forty (Fig. 3c) most of the deep convection in the Arctic has disappeared. This is due to the formation of sea ice in the Arctic. In year 160 (Fig. 3d) deep mixing occurs at the edge of the sea ice in the Arctic and off the coast of Antarctica. This pattern continues to the end of the simulation. If the simulation were extended, it is anticipated that the deep ocean will cool further resulting in a decoupling of deep ocean and surface circulations. This results in slower deep ocean circulation, which may affect proxy indicators, such as sea floor sediments. C. Sea surface temperature and sea ice thickness Having looked at the vertical circulation within the ocean, it is important to look at its effect on the surface. Fig. 4a plots the sea surface temperature ten years into the simulation. Although the Arctic Ocean has cooled, the surface is still around 14 °C. At the equator, surface temperatures have risen to 26 °C. By year forty (Fig. 4b) Arctic waters are below freezing allowing the formation of sea ice. Equatorial waters can now circulate excess heat to the poles and begin to cool. Fig. 4c corresponds to year 160 of the simulation. Surface waters at the freezing point form an arc that extends from Nova Scotia along the coast of Greenland to Great Britain. By the end of the simulation (Fig. 4d), that arc extends further south to form a line between Maine and France. In the Pacific unfrozen ocean extends to the coast of Alaska. Given the shallow connecting basin between the Pacific and Arctic Oceans, circulation begins to restrict by year forty (Fig. 5a). At year 160 (Fig. 5b) sea ice in the Arctic has thickened considerable. This trend continues until the Pacific is cut off from the Arctic. In the Atlantic some circulation with the Arctic continues through the ocean near Iceland. However, most of the Atlantic’s vertical circulation occurs at the boundary between sea ice and open ocean. Looking closely at Fig. 5b, we see a gray region off the coast of Greenland towards Canada. This represents sea ice thickness greater than sixty meters. During the simulation the model stopped several times due to a low ocean mass error. Upon consultation with the model user group, it was revealed that some dynamics of the ocean are not calculated for layers near the surface. As sea ice thickens, it impinges on deeper layers resulting in an error. Initial errors were recoverable; however, by year 165 the model failed. Increasing the depth to layer three, corresponding to 57 meters, extended the life of the model to year 2270. An additional change of depth to layer four, 98 meters, allowed the model to reach year 393. Given the proximity to 400 years, the simulation was ended. By the end of the simulation, the sea ice abnormality off the coast of Greenland had reached the ocean floor, although most of the Arctic Ocean ice was only sixty meters thick. Increasing the depth to layers three and four influences ocean circulation and needs to be addressed in future research. One solution is to remove the sea ice anomaly by Greenland and see if the model will continue running using a minimum depth corresponding to layer two, 30 meters. D. Surface air temperature Surface air temperatures follow an expected pattern. During year one Figure 4. Sea surface Temperature for various years in the simulation. a) Ten years, b) forty years, c) 160 years, and d) 390 years. a) b) c) d) Figure 5. Sea ice thickness. a) Forty years into the model and b) 160 years. Since the model’s sea ice field includes lake ice, there is ice present over the northern continents. GOLLMER Rapid ice age 2023 ICC 271
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