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?
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.
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):
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!
In our paper in Nature Climate Change last year (see our Realclimate post on this) we used sea surface temperature data specifically from the cold blob to diagnose AMOC variations and found an exceptional AMOC slowdown in the 20th Century, and we argued that Greenland mass loss may have made a significant contribution to this slowdown by helping to dilute ocean waters in the subpolar Atlantic.
Modelled response to an AMOC slowdown
Let’s have a look what climate models have to say about this. Fig. 3 shows the sea surface temperature response to a speed-up of the AMOC from a paper by Zhang et al. (2011) from the Geophysical Fluid Dynamics Lab in Princeton. This AMOC speedup was achieved by enhancing salinities near the sea bottom in the overflow of dense waters over the ridge between Greenland, Iceland and Scotland – a nice clean experiment that did not involve any deliberate changes introduced at the sea surface.
Fig. 3 Pattern of modelled sea surface temperature response to an enhanced AMOC. Source: Zhang et al. 2011.
The resulting pattern is very interesting indeed, and similar to what is observed (compare Figs. 1 and 2). For an AMOC slowdown you would get the reverse: a cold blob in the subpolar gyre region of the Atlantic, and a warm region off the US east coast.
In an earlier paper, Zhang (2008) presented this schematic view of the response to an enhanced AMOC (Fig. 4):
Fig. 4 Schematic of surface temperature response to an enhanced AMOC. Reverse the colours for the response to a weakened AMOC.
A very recent study by Saba et al. (2015) specifically analyzed sea surface temperatures off the US east coast in observations and a suite of global warming runs with climate models. They find that the highest resolution climate model can reproduce observed temperatures well, and it projects the following response to increased CO2 (Fig. 5):
Fig. 5 Sea surface temperature response to CO2 doubling in a high-resolution global climate model. Note the cold blob in the subpolar Atlantic and enhanced warming off the US east coast.
They find that the region off the US east coast warms “nearly three times faster than the global average”. They summarize their key finding as follows:
Both observations and the climate model demonstrate a robust relationship between a weakening Atlantic Meridional Overturning Circulation (AMOC) and an increase in the proportion of Warm-Temperate Slope Water entering the Northwest Atlantic Shelf.
There is a strong case that the warm SST off the US coast and the cold blob in the subpolar gyre are linked, both being caused by an AMOC slowdown. This AMOC slowdown thus may have consequences for extreme weather in the US that I did not foresee in the past and only started to think about in the last year.
When Jake Gyllenhaal was snowed in in the New York public library in the film The Day After Tomorrow after an AMOC collapse, the physics may have been wrong, but perhaps there was a grain of truth in that snow storm after all.
The Washington Post on this: The surprising way that climate change could worsen East Coast blizzards
- 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
- S. Rahmstorf, J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford, and E.J. Schaffernicht, "Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation", Nature Climate Change, vol. 5, pp. 475-480, 2015. http://dx.doi.org/10.1038/nclimate2554
- R. Zhang, T.L. Delworth, A. Rosati, W.G. Anderson, K.W. Dixon, H. Lee, and F. Zeng, "Sensitivity of the North Atlantic Ocean Circulation to an abrupt change in the Nordic Sea overflow in a high resolution global coupled climate model", Journal of Geophysical Research, vol. 116, 2011. http://dx.doi.org/10.1029/2011JC007240
- R. Zhang, "Coherent surface-subsurface fingerprint of the Atlantic meridional overturning circulation", Geophysical Research Letters, vol. 35, 2008. http://dx.doi.org/10.1029/2008GL035463
- V.S. Saba, S.M. Griffies, W.G. Anderson, M. Winton, M.A. Alexander, T.L. Delworth, J.A. Hare, M.J. Harrison, A. Rosati, G.A. Vecchi, and R. Zhang, "Enhanced warming of the N orthwest A tlantic O cean under climate change", Journal of Geophysical Research: Oceans, vol. 121, pp. 118-132, 2016. http://dx.doi.org/10.1002/2015JC011346