Journal of the Oceanographical Society of Japan Vol. 42, pp. 299 to 307, 1986 Short Note On the Recent Development of Simple, Coupled Ocean-Atmosphere Models of ENSO* Toshio Yamagata~ Abstract: Based on simple model results, I describe the recent progress in our understanding of physical mechanisms directly associated with E1 Nifio-Southern Oscillation (ENSO) pheno- mena. In particular, I extract two extremes from recent simple coupled models in order to interprete two complementary phases of interannual ENSO events. I also discuss what is most necessary for more complete model studies useful to improve forecasting skills. 1. Introduction As pointed out by Bjerkness (1966) just two decades ago, there is now no doubt that air-sea interaction plays a key role during the E1 Nifio/ Southern Oscillation phenomena. Recent progress in our understanding of the phenomena has been much accelerated by simple modelling studies, and of course, by the best-observed 1982/83 event. Here briefly reviewing the state of the art of simple air-sea coupled models, I direct my attention to examination of underlying physical processes in order to find valid approximation for more complete model studies. The content of the present concise note will be the following: First, I will review briefly some primary elements of a simple, coupled model which possibly simulates or at least ex- plains major features of ENSO events. Then I will extract two extremes from simple, coupled models and examine their stability properties a little in detail. These two extreme models will serve for our understanding of the evolution of two complementary phases of ENSO, that is, warm phase and cold phase. Based on the argument on the stability properties, a simple interpretation of several numerical experiments reported so far and the actual ENSO phenomena will be given. Finally, summarizing my report, I will briefly discuss what is most necessary for further understanding of ENSO events. * Received 2 June 1986; in revised form 21 July 1986; accepted 11 August 1986. ~f Research Institute for Applied Mechanics, Kyushu University, Kasuga 816, Japan. 2. Primary elements of a simple, coupled model for modelling ENSO events Links necessary for air-sea interaction models of the tropics are already well-established, despite the variety of modelling efforts. So, I will summarize briefly some essential points. First, we must clarify how SST affects atmospheric winds. In reality, this process is highly compli- cated. For example, the relation between anomalous SST and evaporation anomalies is not straightforward. It is affected by some factors such as wind speed, air temperature and humidity. Even the relation between anomalous evaporation zones and anomalous condensation zones is not a simple matter. The advection and convergence of low-level winds play signifi- cant roles in determining the above relation. Furthermore, the effect of the anomalous latent heat release on the wind field itself must be clarified. However, there is no doubt that the anomalous SST is somehow strongly related to the heating in the atmosphere. It is clearly necessary that more elaborate future models should take these detailed links into account. Secondly, the process in which winds affect ocean currents, sea levels and depths of the mixed layer is necessary. Thirdly, the relation between SST and oceanic variables must be given. Once one knows all these relations, one can construct a closed set of mechanistic air-sea models. Now, returning to the first point, it is now well-established at the lowest order of approxi- mation that the low-frequency dynamics of the