Atlantic Circular: The Gulf Stream and ocean circulation (Part 2 of 3)

This is part two of a three-part series on Atlantic Circular. You can find part one here.

More than the Gulf Stream

The oceans cover 7/10 of the surface of our planet and contain 97 percent of the water on Earth (if you do not take into account the water that is located in the bowels of the planet). It is not surprising that our knowledge of this giant is incomplete. Some processes in the ocean are often known only in general terms; almost every year this or that phenomenon is clarified.

Scheme of the Atlantic meridional overturning circulation: Lenz (1845). Philip L.Richardson / Progress in Oceanography, 2008

The first observations of the ocean were carried out on sea vessels — at first as companionships, but from the end of the 19th century, they became specialized. Now the ocean observation system includes many more components: in addition to scientific and commercial vessels, these are tide gauges, specialized moored and drifting buoys, gliders, animal trackers, high-frequency radars, and passive and active satellite sensing. For example, using satellite altimetry it was found that the sea level has been rising at an acceleration of up to 0.1 millimeters/year since the end of the 20th century.

Diagram of the Atlantic meridional overturning circulation: Brenneke (1909). Philip L.Richardson / Progress in Oceanography, 2008

It’s not just observations that are important, but also the growing power of our computing machines, which allow us to numerically model the ocean at ever-higher resolution. High resolution is even more important for ocean modeling than for atmospheric modeling. Tropical cyclones have a typical resolution of several hundred kilometers, the usual cyclones up to two thousand kilometers, and the size of eddies in the ocean is only tens of kilometers, while they transfer a significant portion of heat (primarily near the equator).

However, new observation systems and increased computing power by themselves do not lead to discoveries. The most important link remains between scientists and their guesses. Thus, based on just one measurement of the vertical profile of water temperature in the Atlantic, made in 1750 by the captain of a slave trading ship and showing that under a layer of warm surface water at depth there were much colder water masses, the idea of global ocean circulation grew. A circulation that is not limited to surface currents.

Scheme of the Atlantic meridional overturning circulation: Wüst (1949). Philip L.Richardson / Progress in Oceanography, 2008

Half a century after this, Count Rumford suggested that warm water from the equator along the surface of the ocean flows to the poles, and cold water, on the contrary, flows in the depths of the ocean from the poles towards the equator. Russian physicist Emil Lenz developed this idea in 1845, proposing that warm water “overturns” at the poles and cold water rises to the surface at the equator — essentially the first to describe the pattern of the Atlantic Meridional Overturning Circulation (AMOC).

At the beginning of the 20th century, the German oceanographer Brenneke combined the AMOC and surface currents into a single scheme in which the rise of water at the equator was preserved. The next step was taken in 1925–1927 after research by German oceanographers on the Meteor ship: in Georg Wüst’s scheme, the rise of water at the equator disappears, and various levels appear where the water flow is directed to the south or north. In the middle of the 20th century, the American oceanographer Henry Stommel showed that the overturning of warm water occurs in narrow zones, where it cools and, due to active evaporation, becomes more salty — therefore it becomes heavier and sinks. Moreover, in Stommel’s scheme, water flows to the south in a narrow zone in the west of the ocean.

Diagram of the Atlantic meridional overturning circulation: Stommel (1957), showing surface and deep currents. Philip L.Richardson / Progress in Oceanography, 2008

Both Wüst and Stommel showed that in the Atlantic the heat flow is directed across the equator into the Northern Hemisphere. As a result, water temperatures in the North Atlantic are higher than in the North Pacific. But not only the temperature differs: in the north of the Atlantic the salinity is higher, and the water level, on the contrary, is lower than in the north of the Pacific Ocean — by almost a meter! These differences are associated with differences in precipitation (and to a lesser extent with evaporation): due to atmospheric circulation and the size of the oceans, the moisture evaporating over the Pacific Ocean mostly falls over it, and from the Atlantic, it is transferred to the mainland.

Schemes of the Global Ocean Conveyor: Broker (1987). Philip L.Richardson / Progress in Oceanography, 2008

All this independently led two oceanologists in the early 1980s — the American Wallace Brocker and the Russian Sergei Sergeevich Lappo — to the same guess: there is a global thermohaline circulation (that is, determined by density differences due to different temperatures and salinities) connecting all the oceans. In 1982, Broecker compared this circulation to a conveyor belt, and in 1987, Natural History magazine illustrator Joe Le Monier drew its iconic diagram. In 2001, for the third IPCC report, deep water formation zones were added to the same diagram — key zones of oceanic convection, changes in which can slow down the conveyor (by the way, it was in this report that a possible stop of the conveyor was assessed as an unlikely event with significant consequences, but a little later).

