Propagation of the MJO and Associated Moist Dynamics, and the Advantages of Enhanced Resolution in NASA's GEOS-S2S Forecast System

This presentation explores how reliably the NASA’s new subseasonal to seasonal (S2S) forecast system represents the key features of the Madden-Julian Oscillation (MJO). This system is a fully coupled global model that includes the components of atmosphere, ocean, land, and sea ice. The model runs at an approximate spatial resolution of 0.5 degree for the atmosphere and 0.25 degree for the ocean. It employs a two-moment cloud microphysics to enable aerosol-cloud interaction and implements a novel atmosphere-ocean interface layer to enhance the representation of the diurnal warm and cool-skin layer.

Experiment results from multi-decadal nature run reveal that the model captures very successfully the eastward propagation of the MJO in the tropics and the northward propagation of the Boreal Summer Intra-Seasonal Oscillation (BSISO) over the Indian Ocean sector. The eastward MJO propagation speed on average is 5m/sec, aligning with the observed average MJO propagation speed. The model faithfully replicates moist static energy (MSE) growth and MSE advection - a premoistening process crucial for MJO propagation - to the east of the MJO convection anomaly when it approaches the Maritime Continent (MC). However, there is a slight underestimation in the observed amplitude. Comparative analysis of the MJO event cases between the new system and the current operational forecast model reveals that the new system has a potential to perform better in forecasting the MJO than the current operational version, which shows a bi-variate correlation of 0.5 between observation and forecast at a 30-day forecast lead. This study also investigates the MJOs using a higher resolution coupled model (~7 km, 2-4 km) with a specific focus on the MJO propagation across the MC to assess the advantages of increased resolution in simulating the eastward propagating MJO across this MC barrier. The higher resolution model demonstrates improved simulation of eastward MJO propagation into the western Pacific, with more instances of the MJO successfully traversing the MC and exhibiting larger amplitude of anomalies than the coarser resolution run.