Climate change is complicated, as is the modeling of its subparts like the Arctic Ocean and the Beaufort Gyre. In previous posts I have raised the prospect of, in a warming world, the Beaufort Gyre releasing a large volume of relatively freshwater into the North Atlantic Ocean; which would both slow the MOC, and weaken the Arctic's halocline which would cause deeper relatively warm ocean water to rise up in the water column, which would melt more Arctic sea ice. Clearly, this ice-climate feedback mechanism would serve to increase ECS (by both warming the tropical sea surface temperatures due to the slowing MOC and by the albedo flip associated with the accelerated loss of Arctic sea ice); and thus climate science has begun to focus more on this very real and significant climate risk as illustrated by the numerous related paper collected in the first linked JGR: Oceans Special Issue, and by the associated selected linked references (note the JGR: Oceans Special Issue has too many papers to be summarized in this post):
JGR: Oceans Special Issues (2019), "Forum for Arctic Modeling and Observational Synthesis (FAMOS) 2: Beaufort Gyre phenomenon"
https://agupubs.onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)2169-9291.FAMOS2See also:
Proshutinsky, A., and R. Krishfield (2019), In a spin: New insights into the Beaufort Gyre, Eos, 100,https://doi.org/10.1029/2019EO119765. Published on 08 April 2019.
https://eos.org/editors-vox/in-a-spin-new-insights-into-the-beaufort-gyreExtract: "Although the Beaufort Gyre is located in the Arctic region, it has impacts on the climate further afield in two ways:
• First, fresh water accumulates in the Beaufort Gyre which results in a deficit of fresh water flowing into the North Atlantic. This deficit creates the conditions for deep convection of ocean waters and heat release from the ocean to atmosphere in the subpolar regions; it also promotes intensification of the Atlantic Ocean Meridional Circulation (AOMC).
• Second, when there are prevailing counter-clockwise winds over the Arctic, fresh water released from the Beaufort Gyre region inhibits the processes of deep convection, reduces intensity of the AMOC and results in climate cooling. Such periodical releases of fresh water from the Arctic Ocean, which have occurred in the 1970s, 1980s, and 1990s, are known as ‘Great Salinity Anomalies’ (Dickson et al., 1988; Belkin et al., 1998)."
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Edward W. Doddridge et al. (01 April 2019), "A Three‐Way Balance in the Beaufort Gyre: The Ice‐Ocean Governor, Wind Stress, and Eddy Diffusivity", JGR Oceans,
https://doi.org/10.1029/2018JC014897https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JC014897Abstract: "The Beaufort Gyre (BG) is a large anticyclonic circulation in the Arctic Ocean. Its strength is directly related to the halocline depth, and therefore also to the storage of freshwater. It has recently been proposed that the equilibrium state of the BG is set by the Ice-Ocean Governor, a negative feedback between surface currents and ice-ocean stress, rather than a balance between lateral mesoscale eddy fluxes and surface Ekman pumping. However, mesoscale eddies are present in the Arctic Ocean; it is therefore important to extend the Ice-Ocean Governor theory to include lateral fluxes due to mesoscale eddies. Here, a non-linear ordinary differential equation is derived that represents the effects of wind stress, the Ice-Ocean Governor, and eddy fluxes. Equilibrium and time-varying solutions to this three-way balance equation are obtained and shown to closely match the output from a hierarchy of numerical simulations, indicating that the analytical model represents the processes controlling BG equilibration. The equilibration timescale derived from this three-way balance is faster than the eddy equilibration timescale and slower than the Ice-Ocean Governor equilibration timescales for most values of eddy diffusivity. The sensitivity of the BG equilibrium depth to changes in eddy diffusivity and the presence of the Ice-Ocean Governor is also explored. These results show that predicting the response of the BG to changing surface forcing and sea ice conditions requires faithfully capturing the three-way balance between the Ice-Ocean Governor, wind stress and eddy fluxes."
