Forecast Skill for Wintertime Atmospheric Variability over the Northern Hemisphere and its Relationship to Mid-Latitude Teleconnections

Abstract:

This study assessed the sub-seasonal predictability of the weekly mean geopotential height anomaly at 500 hPa and its relationship to teleconnections over the Northern Hemisphere in winter using ensemble reforecast data from the S2S prediction project database. The forecast skill over the North Pacific, Canada, and Greenland is higher than over other areas at the lead times of 3-4 weeks. These peaks correspond to the centers of action for the Pacific–North American (PNA) pattern and the North Atlantic Oscillation (NAO). Forecast skills for the PNA phase and amplitude are higher in El Niño years than in neutral years at lead times of 3–4 weeks. Forecast skills for the NAO phase are also higher in El Niño years than in neutral years at lead times of 2-4 weeks, but skills for the NAO amplitude are worse in El Nino years than in neutral years at these lead times. Although forecast skill for PNA and NAO are higher in La Niña years than in neutral years, the effects of La Niña forcing on PNA and NAO forecasts are small compared with the El Niño forcing. Numerical models have a negative PNA bias at these lead times in La Niña years. Results suggest that the improvement in the mid-latitude upper-level jet rather than in the tropical atmospheric response to the ENSO forcing could lead to better S2S predictions.