Reposted by Dr. Judith Curry’s Climate Etc.
Posted on May 12th 2021 by curryja |
by Alan Longhurst
“Never before in 1000 years has the Atlantic Meridional Overturning Circulation (AMOC), also known as the Gulf Stream System, been as weak as in the last few decades.”
This announcement by the Potsdam Institute for Climate Impact Research was recently cited in my morning paper in France (and dozens of other countries elsewhere) as a result of the anthropogenic “Rechauffment Climatique”. We were warned that the consequences would include harsher winters for us here in Europe. Whoever wrote this cannot have critically questioned the old adage that it is the Gulf Stream that brings Western Europe a pleasant maritime climate, because folklore turns out that it …
Rahmstorf and his colleagues in Potsdam have been dealing with this topic for a long time. Your new paper is short and contains bundles of representatives who are supposed to show that the flow of the Gulf Stream has weakened in recent decades and thus endanger the fair maritime climate of Europe.
However, this is a misleading oversimplification: in reality, the Gulf Stream is only a short segment of the wind-driven subtropical gyre of the North Atlantic and, as Wyville-Thompson wrote of the Gulf Stream in 1871, “after leaving the Florida Straits … for a large part of approx. 42 ° N … turns to the east and south and merges around Lake Sargasso, merges with the northern edge of the equatorial current and flows back into the main circuit. ” 
The North Atlantic Current, which continues towards Europe and eventually flows into the Arctic Ocean, carries only a very small portion of the water that previously flowed as the Gulf Stream along the Florida coast, along with slope and shelf water from the seas around Newfoundland and Newfoundland won the Gulf of Saint Lawrence.
The long-term change in SST is difficult to quantify, but the evolution of sea surface temperature reported in the HadNMAT2 archive for the northern oceans is shown below. Although the authors of this study suggest that these data support “existing estimates of global warming”, it is difficult to imagine that they contain a progressive anthropogenic signal: the data are (i) dominated by the impact of the number of ships at sea (ii )) by the inaccuracy of early bucket-over-the-side observations; and (iii) by the advent of automated free-drifting instruments. These observations do not support the advancement of the North Atlantic temperature / circulation indices, which Potsdam has offered to support its proposal for an extraordinary slowdown in the North Atlantic circulation at approximately the same time period.
And the pattern of SST changes on a much smaller scale obtained from coastal sampling at lighthouses, tide gauges, etc., suggests that the Gulf Stream-washed southeast coast of the United States was in fact not progressively warming. Here are the data for three regions of the Western Atlantic. They do not support the trends that Potsdam’s proxies have indicated over the past 150 years.
Such statements require evidence – not difficult to find in this case: the consequences of a progressive adjustment, purification and homogenization of the original observations, previously shown as individually colored data plots at NASA Goddard, have recently been edited on their public website so that Es became difficult to visualize only the final homogenized data. Fortunately, they seem to have done the job by hand and sloppily.
This enabled me to find – and restore – their original plots for two stations that are very relevant to this discussion: the Valentia Observatory on the shores of Galway and Vestmannaeyjar on an island south of Iceland. In each case, the darker lines of the earlier year of the homogenized data were shifted very much down to introduce significant warming where none was present in the lighter lines representing the unmatched data. NOAA only makes a very modest adjustment to its GHCN-All data, only heating the last decade to create its GHCN-M archive (adjusted).
The pattern of Galway data with warmer periods around 1940 and also at the end of the century is repeated at rural stations in Western Europe. it also perfectly reflects the changing value of the NAO and the evolution of the atmospheric pressure field over the north-east Atlantic.
