Blizzard Jonas and the slowdown of the Gulf Stream System

Blizzard Jonas on the US east coast has just shattered snowfall records. Both weather forecasters and climate experts have linked the high snowfall amounts to the exceptionally warm sea surface temperatures off the east coast. In this post I will examine a related question: why are sea surface temperatures so high there, as shown in the snapshot from Climate Reanalyzer below?

 

GFS-025deg_NH-SAT1_SST_anom_24_Jan_2016

I will argue that this warmth (as well as the cold blob in the subpolar Atlantic) is partly due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), sometimes referred to as the Gulf Stream System, in response to global warming. There are two points to this argument:

(1) The warm sea surface temperatures are not just some short-term anomaly but are part of a long-term observed warming trend, in which ocean temperatures off the US east coast are warming faster than global average temperatures.

(2) Climate models show a “cold blob” in the subpolar Atlantic as well as enhanced warming off the US east coast as a characteristic response pattern to a slowdown of the AMOC.

Observed sea surface temperature change

A comprehensive analysis of the patterns of change in global sea surface temperatures since the 19th Century was performed by Dima and Lohmann (2010). The dominant pattern of change (technically these patterns are called EOF) is global warming – no surprise there. The second-most important pattern is more interesting and shown in Fig. 1.

 

dima-lohmann-global Fig. 1 Second pattern of sea surface temperature change (i.e. EOF2) found in the HadISST global sea surface temperature data set. Source: Dima and Lohmann 2010.

This pattern shows a cold blob (shown in red here) developing in the subpolar North Atlantic, as well as a warm patch (shown in blue) developing off the US east coast. (The colors are reversed compared to what you might expect, as the observed change is composed of this pattern multiplied with a time series which shows a negative trend – so “red is warm” but with a negative trend, i.e. cooling.)

Dima and Lohmann also looked at the pattern correlation between North and South Atlantic, and they found this (Fig. 2):

dima-lohmann-Atlantic

Fig. 2 Pattern of coupled correlation of North Atlantic and South Atlantic sea surface temperatures. The correlation coefficient is 0.93.

This is very interesting, as the physical linkage between South and North Atlantic is the heat transport across the equator from South to North Atlantic, which is dominated by the AMOC. Dima and Lohmann concluded that the patterns shown in Fig. 1 and Fig. 2 indicate a change in the AMOC, and they wrote:

The global conveyor has been weakening since the late 1930s.

(As a side remark, the IPCC in its last report ignored this result and claimed, rather puzzling to me, that there is no evidence for an AMOC slowdown.)

It is noteworthy that in 2015, the “cold blob” region actually registered the coldest sea surface conditions since records began in 1880 – whilst the globe as a whole was record hot!

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References

  1. M. Dima, and G. Lohmann, "Evidence for Two Distinct Modes of Large-Scale Ocean Circulation Changes over the Last Century", Journal of Climate, vol. 23, pp. 5-16, 2010. http://dx.doi.org/10.1175/2009JCLI2867.1