Abstract ID: 202
Prediction skill of the eastward propagating Madden-Julian Oscillation and associated dynamics in NASA’s GEOS-S2S forecast system
Lead Author: Young-Kwon Lim
University of Maryland, Baltimore County, and NASA Goddard Space Flight Center, Global Modeling and Assimilation Office, United States of America
Keywords: MJO, Subseasonal prediction, GEOS model, Moistening, Moist static energy
Abstract: The prediction skill of the eastward propagating MJO in the current operational version of NASA’s Global Earth Observing System Sub-seasonal to Seasonal (GEOS-S2S) forecast system is investigated for winter and summer focusing on moistening-related processes crucial for MJO activity. It is found that the annual bivariate correlation of the Real-time Multi-variate 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 day 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 MJO propagation across the Indo-western Pacific sector is well captured in JJAS, but is a little slower and weaker than observed in JFM. Investigations of the key factors, including mean state sea surface temperature and moisture, moisture advection and moisture sink, Moist Static Energy (MSE) growth, and tropical circulation/pressure responses to the MJO convective heating reveal that, in boreal winter, those responses and moistening processes are underestimated over and to the east of the Maritime Continent (MC) when the MJO anomaly approaches from the west. However, those key factors captured by GEOS-S2S are comparable to or stronger than observed in boreal summer, although moistening is overestimated due to large surface evaporation.
From the moisture mode perspective, this study offers evidence that a dry bias and an underestimated MSE tendency around the MC and western Pacific are responsible for weaker eastward MJO propagation than observed and lowering the model’s prediction skill. Specifically, the underestimated MSE tendency is found to be due to weaker than observed vertical MSE advection and surface latent heat flux. In summer MJO cases, the model predicts greater moisture anomaly and MSE growth over the MC and western Pacific, than in winter MJO cases. This study suggests that improvement of the premoistening tendency to the east of MJO convection in the boreal winter could contribute to further increases in MJO prediction skill of the GEOS-S2S system.
Nathan Arnold, NASA Goddard Space Flight Center, Global Modeling and Assimilation Office, USA
Andrea Molod, NASA Goddard Space Flight Center, Global Modeling and Assimilation Office, USA
Steven Pawson, NASA Goddard Space Flight Center, Global Modeling and Assimilation Office, USA