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An anticyclonic eddy's periphery is characterized by large horizontal density gradients, strain and vertical velocity. In this paper we document the asymmetric distribution of phytoplankton around the periphery of anticyclonic eddies in the western South China Sea based on 432 eddies detected from satellite data. The high level of phytoplankton occurs consistently at the northwestern periphery of eddies, with a maximum positive chlorophyll anomaly greater than 0.01 mg m −3 . The asymmetric distribution of phytoplankton primarily tags the non-uniform surface velocity field varying from 0.15 m s −1 to 0.3 m s −1 along the eddy's periphery. The coastal boundary and off-coast jet may be the primary cause of the non-uniform flow. Associated with the non-uniform flow speed, the combined effects of the velocity convergence and steepened northwestern part of the eddies act to sharpen the density fronts and tend to result in an ageostrophic secondary circulation at the northwestern edge of the eddy. The upward component of the ageostrophic secondary circulation can enhance the nutrient flux into the euphotic layer, thereby increasing phytoplankton productivity. Anticyclonic eddy-induced ageostrophic secondary circulation appears to be an important mechanism for increasing phytoplankton productivity in the oligotrophic waters of the South China Sea. [ABSTRACT FROM AUTHOR]

A support vector machine (SVM) regression algorithm was developed for estimating the chlorophyll-a concentration (C chla) in the turbid estuarine waters of the Pearl River estuary (PRE) based on visible infrared imaging radiometer suite (VIIRS) ocean color data. The proposed SVM algorithm outperformed the OC3 algorithm in the turbid waters. The temporal variation and spatial distribution of SVM-based monthly C chla products were determined during 2012 and 2018. By analyzing the monthly average C chla products and ancillary data, the underlying mechanisms of the C chla coastal summer plume jet and winter bloom were revealed. The C chla coastal plume jet was often observed in June, when the zonal wind direction changed from westward to eastward and the river runoff reached its maximum value of the year. The winter bloom in the ocean area was attributed to strong mixing in the upper layer, which resulted from cooling-induced buoyancy changes. • A SVM model was developed for the estimation of chlorophyll-a concentration. • Spatial distribution and temporal variation of chlorophyll-a concentration was reveal. • The underlying mechanisms of the summer plume jet and winter bloom were discussed. [ABSTRACT FROM AUTHOR]

New evidences provided that the tropical cyclone (TC) Linfa in 2015 induced looping path of Kuroshio invasion into the northeastern South China Sea (NESCS) through the northwestern Luzon Strait (LS), based on the in-situ measurements, satellite data and model output data. This TC-enhanced Kuroshio invasion with low nutrients and denser waters suppressed the TC "Wind Pump" induced upwelling and nutrients uptake, and therefore inhibited the Chlorophyll a concentration (Chl-a) increase from surface to ~50 m in the open ocean of the NESCS. The TC-induced Kuroshio invasion promoted the generation of the strong cyclonic eddy to its left side where weak Ekman Pumping Velocity was observed. This enhancing cyclonic eddy then dominated the nutrients uplift and increased the surface and subsurface (0–50 m) Chl-a through eddy pumping rather than Ekman Pumping. The TC-declined anti-cyclonic eddy, which shoaled the Mixed Layer Depth (MLD), benefited to the nutrient uptake through TC-induced upwelling and thereby increased the surface Chl-a and raised the Chl-a Maximum Layer (CML) to ~20 m over the southwestern LS. The temporal Chl-a variations were also influenced by TC intensities and biochemical processes. The air-sea heat budget analysis indicated that, the air-sea heat exchange contributed to nearly 80% of the sea surface cooling (SST cooling) over the northwestern LS with Kuroshio invasion, while eddy-induced upwelling dominated the SST cooling over the western LS, and the wind-driven upwelling (and mixing) controlled the SST cooling over the southwestern LS. These different formations of SST cooling then played important role in Chl-a variations. This study is the first case of TC "Wind Pump" induced Chl-a variations considering air-sea heat exchange, Kuroshio invasion and mesoscale eddies over LS, which would help to evaluate the influence of TCs over the other major heat transport arteries of the world ocean: The Gulf Stream area. Unlabelled Image • TC "Wind Pump" enhanced Kuroshio invasion which then inhibited the surface Chl-a. • Eddy-induced upwelling dominated Chl-a increase rather than wind-driven upwelling. • TC-enhanced Kuroshio invasion reduced a cyclonic eddy which right on its way. • TC-enhanced Kuroshio conduced to a cyclonic eddy generation in its left side. • Air-sea heat exchange dominated SST cooling which then inhibited Chl-a increase. [ABSTRACT FROM AUTHOR]

