A GCM Study of the 1988 United States Drought

Abstract

June 1988 has been classified as one of the hottest and driest months on record in the United States. This study used the NMC Medium-Range Forecast(MRF) T40 model to simulate circulation features of June 1988 and to investigate the relationship between sea surface temperature anomalies (SSTA) and circulation patterns in the Northern Hemisphere. Three control experiments have been performed using three different initial conditions, separated by one day (21, 22, and 23 May 1988) and using SSTA fixed at the starting date. The three forecasts, and their average, are remarkably skillful in the Northern Hemisphere. The observed anomaly of June 1988, a wave train with a persistent ridge in the north-central United States and a northward shifting of the jet stream in the Pacific?North America area, is very well simulated in each of the integrations. All three experiments were repeated using the same initial conditions, but with climatological SST. The wave train generated is similar to that in the control experiments, but it is not as robust. The simulated jet streams are also similar to those in the control experiments. Two experiments with the 1988 SSTA, but with initial conditions of 22 May 1987 and 22 May 1989 were also run. The circulation patterns generated by these runs are very different from those of 1988, indicating that the persistence of the anomalous ridge in the north-central United States after late May 1998 was not due to the SSTA of the May 1988 alone. A barotropic analysis was done to obtain the normal modes associated with the 300-mb streamfuncton of the June climatology. The analysis indicates the existence of a slowly growing mode with structure similar to the anomalies of 1988. This result, as well as the numerical experiments, suggests that the persistence of the June 1988 wave train may be associated with initial conditions, which were in a rather stable regime. The SSTA may have helped to strengthen the pattern, but the wave train associated with the 1988 drought could not have been generated by SSTA alone.