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Ocean Heat Uptake in Eddying and Non-Eddying Ocean Circulation Models in a Warming Climate

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  • 1 Princeton University, Princeton, New Jersey
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Abstract

Ocean heat uptake is explored with non-eddying (2°), eddy-permitting (0.25°), and eddy-resolving (0.125°) ocean circulation models in a domain representing the Atlantic basin connected to a southern circumpolar channel with a flat bottom. The model is forced with a wind stress and a restoring condition for surface buoyancy that is linearly dependent on temperature, both being constant in time in the control climate. When the restore temperature is instantly enhanced regionally, two distinct processes are found relevant for the ensuing heat uptake: heat uptake into the ventilated thermocline forced by Ekman pumping and heat absorption in the deep ocean through meridional overturning circulation (MOC). Temperature increases in the thermocline occur on the decadal time scale whereas, over most of the abyss, it is the millennial time scale that is relevant, and the strength of MOC in the channel matters for the intensity of heat uptake. Under global, uniform warming, the rate of increase of total heat content increases with both diapycnal diffusivity and strengthening Southern Ocean westerlies. In models with different resolutions, ocean responses to uniform warming share similar patterns with important differences. The transfer by mesoscale eddies is insufficiently resolved in the eddy-permitting model, resulting in steep isopycnals in the channel and weak lower MOC, and this in turn leads to weaker heat uptake in the abyssal ocean. Also, the reduction of the Northern Hemisphere meridional heat flux that occurs in a warmer world because of a weakening MOC increases with resolution. Consequently, the cooling tendency near the polar edge of the subtropical gyre is most significant in the eddy-resolving model.

Corresponding author address: Yu Zhang, 201 Forrestal Rd., Princeton, NJ 08540. E-mail: yuz@princeton.edu

Abstract

Ocean heat uptake is explored with non-eddying (2°), eddy-permitting (0.25°), and eddy-resolving (0.125°) ocean circulation models in a domain representing the Atlantic basin connected to a southern circumpolar channel with a flat bottom. The model is forced with a wind stress and a restoring condition for surface buoyancy that is linearly dependent on temperature, both being constant in time in the control climate. When the restore temperature is instantly enhanced regionally, two distinct processes are found relevant for the ensuing heat uptake: heat uptake into the ventilated thermocline forced by Ekman pumping and heat absorption in the deep ocean through meridional overturning circulation (MOC). Temperature increases in the thermocline occur on the decadal time scale whereas, over most of the abyss, it is the millennial time scale that is relevant, and the strength of MOC in the channel matters for the intensity of heat uptake. Under global, uniform warming, the rate of increase of total heat content increases with both diapycnal diffusivity and strengthening Southern Ocean westerlies. In models with different resolutions, ocean responses to uniform warming share similar patterns with important differences. The transfer by mesoscale eddies is insufficiently resolved in the eddy-permitting model, resulting in steep isopycnals in the channel and weak lower MOC, and this in turn leads to weaker heat uptake in the abyssal ocean. Also, the reduction of the Northern Hemisphere meridional heat flux that occurs in a warmer world because of a weakening MOC increases with resolution. Consequently, the cooling tendency near the polar edge of the subtropical gyre is most significant in the eddy-resolving model.

Corresponding author address: Yu Zhang, 201 Forrestal Rd., Princeton, NJ 08540. E-mail: yuz@princeton.edu
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