This study reveals a possible cause of model bias in simulating the western Pacific subtropical high (WPSH) variability via an examination of an Atmospheric Model Intercomparison Project (AMIP) simulation produced by the atmospheric general circulation model (AGCM) developed at Taiwan's Central Weather Bureau (CWB). During boreal summer, the model overestimates the quasi-biennial (2-3 yr) band of WPSH variability but underestimates the low-frequency (3-5 yr) band of variability. The overestimation of the quasibiennial WPSH sensitivity is found to be due to the model's stronger sensitivity to the central Pacific El Niño-Southern Oscillation (CP ENSO) that has a leading periodicity in the quasi-biennial band. The model underestimates the low-frequency WPSH variability because of its weaker sensitivity to the eastern Pacific (EP) ENSO that has a leading periodicity in the 3-5-yr band. These different model sensitivities are shown to be related to the relative strengths of the mean Hadley and Walker circulations simulated in the model. An overly strong Hadley circulation causes the CWB AGCM to be overly sensitive to the CP ENSO, while an overly weak Walker circulation results in a weak sensitivity to the EP ENSO. The relative strengths of the simulated mean Hadley and Walker circulations are critical to a realistic simulation of the summer WPSH variability in AGCMs. This conclusion is further supported using AMIP simulations produced by three other AGCMs, including the CanAM4, GISS-E2-R, and IPSL-CM5A-MR models.

The most striking feature of the Asian-Australian monsoon associated with the El Niñ o teleconnection is the evolution of anomalous anticyclones over the western North Pacific (WNP) and southeast Indian Ocean (SIO). In this study we investigated the relative role of remote and local SST forcing in shaping the monsoon anomalies with an atmospheric general circulation model (AGCM). Four idealized AGCM experiments were designed to isolate the effect of anomalous SST forcing from the tropical eastern Pacific, tropical western Pacific and tropical Indian Ocean. In the first experiment observed SST is specified in the tropical eastern Pacific, while climatological monthly mean SST is specified elsewhere. In the second experiment the observed SST is specified in the tropical western Pacific only. In the third experiment realistic SST is specified in both the tropical Indian Ocean and eastern Pacific. In the fourth experiment the observed SST is specified across the tropical Indian and Pacific Oceans.Our numerical experiments indicate that the anomalous anticyclone in the WNP is initiated by local SST anomaly (SSTA) forcing in northern fall, and further maintained by both the remote (El Niñ o) and local SSTA forcing. The initiation of the anomalous anticyclone over the SIO is primarily attributed to the local SSTA, though the remote forcing from the eastern Pacific also plays a role, particularly in 1997. The numerical experiments reveal a seasonal-dependent character of inter-basin teleconnection between the tropical Pacific and Indian Oceans, this is, the Indian Ocean SSTA exerts a significant impact on the western Pacific wind in northern summer and fall of the El Niñ o developing year, whereas the eastern Pacific SSTA has a greater impact on the Indian Ocean wind during the mature phase of the El Niñ o (boreal winter), even though the central Pacific heating is stronger in boreal summer. A special feature for 1997-98 El Niñ o is that the meridional wind anomaly over the Indian Ocean in DJF is primarily driven by local SSTA forcing, while the zonal wind component is forced by the remote SSTA in the eastern Pacific.

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 Niñ o (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 di¤erent 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 di¤erent ways. Strong in situ SST anomalies in the tropical IO may directly a¤ect 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.