Can we use ocean temperatures in the Arctic to predict temperatures in Europe?

This question was investigated in a new paper by Erik Kolstad and Marius Årthun in Journal of Climate. The answer is: yes and no. It worked quite well when we used data from the period starting in 1979 (when important weather satellites were launched). But when we used data from earlier in the twentieth century, the relationships that have been valid in recent decades broke down.

An example that we investigated in some detail was central England temperature (CET), for which the observations stretch back to the 17th century. When we used data starting in 1979, we found the following relationship with sea surface temperatures (SSTs) in the Atlantic part of the Arctic:

cet_map_1_0
This picture shows the correlation between SSTs in the Arctic, averaged from October to December, and CET averaged from January to March during the period from October 1979 to March 2017.

The relationship is clearly strongest in the Barents Sea, which is marked with black lines in the picture. When we averaged the SSTs inside that region and created a time series, we could compare it to the CET time series:

cet_timeseries_1_0
Here the time series for Barents Sea SSTs is shown in blue, along with the time series for CET in orange (the latter time series is multiplied by –1 to make the figure more readable).

The correlation between the two time series is –0.44 for the period starting in 1979. But we were interested in what happened during periods starting earlier than 1979. Then we had to use a different data set for the SSTs (CERA-20C, while we used ERA-Interim for the recent period). What we did was to investigate all possible periods with the same length as 1979–2017, starting with 1901–1939. For each period, we computed the correlation between SSTs in the Barents Sea and CET, and found this:

cet_stationarity_0
The correlation between Barents Sea SST and CET for each possible 38-year period starting with 1901–1939. The correlations in blue are based on CERA-20C, and the orange star shows the correlation for the ERA-Interim data.

The recent negative correlation of –0.44 is unprecedented. For periods starting early in the twentieth century the correlations are even positive. One of our main conclusions is that one should take great care when using Arctic SSTs to predict climate in later months.

There is also a Kudos web page about this article.

Erik Kolstad is currently investigating the relationship between Arctic sea ice and the North Atlantic Oscillation.

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How does the ocean influence temperature over land?

Norway is much warmer than most other places at similar latitudes. The picture below shows fishing activity in northern Norway in winter; this could not have been possible without the warm Norwegian Atlantic Current, which brings warm water masses from the south throughout the year.

'Winter_in_Lofoten,_1886'_by_Otto_Sinding,_Bergen_Kunstmuseum
“Vinter i Lofoten” (1886), painted by Otto Sinding

In a new paper, Marius Årthun, Noel Keenlyside and Erik Kolstad from our project group and colleague Tor Eldevik from the Bjerknes Centre for Climate Research investigate the causes of European temperature variations on long time scales. For instance, we know that the temperature in parts of Scandinavia fluctuates with a period of 14 years. The paper shows that this is probably because the sea surface temperatures in the Nordic Seas also fluctuate on the same time scale. The mean westerly winds then transport air masses form over the ocean to over land, and thus the connection between the ocean and land is mediated. We can use this information to predict variations between colder-than-normal and warmer-than-normal, potentially several years ahead.

The title of the paper is Time scales and sources of European temperature variability, and it has now been accepted for publication in Geophysical Research Letters (GRL). Here is the abstract:

Skillful predictions of continental climate would be of great practical benefit for society and stakeholders. It nevertheless remains fundamentally unresolved to what extent climate is predictable, for what features, at what time scales, and by which mechanisms. Here we identify the dominant time scales and sources of European surface air temperature (SAT) variability during the cold season using a coupled climate reanalysis, and a statistical method that estimates SAT variability due to atmospheric circulation anomalies. We find that eastern Europe is dominated by sub-decadal SAT variability associated with the North Atlantic Oscillation, whereas interdecadal and multi-decadal SAT variability over northern and southern Europe are thermodynamically driven by ocean temperature anomalies. Our results provide evidence that temperature anomalies in the North Atlantic Ocean are advected over land by the mean westerly winds, and, hence, provide a mechanism through which ocean temperature controls the variability and provides predictability of European SAT.