JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C12036, doi:10.1029/2011JC007512, 2011
On physical factors that controlled the massive green tide
occurrence along the southern coast of the Shandong Peninsula
in 2008: A numerical study using a particle-tracking experiment
Joon Ho Lee,1 Ig-Chan Pang,1 Il-Ju Moon,1 and Joo-Hyung Ryu2
Received 22 August 2011; revised 15 October 2011; accepted 18 October 2011; published 23 December 2011.
[1] A Lagrangian-particle-tracking experiment has been conducted using Regional Ocean
Modeling System to determine physical factors that controlled the occurrence of the
record-breaking massive green tide along the southern coast of the Shandong Peninsula
(SP) in 2008. The numerical results reveal that the southerly wind in May is responsible
for the offshore movement of the green tide from the Jiangsu Province and the easterly
wind in June is responsible for its extension up to the coast of the SP. From the analysis of
30 year wind fields, it was also found that the wind patterns in 2008, which were very
unique and rare, provided the most favorable conditions for the migration of the bloom to
the SP. Through analyzing the pathway of particles, a recurrent upwelling region due to
tides was found between the Jiangsu coast and the western Yellow Sea where the massive
green tide bloomed. This area seems to provide nutrients for the green tide blooms. In
particular, it is estimated that the nutrient supply in 2008 was large because the upwelling
occurred during a spring tide. These results suggest that the massive green tide along the SP
in 2008 occurred due to the combination of a recent rapid expansion of seaweed
aquaculture, unique wind patterns, and nutrient supplies due to strong tidal forcing in
blooming regions. This implies that the massive green tides in the SP could occur again
as a very rare event if all conditions become favorable for the blooming and migration in
the future.
Citation: Lee, J. H., I.-C. Pang, I.-J. Moon, and J.-H. Ryu (2011), On physical factors that controlled the massive green tide
occurrence along the southern coast of the Shandong Peninsula in 2008: A numerical study using a particle-tracking experiment,
J. Geophys. Res., 116, C12036, doi:10.1029/2011JC007512.
1. Introduction
[2] A massive green tide (previously known as Ulva
prolifera, green macroalgae) [Hayden et al., 2003; Leliaert
et al., 2008; Liu et al., 2009] appeared just weeks before
the opening of the Beijing Olympics around the Qingdao
coast in late June 2008. The green tides were piled up on the
Qingdao coast and over 10,000 people were mobilized to
clean up and remove more than a million tones of algae from
the beach and coast of Qingdao [Lü and Qiao, 2008; China
Ocean News, 2007, available at http://epaper.oceanol.com/
zghyb/20080822, hereinafter referred to as CON, online,
2007] (Figure 1). This is the largest green tide ever reported
in the world, the most extensive translocation of a green tide
and the first case of expansive seaweed aquaculture contributing to a green tide [Liu et al., 2009; Shi and Wang,
2009; Hu et al., 2010]. Media and some scientists reported
that the initial causes of the green tides were attributed to
1
College of Ocean Science, Ocean and Environment Research Institute,
Jeju National University, Jejusi, South Korea.
2
Korea Ocean Satellite Center, Korea Research and Development
Institute, Ansan, South Korea.
Copyright 2011 by the American Geophysical Union.
0148-0227/11/2011JC007512
coastal eutrophication as well as the action of tides and
wind in bringing the algae ashore (China Ocean News,
2008, available at http://epaper.oceanol.com/zghyb/20080822,
hereinafter referred to as CON, online, 2008; Herald Sun,
2008, available at http://www.news.com.au/heraldsun/story/
0,23931047-11088,00html; Daily Mail, 2008, available at
http://www.dailymail.co.uk/news/worldnews/article-1031444/
Chinas-blooming-algae-problem-thats-swamping-Olympics.
html).
[3] Recently, Liu et al. [2009] analyzed the migration of
these green tides using observed satellite images and computed ocean current data. The results suggested that the
expansion of seaweed aquaculture along Jiangsu Province,
which is 250 km away from the Qingdao coast is, a possible
origin of the massive algal bloom in 2008 around the
Shandong Peninsula (SP). They also pointed out that
the coastal eutrophication cannot be the primary cause of
the green tide. N. Ye et al. (China is on the track tackling
Enteromorpha spp forming green tide, 2008, available at
Nature Preceedings, http://hdl.handle.net/10101/npre.2008.
