Seasonality in Prediction Skill of the Madden‐Julian Oscillation and Associated Dynamics in Version 2 of NASA's GEOS‐S2S Forecast System. Issue 18 (22nd September 2021) - Lim, Young‐Kwon
Abstract: The prediction skill of the Madden‐Julian Oscillation (MJO) in Version 2 of NASA's Global Earth Observing System Subseasonal to Seasonal (GEOS‐S2S) forecast system is investigated for winter and summer focusing on moistening‐related processes crucial for eastward propagating MJO activity. It is found that the annual bivariate correlation of the Real‐time Multivariate MJO time series between prediction and observation is ∼0.70, ∼0.57, and ∼0.50 at 20‐, 25‐, and 30‐day forecast leads. Correlation at long‐leads (>30 days) is noticeably higher for boreal summer initial conditions (June‐September [JJAS]), with correlations remaining above 0.5 at 35–40 days leads. Correlations are lower for boreal winter initial conditions from January through March (JFM), dropping to ∼0.5 at 25‐day lead, still comparable to the skills in the other reliable S2S forecast systems. The predicted eastward MJO propagation across the Indo‐western Pacific sector is well captured in JJAS, but is slower than observed in JFM. Investigations of the moisture field and advection and moisture sink, moist static energy (MSE) budget, and tropical circulation/pressure responses to the MJO convective heating reveal that, in JFM, those responses and moistening processes, especially the vertical MSE advection, are underestimated over and to the east of the Maritime Continent when the MJO anomaly approaches from the west. In contrast, those processes are well represented in JJAS, although moistening is overestimated due to large surface evaporation. This study suggests that improvement of the moistening tendency over that region in the boreal winter could contribute to further increases in MJO prediction skill of the GEOS‐S2S system. Plain Language Summary: The Madden‐Julian Oscillation (MJO) is the dominant tropical atmospheric variability at the 30–60‐day time scale. It has major impacts on regional precipitation in the tropics and development of extratropical weather/climate systems through large‐scale teleconnections. It is found that, for boreal summer initial conditions (June‐September), Version2 of NASA's Global Earth Observing System Subseasonal to Seasonal (GEOS‐S2S) forecast model has the prediction skill of the MJO at equatorial region with anomaly correlation remaining above 0.5 at 35–40‐day forecast leads, which is greater than the skills in the other S2S forecast models. However, the correlation is relatively low for boreal winter initial conditions (January‐March) dropping to 0.5 at ∼25‐day lead. The investigation reveals that the GEOS‐S2S captures the eastward MJO propagation speed and intensity over the Maritime Continent and western Pacific more realistically in summer. The moisture‐related key atmospheric factors show that, in winter, the moistening processes that are crucial for maintenance and eastward propagation of the MJO anomalies are underestimated, while they are well represented in boreal summer. This study suggests that improvement of the vertical moistening process ahead of the MJO convection in the boreal winter can contribute to further increases in MJO prediction skill of the GEOS‐S2S system. Key Points: Madden‐Julian Oscillation (MJO) prediction skill in NASA's Global Earth Observing System (GEOS) Sub‐seasonal to Seasonal forecast system is greater than the other forecast models in summer The forecasts represent well the circulation and moistening process ahead of MJO convection, favoring eastward MJO propagation Improvement of vertical moist static energy advection ahead of the MJO anomaly is needed for further increase in forecast skill in winter