The past winter was globally the warmest on record. At the same time it set a new cold record in the subpolar North Atlantic – and it was very cold in the eastern parts of North America. Are these things related?
Two weeks ago NOAA published the following map of temperature anomalies for the past December-January-February (i.e. the Northern Hemisphere winter). One week ago, we published a paper in Nature Climate Change (which had been in the works for a few years) arguing that the cold in the subpolar North Atlantic is indicative of an AMOC slowdown (as discussed in my last post). Immediately our readers started to ask (as we indeed had been asking ourselves): does the cold winter in eastern North America (culminating in the Inhofe snowball incident) have anything to do with what is going on in the Atlantic?
Fig. 1 Temperature anomaly map for the past december-january-february, from NOAA.
Here is a hypothesis for how they may indeed be linked. This is somewhat speculative – I have not investigated this with any special data analysis, I am just connecting the dots of some articles in the published literature, hoping this post might stimulate further investigation. The proposed mechanism has three simple steps, as follows.
1. The AMOC slows down – see my previous post.
2. The slowdown not only leads to a cold patch out in the Atlantic subpolar gyre, but also to warm sea surface temperature anomalies along the east coast of North America. This dipole response was found in the EOF analysis of observed sea surface temperatures by Dima and Lohmann (2011) (Fig.2). An EOF analysis is a standard statistical tool to decompose changes in space and time into a set of characteristic fixed spatial patterns, each of which follows a particular time evolution. That makes sense when a particular physical mechanism of change (such as an AMOC slowdown) has a characteristic spatial pattern. The pattern shown in Fig. 2 is the one identified by Dima and Lohman with the gradual AMOC decline over the 20th Century. (Note you have to reverse colours – as shown in the graph it corresponds to an AMOC increase, because this pattern is then multiplied with a negative time evolution).
Fig. 2 Temperature pattern EOF2 in the HadISST data set, as analysed by Dima and Lohmann and identified with a gradual AMOC decline. Note that in this (and the following) graph the sign is reversed; an AMOC weakening comes with a cold patch south of Greenland and warming along the North American east coast.
Dima and Lohmann also show a second pattern (Fig. 3) associated with the sudden AMOC decline in the 1970s which we also see in the AMOC index in our paper.
Fig. 3 Temperature pattern derived from a correlation analysis and identified by Dima and Lohmann with a rapid 1970s AMOC weakening.
In either case the anomaly in the subpolar North Atlantic is associated with an opposite anomaly along the North American east coast.
This dipole response to an AMOC slowdown is also found in models, as shown by Zhang (2008) – in her paper she presented the schematic shown in Fig. 4. Note you also need to reverse the colors in the diagram of Zhang – she chose to show the effect of an increase, not a slowdown of the AMOC, because she was looking at the increase after 1990 which we also find in our index. Zhang derived this pattern for the subsurface temperatures, so I asked her whether she also found a similar dipole in sea surface temperatures. She responded: “The dipole pattern shown in the subsurface is indeed also expressed in the SST, I use subsurface temperature because it is less noisy than SST.”
Fig. 4 Dipole induced by strengthening the AMOC – for a weakening of the AMOC the reverse response is expected. From Zhang 2008.
Was this warm anomaly along the American east coast present last winter? Definitely – I happen to have saved two snapshots of SST anomalies on my hard disk, shown below.
Fig. 5 SST anomalies for 12 December and 11 February. Note the cold patch in the subpolar Atlantic and the very warm SSTs along the North American east coast. Source: Climate Reanalyzer.
3. Warm SST along the American east coast creates a cold anomaly in the eastern parts of North America by radiating groups of Rossby waves. This was shown in a very elegant paper by Kaspi and Schneider (2011). They set out to explain why the eastern parts of the northern continents are in general much colder than the rest of the hemisphere. They took an idealized climate model, with no continents or other distractions (an “aquaplanet”), and simply pumped a heat anomaly into one small ocean region to mimic the effect of the Gulf Stream. Voila: upstream of this heat anomaly they got a big cold anomaly.
They also performed a clever, fun experiment: they increased the rotation rate of their planet and showed that the size of that cold patch increases in proportion. The theory of Rossby wave propagation explains this.
Fig. 6 Temperature anomalies that result from adding an ocean heat anomaly in the triangular region on an aquaplanet. This triangular heating region is to mimic warm sea surface temperatures (which provide a strong heat source to the atmosphere) along the North American coast. Note the cold anomaly that develops upstream. Left panel shows normal, right panel doubled rotation rate of the Earth.
They conclude in their paper:
The anomalous winter cold of eastern continental boundaries can result at least in part from radiation of nearly stationary Rossby wave groups off the regions of large surface heat fluxes over the warm waters in oceanic western boundary currents.
(They say “in part” because there are other factors like topography – but these don’t change over time, so don’t come into play when explaining why the last winter was colder than usual.)
Of course we need to be cautious – theirs is an idealised experiment which isolates and demonstrates this mechanisms in principle, but does not prove how strong it is in the real world. Certainly, changes in heating from the Gulf Stream will not be the only thing that influences winter weather along the Atlantic seaboard of North America, so we can’t expect a one-to-one relation. But I think this connection is worthy of further investigation.
By the way, this 3-step explanation of the cold of the past winter is not an alternative to the “jet stream meander” or “polar vortex” explanations – rather, it may simply help to explain why there was this persistent strong southward inflow of polar air into the eastern parts of North America. It might just have been helped along by the very warm waters along the coast – and those may well be related to the AMOC slowdown.
p.s. Sorry about the jargon in this post, which was a quick follow-up to my better-prepared previous one. Here a glossary:
NOAA – National Oceanic and Atmospheric Administration of the USA – an agency that e.g. publishes climate data
AMOC – Atlantic Meriodional Overturning Circulation (for lay people the Gulf Stream System as mentioned in my previous post)
SST – Sea surface temperature
HadISST – a particular sea surface temperature data product from the British Met Office
EOF – Empirical Orthogonal Function – you don’t need to understand how this works, just a statistical method to derive patterns of change
Update 1 April: I just read another related paper which I thought would be worth sharing here. Zhang et al. (Journal of Climate 2011) ran a high-resolution climate model with eddy-permitting ocean – the main point here is that the narrow overflows from the Nordic Seas into the Atlantic can be represented in a much better way at such a high resolution. They perturbed the AMOC by altering the overflow so that it increased, which then had the effect of strengthening the entire AMOC. The change in sea surface temperature looks like this:
Note they get a warming due to the stronger AMOC just in the area where we find a cooling in the data (which we interpret as indicator of a weakened AMOC). And they get a cooling along the North American coast north of Hatteras. The arrows show the change in currents and reveal the reason for that cooling: the northern recirculation gyre, located to the north of the Gulf Stream, increases in strength and brings colder waters from the north down along the US/Canadian coast (as sketched in Fig. 4). Thus this connection described above (point 2 in my argument) is also supported by this high-resolution climate model. This experiment is quite neat in that there is no change to the surface forcing; it is pure isolated effect of the AMOC, triggered by a change way below the surface in the deep overflow.
- 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
- 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
- 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
- Y. Kaspi, and T. Schneider, "Winter cold of eastern continental boundaries induced by warm ocean waters", Nature, vol. 471, pp. 621-624, 2011. http://dx.doi.org/10.1038/nature09924