Subpolar Ocean Heat Uptake in CMIP6 Models: Mechanisms, Hemispheric Asymmetry, and Radiative Feedbacks
Ocean heat uptake is fundamentally a coupled atmosphere-ocean problem: how do the dynamics of the atmosphere and ocean evolve together, in response to CO2 forcing, to draw heat into the deeper ocean and slow surface warming?
In response to CO2 forcing, all global climate models take up heat into the ocean, a phenomenon that slows equilibration of the coupled atmosphere-ocean climate system, and likely decreases the effective climate sensitivity of the transient response. While ocean heat uptake is initially more vigorous over tropical oceans in most climate models, its spatial pattern evolves into one that is mostly subpolar: the greatest areas of ocean heat uptake are over the Southern Ocean and the North Atlantic Ocean. Despite these general similarities between climate models, these same models differ wildly in the rate of subpolar ocean heat uptake in response to CO2 warming, and the hemispheric distribution of this ocean heat uptake.
While many models experience greater heat uptake in the SH over the Southern Ocean, for example, a number experience greater heat uptake in the NH over the North Atlantic (see figure above, which shows the hemispheric distribution of the OHU 100 years after abrupt CO2-quadrupling in 13 CMIP5 models). Hemispheric asymmetries in warming appear linked to these differences in ocean heat uptake: models with greater ocean heat uptake in the SH relative to the NH also experience less transient warming in the SH relative to the NH. Furthermore, modes of ocean heat uptake differ substantially in different models: some models directly absorb energy at the sea surface via shortwave radiation, whereas other models converge energy to these regions of ocean heat uptake via dry static energy and latent heat transport, and energy absorption at the ocean surface is through longwave radiation.
The purpose of this project will be to understand what determines ocean heat uptake in climate models, to describe the different mechanisms driving it, and to quantify its consequences. Finally, by comparing the mechanisms we identify in climate models to those that appear to dictate ocean heat uptake in the real world (as represented by observational reanalyses, for example), it may be possible to use observations to constrain the extratropical climate change response.
To understand the governing processes that control the magnitude and hemispheric distribution of subpolar heat uptake in the forced climate system, we will do the following:
- Quantify the rates of subpolar ocean heat uptake in CMIP6 models (in response to abrupt CO2-quadrupling), and its hemispheric distribution.
- Identify the mechanisms of ocean heat uptake in different models by examining the surface and top-of-atmosphere energy budgets and atmospheric energy transport response.
- Assess how the responses of clouds, atmospheric and oceanic static stability, surface winds, and radiative feedbacks may drive differences in the magnitude and the hemispheric distribution of these feedbacks. Could ocean heat uptake itself also impact how clouds, radiation, and extratropical dynamics respond to CO2 forcing?
- Examine the sensitivity of the climate change response (globally and regionally) to differences in the magnitude and hemispheric distribution of ocean heat uptake.
- [pie-in-the-sky] Compare the hemispheric distribution of ocean heat uptake in observational reanalyses to that in the CMIP6 archive to constrain the CO2-forced response in climate models.
Anticipated Data Needs
For both CMIP6 piControl and Ab4XCO2 experiments, monthly model output for the following fields: atmospheric temperature (3d), specific humidity (3d), surface (skin) temperature, winds (3d; u and v), surface fluxes (radiative, sensible, latent; all-sky and clear-sky), top-of-atmosphere fluxes (radiative – longwave and shortwave; clear-sky and all-sky), cloud radiative forcing (LW and SW), cloud fields (fraction, optical depth), ocean potential temperature (3d), ocean salinity (3d), SST, and ocean overturning stream function (in density coordinates).
Anticipated Software Tools
The PANGEO package suite: Xarray, Dask, and Iris.
Also, functionality for reading CMIP6 model output from the cloud (not sure what io package this is).
Anyone with an interest in understanding coupled atmosphere-ocean dynamics in the extratropics, and how this coupled dynamics impacts transient climate sensitivity (both globally and regionally).