The impact of vertical model levels on the prediction of MJO teleconnections. Part II: The stratospheric pathway in the UFS global coupled model

Abstract This study continues the evaluation of the prediction of MJO teleconnections in two versions of the NOAA Unified Forecast System (UFS): prototype 5 (UFS5) and prototype 6 (UFS6). The key difference between the two prototypes is in the number of vertical layers (127 in UFS6 vs. 64 in UFS5) and model top (80 km in UFS6 vs. 54 km in UFS5), and the role of this increased resolution for the stratospheric pathway of MJO teleconnections is explored. The higher resolution prototype (UFS6) displays larger biases in its representation of tropospheric stationary waves, though both simulate a reasonable flux of wave activity entering the stratosphere. The stratospheric polar vortex in both prototypes is overly-sensitive to incoming wave activity, while both simulate a reasonable downward propagation of vortex anomalies within the stratosphere and down to the troposphere. The net effect is that both simulate the stratospheric pathway of the MJO better than has been documented for any forecasting system, though the hindcast period available for UFS differs from that used in previous studies. This allows them to predict, e.g., near-surface temperature over Eurasia with some skill on subseasonal timescales (week 3 to 5). However, this success could be the result of “two wrongs making a right”, and as individual biases are fixed, skill could temporarily be reduced. Overall, the two prototypes show similar performance in simulating the stratospheric route, suggesting that in the UFS, the increase in model vertical resolution has a limited impact on the prediction of MJO teleconnections via the stratosphere.