Dynamical seasonal forecasts during the 1997/98 ENSO using persisted SST anomalies

An evaluation of trial seasonal forecasts during the 1997/98 El Niño, using an atmospheric GCM forced by persisted sea surface temperature and sea-ice anomalies, is presented. Generally, forecasts of seasonal anomalies of precipitation, surface air temperature. 200-hPa geopotential height, and mean sea level pressure (MSLP) are shown to have statistically significant skill in the Tropics and subtropics, but predominantly over the oceans. Surface air temperature and 200-hPa height anomalies are also skillfully forecast over land in the 30°S-30°N latitudinal band, and in contrast to precipitation and MSLP, also show significant skill in the extratropics. The global pattern of significant skill seems not to be oversensitive to the use of a Kuo or a mass-flux convection scheme (Tiedtke), although the global root-mean-square errors are consistently larger, in the latter case. Results from multidecadal simulations of the model, when forced by observed sea surface temperature and sea-ice, show that the model reproduces quite well the observed global Southern Oscillation index relationships and that these go some way to explaining the skill in the model forecasts. In addition, the global patterns of skill are consistent with those seen in the model forecasts. An estimate of the role of sea surface temperature and sea-ice in forcing interseasonal climate variations, suggests that the model displays forecasts skill in those areas where this forcing plays a large, if not dominant, role. In areas where internal, or chaotic, variability plays a dominant role, the model shows little statistically significant skill.