Abstract:
Forecast performance by coupled ocean–atmosphere or one-tiered models predicting seasonal rainfall totals
over South Africa is compared with forecasts produced by computationally less demanding two-tiered systems
where prescribed sea surface temperature (SST) anomalies are used to force the atmospheric general circulation
model. Two coupled models and one two-tiered model are considered here, and they are, respectively, the
ECHAM4.5–version 3 of the Modular Ocean Model (MOM3-DC2), the ECHAM4.5-GML–NCEP Coupled
Forecast System (CFSSST), and the ECHAM4.5 atmospheric model that is forced with SST anomalies predicted
by a statistical model. The 850-hPa geopotential height fields of the three models are statistically
downscaled to SouthAfrican Weather Service district rainfall data by retroactively predicting 3-month seasonal
rainfall totals over the 14-yr period from1995/96 to 2008/09.Retroactive forecasts are produced for lead times of
up to 4 months, and probabilistic forecast performance is evaluated for three categories with the outer two
categories, respectively, defined by the 25th and 75th percentile values of the climatological record. The
resulting forecast skill levels are also compared with skill levels obtained by downscaling forecasts produced by
forcing the atmospheric model with simultaneously observed SST in order to produce a reference forecast set.
Downscaled forecasts from the coupled systems generally outperform the downscaled forecasts from the twotiered
system, but neither of the two systems outscores the reference forecasts, suggesting that further improvement
in operational seasonal rainfall forecast skill for South Africa is still achievable.
