Ocean circulation changes in a warming climate: the South Atlantic coupled response to wind and thermohaline forcings

As human activity warms the planet, ocean density changes and shifted atmospheric patterns combine to reshape South Atlantic circulation dynamics: large ensemble simulations suggest that the “Atlantic conveyor belt” is losing its source waters to a distorted South Atlantic subtropical gyre
Published in Earth & Environment

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First of all, why large-scale ocean circulation changes matter?

I would say, because they silently carry the imprints of human influence on Earth’s climate — eventually showing us that indeed what comes around goes around.

Because water is ~1,000 times denser than air, the mass of the ocean is a lot greater than that of the atmosphere, which makes the ocean the real climate regulator — acting as a stabilizer thermal-reservoir, storing excess heat from the atmosphere and slowly moving and releasing it over long periods of time, with the potential to make significant changes in the climate system on a variety of spatial and timescales. The ocean holds our history and a whole lot of power.

The global-scale motion of the ocean includes both wind-driven and thermohaline fluxes linking upper and deeper layers to circulate and redistribute fundamental properties around the globe. It functions as a great “conveyor belt”, formed by two interconnected cells encompassing all ocean basins.

And what is special about the South Atlantic ocean basin?

South Atlantic schematic circulation.

It happens that this great conveyor belt (or, as formally called, the “overturning circulation”) is centered in the Atlantic — termed as the “active basin”, where the sinking of denser waters primarily occurs at its northern end. The Indo-Pacific basin, in turn, is considered the “passive basin”, where most waters rise back to the upper layers.

Therefore a transfer of deep waters from the active into the passive basin is balanced by a transfer of shallower waters in the opposite direction — because, eventually, deep waters that have risen towards the surface must return to their source regions, meeting mass conservation constraints.

And, finally, this return flow of Pacific and Indian ocean waters to the North Atlantic (the dominant site of deepwater formation) is originated within, and happens via, the South Atlantic basin — the critical crossroad for the global overturning circulation.

Pacific and Indian contributions are received at the southern opposing corners of the South Atlantic basin, then blended together and incorporated into the anticlockwise loop of the South Atlantic subtropical gyre — until reaching the South American coast at the South Atlantic western boundary. 

From there on, waters are forced to bifurcate to the north and south: the overturning circulation return flow continues towards the equator and the North Atlantic, and the subtropical gyre circulation is closed by its western boundary current flowing poleward.

How ocean circulation is changing in a warming climate and how is the South Atlantic affected? What is the deal with this paper, anyway?

Under global warming, a more sluggish Atlantic overturning circulation is expected in response to high-latitude ocean density changes, while Southern Hemisphere wind-driven ocean gyres are suggested to be spinning up and moving poleward due to shifted mid-latitude atmospheric patterns. 

The South Atlantic is subject to both these concomitant changes. Its subtropical gyre is coupled to the overturning circulation and is influenced by wind and thermohaline forcings. So that variations in the subtropical gyre and the overturning circulation are intimately intertwined.

Therefore, our paper aimed at investigating how these integrated changes in high-latitude overturning and hemispheric-scale atmospheric forcing are communicated along South Atlantic pathways composing the subtropical gyre and overturning circulations.

In other words, what is the interplay between changes in the overturning return flow and the subtropical gyre circulation? How does the South Atlantic respond to both thermohaline and wind drivers in a warm future?

To answer these, we relied on large ensemble simulations generated from a single global climate model.

What on Earth is a “single-model large ensemble” and what is it useful for?

It is unequivocal that the rise of greenhouse gases from burning fossil fuels to sustain humanity’s modern lifestyle is the dominant factor among external forcings influencing the climate system (such as variations in solar radiation, orbital parameters and volcanic eruptions).

In the real world, human-caused climate change is superposed by chaotic variability that is internally generated by the interacting components of the climate system — in other words, by exchanged energy between Earth’s five spheres: the atmosphere, hydrosphere (such as the oceans, lakes and rivers), cryosphere (containing all frozen parts of the planet), lithosphere (solid earth) and biosphere (living organisms and ecosystems). 

This means that, even though we know rising greenhouse gases are causing globally averaged temperatures to rise, there is substantial lack of knowledge concerning how the climate system components are to interact causing heat to move within the climate system itself, assuming other forms of energy that flow altering weather and climate patterns — shaping the state, variability and change of Earth’s climate.

The way Earth’s climate has precisely evolved so far (the sequence of variability that actually occurred) corresponds to just one amongst many possibilities that could have happened, because even tiny differences in Earth’s initial state (like slightly different atmospheric conditions) would have triggered a totally different sequence of internal climate variability.

Thus, a “single-model large ensemble” is a tool designed to isolate the uncertainty arising from internal variability, consisting of an ensemble of simulations created with a single climate model, with the same time-evolving external forcing conditions, which differ only due to the effect of unpredictable internal variability.

More specifically, these simulations differ only in terms of their initial condition (small round-off level differences in atmospheric temperature). This is enough to ensure that each ensemble member has a unique climate trajectory that represents a possible realization of real-world climate change. As individual ensemble members lose their initial condition memory, after a few years to decades, their trajectories randomly diverge due to internal variability —  causing ensemble spread.

The ensemble mean (averaging together all ensemble members) thus cancels out internal variability, resulting in changes only due to human-caused climate change. Our results represent the ensemble mean of 40 different simulations along 1920-2100, where future projections are derived under the realistic assumption of “business-as-usual” greenhouse gas emissions.

Our results suggest that a major reorganization of South Atlantic circulation dynamics is happening in response to human activity

We show that in between a declining Atlantic overturning and modified Southern Hemisphere surface wind-stress patterns, the South Atlantic circulation is strangely reshaped in adjustment to these concurrent changes.

The ongoing atmospheric trends induce a southward shifting, intensifying subtropical gyre circulation — besides a strengthened interbasin connectivity which favors the entry of Pacific and Indian ocean waters into the South Atlantic.

However, superimposed on the surface wind forcing, the weakened overturning return flow acts to decrease oceanic transports along its shared route with the subtropical gyre: the gyre’s eastern and northern boundaries.

The interplay between wind and thermohaline forcings results in the following:

   While a sustained increase in the inflow of Pacific and Indian contributions to form the Atlantic overturning return flow is expected until the end of the 21st century,

   these increased interbasin inflows are not precisely being destined to join the cross-equatorial overturning return flow up to the subpolar North Atlantic. 

   Instead, these waters seem to be either lost to the Antarctic Circumpolar Current system continuing eastward to exit the South Atlantic basin (without having even been merged with South Atlantic intrinsic circulations),

   or incorporated into the southwestern portion of an enhanced South Atlantic subtropical gyre circulation.

Both the wind-stress trends and the slow-down of the Atlantic overturning work in synergy to favor the recirculation of waters in the subtropical gyre to the south,

rather than their northward extension into the equator and the Northern Hemisphere.

In response to the altered transport of its boundary currents, the subtropical gyre spatial structure is predicted to change non-homogeneously across the horizontal-depth plane. In other words, the South Atlantic subtropical gyre is distorted from its mean state.

This finally leads us to the conclusion that the “Atlantic conveyor belt” is losing its source waters to a distorted South Atlantic subtropical gyre.

In summary, the accumulated effect of human activity so far is ultimately being able to change the way waters circulate and redistribute physical, chemical and biological properties around the global ocean.

As if the Earth’s crust was your bowl and the global ocean was the soup lying in it… If you happened to accidentally spill too much hot spicy sauce into your precious soup, I guess you should be concerned with the way you are going to stir it around...

It seems like a big deal to me, don’t you think?!

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