Implied ocean heat transport in CMIP6 models

The importance of heat transport in the ocean to maintain energy balance between different regions is well known, with heat typically being transported from the Equator to high latitudes. Ocean heat transport (OHT) can be separated into two different components; a divergent component which contributes directly to the Earths’ energy budget as it is the energy that converges in an ocean basin to balance the release of heat into the atmosphere, and a rotational component which does not affect the energy budget. Climate models show significant uncertainty in projections of ocean heat uptake, both in terms of the magnitude and geographical pattern. Since the oceans’ response under climate changes depends on the patterns of surface energy fluxes, it is important to assess the simulation of surface fluxes as a potential constraint of transient and long-term responses of the Earths’ climate. Assuming that the ocean absorbs all of the excess energy within the Earth system, it is possible to directly relate the net surface flux (NSF) over the ocean to divergent OHT, potentially providing a metric to quantify how well climate models are able to reproduce observed patterns of NSF and OHT. In this work, we present a detailed comparison of different methods used to calculate divergent OHT from the NSF over the ocean using data from various CMIP6 models. The methods investigated include a least-squares solution to a matrix equation in which energy convergence is related to NSF via the Earths’ energy imbalance, and solving a Poisson equation over the ocean surface (see Forget and Ferreira 2020). Comparison to observational estimates of OHT requires that the observational data set includes only sources of divergent heat transport, which is often not the case. Therefore, we intend to produce a data set of radiative energy fluxes that are consistent with both energy and water constraints (see Rodell et al. 2015, L’Ecuyer et al. 2015, Thomas et al. 2020) which can be subject to the same methods of determining OHT, and see how these estimates compare to the results from climate models.