2352.1, hereinafter referred to as Ye et al., online report,
2008) analyzed the observed data from oceanographic
comprehensive surveys from the Subei bank to the center of
the Yellow Sea (YS) during the bloom and postbloom
periods in 2008. They also suggested that the massive green
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Figure 1. Massive green tide bloom in Qingdao, late June 2008 (CON, online, 2008).
algal bloom that occurred in the YS in 2008 did not originate from the coastal area of Qingdao, but originated from
between 33 and 34°N along the Jiangsu coast, then drifted
to the coastal area of Qingdao by the seasonal south wind
and currents. Shi and Wang [2009] reported the cause of the
macroalgae bloom in 2008 is attributed to the eutrophication and bloom-favorable physical conditions such as sea
surface temperature, southeast winds and northward winddriven currents using a new algorithm for the detection of
macroalgae bloom. Qiao et al. [2009, 2011] also emphasized that floating and piling up of a large amount of
floating algae to the Qingdao coast and beaches in June
2008 was mainly caused by the wind-induced onshore
surface current. Leliaert et al. [2009] illustrated that the
Qingdao bloom is a typical green tide, which occurs
increasingly along several coasts worldwide. Recently, Liu
et al. [2010] reported a recurrence of the massive green
tide in 2009 based on satellites and fields images. Hu et al.
[2010] also suggested that the green tide occurred not only
in 2008 but almost every year since 2000 from April to July
based on the analysis of 10 year green tide distributions
along the Chinese coast using high-resolution satellite
images.
[4] Throughout the review of previous studies, three fundamental questions are raised here. First, is the Jiangsu coast
really the original source area of the recent algal bloom? If
so, what are their pathways and the controlling processes of
migration? Second, is the massive algal bloom in Qingdao
2008 a sporadic event or does it have the potential to occur
again? Third, during the migration of the green tide from the
source region to the massively bloomed area, was (were)
there any possible nutrient supplier(s)? To address these
questions, a three-dimensional numerical ocean model was
introduced in this study with a special focus on exploring the
physical process that controlled the recent green tide occurrence using a Lagrangian particle tracking experiment. In
order to simplify the model structure, the biological and
ecological variations of the green tide were neglected in this
model.
[5] The general features and setup of the numerical model
are described in section 2. The simulation results using the
Lagrangian particle-tracking experiment and comparisons
with the satellite images from Moderate Resolution Imaging
Spectroradiometer (MODIS) are presented in section 3.1.
The simulated ocean current patterns and the migration
processes of green tide in 2008 are described in section 3.2.
Additional experiments for the green tide movement in
2007 and 2009 are presented and analyzed in section 3.3. A
possible nutrient supply during the massive bloom is suggested in section 3.4. Possibility of reoccurrence of the
Qingdao event is addressed in section 3.5. Finally, discussion and conclusions are given in section 4.
2. Numerical Model and Setup
2.1. General Description of Model
[6] A numerical ocean model, Regional Ocean Modeling
Systems (ROMS) [Shchepetkin and McWilliams, 2005], is
used for this study. The ROMS is a three-dimensional,
free surface, primitive equation, finite-volume numerical
model based on the nonlinear terrain-following coordinate
(s coordinate) of Song and Haidvogel [1994]. The s coordinate is an extension of the classical sigma-coordinate used
in the Princeton Ocean Model [Blumberg and Mellor, 1987]
and is able to provide enhanced resolution at either the sea
surface or seafloor, which is a valuable feature for handling
steep topography in coastal oceans. In ROMS, the hydrostatic primitive equations for momentum are solved using a
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Figure 2. Model domain (blue box) and bottom topography (contour, unit: meter) with location of the
initial releasing points for the particle-tracking experiments (green dots).
split-explicit time stepping scheme, in which the barotropic
fast and baroclinic slow modes are separately advanced
in time. The horizontal diffusion is calculated using the
Smagorinsky diffusion parameterization. Details of the ROMS
computational algorithms are described by Shchepetkin and
McWilliams [2005].
