Interval-based statistical validation of operational seasonal forecasts in Spain condition to El Niño-Southern Oscillation events

Abstract

As opposed to the tropics, operational seasonal forecasting systems have shown little or no skill in European midlatitudes. In this paper we explore the potential source of predictability in this region given by El Niño-Southern Oscillation (ENSO) events; in particular we analyze winter rainfall in Spain. First, we apply a simple statistical method to assess the teleconnections between rainfall records in 123 gauges over Spain and ENSO events during the last 40 years. A significant teleconnection for dry winter episodes is found associated with La Niña events, extending the results obtained in previous studies. Then, we adapt the statistical method to perform operational seasonal forecasts validation conditioned to ENSO events; in particular we consider a state-of-the-art operational model, the System2 from ECMWF. The validation method defines a forecast interval to account for the ensemble spread, and applies a simple skill measure based on the proportion of hits (observations falling into the forecast interval) compared with a random forecast. As a result, we uncover the significant skill of operational seasonal predictions for reproducing the dry winter episodes associated with La Niña events (a window of opportunity for operational seasonal forecast in midlatitudes). Finally, the results are improved using statistical downscaling methods and some sensitivity studies are conducted. The analysis presented in this paper can be extended to other regions under the influence of any seasonal predictability-driving factor. Copyright 2008 by the American Geophysical Union.