In addition to remote SST forcing, realistic representation of land forcing (i.e., soil moisture) over the US is critical for a prediction of US severe drought events approximately one season in advance. Using “identical twin” experiments with different land initial conditions (ICs) in the 32-year (1979-2010) CFSv2 reforecasts (NASA GLDAS-2 reanalysis vs. NCEP CFSR), sensitivity and skill of US drought predictions to land ICs are evaluated. Although there is no outstanding performer between the two sets of forecasts with different land ICs, each set shows greater skill in some regions, but their locations vary with forecast lead-time and season.
The 1999 case study demonstrates that although a pattern of below normal SST in the Pacific in the fall and winter is realistically reproduced in both reforecasts, GLDAS-2 land initial states display a stronger east-west gradient of soil moisture, particularly drier in the eastern US and more consistent with observations, leading to warmer surface temperature anomalies over the US. Anomalies lasting for one season are accompanied by more persistent barotropic (warm core) anomalous high pressure over CONUS, which results in better prediction skill of this drought case up to 4-months in advance in the reforecasts with GLDAS-2 land ICs. Therefore, it is essential to minimize the uncertainty of land initial states among the current land analyses for improving US drought prediction on seasonal timescales.