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Summer wind gusts modulate transport through a narrow strait, Bohai, China
Estuarine, Coastal and Shelf Science 233 (2020) 106526
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Summer wind gusts modulate transport through a narrow strait, Bohai, China Haiqin Duan a, Chenghao Wang a, Zhiqiang Liu b, Houjie Wang a, c, Xiao Wu a, c, Jingping Xu b, c, * a
College of Marine Geosciences, Key Laboratory of Submarine Geosciences and Prospecting Technology, Ocean University of China, 238 Songling Road, Qingdao, 266100, China b Department of Ocean Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China c Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology (QNLM), Qingdao, 266061, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bohai strait Wind gust Exchange flow Volume transport
The hydrological and hydrodynamic response of Bohai Strait to an intense wind gust in the summer of 2016 was synthesized based on a 9-day in-situ observation at two mooring stations across the strait. At the north station, the sea level abnormally rose by 0.4 m during the wind gust. Strong subtidal inflow (into Bohai Sea) was observed during the gust, then changed to outflow after the gust subsided. At the southern station, the hydro graphic property of water column changed rapidly with a strong outflow after the gust subsided. The Regional Ocean Modeling System model was employed to reconstruct the dynamic processes governing the event. It was found that wind-triggered sea level fluctuation caused the modulation of barotropic pressure gradient force, and thereby altered the exchange flow through the strait. Statistical analysis showed that a wind-triggered quasibarotropic mode (controlled by sea level variation) contributed nearly 47% of the total variance. This suggests that during summer seasons that are normally dominated by baroclinic processes, the barotropic processes related to intermittent wind gusts also contribute considerably to the exchange flow and volume transport through the Bohai Strait.
1. Introduction A strait is a natural channel that connects two large bodies of waters between which water and particle material are exchanged. Such ex changes of material and energy play a critical role in the determining redistribution of heat, salt, pollutants and sediment mass, and biogeo chemical cycles (Hoshika et al., 1999; Malikides et al., 1989; Tian et al., 2006). For those reasons, studies have been carried out for numerous straits around the globe, for instance, Gibraltar (Baschek et al., 2001), Otranto (Ferentinos and Kastanos, 1988; Ursella et al., 2012) and Tsushima Straits (Takikawa and Yoon, 2005). In-situ measurements by subsurface moorings and various numerical models are used to quantify the volume transport through those straits. The Bohai Strait (BHS) between Shandong and Liaodong peninsula is the only pathway connecting two epeiric seas in the northern China seas, the Bohai Sea (BH) and the North Yellow Sea (NYS) (Fig. 1). The ~104 km wide BHS is the shallowest in the southern end, deepens northward with a channel as deep as 70 m offshore Liaoning Peninsula. (Bi et al., 2011; Cheng et al., 2004; Fig. 1(a)). BH is a semi-enclosed shallow (<18
m) marginal sea that positions to the west of BHS (Qin and Li, 1983). Its water depth increases eastward towards central NYS. The hydrographic properties of waters in BH are concurrently influenced by the riverine freshwater influx mainly from Yellow River and the oceanic waters intruding from Yellow Sea (YS) (Zhang et al., 1994; Ren and Shi, 1986; Wang et al., 2010). Yellow River is the world’s 2nd largest river in terms of sediment discharge: 1.2 to 1.3 Gt/yr, which shapes the shelf-sea sedimentary processes around the BH (Lee et al., 2009; Milliman and Meade, 1983) and interpreted as the primary source to several mud deposits in NYS (Yang et al., 2002). The exchange flow in BHS plays a critical role in the evolution of marine environment of the regional and global ecosystems (Ding et al., 2019; Wang et al., 2014). It has long been recognized that the mixture of these waters critically determines not only the general circulation, but also the associated sediment distribu tion and biogeochemical processes (Bi et al., 2011; Wang et al., 2014). Historic simulations and observations suggest that the exchange flow in BHS is primarily composed of an extensive inflow in the north and an outflow in the south of the strait and the peak water transport through the BHS usually occurs in summer (Lu et al., 2011; Lin et al., 2002;
* Corresponding author. Department of Ocean Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China E-mail address: [email protected] (J. Xu). https://doi.org/10.1016/j.ecss.2019.106526 Received 27 June 2019; Received in revised form 16 September 2019; Accepted 4 December 2019 Available online 14 December 2019 0272-7714/© 2019 Elsevier Ltd. All rights reserved.
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characteristics in BHS responded to a summer gust by coupling in-situ time-series data and numerical models. Besides, a statistical method was employed to discuss the contribution of wind-affected transport to the volume transport through the BHS in summer.
Zhang et al., 2018). The winter circulations in BH and YS Sea are mainly driven by the intense winter monsoon, while the circulations in summer when the wind is much weaker and heat flux is much stronger are maintained by the tilting thermohaline (Zhao and Shi, 1993; Wei et al., 2003; Wan et al., 2004; Lin et al., 2002). Circulations from these pre vious simulations are usually driven by the monthly or seasonally averaged wind stress, which is feasible for revealing the seasonally averaged flow pattern without sufficiently resolving the variabilities imposed by synoptic events (Ding et al., 2019; Wu et al., 2016). Recent studies that mainly focused on the effect of winter storm events showed that synoptic-scale events have a greater impact on the circulation and exchange flow in the Bohai Strait (Yuan et al., 2008; Qu et al., 2017; Ding et al., 2019). However, the response of exchange flow and circu lation in BHS to synoptic variability of the summer monsoon is still not well understood. In summer, a cold water mass with high salinity pre sent in the middle of the YS invades the BH along the Laotieshan Channel in the northern BHS (He et al., 1959; Lin et al., 2002). The baroclinic circulation is characterized by a cyclonic circulation around this cold water mass. The inflow is strong and mainly confined to the narrow northern passage, while the outflow is broad and weak in the rest of the strait (Huang et al., 1999; Zhou et al., 2017). Huang et al. (1999) and Zhou et al. (2017) analyzed the effects of different forces (tide, varying wind and baroclinicity) on the summer circulation and found that the stratification and wind were the main forces on the circulation in BH, and inferred that the varying wind forces might change the in tensity of the exchange flow though the BHS. However, what role the summer wind gusts play and how they affect the exchange flow and circulation remain unclear. This study analyzes the variation of hydrological and hydrodynamic
2. Data and method 2.1. Field measurements In this study, both moored time-series and ship-based hydrographic surveys (for spatial coverages) are used to characterize the hydrological properties and hydrodynamic processes related to exchange flows and material transport through the strait. All data were collected on the R/V DongFangHong2 during the cruise from June 28 to July 22, 2016. Two key stations were located in the south (T01, 30m) and south sides (T02, 50m) of the BHS, respectively. The R/V DongFangHong2 was first anchored at the southern site T01 for more than 3 days (July 15, 8:00 to July 18, 12:00). The wind 10 m above the sea surface was recorded by the meteorological instrument onboard with the measurement interval of 10 s. A 75 kHz shipborne Acoustic Doppler Current Profiler (ADCP) downward-looking at T01 recorded the current velocities (u, v) once every 5 min. Considering both the waterline and blanking distance of ADCP, the first bin was about 16 m. The hydrological parameters were measured by the conductivity/ temperature/pressure (CTD) sensor. Water samples were collected with CTD cast once every 2 h at 4 water depths (4, 9, 16, 28 m; Table 1). The 4 depths chosen represent the surface, the subsurface, the middle and the bottom layers respectively. The ship was then moved to and anchored at the northern site T02, to collect 9 hourly CTD casts.
Fig. 1. (a) A local bathymetric map showing the location of the observation sites (red stars). The red solid line is the transect across BHS. The dashed contours in Fig. 1(a) are the thickness of the Holocene mud in NYS (Yang and Liu, 2007); (b) A schematic drawing showing the instrument placements at the two sites (T01, T02). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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Table 1 Parameters and instrument settings. Stations
Observation Method
Data
Measurement Period
Interval
Depth (m)
Sampling Levels/Vertical Bin size (m)
Blind zone (m)
T01
Anchor station
SSC Salinity, Temperature, pressure Velocity (u, v)
15 July 08:00 to 18 July 12:00
2h
Profile
4/-
–
15 July 08:00 to 18 July 12:00 18 July 18:00 to 19 July 02:00
5 min
8
1/8
16.54
1h
Profile
3/-
–
11 July 16:00 12:00 11 July 16:00 12:00 11 July 16:00 12:00 11 July 16:00 12:00
to 19 July
3s
53
1/-
–
to 19 July
1 min
53
24/2
4.23
to 19 July
3s
26,34
2/-
–
to 19 July
3s
16,24, 32,40
4/-
–
T02
Anchor station Bottom Mount
Subsurface Mooring
SSC, Salinity, Temperature, pressure Salinity, Temperature, pressure Velocity (u, v) Turbidity Salinity Temperature
A subsurface mooring and an instrumented bottom mount were deployed at station T02 to measure flows and hydrological parameters for 9 days. A 300 kHz ADCP on the bottom mounted system facing up ward recorded the current velocities (u, v) once every minute with a vertical resolution of 2 m. Considering both the height and blanking distance of ADCP, the first layer was about 5 m above the seafloor. In addition, a CTD was about 1 m above the bed. Next to the bottom mount was a subsurface mooring that hosted a CTD sensor at 16 m, two turbidity sensors at 26 m and 34 m respectively and three conductivity/ temperature (CT) sensors at 24, 32 and 40 m respectively. The sampling intervals for the CTD, turbidity sensors and CT sensors were 10, 3, and 1 s respectively (Fig. 1(b)). Due to insufficient buoyancy of float balls, the bobbing amplitude of the instruments on the mooring were over influ enced by the tide, thus the data of this mooring was excluded.
