BGBiogeosciencesBGBiogeosciences1726-4189Copernicus GmbHGöttingen, Germany10.5194/bg-12-299-2015Satellite observations of the small-scale cyclonic eddies in the western
South China SeaLiuF.TangS.sltang2009@gmail.comChenC.School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006,
ChinaState Key Laboratory of Tropical Oceanography, South China Sea
Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301,
ChinaS. Tang (sltang2009@gmail.com)16January20151222993058August201419September201418November20141December2014This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.biogeosciences.net/12/299/2015/bg-12-299-2015.htmlThe full text article is available as a PDF file from https://www.biogeosciences.net/12/299/2015/bg-12-299-2015.pdf
High-resolution ocean color observations offer an opportunity to investigate
the oceanic small-scale processes. In this study, the Medium Resolution
Imaging Spectrometer (MERIS) daily 300 m data were used to study small-scale
processes in the western South China Sea. It is indicated that the cyclonic
eddies with horizontal scales of 10 km are frequently observed during the
upwelling season of each year over the 2004–2009 period. These small-scale eddies were
generated in the vicinity of the southern front of the cold tongue, and then
propagated eastward with a speed of approximately 12 cm s-1. This
propagation speed was consistent with the velocity of the western boundary
current. As a result, the small-scale eddies kept the high levels of
phytoplankton rotating away from the coastal areas, resulting in the accumulation of
phytoplankton in the interior of the eddies. The generation of the
small-scale eddies may be associated with strengthening of the relative
movement between the rotation speed of the anticyclonic mesoscale eddies and
the offshore transport. With the increases of the normalized rotation speed
of the anticyclonic mesoscale eddies relative to the offshore transport, the
offshore current became a meander under the impacts of the anticyclonic
mesoscale eddies. The meandered cold tongue and instability front may
stimulate the generation of the small-scale eddies. Unidirectional uniform
wind along the cold tongue may also contribute to the formation of the
small-scale eddies.
Introduction
(a) Bathymetry of the South China Sea (m), the red
rectangle represents the study area. (b) Mean surface geostrophic
currents in June–October of 2002–2008.
Approximately 90 % of the kinetic energy of ocean circulation is
contained in small-scale features, and 50 % of the vertical exchange of
water mass properties between the upper and the deep ocean may occur at the
submesoscale and mesoscale (Bouffard et al., 2012). Mesoscale eddies with
horizontal scales of 50–500 km can be observed using altimeters. However,
the smaller-scale eddies (with horizontal scales below 50 km) cannot be
resolved by conventional altimeters (Liu et al., 2008). Satellite ocean color
sensors provide high-quality observations of the bio-optical constitute at a
spatial resolution better than from altimeters. The spatial resolutions of most
ocean color satellites fall in the range from 300 m to 1.1 km (at nadir
viewing). The high-resolution bio-optical observations reveal more details of
small-scale phytoplankton structures. By tracking these small-scale
biological features, one can determine the circulation pattern if the motion
speed is large with respect of the growth and grazing of the phytoplankton
(Pegau et al., 2002). Recently, the Medium Resolution Imaging Spectrometer
(MERIS) full-resolution (FR, 300 m) data set is available publicly. The
MERIS FR (300 m) phytoplankton fields are rich in smaller-scale biological
features and provide opportunities to study the small-scale processes.
Generally, the time period of the small-scale ocean variability ranges from
several days to several weeks. However, the widely used ocean color data are
usually averaged into weekly or monthly products in order to obtain a large
spatial coverage. This time-averaging may smooth the phytoplankton
variability on day-scale (Genin and Boehlert, 1985). Therefore, the study of
the small-scale processes requires higher space–time resolution of ocean
color observation.
The South China Sea (SCS) is the largest marginal basin within the western
Pacific, with a total area of 3.5 million square kilometers and a basin depth of
> 3000 m (0∘–25∘ N, 100∘–125∘ E,
Fig. 1). The SCS is oligotrophic with limited nitrogen and phosphorus within
the euphotic layer. A high abundance of phytoplankton mainly occurs in the
Gulf of Tonkin, the western SCS and the Sunda Shelf in
summer (Ning et al., 2004). It was reported that a phytoplankton filament in
the western SCS is consistent with the mesoscale eddies transportation and
Ekman upwelling (Tang et al., 2004; Xie et al., 2003; Xiu and Chai, 2011).
