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.

The future changes in the tropical cyclone (TC) intensity and frequency over the western North Pacific (WNP) under global warming remain uncertain. In this study, we investigated such changes using 20-km resolution HiRAM and MRI models, which can realistically simulate the TC activity in the present climate. We found that the mean intensity of TCs in the future (2075−2099) would increase by approximately 15%, along with an eastward shift of TC genesis location in response to the El-Niño like warming. However, the lifetime of future TCs would be shortened because the TCs tend to have more poleward genesis locations and move faster due to a stronger steering flow related to the strengthened WNP subtropical high in a warmer climate. In other words, the enhancement of TC intensity in future is not attributable to the duration of TC lifetime.To understand the processes responsible for the change in TC intensity in a warmer climate, we applied the budget equation of synoptic-scale eddy kinetic energy along the TC tracks in model simulations. The diagnostic results suggested that both the upper level baroclinic energy conversion (CE) and lower-level barotropical energy conversion (CK) contribute to the intensified TCs under global warming. The increased CE results from the enhancement of TC-related perturbations of temperature and vertical velocity over the subtropical WNP, whereas the increased CK mainly comes from synoptic-scale eddies interacting with enhanced zonal-wind convergence associated with seasonal mean and intraseasonal flows over Southeast China and the northwestern sector of WNP.

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The population in the coastal areas of East Asian countries has increased significantly over the past several decades. The economic damage and loss of human life caused by the passage of intense tropical cyclones (TCs) may thus increase dramatically with the growth of the population and economy in these coastal regions of East Asia (Zhang et al., 2009). Understanding future changes in the characteristics (i.e., frequency and intensity) of TC activity, particularly landfalling TCs, in the context of global warming, and the mechanisms responsible for these changes, is of great importance for climate change adaptation and disaster mitigation. TCs and climate change studies have been thoughtfully reviewed in a number of published papers (Knutson et al., 2010; Walsh et al., 2016), as well as the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (Christensen et al., 2013). The projected changes in TCs under a 2°C warming scenario have very recently been reported by Knutson et al. (2020). A new paper of Roberts et al. (2020) discussed the TC projections by the high-resolution coupled models. On a global scale, a decrease in TC frequency but an increase in TC intensity are commonly projected and simulated by models under global warming scenarios. However, future changes in TC genesis count, intensity and tracks over individual basins show less consensus and remain uncertain (Camargo, 2013; Knutson et al., 2010; Murakami et al., 2012; Roberts et al., 2020). As for the western North Pacific (WNP), the basin with the highest frequency of TCs worldwide, a reduction in TC frequency has been projected by some atmospheric general circulation models (AGCMs) (Murakami et al., 2011; Tsou et al., 2016), while an increased TC frequency was revealed by

This study addressed the abrupt increase in the development speed of tropical storms (TSs) to severe TSs (≥ category 3, referred to as STSs) in the western North Pacific (WNP) during the late 1990s. Our investigation indicated that the TSs developed into STSs with a faster speed since the late 1990s. The eddy kinetic energy budget of synoptic‐scale eddy (SSE) indicated that the enhancement of energy conversion from intraseasonal oscillation (ISO) to SSE played a critical role in accelerating the TS‐to‐STS development. The increase of ISO‐SSE interaction was attributed to the mega La Niña–like mean state change. The anticyclone anomaly associated with mega La Niña and warm oceanic condition in the WNP substantially modified the mean TS genesis location (northwestward shift) and enhanced the ISO magnitude in the South China Sea and Philippine Sea, thereby amplifying the ISO‐SSE interaction in the WNP in the late 1990s.