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Using regional wind fields to improve general circulation model forecasts of July–September Sahel rainfall
Author(s) -
Ndiaye Ousmane,
Goddard Lisa,
Ward M. Neil
Publication year - 2009
Publication title -
international journal of climatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.58
H-Index - 166
eISSN - 1097-0088
pISSN - 0899-8418
DOI - 10.1002/joc.1767
Subject(s) - climatology , gcm transcription factors , empirical orthogonal functions , environmental science , general circulation model , tropical atlantic , atmospheric circulation , sea surface temperature , forecast skill , walker circulation , climate change , geology , oceanography
This study develops and applies a model output statistics (MOS) approach for correcting poor general circulation model (GCM) seasonal rainfall predictions over the Sahel region of West Africa. It illustrates a methodology for approaching the MOS prediction of regional rainfall, drawing on knowledge of the regional circulation system. The ECHAM4.5 GCM has very little skill in predicting July–September Sahel rainfall. However, the GCM is much more capable of reproducing the regional wind circulation at 925 hPa, especially over the tropical Atlantic. This is capitalized upon using a MOS approach that applies empirical orthogonal functions (EOFs) of the model's regional (tropical Atlantic and West Africa) 925 hPa wind as predictors in a regression with observed Sahel rainfall as the predictand. Over 1968–2002, the MOS system requires only the first wind EOF and improves the correlation skill of July–September Sahel rainfall from 0.07 (raw GCM) to 0.57. The MOS system is applied to GCM experiments using persisted sea‐surface temperature (SST) anomalies from the months of June, May and April respectively, to estimate the potential of the system to make forecasts with lead times between 0 and 2 months ahead of the July–September season. Almost identical skill is achieved from the June SST, but May and April SSTs show a substantial decline in skill. This is mainly associated with a tendency in tropical Pacific SST anomalies from May to June, highlighting development of El Niño (La Niña) in Sahel dry (wet) years, though statistically significant tendencies are also found in the tropical Atlantic and Indian Oceans. The MOS approach is robust in simulation experiments over the 1950–2002 period, when one additional EOF in the MOS system is found to best capture the tropical Atlantic influence. For increasing forecast lead time, targeted prediction of SST from April to June is motivated by these findings. Copyright © 2008 Royal Meteorological Society