Reply to the comment by T.J. McDougall, P.R. Gent and S. Drijfhout

Abstract

McDougall, Gent, and Drijfhout’s [McDougall, T.J., Gent, P., Drijfhout, S.S., in press. Comment on “Dynamical model of mesoscales in z-coordinates” and “The effect of mesoscales on the tracer equation in z-coordinates OGCMs” by V.M. Canuto and M.S. Dubovikov. Ocean Modell. doi:10.1016/j.ocemod.2006.11.004, cited as MGD] opening sentence is that we have suggested that OGCMs have ignored a large diapycnal flux Σ in the mean buoyancy equation. As everybody knows, they have. The fact that in the Eulerian framework Σ exists and is large is uncontroversial. The real question is whether one should include Σ in OGCMs or whether there exists a formalism in which Σ does not show up in the mean buoyancy equation while at the same time the momentum equations are the same as the ones solved by OGCMs. MGD present a defense of the TRM formalism which they claim does exactly that. The belief in TRM, they write, is widely spread among oceanographers and thus they cast our work as going against popular wisdom. Since we are not dealing with a popular context but with a scientific issue that is either correct or incorrect, the MGD claim about TRM must be scrutinized in detail. We have done so and our result, which we present in detail so that it can be checked, tested and verified, is that a key assumption concerning the TRM momentum equation is badly violated. Specifically, when compared with the OGCMs mean momentum equations, the TRM version of the Eulerian mean momentum equations contain an additional term which MGD claim is smaller than other terms and thus negligible. By contrast, we show it is much larger than the non-linear terms. Our conclusion is that MGD’s contention that TRM exonerates OGCMs from having to include Σ is true but incomplete. The question concerning the presence or not of Σ in the mean buoyancy equation remains unresolved. A final comment may be of interest. MGD’s lengthy and fervent defense of TRM is at odds with the fact that in our papers [Canuto, V.M., Dubovikov, M.S., 2006. Dynamical model of mesoscales in z-coordinates. Ocean Modell. 11, 123–166; Canuto, V.M., Dubovikov, M.S., 2007. The effect of mesoscales on the tracer equation in z coordinates OGCMs. Ocean Modell. 16, 17–27] we never criticized it since it had little overlap with our mesoscale model. On the other hand, the Comments by MGD forced us to dwell into TRM and what we found is that it does not do what the authors promised and the community, including us, took to be true. At the same time, none of the criticisms of MGD has any effect on any results we have derived in our work.