2.2. Model Domain and Setup
[7] The model region covers the entire YS and some parts
of the East China Sea (ECS) with 1/20 degree (5 km)
horizontal resolution and 20 terrain-following vertical levels
(Figure 2). The bottom topography is extracted from the
work of Seo [2008].
[8] The model initialization for the spin-up is obtained
from the Northwestern Pacific Model (henceforth, NWP
model) with 1/8° horizontal resolution [Moon et al., 2010].
The NWP model is spun up for 17 years with climatological
temperature and salinity data of the World Ocean Atlas
(WOA2001) [Stephens et al., 2002]. Atmospheric forcing is
extracted from the Comprehensive Ocean-Atmosphere Data
Set (COADS) [Da Silva et al., 1994]. The COADS provides
the surface heat and freshwater flux data as well as the heat
flux sensitivity data to the sea surface temperature (SST).
The total heat flux Q is applied to the surface grid level and
formulated as in the work of Barnier et al. [1995]. In order
to introduce thermal feedback, a correction with respect to
surface temperature, where dQ/dT is derived from bulk formulas, is used [Marchesiello et al., 2001]. The present
model includes a river source to take into account the
Changjiang River discharge. The monthly discharge data is
obtained from the Datong station.
[9] Warner et al. [2005a] have incorporated a number of
turbulence closure schemes into ROMS. In this study, the
Generic Length Scale (GLS) mixing scheme of Umlauf and
Burchard [2003], in which the background diffusivity is
chosen to be 10 5 m2 s 1, is used as a vertical mixing
parameterization. The scheme is successfully applied for
coastal and estuary area [Warner et al., 2005b; Banas
et al., 2009; MacCready et al., 2009]. The barotropic and
baroclinic time steps are set to be 80 and 10 s, respectively.
At the bottom, the quadratic bottom friction law was applied.
[10] Daily mean temperature, salinity and velocity of the
NWP model based on Moon et al. [2010] are used for the
lateral boundary conditions. At the boundaries, clamped
conditions are introduced for the three-dimensional velocity,
temperature and salinity fields. Ten tidal harmonics (M2,
S2, N2, K2, K1, O1, P1, Q1, Mf, Mm) are extracted from a
global tide simulation [Egbert et al., 1994; Egbert and
Erofeeva, 2002] and are used as a surface height and depthaveraged velocity boundary condition [Chapman, 1985;
Flather, 1976]. The model was spun up with the climatology
fields for 1 model year and then run for an additional 1 year
with the Quik scatterometer (QuickSCAT) daily wind of
2008. In order to investigate the yearly variability of the algal
bloom, additional experiments were conducted for 2007 and
2009 using QuikSCAT daily wind of 2007 and 2009 after
a 1 year spin up.
2.3. Particle Tracking Experiments
[11] The particles are tracked through the process that
integrates three-dimensional velocity fields calculated from
ROMS. A fourth-order Runge-Kutta scheme is used for the
integration. As shown in Figure 2, 75 particles per day
were continuously released at the northern region of the
Changjiang River from 20 April to 10 May 2008. The released
time and place is determined in consideration of the harvesting period and place of seaweed aquaculture [Liu et al.,
2009; Hu et al., 2010]. All particles are released at the surface layer (top sigma level) and are set to migrate along the
surface layer, assuming no vertical movement.
3. Simulation Results
3.1. Comparison of Green Algae Distribution
Between Model and Satellite Images
[12] Model simulations are compared with the observed
green tide distributions, which are based on the normalized
difference vegetation index (NDVI) from the MODIS
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Figure 3. (a–d) Comparisons of observed satellite images (MODIS, NASA data distribution system,
available at http://ocean-color.gsfc.nasa.gov; adopted from Liu et al. [2009]) and (e–h) model-simulated
green tide distributions. Contours (Figures 3e–3h) denote water depth (unit: meter). Note that green dots
are the accumulated particles that are released from 20 April to 10 May 2008.
satellite images. The NDVI is the satellite-derived normalized difference of brightness values, which is related to the
relative distribution and amount for vegetation biomass. For
the NDVI processing, the first and second bands of MODIS
data with a 250 m spatial resolution were used. In this study,
the processed satellite images are obtained from the OceanColor Web (available at http://oceancolor.gsfc.nasa.gov) and
the Korea Ocean Satellite Center (KOSC, available at http://
kosc.kordi.re.kr).
