Impact of Large-Scale Ocean Circulation Changes on Climate

Abstract:

Oceanic meridional heat transport is thought to have an overall warming effect on global climate due to feedbacks between the ocean, the cryosphere, and the atmosphere. Thus, if anthropogenic warming slows the ocean’s meridional transport of heat, as is widely predicted by climate models, then an expected consequence is a reduced pace of transient climate change. However, many studies treat ocean heat uptake as a passive process whereby the ocean reduces the rate of global warming exclusively by storing heat via altered fluxes at the air-sea boundary, ignoring dynamical ocean changes and their associated radiative feedbacks. Here, we quantify the impact on climate of changes to the ocean’s large-scale circulation in response to a doubling of CO₂ in a suite of idealized simulations of a course resolution version of NOAA-GFDL’s CM2.1. In these simulations, we hold ocean velocities, various atmospheric and cryospheric feedback processes, or a combination of each fixed. A comparison of the climate and ocean circulation responses to a doubling of CO₂ in these idealized configurations to the freely evolving model reveals the mechanisms by which the ocean circulation-radiative feedback processes operate. Moreover, this comparison exposes the degree to which the ocean and atmosphere’s responses to CO₂ forcing are independent. Thus, we begin to answer the question of what climate and circulation responses are missed when an atmospheric model is coupled to a slab ocean, or an ocean model is warmed by a prescribed flux perturbation. As in previous studies, we find that holding ocean velocities fixed increases the rate of atmospheric warming relative to an ocean where the circulation responds dynamically to warming, particularly in the Northern Hemisphere. We can attribute the change primarily to a greater shortwave flux out of the top of the atmosphere in the freely evolving simulation relative to the fixed ocean velocities simulations.