data with Orlanski-type radiation condition for active tracers and bar oclinic velocities. Eight principle tidal constituents (M2, S2, N2, K2, K1, O1, P1, and Q1) from TPXO8 tidal model were prescribed along the eastern and southern open boundaries by using Flather-typed radiation condition (Flather, 1976). High spatial (0.125� � 0.125� ) and temporal (3 h) resolution atmospheric data such as surface wind, evaporation, precipitation, air temperature and radiation flux were from the Euro pean Centre for Medium-range Weather Forecasts (ECMWF) and was applied to the model to capture episodic events. The calculation was based on bulk formulas (Fairall et al., 1996). The water level and vertically averaged velocities from observation were used to validate the simulation with the coefficient of determina tion (R2). It was shown that the modeled water level matched the ob servations fairly well (R2 ¼ 0.97) (Fig. 2(a)). According to harmonic analysis, the principle tidal constituents were M2 and K1. The absolute errors between the simulated and observed M2 amplitude were 0.07 m, and amplitude differences for K1 between simulation and observations were 0.01 m. The discrepancy from observations in phase lag of M2 and K1 were 9 and 3 min, respectively. Theses comparison evidenced that the tidal current was well simulated (Fig. 2(c)), and the simulated velocity magnitude was also consistent with the observation (R2 ¼ 0.75). Observed and simulated velocities also well agreed with each other in the east-west direction, which represents of the orientation of the ex change flow (Fig. 2(b)). Furthermore, the simulated sea surface tem perature (SST) was comparable with that retrieved from remote sensing data (MODIS) in June, July and August (Figs. 3(a) to 3(f)). The temporal variation of SST was also well reproduced in our simulation. Fig. 3(g) showed the consistency between observed and simulated SST with R2 ¼ 0.89. Thus, we conclude that the simulations not only reproduced the
2.2. ROMS modeling This study uses the Regional Ocean Modeling System (ROMS) to simulate the regional hydrodynamic processes during the cruise period. The computational domain covered the region between 117� E and 127.5� E, 31� N and 41� N with horizontal resolution 1/30� . The bottom topography was obtained from the ETOPO1 Global Relief Model. To better resolve the currents in the bottom and surface boundary layers, we vertically discretized the water column into 20 sigma levels using the s-coordinate with θs ¼ 4 and θb ¼ 0.8 (Song and Haidvogel, 1994). Three dimensional temperature and salinity data from the Hybrid Coordinate Ocean Model (HYCOM) with resolution 1/12� were interpolated to ROMS grid as initial conditions. Meanwhile, daily averaged tempera ture, salinity and velocities were imposed as the open boundaries forcing
Fig. 2. Comparison of the (a) water level, (b) current magnitude and direction of in-situ observed and model simulated bathymetry averaged current in T02 during the observation period; (c) The harmonic analysis results of water level and the error between the observation and simulation. 3
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Fig. 3. Comparison of the simulated and satellite remote sensing retrieved monthly SST in June ((a) and (b)), July ((c) and (d)) and August ((e) and (f)) 2016 respectively; (g) The correlation between MODIS and ROMS SST. The SST datasets are retrieved from the MODIS of NASA, available at https://oceandata.sci.gsfc.na sa.gov/.
observed velocity field, including both barotropic and baroclinic com ponents, but also captured the responses in hydrographic properties, especially the temperature field.
north and south of BHS, we used the buoyancy frequency N2 . The buoyancy frequency is usually used to describe the static stability of water column, which is associated with the density change with depth. One common buoyancy frequency calculation method is:
3. Results
N2 ¼
3.1. The observed hydrological characteristics in Bohai Strait
g dρ ρ0 dz
(1)
where g is the gravitational acceleration, ρ0 is the average density value, and z is the vertical coordinate. N2 was overall positive during the observation, indicating that the water column had a stable stratified structure which should be typical in summer. The hydrological properties between the north and south of BHS were different. The maximum value of N2 of both sides were 6–12 m (position of pycnocline) and the north side was stronger than the south by 2–4 times. Below the pycnocline, N2 tended to be 0 and the properties were relatively uniform (Fig. 4(c)). The density in the north was significantly higher than that in the south. It shows there was a strong meridional-direction baroclinic gradient between the north and south of BHS (Fig. 4(b)).
At station T01, water temperature decreased with depth, ranging between 10 and 24 � C. The salinity decreased slightly, ranging from 31.8 to 30.5 PSU (Fig. 4(a)). At station T02, water temperature varied be tween 6 and 26 � C. A significant temperature drop appeared at about 10 m depth. Below 10 m, there was an extensive and relatively uniform cold water mass averaged at only 7 � C (Fig. 4(a)), which could be considered as the NYS cold water mass (He et al., 1959; Guan, 1963). The salinity varied slightly with time, ranging from 31.2 to 32.3 PSU, which was a little higher than T01 (Fig. 4(a)). The above comparison shows that the difference in temperature and salinity between north and south of BHS was small in the surface layer. While the temperature below 10 m depth decreased from south to north with an increase of salinity. To further explore the differences in hydrological properties between
Fig. 4. Vertical distribution of (a) temperature, salinity, (b) density (ρ0) and (c) buoyancy frequency. The data are from hourly CTD cast at the two sites. 4
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3.2. Wind gust and response of hydrological characteristics in Southern Bohai Strait
with the wind subsided, accompanied by rapid change of temperature and salinity as mentioned previously (Fig. 5(c)). The v component of currents along the transect of BHS had no obvious relation with the wind gust, but showed a weak southward flow during most of the observation period. In northern BHS, the residual sea level is relatively stable except for the wind gust (Fig. 7). An abnormal sea level rise with the maximum of 0.4 m was recorded during July 15 to July 17. In terms of temporal scale, the abnormal variation of sea level was consistent with the wind gust that sea level continued to rise as the wind got stronger, and dropped as the wind weakened. The u component of subtidal current had a similar response. During the sea level rise, the residual current flowed into BH vertical uniformly. When the sea level declined, a strong flow gushed out of BH and the residual current reached its peak. However, the v component showed no obvious relation with sea level variation. A ver tical two-layer structure residual current was obvious during the observation period. The upper layer showed a strong southward flow, while the bottom layer showed a weak northward flow (Fig. 7(b) and (c)). According to the above observations, the abnormal sea level rise and the variation of exchange flow through BHS were linked with the wind gust and cause the hydrological characteristics variation. Besides, the wind gust mainly affected the cross-transect (mainly in the meridional direction) flow rather than the along-transect (mainly in the zonal di rection) flow.
In summer, weak southerly wind prevailed in BH and YS (Wang et al., 2014). But an abnormal northeasterly wind gust was recorded from July 15, 18:00 to July 16, 18:00 with a maximum speed over 12 m/s (Fig. 5(a)). Records showed that the salinity decreased from 31.7 to 31.5 PSU and the temperature increased from 11.5 to 14 � C in the bot tom layer of T01 from July 15 to 18. This was a rapid transition within a short period of time after the wind gust weakened (Fig. 5(b)). This transition can also be seen in the T-S diagram. Prior to the northeasterly wind, the water mass of this area was mainly cold and salty water; after the wind gust, this area was occupied by warm and fresh water (Fig. 6). This water mass properties variation was linked with the wind gust. 3.3. Response of hydrodynamic characteristics to the wind gust in Southern and Northern Bohai Stait To better demonstrate how hydrodynamic characteristics responded to the wind gust, this study only analyzed subtidal currents. The following processing methods were used: first, we used the T_Tide (Pawlowicz et al., 2002) tool box to predict the time series tidal current during the observation period. Then, the predicted tidal currents were subtracted from the raw current field (u and v) to remove the tidal component. Lastly, the residual currents were smoothed by moving average with a 1-h window to filtering the high frequency variability. The residual currents were considered as the subtidal currents. At T02, we used the bottom pressure record to retrieve the sea level height and then processed by the same method to obtain the residual sea level height. In southern BHS, a slight eastward flow occurred in the bottom layer during the wind. Subsequently, a strong eastward flow came out of BH
3.4. Regional subtidal circulation response to the wind gust recreated by ROMS Our collected data only gives us a glimpse into the local wind gust process. The existing evidence shows us part of the process, but the full picture is still unclear. Thus, we used ROMS model to offer the regional
Fig. 5. (a) Wind data of 10 m above the surface at anchor station T01; (b) Temperature, salinity variation in bottom layer (2 m above bed) of T01; (c) Current velocity of bottom layer at T01. The solid black line is the u component of residual velocity. 5
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Fig. 6. T-S diagram of CTD data acquired at T01 during the observation period.
Fig. 7. Time-series of (a) water level and residual velocity profiles of (b) u and (c) v component at site T02.
daily averaged sea level and current field during the wind gust (July 15 to July 17, 2016). On July 15, BH and YS were dominated by a weak northerly wind of lower than 5 m/s in speed, and the sea level varied within 0.2 m. The surface current field was a weak northwestward flow, forming a coun terclockwise circulation in NYS and a clockwise circulation in the mid dle of BH. The bottom velocities presented a weak northward flow in the South Yellow Sea (SYS) and an eastward flow in BH (Fig. 8(a)). During the wind gust (July 16), a regional cyclone with its center over Shandong peninsula strengthened gradually from north to south. BH was domi nated by a weak northeasterly wind which was the same as July 15, while NYS presented a strong southeasterly wind with a maximum speed over 10 m/s and the SYS was affected by an intense southerly wind with a maximum speed over 13 m/s. The sea level rapidly rose over 0.4 m, especially in BH. Surface current corresponded well with the cyclone.
