The variation in presummer precipitation in South China from 1979 to 2015 and its relationship with urbanization were analyzed. The results reveal that the intensity of precipitation and the occurrence of extreme precipitation events during the presummer season in South China have increased significantly, and the upward trend is much more significant in urban areas than in nonurban areas. The mean trends in urban and nonurban areas in South China are, respectively, 1.34 and 0.97 mm/day/year for maximum daily precipitation, 4.41 and 2.79 mm/year for the top 5% of daily precipitation, and 0.26 and 0.16 day/year for extremely heavy precipitation days during the presummer season. In addition to the variability of large‐scale atmospheric circulation, urbanization appears to have a significant effect on the variability of presummer precipitation in South China, especially with regard to the intensity of precipitation and the occurrence of extremely heavy precipitation. From 1979 to 2015, the upward trends of maximum daily precipitation, the top 5% of daily precipitation, and extremely heavy precipitation days during presummer season in urban areas are, respectively, between 38.14% and 39.18%, 55.97% and 59.14%, and 43.75% and 68.75% higher than those in nonurban areas during the investigated period. Urban areas in South China are more exposed to extreme precipitation than nonurban areas during the presummer season.

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Marginal seas, surrounded by continents with dense populations, are vulnerable and have a quick response to climate change effects. The seas typically have alternatively rotating layered circulations to regulate regional heat and biogeochemical transports. The circulations are composed of dynamically active hotspots and governed by the couplings between unique extrinsic inflow and intrinsic dynamic response. Ambiguities about the circulations’ structure, composition, and physics still exist, and these ambiguities have led to poor numerical simulation of the marginal sea in global models. The South China Sea is an outstanding example of a marginal sea that has this typical rotating circulation. Our study demonstrates that the rotating circulation is structured by energetic hotspots with large vorticity arising from unique dynamics in the marginal sea and is identifiable by the constraints of Stokes Theorem. These hotspots contribute most of the vorticity and most of energy needed to form and maintain the rotating circulation pattern. Our findings provide new insights on the distinguishing features of the rotating circulation and the dominant physics with the objectives of advancing our knowledge and improving modeling of marginal seas.

Based on a physics-oriented modeling study, we investigate the underlying forcing processes of the North Equatorial Undercurrent (NEUC). Made up of large-scale (~90%) and mesoscale (~10%) components, the NEUC weakens eastward with a longitude-independent seasonality. The large-scale component reflects the effect of the meridional baroclinic pressure gradient force (PGF_BC). The vertical velocity shear forms the eastward NEUC, when the PGF_BC exceeds the meridional barotropic pressure gradient force (PGF_BT). The mesoscale variability with alternating jets is linked to the wind stress curl in different regions of the tropical North Pacific. Spatially, the NEUC has a northern (NEUC_N) and a southern branch (NEUC_S), which are mainly attributed to the transports from Luzon Undercurrent (LUC) and Mindanao Undercurrent (MUC), respectively. The LUC of ~3 Sv (1 Sv ≡ 106 m3 s−1) feeds the NEUC_N in summer, while the MUC of ~4 Sv fuels the NEUC_S in autumn and the two branches do not coexist. The total NEUC transport peaks in August/September, and there exist three distinct periods in a 1-yr cycle: the non-NEUC period in winter, the LUC-driven period in summer, and the MUC-driven period in autumn. Based on the layer-integrated vorticity equation, we diagnose quantitatively that the variation of the NEUC is dominated by the lateral planetary vorticity influx from the LUC and the MUC. These external influxes interact with the internal dynamics of pressure torques and stress curls in the NEUC layer, to jointly govern the NEUC and its variability. Meanwhile, the nonlinearity due to relative vorticity advection near the coast modulates the strength of the NEUC.

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The KC and MC are strong and narrow western boundary currents, which are crucial to the freshwater/heat/energy transport and dynamic interaction between the Pacific and neighboring marginal seas (i.e., the South China Sea (SCS) and the East China Sea)

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