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Frontal convection (FC) and diurnal convection (DC) are the two most frequently observed weather systems affecting the pre-summer (May and June, MJ) rainfall formation over Taiwan and Southeast China. Focusing on the time period of 1982-2012 MJ months, this study found that the occurrence frequency of FC has declined, but the occurrence frequency of DC has significantly increased over Taiwan and Southeast China. Diagnoses of the atmospheric thermodynamic conditions over the East Asian-Western North Pacific (EAWNP) region indicate that the area favourable for the FC formation (i.e. the area with a locally enhanced meridional temperature gradient) has shifted northward from 20 o -30 o N to north of 30 o N during the recent three decades; this shift has led to a decline in FC numbers over Taiwan and Southeast China. Analyses also indicate that the recent increase in DC activities over Taiwan and Southeast China after 1982 occurred in association with an observed strengthening of the daytime land-sea thermal contrast coupled with an intensification of the afternoon sea-breeze over the EAWNP region and a locally enhanced moisture flux convergence over Taiwan and Southeast China. Possible causes for the observed changes in the atmospheric thermodynamic conditions over the EAWNP region are also discussed.KEY WORDS Mei-Yu season rainfall; frontal activity; diurnal activity; long-term variations.

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