the spatial pattern of the wintertime Pearl River plume front (PRPF), and its variability on diurnal and spring-neap time scales are characterized from the geostationary meteorological Himawari-8 satellite, taking advantage of the satellite’s unique 10-minutely sea surface temperature sequential images. Our findings suggest that the PRPF in winter consists of three subfronts: the northern one north of 22°N 20′, the southern one south of 21°N 40′, and the middle one between 22°N 20′ and 21°N 40′. The time-varying trend of the frontal intensity generally exhibits a strong-weak-strong pattern, with the weakest plume front occurring at about 06:00 UTC, which is closely associated with net surface heat flux over the region. The comparison in frontal variability between the spring and neap tides shows that the plume front during the spring tide generally tends to be more diffuse for the frontal probability, move further offshore for the frontal position, and be weaker for the frontal intensity than those found during the neap tide. These great differences largely depend on the tidally induced stronger turbulent mixing during the spring tide while the wind stress only plays a secondary role in the process. To best of our knowledge, the distinct diurnal variations in PRPF with wide coverage are observed for the first time. This study demonstrates that the Himawari-8 geostationary satellite has great potential in characterizing high-frequency surface thermal fronts in considerable detail.
Water masses and their variability play vital roles in regulating ocean circulation, material exchanges and biogeochemical processes. However, there is still a lack of quantitative analysis of water mass distributions in coastal waters of the South China Sea. Here, two oceanographic cruise observations in April and May 2016 are used to quantify water mass distributions, pathways and mixture, and their intraseasonal variability off western Guangdong during the spring monsoon transition. Temperature and salinity observations qualitatively reveal that there are three types of water masses: the Pearl River diluted water (PRDW, salinity (S) = 22 psu, potential temperature (θ) = 25 °C), the South China Sea surface water (SCSSW, S = 34 psu, θ = 28 °C) and the South China Sea subsurface water mass (SCSSUW, S = 34.5 psu, θ = 17 °C). Their relative contributions and intraseasonal variability are quantified using the Optimum Multiparameter (OMP) method. The PRDW is largely confined to the upper 10 m layer in shallow nearshore waters (depths < 30 m), with a maximum contribution >90% near the Pearl River Estuary. The SCSSW mainly dominates the rest of the surface layer above 20 m, with a contribution >50% in offshore regions. The layer below 20 m is primarily composed of ~60% SCSSW and ~40% SCSSUW. A comparison between the two different observations suggests that the PRDW tends to expand southwestward and the SCSSUW spreads offshore, whereas the SCSSW moves landward and is situated underneath the surface fresh PRDW. These characteristics are very likely associated with the wind transition from weak southeasterly in April to strong northeasterly in May, which enhances the southwestward coastal current and the onshore surface Ekman transport from offshore waters.
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