Prediction and predictability of boreal winter MJO using a multi-member subseasonal to seasonal forecast system of NUIST (NUIST CFS 1.1)

• Published in: Climate dynamics 2024-05, Vol.62 (5), p.3003-3026
• Main Authors: Wu, Jiye, Li, Yue, Luo, Jing-Jia, Zhang, Yi, Doi, Takeshi, Yamagata, Toshio
• Format: Article
• Language: English
• Subjects: Advection; Atmosphere; Bias; Boundary layers; Climatology; Convection; Convection heating; Diabatic heating; Earth and Environmental Science; Earth Sciences; Errors; Forecast errors; Geophysics/Geodesy; Heat transfer; Kelvin waves; Latent heat; Latent heat release; Madden-Julian oscillation; Moisture; Moisture gradient; Oceanography; Original Article; Physics; Planetary boundary layer; Planetary waves; Predictions; Propagation; Rossby waves; Seasonal forecasting; Weather; Winds
• ISSN: 0930-7575; 1432-0894
• DOI: 10.1007/s00382-023-07047-4

The Madden–Julian Oscillation (MJO) provides an important source of global subseasonal-to-seasonal (S2S) predictability, while its prediction remains great challenges. Based on an atmosphere–ocean coupled model and the widely-used nudging method, suitable initialization and ensemble schemes are explored toward an improved MJO prediction. Results show that the ensemble strategy with perturbed atmospheric nudging coefficients facilitates adequate ensemble spread and hence improves the prediction skill, particularly for weak cases. Finally, an 18-member ensemble subseasonal prediction system called NUIST CFS1.1 is developed. Skill evaluation indicates that the NUIST CFS1.1 can extend the MJO prediction to 24 days lead, which reaches the world-average level but is far from the estimated potential predictability (~ 45 days). The limited skill at longer lead times corresponds to forecast errors exhibiting slower propagation and weaker intensity, which are jointly caused by the initial errors and model’s imperfections. The latter is associated with unrealistic representations of MJO-related physical processes. The model underestimates the diabatic heating (mostly contributed by the latent heat release) of enhanced convection and fails to reproduce the suppressed convection within the MJO structure, collaboratively weakening the Kelvin/Rossby waves. This causes weaker horizontal winds and ultimately reduces the horizontal moisture advection on the two flanks of MJO convection. Furthermore, the underestimated Kelvin wave induces insufficient planetary boundary layer (PBL) convergence and thereby results in poor simulation of PBL premoistening ahead of MJO convection. Interestingly, the commonly dry bias is absent in our model, and the bias in horizontal moisture gradients may have a minor impact on the MJO propagation. Further efforts are urged to improve the model physics involving cumulus and PBL processes.