A comprehensive investigation using the air quality network and meteorological data of China in 2015 showed that PM_2.5 driven by cold surges from the ground level could travel up to 2000 km from northern to southern China within two days. Air pollution is more severe and prominent during the winter in north China due to seasonal variations in energy usage, trade wind movements, and industrial emissions. In February 2015, two cold surges traveling from north China caused a temporary increase in the concentration of PM_2.5 in Shanghai. Subsequently, the concentration of PM_2.5 in Xiamen increased to a high of 80 µg/m³_, which is double the average PM_2.5 concentration in Xiamen during the winter. This finding is a new long-range transport mechanism comparing to the well-established mechanism, with long-range transport more likely to occur in the upper troposphere than at lower levels. These observations were validated by results from the back trajectory analysis and the RAMS- CMAQ model. While wind speed was found to be a major facilitator in transporting PM_2.5 from Beijing to Xiamen, more investigation is required to understand the complex relationship between wind speed and PM_2.5 and how it moderates air quality in Beijing, Shanghai, and Xiamen.

Using high‐resolution full‐water column velocity data at a site of 60‐m depth on the northeastern shelf of the South China Sea, we observed high‐frequency internal waves (HIWs) occurring continuously on the continental shelf during the 15‐hr observation on 13‐14 July 2011. This phenomenon is in contrast to the regular occurrence of internal solitary waves (ISWs) that are generally phase‐locked to the tides. We hypothesize that the continuously occurring HIWs are generated from shoaling ISWs via fission. Analysis of the Moderate Resolution Imaging Spectroradiometer (MODIS) true‐color images supports this hypothesis. The mechanism of ISW fission is then investigated using an analytical model as well as a two‐layer weakly nonlinear numerical model based on the regularized long‐wave (RLW) equation. The results are also compared with those from a fully nonlinear continuously stratified numerical model. Simulation results are generally consistent with the field measurements, explaining the observations of the continuous occurrence of HIWs. Sensitivity experiments are conducted to investigate the role and effects of the initial wave amplitude, pycnocline depth and thickness, bathymetry, and viscosity on ISW fission. Although all these factors impact the properties of the shoaling ISWs, both the two‐layer and continuously stratified models suggest that fission mainly occurs near the critical point region, highlighting the importance of pycnocline depth. However, simulations using the two‐layer model suggest that fission occurs after ISWs pass through critical points, whereas the fission location predicted by the continuously stratified model is generally offshore of critical points. This points to the deficiency of the two‐layer model in simulating small‐amplitude short trailing waves, large‐amplitude ISWs, wave breaking, and higher‐mode waves.

Upwelling and internal waves are known to affect the physical and chemical characteristics of marine ecosystems, yet the processes and mechanisms by which internal waves influence phytoplankton biomass and community composition in upwelling regions are still unclear. In this study, a 72‐h time series of observations was conducted in an upwelling system in the northern South China Sea during the summer of 2014. The results showed that the intensity of upwelling was affected by internal waves, which caused nutrient fluctuations in the upper water column. The phytoplankton total chlorophyll a responded positively to the increase of nutrient concentrations, but only after a time lag of 12–16 h. This overall response was the net result of four different types of responses displayed by nine specific phytoplankton groups. All groups that displayed immediate positive responses continued to respond positively at least 12 h later, whereas all groups that responded negatively showed no time‐lagged responses. Based on the vertical distributions of the nine phytoplankton groups and their known physiological traits, we suggest that these different types of response were the net result of a rapid physical transport effect and a time‐lagged, physiological effect via bottom‐up control.

This study investigates along‐shelf propagating internal solitary waves (ISWs) on the Amazon Shelf using satellite observations and numerical modeling. These ISWs appear as streak‐like patterns in satellite images along a narrow path within the North Brazilian Current (NBC), propagating against the current. The streak‐like patterns of the along‐shelf propagating ISWs are a result of the quasi‐two‐dimensional bathymetry below, suggesting the possibility of using sea surface imprints to detect the topographic features beneath. In this study, both the effects of the NBC and tidal current on ISW generation are considered over irregular seafloor. Near‐critical conditions (internal Froude number close to 1) created by the joint effects of the NBC and the tides result in ISW generation, while the dominant subcritical conditions result in the upstream propagation of these ISWs. Both the NBC and tidal currents are needed to continuously generate ISWs. This study demonstrates that small‐scale topographic features can result in the generation of large numbers of ISWs, which are expected to significantly contribute to ocean mixing and, potentially, sediment resuspension. The ISW‐induced current also contributes to sea surface wave breaking as observed by satellites.