Warnings like the one from Potsdam, however, have a long history, despite refutation: The alleged effects of a slowdown in deep convection in the North Atlantic on the climate, especially in Europe, were discussed in Nature in 2005 shortly before a climate meeting after Kyoto. in relation to observations of deep currents at lower latitudes. This idea was quickly followed by a comment from Carl Wunsch titled “The Atlantic Fund may have slowed down, but don’t panic yet!” Questioned. And it is not surprising that he should have reacted as dogmatically as he did: “European readers should be reassured that the existence of the Gulf Stream is a consequence of the extensive wind system over the North Atlantic and the nature of the fluid movement on a rotating planet. The only way to create ocean circulation without the Gulf Stream is to either turn off the wind system or stop the earth’s rotation, or both. ‘
Wish could also have commented that the greatest heat flow from the Gulf Stream into the atmosphere occurs in the western ocean south of Nova Scotia, where sun-warmed water from low latitudes meets very cold polar air. This process is dominated by latent heat of evaporation and is an anomaly of global proportions. On the coasts of Western Europe, however, little or no heat remains in the ocean that can be released into the atmosphere.
As a result, it is clear that the popular image of a broad Gulf Stream flowing east across the Atlantic at around 45 ° N, carrying the heat gained in the tropics to maintain the European maritime climate, is of no help.
The fact is that apparently everyone has forgotten that there is a perfect maritime climate at similar latitudes on the Pacific coast of North America – where there is no equivalent to the Gulf Stream: in fact, the cold water mass along this coast is at roughly the same latitude as Europe, comes from the Gulf from Alaska and brings no warmth. But the winters are warm and the summers cool here, as Richard Seager, an apparently naive English oceanographer, discovered to his surprise when he first visited the west coast of the United States. This maritime climate with no Gulf Stream analogue is the result of the heat that southwest winds carry from the distant subtropical gyrus over the cold coastal currents.
The heat that these winds bring is not lost to the ocean when crossing the cold coastal waters of Alaska, since a sensitive heat flow between the ocean and the atmosphere is a loss concept almost everywhere. This is due to the microstructure of the oceanic skin layer, over which molecular diffusion dominates. Only in autumn and winter at high latitudes, when strong wind stress creates breaking waves that destroy the molecular skin layer, is there a significant flow of sensible heat from the ocean into the atmosphere. 
When these “anti-Passat” winds pass over the Rocky Mountains, their gyroscopic structure compresses vertically and expands horizontally to maintain angular momentum. This forces the storm tracks to curve south over the continent so that they meet the ocean again at a significantly lower latitude – in the northern Caribbean and east of Florida, where sensible heat is transferred very quickly to the cool atmosphere. During its north-easterly movement across the Atlantic towards Europe, the surface air mass absorbs further heat from the sea surface through a noticeable flow of heat and transports it at a speed that significantly exceeds that transmitted in the ocean. If these winds were to cross the ocean at the same latitude as the western Rocky Mountains to Europe, their consequences for the European climate would be much less positive.
These images from Ventusky simulations for (left) near-surface air temperature on November 30, 2011 and (right) for July 31, 2020. were selected to show the classic weakening of the Western European winter climate by warm southwest winds (left) and also a whole another situation that is not often talked about in the context of the European climate, as regards our very uncomfortable southern neighbors, North Africa and the Sahara.
Such conditions can also develop in winter, like on February 6th of this year when my car and everyone else in town were speckled with raisin-sized clumps of moist Sahara mud, aggregated from dust particles carried in (and by this have fallen) Cumulus Deck.
The myth of the maritime, fair climate that the Gulf Stream brought to Europe must really be seen for what it is – as a myth. It may be appropriate for the British Isles and Scandinavia, but certainly not for the entire subcontinent. And this “Gulf Stream at its lowest level for 1000 years” in Potsdam, which was trumpeted by a fear-hungry press, is perhaps also mythical – but certainly not helpful in understanding how the real world actually works.
I fear, however, that we will hear more, and probably even more, from the Potsdam Institute for Climate Impact Research, because they advocate a failing Gulf Stream and a cool Europe – a concept that does not need to be examined in detail: after all, financed to address the areas of ” Global Change, Climate Impacts and Sustainable Development ”, and I conclude from your writings that this is not interpreted as a mandate to try to understand the natural world – and sometimes so. In the present case, they get it wrong.
46160 Cajarc, France
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