Remote sensing observation and numerical modeling of an upwelling jet in Guangdong coastal water (original) (raw)
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Coastal upwelling in summer 2000 in the northeastern South China Sea
Journal of Geophysical Research, 2012
1] Using a combination of hydrographic, tide-gauge, near-bottom mooring, and satellite observations; and a numerical circulation model, we investigate the coastal upwelling in the northeastern South China Sea (NSCS) off the coast of Fujian and Guangdong Provinces, China, in the summer of 2000. Subsurface upwelling phenomenon exists mainly near the bottom boundary in the whole region investigated. It is closely related to the coastal sea level fluctuations, which are evidently modulated by both the local wind-forcing and the large-scale circulation. The northeastward interior flow following the bathymetry is accelerated by the drop of coastal sea level and leads to onshore transport and subsequent cooling in the bottom boundary layer (BBL) over the shelf west of Shantou. To the east of Shantou, the near-bottom flow veers more eastward, parallel to the coastline, and transports the nearshore cold water mass farther to the southern Fujian coast. The cross-shelf advected cold water does not always penetrate through the stratification and reach the surface. The local wind exhibits considerable synoptic variability. The decrease in sea surface temperature (SST) is mostly significant near Dongshan-Shantou, intermittent in time and intensifies preferably during weather events that bring southwesterly alongshore wind. To the west a freshwater tongue originating from the Pearl River forms a barrier layer, which results in high surface temperature in the freshwater plume. The observational evidences and modeled results shown in this study provide important information for further understanding the ecological effects associated with the upwelling processes in the NSCS.
Ocean Modelling, 2011
We analyze four-dimensional structures of upwelling and Pearl River plume in the northern South China Sea (NSCS) during the summer of 2008 based on data assimilation. An Ensemble Kalman Smoother scheme is employed in the Princeton Ocean Model. It is found that the Pearl River plume axis extended eastward along with the surface current and swerved offshore twice near (116°E, 22.6°N) and (117.5°E, 22.8°N) before reaching the Taiwan Strait. The vertical transect of salinity along the plume axis indicates that the Pearl River freshwater could affect salinity distribution down to a depth of 10-20 m. Anomalously warm water is found in the upper layer, which could be attributed to the intensified stratification and suppressed vertical mixing caused by the freshwater of the plume capping the upwelling west of 116°E. The varying winds from upwelling favorable to downwelling favorable could induce a low-salinity water lens at the center of the model domain. Upwelling in the NSCS initially occurred at 114.5°E, to the east of the Pearl River Estuary, intensified eastward, and reached its maximum near Shantou (116.7°E, 23.2°N). Since current-induced upwelling appeared mainly in Shantou due to the widened shelf, it is found that even if the wind-induced upwelling was shut down in Shanwei by downwelling favorable wind on July 4, the upwelling still existed in Shantou. Moreover, because the direction of large-scale current was in favor of upwelling in the NSCS that cannot be reversed by varying local winds over a short time period, the upwelling shutdown time is longer for both wind-induced and current-induced upwelling in Shantou than for mainly wind-induced upwelling in Shanwei. The steeper slope in Shanwei also shortens the upwelling shutdown time there.
East Hainan upwelling fronts detected by remote sensing and modelled in summer
International Journal of Remote Sensing, 2014
Using the Belkin and O'Reilly algorithm and high-resolution (1 km) satellite sea surface temperature (SST) and chlorophyll-a (chl-a) data from 2002 to 2011, fronts were detected off the east/northeast coast of Hainan Island, South China Sea. These fronts were mainly produced by upwelling off eastern Hainan Island, through which cold, high-salinity, high-density, and nutrient-rich bottom water was brought to the surface and subsurface and then transported to the northeast of Hainan Island by the along-shore currents. The fronts are anisotropic, with a dominant orientation SSW-NNE. A three-dimensional ocean model forced by the Quick Scatterometer (QuikSCAT) winds was employed to study the three-dimensional structure of these fronts as well as the relationship between the fronts and upwelling or summer monsoon. The results show that the front intensity (cross-frontal gradient) is strongly correlated with the along-shore local winds, and has a strong seasonal and a weak inter-annual variation with a maximum of about 0.5°C km -1 at the subsurface (about 15 m) rather than the surface.
Evolution of a coastal upwelling event during summer 2004 in the southern Taiwan Strait
Acta Oceanologica Sinica, 2011
A coastal upwelling event in the southern Taiwan Strait (STWS) was investigated using intensive cruise surveys (four repeated transects in a month) and satellite data in July and early August 2004. The extensive upwelling-associated surface cold water was first observed in early July (∼2.0×104 km2) along the STWS coast. Then, the cold surface water reduced in size by ∼50% with decreased chlorophyll concentrations after 15 days, indicating the weakening of the upwelling event. At the end of July, the cold surface water disappeared. The temporal variations of the surface cold water and the 3-D hydrography around Dongshan Island are thought to be mainly attributed to the weakened upwelling-favorable southwestern wind, the asymmetric spatial structure of the wind field and the intrusion of warm water from the northern South China Sea.
