Convectively Coupled Equatorial Waves and the MJO in Subseasonal Forecasts from Global Coupled Atmosphere–Ocean Models: Activity and Predictive Skill

Subseasonal-to-seasonal (S2S) prediction has the potential to bridge the gap between medium-range weather forecasting and seasonal climate outlooks. Coupled atmosphere-ocean models have shown skill in predicting precipitation in the tropics and tropical cyclones at S2S time scales. Sources of predictability in the tropics on S2S time scales include the El Nińo Southern Oscillation (ENSO), the Madden Julian Oscillation (MJO), and equatorial Rossby (ER) waves. In this study, we examine the contribution of these different modes of variability to skill in predicting convection and the large-scale atmospheric flow in the tropics at S2S time scales using hindcasts from the Navy Earth System Model (NESM) and other coupled models in the S2S database during boreal summer 1999-2015.

Regional biases in the simulated standard deviation of OLR filtered for different wavenumbers-frequency bands are examined for each of the models. Some models dramatically underestimate convectively coupled Kelvin wave activity while others have large regional biases in Mixed-Rossby Gravity (MRG) / Tropical Depression (TD) activity. The models have similar skill in predicting low-frequency variability but large differences in MJO skill, especially over and downstream of Southeast Asia.