Climatology and mechanisms of low-level jets over southeastern coast of mainland China during the early-summer rainy season

Abstract Low-level jets (LLJs) significantly impact early-summer rainy season over the southeastern coast of mainland China (SECC), yet northerly LLJs over this region remain understudied compared to their southerly counterparts. Utilizing decade-long hourly ERA5 reanalysis, this study establishes a comprehensive climatology and dynamical framework for distinct LLJ types over the SECC. Climatologically, northerly boundary layer jets (NBLJs) peak in April with a shallow core (~ 975 hPa) concentrated over the central Taiwan Strait. Conversely, southerly boundary layer jets (SBLJs) and southerly synoptic-system-related LLJs (SSLLJs) peak in June. SBLJs feature a hotspot north of the strait (~ 950 hPa), while SSLLJs exhibit a dispersed, elevated dual-core structure (700–900 hPa). The driving mechanisms of NBLJs and SBLJs diverge markedly. Despite their shared dependence on low-level pressure gradient forcing, NBLJs and SBLJs develop under fundamentally different synoptic backgrounds. The former are primarily forced by cold high intrusions, while the latter are supported by the northwestern flank of subtropical high. Under the influence of midlatitude cold highs, NBLJs are exceptionally long-lived and exhibit negligible diurnal variations, which can be primarily attributed to modest land–sea thermal gradients and reduced nocturnal radiative cooling. In contrast, SBLJs develop under warm monsoonal southwesterlies and manifest a pronounced bimodal diurnal cycle. Their dominant nocturnal peak is driven by classic inertial oscillations, while a secondary late-afternoon peak is fueled by enhanced cross-shore baroclinicity and thermal winds induced by intense land–sea thermal contrasts. Furthermore, the coastal hills over southeastern mainland China serve as an effective blocking barrier under low Froude-number condition, which redirects northerly low-level flows and contributes to the formation of NBLJs. These findings enrich the conceptual framework for East Asian coastal LLJs, highlighting that NBLJs are primarily associated with synoptic cold highs and terrain blocking, whereas SBLJs are modulated by boundary-layer dynamics and local thermal gradients.