I have always been passionate about perspectives and about the functioning of the Earth system under the Conservation Laws. This passion of mine is the key ingredient behind this paper. Please follow me.
Sun’s energy powers the whole Earth system, and is redistributed across the spherical, rotating Earth by the circulation of its fluidic components — the atmosphere and the ocean.
In this context, energy flow through the atmosphere and ocean follows a timescale hierarchy. The atmosphere dominates shorter timescales, shaping the daily weather conditions we experience directly. The ocean in turn dominates more stable, long-term climate conditions, serving as an energy reservoir for the atmosphere. However, the ocean’s surface is forced by the atmosphere, resulting in its upper ~1 km being governed by the wind-driven ocean circulation.
In the background of this dynamic upper-ocean wind-driven circulation, lies a slower movement of the ocean’s waters, that is density-driven.
At the subpolar North Atlantic, cold, salty surface waters become dense enough to sink, through a process called deepwater formation.
By mass conservation, deepwater formation creates a meridional, large-scale movement in the ocean — a continuous conveyor belt system. As surface waters are converted to deep waters, these spread toward the Equator, putting the deep ocean into motion by pushing the old waters ahead of it. Similarly, surface waters that sank must be replaced, by new surface waters drawn from lower latitudes toward the polar regions.
The Atlantic conveyor belt therefore refers to the northward transport of warm waters in the upper Atlantic ocean, overlying deep southward transport of cold waters — with the northward and southward limbs connected through deepwater formation to the north.
In this framework, the main property being transported is heat. Think of the Atlantic conveyor belt as a massive heat engine, modulating the global heat distribution.
Just as northward transport of waters is required to balance deepwater formation, northward ocean heat transport is required to balance the heat that is lost to the atmosphere in the process of transforming light surface waters into dense deep waters.
That heat, once released to the atmosphere, is then used to help moderate northwestern European climate, as prevailing winds carry the warm air eastward toward the continent. As a result, European and North American cities located at equivalent high latitudes tend to experience winter temperatures that are more than 10ºC apart.
The heat that is ‘consumed’ in the North Atlantic, is said to be ‘stolen’ from the South Atlantic. As a consequence, South Atlantic net meridional heat transport is toward the Equator — counterintuitively, and contrary to all other ocean basins.
For deepwater to be formed, strong local density contrasts are needed. Without northward ocean heat transport, there would be no warm water to cool.
In the context of the seawater density equation, deepwater formation depends on a delicate balance of heat and also salt.
When the Earth’s radiative balance changes — that is, when the Earth receives or stores more or less energy from the Sun — atmospheric and oceanic heat content are directly affected, while the concentration and spatial distribution of salt in sea water are reshaped indirectly by changes in evaporation/precipitation patterns and sea ice melting/formation.
Changes in Earth’s radiative balance are triggered by so-called external forcings, which correspond to any forcing agent that is external to the climate system’s natural internal dynamics and capable of altering Earth’s energy balance — without necessarily being physically outside of the Earth system per se.
* In constrast, natural, internal climate system dynamics act by just redistributing the available energy among internal Earth system components (atmosphere, hydrosphere, cryosphere, lithosphere, biosphere) — without modifying Earth’s energy balance. This process is instead called “internal climate variability”.
Currently, human activity is the critical external forcing agent causing Earth’s radiative imbalance, which is driving global warming and its consequent internal climate system changes.
Besides its “heating effect”, global warming is increasing precipitation over evaporation (with relevant regional contrasts), and causing subpolar ice to melt — both leading to decreased salinity and preventing the densification of waters that must sink to depth — in order to keep the Atlantic conveyor belt engine running regularly.
All this reasoning left me with the question:
Earth is warming. But its greatest ocean heat engine is about to weaken. Where is the excess heat heading to?
* This is not to suggest that all the excess heat from human-induced global warming that is captured by the ocean (currently more than 90%) is handled solely by the Atlantic conveyor. But, instead, considering the large amount of heat that is indeed involved in the Atlantic conveyor…:
The Atlantic conveyor belt northward heat transport amounts to ~0,5 petawatt (1 PW = 1015 Joules per second) across the Equator, and ~1 PW across the subtropical North Atlantic. Over a year, 1 PW is equivalent to 50 times more power than annual global human energy consumption.
