Monthly Difference in the Prediction Skill of the Boreal Winter ENSO Response over North America in Coupled and Uncoupled NASA GEOS-5 Model Simulations

This study explores the monthly difference in the prediction skill of the ENSO response in boreal winter over North America reported in earlier studies. This monthly difference in the prediction skill is also revealed in atmosphere-ocean coupled and uncoupled NASA GEOS-5 model. Observational features of the ENSO response each month are analyzed using Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) data.

The results show that the prediction skill of the temperature and precipitation anomalies associated with the ENSO response over the western North America tends to be greater in February whereas lower in January. Coupled and uncoupled models both put negative geopotential height anomaly over the northeast Pacific to the west of observation in January, causing that the precipitation anomalies to be off the coast instead of the coast. This discrepancy between observation and model is negligible in February. West coast precipitation anomalies are shown to be sensitive to the location of this anomalous height anomaly, but the precipitation response all the southern tier of the US appear less sensitive to errors in the zonal wind response.

This study suggests that the character of the northeast Pacific height anomaly (strength/orientation) is largely determined by the anomalous stationary wave patterns. Precipitation anomalies over North America are sensitive to errors in that wave pattern in the model. This error seems to reflect in part an unrealistic response to SST anomalies in the eastern Indian Ocean or tropical Pacific, leading to the above westward shift of the height response over the Pacific.