by Alan Longhurst
“Never in the last 1000 years has the Atlantic meridian circulation (AMOC), also known as the Gulf Stream System, been as weak as it has been in the past decades.“.
This announcement from the Potsdam Institute for Climate Impact Research was recently published in my morning paper in France (and in dozens of other newspapers elsewhere) as a result of man-made’the climax of re-creation. ‘ We have been warned that the consequences will include harsher winters for us here in Europe.[1] Whoever wrote it couldn’t help but seriously consider the old adage that it is the Gulf Stream that gives Western Europe a comfortable maritime climate, because the folklore turns out to be…
Rahmstorf and his colleagues in Potsdam have long been interested in this issue.[2] Their new paper succinctly presents trust packages aimed at demonstrating that the flow of the Gulf Stream has weakened in recent decades, and thus puts Europe’s egalitarian maritime environment at risk. ro.
But this is a false simplification: in fact, the Gulf Stream is more than a short stretch of the Subtropical Gyre driven by the winds of the North Atlantic and, as Wyville-Thompson wrote in 1871 in relation to Gulf Stream”after leaving the Florida Straits… at about 42oN a a large part… rotates east and south and swirls around the Sargasso Sea, merging with the northern edge of the equator and rejoining the main circulation.” [3]
So, the North Atlantic Current continues toward Europe and eventually enters the Arctic Ocean carrying only a tiny fraction of the water that had previously traveled along the Florida coast as the Gulf Stream, along with its steep gradient. and the water shelf received from the seas around Newfoundland and the Gulf of Saint Lawrence.
Long-term variation in SST is difficult to quantify, but the evolution of sea surface temperatures indicated by the HadNMAT2 archive for the northern oceans is presented below.[4] Although the authors of this study argue that these data support “existing estimates of global warming“It is difficult to imagine that they contain a progressive human signal: the data are dominated (i) by the consequences of the number of ships at sea (ii) by the imprecision of the locomotive observations and ( iii) by the introduction of automatic measuring equipment. These observations do not support the evolution of the North Atlantic circulation/temperature indices that were put forward by Potsdam to support their suggestion of a particular deceleration in the North Atlantic circulation during the interval. almost the same time.[5]
And the model of SST changes on a much smaller scale obtained by coastal sampling at lighthouses, tide gauges, etc. shows that the southeastern coast of the United States is washed by the Gulf Stream. drifting in fact does not warm up at all. This is data for three regions of the western Atlantic Ocean.[6] They do not support the trends indicated by the Potsdam mandates over the past 150 years.
Such claims require proof – in this case, not difficult to find: the consequences of a gradual adjustment, cleaning, and homogenization of the original observations that had previously been shown as The individually colored data plots at NASA Goddard have recently been edited on their public website to make it difficult to visualize any but final homogenous data. Luckily for us, they seem to have done the job by hand and sloppily.
So I was able to find – and recover – their original drawings for two stations very relevant to this discussion: the Valentia Observatory on the Galway coast and Vestmannaeyjar on an island south of Iceland. In each line, the previous year’s darker lines of homogenous data have been shifted down very significantly to induce significant warming with none of the lighter lines representing unadjusted data. correction. NOAA made only a very modest adjustment to all of their GHCN data by warming only the last decade to create their (modified) GHCN-M archive.
The pattern of the Galway data, which has a warmer period around 1940 and also later in the century is repeated in rural stations in Western Europe; it also perfectly reflects the changing value of the NAO and the evolution of the barometric pressure field over the northeastern Atlantic.
But warnings like the one in Potsdam have been around for a long time, though refuted: the supposed effects on climate, especially on Europe, of a decline in North Atlantic deep convection have been discussed. essay in nature in 2005 just before the Kyoto climate meeting, which involved observations of deep currents at lower latitudes.[8] This notion was quickly challenged by a comment from Carl Wunsch titled ‘The Atlantic Conveyor may have slowed down, but don’t panic!‘And it’s no surprise that he should have reacted as dogmatically as he did:’European readers should rest assured that the existence of the Gulf Stream is a consequence of the large-scale wind system over the North Atlantic and the nature of fluid motion on a rotating planet. The only way to create ocean circulation without the Gulf Stream is to shut down the wind system, or stop the Earth’s rotation, or both. ‘[9]
Wunsch may also have observed that the strongest heat transfer from the Gulf Stream to the atmosphere occurred in the western ocean south of Nova Scotia, where warm water from low latitudes meets extremely cold air. This process is governed by the latent heat of evaporation and is an unusual phenomenon on a global scale – but along the coasts of Western Europe there is little or no heat left in the ocean to transfer to the atmosphere.[10]
Thus, it is clear that the common image of the vast Gulf Stream sweeping eastward across the Atlantic at about 45oN, which brings the heat captured in the tropics to sustain Europe’s maritime climate, is useless.