On the east side of SYS, a strong northward flow extended to NYS and entered into BH through BHS. The bottom flow was divided into two branches on the east side of SYS. One stronger branch ran along the northern coast of Shandong peninsula and the coast of Subei towards the south, forming a counterclockwise circulation, and the other weaker branch extended to NYS and entered into BH. As the cyclone moved northeast, the weakened northerly wind dominated the region. Only in parts of SYS the wind speed exceeded 10 m/s. Meanwhile, water piled up in the west coast of BH and the coast of Shandong peninsula and Subei, while the low sea level began to show in the east side of NYS and SYS. A strong outflow through BHS bypassed Shandong peninsula and propa gated southward (Fig. 8(b) and (c)). According to the simulation, the regional sea level distribution and circulation will reconstruct due to the summer wind gusts, especially the exchange flow in BHS. Besides, although the local wind of BH was steady 6
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Fig. 8. The spatial patterns of (a) the ECWMF wind field and sea level, (b) surface and (c) bottom current field produced by ROMS for the period of July 15 to 17. Collectively they show the processes of water exchange between BH and YS during the wind gust.
during the observation, the sea level height and circulation still changed due to the wind of NYS and SYS. Therefore, the effect of regional wind, including the SYS wind field, cannot be ignored when studying the ex change flow in BHS. Next, we will focus on the mechanism of the wind gusts effect over exchange flow and the role of the wind gusts in the volume transport through BHS in summer.
direction respectively; ρ0 represents the reference density; ρ is the water density other terms in Eqs. (2) and (3) are: acceleration (ACCEL); Coriolis force (COR); pressure gradient force (PGF), barotropic and baroclinic PGF included; horizontal and vertical nonlinear advection (HADV þ VADV); and the horizontal and vertical viscous term (HVISC þ VVISC), in which HVISC is represented by Fx and Fy , and Kv stand for the vertical viscosity coefficient. In this study, the focus is on the geostrophic balance terms (PGF and COR) since the other terms are one or two orders of magnitude lower. Regional horizontal distribution of SSTR, and vertical averaged PGF and COR in x and y directions during the wind gust period are shown in Fig. 9. During the observation period, the zonal current (u component) through BHS (focus of this research) was dominated by geostrophic balance in the meridional direction, which is indicated by the notation y in equations (2) and (3). On July 16, the wind stress blew northwest ward, and under direct wind stress and Ekman transport, sea surface height gradually piled up to NYS and BH, forming a gentle sea level slope. Then a geostrophic inflow through BHS (represented by positive CORy ) was triggered and was balanced by PGFy . As the wind continued, sea surface height slope gradually became steeper and PGF in x=y direction gradually turned to positive/negative. After the wind weakened, the intense PGF was released, and there was a strong outflow through BHS along the northern coast of Shandong peninsula under the balance of the opposite COR. Previous results suggest that the wind gust mainly affects barotropic PGF (PGFT ) by influencing sea level dis tribution, while the baroclinic PGF (PGFC ) is mainly controlled by the hydrographic properties of waters (Guo, 1994). To better distinguish the impact of surface elevation and baroclinicity of the waters, we decom posed the term PGF into PGFT and PGFC , as shown in Fig. 9(d) and (e). We observed that the variation of PGF during the wind gust was mainly the modulated process of PGFT , whereas the PGFC was rela tively stable and far smaller than the PGFT . This was because the modulation rate of the density field was much slower than the sea sur face height field. In summer, the relatively weak monsoon only affects the surface layer circulation (Zhao and Shi, 1993), and the intense vertical
4. Discussion 4.1. The momentum balance After ROMS demonstrated the regional hydrodynamic processes during the wind gust, the embedded dynamic mechanism was further investigated using the terms balances in depth-independent momentum equations, which showed the possible governing dynamic characteris tics of these processes (Gan et al., 2009; Signorini et al., 1997). The momentum equation in Cartesian coordinates is written as: ACCEL
z}|{ COR z}|{ ∂u ¼ fv ∂t
HADVþVADV
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂u ∂u ∂u u þv þw ∂x ∂y ∂z
PGF
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflZfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂η g ∂ g þ ρdz ∂x ρ0 ∂x
HVISCþVVISC
zfflfflfflfflfflfflfflfflfflffl }|fflfflfflfflfflfflfflfflfflffl { 0 1 ∂u ∂ K v ∂z C B þ @Fx þ A ∂z ACCEL
z}|{ COR z}|{ ∂v ¼ fu ∂t
(2)
PGF
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflZfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂η g ∂ g þ ρdz ∂y ρ0 ∂y
HADVþVADV
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂v ∂v ∂v u þv þw ∂x ∂y ∂z
HVISCþVVISC
zfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflffl{ 0 1 ∂v ∂ K v ∂zC B þ @Fy þ A ∂z
(3)
Subscripts x and y denote the momentum in east-west and north-south 7
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Fig. 9. Terms of the depth-averaged momentum equations ((a), (b) and (c)) (Eq. (3)) and the PGFT and the PGFC ((d) and (e)) in zonal direction (left panels) and meridional direction (right panels) respectively. Positive values represent the eastward in the left panels and the northward in the right panels. All the variables are daily averaged.
stratification plays an important role in water exchange through BHS with the baroclinic pattern (Wang et al., 2010). However, there are still intermittently wind gusts events like in this study that may last for many days. Such intense wind gusts would trigger regional sea surface height variation, thereby causing the PGFT field modulation. During the gusts, the barotropic current would dominate the water exchange processes through BHS. In addition to enhancing the wind-driven currents, we also found that the strong wind burst modulates the barotropic pressure gradient field in the Bohai Sea and Yellow Sea even after the wind subsided, a result that has not been seen in previous studies.
remote wind field over the neighboring seas. In addition, the residence time in BH under the normal summer condition was about 400 days which was consistent with the previous studies (Liu et al., 2012, 2017). In contrast, it was only 138 days during the wind gusts, a 65% reduction. 4.2.2. The barotropic processes extracted from the subtidal currents by EOF method To further understand the properties of the subtidal currents through BHS, a conventional Empirical Orthogonal Function (EOF) method was applied to analyze the observed and simulated subtidal flows respec tively (for further details see Emery and Thomson, 2001). For the wind gust process in T02, the first several modes of subtidal current were separated from the others. According to the method pro posed by North et al. (1982), the first six modes were effective in 95% confidence interval. The greatest EOF mode of u component accounted for more than 74% of the total current and the rest of the modes accounted for 26% (Fig. 11(a) and (b)). The time series of the first and the rest of the current profiles were obtained from the EOF modes and their corresponding principal component (PC). And the parameter of PC indicates the proportion of the EOF mode in the raw data in different time. The first mode of the cross-transect current mainly showed a uniform distribution in the water column. Meanwhile, there was a sig nificant negative correlation between the first mode PC and the sea level variation rate ðdη =dtÞ (Fig. 11(c)). This suggested that the sea level variation was the main controlling factor of the first mode of the cur rents in BHS. Thus the first mode of current profile was considered as quasi-barotropic mode according to the definition of barotropic currents (Guo, 1994). The rest of u component modes presented a layered structure in the water column with upper layers dominated by the outflow and lower layers dominated by the inflow, and we defined it as the baroclinic mode. BHS is oriented approximately north-southward and the zonal ve
4.2. The wind-triggered barotropic process through the Bohai Strait in summer 4.2.1. Summer wind characteristics and its corresponding sea level variation Previous results give us a providential observed wind gust event in summer and analysis of its process and mechanism. We focus more on the role of wind gusts and their contribution to the volume transport through BHS throughout summer. The regional averaged wind field data of BH, NYS and SYS in summer (June, July and August) 2016 were shown in Fig. 10 (a), (b) and 10(c). In summer, although it is mainly weak southerly wind that prevailed the BH and YS, intense wind gusts still episodically occurred with about nine events during which the regional averaged wind speed exceeds 10 m/s (red sticks in Fig. 10). By comparing the regional averaged sea level height in BH, NYS and SYS simulated by ROMS (the histograms in Fig. 10(d)), the strong wind events (the red line in Fig. 10(d)) were often accompanied by pro nounced fluctuations of sea level height. Such concomitant relationship between wind fields and sea level showed that sea level rose during intense southerly wind while fell during intense northerly wind. Meanwhile, sea level variation was affected by both the local wind and 8
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Fig. 10. Temporal variation of region-averaged wind fields in (a) BH, (b) NYS and (c) SYS in summer season (June, July and August 2016) obtained from ECWMF mentioned in the text. Wind speed exceeding 10 m/s are in red; (d) The histograms indicate the regional averaged sea level height of BH, NYS and SYS respectively during 2016 summer. The red line indicates the regional averaged wind velocity in meridional direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 11. EOF mode 1 (a) and other modes (b) of the u component of residual current in T02, with the gray solid line representing the corresponding PC; (c) The scatter plot of the sea level variance rate and the PC of the first mode of u component.
