Tuesday 20 November 2012

A shift in the European climate in the 1990 linked to the AMO

During the 1990s there was a substantial shift in the European climate. We experienced more wet summers in northern Europe and more hot and dry summers in southern Europe relative to earlier summers. What caused this anomalous shift in European climate? Was it related to anthropogenic climate change or is it just a case of natural variability in the climate system? 

A recently published paper in Nature by Sutton & Dong explains that it is the North Atlantic Ocean and a pattern of variability known as the Atlantic Multidecadal Oscillation (AMO) that is responsible for this shift in European climate. The AMO is a multidecadal variation in North Atlantic sea surface temperature (SST) which fluctuates between anomalously warm and anomalously cool phases, each lasting several decades at a time. Its instrumental record is shown below, where you can see the shifts about the mean state over the years.


As you can see there was a shift towards a positive phase in the mid 1990s, and Sutton & Dong finds evidence that this has caused the climate shift in European summers from 1996-2010. AMO-like variations in SST are closely related to the variations in the Atlantic Meridional Overturning Circulation (AMOC) (an Atlantic circulation feature part of the global ocean circulation which transports heat from lower- to higher latitudes, where the water gets so dense that it sinks and deepwater is formed, thereby overturning circulation). Evidence suggests that the 1990s warming of the Atlantic Ocean was largely caused by an acceleration of the AMOC in response to the persistent positive phase of the winter NAO, which I’ve talked about in previous posts.
As you also can see from the above figure, there was another warm period between about 1930 and 1960, with very similar pattern of North Atlantic SST as for the recent warm period. Previous research has shown that this warm state, relative to the cold period 1960-1990, forced a certain pattern of sea level pressure (SLP), surface air temperatures (SATs) and precipitation over Europe. So the similarities in North Atlantic SST anomalies between the two warm periods suggest that similar climate impacts may have been excited in the recent warm period. And that is true – very similar patterns in SLP, SATs and precipitation does exist for the two warm periods for spring (MAM), summer (JJA) and autumn (SON). This is shown in the model simulation figures below (which show anomalies relative to the intervening cool phases), which also show the typical climate pattern associated with a warm phase of the AMO:

SLP

SAT

Precipitation


This is in close agreement with the observed recent wet northern Europe summers and hot and dry southern Europe summers. It is also consistent with the observed conditions for recent springs and autumns. As Sutton & Dong explains;
“The consistency between the two warm North Atlantic periods in the patterns of anomalies in SAT, precipitation and SLP is strong circumstantial evidence that the North Atlantic Ocean was an important driver of these decadal changes in European climate.”

So what can we expect into the future? How long will the AMO stay in its warm phase, keeping the European climate locked in the same pattern? And are there also external forcings contributing to the recent warming, like anthropogenic greenhouse gasses? These are hard questions to answer given that the AMO is a natural mode of variability internal to the climate system, varying with no particular pattern, which makes it hard to predict in the future. I will try to dig deeper into this for my next blog post and further discuss these questions.


2 comments:

  1. Hi Emilie,
    In addition to your interesting considerations on the impact of the North Atlantic on weather changes in Europe and elsewhere during the last decades, a stronger focus on the changes that occurred since winter 1939/40 may help to understand climatic mechanism better. It started with three extreme cold winters in Northern Europe, which had been the coldest for more than 100 years. Than followed global temperatures decrease for almost three decades. By a thorough analysis at: http://www.seaclimate.com/ the naval war in Western European sea areas is the most serious source for the occurrence of the three extreme winter, while the Atlantic Battle and the naval war in the Pacific are likely to have contributed to the global cooling. The naval war during WWII is a unique opportunity to investigate the relevance of the oceans and seas in weather and climate matters, and man’s activities in the marine environment. The reference website is available as book and covers about 220 pages.

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  2. Hi aber,
    Thank you for commenting and giving feedback, I apprechiate it! I'm also taking a note on your tip, that is a smart thought!
    I will actually put up a post on extreme winters later on, but not that far back in time. I've choosen to only focus on the past couple of decades to narrow down an else broad topic.
    Cheers!

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