Impact of boreal summer intra-seasonal oscillations on warm season diurnal convection activity in Taiwan

Diurnal convection (DC), which is characterized by rainy days with maximum rainfall during the afternoon without strong synoptic disturbances, is the most frequently observed weather system that affects local rainfall in Taiwan. In this study, we investigate the impact of boreal summer intra-seasonal oscillations (BSISOs) on the modulation of warm season (May–September) DC activities in Taiwan. Our analyses focus on (1) identifying the relationships between changes in DC characteristics (e.g. DC days and the mean rainfall intensities on DC days) and the phase evolutions of two types of BSISOs (including the 30–60-day ISO named BSISO1 and the 10–30-day ISO named BSISO2) and (2) the related maintenance mechanisms. Analyses of DC days demonstrate that the contribution of DC days to the total quantity of rainy days in Taiwan generally reaches a maximum value when Taiwan is being modulated by intensified southwesterly winds, intensified local sea breeze circulations, and intensified local thermal instabilities (i.e. phase 3 of BSISO1 and phase 7 of BSISO2). Analyses of the mean rainfall intensities on DC days show that maximum values are generally reached when the moisture convergence line propagates northwards over Taiwan (i.e. phase 1 of BSISO1 and phase 3 of BSISO2). Further examinations indicate that the abovementioned findings are not dependent upon the strengths of the BSISOs.