Potential Predictability of Tropical Low-level Circulation in CWB GFS Ensemble Hindcast

Abstract

This study investigates the major sources of potential predictability and associated regulating processes for summer (JJA) low-level tropical circulations using a 1979?2003 ensemble hindcast made by the Central Weather Bureau (CWB) Global Forecast System (GFS) model. This hindcast is conducted with a two-tier system using the external SST forcing so that it lacks the processes of air-sea interactions. This study focuses on three tropical regions: the eastern Pacific Niño (EPN; 160°E-80°W, 30°S-30°N), the western Pacific monsoon (WPM; 100°E-160°E, 30°S-30°N), and the Indian Ocean monsoon (IOM; 40°E-100°E, 30°S-30°N).
The WPM and IOM circulations are found to have different predictability sources and should be examined separately. The predictability source for the former is primarily from SST anomalies in the tropical eastern Pacific, while SST anomalies in the tropical central Indian Ocean (IO) for the latter. Strong SST anomalies tend to induce persistent and large-amplitude circulation anomalies and by so doing enhance potential predictability. Circulation predictability is generally higher over the WPM and EPN regions than over the IOM region.
The eastern Pacific SST anomalies induce a pair of low-level divergence-convergence anomalies over the Pacific to modulate the EPN and WPM rotational circulations simultaneously via Rossby-wave response. The predictability of these two circulations tends to be temporally coherent. For the IOM circulation, its predictability is regulated in two different ways. Strong in situ SST anomalies in the tropical IO may directly affect the IOM circulation via Rossby-wave response. In the absence of strong SST anomalies, the IOM circulation is mainly maintained by a local SST dipole pattern via changing local Walker circulation cells.