The main motivation for ENS extended (monthly forecasting) is to investigate predictability in the time range of 10 to 46 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 mostly an atmospheric initial value problem. Seasonal forecasting, on the other hand, is justified by the long predictability of the atmospheric boundary conditions (ocean/land/ice) which can of the order of several months and by their impacts on the atmospheric circulation. 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 46 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 10-46 day range originates from the Madden-Julian Oscillation (MJO) (see for instance Cassou 2008). The MJO is a 40-50 day tropical oscillation. Several papers (see for instance Woolnough et al. 2003) 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 helps capture some aspects of the MJO variability. Other important sources of extended-range predictability include the stratospheric initial conditions (e.g. Baldwin and Dunkerton 2001) and the soil moisture initial conditions (e.g. Koster et al. 2010).
Baldwin, M. P., and T. J. Dunkerton (2001), Stratospheric harbingers of anomalous weather regimes, Science, 294, 581–584.
Cassou C. 2008. Intraseasonal interaction between the Madden–Julian Oscillation and the North Atlantic Oscillation. Nature 455: 523–527, doi:10.1038/nature07286.
Koster, R.D., S.P.P. Mahanama, T.J. Yamada, G. Balsamo, A.A. Berg, M. Boisserie, P.A. Dirmeyer, F.J. Doblas-Reyes, G. Drewitt, C.T. Gordon, Z. Guo, J.-H. Jeong, D.M. Lawrence, W.-S. Lee, Z. Li, L. Luo, S. Malyshev, W.J. Merryfield, S. Seneviratne, T. Stanelle, B.J.J.M. van den Hurk, F. Vitart and E.F. Wood, 2010: The contribution of land surface initialization to subseasonal forecast skill: First results from a multi-model experiment. Geophys. Res. Lett., 37, L02402,doi:10.1029/2009GL041677.
Woolnough, S. J., F. Vitart and M. A, Balmaseda, 2007: The role of the ocean in the Madden-Julian Oscillation: Implications for MJO prediction Quart. J. Roy. Meteor. Soc., 133, 117-128.