Guest blog – How the Atlantic may influence wet summers

19 06 2013

This morning there has been a lot of media coverage following a workshop held here at the Met Office HQ in Exeter on a recent run of unusual seasons in the UK.

Much of this centred around recent research by the University of Reading, presented at the workshop yesterday, which suggested Atlantic ocean cycles – specifically one known as the Atlantic Multidecadal Oscillation (AMO) – can have an influence on UK summer weather.

Here Professor Rowan Sutton, from the University of Reading, explains that research in a bit more detail:

 

“Last year, Buwen Dong and I at the National Centre for Atmospheric Science published a paper in Nature Geoscience about the link between slow changes in the temperature of the North Atlantic Ocean and weather patterns.

In particular, we presented evidence of a link between warm surface temperatures in the North Atlantic and a higher frequency of wet summers in the UK and Northern Europe.

This research built on earlier research I published with another colleague, Dan Hodson, in Science in 2005 and an important study by Jeff Knight and colleagues at the Met Office, which was published in 2006.

In our 2012 paper we showed that a rapid warming of the North Atlantic Ocean which occurred in the 1990s coincided with a shift to wetter summers in the UK and northern Europe and hotter, drier summers around the Mediterranean. The pattern identified matched that of summer 2012, when the UK had the wettest summer in 100 years.

Observational records show that the surface temperature of the North Atlantic has swung slowly between warmer and cooler conditions, and the present warm phase has a similar pattern to warm conditions that persisted throughout the 1930s, 40s and 50s. During the 1960s, 70s and 80s cooler conditions prevailed.

Computer simulations suggest that these changes in ocean temperature affect the atmosphere above. Warmth in the North Atlantic causes a trough of low pressure over western Europe in summer and steers rain-bearing weather systems into the UK.

An important question of interest to many people is how long will the current pattern of wet summers in northern Europe persist? This is a key research question and we don’t yet have precise answers.

In our 2012 paper we stated: “Our results suggest that the recent pattern of anomalies in European climate will persist as long as the North Atlantic Ocean remains anomalously warm.”

How long might this be?  There is strong evidence linking the swings in the Atlantic Ocean surface temperature to the “overturning” or “thermohaline” circulation of the Atlantic.

This circulation appears to have intensified in the 1990s. Following such a strengthening, a subsequent weakening is expected, as various feedbacks exert their influence.

For example, the surface warm waters transported northward by the overturning circulation have relatively low density which inhibits their tendency to sink, and acts to slow the circulation. Such a slowing cools the North Atlantic.

The time scales involved are in the range between a few years and a decade or two.  Progress in Decadal Forecasting, such as the pioneering work at the Met Office, and critical observations such as from the NERC-funded “RAPID” array, should help us to reduce this large range of uncertainty, but it is a challenging problem and advances may take some years.”





Media coverage on ‘wet summers for a decade’

19 06 2013

There has been a lot of media coverage today following a science workshop held at the Met Office HQ in Exeter yesterday.

Most of the articles go some way to capturing the science as it was delivered in the press briefing following the event – such as this article on the BBC News website. However, some stories, and particularly some headlines, do not.

The key point revolves around discussion of Atlantic ocean cycles, specifically one known as the Atlantic Multidecadal Oscillation (AMO), which can have an influence on UK summer weather.

Professor Stephen Belcher, Head of the Met Office Hadley Centre, and Dr James Screen, a NERC Research Fellow at the University of Exeter, were careful in their messaging about the AMO.

They talked about initial research which suggests this cycle, which can last for 10-20 years, can ‘load the dice’ to mean we may see a higher frequency of wetter than average summers before switching to its opposite phase, where we may see the opposite effect.

Currently, they said, it appears we are well into the ‘wet’ phase of this cycle, so it may continue to have an influence for a few more years to come.

That does not mean every summer will be a ‘washout’ for the next decade and shouldn’t be taken as a deterministic forecast for what we will see in the years to come.

First of all, we’ve seen five summers of higher than average rainfall in the last six years (with 2010 being the exception, which had average levels of rainfall). Even within each of those years we have seen periods of decent weather – so there’s no expectation of total washouts for the whole summer.

Secondly, the research suggests there is a tendency towards a higher frequency of wetter than average summers – so we could still see summers which buck this trend.

And finally, this research is still at an early phase and more work needs to be done to see exactly how this process works and how we can predict its influence on future seasons.

So, much like a blog we recently wrote about this year’s summer, it’s fair to say that you shouldn’t write off summers for the next decade or so.

We expect to be publishing a guest blog from scientists at the University of Reading on the science they presented to the workshop yesterday, so look out for that in the coming days.

You can now see video of the Professor Stephen Belcher speaking at the press conference which followed the science workshop on 18 June on our Youtube channel.








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