Variability of major oceanic currents driven by climate change

March 17, 2016 by Christopher Packham report
Topographic map of the Nordic Seas and subpolar basins with schematic circulation of surface currents (solid curves) and deep currents (dashed curves) that form a portion of the Atlantic meridional overturning circulation. Colors of curves indicate approximate temperatures. Credit: R. Curry, Woods Hole Oceanographic Institution/Science/USGCRP.

(Phys.org)—The Earth's oceanic system isn't just the big, blue puddle of water that globes suggest; its waters are stirred by a vast system called thermohaline circulation, a process driven by varying water densities, heat, and the interactions of freshwater and salt water. Thermohaline circulation moves energy and matter around the globe and drives the planet's climate, but it also responds to changing climate conditions.

For example, one such oceanic conveyor is the Atlantic Meridional Overturning Circulation (AMOC-IV), which drives warm water poleward from the equator, cooling toward the North, and sinking as it becomes more dense at high latitudes. This cold, dense water then empties into ocean basins before upwelling in in the Antarctic.

Its interdecadal variability, measured by the oscillation of its period and amplitude, modulates climate changes worldwide. However, it's unclear how affects AMOC-IV, and an international collaborative of researchers has now investigated the effects of climate change on AMOC-IV amplitude and time scale.

The researchers conducted an analysis of 19 experiments from five models of the Coupled Model Intercomparison Project Phase 5 simulations; each experiment had one preindustrial control simulation and four future warming scenarios. Under all future global warming scenarios, AMOC transport is reduced by 5 percent to 48 percent, depending on the scenario. "In response to future global warming, AMOC-IV shows a robust change, with its major period shortened and its amplitude reduced," the authors write.

They note that the reduced amplitude of AMOC is consistent with other modeling studies. The mechanism of its changes under global warming remains unclear, however, due to the challenging complexities and interactions of global oceanic circulation. As one candidate for such a mechanism, they cite baroclinic Rossby waves. These are huge, slow waves in the troposphere generated by temperature differences of the oceans and continents.

"With global warming, oceanic stratification is projected to be enhanced over the globe owing to a weaker warming with depth, and the stratification enhancement is more robust in a stronger warming scenario," they write. "A stronger stratification should lead to faster baroclinic Rossby waves and, potentially, a shorter period of AMOC-IV."

Even continental regions situated far from the coasts are strongly influenced by ocean currents. Since most of the sun's radiation is absorbed by the ocean, the movement of this energy in the Gulf Stream and other currents serves to modulate weather; through evaporation, heat from the ocean is exchanged with the air. A failure of the conveyance of heat away from warm regions and toward cooler regions stands to throw the climate further out of balance. Thus, research into climate change-related disruptions of oceanic conveyors is important as CO2 and methane contribute to atmospheric heat retention.

The authors conclude, "Our study suggests that AMOC-IV may be significantly weakened in amplitude and shortened in period under future global warming, and that these responses could be caused by strengthened ocean stratification and, in turn, the speedup of baroclinic Rossby waves."

Explore further: Agulhas Current leakage could stabilize Atlantic overturning circulation

More information: Reduced interdecadal variability of Atlantic Meridional Overturning Circulation under global warming. PNAS 2016 ; published ahead of print March 7, 2016, DOI: 10.1073/pnas.1519827113

Abstract
Interdecadal variability of the Atlantic Meridional Overturning Circulation (AMOC-IV) plays an important role in climate variation and has significant societal impacts. Past climate reconstruction indicates that AMOC-IV has likely undergone significant changes. Despite some previous studies, responses of AMOC-IV to global warming remain unclear, in particular regarding its amplitude and time scale. In this study, we analyze the responses of AMOC-IV under various scenarios of future global warming in multiple models and find that AMOC-IV becomes weaker and shorter with enhanced global warming. From the present climate condition to the strongest future warming scenario, on average, the major period of AMOC-IV is shortened from ∼50 y to ∼20 y, and the amplitude is reduced by ∼60%. These reductions in period and amplitude of AMOC-IV are suggested to be associated with increased oceanic stratification under global warming and, in turn, the speedup of oceanic baroclinic Rossby waves.

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6 comments

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cantdrive85
1.8 / 5 (12) Mar 17, 2016
Variability of major oceanic currents driven by climate change

And that variability is driven by the Sun.
philstacy9
1.9 / 5 (9) Mar 17, 2016
Climate change science driven by politics.
Caliban
3.3 / 5 (7) Mar 17, 2016
Variability of major oceanic currents driven by climate change

And that variability is driven by the Sun.


Right.

And that variability, driven by the Sun(or at least Earth's proximity to it) is heated and made more energetic, on an increasing basis --by increasing GHGs emitted into our atmosphere by human and human-induced action.

Simples.
Da Schneib
5 / 5 (1) Mar 19, 2016
Hmmm, @Caliban, global warming is about how much energy comes in vs. how much goes out. More CO2 means less goes out. It doesn't have to do with how close we are to the Sun, it has to do with how much heat escapes. If the amount that escapes is reduced, then the temperature must increase.

And that's simples.
Caliban
3.7 / 5 (3) Mar 20, 2016
Hmmm, @Caliban, global warming is about how much energy comes in vs. how much goes out. More CO2 means less goes out. It doesn't have to do with how close we are to the Sun, it has to do with how much heat escapes. If the amount that escapes is reduced, then the temperature must increase.

And that's simples.


Correct, regarding principle.

But there is further instantaneous variability due to Milankovitch Cycles, as I pointed out, thinking it unnecessary to name what was already implied.

Da Schneib
5 / 5 (2) Mar 21, 2016
Ahh, now I understand. The context was unclear to me.

Also, the proximity of the Sun isn't (theoretically speaking) the most important signal in the Milankovic Cycles; it's the axial tilt that does the most (though there is some mild controversy regarding which particular element is most important). Interestingly, this study may provide some more data to crunch to address some of that controversy.

For the purists out there, there is also a strong influence based on the positions of the continents; when the pole-to-pole flow of ocean currents is substantially obstructed, either by most of the land being around the equator, or by continents covering one or both poles, ice ages are more likely to happen when the Milankovic Cycles reduce insolation; when the ocean currents can flow freely, ice ages are less likely. But this happens on time scales of tens or hundreds of millions of years, rather than tens and hundreds of thousands.

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