However, there have been only limited studies on the small-scale process and
its phytoplankton footprints (Nicholson, 2012). In this study, the daily
MERIS FR data were used to identify the phytoplankton variability associated
with the small-scale dynamic processes. In this paper, we will call eddies
with diameters smaller than 50 km the small-scale eddies although, in the
literature, they are often called submesoscale eddies (Bassin et al.,
2005; Burrage et al., 2009).
Daily 300 m MERIS chlorophyll (mg m-3) on (a, b) 5 September 2004, (c, d) 22 June 2005, (e,f )7 June
2006, (g, h) 21 July 2007, (i, j) 16 July 2008, (k, l) 29 July 2009. The cloud covered area was masked by the white color.
Daily 300 m MERIS chlorophyll (mg m-3) on (b)
9 July 2008, (c, d) 12 July 2008, and (e, f) 13 July 2008. The cloud
covered area was masked by the white color. A and B indicate two
small cyclonic eddies, respectively. The pink circle in (a) denotes
the anticyclonic mesoscale eddy (AME) on 9 July 2008, which was derived from
AVISO MSLA data following the method of Chelton et al. (2011).
MODIS 1 km sea surface temperature distribution (unit: ∘)
on 13 July 2008.
The offshore transport (Mx, kg m-1 s-1), rotation
speed of the mesoscale anticyclonic eddy (U, cm s-1) and the
normalized rotation speed to Mx (U/Mx), indicating the relative
importance of the mesoscale anticyclonic eddy and offshore Ekman transport in
the form of small-scale eddies.
Wind fields on 2 July 2008 (blue arrows), 12 July 2008 (red
arrows) and 26 July 2008 (green arrows).
The western SCS is one of the dynamically active regions in the SCS (Liu et
al., 2000). A northeastward alongshore current in summer (Fig. 1) and a
southwestward alongshore current in winter off the east coast of Vietnam are
in accordance with wind stress (Hwang and Chen, 2000; Morimoto et al., 2000;
Yuan et al., 2005). The northeastward alongshore current meanders off the
southeastern coast of Vietnam and leaves the Vietnam coast forming an
eastward current driven by the southwest wind paralleled to the coast of
eastern Vietnam (Hwang and Chen, 2000; Kuo et al., 2000; Barthel et al.,
2009). The southwesterly monsoon and Ekman transport drive seasonal upwelling
off southeastern Vietnam coast in summer, leading to more than 1∘ C
drop in sea surface temperature (SST) (Wyrtki, 1961; Kuo et al., 2000;
Metzger, 2003; Tang et al., 2006). A cold SST tongue around 12∘ N
extends eastward. The orographic effect of the coastal mountain ridge in
Vietnam can further intensify the southwesterly wind and, thus, significantly
enhances the coastal upwelling (Xie et al., 2003, 2007). The local orographic
wind forces the coastal jet separation. This deformation and movement of
coastal water induce mesoscale eddy activities (Gan et al., 2006; Wang et
al., 2008; Chen et al., 2010). An eddy pair in the western SCS during
the upwelling season is generated probably due to the vorticity transports
from the nonlinear effect of the western boundary currents (Xie et al., 2003;
Ning et al., 2004; Wang et al., 2006; Chen et al., 2010). Moreover, a pair of
anticyclonic eddies (A–A eddy pairs) in the western SCS during the
upwelling season was mentioned by Kuo et al. (2000) and Xie et al. (2003).
Data
The study area is located in the western SCS, covering
5∘–18∘ N, 105∘–115∘ E (Fig. 1). The daily
MERIS FR chlorophyll data from 2004 to 2009 were obtained from the European
Space Agency (ESA). The daily 1 km Moderate-resolution Imaging
Spectroradiometer (MODIS) SST data were obtained from National Aeronautics
and Space Administration (NASA) Ocean Color project.
The mean sea level anomaly (MSLA) and geostrophic velocity data used here
were extracted from the Delayed Time reference series provided by Archiving,
Validation and Interpretation of Satellite Data in Oceanography (AVISO). The
mesoscale eddies were identified by a new SSH-based (sea surface height)
method developed by Chelton et al. (2011). Rotational speed was computed as
U=gf-1A/Ls,
where g is the gravitational acceleration, f is the Coriolis parameter,
A is the eddy amplitude (in centimeters) and Ls is the eddy
length scale (in kilometers), defined by the radius of the circle that has
the same area as the region within the closed contour of MSLA with maximum
average geostrophic current speed (Chelton et al., 2011).
