Operational 15-day forecasts with the global prediction model of the Japan Meteorological Agency (JMA) are utilized to investigate the relationship between the eastward-propagating 30-60 day modes in the tropics and the predictability of the northern hemisphere extratropical circulation for the period from March 1988 to February 1989.
It is found that the global model successfully predicts the eastward-propagating 30-60 day modes both in amplitude and phase when they are significant in the observation. There is a tendency that the forecast skill of the 10-day mean 500 mb geopotential height north of 20°N for days 6-15 becomes relatively high in such cases during the warm season (May-October). In these skilful forecasts, wavelike height anomaly patterns are found in middle latitudes in 500 mb maps of the analysis and the forecast, and wave propagations from low latitudes are seen along these wave-like patterns.
The above results indicate that when the tropical 30-60 day oscillation is well predicted the skill of extended-range forecast for the northern hemisphere tends to increase due to a successful prediction of the influence of the 30-60 day modes on the extratropical circulation.