Assessing the Influence of Tropical Forecast Errors on Higher-Latitude Predictions Using Nudging Experiments

The atmospheric response to variations in tropical latent heating extends well beyond its source region, and therefore it is thought that a reduction of tropical forecast errors should also benefit subsequent forecasts over the extratropics. In this presentation, mechanisms underlying tropical-to-extratropical teleconnections on subseasonal timescales are reviewed. We employ the use of “relaxation experiments” to quantify the remote influence of tropical forecast errors, as well as the implementation of this technique on the National Centers for Environmental Prediction (NCEP) forecast system (FV3-GFS). This approach involves nudging forecasts towards analyses or reanalyses over a tropical region, while allowing the model to run freely elsewhere. By comparing nudged to global free running forecasts, this type of experiment generally shows that midlatitude forecasts are improved in association with reducing tropical forecast errors. For example, Week 2-4 forecast errors over the North Pacific and North America in particular are reduced by tropical nudging. The pattern and amplitude of remote error reductions depend on ENSO state as well as on the Madden Julian oscillation. The sensitivity of changes in remote forecast errors to nudging parameters is also discussed with focus on the location of the nudging region as well as on which state variables are nudged.