locity is much larger than the meridional velocity, therefore only the u component was considered when calculating the net volume transport across BHS during the observation. Based on the current velocity, the unit width water flux (WF) of different modes were calculated as follow: PT Pn t¼1 i¼1 Hit ⋅uit WFu ¼ (4) T
the surface layer; u represents the u components of the current velocity; t is the interval between two continuous sampling casts. The unit width net WF of baroclinic mode ( 0.23 � 10 7 Sv) due to the stratified structure was negligible compared to the barotropic mode (2.63 � 10 7 Sv) (Table 2). Therefore, although the baroclinic mode influences the water exchange between BH and NYS, the barotropic mode is the main controlling factor of the net water volume transport during the obser vation period. We applied EOF analysis to the daily averaged u component of cur rent profiles of BHS (transect a-a’ in Fig. 1(a)) in summer, 2016. The first
Where H is the thickness area of each layer, which depends on the water depth and layer division; i is the layer number, i ¼ 1 indicating 9
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transport processes would be enhanced during gusty weather as the WFT. We further analyzed the net WF through BHS in June, July and August by separating the barotropic and baroclinic modes. It is found that the WFT accounted for a considerable proportion of the total WF (Table 2). Meanwhile, due to the contingency of gusts, the WFT varied greatly among different months. We proposed the importance of barotropic currents in impacting the exchange flow in BHS based on EOF analysis and the previously pro posed dynamic analysis with both observation and simulation. And the results were consistent. We also found that the barotropic exchange flow was determined by the intermittently wind gusts in summer. It is concluded that although previous studies have proposed the importance of the thermohaline circulation in summer (Huang et al., 1999; Wei et al., 2003) as also shown in our previous analyses, the contribution of wind-triggered barotropic processes have a considerable impact on the water exchanges and volume transport through BHS.
Table 2 Net water flux of barotropic and baroclinic modes in summer 2016. Periods
Water Flux (10 2⋅Sv) (þ/ : Out/in) Barotropic
Observed data Simulated data
observation June July August
2.63 � 10 1.55 1.72 2.87
Baroclinic 5
0.23 � 10 1.74 3.00 0.17
5
mode of current field was approximately uniform in the water column, with the simultaneous entry or exit pattern and a gradually decreasing velocity from west to east (not shown here). According to the previous analysis, the barotropic exchange processes through BHS was mainly dominated by the meridional PGFT . We find a remarkable positive cor relation (R2 ¼ 0.75) between the PC of the first mode and the sea level difference between southern ðηS Þ and northern (ηN Þ BHS (Fig. 12(a)). This finding suggests that the first mode of the cross-transect current was also governed by the wind-triggered modulation of sea level in the scope of quasi-barotropic geostrophic current. The contribution of barotropic mode to fluctuation of the exchange flow was nearly 47%. The baro clinic mode was obtained by subtracting the barotropic mode from the raw current profiles. Fig. 12(b) showed the daily averaged barotropic and baroclinic exchange flux, respectively. The baroclinic exchange flow was relative stable with an exchange flux of ~0.12 Sv. The variations of barotropic exchange flow were consistent with that of the wind field (for example, wind gusts). It was shown that the relatively stable baroclinic exchange flow dominated the structure of the water exchange in the water column, and the barotropic exchange could be enhanced to exceed the intensity of the baroclinic circulation and dominate the exchange flow during the periods of wind gusts. The averaged exchange flux of barotropic processes was 0.07 Sv in summer, accounting for 36.8% of the total exchange fluxes. This indicated that besides the wind gusts, the wind-triggered barotropic processes also played a non-negligible role in impacting the exchange flow in the BHS in summer. Moreover, the net WF from barotropic current (WFT) and baroclinic current (WFC) showed in Fig. 12(c). Due to the two-layer structure, the WFC was small, and the
5. Conclusions A wind gust event was captured by hydrodynamic and hydrographic observation at two stations (T01 and T02) in BHS in summer 2016. The response of sea level, temperature and salinity were analyzed. The quasibarotropic and baroclinic residual currents at T02 station were effi ciently separated by using an EOF method, while their effect on water exchanges through BHS were quantitatively assessed. The ROMS model was used to recreate the regional dynamic process, and the effect of quasi-barotropic mode was confirmed. Based on these results, we came to the following conclusions: 1) The process of exchange flow in the BHS modulated by the summer wind gust was described. At the onset of the southerly wind gust, water level in BH increased due to water pile up, which in effect enhanced the inflow through BHS. The sea level difference between BH and YS formed a pressure gradient field pointing to the YS. As the wind gust subsides, the intense pressure gradient force released with the warmer and fresher water extrudes out of BH.
Fig. 12. (a) The sea level differences between southern and northern BHS (Blue one) and the first mode PC of the current profiles (red one) through the BHS during the summer season 2016; (b) The daily exchange flux of barotropic pattern and baroclinic pattern through BHS. The red line is the seasonal averaged exchange flux during summer 2016; (c) The daily barotropic and baroclinic net WF through BHS. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 10
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2) The regional dynamic processes under the effect of summer monsoon mainly present as the fluctuation of sea surface height in the BH and YS caused by wind gusts. Such fluctuation modulates the pressure gradient force field in the meridional direction. The exchange flow in BHS is thereby significantly changed under the effect of geostrophic balance. The variation of pressure gradient force in BHS is mainly manifested as the modulation of the barotropic pressure gradient force. 3) A statistical method is employed to analyze the seasonal current profile across the BHS, and a quasi-barotropic mode and a baroclinic mode are effectively separated. Barotropic mode contributes nearly 47% of the total variance. The wind-triggered barotropic intrusion/ extrusion of volume transport in the BHS is thereby cannot be ignored. It even plays a more important role during the period of wind gusts in summer.
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Declaration of competing interest This is to declare that there is no conflict of interest among the coauthors, between the authors of this manuscript and their co-authors on other publications of similar subjects, and between the authors and their funding agencies. Acknowledgement We appreciate the editor and the reviewers for their constructive comments on improving the original manuscript. Thanks are extended to the crew member of R/V ‘Dongfanghong 2’ for their support to field observation. This work was supported by the National Natural Science Foundation of China [NSFC, grants No. 41530966, 41476070 and 41806101] and NSFC Open Research Cruise [No. NORC2016-01]. Wind and sea surface temperature datasets used in this study are freely available from European Centre for Medium-Range Weather Forecasts (http://apps.ecmwf.int/datasets/) and Aqua-MODIS SST of NASA Ocean Color (http://oceancolor.gsfc.nasa.gov/). References Baschek, B., Send, U., Lafuente, J., Candela, J., 2001. Transport estimates in the Strait of Gibraltar with a tidal inverse model. J. Geophys. Res. Oceans 106 (C12), 31033–31044. https://doi.org/10.1029/2000JC000458. Bi, N., Yang, Z., Wang, H., Fan, D., Sun, X., Lei, K., 2011. Seasonal variation of suspended-sediment transport through the southern Bohai Strait. Estuar. Coast Shelf Sci. 93 (3), 239–247. https://doi.org/10.1016/j.ecss.2011.03.007. Cheng, P., Gao, S., Bokuniewicz, H., 2004. Net sediment transport patterns over the Bohai Strait based on grain size trend analysis. Estuar. Coast Shelf Sci. 60 (2), 203–212. https://doi.org/10.1016/j.ecss.2003.12.009. Ding, Y., Bao, X., Yao, Z., Bi, C., Wan, K., Bao, M., Jiang, Z., Song, J., Gao, J., 2019. Observational and model studies of synoptic current fluctuations in the Bohai Strait on the Chinese continental shelf. Ocean Dyn. 69, 323–351. https://doi.org/10.1007/ s10236-019-01247-5. Emery, W., Thomson, R., 2001. Data Analysis Methods in Physical Oceanography. Elsevier Science. https://doi.org/10.2307/1353059. Fairall, C., Bradley, E., Rogers, D., Edson, J., Young, G., 1996. Bulk parameterization of air-sea fluxes for tropical ocean - global atmosphere coupled - ocean atmosphere response experiment. J. Geophys. Res. Oceans 101, 3747–3764. https://doi.org/ 10.1029/95JC03205. Ferentinos, G., Kastanos, N., 1988. Water circulation patterns in the Otranto straits, eastern mediterranean. Cont. Shelf Res. 8 (9), 1025–1041. https://doi.org/10.1016/ 0278-4343(88)90037-4. Flather, R., 1976. A tidal model of the northwest European continental shelf. Mem. Soc. R. Sci. Li� ege 10 (6), 141–164. Gan, J., Cheung, A., Guo, X., Li, L., 2009. Intensified upwelling over a widened shelf in the northeastern South China Sea. J. Geophys. Res. 114, C09019. https://doi.org/ 10.1029/2007JC004660. Guan, B., 1963. A preliminary study of the temperature variations and the characteristics of the circulation of the cold water mass of the Yellow Sea. Oceanol. Limnol. Sinica 5 (4), 255–284 (in Chinese with English abstract). Guo, B., 1994. Discussion on the barotropic current and baroclinic current in the ocean. J. Oceanogr. Huanghai Bohai 12 (3), 65–69 (in Chinese with English abstract). He, C., Wang, Y., Lei, Z., Xu, S., 1959. A preliminary study of the formation of Yellow Sea Cold Mass and its properties. Oceanol. Limnol. Sinica 11 (1), 11–15.
