Northern and Southern Hemisphere Seasonal Variability of Blocking Frequency and Predictability

Abstract

Seven years of analyses and forecasts from the operational archives of the European Centre for Medium-Range Weather Forecasts have been analyzed to assess the performance of the model in forecasting blocking events. This paper extends the previous work by Tibaldi and Molteni to the other seasons of the year and to the Southern Hemisphere. The dataset covers the period from 1 December 1980 to 30 November 1987 and consists of 5OO-hPa geopotential height daily analyses and the 120 corresponding forecasts verifying on the same day, a dataset commonly known as the ?Lorenz files.? Local blocking and sector blocking have been defined as in Tibaldi and Molteni, using a modified version of the Lejenas and ?kland objective blocking index. The results broadly confirm the conclusions previously reached for the winter season alone, extending their validity to the rest of the year and, mutatis mutandis, to the other hemisphere. The main observational difference between blocking in the two hemispheres is in the number of preferred locations: Atlantic and Pacific blocking in the Northern Hemisphere, and only one broad region in the Southern Hemisphere, around 180° longitude. Forecasting the onset of blocking events is in general a task that the model finds difficult, whereas if the integration starts from an already blocked initial condition, the performance of the model is usually better. The poor observational data coverage in the Southern Hemisphere is likely to produce initial conditions affected by larger errors, making the correct forecast of the onset of a blocking event an even more difficult task than it is in the Northern Hemisphere. In the Northern Hemisphere, although the dynamical characteristics of Atlantic and Pacific blocks are inferred from the respective model errors to be different, their detrimental effects on forecast performance are similar in the two cases.