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Gianluca Meneghello, John Marshall, Jean-Michel Campin, Edward Doddridge, Mary-Louise Timmermans. The Ice-Ocean governor: ice-ocean stress feedback limits Beaufort Gyre spin up. Geophysical Research Letters, 2018; DOI: 10.1029/2018GL080171
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018GL080171AbstractThe Beaufort Gyre is a key circulation system of the Arctic Ocean and its main reservoir of freshwater. Freshwater storage and release affects Arctic sea ice cover, as well as North Atlantic and global climate. We describe a mechanism that is fundamental to the dynamics of the gyre, namely, the ice‐ocean stress governor. Wind blows over the ice, and the ice drags the ocean. But as the gyre spins up, currents catch the ice up and turn off the surface stress. This governor sets the basic properties of the gyre, such as its depth, freshwater content, and strength. Analytical and numerical modeling is employed to contrast the equilibration processes in an ice‐covered versus ice‐free gyre. We argue that as the Arctic warms, reduced sea ice extent and more mobile ice will result in a deeper and faster Beaufort Gyre, accumulating more freshwater that will be released by Ekman upwelling or baroclinic instability.
Plain Language SummaryThe Beaufort Gyre, located north of Alaska and Canada, is a key circulation system of the Arctic Ocean. Changes in its depth and circulation influence the evolution of the Arctic sea ice cover, the North Atlantic circulation, and the global climate. The gyre is driven by persistent, ice‐mediated winds, accumulating surface freshwater toward the center, deepening the gyre, and spinning up its currents. We describe a mechanism, dubbed here the ice‐ocean governor, in which the interaction of surface currents with the ice regulates the depth of the Beaufort Gyre: The spinning up of the gyre reduces the relative speed between the ocean and the ice, and hence the freshwater accumulation. This competes with, and we argue is more important than, the release of freshwater by flow instability, which moves water from the center toward the periphery. In the current climate the depth and speed of the Beaufort Gyre are mainly set by the governor, but this may change in a warming world where reduced ice cover will render the ice‐ocean governor less effective. The resulting deeper, swifter gyre will likely exhibit more variability in its freshwater storage and flow speeds.
See also:
Title: "Arctic ice sets speed limit for major ocean current"
https://www.sciencedaily.com/releases/2018/10/181017140925.htmExtract: "There have been a handful of times in the recorded past when the Beaufort Gyre has spilled over, beginning with the Great Salinity Anomaly in the late 1960s, when the gyre sent a surge of cold, fresh water southward. Fresh water has the potential to dampen the ocean's overturning circulation, affecting surface temperatures and perhaps storminess and climate.
Similar events could transpire if the Arctic ice controlling the Beaufort Gyre's spin continues to recede each year.
"If this ice-ocean governor goes away, then we will end up with basically a new Arctic ocean," Marshall says.
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In this new paper, the researchers studied the interplay of ice, wind, and ocean currents in more depth, using a high-resolution, idealized representation of ocean circulation based on the MIT General Circulation Model, built by Marshall's group. They used this model to simulate the seasonal activity of the Beaufort Gyre as the Arctic ice expands and recedes each year.
They found that in the spring, as the Arctic ice melts away, the gyre is exposed to the wind, which acts to whip up the ocean current, causing it to spin faster and draw down more fresh water from the Arctic's river runoff and melting ice. In the winter, as the Arctic ice sheet expands, the ice acts as a lid, shielding the gyre from the fast-moving winds. As a result, the gyre spins against the underside of the ice and eventually slows down.
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Marshall and Meneghello note that, as Arctic temperatures have risen in the last two decades, and summertime ice has shrunk with each year, the speed of the Beaufort Gyre has increased. Its currents have become more variable and unpredictable, and are only slightly slowed by the return of ice in the winter.
"At some point, if this trend continues, the gyre can't swallow all this fresh water that it's drawing down," Marshall says. Eventually, the levee will likely break and the gyre will burst, releasing hundreds of billions of gallons of cold, fresh water into the North Atlantic.
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An increasingly unstable Beaufort Gyre could also disrupt the Arctic's halocline -- the layer of ocean water underlying the gyre's cold freshwater, that insulates it from much deeper, warmer, and saltier water. If the halocline is somehow weakened by a more instable gyre, this could encourage warmer waters to rise up, further melting the Arctic ice."
Edit: It goes without saying that a pulse of cold, relatively fresh, water from the Beaufort Gyre into the North Atlantic, would trigger a bipolar seesaw mechanism that would contribute to the destabilization of key Antarctic marine glaciers.