A summertime hypoxia sporadically occurred in the lower Pearl River Estuary (PRE) for more than three decades. Although its mechanism has already been extensively studied, the topic on why seasonal hypoxia is persistent in patchy waters is still an open question. Here, we presented the investigation of physical dynamics structures and dissolved oxygen (DO) processes for controlling the spatial distribution and maintenance of coastal hypoxia. Field observations were conducted in the 2015 summer in the PRE and adjacent shelf sea. High river discharge forms intense haloclines in the river plume, while salinity intrusion of shelf benthic waters results in a notable pycnocline at the top of salt wedge. A mid‐depth transitional layer with the weakest mixing over water column functions as a barrier for DO vertical exchange between river plume and shelf salt wedge. A benthic hypoxia in the 2015 summer appears at the overlapping zone between river plume and shelf salt wedge. Based on physical and biological processes, a DO budget for the hypoxic system was established. The DO advection by gravitational circulation from shelf benthic waters is roughly balanced by bacterial respiration in water column. The DO diffusion from river plume to benthic hypoxia is completely inhibited by the barrier layer. The patchy distribution of benthic hypoxia for the 30‐yr period in the PRE can be satisfactorily predicted by the numerical simulations of the overlapping zones between river plume and shelf salt wedge. These findings will have an important implication for predicting and mitigating coastal hypoxia. Physical structures and processes of DO dynamics were investigated to understand the spatial distribution and maintenance of coastal hypoxia. Summertime hypoxia appear near the head of shelf salinity intrusion, where a mid‐depth barrier layer inhibits the vertical exchange between river plume and shelf salt wedge. DO advection by gravitational circulation from DO‐rich shelf benthic waters is roughly balanced by bacterial respiration in water column. The spatial distribution of coastal hypoxia can be well predicted by the overlapping zone between river plume and shelf salt wedge.

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The disappearance mechanisms of subtropical sea surface temperature (SST) fronts occurring from May to August were examined quantitatively using a simple mixed-layer model. Weekly 2.5 data sets were matched up between satellite and in situ observations, including cloud-free SST from Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) and Global Temperature and Salinity Profile Program (GTSPP) data. A 1.5 mixed-layer model used in this study assumed that the temporal variation of the SST gradient was controlled by the resultant effect among the net heat flux, temperature advection (including Ekman and geostrophic), and the temperature entrainment at the bottom of the mixed layer. The net heat flux was found to provide a dominant contribution to the weakening of the SST front (decreasing SST gradient), while the temperature advection and the bottom entrainment were relatively weak. Decomposition of the net heat flux revealed that the meridional gradient of the latent heat flux is a direct factor in the weakening of the SST front, while the shortwave radiation could have indirect effects. The meridional gradient of the latent heat flux is induced by southerly winds, which in turn causes the weakening and disappearance of the SST front. Comparison of weekly and monthly averaged SST gradient modeling results with in situ observations demonstrated that the weekly SST gradient in the model agrees closely with AMSR-E observations, but there was a large difference between the monthly SST gradient in the model and in the observations.