A Review of Coastal Upwelling Research in the South China Sea: Challenges, Limitations and Prospects
Since it was first observed in the 1950s a number of coastal upwelling systems have been identified and rigorously studied in the continental shelf of the South China Sea (SCS). Northern SCS in particular has been the predominant focal region while Southern Vietnam and Luzon Strait Upwelling Zones have been the locations that have received disproportionately greater attention from the international research community. Most studies of the phenomenon find it to be predominantly a seasonal occurrence, driven by alongshore wind stress, wind stress curl, bottom topography, coastline orientation, shelf circulation, eddies, islands and capes, and the shape of the coastline. However, the present review finds that since focus has generally been on localized areas, usually at seasonal scale and based largely on proxy indicators such as anomalies of sea surface temperature (SST), sea surface salinity (SSS), nutrients and Chlorophyll-a, it is extremely difficult to directly quantify upwelling. Consequently, long time-series of the phenomenon are inexistent in the SCS and most other marginal seas, rendering analysis of long term dynamics impossible. Moreover, despite the significance of the opposite process of downwelling, this phenomenon has been largely neglected. Finally, this study recommends the development of a time-series of upwelling indices which directly represent the phenomenon, based on method adopted NOAA's Pacific Fisheries Environmental Lab (NOAA PFEG/PFEL).
Diagnosis of 3D Vertical Circulation in the Upwelling and Frontal Zones East of Hainan Island, China
Journal of Physical Oceanography, 2017
Using the generalized omega equation and cruise observations in July 2012, this study analyzes the 3D vertical circulation in the upwelling region and frontal zone east of Hainan Island, China. The results show that there is a strong frontal zone in subsurface layer along the 100-m isobath, which is characterized by density gradient of O(10−4) kg m−4 and vertical eddy diffusivity of O(10−5–10−4) m2 s−1. The kinematic deformation term SDEF, ageostrophic advection term SADV, and vertical mixing forcing term SMIX are calculated from the observations. Their distribution patterns are featured by banded structure, that is, alternating positive–negative alongshore bands distributed in the cross-shelf direction. Correspondingly, alternating upwelling and downwelling bands appear from the coast to the deep waters. The maximum downward velocity reaches −5 × 10−5 m s−1 within the frontal zone, accompanied by the maximum upward velocity of 7 × 10−5 m s−1 on two sides. The dynamic diagnosis indi...
PLOS ONE, 2017
This study analyzes two wind-induced upwelling mechanisms, namely, Ekman transport and Ekman pumping that occur during the southwest monsoon. The results suggest that the coastline of the east coast of Peninsular Malaysia (ECPM) is affected by upwelling with spatiotemporal variations. Characterization of upwelling by using wind-induced upwelling indexes (UI W) indicate the existence of favorable upwelling conditions from May to September. Upwelling intensity increased in May and peaked in August before declining in September, decreasing intensity from the southern tip towards the northern tip along the coastline of the ECPM. The existence of upwelling along the ECPM has resulted in an important difference between the SSTs of the inshore and the oceanic regions. Nonetheless, the use of the SST gradient between the inshore and the oceanic SSTs to characterize upwelling (UI SST) was found to be unsuitable because the SST along the ECPM was affected by water advection from the Java Sea and incessant changes in the SST. In order to indicate the major contributor of wind-induced upwelling along the ECPM in terms of the spatiotemporal scale, a comparison between Ekman transport and Ekman pumping was drawn by integrating Ekman pumping with respect to the distance where the positive wind stress curl existed. The estimation of Ekman transport and Ekman pumping indicated that Ekman pumping played a major role in contributing towards upwelling in any particular month during the southwest monsoon along the entire coastline of the ECPM as compared to Ekman transport, which contributed towards more than half of the total upwelling transport. By dividing the ECPM into three coastal sections, we observed that Ekman pumping was relatively predominant in the middle and northern coasts, whereas both Ekman transport and Ekman pumping were equally prevalent in the southern coast.
Persistence of coastal upwelling after a plunge in upwelling-favourable wind
Scientific Reports, 2020
Unprecedented coastal upwelling off the southern coast of the Korean Peninsula was reported during the summer of 2013. The upwelling continued for more than a month after a plunge in upwelling-favourable winds and had serious impacts on fisheries. This is a rare phenomenon, as most coastal upwelling events relax a few days after the wind weakens. In this study, observational data and numerical modelling results were analysed to investigate the cause of the upwelling and the reason behind it being sustained for such an extended period. Coastal upwelling was induced by an upwelling-favourable wind in July, resulting in the dynamic uplift of deep, cold water. The dynamic uplift decreased the steric sea level in the coastal region. The sea level difference between the coastal and offshore regions produced an intensified cross-shore pressure gradient that enhanced the surface geostrophic current along the coast. The strong surface current maintained the dynamic uplift due to geostrophic ...
Summertime sea surface temperature fronts associated with upwelling around the Taiwan Bank
Continental Shelf Research, 2009
It is well known that upwelling of subsurface water is dominant around the Taiwan Bank (TB) and the Penghu (PH) Islands in the southern Taiwan Strait in summertime. Sea surface temperature (SST) frontal features and related phenomena around the TB upwelling and the PH upwelling were investigated using long-term AVHRR (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) and SeaWiFS (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) data received at the station of National Taiwan Ocean University. SST and chlorophyll-a (Chl-a) images with a spatial resolution of 0.011 were generated and used for the monthly SST and Chl-a maps. SST fronts were extracted from each SST images and gradient magnitudes (GMs); the orientations were derived for the SST fronts. Monthly maps of cold fronts where the cooler SSTs were over a shallower bottom were produced from the orientation.