Abstract
Subseasonal forecast skill over the broadly defined North American (NAM), West African (WAM) and Asian (AM) summer monsoon regions is investigated using three Ensemble Prediction Systems (EPS) at sub-monthly lead times. Extended Logistic Regression (ELR) is used to produce probabilistic forecasts of weekly and week 3-4 averages of precipitation with starts in May-Aug, over the 1999-2010 period. The ELR tercile category probabilities for each model gridpoint are then averaged together with equal weight. The resulting Multi-Model Ensemble (MME) forecasts exhibit good reliability, but have generally low sharpness for forecasts beyond one week; Multi-model ensembling largely removes negative values of the Ranked Probability Skill Score (RPSS) seen in individual forecasts, and broadly improves the skill obtained in any of the three individual models except for the AM. The MME week 3-4 forecasts have generally higher RPSS and comparable reliability over all monsoon regions, compared to week 3 or week 4 forecast separately. Skill is higher during La Nina compared to El Nino and ENSO-neutral conditions over the 1999-2010 period, especially for the NAM. Regionally averaged RPSS is significantly correlated with the Maden-Julian Oscillation (MJO) for the AM and WAM. Our results indicate potential for skillful predictions at subseasonal time-scales over the three summer monsoon regions of the Northern Hemisphere.
Highlights
Monsoon systems of the Northern Hemisphere including the North American (NAM), West African (WAM) and Asian (AM) summer monsoons, are all characterized by marked seasonality and strong variability on seasonal, annual and decadal time-scales (Adams and Comrie, 1997; Lebel et al, 2000; Krishnamurthy and Kinter, 2003)
Similar findings are found for the WAM and AM regions, where individual models (Figures 3A,B, 4A,B and Model Ensemble (MME) forecasts (Figures 3C,D, 4C,D) display good reliability but lower sharpness at week 1 lead when compared to the NAM, especially for the WAM
Skill decreases at higher leads with decreasing sharpness, but greater slopes for the MME indicate more reliability than for individual forecasts
Summary
Monsoon systems of the Northern Hemisphere including the North American (NAM), West African (WAM) and Asian (AM) summer monsoons, are all characterized by marked seasonality and strong variability on seasonal, annual and decadal time-scales (Adams and Comrie, 1997; Lebel et al, 2000; Krishnamurthy and Kinter, 2003). NAM precipitation is influenced by the MJO over southern Mexico (Higgins and Shi, 2001), but Lorenz and Hartmann (2006) found that associated westerly wind anomalies in the eastern tropical Pacific lead, from several days to over a week later, to above-normal rainfall in the monsoon regions of Arizona, New Mexico and northwest Mexico. These changes occur through modulations in the strength of lowlevel easterly waves off the coast of Mexico, which in turn trigger the development of moisture surges from the Gulf of California and could indicate potentials for predictability. RFC size d0-46 Tco639/319 L91 51 2/week On the fly Past 20 years 2/week 11
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