Changes in the Atlantic conveyor strength inevitably reshape how — and where to — heat is redistributed across the planet.
If the Atlantic conveyor strength undergoes a critical weakening (a possibility that is still under scrutiny, but it might already be underway), as Earth continues to warm, the excess heat uptaken by the ocean in the tropics will no longer be efficiently advected northward to be released to the atmosphere in the North Atlantic — risking a profound cooling effect on Northwestern Europe as well as eastern North America, even with the rest of the world continuing to warm. [ A scenario which is exaggeratedly illustrated by the movie ‘The Day After Tomorrow”. ]
As a consequence of reduced northward advection, heat accumulates in the South Atlantic.
* As northward heat advection reduces, heat may also be released to the atmosphere earlier than expected, along the Equatorial Atlantic. And, after accumulating in the South Atlantic, heat is stored in the global deep ocean — as it propagates out of the South Atlantic, along continental boundaries, into the Indian and Pacific.
This is known from studies of past climate changes. “When the Atlantic conveyor belt weakens, the North Atlantic cools, and the South Atlantic warms, and vice versa.” — a concept popularly known as the Atlantic thermal bipolar seesaw.
However, there is more than just the meridional seesaw heat redistribution. How the heat is actually redistributed across the horizontal Atlantic circulation, which is shaped by dynamic upper-ocean wind-driven currents and their mean-state configuration, is largely unexplored.
The majority of studies on the Atlantic conveyor are focused on the North Atlantic basin — such as on the processes governing deepwater formation and on the threat of vanishing heat to keep typically mild European and North American climates.
And, finally, this is where the paper ahead of this blog post comes.
The paper investigated the Atlantic conveyor response to changes in orbital parameters (the dominant external forcing shaping Earth’s climate before human activity), which drove Earth out of the last ice age — a process started ~22,000 years ago.
The last deglaciation corresponds to the most recent episode of natural global warming. As periodic changes in Earth’s orbital parameters culminated in increasing the amount of energy that is received from the Sun, this triggered amplifying responses of internal climate system components — which ultimately regulate heat and salt balances in the North Atlantic, affecting deepwater formation and the Atlantic conveyor strength.
As the Earth system components adjusted internally to accommodate the extra energy, the Atlantic conveyor strength varied from strong to weak states from time to time, until stabilizing during the Late Holocene, ~6,500 years ago.
In this context, the paper’s primary findings are:
- Longer-term, large and persistent changes in deepwater formation — essentially triggered by external, orbital forcing — generate strong and systematic changes in the Atlantic conveyor strength, that propagate far south, up to the southern South Atlantic.
- In turn, shorter-term, smaller and intermittent variations in deepwater formation resulting from the natural exchange of energy among internal Earth system components — which occurs continuously, regardless of changes in Earth’s radiative balance — produce weaker changes in the Atlantic conveyor strength. In other words, the conveyor strength changes mostly over the North Atlantic, but these changes are not strong enough to propagate far south, and are therefore largely dissipated toward the South Atlantic.
The former are called forced, while the latter are called unforced changes.
Remarkably, the paper’s novelty derives from the finding that forced and unforced changes in the Atlantic conveyor strength are compensated in fundamentally different ways by upper-ocean horizontal circulation pathways, on the longitude-latitude plane.
- When the Atlantic conveyor is forced to speed up, for example: as forced changes are stronger, they require more than just the strengthening of northward-flowing ocean currents to compensate for increased deepwater formation (that is, to replace the waters that are sinking faster). They require also the weakening of southward-flowing ocean currents — which are associated with wind-driven gyres in the upper ocean — in order to sufficiently increase net northward transports to balance the accelerated formation and export of deepwaters.
The opposite occurs when the Atlantic conveyor is forced to slow down, conversely inducing the strengthening of southward-flowing ocean currents in the upper-ocean.
- In contrast, “unforced” variations in the Atlantic conveyor strength are usually sufficiently balanced mainly by adjustments in the strength of northward-flowing ocean currents, not requiring substantial opposed changes in the transport of the southward-flowing ocean currents.
However, there are key structural differences between the North, and the South Atlantic wind-driven circulations, which consequently make that forced changes in the Atlantic conveyor strength impact them differently.