[13] Simulated model results generally show a good
agreement with the observed green tide distributions
(Figure 3). On 20 May 2008, the observed green tide was
found near 34.5°N as small green patches (Figure 3a).
Below them, the dark blue color represents the suspended
sediment rich region. Both algae and sediments are suspended in water and show higher reflectances at NDVI,
which makes it difficult to detect green algae in this area [Hu
and He, 2008; Shi and Wang, 2009, 2010]. Model results
show that the particles are distributed along the Jiangsu
coastline initially and then they started to move into the
center of the YS. The distribution of the green patches at
35°N is quite well simulated in the model (Figure 3e).
[14] Ten days later, on 20 May, the green patches moved
into the center of the YS and widely spread in the YS
(Figure 3b), covering about 1,200 km2 and impacting
40,000 km2 [Liu et al., 2009]. These are the largest green
patches ever reported based on satellite image analysis in
2008. The model reproduced the large-spreading distributions realistically. Now a key question is raised. What are
the main processes controlling the generation of the massive
bloom? Are these processes related to the biological characteristics or the physical environmental factors? The
answer will be addressed in section 3.3 and 3.4.
[15] On 18 June, these green patches are distributed along
the coasts of the SP, spreading into the coast of Rizhao and
Qingdao city (Figure 3c). These patches are well replicated
by the model-tracked particles (Figure 3g). Sixteen days
later, on 6 July, these green tides moved northeastward along
the SP, as shown in both observations and model results
(Figures 3d and 3h).
[16] From late June to early July, it is reported that a large
amount of green tide moved into the coast of Qingdao
(CON, online, 2008). This is well seen in satellite images
and model simulations. In particular, the simulated results
not only capture the general pattern of the green tide
movement into the coast of Qingdao but also replicate the
location and arrival time of the green patches. This result
clearly suggests that the northern parts of the Changjiang
River near the Jiangsu Province is a possible source of the
green algae bloom in 2008 as reported in the work of Ye
et al. (online report, 2008) and Liu et al. [2009].
3.2. Spatial Distribution of Surface Currents
in the Western YS
[17] Figures 4a–4e display the spatial distributions of
the simulated 10 day mean surface currents from 1 May to
30 June 2008. The calculated current pattern of the YS
generally coincides with the previous result [Naimie et al.,
2001; Xia et al., 2006; Qiao et al., 2011]. The most prominent feature is the northward flow from the north of the
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Figure 4. (a–e) Ten day mean surface currents from 11 May to 30 June 2008 and (f) tidal residual current.
Contours denote depth (Contour interval is 5 m).
Changjiang River to the northern YS although its speed and
direction vary in region to region due to the topographic
effect and local wind distribution. In northern regions off
the Changjiang River mouth, the current flows northwestward along the coast of Jiangsu Province until they meet the
curved coastline near 34.5°N. Then, these currents diverge
into three currents which are directed northwestward,
northward and northeastward, respectively.
[18] First, the northwestward current flows along the
coastline before turning clockwise near Lianyungang. Then
it passes by the Qingdao coast and eventually moves to the
northern part of the YS. Second, the northward current is
directed almost straight forward until it encounters the
Qingdao coast and then it flows to the northern part of
the YS and finally merges with the first current. Last, the
northeastward current flows northward until it meets the
sloping bottom where the depth is deeper than 30 m and then
turns clockwise. Sometimes this current becomes strong and
flows eastward or southeastward even though the wind and
current directions are almost opposite (Figures 4b and 4c).
This is because the tidal residual current in this region
(Figure 4f) flowing southeastward at the sloping bottom as
discussed by Lee and Beardsley [1999] is strong enough to
overcome the wind-induced current.
3.3. Migration Process of Green Tide
[19] During 20–30 May 2008, the green patches grew
rapidly and extensively (Figures 3a and 3b). This suggests
that the environmental conditions were favorable for their
rapid growth. Previous studies on eight marine algal species
associated with green tides have reported that ocean temperature plays a crucial role in controlling their growth rates.
