This post is in response to a press release issued in recent days by Reading University which discusses the link between a warm North Atlantic and wetter UK summers.

If you’ve been following me back to when I first started blogging back in July 2009, you’ll be aware of my stance on why our summers here in the UK have been turning wetter and wetter. While I have always said that the warmer North Atlantic has been a large part of the reason for our increase in summer rainfall, however the Pacific plays an important role also. In fact, one could argue that it may play an even greater role when you think about it. The Pacific is where the El Nino and La Nina is and these play a GLOBAL role in distributing or redistributing ridge and trough positions and whether a region may see drier or wetter. It is the overall decadal warm/cold cycle within the Pacific which controls amount, intensity and length of time an El Nino/La Nina is alive for.

Right now the PDO is cold, so La Ninas are favoured. They last longer and their stronger. El Ninos which develop in between these longer, stronger La Ninas are more reactive to the colder signal. We saw this over the past two years with the dominance of the La Nina up until the warming and reintroduction to the El Nino took place but because the Pacific is rather cold, this Nino is weak just like it was back in 2009.

My theory is that since 2007, our summers have trended wetter. How do I come up with that year in particular? The reason, after several warm/dry summers here in the UK, we all of a sudden endured our second wettest summer on record. Remember the epic floods? The summer before was hot, dry and we recorded our warmest July day on record here in Britain.

Why the sudden flip between the warmth and dry weather of summer 2006 and the epic rains of 2007?

In 2007, the Pacific is said to have flipped into a cold phase, also, 2007 was a year where an El Nino was in control. The combination of a VERY warm North Atlantic, similar to this year in which back then, like this year, the arctic sea ice shrunk to it’s lowest levels ‘on record’.

So, el nino, cold PDO, very warm North Atlantic caused a very wet summer in 2007. Back then it was the 2nd wettest on record.

Take this year.. an even colder PDO than 2007, a slightly warmer North Atlantic (warmer because the sea ice shrinkage in the arctic surpassed that of 2007) AND… the El Nino, ALL these factors helped us beat 2007 in terms of summers rains. There is a clear link there and a fundamental point I am trying to get at and that is, yes the warm North Atlantic plays a vital role but so does the Pacific.

Afterall, is it a coincidence that the past 2 strong and back to back La Ninas which favour drier conditions here in the UK, did we see the driest two years on record. This is despite the warm AMO.

The Pacific has the power to override the trend of wetter.

Sept 2007

Sept 2008

Sept 2009

Sept 2010

Sept 2011

Sept 2012

Reading university claim this wet cycle may change in the next 2-3 years

According to the article, Reading university claim there could be clange in this cycle over the next 2-3 years. That I dissagree with. However, what appears likely is that we return to a La Nina next spring and so that could well lead to a drier summer in the UK, when going by recent years. However, the North Atlantic while could be peaking right now or over the next few years, should keep summers wetter given the Pacific should remain in a cold phase. If you continue to keep the Pacific cold and the Atlantic warm, our summers will remain wet. The slight alteration within this cycle will be the behavior of the ENSO and when in a cold mode (La Nina), it should be drier in the UK but when warm (El Nino), expect increased summer and autumn rains in the UK.

The rains are certainly down to a warm North Atlantic but the tigger to the major rains is most likely down to the warm mode of the ENSO index within a cold PDO.

The turnaround comes when the Atlantic cools and or the Pacific warms and that may be several years away yet.

Of course there is the link to winter and the increased blocking caused by the feedback of warm waters. That is another discussion for another time. If you want to read more on that however, have a read at my winter related posts which you can go directly to by clicking on the winter 2012-13 tab at the top of the homagepage.

Is global warming the cause of our wetter summers?

If you look back at climate history, the answer is clearly no.

Yes, this is the wettest sustained period within summer months here in the UK basically in our lifetime, but the history of our planet, atmosphere and oceans go back LONG before we were around and so my point is simple. This has happened before and we have seen this between back between 1930-50 when summer rains were increased too due to a warm AMO. Of course what can often be ignored is that after these increased rains, we entered a cold period in which the Atlantic cooled and here in Britain, we saw multiple severe winters through the 60s and 70s.

Within that period, we saw several flood disasters.

According to the study and the book ‘Storm Force, August 1948 saw severe flooding which shutdown the main east coast line for 3 months. In August 1952 the Lynmouth flood disaster killed 34 people.

It happened before and it will happen again!

This year!

1952!

Why the increase in flash flooding during summer?

In account of this summers soaking, I have made reference to the role the warmth of the North Atlantic has on energizing low pressure systems crossing the Atlantic. The reason also for a more active storm track during summer when it should be much weaker is a lot due to an increase in a NEGATIVE summer NAO (North Atlantic Oscillation). Another important aspect which fuels bigger rains which result in flash flooding is the role in which warmer air and SST’s have down near the Azores. As low’s swing into the UK, trailing cold fronts drag all the way down to the Azores where it is naturally warmer and moister but with the warm AMO, this region tends to be warmer than normal and this energy does get transported north into circulations which cross the UK and so the rainfall rate and intensity is increased over the UK resulting in FLASH flooding as well as regard flooding.

To conclude

To round up, what won’t be highlighted in these publications, is the clear linkage between these increased summer rains and the colder UK winters. These are BOTH due to Warm North Atlantoc waters caused by the NATURAL warm cycle on the AMO.