Effect of ocean mixing on the transient climate response to a CO2 increase: Analysis of recent model results

Abstract

Results of several recent reports on the role of ocean mixing in the transient response to a CO2 doubling are analyzed and compared. Wigley and Schlesinger (1985) obtained an analytical solution to a one‐dimensional box‐diffusion (BD) ocean model in which the mixed layer e‐folding time τe ∝ ƒ02KΔTeq2, where ƒ0 is the planetary ocean fraction, K is the ocean diffusivity, and ΔTeq is the equilibrium temperature response. The dependence of τe obtained by Wigley and Schlesinger is compared with that obtained using simple land‐sea resolved and box‐advection‐diffusion (BAD) models. For a BAD model the dependence of τe on both K and ΔTeq is significantly less than for a BD model. A comparison of the transient response of BD and BAD models using the same effective K indicates that the behavior of the BAD model cannot be captured using a BD model, except during the first few decades of the transient response. Schlesinger et al. (1985) estimated atmospheric and mixed layer values of τe for their coupled atmosphere‐ocean general circulation model (A/O GCM) by fitting a BD model to the first 16 years of their A/O GCM transient response and then integrating the BD model for 200 years. However, Schlesinger et al. appear to have overestimated values of τe for their long‐term A/O GCM by fitting their short‐term A/O GCM transient response to a model in which the equilibrium globally averaged atmospheric and mixed layer temperature responses are equal, whereas this is apparently not true for their A/O GCM. Hansen et al. (1984) estimated τe by modeling three‐dimensional ocean mixing as a series of isolated BD models, one below each horizontal grid point, and by using tritium‐based diffusion coefficients, which probably are too large when applied to heat and probably lead to an overestimate of the actual τe.