The main motivation for ENS extended (monthly forecasting) is to investigate predictability in the time range of 10 to 30 days. It is a time scale in between medium-range weather forecasting and seasonal forecasting. Medium-range weather forecasting and seasonal forecasting represent two different problems. Medium-range weather forecasting is essentially an atmospheric initial value problem, since the time scale is too short for variations in the ocean to have significant impact on the atmospheric circulation. The ECMWF medium-range weather forecasting system is based on atmospheric-only integrations: SSTs are simply persisted. Seasonal forecasting, on the other hand, is justified by the long predictability of the oceanic circulation (of the order of several months) and by the fact that the variability in tropical SSTs has a significant global impact on the atmospheric circulation. Since the oceanic circulation is a major of source of predictability in the seasonal scale, the seasonal forecasting system is based on coupled ocean-atmospheric integrations. Seasonal forecasting is also an initial value problem, but with much of the information contained in the initial state of the ocean.
The time range of 10 to 30 days is probably short enough that the atmosphere still has a memory of its initial condition and long enough that the ocean variability could have an impact on the atmospheric circulation. Therefore, the ECMWF monthly forecasts are produced from coupled ocean-atmosphere integrations.
An important source of predictability over Europe in the 10-30 day range is believed to originate from the Madden-Julian Oscillation (MJO) (see for instance Ferranti et al, 1990). The MJO is a 40-50 day tropical oscillation. Several papers (see for instance Flateau et al, 1997) suggest that the ocean-atmosphere coupling has a significant impact on the speed of propagation of an MJO event in the equatorial Indian and western Pacific oceans. Therefore, the use of a coupled ocean-atmosphere system may help capture some aspects of the MJO variability.
Ferranti, L., T. N. Palmer, F. Molteni, E. Klinker, 1990: Tropical-Extratropical Interaction Associated with the 30-60 Day Oscillation and Its Impact on Medium and Extended Range Prediction. Journal of the Atmospheric Sciences:Vol. 47, No. 18, pp. 2177-2199.
Bender, Morris A., Isaac Ginis, 2000: Real-Case Simulations of Hurricane-Ocean Interaction Using A High-Resolution Coupled Model: Effects on Hurricane Intensity. Monthly Weather Review: Vol. 128, No. 4, pp. 917-946.
Flatau, Maria, Piotr J. Flatau, Patricia Phoebus, Pearn P. Niiler, 1997: The Feedback between Equatorial Convection and Local Radiative and Evaporative Processes: The Implications for Intraseasonal Oscillations. Journal of the Atmospheric Sciences: Vol. 54, No. 19, pp. 2373-2386.