Abstract Spatio-temporal variability of the suspended sediment concentration (SSC) in the Pearl River Estuary (PRE) is analyzed over 13 years (2003–2015) using MODIS-Aqua Level-1B data. A unique seasonal SSC variability was found. During the wet season, the SSC in the northwestern estuary (NWE) is high, which could reach up to 100 mg/L, while during the dry season it suffers from a markedly decrease. Over the southeastern estuary (SEE), however, the pattern is reversed. This seasonality is mainly affected by freshwater discharge, winds and water column stratification. During the wet season, freshwater discharge dominates the SSC in the NWE while stratification and water discharge control the SSC variation in the SEE. During the dry season, freshwater discharge also plays a more important role in changing the SSC in the NWE, while the combined effect of wind mixing and tidal mixing dominates the SSC in the SEE. The interannual variability of the SSC is revealed by an EOF analysis. The first mode is highly related to water discharge of the Pearl River, indicating that the interannual variability is mainly subject to the rainfall over the Pearl River basin. The long-term SSC trend is characterized by a general decrease across the entire estuary and proved to be closely related to human activity. The reduction rate of the SSC varies spatially and seasonally. Spatially, the northern West Shoal and upper eastern part of the PRE suffer from a much greater SSC decrease, while the rest of the estuary, especially the southern West Shoal, is relatively less influenced. Seasonally, the decreasing rate in the dry season (49.3%) is higher than that in the wet season (30.1%). Highlights • Seasonal variability of the SSC in the NWE and SEE are controlled by different factors. • Interannual variability of the SSC is subject to the freshwater discharge. • A significant decreasing trend of the SSC is detected, presumably due to dam constructions. • The reduction rate of the SSC in the dry season is higher than that in the wet season. [ABSTRACT FROM AUTHOR]

The influence of oceanic mesoscale eddies on their internal phytoplankton levels varies at different developmental stages. Based on 12 years of satellite ocean color observations, we investigated the variability of phytoplankton inside 4564 anticyclonic eddies and 3675 cyclonic eddies off eastern Australia in different development stages using the method of composited average analysis. The results indicated that the lowest level of chlorophyll was observed in the forth (decay) development stage for anticyclonic eddies, which was associated with the warmest SST, largest eddies amplitude, rotation speeds, angular velocities and surface water convergence. It is indicated that downwelling induced by anticyclonic eddies dominates the chlorophyll variations. Because the convergence induced by the ageostrophic velocity components occurred throughout the eddy's development stage, relaxation of the density perturbations (upwelling) associated with eddy decay was not observed in this study. Chlorophyll concentration near the center of cyclonic eddies decreased from the first to the middle stage, and then increased to the largest levels at the last stage. Although vertical motions induced by the ageostrophic velocity components varied from divergence to convergence during the development of an eddy, a higher eddy-ambient water exchange occurred and dominated the increases in total divergence and chlorophyll concentration in the last stage. [ABSTRACT FROM AUTHOR]

• A satellite concept Ocean Surface Current multiscale Observation Mission is proposed. • OSCOM will directly measure surface currents with a resolution of ~10 km. • OSCOM will observe ocean surface current, vector wind, and temperature simultaneously. • OSCOM will help the study on non-equilibrium ocean state and air-sea interaction. • OSCOM will provide an improved pathway to data assimilation for ocean prediction. Ocean current is one of the major drivers of water mass, energy, and biogeochemical cycles in the global ocean–atmosphere boundary layer and also a key variable in the formation of extreme climate events (e.g., El Niño). Direct measurement of the global ocean surface current is of great scientific interest and application value for understanding multiscale ocean dynamics, air-sea interaction, ocean mass and energy balance, and ocean carbon budget, as well as their variabilities under climate change. This paper reviews the state-of-the-art developments of ocean dynamics and technology on the observation of multiscale ocean circulation and related scientific frontiers. Presently, measurements of global ocean surface currents, which are mainly geostrophically derived from satellite altimeter data, are only available to resolve quasi-geostrophic current at large- to meso-scale in the off-equatorial open ocean. This becomes a bottleneck impeding the application and the development of ocean circulation dynamics. With the ambition of breaking the bottleneck, Ocean Surface Current multiscale Observation Mission (OSCOM) will launch a satellite equipped with a Doppler Scatterometer to directly measure ocean surface currents for the first time with a very high horizontal resolution of 5–10 km and a 3-day global coverage. Through carrying a Surface Temperature Infrared Radiometer and a Surface Temperature Microwave Radiometer, the OSCOM satellite is supposed to have the capability of observing ocean surface current, ocean surface vector wind, and sea surface temperature simultaneously. With a cutting-edge design, OSCOM will provide an in-depth picture of non-equilibrium ocean state and air-sea interaction from mesoscale to submesoscale, and helps to construct the fine structure of deep ocean current through a combination with Array for Real-time Geostrophic Oceanography profiling. Those direct measurements and derived dynamic parameters will further facilitate analyses of ocean carbon budget and ocean biogeochemical cycle, and provide a novel and improved pathway to data assimilation, coupling of General Circulation Models, and the Earth System Modelling for ocean prediction and climate change. [ABSTRACT FROM AUTHOR]