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Abstract We evaluate five commonly‐applied criteria to validate that a climate model is in so‐called “quasi‐equilibrium,” using a suite of five simulations with CO2concentrations between 1× and 16× Pre‐Industrial values. We find that major changes in ocean circulation can occur after common thermal equilibrium criteria are reached, such as a small Top of Atmosphere radiative flux imbalance, or weak trends in surface air temperature, sea surface temperature, and deep ocean temperature. Ocean circulation change, in turn, impact high‐latitude SAT, sea ice, and the Inter‐tropical Convergence Zone position. For future modeling studies and intercomparison projects aiming for an ocean in quasi‐equilibrium, we suggest that time series of key meridional overturning circulation (MOC) metrics in the Atlantic, Pacific, and Southern Ocean are saved, and that MOC trends are less than 1 Sv/1000 years, and DOT trends less than 0.1°C/century for the final 1000 years of the simulations.
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Ocean Meridional Overturning Circulation During the Early and Middle Miocene
Abstract The Miocene (∼23–5 Ma) is a past warm epoch when global surface temperatures varied between ∼5 and 8°C warmer than today, and CO2concentration was ∼400–800 ppm. The narrowing/closing of the tropical ocean gateways and widening of high‐latitude gateways throughout the Miocene is likely responsible for the evolution of the ocean's overturning circulation to its modern structure, though the mechanisms remain unclear. Here, we investigate early and middle Miocene ocean circulation in an opportunistic climate model intercomparison (MioMIP1), using 14 simulations with different paleogeography, CO2, and vegetation. The strength of the Southern Ocean‐driven Meridional Overturning Circulation (SOMOC) bottom cell is similar in the Miocene and Pre‐Industrial (PI) but dominates the Miocene global MOC due to weaker Northern Hemisphere overturning. The Miocene Atlantic MOC (AMOC) is weaker than PI in all the simulations (by 2–21 Sv), possibly due to its connection with an Arctic that is considerably fresher than today. Deep overturning in the North Pacific (PMOC) is present in three simulations (∼5–10 Sv), of which two have a weaker AMOC, and one has a stronger AMOC (compared to its PMOC). Surface freshwater fluxes control northern overturning such that the basin with the least freshwater gain has stronger overturning. While the orography, which impacts runoff direction (Pacific vs. Atlantic), has an inconsistent impact on northern overturning across simulations, overall, features associated with the early Miocene—such as a lower Tibetan Plateau, the Rocky Mountains, and a deeper Panama Seaway—seem to favor PMOC over AMOC.
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- PAR ID:
- 10587547
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 40
- Issue:
- 4
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The Miocene (∼23–5 Ma) experienced substantial paleogeographic changes, including the shoaling of the Panama Seaway and closure of the Tethys Seaway, which altered exchange pathways between the Pacific and Atlantic Oceans. Changes in continental configuration and topography likely also influenced global wind patterns. Here, we investigate how these changes affected surface wind‐driven gyre circulation and interbasin volume transport using 14 fully coupled climate model simulations of the early and middle Miocene. The North and South Atlantic gyres, along with the South Pacific gyre, are weaker in the Miocene simulations compared to pre‐industrial (PI), while the North Pacific gyres are stronger. These changes largely follow the wind stress curl and basin width changes. Westward flow through the Panama Seaway occurs only in early Miocene simulations when the Tethys Seaway is open and transports are strongly westward. As the Tethys transport declines, flow across the Panama Seaway gradually reverses from westward (into the Pacific) to eastward (into the Atlantic). In simulations with a closed Tethys Seaway, the Panama transport is consistently eastward. The Southern Hemisphere westerlies are weaker than PI in all simulations, contributing to a reduced Antarctic Circumpolar Current (ACC) in 11 of the 14 cases. In the remaining three, a stronger ACC is simulated, likely due to a combination of enhanced meridional density gradients and model‐dependent sensitivities. These findings highlight how changes in Miocene seaways and wind patterns reshaped ocean circulation, influencing interbasin exchange, thermohaline properties, and global climate.more » « less
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2024PA005055
