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Understanding the variability and change of monsoon onset is of utmost importance for agriculture planning and water management. In the last three decades, the Asian summer monsoon onset (ASMO) has remarkably advanced, but the physical mechanisms underlying the change remain elusive. Since the overall ASMO occurs in May, this paper focuses on the change of mean fields in May and considers enhanced mean precipitation and monsoon westerly winds as signs of advanced ASMO. The results reveal that the advanced ASMO mainly represents a robust decadal shift in the mid-to-late 1990s, which is attributed to the mean state change in the Pacific basin characterized by a grand La Niña-like pattern. The La Niña-like mean state change controls the ASMO through the westward propagation of Rossby waves and its interaction with the asymmetric background mean states in the Indian Ocean and western Pacific, which intensifies the Northern Hemispheric perturbations and westerly winds. Intriguingly, the abrupt decadal shifts of monsoon onset in the Arabian Sea and Bay of Bengal occur in 1999, in contrast to the South China Sea with a decadal shift in 1994. Numerical experiments with a coupled climate model demonstrate that the advanced monsoon onset in the Arabian Sea and Bay of Bengal is governed by the enhanced zonal sea surface temperature (SST) gradients in the equatorial Pacific, while that in the South China Sea is primarily determined by the abrupt SST warming near the Philippine Sea. [ABSTRACT FROM AUTHOR]

The western Pacific subtropical high (WPSH) is regarded as the key circulation system that dominates the summer heat waves over eastern China, but whether the WPSH–summer heat wave connection changes with time remains unknown. In this study, decadal variations in the WPSH–heat wave connection were examined for the period 1959–2016 using daily maximum temperature data from 654 observational stations across China and global reanalysis datasets. The results show that the correlation coefficient between the WPSH intensity (WPSHI) and the number of heat-wave days (NHD) was 0.65 (>99% confidence level) during positive phases of the Pacific decadal oscillation (PDO), whereas that during negative phases of the PDO was only 0.12 (<80% confidence level). The remarkable difference in correlations is due to the more westward extension of a stronger WPSH in El Niño decaying summers during the positive phases of PDO. The stronger Indian Ocean warming in El Niño decaying-year summers for PDO positive phases in comparison to PDO negative phases is associated with enhanced convection and heating, which further drive a stronger anticyclone over the northwestern Pacific, leading to a stronger and more westward-extending WPSH, which is favorable for more heat waves over eastern China. [ABSTRACT FROM AUTHOR]

Feature-tracking techniques are employed to investigate why there is a relative minimum in storminess during winter within the Pacific storm track (the midwinter suppression). It is found that the frequency and amplitude of disturbances entering the Pacific storm track from midlatitude Asia are substantially reduced during winter relative to fall and spring and that the magnitude of this reduction is more than sufficient to account for the midwinter supression. Growth rates of individual disturbances are calculated and compared to expectations from linear theory for several regions of interest. Although there are discrepancies between linear expectations and observed growth rates over the Pacific, the growth of disturbances within the Pacific storm track cannot explain why the midwinter suppression exists. Furthermore, it is determined that the development of a wintertime reduction in storminess over midlatitude Asia is consistent with linear expectations, which predict a wintertime minimum in Eady growth rates in this region, mainly because of increased static stability. Several other mechanisms that may contribute to the initiation of the midwinter suppression over midlatitude Asia are discussed, including the interaction between upper-level waves and topography, the behavior of waves upwind of the Tibetan Plateau, and the initiation of lee cyclones. [ABSTRACT FROM AUTHOR]