The Weekend Wonk: Stefan Rahmstorf on the Gulf Stream, and the “Cold Blob”
October 22, 2016
New computer is ordered – won’t be here till next week, and given my work/travel schedule, it will take a week or so to get set up. For now, limping along – but I was glad to find this lecture and QA from Stefan Rahmstorf on the mysterious “cold blob” and impacts on the Gulf stream. My video on the issue at bottom.
What is the cause of the cold blob?
In principle, there can be two reasons for a change in ocean temperature: heat exchange through the surface or heat transports within the ocean. Halldór Björnsson of the Icelandic weather service showed in his lecture on Saturday that the short-term temperature fluctuations from year to year correlate with the heat exchange through the sea surface, but that this does not explain the longer-term development of the ‘cold blob’ over decades. He concluded that the latter is caused by changes in the North Atlantic ocean circulation, also called the Gulf Stream System. That’s exactly what one expects. Weather dominates the short-term fluctuations, but the ocean currents dominate the long-term development.
One suggestion that had been made some years ago – that the cooling may be caused by shading the sun by aerosol pollution – did not show up in the discussion on Saturday. In the scientific literature that idea was rapidly contradicted at the time, for good reasons (we discussed this in more detail in our paper).
What evidence speaks for a slowdown of the Gulf Stream System?
The basic problem is the lack of direct, continuous measurements of the key circulation in the Atlantic, the so-called AMOC (Atlantic Meridional Overturning Circulation). Such measurements are only available since 2004 through a series of moorings at 26°N (RAPID project). For the longer term development, one must therefore use indirect indicators of the flow.
My colleagues Mihai Dima and Gerrit Lohmann of the Alfred Wegener Institute in Germany in a 2010 study analysed the patterns of changes in global sea surface temperatures. They were the first to conclude that the AMOC has been weakening since the 1930’s. The evidence for this is the trend towards cooling in the subpolar North Atlantic which anti-correlates with temperatures in the South Atlantic (suggesting reduced heat transport from the South Atlantic to the North Atlantic). In addition, Dima and Lohmann found an anti-correlation to the temperatures off the US East Coast, to the south-west of the ‘cold blob’. This is not seen in Fig. 1 above, since the NASA data use a smoothing radius of 1200 km, but you can see it, for example, in the currently high temperatures in Fig. 2.
The latest high resolution simulations of the GFDL in Princeton show precisely this pattern in response to a CO2 increase in the atmosphere (discussed more in this RealClimate post). In the model the cause is a slowdown of the Gulf Stream system. There are also coral data from the Gulf of Maine off the US coast, which indicate a similar time evolution of water mass changes there as the ‘cold blob’ (discussed further in the same post).
For the most recent past, the Atlantic flow index we calculated from the temperature pattern is consistent with other data. For the time since 2004, for which there are direct measurements of the AMOC from the RAPID array, the downward trend by 3 Sv measured there agrees with our indirect estimate. The significant slowdown after 1970 and then following recovery from about 1990 in our curve has been confirmed by other studies with other methods (see e.g. Haine 2016 and its schematic diagram).
What speaks against a slowdown of the Gulf Stream System?
As a counter-argument against a weakening of the Gulf Stream system, Steingrímur Jónsson on Saturday brought up the measurements of the so-called “overflow” from the Nordic Seas across the sills between Greenland, Iceland and Scotland, which do not show any trend. Here one must simply distinguish between different parts of the Atlantic ocean circulation. In our study, we argue that the AMOC in the open Atlantic has weakened – i.e. to the south of the ‘cold blob’, where the heat comes from. This is what’s measured by the RAPID array. The overflows further north are (i) unlikely to have an influence on the temperatures in the ‘cold blob’, and (ii) are largely independent of the AMOC in the open Atlantic – at least that is suggested by a model simulation of the Max Planck Institute for Meteorology in Hamburg, for which we show a correlation analysis in Fig. 2b in our paper.
Another counterargument (though not brought up in professional discourse but on a “climate skeptics” website) is that the measurements on the Oleander line across the Gulf Stream show no slowdown (Rossby et al. 2014). However, these cover only a 20-year period for which our AMOC index also does not show any slowdown. And as Tal Ezer showed in a study in 2015, these measurements of the Gulf Stream don’t correlate with the AMOC measurements of the RAPID array – which is not surprising because the AMOC is only a minor component of the mainly wind-driven Gulf Stream. Therefore these diverse measurements of other aspects of the complex Atlantic ocean circulation are by no means inconsistent with a general long-term slowdown of the AMOC as proposed by Dima and Lohmann.
What impact does a Gulf Stream System slowdown have?
The potential impacts are increasingly studied, here just briefly a few examples. Haarsma et al. (2015) argue on the basis of model simulation that the weakening of the Gulf Stream system will in the future be the main cause of changes in the atmospheric summer circulation over Europe. Jackson et al. (2015) found that a slowdown is likely to lead to increased storm activity across Britain and parts of mainland Europe. And a new study by Duchez et al. (2016) connects the ‘cold blob’ in the summer of 2015 to the heatwave across Europe that year, because the cold subpolar Atlantic favors a certain air pressure distribution.
Could the AMOC break down entirely?
This risk has been discussed since the 1980s, originally due to paleoclimatic data showing a number of abrupt AMOC changes in the course of Earth’s history. It is now well understood that there is a critical tipping point in the system. How far we are from this, however, is not known. Earlier model intercomparisons suggest that a freshwater flow in the order of 0.1 Sv (the equivalent of 3,000 cubic kilometers per year) could be critical. There are some arguments suggesting that models might systematically overrate the stability of the AMOC, which we summarized in PNAS in 2009. An assessment from 2011, commissioned by the European Environment Agency, concluded that the system may be viewed as more sensitive than suggested by earlier assessments.