The wind stress was obtained from the National Oceanic and Atmospheric
Administration (NOAA) Environmental Research Division's Data Access Program
(ERDDAP). The offshore transport (Mx) was calculated from
Mx=τy/f,
where τy is the wind stress parallel to the coastline, positive in
the northward direction. This was replaced with the meridional direction wind stress since
the most significant offshore transport perpendicular to the Vietnam coast is
approximately in the zonal direction.
Results and discussion
A series of small cyclonic phytoplankton tendrils at the southern edge of the
phytoplankton filament were found during June and October of each year over
the 2004–2009 period (Fig. 2). The phytoplankton tendrils had a mean diameter of 25 km
and obviously rotated out of the filament as the concentration variability of
the phytoplankton tendril seemed consistent with the phytoplankton filament
concentration variability. It is implied that the phytoplankton tendril is
rotated by the small-scale cyclonic eddy. High levels of phytoplankton were
frequently observed in the center of the small-scale cyclonic eddies. The
reason for this phenomenon will be discussed in the next section.
Figure 3 shows an evolution of two cyclonic phytoplankton tendrils during
9 July 2008 and 13 July 2008. It seems that these phytoplankton tendrils have
a timescale of several days. The phytoplankton tendril A was less
obvious on 9 July 2008. Three days later, the concentration of phytoplankton
tendril A increased about 0.1 mg m-3. This high level of
phytoplankton mainly occurred at the edge. The phytoplankton levels in the
center were relatively low (approximately 0.07 mg m-3). Only 1 day
later, the phytoplankton concentration in the center increased to
approximately 0.3 mg m-3 and became greater than the level of
phytoplankton at the periphery. Another feature was that the cyclonic A
tended to propagate eastward. It propagated approximately 0.1∘
(∼ 10 km) from 12 July 2008 to 13 July 2008. The western boundary
current had a speed of about 12 cm s-1 (10.4 km day-1) in the
western SCS during summer (Cai et al., 2007), which was consistent with the
propagation velocity of the small cyclonic eddy A. Therefore, the
eastward propagating cyclonic eddy may be driven by the western boundary
current. The small cyclonic eddy B strengthened on 12 July 2008, with high
levels of phytoplankton within its interior, and then it disappeared on
13 July 2008.
The observation of more detailed phytoplankton distribution in the tendrils
was attributed to the much finer resolution (300 m). We found that there
were relatively high phytoplankton levels in the center of the small cyclonic
eddies. One possible mechanism is that the small cyclonic eddies keep
rotating high phytoplankton and perhaps nutrients, leading to the
accumulation of phytoplankton in their center. Another possible mechanism is
that the vertical velocity of these small-scale cyclonic eddies may drive
episodic nutrient pulses to the euphotic zone to stimulate phytoplankton
growth (Lévy et al., 2012). Figure 4 shows the sea surface temperature
distribution associated with the phytoplankton tendril A. It is obvious
that the cold water was transported away from the cold tongue by the
small-scale eddies, and that the low-temperature water firstly occurred in the
periphery of the eddies. Different from the majority of mesoscale cyclonic
eddies, there was no significantly lower-temperature water in the center. It is
implied that there is no upwelling or vertical mixing in the center.
Therefore, the phytoplankton distribution over this small-scale eddy may be
dominated by horizontal movement, and the relatively high phytoplankton level
in the center of the cyclonic eddy A could be attributed to the
accumulation of phytoplankton or nutrients from the outer edge to the interior
under the rotation effect.
The small-scale eddies strengthened the horizontal diffusion of the nutrients
and phytoplankton (Capet et al., 2008a). These small cyclonic eddies were
mainly observed at the front of the filament, where strong differences in
water mass properties resulted in high strain rates and instabilities.
Meanwhile, the small-scale eddies were also associated with the occurrence of
an anticyclonic mesoscale eddy to the south of the filament (Fig. 3a).