These species included Enteromorpha and Ulva in which
their origins are similar to Enteromorpha prolifera known as
a dominant species of green tide bloom in 2008 [Liu et al.,
2009]. It is also reported that the optimal temperature
ranges for their growth are between 15°C and 20°C [Taylor
et al., 2001; Largo et al., 2004]. Monthly SST from averaged daily microwave optimal interpolated data (available
at http://www.ssmi.com/sst/microwave_oi_sst_browse.html)
reveals that the temperature ranged from 13 to 16°C in May
and from 16 to 20°C in June during the growth and movement period of green patches in 2008 (Figures 5c and 5g).
This implies that the temperature in 2008 was optimum for
their growth. However, the SST distributions in other years
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Figure 5. Monthly mean SST distributions of (a–d) May and (e–h) June for 2007 (Figures 5b and 5f),
2008 (Figures 5c and 5g), 2009 (Figures 5d and 5h), and climatology data (Figures 5a and 5e). The climatology and 2007–2009 SSTs are obtained from the AVHRR data during 1985–1997 [Casey and Cornillon,
1999] and the microwave optimal interpolated data (available at http://www.ssmi.com/sst/microwave_
oi_sst_browse.html), respectively.
(2007 and 2009) including the Advanced Very High
Resolution Radiometer (AVHRR) climatology [Casey and
Cornillon, 1999] show that the 2008 was not the only
optimal year for the green algae blooming (Figure 5). This
suggests that the SST was not a controlling factor for the
massive bloom of green tide in 2008.
[20] Figure 6 compares all 10 day mean QuikSCAT winds
from 1 May to 30 June for 2007, 2008 and 2009. In May
2008, the wind directions around Jiangsu Province and the
SP exhibit almost the same direction while their directions in
2007 and 2009 vary over time (Figures 6a, 6b, and 6c). In
early June, the wind near the SP in 2008 blew strongly from
the east, while the winds in 2007 and 2009 from the south
(Figure 6d). From the middle of June, winds in both 2007
and 2008 blew northwest, while those in 2009 kept blowing
north (Figures 6e and 6f).
[21] To investigate the effect of the different wind distribution on the migration of green tides, additional experiments for 2007 and 2009 were carried out using QuikSCAT
wind field. Here, all conditions are the same as in 2008
except using 2007 and 2009 wind forcing. Figures 7b and 7d
as well as Figures 7f and 7h compare the modeled particle
distributions with satellite images, revealing that the model
simulations are in good agreement with the observation.
Here, since satellite images can be obtained in only cloudfree conditions, the comparisons between model and satellite
images are very limited and made at different times each
year. The simultaneous comparisons among 2007, 2008, and
2009 are only made using simulated particle distributions for
30 May (Figures 3f, 7a, and 7e) and 6 July (Figures 3h, 7c,
and 7g). From these figures, it is seen that the movement of
particles was different each year. On 30 May 2008, the
particles in 2008 were extended to the center of the YS,
while those in other years stayed near the Jiangsu coast.
Particularly in 2007, their extension was limited to near the
coast. These distributions can be explained by the wind
pattern. During 11–20 May (Figure 6b), the wind blew
northeast in 2008, west in 2009, and northeast in 2007 (with
fairly small wind speed) near the Jiangsu coast. It is clear
that the particles migrated according to the wind direction
since water depths in these areas were very shallow (this is
based on the Ekman theory in shallow water).
[22] During the next ten days, the wind in 2007 begins to
blow northwest or north, which allows the particles to
migrate to the center of the YS (Figure 6c). The 2009 wind
pattern during the same period was very different from other
years, that is, the wind blew south or southeast (Figure 6c).
This may prevent the particles from migrating to the north.
