cc: mcdougall@marine.csiro.au, David.Jackett@marine.csiro.au, m.e.mcintyre@damtp.cam.ac.uk, John.Church@marine.csiro.au, jmgregory@meto.gov.uk, wigley@meeker.ucar.edu, s.raper@uea.ac.uk, spo@dar.csiro.au, m.england@unsw.edu.au, ach@dar.csiro.au date: Mon, 30 Nov 1998 15:31:48 +1100 from: Trevor McDougall subject: Re: Thermal expansion to: Jonathan Gregory , t.osborn@uea.ac.uk Dear All, A few comments on the thermal expansion issues. First, Tim suggests that we may have prejudged the answer when we reduced the Greens function in the lowest layer of our model. Without claiming that that model is the last word, this is best answered by imagining the result we would have obtained if we had not reduced the Greens function in that way. Then the thermal expansion would have been overestimated with a model that had been tuned to today's ocean circulation. So we had not prejudged the issue:- to get the best fit we might have had to increase the Green's function rather than decrease it. Of course, in our present and continuing work we are adressing the issue of the slowing THC in a more comprehensive fashion by allowing the response functions of all layers to be a function of the globally averaged sea surface temperature. In some layers we fully expect the ventillation rate to increase while in others it will decrease. In the meantime, we intend to be more circumspect in the discussion of these issues in the next draft of the present Jackett et al manuscript. Then there is the issue about the existing THC giving up heat to the atmosphere and warming Europe. When the THC slows there will be less heat delivered in this way to the atmosphere which would tend to increase the thermal expansion. But, if there is less heat delivered to the atmosphere in this latitude band, then there will also be correspondingly less heat taken from the atmosphere at low latitudes, so this argument is not conclusive. The argument about the Deacon Cell is very unsatisfying to a physical oceanographer. The Deacon Cell is a phenomenon that occurs when averaging in Cartesian coordinates and appears to be a diabatic circulation. However, the Deacon Cell, and in particular, its diabatic nature, is simply an artifact of the averaging. As Doos & Webb, and McIntosh & McDougall have explained, when doing the zonal averaging in density coordinates, the Deacon Cell disappears. In fact Doos & Webb have defined the Deacon Cell in just this fashion as the cell that appears under z-coordinate averaging but not under density-coordinate averaging. The Deacon Cell corresponds to the Ferrel cells in the atmosphere (see Karoly et al, Q. J. R. Meteorol. Soc., 1997, 123, 519-526). The Deacon Cell is primarily wind-driven and since it is the result of a three-dimensional circulation that is adiabatic, we would not want to model it in a diffusive way. It is this desire to (a) not prejudge the signs of the answers (of SST increase and thermal expansion trend), and (b) have a closer resemblence between the physical balance of the model and of reality that has driven us to embark on the present project. At the end of the day, any simplified model will have an element of tuning involved in it as it strives to mimick a fully coupled three-dimensional model. As physical oceanographers, we are very uncomfortable with a model that is knowingly constructed as diffusive when we all agree that this is not the primary balance in the ocean or in the coupled models. It is our intuition that the slowing circulation will lead the a smaller-than-otherwise thermal expansion and probably a higher-than-otherwise SST trend. However, we are prepared to be surprised by the fully coupled model results. The work we are doing will answer this question, and we will not be prejudging the answer. Given the interest this discussion has generated in the above email group, it is our intention to write up some notes about the work we are embarked upon and to circulate it among the group for comment. With best wishes, Trevor & David. ______________________________________________________________________ From: Dr Trevor J McDougall [address for couriers] CSIRO Marine Research CSIRO Marine Research GPO Box 1538 Castray Esplanade Hobart, TAS 7001 Hobart, TAS 7000 Australia Australia +61-3-6232-5250 (telephone) +61-3-6232-5123 (fax)