Tropical Influences on the Predictability of Ultralong Waves

Abstract

Some implications of predictability theory for ultralong waves are examined in an ensemble of real-data forecasts carried out with a primitive-equation numerical model in both global and hemispheric configurations. Although the model is adiabatic and almost inviscid, its skill at forecasting the 5-day evolution of ultralong waves in middle latitudes of the Northern Hemisphere is approximately equivalent to that of a physically comprehensive general circulation model. The ultralong wave forecasts produced by a hemispheric version of the model are markedly less skillful than those made by the global version, especially in the latter part of the 5-day period. When the initial state of the hemispheric version is modified by using a smooth field in the tropics in place of analyzed observed data, the skill of the prediction is degraded further, and the effect is apparent early in the 5-day period. These adverse tropical influences on middle-latitude forecast skill are essentially confined to the ultralong waves (zonal wavenumbers 1–3). They appear to be typical of hemispheric integrations with conventional numerical weather prediction models and conventional analysis and initialization techniques. The resulting forecast errors may be associated with the spurious excitation of large-amplitude external modes. These effects of tropical deficiencies in the prediction model and in the initial data provide a partial explanation for the poor skill of typical actual forecasts of ultralong waves, relative to the skill expected on the basis of predictability theory. The results also suggest that improvements in hemispheric analysis and initialization procedures are urgently required. Until such improvements are implemented, the use of global rather than hemispheric models, even for forecasts of only a few days, might be beneficial in operational practice.