The 4-D structure of upwelling and Pearl River plume in the northern South China Sea during summer 2008 revealed by a data assimilation model (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 Dynamics, 2014
Analyses of moored current meter data from the South China Sea (SCS) Coastal Oceanographic Process Experiment Pilot Project (SCOPE-PP) reveal that strong spatial variation in the vertical structure of coastal currents in the northern SCS (NSCS) was caused by the interaction between the Pearl River plume (PRP) and the alongshore coastal jet. Observations indicate that the NSCS coastal current was generally eastward, spreading the PRP water eastward. The lighter brackish plume residing on the top of the saline, coastal water strengthened the stratification near the surface and blocked the downward momentum transport, resulting in a strong vertical shear of the velocity at the mooring site S205. Meanwhile, the plume water raised the sea level. Because of the geostrophic balance in the across-shore direction, the eastward coastal current in the upper layer strengthened on the onshore side (near S205) and weakened on the offshore side (near S206 and S305) due to the plume water intrusion. Therefore, the vertical shear of the velocity was enhanced at S205 and reduced at S206 and S305. Moreover, because the plume varied substantially with the wind, the vertical shear of the velocity responding to the wind was different between S205 and S206. The numerical simulation during summer 2008 confirms that the PRP enhances (weakens) the vertical shear of the velocity on the onshore (offshore) side by increasing (decreasing) the upper-layer current by about 0.2 m s -1 and decreasing (increasing) the middle-layer current slightly.
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.
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.
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).
Deep Sea Research Part II: Topical Studies in Oceanography, 2015
An array of four bottom-mounted acoustic Doppler current profilers (ADCPs) were deployed during the winter of 2008 (28 December 2008 to 12 March 2009) along a cross-shelf section in the western East China Sea to investigate the winter circulation and its response to wind. During the observation period, the observed subtidal currents exhibit coherent spatial structure and temporal variation in terms of their mean (seasonal), trend (intra-seasonal), and synoptic variability. The subtidal currents are polarized roughly in the alongshore direction parallel to local isobaths, and the weak cross-shore current is closely linked to the alongshore component. The temporal variation of the currents follows the rhythm of wind stress, sea level, and sea level difference at the synoptic scale.
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.
Remote sensing observation and numerical modeling of an upwelling jet in Guangdong coastal water
Journal of Geophysical Research, 2012
1] An upwelling event that occurred in Guangdong coastal water on July [14][15][16] 2003 is observed by using satellite multisensor data including the Moderate Resolution Imaging Spectroradiometer (MODIS) sea surface temperature (SST) and QuikSCAT ocean surface winds. Successive MODIS SST images reveal expansion process of a jet-like upwelling cold water body in surface layer. The regional ocean model system (ROMS) is used to explore the upwelling dynamics. The modeling successfully reproduces the jet-like shape of the surface upwelling water as well as the upwelling developing process. Analyses of modeling momentums reveal that the large offshore transport appeared on the west side of Honghai Bay as a result of high alongshore pressure gradient and nonlinear advections, and the enhanced horizontal advection also played an important role in developing the prominent upwelling in Honghai Bay. A numerical experiment is conducted to confirm that it was the wind driven upwelling rather than the wind-induced vertical turbulent mixing that induced the surface cold water. Further numerical analyses suggest that strong internal tides occurred in Honghai Bay caused by the local bottom topography. The interaction between the upwelling and internal tides enhances the bottom water uplifting. The offshore expansion of the upwelling water is controlled by the cross-shore topography slope: a gentle and offshore extended slope helps the bottom water climbing up to the surface in a wide range in cross-shore direction, whereas a steep and narrow slope restricts the expansion of the upwelling water and confines the cold water in a narrow band along the shore.