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Summer wind gusts modulate transport through a narrow strait, Bohai, China Haiqin Duan a, Chenghao Wang a, Zhiqiang Liu b, Houjie Wang a, c, Xiao Wu a, c, Jingping Xu b, c, * a
College of Marine Geosciences, Key Laboratory of Submarine Geosciences and Prospecting Technology, Ocean University of China, 238 Songling Road, Qingdao, 266100, China b Department of Ocean Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China c Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology (QNLM), Qingdao, 266061, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Bohai strait Wind gust Exchange flow Volume transport
The hydrological and hydrodynamic response of Bohai Strait to an intense wind gust in the summer of 2016 was synthesized based on a 9-day in-situ observation at two mooring stations across the strait. At the north station, the sea level abnormally rose by 0.4 m during the wind gust. Strong subtidal inflow (into Bohai Sea) was observed during the gust, then changed to outflow after the gust subsided. At the southern station, the hydro graphic property of water column changed rapidly with a strong outflow after the gust subsided. The Regional Ocean Modeling System model was employed to reconstruct the dynamic processes governing the event. It was found that wind-triggered sea level fluctuation caused the modulation of barotropic pressure gradient force, and thereby altered the exchange flow through the strait. Statistical analysis showed that a wind-triggered quasibarotropic mode (controlled by sea level variation) contributed nearly 47% of the total variance. This suggests that during summer seasons that are normally dominated by baroclinic processes, the barotropic processes related to intermittent wind gusts also contribute considerably to the exchange flow and volume transport through the Bohai Strait.
1. Introduction A strait is a natural channel that connects two large bodies of waters between which water and particle material are exchanged. Such ex changes of material and energy play a critical role in the determining redistribution of heat, salt, pollutants and sediment mass, and biogeo chemical cycles (Hoshika et al., 1999; Malikides et al., 1989; Tian et al., 2006). For those reasons, studies have been carried out for numerous straits around the globe, for instance, Gibraltar (Baschek et al., 2001), Otranto (Ferentinos and Kastanos, 1988; Ursella et al., 2012) and Tsushima Straits (Takikawa and Yoon, 2005). In-situ measurements by subsurface moorings and various numerical models are used to quantify the volume transport through those straits. The Bohai Strait (BHS) between Shandong and Liaodong peninsula is the only pathway connecting two epeiric seas in the northern China seas, the Bohai Sea (BH) and the North Yellow Sea (NYS) (Fig. 1). The ~104 km wide BHS is the shallowest in the southern end, deepens northward with a channel as deep as 70 m offshore Liaoning Peninsula. (Bi et al., 2011; Cheng et al., 2004; Fig. 1(a)). BH is a semi-enclosed shallow (<18
m) marginal sea that positions to the west of BHS (Qin and Li, 1983). Its water depth increases eastward towards central NYS. The hydrographic properties of waters in BH are concurrently influenced by the riverine freshwater influx mainly from Yellow River and the oceanic waters intruding from Yellow Sea (YS) (Zhang et al., 1994; Ren and Shi, 1986; Wang et al., 2010). Yellow River is the world’s 2nd largest river in terms of sediment discharge: 1.2 to 1.3 Gt/yr, which shapes the shelf-sea sedimentary processes around the BH (Lee et al., 2009; Milliman and Meade, 1983) and interpreted as the primary source to several mud deposits in NYS (Yang et al., 2002). The exchange flow in BHS plays a critical role in the evolution of marine environment of the regional and global ecosystems (Ding et al., 2019; Wang et al., 2014). It has long been recognized that the mixture of these waters critically determines not only the general circulation, but also the associated sediment distribu tion and biogeochemical processes (Bi et al., 2011; Wang et al., 2014). Historic simulations and observations suggest that the exchange flow in BHS is primarily composed of an extensive inflow in the north and an outflow in the south of the strait and the peak water transport through the BHS usually occurs in summer (Lu et al., 2011; Lin et al., 2002;
* Corresponding author. Department of Ocean Science and Engineering, Southern University of Science and Technology, Shenzhen, 518055, China E-mail address: [email protected] (J. Xu). https://doi.org/10.1016/j.ecss.2019.106526 Received 27 June 2019; Received in revised form 16 September 2019; Accepted 4 December 2019 Available online 14 December 2019 0272-7714/© 2019 Elsevier Ltd. All rights reserved.
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characteristics in BHS responded to a summer gust by coupling in-situ time-series data and numerical models. Besides, a statistical method was employed to discuss the contribution of wind-affected transport to the volume transport through the BHS in summer.
Zhang et al., 2018). The winter circulations in BH and YS Sea are mainly driven by the intense winter monsoon, while the circulations in summer when the wind is much weaker and heat flux is much stronger are maintained by the tilting thermohaline (Zhao and Shi, 1993; Wei et al., 2003; Wan et al., 2004; Lin et al., 2002). Circulations from these pre vious simulations are usually driven by the monthly or seasonally averaged wind stress, which is feasible for revealing the seasonally averaged flow pattern without sufficiently resolving the variabilities imposed by synoptic events (Ding et al., 2019; Wu et al., 2016). Recent studies that mainly focused on the effect of winter storm events showed that synoptic-scale events have a greater impact on the circulation and exchange flow in the Bohai Strait (Yuan et al., 2008; Qu et al., 2017; Ding et al., 2019). However, the response of exchange flow and circu lation in BHS to synoptic variability of the summer monsoon is still not well understood. In summer, a cold water mass with high salinity pre sent in the middle of the YS invades the BH along the Laotieshan Channel in the northern BHS (He et al., 1959; Lin et al., 2002). The baroclinic circulation is characterized by a cyclonic circulation around this cold water mass. The inflow is strong and mainly confined to the narrow northern passage, while the outflow is broad and weak in the rest of the strait (Huang et al., 1999; Zhou et al., 2017). Huang et al. (1999) and Zhou et al. (2017) analyzed the effects of different forces (tide, varying wind and baroclinicity) on the summer circulation and found that the stratification and wind were the main forces on the circulation in BH, and inferred that the varying wind forces might change the in tensity of the exchange flow though the BHS. However, what role the summer wind gusts play and how they affect the exchange flow and circulation remain unclear. This study analyzes the variation of hydrological and hydrodynamic
2. Data and method 2.1. Field measurements In this study, both moored time-series and ship-based hydrographic surveys (for spatial coverages) are used to characterize the hydrological properties and hydrodynamic processes related to exchange flows and material transport through the strait. All data were collected on the R/V DongFangHong2 during the cruise from June 28 to July 22, 2016. Two key stations were located in the south (T01, 30m) and south sides (T02, 50m) of the BHS, respectively. The R/V DongFangHong2 was first anchored at the southern site T01 for more than 3 days (July 15, 8:00 to July 18, 12:00). The wind 10 m above the sea surface was recorded by the meteorological instrument onboard with the measurement interval of 10 s. A 75 kHz shipborne Acoustic Doppler Current Profiler (ADCP) downward-looking at T01 recorded the current velocities (u, v) once every 5 min. Considering both the waterline and blanking distance of ADCP, the first bin was about 16 m. The hydrological parameters were measured by the conductivity/ temperature/pressure (CTD) sensor. Water samples were collected with CTD cast once every 2 h at 4 water depths (4, 9, 16, 28 m; Table 1). The 4 depths chosen represent the surface, the subsurface, the middle and the bottom layers respectively. The ship was then moved to and anchored at the northern site T02, to collect 9 hourly CTD casts.
Fig. 1. (a) A local bathymetric map showing the location of the observation sites (red stars). The red solid line is the transect across BHS. The dashed contours in Fig. 1(a) are the thickness of the Holocene mud in NYS (Yang and Liu, 2007); (b) A schematic drawing showing the instrument placements at the two sites (T01, T02). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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Table 1 Parameters and instrument settings. Stations
Observation Method
Data
Measurement Period
Interval
Depth (m)
Sampling Levels/Vertical Bin size (m)
Blind zone (m)
T01
Anchor station
SSC Salinity, Temperature, pressure Velocity (u, v)
15 July 08:00 to 18 July 12:00
2h
Profile
4/-
–
15 July 08:00 to 18 July 12:00 18 July 18:00 to 19 July 02:00
5 min
8
1/8
16.54
1h
Profile
3/-
–
11 July 16:00 12:00 11 July 16:00 12:00 11 July 16:00 12:00 11 July 16:00 12:00
to 19 July
3s
53
1/-
–
to 19 July
1 min
53
24/2
4.23
to 19 July
3s
26,34
2/-
–
to 19 July
3s
16,24, 32,40
4/-
–
T02
Anchor station Bottom Mount
Subsurface Mooring
SSC, Salinity, Temperature, pressure Salinity, Temperature, pressure Velocity (u, v) Turbidity Salinity Temperature
A subsurface mooring and an instrumented bottom mount were deployed at station T02 to measure flows and hydrological parameters for 9 days. A 300 kHz ADCP on the bottom mounted system facing up ward recorded the current velocities (u, v) once every minute with a vertical resolution of 2 m. Considering both the height and blanking distance of ADCP, the first layer was about 5 m above the seafloor. In addition, a CTD was about 1 m above the bed. Next to the bottom mount was a subsurface mooring that hosted a CTD sensor at 16 m, two turbidity sensors at 26 m and 34 m respectively and three conductivity/ temperature (CT) sensors at 24, 32 and 40 m respectively. The sampling intervals for the CTD, turbidity sensors and CT sensors were 10, 3, and 1 s respectively (Fig. 1(b)). Due to insufficient buoyancy of float balls, the bobbing amplitude of the instruments on the mooring were over influ enced by the tide, thus the data of this mooring was excluded.
