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.”


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8 responses

19 06 2013
nuwurld

“The AMO is an ongoing series of long-duration changes in the sea surface temperature of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time and a difference of about 1°F between extremes. These changes are natural and have been occurring for at least the last 1,000 years.”

That is a quote from NOAA. So why do you suggest we need ‘anomalously’ high Atlantic temperatures to drive this ‘natural cycle’?

26 06 2013
John Havery Samuel

From the same NOAA page,

s the AMO a natural phenomenon, or is it related to global warming?

Instruments have observed AMO cycles only for the last 150 years, not long enough to conclusively answer this question. However, studies of paleoclimate proxies, such as tree rings and ice cores, have shown that oscillations similar to those observed instrumentally have been occurring for at least the last millennium. This is clearly longer than modern man has been affecting climate, so the AMO is probably a natural climate oscillation. In the 20th century, the climate swings of the AMO have alternately camouflaged and exaggerated the effects of global warming, and made attribution of global warming more difficult to ascertain.

30 06 2013
nuwurld

Why are you using the phrase ‘global warming’, John? Everyone else knows it’s ‘climate change’ now, ’cause no instrumental data set shows warming for at least 16 years. Nothing can be due to ‘warming’ that hasn’t happened. There is no ‘camouflage or exaggeration ‘ beyond data manipulation. There is NO RADIATIVE GREENHOUSE EFFECT.

3 07 2013
nebakhet

“cause no instrumental data set shows warming for at least 16 years”

That’s wrong

19 06 2013
John Havery Samuel

Given the ocean is warming as per http://www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/, may this strengthen the periodic effect?

24 06 2013
nuwurld

John, you are confusing heat content with temperature. Global warming is about temperature, not heat content. For instance, if we add more water to the atmosphere the temperature will drop for the same heat content. Or if there is less water in the atmosphere the measured temperature will rise for the same heat content.

Sea surface temperature is the ‘only’ thing we need to know about the ocean. It is the only way the ocean interfaces with the atmosphere and and land. Heat within the ocean is ‘lost’ to the entropic world of millennium scale approach to equilibrium. Long wave radiation and evaporation from the ‘surface’ is the only way ocean heat can affect climate. Changes at depth are measurements regarding thermal inertia.

The global sea surface temperature is not rising. The sea level is not rising to any significant degree, and that is the sum of land ice melt and thermal expansion. All the worlds oceanographers agree on that. Tidal gauges have error bars, so do satellite measurements.

These conditions are similar to the late 1950’s, with the PDO negative and the AMO following. Except this time the driving energy, the solar flux is heading for a much deeper decline (a solar Grand Minimum, the sum of the sixty year and 172 year downturn in activity). From the 1950’s to the 1970’s the global temperatures fell and sea ice returned to the Arctic after the melt during the 1930’s.

Wait and see.
Regards.

24 06 2013
nuwurld

John, you are confusing heat content with temperature. Global warming is about temperature, not heat content. For instance, if we add more water to the atmosphere the temperature will drop for the same heat content. Or if there is less water in the atmosphere the measured temperature will rise for the same heat content.

Sea surface temperature is the ‘only’ thing we need to know about the ocean. It is the only way the ocean interfaces with the atmosphere and and land. Heat within the ocean is ‘lost’ to the entropic world of millennium scale approach to equilibrium. Long wave radiation, conduction and evaporation from the ‘surface’ are the only ways ocean heat can affect climate. Changes at depth are measurements regarding thermal inertia.

The global sea surface temperature is not rising. The sea level is not rising to any significant degree, and that is the sum of land ice melt and thermal expansion. All the worlds oceanographers agree on that. Tidal gauges have error bars, so do satellite measurements.

These conditions are similar to the late 1950’s, with the PDO negative and the AMO following. Except this time the driving energy, the solar flux is heading for a much deeper decline (a solar Grand Minimum, the sum of the sixty year and 172 year downturn in activity). From the 1950’s to the 1970’s the global temperatures fell and sea ice returned to the Arctic after the melt during the 1930’s.

Wait and see.
Regards.

3 07 2013
nebakhet

“The global sea surface temperature is not rising. The sea level is not rising to any significant degree, and that is the sum of land ice melt and thermal expansion.”

wrong on both counts

http://sealevel.colorado.edu/

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