The influence of tropical forecast errors on higher latitude predictions

Juliana Dias

CIRES and ESRL Physical Sciences Division

Thursday, Apr 25, 2019, 11:00 am
DSRC Room 1D403


Abstract

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, we first review the mechanisms underlying tropical-to-extratropical teleconnections on subseasonal timescales. Next, we discuss the use of “relaxation experiments” to quantify the remote influence of tropical forecast errors, as well as the implementation of this technique on NOAA’s 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, these studies generally show 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. This relationship is then evaluated using a conditional skill analysis applied to subseasonal reforecasts from the National Centers for Environmental Prediction Coupled Forecast System (NCEP CFSv2) and the European Centre for Medium-Range Weather Forecasts Integrated Forecast System (ECMWF IFS). We show that there is enhanced or attenuated skill in Northern Hemisphere Week 2-4 forecasts when tropical short range precipitation forecasts are “good” or “poor”, respectively. This conditional skill is modulated by both El Nino Southern Oscillation (ENSO) and the Madden and Julian Oscillation (MJO), particularly in the IFS. The results presented here indicate that midlatitude Week 2-4 predictive skill would benefit from improvements in Week 1 tropical performance, particularly for the NCEP system.

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Seminar Contact: Tom.Statz@noaa.gov