Medium-Range Forecast Errors Associated with Active Episodes of theMadden–Julian Oscillation

Abstract

Systematic forecast errors associated with active episodes of the tropical Madden?Julian oscillation (MJO) are examined using five winters of dynamical extended range forecasts from the National Centers for Environmental Prediction reanalysis model. Active episodes of the MJO are identified as those periods when the amplitude of either of the first two empirical orthogonal functions of intraseasonally filtered outgoing longwave radiation, which efficiently capture the MJO, is large. Forecasts initialized during active episodes of the MJO are found not to capture the eastward propagation of the tropical precipitation and circulation anomalies associated with the MJO. Rather, the MJO-induced anomalies of precipitation and winds are systematically forecast to weaken and even retrograde. By about day 7 of the forecast the convectively coupled, tropical circulation anomalies produced by the MJO are largely gone. Systematic errors in the extratropical 200-mb streamfunction also fully develop by day 10. The initial development of these errors is argued to result from the collapse of the tropical divergence forcing produced by the MJO and, thus, the lack of correct Rossby wave source. Forecast skill in the Tropics and Northern Hemisphere extratropics is found to be systematically reduced during active periods of the MJO as compared to quiescent times. This reduced skill is suggested to result because the MJO is the dominant mode of convective variability and not because the model is better able to forecast intraseasonal convection unrelated to the MJO.