Content area
Full Text
ABSTRACT
A monthly forecasting system based on 32-day coupled ocean-atmosphere integrations has been set up at ECMWF. This system has run routinely since March 2002 every 2 weeks, and 45 cases from March 2002 to December 2003 have been verified. Results of this validation suggest that the model displays some skill in predicting weekly averaged 2-m temperature, precipitation, and mean sea level pressure anomalies relative to the climate of the past 12 years. For days 12-18, probabilistic scores indicate that the monthly forecasting system performs generally better than both climatology and the persistence of the previous weekly probabilities, suggesting that forecasts at that time range could be useful. After about 20 days of forecast, the model displays some skill in predicting events with a large threshold. At that time range, the performance of the system depends strongly on the geographical location, with Europe being a particularly difficult region. However, the model displays some useful skill after 20 days over North America, Asia, and the southern extratropics.
In order to calibrate the monthly forecasting system, a 5-member hindcast over the 12 years preceding the real-time forecast has been produced. Probabilistic scores computed with the hindcast confirm the main results obtained with the real-time forecast. The scores display strong seasonal variability, with the model being particularly skillful in winter.
1. Introduction
Medium-range weather forecasting is essentially an atmospheric initial value problem. Sea surface temperatures (SSTs) are usually simply persisted. Seasonal forecasting, on the other hand, is justified by the long time scale of the predictability of the oceanic circulation (of the order of several months) and by the fact that the tropical SSTs have a significant global impact on the atmospheric circulation. Since the oceanic circulation is a major source of atmospheric predictability at seasonal time scales, the European Centre for Medium-Range Weather Forecasts (ECMWF) seasonal forecasting system is based on coupled ocean-atmosphere integrations.
Monthly forecasting (forecasts between 10 and 30 days) fills the gap between medium-range weather forecasting and seasonal forecasting. It is often considered a difficult time range for weather forecasting, since the time scale is sufficiently long so that much of the memory of the atmospheric initial conditions is lost, and it is probably too short so that the variability of the ocean...