May is usually the start of summer with rising temperatures and softer rain. Here in Ireland, and across Northern Europe this month of May has been terrible weather. There has been a lot of rain much of it incredibly intense. At the same time the temperature has remained low – in the first days of May there was early morning frost. As a vegetable grower it’s normal to plant out pretty much everything in May and get good growth as the temperature rises. But we all noticed that there was little of no growth. Instinctively this seems very unusual. So what’s going on?
A report in the Irish Times earlier this year suggested that the Gulf Stream is at its weakest in more than a millennium. “ . . researchers from Ireland, Britain and Germany have compiled data taken mainly from natural archives in the form of ocean sediment or ice cores – going back hundreds of years – to reconstruct the flow history of the Gulf Stream. They found consistent evidence its slowdown during the 20th century is unprecedented in the past millennium and is “likely linked to human-caused climate change”. The giant ocean circulation has an impact on the climate of northwestern Europe.”
Most of us will be aware that the Gulf Stream acts like a huge conveyor belt carrying warmer water from the Equator North East across the Atlantic before then sending cooler, deeper water back down South. Similarly it’s well known that the Arctic is melting and sending vast volumes of cooler water down into the Atlantic. Similarly there have been predictions that, as a result Northern Europe will actually become cooler with less precipitation.
So I looked at the data. And to make the job a little less daunting, and perhaps more topical, I just looked at the month of May. I brought down actual rainfall and mean temperatures for the last 50 years. Note the May 2021 numbers are (naturally) incomplete and I will adjust in due course. But you will see below the results are very interesting.
The first thing you notice is that the rainfall numbers are all over the place. The 50 year May mean is 82mm. But very high rainfall numbers are common. The eleven years 2015, 2011, 2006, 2002, 1994, 1993, 1988, 1986, 1981, 1979, 1976, are all over 100mm. Conversely 15 years are under 50mm. So the mean average deviation is huge.
Now looking at the mean temperatures, 2021 is very obviously in line with expectation. We will probably end up with over 140mm precipitation and the mean temperature is currently 9.1. This may alter in the last 9 days, but I guess not sufficient to change that dramatic downwards curve! Comparing the mean temperature with rainfall is less clear.
Above the mean temperature line I have added indicators of La Nina years and indicated the relative strength of the phenomenon for the April, May, June period in the size of the arrows. The La Nina effect is in the year following the El Nino years and is usually expected to cause lower temperatures. Certainly true for this year, but less clear for 1999, 1989 etc.
So can we draw any conclusions from this exercise? As I write the heavens open again and there is a heavy downpour. This must be the tenth or eleventh torrential shower so far today. This pattern of frequent, intense showers is certainly a change in the normal May weather. Unfortunately Met Eireann don’t record these effects in their data.
The researchers believe the finger of blame for the disruption points to global warming caused by the build-up of greenhouse gases in the atmosphere. Yet even as other places warm up because of global warming, our part of the world would probably cool down.
But there are dissenting voices. For example, Richard Seager Lamont-Doherty of Earth Observatory of Columbia University has commented that A few times a year the media goes into a tizzy of panic that there is a possibility that the Gulf Stream will slow down in coming years or even stop. He says, “We now know this is a myth, the climatological equivalent of an urban legend. In a detailed study published in the Quarterly Journal of the Royal Meteorological Society in 2002, we demonstrated the limited role that ocean heat transport plays in determining regional climates around the Atlantic Ocean.”
However, myself and indeed other friends who watch the climate, are convinced that the ocean is in fact a huge energy store, and plays a massive role in weather formation. Just think of how hurricanes are formed over warm water.
In conclusion, we have hugely complex weather patterns here in Ireland and Northern Europe. There are clearly changes in our patterns, particularly the levels of intensity of rainfall, but given the data I am minded to say prediction is a mugs game.
Irish Times: Gulf Stream lifeline at its weakest in more than a millennium
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