Introduction

ECMWF started an experimental programme of monthly forecasting (time range from 10 to 30 days) in March 2002. The monthly forecasting system is operational since October 2004.

Why Monthly Forecasting?

Two ensemble forecasting systems are currently operational at ECMWF: EPS for medium-range weather forecasting and seasonal forecasting. EPS produces weather forecasts out to 15 days, whereas seasonal forecasting produces forecasts out to 7 months (13 months 4 times a year). The two systems have different physical bases. Medium-range weather forecasting is essentially an atmospheric initial value problem. Since the time scale is too short for variations in the ocean significantly to affect 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 source of predictability in the seasonal scale, the ECMWF seasonal forecasting system is based on coupled ocean-atmosphere integrations. Seasonal forecasting is also an initial value problem, but with much of the information contained in the initial state of the ocean.

The main goal of monthly forecasting is to fill the gap between these systems and produce forecasts for the time range 10 to 30 days. The time range 10 to 30 days is probably still short enough that the atmosphere retains some memory of its initial state and it may be long enough that the ocean variability has an impact on the atmospheric circulation. Therefore, the monthly forecasting system has been built as a continuation of the medium-range VArEPS, but in ocean-atmospheric coupled mode after day 10. For technical reasons it is not possible to coupled EPS from day 0 at present but shoudl be possible when the next ocean system will be implemented.

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 upon the speed of propagation of an MJO event in the Indian Ocean and western North Pacific. The use of a coupled system after day 10 may therefore help to capture some aspects of the MJO variability.

The Ocean Analysis

In order to initiate monthly forecasts, initial conditions for both the ocean and atmosphere are required. Atmospheric and land surface initial conditions are obtained from the ECMWF operational atmospheric analysis/reanalysis system. Oceanic initial conditions originate from the oceanic data assimilation system used to produce the initial conditions of the seasonal forecasting system 3. However, this oceanic data assimilation system lags about 12 days behind real-time. The lag is partially due to the fact that the SST, obtained by interpolating in time the weekly OIv2 SSTs produced by NCEP, can be up to 11 days behind real-time. A first option would be to wait for the oceanic initial condition to be created by the data assimilation system to start the forecast, as in seasonal forecasting. This would mean losing 12 days of forecast and is not therefore suitable for monthly forecasting. A second option would be to persist the SST anomalies of the latest ocean analysis. However, we have some informations about the wind stress and heat fluxes during the last 12 days of the ECMWF atmospheric analysis and also some subsurface information; this information can be used to help determine the present ocean initial state. Therefore, the option that has been chosen for monthly forecasting consists in producing an additional ocean assimilation during those 12 days with all the observatiosn which are available. During this 'ocean data assimilation', the sea surface temperature is relaxed towards persisted SST, with a damping rate of 100 W/m2/K.

The Coupled Model

The coupled model consists of the ECMWF atmospheric model (the same cycle as the deterministic forecast), coupled to an ocean general circulation model (NEMO). Currently, the atmospheric model is run at TL639 resolution from day 0 to day 10 and at T319 from day 10 to 32 with 62 levels in the vertical. The ocean model has lower resolution in the extratropics but higher resolution in the equatorial region, in order to resolve ocean baroclinic waves and processes which are tightly trapped at the equator. The atmosphere and ocean communicate with each other through a coupling interface, called OASIS, developed at CERFACS, France. The atmospheric fluxes of momentum, heat and fresh water are passed to the ocean every 3 hours and, in exchange, the ocean sea surface temperature (SST) is passed to the atmosphere.

Twice a week (Monday and Thursday at 00Z), the coupled model is integrated forward to make a 32 day forecast with 51 different initial conditions, in order to create a 51-member ensemble. Full coupling is applied between the ocean and atmosphere after day 10 (the atmospheric model is forced by persisted SST anomalies from day 0 to day 10). Because of model errors, a drift occurs in the coupled system. In order to evaluate this model drift, the coupled model is integrated with 5 different initial conditions (5-member ensemble) at the same day and month as the thursday real time forecast, but over the past 18 years, creating a 90-member climate ensemble.

References:

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.

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