From the middle of June (Figures 6e and 6f), the wind
direction in 2008 and 2007 became almost the same
(southeast wind). The wind patterns possibly contributed to
the migration of the particles to the SP (Figure 7c and
Figure 3h). Actually, there were blooms in Qingdao city in
July 2007 although the amount and extension area of
the bloom was not large as in 2008 [Liu et al., 2009; CON,
online, 2007]. On the other hand, both observed green
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Figure 6. Ten day QuikSCAT mean wind speed vectors from 1 May to 30 June. Blue, red, and green
arrows indicate 2007, 2008, and 2009, respectively (Data source: http://www.ifremer.fr/cersat/en/data/
download/gridded/mwfqscat.htm).
patches and model tracked particles show that the bloom
in 2009 was transported to the center of the YS, not
approaching the SP (Figure 7g). This is because the wind
during June 2009 kept blowing north in the western YS
(Figures 6d, 6e, and 6f), which led to moving the particles
to the northeast, i.e., to the center of the YS (this is based on
the Ekman theory in deep water). This result coincides with
the previous research by Qiao et al. [2011] in the sense that
wind forcing plays a key role controlling the movement of
green tide each year.
[23] In order to analyze the detailed tracks of particles, we
selected a particle which represents an average pathway for
each year and tracked the particle for the study period
(Figure 8). For all years, the selected particles moved
northward until they encountered a sloping topography near
34.8°N. After that, the particle in 2009 moved to the northeast, to the center of the YS, whereas those in 2007 and 2008
moved in the same direction, to the north, although in 2008
it moved more straightforward and quickly than in 2007
(Figures 8a and 8b).
[24] For selected particles, zonal and meridional moving
distances were calculated at 1 day intervals and they were
compared with the wind stress components at each point
(Figure 9). In the inner shelf where the water depth is shallow
and friction is large, the particle’s movement responds
almost instantaneously to the local wind stress (Figure 9,
yellow shaded areas), that is, particles move in the same
direction as the wind. However, when they reached relatively deep water above 30 m depth, the particles do not
follow the wind any more (Figure 9, blue and red shaded
areas). This is because of the direction difference between
the wind and the Ekman flow in deeper water as well as the
dominant tidal residual current in this region as discussed in
section 3.2.
[25] Throughout the analysis of the particle tracking
information, one can see that the surface currents (Figure 9,
red dots) in the study region are affected by various factors
such as wind-induced Ekman flow, topographic effect and
tidal rectification flow. Nevertheless, it is believed that
wind forcing plays the most crucial role in determining the
particle’s distributions each year simply because the other
factors are not changing much yearly. The role of wind
forcing in determining yearly patterns of green tide will be
discussed in section 3.5.
3.4. Nutrient Supply by Upwelling
[26] The SST along the particle’s track (Figures 9g, 9h,
and 9i) tends to increase as time goes on mainly due to an
increasing heat transfer from the atmosphere, which allows
the optimal temperature to be reached for the green tide’s
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Figure 7. Distributions of accumulated model particles during (a–c) 2007 and (e–g) 2009 from 20 May
to 6 July and their comparisons with observed green patches during (d) 2007 and (h) 2009 that are estimated from satellite images (available from KOSC, at http://kosc.kordi.re.kr). Contours denote water
depth (unit: meter).
growth and germination. During the life span of each particle,
an interesting feature was found when it migrated to deeper
water. The temperature suddenly dropped at the sloping
topography and increased again as the particles migrated
farther (Figures 9g, 9h, and 9i, blue shaded areas). Considering the ordinary wind patterns during the period, this does
not seem to be related to the wind-induced upwelling or
mixing (Figures 9a–9f).
[27] A possible reason to produce this type of surface
cooling is the tidal forcing. Lü et al. [2006, 2010] suggested
two mechanisms on the tide-induced surface cooling in these
areas. First, the tidal mixing front induces the upwelling
which supplies cold water from the deep layer to the surface.
Second, tidal mixing itself can stir the bottom water upward
and homogenize the water column vertically, resulting in the
surface cooling. It was also reported that the upwelling
region was located between the Jiangsu coast and the
Figure 8. Trajectories of a selected Lagrangian particle from 30 April to 12 July of each year (a) 2007,
(b) 2008, and (c) 2009. Contour denotes water depth.
