The Impact of Tropical Forecast Skill on Extratropical Skill In Two S2S Weather Prediction Systems

It is well established that variations in the location and duration of tropical convective heating can substantially affect extratropical circulation patterns. It follows that a reduction of tropical forecast errors in operational prediction systems should also benefit subsequent forecasts over the extratropics. This relationship is 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 forecasts for Quantitative Precipitation Forecasts (QPF), 500 hPa geopotential height, and a variety of other metrics when tropical short-range precipitation forecasts are “good” or “poor”, respectively, as represented by terciles of the tropical forecast QPF skill distribution. Good tropical QPF skill for three day forecasts maintains above average skill within the tropics for out to 4 weeks, and vice-versa for poor tropical QPF forecasts. However, in the extratropics, skill metrics remain close to average for the first few days regardless of tropical skill, but then show better than average skill in the 1 to 4 week range, and conversely show worse than average skill for the poor tropical forecasts over the same range. The delay in the forecast skill signal is expected based on the theoretical timescale for setting up tropical to extratropical teleconnections due to Rossby wave propagation. The 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, and strongly suggest that improvements in the treatment of organized convection by equatorial wave activity will lead to improved extratropical forecasts on the weekly to seasonal timescale.