Regional Downscaling of S2S Prediction: Past Lessons and Future Prospects
Skillful subseasonal-to-seasonal (S2S) prediction is a major gap in the weather-climate nexus. Predictability of S2S large-scale circulation anomaly at mid-latitudes is largely associated with tropical disturbance, which excites Rossby waves that propagate to the extra-tropics (Hoskins and Karoly 1981; Sardeshmukh and Hoskins 1988). Small errors in representing tropical convection and associated heating can quickly upscale to large errors in extra-tropical weather patterns and limit skillful extended-range forecast (Jung et al. 2010). A necessary requirement for useful S2S forecasts is a combination of skillful forecast of the large-scale circulation anomaly and its regional response in temperature and precipitation, as modulated by regional forcings such as topography and land cover (Leung et al. 2003). In this simple framework (Figure 1a), dynamical downscaling using regional climate models (RCMs) driven by boundary conditions from global forecasts may enhance regional forecast skill by better resolving the regional forcings and processes that generate the regional response from the large-scale circulation anomaly.