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Figure 9. Time series for (a–c) zonal and (d–f) meridional component of distance (black lines), surface
current (red dots) and wind stress (blue lines) as well as for (g–i) depth (black line) and temperature (red
line) along the particle’s track in 2007 (Figures 9a, 9d, and 9g), 2008 (Figures 9b, 9e, and 9h), and 2009
(Figures 9c, 9f, and 9i). The half full circle (Figure 9g) and full circle (Figures 9h and 9i) represent the
neap and spring tide, respectively.
western YS [see Lü et al., 2010, Figure 9a] and stronger
tides produced larger cooling. In the present model, we also
found a strong upward vertical velocity at the same place
during the largest blooming period, on 26 May 2008
(Figure 10). Although this upwelling was not strong enough
to destroy the thermocline, it is believed that the upwelling
works as a possible nutrient supplier for the green algae
bloom. In the magnitude of the upwelling, 2009 was comparable with 2008, but 2007 was much smaller than 2008
(not seen here). This is because particles in both 2008 and
2009 arrived in the upwelling region during a spring tide,
whereas those in 2007 arrived during a neap tide (Figure 9).
[28] Zhu and Iversen [1990] and Zhou et al. [2008]
reported that there is an anchovy-rich region near tidal
fronts around 120–121°E and 35°N (Figure 11), implying
that nutrients are abundant in this area. Chen [2009] also
showed a maximum phosphate distribution in that area.
These results support our hypothesis that the upwelling in
Figure 10. (a) Simulated SST distribution (white contour denotes depth) on 26 May 2008 and vertical
sections of (b) water temperature and (c) vertical velocity (10 5 ms 1) along the black line in Figure 10a
at the front of Subei bank (SB).
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June. Since the easterly wind in June is responsible for the
migration of green tide to the SP coasts, easterly winds with
more than 2 ms 1 velocity in negative x direction were
chosen (Figure 13, right). The years, 2008, 2007, 2002,
1999, and 1985 satisfied the condition for June. In particular, the easterly wind component in 2008 was the largest in
32 years. From this analysis, it is seen that 2007 and 2008
during the last 32 years are the only years satisfying the
criteria of both months, implying these years are very
unique. In 2008, particularly, the particle passed over the
upwelling region during a spring tide during the blooming
period as mentioned in section 3.4, which provided more
nutrients to algae than in 2007. The coincidence in 2008
may partly contribute to the record-breaking massive green
tide occurrence along the coast of the SP. These overall
results suggest that the 2008 event in the SP was a very rare
case.
Figure 11. Tidal front and its relation to the anchovy egg
[adopted from Zhou et al., 2008].
this region is an important nutrient source for the green tide
blooming. A clear SST image from NOAA AVHRR on
23 July 1997 also reveals that the region showing a low-SST
coincided with the tide-induced upwelling area (Figure 12).
Considering the upwelling region was a pathway of particles
during three years (Figure 12), the tide-induced upwelling
seems to contribute to the recent massive algae blooms as a
nutrient supply source. In particular, it is found that the
effects in 2008 and 2009 were significant because the
upwelling occurred during a spring tide.
3.5. Possibility of Reoccurrence
[29] It is reported that the massive green tide around
Qingdao in 2008 was one of the largest bloomings in
recorded history [Hu and He, 2008; Liu et al., 2009].
Although the green tide was also observed in Qingdao in
2007, its size and amount was much smaller than in 2008
(CON, online, 2007). This may be partly attributed to a
dramatic increase in cultured seaweed along the coastal area
of Jiangsu Province in 2008 as reported by Hu et al. [2010].
Actually, 2007 was the first year for the seaweed expansion
in the Jiangsu coast and so the initial biomass of green algae
in 2007 was much lower than 2008 [Liu et al., 2010].
However, as we discussed above, it is more likely believed
that wind-forcing is a crucial factor determining the migration of the green tide to the SP. Then, what types of wind in
2008 contributed to migration of the green tide to the
Qingdao region? How unique was the wind in 2008? To
address these questions, the monthly mean wind data from
the National Centers for Environmental Prediction (NCEP)
are analyzed for 32 years (1980–2011). Figure 13 reveals the
30 year wind vectors in May and June, which are averaged
for the western YS (34–37°N, 119–123°E).