data with Orlanski-type radiation condition for active tracers and bar oclinic velocities. Eight principle tidal constituents (M2, S2, N2, K2, K1, O1, P1, and Q1) from TPXO8 tidal model were prescribed along the eastern and southern open boundaries by using Flather-typed radiation condition (Flather, 1976). High spatial (0.125� � 0.125� ) and temporal (3 h) resolution atmospheric data such as surface wind, evaporation, precipitation, air temperature and radiation flux were from the Euro pean Centre for Medium-range Weather Forecasts (ECMWF) and was applied to the model to capture episodic events. The calculation was based on bulk formulas (Fairall et al., 1996). The water level and vertically averaged velocities from observation were used to validate the simulation with the coefficient of determina tion (R2). It was shown that the modeled water level matched the ob servations fairly well (R2 ¼ 0.97) (Fig. 2(a)). According to harmonic analysis, the principle tidal constituents were M2 and K1. The absolute errors between the simulated and observed M2 amplitude were 0.07 m, and amplitude differences for K1 between simulation and observations were 0.01 m. The discrepancy from observations in phase lag of M2 and K1 were 9 and 3 min, respectively. Theses comparison evidenced that the tidal current was well simulated (Fig. 2(c)), and the simulated velocity magnitude was also consistent with the observation (R2 ¼ 0.75). Observed and simulated velocities also well agreed with each other in the east-west direction, which represents of the orientation of the ex change flow (Fig. 2(b)). Furthermore, the simulated sea surface tem perature (SST) was comparable with that retrieved from remote sensing data (MODIS) in June, July and August (Figs. 3(a) to 3(f)). The temporal variation of SST was also well reproduced in our simulation. Fig. 3(g) showed the consistency between observed and simulated SST with R2 ¼ 0.89. Thus, we conclude that the simulations not only reproduced the
2.2. ROMS modeling This study uses the Regional Ocean Modeling System (ROMS) to simulate the regional hydrodynamic processes during the cruise period. The computational domain covered the region between 117� E and 127.5� E, 31� N and 41� N with horizontal resolution 1/30� . The bottom topography was obtained from the ETOPO1 Global Relief Model. To better resolve the currents in the bottom and surface boundary layers, we vertically discretized the water column into 20 sigma levels using the s-coordinate with θs ¼ 4 and θb ¼ 0.8 (Song and Haidvogel, 1994). Three dimensional temperature and salinity data from the Hybrid Coordinate Ocean Model (HYCOM) with resolution 1/12� were interpolated to ROMS grid as initial conditions. Meanwhile, daily averaged tempera ture, salinity and velocities were imposed as the open boundaries forcing
Fig. 2. Comparison of the (a) water level, (b) current magnitude and direction of in-situ observed and model simulated bathymetry averaged current in T02 during the observation period; (c) The harmonic analysis results of water level and the error between the observation and simulation. 3
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Fig. 3. Comparison of the simulated and satellite remote sensing retrieved monthly SST in June ((a) and (b)), July ((c) and (d)) and August ((e) and (f)) 2016 respectively; (g) The correlation between MODIS and ROMS SST. The SST datasets are retrieved from the MODIS of NASA, available at https://oceandata.sci.gsfc.na sa.gov/.
observed velocity field, including both barotropic and baroclinic com ponents, but also captured the responses in hydrographic properties, especially the temperature field.
north and south of BHS, we used the buoyancy frequency N2 . The buoyancy frequency is usually used to describe the static stability of water column, which is associated with the density change with depth. One common buoyancy frequency calculation method is:
3. Results
N2 ¼
3.1. The observed hydrological characteristics in Bohai Strait
g dρ ρ0 dz
(1)
where g is the gravitational acceleration, ρ0 is the average density value, and z is the vertical coordinate. N2 was overall positive during the observation, indicating that the water column had a stable stratified structure which should be typical in summer. The hydrological properties between the north and south of BHS were different. The maximum value of N2 of both sides were 6–12 m (position of pycnocline) and the north side was stronger than the south by 2–4 times. Below the pycnocline, N2 tended to be 0 and the properties were relatively uniform (Fig. 4(c)). The density in the north was significantly higher than that in the south. It shows there was a strong meridional-direction baroclinic gradient between the north and south of BHS (Fig. 4(b)).
At station T01, water temperature decreased with depth, ranging between 10 and 24 � C. The salinity decreased slightly, ranging from 31.8 to 30.5 PSU (Fig. 4(a)). At station T02, water temperature varied be tween 6 and 26 � C. A significant temperature drop appeared at about 10 m depth. Below 10 m, there was an extensive and relatively uniform cold water mass averaged at only 7 � C (Fig. 4(a)), which could be considered as the NYS cold water mass (He et al., 1959; Guan, 1963). The salinity varied slightly with time, ranging from 31.2 to 32.3 PSU, which was a little higher than T01 (Fig. 4(a)). The above comparison shows that the difference in temperature and salinity between north and south of BHS was small in the surface layer. While the temperature below 10 m depth decreased from south to north with an increase of salinity. To further explore the differences in hydrological properties between
Fig. 4. Vertical distribution of (a) temperature, salinity, (b) density (ρ0) and (c) buoyancy frequency. The data are from hourly CTD cast at the two sites. 4
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3.2. Wind gust and response of hydrological characteristics in Southern Bohai Strait
with the wind subsided, accompanied by rapid change of temperature and salinity as mentioned previously (Fig. 5(c)). The v component of currents along the transect of BHS had no obvious relation with the wind gust, but showed a weak southward flow during most of the observation period. In northern BHS, the residual sea level is relatively stable except for the wind gust (Fig. 7). An abnormal sea level rise with the maximum of 0.4 m was recorded during July 15 to July 17. In terms of temporal scale, the abnormal variation of sea level was consistent with the wind gust that sea level continued to rise as the wind got stronger, and dropped as the wind weakened. The u component of subtidal current had a similar response. During the sea level rise, the residual current flowed into BH vertical uniformly. When the sea level declined, a strong flow gushed out of BH and the residual current reached its peak. However, the v component showed no obvious relation with sea level variation. A ver tical two-layer structure residual current was obvious during the observation period. The upper layer showed a strong southward flow, while the bottom layer showed a weak northward flow (Fig. 7(b) and (c)). According to the above observations, the abnormal sea level rise and the variation of exchange flow through BHS were linked with the wind gust and cause the hydrological characteristics variation. Besides, the wind gust mainly affected the cross-transect (mainly in the meridional direction) flow rather than the along-transect (mainly in the zonal di rection) flow.
In summer, weak southerly wind prevailed in BH and YS (Wang et al., 2014). But an abnormal northeasterly wind gust was recorded from July 15, 18:00 to July 16, 18:00 with a maximum speed over 12 m/s (Fig. 5(a)). Records showed that the salinity decreased from 31.7 to 31.5 PSU and the temperature increased from 11.5 to 14 � C in the bot tom layer of T01 from July 15 to 18. This was a rapid transition within a short period of time after the wind gust weakened (Fig. 5(b)). This transition can also be seen in the T-S diagram. Prior to the northeasterly wind, the water mass of this area was mainly cold and salty water; after the wind gust, this area was occupied by warm and fresh water (Fig. 6). This water mass properties variation was linked with the wind gust. 3.3. Response of hydrodynamic characteristics to the wind gust in Southern and Northern Bohai Stait To better demonstrate how hydrodynamic characteristics responded to the wind gust, this study only analyzed subtidal currents. The following processing methods were used: first, we used the T_Tide (Pawlowicz et al., 2002) tool box to predict the time series tidal current during the observation period. Then, the predicted tidal currents were subtracted from the raw current field (u and v) to remove the tidal component. Lastly, the residual currents were smoothed by moving average with a 1-h window to filtering the high frequency variability. The residual currents were considered as the subtidal currents. At T02, we used the bottom pressure record to retrieve the sea level height and then processed by the same method to obtain the residual sea level height. In southern BHS, a slight eastward flow occurred in the bottom layer during the wind. Subsequently, a strong eastward flow came out of BH
3.4. Regional subtidal circulation response to the wind gust recreated by ROMS Our collected data only gives us a glimpse into the local wind gust process. The existing evidence shows us part of the process, but the full picture is still unclear. Thus, we used ROMS model to offer the regional
Fig. 5. (a) Wind data of 10 m above the surface at anchor station T01; (b) Temperature, salinity variation in bottom layer (2 m above bed) of T01; (c) Current velocity of bottom layer at T01. The solid black line is the u component of residual velocity. 5
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Fig. 6. T-S diagram of CTD data acquired at T01 during the observation period.
Fig. 7. Time-series of (a) water level and residual velocity profiles of (b) u and (c) v component at site T02.
daily averaged sea level and current field during the wind gust (July 15 to July 17, 2016). On July 15, BH and YS were dominated by a weak northerly wind of lower than 5 m/s in speed, and the sea level varied within 0.2 m. The surface current field was a weak northwestward flow, forming a coun terclockwise circulation in NYS and a clockwise circulation in the mid dle of BH. The bottom velocities presented a weak northward flow in the South Yellow Sea (SYS) and an eastward flow in BH (Fig. 8(a)). During the wind gust (July 16), a regional cyclone with its center over Shandong peninsula strengthened gradually from north to south. BH was domi nated by a weak northeasterly wind which was the same as July 15, while NYS presented a strong southeasterly wind with a maximum speed over 10 m/s and the SYS was affected by an intense southerly wind with a maximum speed over 13 m/s. The sea level rapidly rose over 0.4 m, especially in BH. Surface current corresponded well with the cyclone.
