Analyzed and Forecast Large-Scale Tropical Divergent Flow

Abstract

Analyzed and forecast 200-mb velocity potential statistics from the ECMWF analysis-forecast cycle for the December?February and June?August seasons of the years 1986?91 are considered. The forecast skill of this representation of the tropical divergent flow is analyzed as is its connection with the skill of forecasting both the tropical and extratropical rotational flow. The mean structure of the velocity potential is concentrated in the tropical region as is its transient variability, which is dominated by the large-scale wavenumber 1 component. Hovmöller diagrams of the velocity potential averaged over the equatorial region show episodic propagating features identified as the Madden-Julian oscillation (MJO) in both analyses and forecasts. Local deterministic forecasts of velocity potential show a comparatively rapid decrease of skill in the tropical region. There is a general loss of transient variance with forecast range. The forecast skill of tropical velocity potential decreases more rapidly with forecast range than does that of tropical or extratropical streamfunction. Tropical velocity potential forecasts are more skillful in December?February than in June?August and levels of skill differ from year to year. There is little evidence of a dependence of forecast skill on the state of the MJO or the E1 Niño/Southern Oscillation (ENSO). There is some evidence of a weak connection between the skill of the forecast of the tropical divergent component and that of rotational component in both the Tropics and extratropics. This connection is apparently independent of forecast range and also of the state of the MJO and ENSO.