[30] Since the southerly wind in May is responsible for the
migration of green tide to the north along the Jiangsu coast,
as described in section 3.3, southerly winds with more than
2 ms 1 velocity in y direction were chosen for the last
32 years and marked with colors (Figure 13, left). As a result,
a total of 7 years including 2008 are selected based on the
criterion. A similar analysis has been done for winds in
4. Discussion and Conclusion
[31] The present research was conducted to investigate
physical factors that controlled the occurrence of the massive green tide along the southern coast of the Shandong
Peninsula (SP) in 2008 using a Lagrangian-particle-tracking
experiment by ROMS (Regional Ocean Modeling System).
For the comparison, 75 particles per day for a total of 3 years
(2007–2009) were continuously released along the coast of
Jiangsu Province, a possible origin of the green tide, from
25 April to 10 May. The results show that although the
present model could not consider the biological processes
such as germination, sinking and decaying, it reproduced
yearly distributions of the observed green patches with a
high accuracy. Based on the simulation results, the following important conclusions were made in this study.
[32] First, the simulated trajectory of particles revealed
that the Jiangsu coast is a possible origin of the recent green
tide blooming as suggested by Ye et al. (online report, 2008)
and Liu et al. [2009]. Second, the wind pattern plays a
Figure 12. SST distribution on 23 July 1997 from NOAA
pathfinder AVHRR with model-tracked particles of each
year.
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LEE ET AL.: THE MASSIVE GREEN TIDE IN 2008
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Figure 13. Monthly averaged wind vectors within the study area (34–37°N, 119–123°E) in (left) May
and (right) June during 1980–2011. Winds larger than 2 ms 1 are plotted with color arrows. Unit is ms 1.
crucial role in determining the spatial and temporal distributions of the green tide and particularly in migrating
blooms to the SP. In May, a southerly wind was responsible
for the offshore movement of green patches and in June an
easterly wind was responsible for their migrations to the
southern coast of the SP in 2008. A long-term wind analysis
showed that the wind pattern in 2008 seemed to be a very
unique and favorable for the migration to the SP throughout
32 years. Third, the analysis of SST data for May and June
revealed that the temperatures during 3 years were all warm
enough for the green tide blooming. It means the SST, which
was previously suggested as a main factor for the blooming
and germinating, was likely not a critical factor in generating
the 2008 massive green tide. Fourthly, an upwelling region
between the Subei bank and the center of the YS seems to
provide nutrients for the green tide blooming. Since the
upwelling is known to be produced by a tidal forcing [Lü
et al., 2010], it is believed that the timing of the green tide
passing over the upwelling region, whether it is neap or
spring tide, is also a crucial factor determining the magnitude of the blooming. In 2008, it is revealed that the green
patches went through the upwelling region during the spring
tides, which seems to provide more favorable conditions for
the blooming with a sufficient nutrient supply.
[33] To sum up, the massive green tides along the southern
coast of the SP in 2008 resulted from the combination of a
recent rapid expansion of seaweed aquaculture, unique wind
patterns, and nutrient supplies due to a strong tidal forcing in
blooming regions. In particular, the roles of wind and tidal
forcing are likely to be essential not only in understanding
and predicting the migration of green patches but also finding a possible nutrient source for the blooming. The occurrence of the massive green tide along the SP was a very
infrequent and unusual event based on the analysis of the
32 year wind data. However, with growing seaweed aquaculture and the resultant increase of impact areas and biomass since 2008 [Liu et al., 2010], the record-breaking
green tide in the YS could occur more frequently in the
future. It should be also noticed that the biological and
ecological processes of the green tide blooming were
neglected in this model. Some parts which could not be
fully explained only by physical processes are probably
related with them, which remain not clear enough.
[34] Acknowledgments. This work is supported by the KORDI
Projects, PM51500, “Construction of Ocean Research Station and their
Application Studies” and “Support for Research and Applications of Geostationary Ocean Color Imager (GOCI),” funded by the Ministry of Land
Transport and Maritime Affairs in Korea and the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research
Promotion Fund) (KRF-2007-331-C00255).
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J. H. Lee, I.-J. Moon (corresponding author), and I.-C. Pang, College of
Ocean Science, Ocean and Environment Research Institute, Jeju National
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kr)
J.-H. Ryu, Korea Ocean Satellite Center, Korea Research and
Development Institute, Ansan, 426-744 South Korea.
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