The Role of Convectively Coupled Equatorial Waves in Sub-Seasonal Predictions

Convectively coupled equatorial waves (CCEWs) are modes of tropical variability that are often seen as potentially beneficial for sub-seasonal predictions. Quantifying this potential has been proven difficult in operational forecast systems because tropical variability tends to be poorly represented in such models. The European Center for Medium-Range Weather Forecasts (ECMWF) is a potential exception because it has been shown to have much improved representation of the tropics. Here, ECMWF reforecasts are used to investigate the predicted evolution of CCEWs and how they might impact sub-seasonal predictions. It is shown that the ECMWF system is able to generate variability consistent with various modes of CCEWs, but CCEWs tend to be weaker in amplitude and their propagation characteristics often deviate from observations. This study suggests that model advancements aimed at improving the deterministic representation of CCEWs are still needed in order to better understand and utilize tropical sources of sub-seasonal predictive skill. Accurate weather forecasts leverage on proper characterization of the current state of the atmosphere and on predictable atmospheric phenomena that constrain the evolution of that state. Tropical weather forecasting is challenging in both aspects: observations are more sparse than in the extratropics, and the evolution of the atmosphere is constrained by large-scale forcings such as tropical rainfall. This study focuses on the contribution of equatorial waves to low latitude weather forecast skill because these types of disturbances are known to modulate tropical rainfall, which feed back strongly onto the disturbances themselves. The main finding is that the initial state tends to be fairly well represented in forecast systems; however, within a day or two the amplitude of these waves becomes weaker than in observations, which is a decay time much faster than their typical one-to-three week lifetimes. Along with errors in the waves propagation characteristics, this loss of amplitude suggests that equatorial waves do not impact precipitation forecast within the models as much as theory and observations imply they should. Convectively coupled equatorial waves (CCEWs) are a potential source of global sub-seasonal precipitation skillOperational forecast model's representation of CCEWs has greatly advanced over the last two decadesDeviations from observations often attenuate the role of CCEWs as sources of sub-seasonal predictive skill