Analysis of the retrospective ensemble predictions (hindcasts) of the NCEP Climate Forecast System (CFS) indicates that the model successfully simulates many major features of the Asian summer monsoon including the climatology and interannual variability of major precipitation centers and atmospheric circulation systems. The model captures the onset of the monsoon better than the retreat of the monsoon, and it simulates the seasonal march of monsoon rainfall over Southeast Asia more realistically than that over South Asia. The CFS predicts the major dynamical monsoon indices and monsoon precipitation patterns several months in advance. It also depicts the interactive oceanic-atmospheric processes associated with the precipitation anomalies reasonably well at different time leads. Overall, the skill of monsoon prediction by the CFS mainly comes from the impact of El Niño-Southern Oscillation (ENSO).The CFS produces weaker-than-observed large-scale monsoon circulation, due partially to the cold bias over the Asian continent. It tends to overemphasize the relationship between ENSO and the Asian monsoon, as well as the impact of ENSO on the Asian and Indo-Pacific climate. A higher-resolution version of the CFS (T126) captures the climatology and variability of the Asian monsoon more realistically than does the current resolution version (T62). The largest improvement occurs in the simulations of precipitation near the Tibetan Plateau and over the tropical Indian Ocean associated with the zonal dipole mode structure. The analysis suggests that NCEP's next operational model may perform better in simulating and predicting the monsoon climate over Asia and the Indo-Pacific Oceans.

Based on the 1979-95 mean pentad reanalysis data from the US National Centers for Environmental Prediction, the climatological characteristics and physical mechanism of the Asian summer monsoon (ASM) onset are investigated. Special focus is given to whether the ASM onset starts earlier over the Indochina Peninsula than over the South China Sea (SCS) and why the ASM is established the earliest over Southeast Asia.An examination of the composite thermodynamic and dynamic quantities confirms that the ASM onset commences earliest over the Indochina Peninsula, as highlighted by active convection and rainfall resulting from the convergence of southwesterly flow from the Bay of Bengal (BOB) vortex and easterly winds associated with the subtropical anticyclone over the SCS. Two other important characteristics not previously noted are also identified: the earliest reversal of meridional temperature gradient throughout the entire troposphere and the corresponding establishment of an easterly vertical wind shear, which are due to upper level warming caused by eddy (convective) transport of latent heat.These changes in the large-scale circulation suggest that, in addition to rainfall, a reversal in the planetary-scale circulation should be included in determining the timing of the ASM onset. With such a consideration, the climatological ASM onset occurs first over southeastern BOB and southwestern Indochina Peninsula in early May, and then advances northeastward to reach the SCS by the fourth pentad of May (16-20 May). The monsoon then covers the entire Southeast Asia region by the end of May. Subsequently, a similar onset process begins over the eastern Arabian Sea, India and western BOB, and the complete establishment of the ASM over India is accomplished in mid June. In the process of the onset of each ASM component, the reversal of the upper level planetary-scale circulation depends strongly on that of the meridional temperature gradient. Over the Indochina Peninsula, the seasonal transition of upper level temperature results from convection-induced diabatic heating, whereas over western Asia it is attributed to subsidence warming induced by the active ascending motion over the former region.The steady increase in surface sensible heating over the Indian subcontinent and the latent heating over the tropical Indian Ocean in April to early May appear to be the major impetus for the development of the cyclonic vortex over the BOB. A similar enhancement over the Arabian Peninsula and the surrounding regions is also identified to be crucial to the development of the so-called onset vortex over the Arabian Sea, and then ultimately to the ASM onset over India.

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The authors examine different evolution features of the low-level anticyclone over the tropical northwestern Pacific between eastern Pacific (EP) El Niño events and central Pacific (CP) El Niño events. During EP El Niño, the low-level anticyclone shows an eastward movement from the northern Indian Ocean to the east of the Philippines. During CP El Niño, however, the anticyclone is mostly confined to the west of the Philippines. It is weaker, exhibits a shorter lifetime, and lacks eastward movement compared to the Philippine Sea anticyclone (PSAC) during EP El Niño. Investigation into the possible impact of Indian Ocean (IO) sea surface temperature (SST) on the evolution of the low-level anticyclone during EP and CP El Niño indicates that both SST and low-level atmospheric circulation over the IO are related more strongly with EP El Niño than with CP El Niño. The IO SST tends to exert a more prominent influence on PSAC during EP El Niño than during CP El Niño. During the developing summer and autumn of EP El Niño, the anomalous anticyclone over the northern Indian Ocean excited by positive IO dipole may contribute to an early development of the PSAC. During the winter and decaying spring, the anomalous anticyclone to the east of the Philippines instigated by the IO basin-wide warming mode also favors a larger persistence of the PSAC. During CP El Niño, however, IO SST shows a negligible impact on the evolution of the anticyclone.