In order to study influence of greenhouse warming on tropical cyclone frequency, we conducted six numerical experiments with a high-resolution atmospheric GCM, which is a T106-L21 version of the Japan Meteorological Agency global spectral model (JMA-GSM9603), with an Arakawa-Schubert-type cumulus parameterization. Effect of SST spatial pattern difference is also investigated. For each experiment, the model was integrated for five years. The criteria used for definition of tropical cyclones in the model are same as those of the previous study (Sugi and Sato, 1997), that is similar to those used by Bengtsson et al. (1996).
Result of a control experiment, with climatological SST, shows reasonably realistic geographical distribution of tropical cyclone formation, as well as seasonal variation. In both of other two runs, added SST anomaly of composite El Nino and composite La Nina respectively, total number of tropical cyclones is not significantly different from that of the control run.
All of the other three experiments are for greenhouse-warmed climates, with doubled CO2 concentration in the model atmosphere, and spatial patterns of their prescribed SST are different from each other: One is increased by +2K uniformly from the climatological SST. For another one, a natural-variation pattern of MRI coupled model is added to the uniformly increased SST, that is similar to the global warming pattern predicted by the coupled model. For the last one, the same natural-variation pattern is subtracted from the uniformly increased SST. Each result of the three greenhouse climates shows significant reduction (by about 22%) in total frequency of tropical cyclones, compared with that of the control run. It is qualitatively consistent with other high-resolution numerical simulations of global warming. For the uniformly +2K increased SST case, such reduction in tropical cyclones is not confined to specific regions, but it is seen almost all over the longitudes and the latitudes where tropical cyclones occur.
Relationship between tropical cyclone frequency and large-scale fields, such as precipitation, vertical motion and other dynamical fields, is investigated. Using a modified version of Gray's Yearly Genesis Parameter (YGP), proposed by Royer et al. (1998), the frequency reduction is not reproduced, mainly because precipitation, used in the modified YGP calculation, is generally increased in the tropics.