On the east side of SYS, a strong northward flow extended to NYS and entered into BH through BHS. The bottom flow was divided into two branches on the east side of SYS. One stronger branch ran along the northern coast of Shandong peninsula and the coast of Subei towards the south, forming a counterclockwise circulation, and the other weaker branch extended to NYS and entered into BH. As the cyclone moved northeast, the weakened northerly wind dominated the region. Only in parts of SYS the wind speed exceeded 10 m/s. Meanwhile, water piled up in the west coast of BH and the coast of Shandong peninsula and Subei, while the low sea level began to show in the east side of NYS and SYS. A strong outflow through BHS bypassed Shandong peninsula and propa gated southward (Fig. 8(b) and (c)). According to the simulation, the regional sea level distribution and circulation will reconstruct due to the summer wind gusts, especially the exchange flow in BHS. Besides, although the local wind of BH was steady 6
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Fig. 8. The spatial patterns of (a) the ECWMF wind field and sea level, (b) surface and (c) bottom current field produced by ROMS for the period of July 15 to 17. Collectively they show the processes of water exchange between BH and YS during the wind gust.
during the observation, the sea level height and circulation still changed due to the wind of NYS and SYS. Therefore, the effect of regional wind, including the SYS wind field, cannot be ignored when studying the ex change flow in BHS. Next, we will focus on the mechanism of the wind gusts effect over exchange flow and the role of the wind gusts in the volume transport through BHS in summer.
direction respectively; ρ0 represents the reference density; ρ is the water density other terms in Eqs. (2) and (3) are: acceleration (ACCEL); Coriolis force (COR); pressure gradient force (PGF), barotropic and baroclinic PGF included; horizontal and vertical nonlinear advection (HADV þ VADV); and the horizontal and vertical viscous term (HVISC þ VVISC), in which HVISC is represented by Fx and Fy , and Kv stand for the vertical viscosity coefficient. In this study, the focus is on the geostrophic balance terms (PGF and COR) since the other terms are one or two orders of magnitude lower. Regional horizontal distribution of SSTR, and vertical averaged PGF and COR in x and y directions during the wind gust period are shown in Fig. 9. During the observation period, the zonal current (u component) through BHS (focus of this research) was dominated by geostrophic balance in the meridional direction, which is indicated by the notation y in equations (2) and (3). On July 16, the wind stress blew northwest ward, and under direct wind stress and Ekman transport, sea surface height gradually piled up to NYS and BH, forming a gentle sea level slope. Then a geostrophic inflow through BHS (represented by positive CORy ) was triggered and was balanced by PGFy . As the wind continued, sea surface height slope gradually became steeper and PGF in x=y direction gradually turned to positive/negative. After the wind weakened, the intense PGF was released, and there was a strong outflow through BHS along the northern coast of Shandong peninsula under the balance of the opposite COR. Previous results suggest that the wind gust mainly affects barotropic PGF (PGFT ) by influencing sea level dis tribution, while the baroclinic PGF (PGFC ) is mainly controlled by the hydrographic properties of waters (Guo, 1994). To better distinguish the impact of surface elevation and baroclinicity of the waters, we decom posed the term PGF into PGFT and PGFC , as shown in Fig. 9(d) and (e). We observed that the variation of PGF during the wind gust was mainly the modulated process of PGFT , whereas the PGFC was rela tively stable and far smaller than the PGFT . This was because the modulation rate of the density field was much slower than the sea sur face height field. In summer, the relatively weak monsoon only affects the surface layer circulation (Zhao and Shi, 1993), and the intense vertical
4. Discussion 4.1. The momentum balance After ROMS demonstrated the regional hydrodynamic processes during the wind gust, the embedded dynamic mechanism was further investigated using the terms balances in depth-independent momentum equations, which showed the possible governing dynamic characteris tics of these processes (Gan et al., 2009; Signorini et al., 1997). The momentum equation in Cartesian coordinates is written as: ACCEL
z}|{ COR z}|{ ∂u ¼ fv ∂t
HADVþVADV
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂u ∂u ∂u u þv þw ∂x ∂y ∂z
PGF
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflZfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂η g ∂ g þ ρdz ∂x ρ0 ∂x
HVISCþVVISC
zfflfflfflfflfflfflfflfflfflffl }|fflfflfflfflfflfflfflfflfflffl { 0 1 ∂u ∂ K v ∂z C B þ @Fx þ A ∂z ACCEL
z}|{ COR z}|{ ∂v ¼ fu ∂t
(2)
PGF
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflZfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂η g ∂ g þ ρdz ∂y ρ0 ∂y
HADVþVADV
zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl� ffl{ � ∂v ∂v ∂v u þv þw ∂x ∂y ∂z
HVISCþVVISC
zfflfflfflfflfflfflfflfflfflffl}|fflfflfflfflfflfflfflfflfflffl{ 0 1 ∂v ∂ K v ∂zC B þ @Fy þ A ∂z
(3)
Subscripts x and y denote the momentum in east-west and north-south 7
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Fig. 9. Terms of the depth-averaged momentum equations ((a), (b) and (c)) (Eq. (3)) and the PGFT and the PGFC ((d) and (e)) in zonal direction (left panels) and meridional direction (right panels) respectively. Positive values represent the eastward in the left panels and the northward in the right panels. All the variables are daily averaged.
stratification plays an important role in water exchange through BHS with the baroclinic pattern (Wang et al., 2010). However, there are still intermittently wind gusts events like in this study that may last for many days. Such intense wind gusts would trigger regional sea surface height variation, thereby causing the PGFT field modulation. During the gusts, the barotropic current would dominate the water exchange processes through BHS. In addition to enhancing the wind-driven currents, we also found that the strong wind burst modulates the barotropic pressure gradient field in the Bohai Sea and Yellow Sea even after the wind subsided, a result that has not been seen in previous studies.
remote wind field over the neighboring seas. In addition, the residence time in BH under the normal summer condition was about 400 days which was consistent with the previous studies (Liu et al., 2012, 2017). In contrast, it was only 138 days during the wind gusts, a 65% reduction. 4.2.2. The barotropic processes extracted from the subtidal currents by EOF method To further understand the properties of the subtidal currents through BHS, a conventional Empirical Orthogonal Function (EOF) method was applied to analyze the observed and simulated subtidal flows respec tively (for further details see Emery and Thomson, 2001). For the wind gust process in T02, the first several modes of subtidal current were separated from the others. According to the method pro posed by North et al. (1982), the first six modes were effective in 95% confidence interval. The greatest EOF mode of u component accounted for more than 74% of the total current and the rest of the modes accounted for 26% (Fig. 11(a) and (b)). The time series of the first and the rest of the current profiles were obtained from the EOF modes and their corresponding principal component (PC). And the parameter of PC indicates the proportion of the EOF mode in the raw data in different time. The first mode of the cross-transect current mainly showed a uniform distribution in the water column. Meanwhile, there was a sig nificant negative correlation between the first mode PC and the sea level variation rate ðdη =dtÞ (Fig. 11(c)). This suggested that the sea level variation was the main controlling factor of the first mode of the cur rents in BHS. Thus the first mode of current profile was considered as quasi-barotropic mode according to the definition of barotropic currents (Guo, 1994). The rest of u component modes presented a layered structure in the water column with upper layers dominated by the outflow and lower layers dominated by the inflow, and we defined it as the baroclinic mode. BHS is oriented approximately north-southward and the zonal ve
4.2. The wind-triggered barotropic process through the Bohai Strait in summer 4.2.1. Summer wind characteristics and its corresponding sea level variation Previous results give us a providential observed wind gust event in summer and analysis of its process and mechanism. We focus more on the role of wind gusts and their contribution to the volume transport through BHS throughout summer. The regional averaged wind field data of BH, NYS and SYS in summer (June, July and August) 2016 were shown in Fig. 10 (a), (b) and 10(c). In summer, although it is mainly weak southerly wind that prevailed the BH and YS, intense wind gusts still episodically occurred with about nine events during which the regional averaged wind speed exceeds 10 m/s (red sticks in Fig. 10). By comparing the regional averaged sea level height in BH, NYS and SYS simulated by ROMS (the histograms in Fig. 10(d)), the strong wind events (the red line in Fig. 10(d)) were often accompanied by pro nounced fluctuations of sea level height. Such concomitant relationship between wind fields and sea level showed that sea level rose during intense southerly wind while fell during intense northerly wind. Meanwhile, sea level variation was affected by both the local wind and 8
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Fig. 10. Temporal variation of region-averaged wind fields in (a) BH, (b) NYS and (c) SYS in summer season (June, July and August 2016) obtained from ECWMF mentioned in the text. Wind speed exceeding 10 m/s are in red; (d) The histograms indicate the regional averaged sea level height of BH, NYS and SYS respectively during 2016 summer. The red line indicates the regional averaged wind velocity in meridional direction. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 11. EOF mode 1 (a) and other modes (b) of the u component of residual current in T02, with the gray solid line representing the corresponding PC; (c) The scatter plot of the sea level variance rate and the PC of the first mode of u component.
