Abstract
In this paper, results are presented of an investigation to study the tropical intraseasonal oscillation (ISO) and its impact on the extended-range forecast in the NMC operational model during Phase II (14 December 1986–31 March 1987) of the Dynamical Extended Range Forecast (DERF). Based on principal component analysis of the velocity potential and streamfunction, evidence was found of tropical–extratropical interaction associated with the ISO. The NMC model possesses significant forecast skills for the principal streamfunction and velocity potential modes up to the first ten days. Results of the error growth analysis suggest that the principal modes of velocity potential have large initial errors comparable to the model random errors. By comparison, the initial errors in the streamfunction are much smaller. The error growth for both tropical and extratropical modes are found to be significantly suppressed during periods of strong ISO relative to periods of weak ISO. During strong ISO, a reduction of forecast error, implying a recovery of forecast skill, is also found at about three weeks into the forecast. This is presumably due to the oscillatory nature of the ISO.
The overall results suggest that the forecast skill of ISO and of low-frequency extratropical modes may be considerably enhanced during a special period of DERF when the ISO was strong. The increase in extratropical forecast skill is likely due to (a) the model's ability to better capture ISO signals in the tropics and (b) the increased coupling between the tropics and extratropics during periods of strong ISO. However, because of the limited data, the applicability of the present results to a more general case needs to be ascertained by further studies.