Blueprints for the global ocean pipeline: IPCC (2001). Philip L.Richardson / Progress in Oceanography, 2008

In the Atlantic, the meridional circulation at latitude 26.5º north transports about 18 sverdrups of water to the north (1 sverdrup = 106 cubic meters per second) in the upper layers of the ocean, and the lower layers transport the same amount to the south. For comparison, the largest river in the world, the Amazon, carries 0.2 sverdrup, and the strongest current in the ocean, the Antarctic Circumpolar Current, which encircles the sixth continent, carries 130 sverdrup. The Gulf Stream is not so much inferior to it: it carries from 85 to 105 sverdrup. That is five times more than AMOC! Why is it the latter, and not the Gulf Stream, that is important for heat transfer to the North Atlantic? After all, on the maps and diagrams, there is a “river” (although it is, of course, not a river, but many separate eddies), which carries heat to Europe, as it once carried galleons with gold towards the Old World.

Scientists experimented: from 1990 to 2002, they launched hundreds of drifters into the water in the subtropics and temperate latitudes of the Atlantic and looked at how they drifted along with surface currents. Of the 273 drifters who passed through the Gulf Stream region, only one reached Northern Europe.

Top: Drifter trajectories on the surface of the Atlantic Ocean from 1990 to 2002, passing through the Gulf Stream region (shown as a rectangle). Bottom: Trajectories of drifters passing through the Icelandic Sea (shown as a rectangle). The trajectories of drifters before entering the region are shown in green, and after they are in blue. Elena Brambilla et al. / JGR Oceans, 2006

A similar result was obtained with model drifters in a numerical ocean model: it was shown that only 5 percent of drifters from the surface waters of the subtropical gyre enter the subpolar gyre. The signal from water surface temperature anomalies in the Gulf Stream region is not traced in the water surface temperature in the North Atlantic — the subtropical and subpolar gyres are generally weakly connected. As a result, many piles of warm water, carried by the Gulf Stream and driven mostly by the wind, circulate in the subtropical gyre, passing through the Gulf Stream region again and again, and are in no hurry to warm the shores of Europe.

Scheme of the movement of waters in the Atlantic — warm surface (red arrows) and cold deep (blue arrows). The water cycles are also indicated — subtropical (STG — subtropical gyre) and subpolar (SPG — subpolar gyre), the sign © indicates regions of convection (formation of deep water). Janne Repschläger et al. / Climate of the Past, 2017

At depth, the connection is stronger: modeling shows that 30 percent of drifters launched in the Gulf Stream region at a depth of 700 meters penetrate from the subtropical gyre to the subpolar one. The characteristic time of such a deep exchange is from two to seven years.

In the northeastern part of the subpolar gyre, the heat influx gives up to 0.3 petawatts, of which 0.1 petawatts are released into the atmosphere (the atmosphere transfers this heat to the mainland), and the rest goes further — to the northwest, into the Labrador Sea, where one of the zones of convection and formation of the upper deep Atlantic waters at a depth of 1.5–3 kilometers), and to the northeast, towards the Norwegian, Icelandic and Greenland seas, where the second convection zone is located and where the lower deep Atlantic waters are formed (located below three kilometers).

Ultimately, 0.045 petawatts reach the Barents Sea. This heat is enough to keep the sea ice-free all year round. And it is precisely this heat that is primarily associated directly with the AMOC, which sets in motion the continuation of the Gulf Stream — the North Atlantic Current. So if we are interested in the fate of Norway, the question is not whether the Gulf Stream is slowing down, but whether the AMOC is slowing down. And if so, why?

Is the circulation of water in the Atlantic slowing down?