locity is much larger than the meridional velocity, therefore only the u component was considered when calculating the net volume transport across BHS during the observation. Based on the current velocity, the unit width water flux (WF) of different modes were calculated as follow: PT Pn t¼1 i¼1 Hit ⋅uit WFu ¼ (4) T
the surface layer; u represents the u components of the current velocity; t is the interval between two continuous sampling casts. The unit width net WF of baroclinic mode ( 0.23 � 10 7 Sv) due to the stratified structure was negligible compared to the barotropic mode (2.63 � 10 7 Sv) (Table 2). Therefore, although the baroclinic mode influences the water exchange between BH and NYS, the barotropic mode is the main controlling factor of the net water volume transport during the obser vation period. We applied EOF analysis to the daily averaged u component of cur rent profiles of BHS (transect a-a’ in Fig. 1(a)) in summer, 2016. The first
Where H is the thickness area of each layer, which depends on the water depth and layer division; i is the layer number, i ¼ 1 indicating 9
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transport processes would be enhanced during gusty weather as the WFT. We further analyzed the net WF through BHS in June, July and August by separating the barotropic and baroclinic modes. It is found that the WFT accounted for a considerable proportion of the total WF (Table 2). Meanwhile, due to the contingency of gusts, the WFT varied greatly among different months. We proposed the importance of barotropic currents in impacting the exchange flow in BHS based on EOF analysis and the previously pro posed dynamic analysis with both observation and simulation. And the results were consistent. We also found that the barotropic exchange flow was determined by the intermittently wind gusts in summer. It is concluded that although previous studies have proposed the importance of the thermohaline circulation in summer (Huang et al., 1999; Wei et al., 2003) as also shown in our previous analyses, the contribution of wind-triggered barotropic processes have a considerable impact on the water exchanges and volume transport through BHS.
Table 2 Net water flux of barotropic and baroclinic modes in summer 2016. Periods
Water Flux (10 2⋅Sv) (þ/ : Out/in) Barotropic
Observed data Simulated data
observation June July August
2.63 � 10 1.55 1.72 2.87
Baroclinic 5
0.23 � 10 1.74 3.00 0.17
5
mode of current field was approximately uniform in the water column, with the simultaneous entry or exit pattern and a gradually decreasing velocity from west to east (not shown here). According to the previous analysis, the barotropic exchange processes through BHS was mainly dominated by the meridional PGFT . We find a remarkable positive cor relation (R2 ¼ 0.75) between the PC of the first mode and the sea level difference between southern ðηS Þ and northern (ηN Þ BHS (Fig. 12(a)). This finding suggests that the first mode of the cross-transect current was also governed by the wind-triggered modulation of sea level in the scope of quasi-barotropic geostrophic current. The contribution of barotropic mode to fluctuation of the exchange flow was nearly 47%. The baro clinic mode was obtained by subtracting the barotropic mode from the raw current profiles. Fig. 12(b) showed the daily averaged barotropic and baroclinic exchange flux, respectively. The baroclinic exchange flow was relative stable with an exchange flux of ~0.12 Sv. The variations of barotropic exchange flow were consistent with that of the wind field (for example, wind gusts). It was shown that the relatively stable baroclinic exchange flow dominated the structure of the water exchange in the water column, and the barotropic exchange could be enhanced to exceed the intensity of the baroclinic circulation and dominate the exchange flow during the periods of wind gusts. The averaged exchange flux of barotropic processes was 0.07 Sv in summer, accounting for 36.8% of the total exchange fluxes. This indicated that besides the wind gusts, the wind-triggered barotropic processes also played a non-negligible role in impacting the exchange flow in the BHS in summer. Moreover, the net WF from barotropic current (WFT) and baroclinic current (WFC) showed in Fig. 12(c). Due to the two-layer structure, the WFC was small, and the
5. Conclusions A wind gust event was captured by hydrodynamic and hydrographic observation at two stations (T01 and T02) in BHS in summer 2016. The response of sea level, temperature and salinity were analyzed. The quasibarotropic and baroclinic residual currents at T02 station were effi ciently separated by using an EOF method, while their effect on water exchanges through BHS were quantitatively assessed. The ROMS model was used to recreate the regional dynamic process, and the effect of quasi-barotropic mode was confirmed. Based on these results, we came to the following conclusions: 1) The process of exchange flow in the BHS modulated by the summer wind gust was described. At the onset of the southerly wind gust, water level in BH increased due to water pile up, which in effect enhanced the inflow through BHS. The sea level difference between BH and YS formed a pressure gradient field pointing to the YS. As the wind gust subsides, the intense pressure gradient force released with the warmer and fresher water extrudes out of BH.
Fig. 12. (a) The sea level differences between southern and northern BHS (Blue one) and the first mode PC of the current profiles (red one) through the BHS during the summer season 2016; (b) The daily exchange flux of barotropic pattern and baroclinic pattern through BHS. The red line is the seasonal averaged exchange flux during summer 2016; (c) The daily barotropic and baroclinic net WF through BHS. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 10
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2) The regional dynamic processes under the effect of summer monsoon mainly present as the fluctuation of sea surface height in the BH and YS caused by wind gusts. Such fluctuation modulates the pressure gradient force field in the meridional direction. The exchange flow in BHS is thereby significantly changed under the effect of geostrophic balance. The variation of pressure gradient force in BHS is mainly manifested as the modulation of the barotropic pressure gradient force. 3) A statistical method is employed to analyze the seasonal current profile across the BHS, and a quasi-barotropic mode and a baroclinic mode are effectively separated. Barotropic mode contributes nearly 47% of the total variance. The wind-triggered barotropic intrusion/ extrusion of volume transport in the BHS is thereby cannot be ignored. It even plays a more important role during the period of wind gusts in summer.
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Declaration of competing interest This is to declare that there is no conflict of interest among the coauthors, between the authors of this manuscript and their co-authors on other publications of similar subjects, and between the authors and their funding agencies. Acknowledgement We appreciate the editor and the reviewers for their constructive comments on improving the original manuscript. Thanks are extended to the crew member of R/V ‘Dongfanghong 2’ for their support to field observation. This work was supported by the National Natural Science Foundation of China [NSFC, grants No. 41530966, 41476070 and 41806101] and NSFC Open Research Cruise [No. NORC2016-01]. Wind and sea surface temperature datasets used in this study are freely available from European Centre for Medium-Range Weather Forecasts (http://apps.ecmwf.int/datasets/) and Aqua-MODIS SST of NASA Ocean Color (http://oceancolor.gsfc.nasa.gov/). References Baschek, B., Send, U., Lafuente, J., Candela, J., 2001. Transport estimates in the Strait of Gibraltar with a tidal inverse model. J. Geophys. Res. Oceans 106 (C12), 31033–31044. https://doi.org/10.1029/2000JC000458. Bi, N., Yang, Z., Wang, H., Fan, D., Sun, X., Lei, K., 2011. Seasonal variation of suspended-sediment transport through the southern Bohai Strait. Estuar. Coast Shelf Sci. 93 (3), 239–247. https://doi.org/10.1016/j.ecss.2011.03.007. Cheng, P., Gao, S., Bokuniewicz, H., 2004. Net sediment transport patterns over the Bohai Strait based on grain size trend analysis. Estuar. Coast Shelf Sci. 60 (2), 203–212. https://doi.org/10.1016/j.ecss.2003.12.009. Ding, Y., Bao, X., Yao, Z., Bi, C., Wan, K., Bao, M., Jiang, Z., Song, J., Gao, J., 2019. Observational and model studies of synoptic current fluctuations in the Bohai Strait on the Chinese continental shelf. Ocean Dyn. 69, 323–351. https://doi.org/10.1007/ s10236-019-01247-5. Emery, W., Thomson, R., 2001. Data Analysis Methods in Physical Oceanography. Elsevier Science. https://doi.org/10.2307/1353059. Fairall, C., Bradley, E., Rogers, D., Edson, J., Young, G., 1996. Bulk parameterization of air-sea fluxes for tropical ocean - global atmosphere coupled - ocean atmosphere response experiment. J. Geophys. Res. Oceans 101, 3747–3764. https://doi.org/ 10.1029/95JC03205. Ferentinos, G., Kastanos, N., 1988. Water circulation patterns in the Otranto straits, eastern mediterranean. Cont. Shelf Res. 8 (9), 1025–1041. https://doi.org/10.1016/ 0278-4343(88)90037-4. Flather, R., 1976. A tidal model of the northwest European continental shelf. Mem. Soc. R. Sci. Li� ege 10 (6), 141–164. Gan, J., Cheung, A., Guo, X., Li, L., 2009. Intensified upwelling over a widened shelf in the northeastern South China Sea. J. Geophys. Res. 114, C09019. https://doi.org/ 10.1029/2007JC004660. Guan, B., 1963. A preliminary study of the temperature variations and the characteristics of the circulation of the cold water mass of the Yellow Sea. Oceanol. Limnol. Sinica 5 (4), 255–284 (in Chinese with English abstract). Guo, B., 1994. Discussion on the barotropic current and baroclinic current in the ocean. J. Oceanogr. Huanghai Bohai 12 (3), 65–69 (in Chinese with English abstract). He, C., Wang, Y., Lei, Z., Xu, S., 1959. A preliminary study of the formation of Yellow Sea Cold Mass and its properties. Oceanol. Limnol. Sinica 11 (1), 11–15.
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