However, the small-scale cyclonic eddy did not occur for the entire period of
the offshore Ekman transport and the anticyclonic eddy. It only arose at
certain stages. We analyzed the offshore Ekman transport (Mx) and rotation
speed of the anticyclonic eddies during the development of the small-scale
eddies over the period of July 2008 shown in Fig. 3. Due to the limits of the
cloud coverage and satellite passing time, the image showing the declination
of small-scale eddies was not available. However, it is found that the
small-scale eddies disappeared on 22 July 2008. Figure 3a and b imply that the
small-scale eddies may initially form on 9 July 2008. Therefore, we presumed
that the small-scale eddies occurred during 9–22 July. Figure 5
indicates that the offshore transport (Mx) decreased first and then increased
rapidly on 16 July. Different from the variability of Mx, the rotation speed
increased from 0.33 m s-1 on 2 July to 0.42 m s-1 on 12 July,
and then started to decrease to approximately 0.4 m s-1 on 16 July.
Lastly, the rotation speed increased, which is associated with the strengthening of
Mx. The variability of Mx seemed not to be consistent with the variability of
the levels of phytoplankton (Fig. 3). The levels of phytoplankton had a
significant increase from 9 July to 13 July, accompanying the decreases
of Mx. This may be due to a lag between nutrient inputs and
phytoplankton growth. The normalized rotation speed of the anticyclonic eddy
was defined as the ratio of the rotation speed and the Mx, which indicated
the relative movement between the anticyclonic eddy and the offshore
transport. The variability of the normalized rotation speed shows that the
small-scale eddies were associated with the greater relative movement between
the anticyclonic eddy and the offshore transport (Fig. 5). The offshore
current became a meander under the influence of the anticyclonic eddy when the
offshore transport turned weaker and the rotation speed of the anticyclonic
eddy increased. The meandering current may stimulate the generation of the
small-scale process (Capet et al., 2008a, b).
The phytoplankton filament was consistent with the cold tongue induced by the
offshore Ekman transport, which was associated with the negative sea surface
height anomaly relative to the surrounding light and warm water. The small-scale
eddies extended from the cold tongue along the front. Therefore, the heavy and
cold water firstly occurred in the periphery of small-scale eddies. Along the
front, the transport from the surface, heavy water to the light water may be
forced by the wind. Throughout the development of the small-scale eddies, the
wind direction exhibited some variations (Fig. 6). Wind direction varied from
west-southwest (WSW) on 2 July 2008 (before the generation of the small-scale
eddies) to southwest (SW) on 9–13 July 2008 (during the presence of the
small-scale eddies). This implies that the small-scale eddies tend to be
associated with the more unidirectional uniform wind blowing along the
phytoplankton filament. Under spatially uniform wind forcing, the changed
meandering current may be more likely to generate the small-scale structure
(McGillicuddy et al., 2007; Mahadevan et al., 2008).
Conclusions
This paper describes the small-scale cyclonic eddies in the western SCS.
Driven by the small-scale cyclonic eddies, a series of phytoplankton tendrils
occurs at the southern front of the wind-driven offshore current. These
small-scale eddies have a horizontal extent of less than 50 km and propagate
eastward at the speed of 12 cm s-1, accompanying an offshore
current. The offshore current, mesoscale anticyclonic eddies and wind field
may contribute to the generation of the small-scale cyclonic eddies.
Horizontal transport by the small-scale cyclonic eddies stimulates the
diffusion of the nutrients and phytoplankton of the western SCS.
Acknowledgements
We gratefully thank Ruixin Huang and Ian Jones for helpful comments and
suggestions. The MERIS 300 m chlorophyll data were provided by ESA-MOST
Dragon 3 Cooperation Programme from the European Space Agency. The sea surface
height and geostrophic current data were obtained from the Archiving
Validation and Interpretation of Satellite Data in Oceanography. The
MODIS sea surface temperature was obtained from the NASA ocean color project.
The wind stress was obtained from the National Oceanic and Atmospheric
Administration Environmental Research Division's Data Access Program.
The research was supported by the Strategic Priority Research
Program of the Chinese Academy of Sciences (no. XDA11010302), the Public
science and technology research funds projects of ocean (no. 201205040–6),
the Innovation Group Program of State Key Laboratory of Tropical
Oceanography, South China Sea Institute of Oceanology, Chinese Academy of
Sciences (no. LTOZZ1201) and the National Natural Science Foundation of China
(no. 41006111). Edited by: K. Suzuki
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