently powerful (depending on sulphur content of its emissions) to affect the climate. There’s be another blip on the SST graph.

And maybe the June CET mean temperature will revert to warming slower than the mean temperature for other months of the year.