A recent article by German oceanographer-climatologist Stefan Rahmstorff and his colleagues, which was actively discussed by everyone in February, suggests that the AMOC circulation is now the weakest in the last 1600 years (by the way, there is not a word about the Gulf Stream in this article!). Scientists concluded this based on independent proxy data, one way or another showing the intensity of various parts of the AMOC or processes in the atmosphere and ocean associated with the AMOC (but not the AMOC as such): the ratio of various isotopes in the shells of fossil invertebrates (foraminifera) at the bottom seas, the characteristic size of muddy sediments, the content of methanesulfonic acid in Greenland ice cores, and so on. The entire set of data used indicates that the intensity of the AMOC is, with a high probability, now the weakest over the past 1600 years.

Changes in various paleo data indirectly indicate the current state of AMOC intensity — the weakest over the last 1600 years. L. Caesar et al. / Nature Geoscience, 2021

The idea that the global thermohaline circulation conveyor and the AMOC along with it may weaken as a result of the strengthening of the greenhouse effect due to rising CO2 concentrations was expressed by American climatologists Syukuro Manabe and Ronald Stouffer in the early 1990s. Based on numerical experiments with a climate model with doubling and quadrupling of CO2 concentration in the atmosphere, scientists have found that in the North Atlantic, as a result of melting ice in the Arctic and Greenland and increased precipitation, surface waters will desalinate. This led to a weakening of convection (lowering of water) and a slowdown in the thermohaline circulation. The desalination predicted 30 years ago is already happening. Does this mean the AMOC is also slowing down?

In 2010, the weakening of the global ocean circulation was indirectly confirmed by observational data on the ocean surface temperature field, highlighting various modes of variability in it. Later, as a measure of AMOC intensity, it was proposed to estimate the water surface temperature in the subpolar North Atlantic Gyre, one of the most sensitive regions to the AMOC. While the rest of the world was warming, this region was cooling. The term “warming hole” even appeared. Using this indicator, scientists showed that the AMOC has weakened by 15 percent since the mid-20th century.

Linear trend of Earth’s surface temperature (ºC/century) according to NASA-GISS data for 1901–2013. (regions with insufficient data are shown in white). Stefan Rahmstorf et al. / Nature Climate Change, 2015

True, this weakening cannot yet be confirmed by direct observations of water transport in the ocean. In the spring of 2004, the RAPID observation network was deployed at 26.5 degrees north latitude to monitor the AMOC, which included a whole range of observations: a submarine cable in the Florida Straits (to measure the Gulf Stream flow), an array of moored buoys in the open ocean and pressure sensors on the ocean floor (to measure the flow in the ocean column), and data from satellite measurements of the wind at the ocean surface (to determine the so-called Ekman transport of water resulting from the action of wind and Coriolis force in the near-surface layer of the ocean).

RAPID observation scheme for the AMOC. M. A. Srokosz et al. / Science, 2015

Direct measurements made it possible to identify the strongest variability of the AMOC (from 4 to 35 sverdrup per ten days, and this is on average), due to which it is impossible to clearly “feel” in the data a tendency towards weakening circulation from year to year. A serious weakening of the AMOC was recorded in 2009–2010, but since then the circulation has recovered.

The most recent studies based on various oceanographic observations (including RAPID data) show (1, 2, 3) that the AMOC is quite stable and is not weakening. Stability is also indicated by direct observations of the Gulf Stream transport by acoustic Doppler profilers and numerous oceanographic data on the position of the Gulf Stream (1, 2).

However, data from satellite altimetry and coastal stations monitoring sea level indicate (1, 2) a slight weakening and shift of the Gulf Stream to the south. The weakening of the Gulf Stream is accompanied by a higher rise in sea level off the northeastern coast of the United States — because the stronger the Gulf Stream, the stronger the Coriolis force acts on it, which, as it were, takes it away from the coast.

Reconstructed (orange) and measured values (blue and gray) of transport by AMOC components (through the Straits of Florida, in the Ekman layer, in the lower and upper parts of the ocean) and by the entire AMOC. Emma L. Worthington et al. / Ocean Science, 2021

Thus, scientists do not have a clear conclusion about whether the AMOC (and the Gulf Stream, as part of it) is weakening or not. More often, the conclusion is made about the presence of long-period AMOC oscillations, which are closely related to the 60-year cyclicity of water temperature in the North Atlantic (although hypotheses are put forward that this cyclicity is either a random process or due to the influence of volcanoes), in the new — cold — a phase we are now entering.

But why do scientists point to a possible shutdown of the AMOC as a risk (albeit unlikely) with serious consequences? They are alarmed by examples from the past.

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