The fact that people seem to have forgotten that a perfectly good maritime climate exists at similar latitudes on the Pacific coast of North America – where there is no equivalent of the Gulf Stream: in fact In fact, this cold body of water lying along this coast at the same latitude as Europe, originates in the Gulf of Alaska and gives it no warmth. But winters are warm and summers are cool here, as Richard Seager, a seemingly innocent British oceanographer, discovered to his surprise when he first visited the West Coast of the United States. . This Gulf Stream analogy lacking in maritime climate is the result of heat from southwesterly winds from distant subtropical gyres across cold ocean currents.[11]
The warmth these winds bring is not lost to the ocean as it passes through frigid Alaskan coastal waters, because the proper heat flux between the ocean and the atmosphere is almost everywhere a loss. cool. This is due to the microstructure of the oceanic skin over which molecular diffusion predominates; It is only in the fall and winter at high latitudes, when strong wind pressure produces breaking waves that disrupt the molecular skin, that there is any significant perceptible heat flux from the ocean to the atmosphere. [12]
As these ‘anti-commercial’ winds pass through the Rocky Mountains, their gyroscopic structure is vertically compressed, expanding horizontally to preserve angular momentum. This forces the storm’s paths to curve south across the continent so that they re-enter the ocean at significantly lower latitudes – in the northern Caribbean and east of Florida, where the heat felt is transferred to the atmosphere. The book freezes very quickly. Then, during its north-eastward movement across the Atlantic Ocean toward Europe, the surface air mass will pick up additional heat from the ocean surface by the perceptible heat flux and carry it at a faster rate. significantly exceeds the transmission rate in the ocean.[13] If these winds crossed the ocean to Europe at the same latitude as they encountered the western Rocky Mountains, their consequences for European climate would be much less positive.
These images from Ventusky’s simulations for air temperatures (left) near the surface on November 30, 2011 and (right) on July 31, 2020 were chosen to represent classical softening of the western European winter climate by warm southwesterly winds (left) and a very different, not often talked about situation related to the European climate, involving their very unpleasant southern neighbors us, North Africa and the Sahara Desert.
Such conditions can also develop in winter as they did on February 6 of this year when my car, and everyone else in town, was dotted with blobs as small as the seeds of a raisin. The humid Saharan mud accumulates from dust particles that are carried into (and landed on) the storage deck.[14]
So the maritime, climate-fair myth brought to Europe by the Gulf Stream really needs to be seen for what it is – a myth. It is perhaps appropriate for the British Isles and Scandinavia, but certainly not for the entire subcontinent. And, that “Gulf Stream at 1000-year low“Potsdam’s being blown away by a threatening press is perhaps also myth – but certainly not helpful in understanding how the real world works.
But I fear that we will hear more, and perhaps even more, from the Potsdam Institute for Climate Impact Research because they endorse the failed Gulf Stream and a frigid Europe – a concept are not closely examined: they are, after all, funded to research the fields of “Global change, climate impactand sustainable developerst” and I infer from their writings that this is not to be understood as the quest to understand the natural world – and so, sometimes, as in the present case, they get it wrong.
Alan Longhurst
Place de l’Eglise
46160 Cajarc, France
[1] Caesar, Feast e al .; (In 2021). Natural Geosciences14, 118-120. 10.1038/s41561-021-00699-z.
[2] Rahmstorf, S. et al. (2015) Natural Climate Change 5, 475-480, DOI.10.1038NCLIM2554
[3] Nature (1871) vol. 4, 251-253. Images from the indispensable 2002 “Regional Oceanography” by Tomczac and Godfrey, freely available online.
[4] Kent, EC et al. (2013) J. Geophysics. Res. 118, 1281-1298.
[5] Rahmstorf, S. et al. (2015) Nat. Climb up. Change 6, DOE. 10.1038 / NCLIMATE2554
[6] from Shearman, RK and SJ Lutz (2010) J. Phys. Ocean. 40, 1004-1017
[7]Neto, AG et al. (2021) Communications Earth Envir. | https://doi.org/10.1038/s43247-021-00143-
[8] Bryden, HL et al. (2005) Nature, 438, 655-657.
[9] Wuntsch, C. (2004) Nature, 428, 601.
[10] Broecker, WS (1991) Oceanography 4, 79-89; image, left – Tomczac and Godfrey again and rightly, from Bedford Inst. Ocean. 1984 version.
[11] Seager, R. et al. (2002) Quart. Roy. J. Meteor. Soc. 128, 2563-2586.
[12]image from Seager, R. (2006) Am. Science. 94, 334341 and from Tomczak (2003). Average annual heat flux from ocean to atmosphere (Wm2)
[13] Seager, R. et al. (2002) Quat. J. Roy. Meet. Soc. 128, 2563 doi.10.1256 / qj.01.128
[14] Cuadros, J. et al. (2015) Atmosphere. Envir. 120, 160-172