(1) As a result of the Earth’s rotation, the wind-driven subtropical gyre circulates clockwise in the North Atlantic, while counterclockwise in the South Atlantic.
(2) This dynamical setting combines with a unique configuration in the tropical South Atlantic (just north of the South Atlantic subtropical gyre), where changes in the conveyor strength tend to be compensated exclusively by adjustments in northward transports, which are concentrated over a narrow band off the South American coast — at the South Atlantic western boundary. This owes to the predominance of eastward/westward ocean currents off the South Atlantic western boundary, to the east, which are part of the zonal equatorial current system.
* Notably, changes in net meridional transports required from changes in the Atlantic conveyor strength are balanced among individual east-to-west basin-scale components affecting total volumetric transports at a same given latitude.
Factors (1) and (2) combined result in Atlantic conveyor northward transports occurring at the South Atlantic western boundary over tropical South Atlantic latitudes (through the NBUC), in opposition to the southward-flowing ocean current which closes the South Atlantic subtropical gyre circulation over subtropical South Atlantic latitudes (the BC).
Consequently, when the Atlantic conveyor is forced to speed up or slow down, changes in its strength are predominantly compensated across the entire meridional extent of the South Atlantic western boundary current system.
For example, when the Atlantic conveyor is forced to slow down, the South Atlantic western boundary experiences decreased northward transports over tropical South Atlantic latitudes, which are in turn compensated by increased southward transports over subtropical South Atlantic latitudes.
In such three-dimensional context, continued global warming combined with a weakening of the ocean’s greatest heat engine — the Atlantic conveyor belt — would possibly result in most of the excess heat heading not only toward the South Atlantic, but specifically toward the southern extension of the South Atlantic western boundary, over subtropical South Atlantic latitudes, off the South American continent (spanning the east coast of southern Brazil, Uruguai and Argentina).
Moreover, in contrast with the North Atlantic, the singular configuration of the upper-ocean wind-driven circulation over the tropical and subtropical South Atlantic, and the particular manner that it is superposed onto the density-driven Atlantic conveyor, creates a scenario where atmospheric wind patterns have a small influence on the general upper South Atlantic circulation when compared to the Atlantic conveyor belt (usually on decadal and longer timescales).
Importantly, as the dynamic wind-driven circulation governs the horizontal redistribution of waters in the upper ocean, it has the potential to shape the shorter-term response of ocean current systems to their underlying longer-term variability, given in response to variations in the Atlantic conveyor strength.
However, because wind variability exerts a weak control on South Atlantic ocean currents at longer timescales, when the Atlantic conveyor strength is forced to change, the western South Atlantic responds loud and clear.
Loud, because changes in the strength of South Atlantic western boundary transports are as large as changes in the Atlantic conveyor strength (relative to their particular magnitudes).
Clear, because such transport changes tend not to get masked or obscured by superposing wind-driven transport changes.
Contrastingly, North Atlantic counterparts are more closely associated with unforced variations in the Atlantic conveyor strength, which are weaker. While a forced slowdown of the Atlantic conveyor would indeed decrease the overall amount of heat reaching the North Atlantic, it would do so in a more balanced way between western and eastern portions of the basin. Besides, North Atlantic ocean currents are more strongly influenced by longer-term wind variability, which shapes and obscures background conveyor strength changes.
And here’s my closing perspective:
The timescales of changes in deepwater formation, which generate changes in the Atlantic conveyor strength, critically depend on the rate of external forcing variation — that is, the velocity at which external forcing agents are changing Earth’s radiative balance and consequently heat and salt balances in the North Atlantic.
Current warming — forced by human activity — is estimated to be occurring at least 25 times faster than the average rate of warming from past deglacial periods (when Earth was naturally recovering from an ice age) — in turn governed by slowly evolving orbital forcing.
Over the last deglaciation, forced changes in the Atlantic conveyor strength propagated to the South Atlantic on decadal and longer timescales. Still, during abrupt episodes of melting ice released to the ocean, Atlantic conveyor changes generated large-scale climate impacts within a single decade.
This leads to the assumption that, in the context of modern global warming conditions, forced changes in the Atlantic conveyor strength may propagate even faster to the South Atlantic, possibly within less than a decade.