Understanding Ocean Heat Uptake Using Simple Models and GCMs

The ocean heat uptake is one of the most important factors in determining the transient climate response to increases in greenhouse gases, and in particular in how the transient climate sensitivity differs from the equilibrium climate sensitivity. To better understand this uptake we have performed several numerical experiments with an idealized three-dimensional primitive equation ocean model (MOM), and we tried to interpret the results using box models and upwelling-diffusion models of varying degrees of complexity. The simple models all have various unknown coefficients which we try to estimate using the primitive equation model, and also determine which of our idealized models may be regarded as 'good' ones for our purposes. We are helped in this by the construction of analytic solutions for the box models for certain forcing scenarios.

In many circumstances we find multiple time-scales appearing in the ocean response that can be fairly well explained using box model solutions. The solutions typically depend on the heat exchange rate between mixed layer, thermocline and deep ocean. Heat exchange coefficients may be calculated by comparing the numerical experiments with the analytic solutions, and then related to the physical mechanisms for heat exchange between different ocean regions. We have also examined the oceanic response in various global warming scenarios of the GFDL coupled climate model, and these are also compared to our simple model. The results indicate that, for the purposes of heat uptake, some aspects of ocean may be parameterized using relatively simple box models.