TRANSIENT-INDUCED CLIMATE DRIFT

The climate drift of various quantities associated with deep, planetar y-scale, equilibrated, transient Rossby waves are estimated for the So uthern Hemisphere extratropical summer as revealed by the DERF II (Dyn amical Extended Range Forecasting) dataset. It is found that the verti cal structures of these waves systematically become too baroclinic dur ing the course of integration. There are two time scales associated wi th this climate drift. There is one very short time scale, estimated t o be of the order of one day, when the waves become more barotropic. I t is followed by a period when the wave baroclinicity monotonically in creases, and after roughly 10 days the model structures appear to have reached their statistically equilibrated state. In the meantime, the kinetic energy of the transient waves decreases substantially to rough ly half the observed value. After this initial drop, however, the tran sient kinetic energy increases again, and it is not clear if an equili brium value has been reached after 30 days, which is the limit of the DERF II dataset. This third time scale is not found in the quantities directly associated with the vertical structures per se, but it is hyp othesized to be a consequence of these errors. A theory is utilized th at in a simplified way takes into account the processes that determine the vertical structure of baroclinic waves as well as their robustnes s as a means of understanding the processes leading to these errors. T he implications from this theory are that the formulation and magnitud e of the dissipative and diffusive processes in the model are the most likely problem, but there are other possibilities.