The influence of meteorological variation on the upwelling system off eastern Hainan during summer 2007–2008
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The influence of meteorological variation, i.e., typhoon and precipitation events, on the coastal upwelling off the eastern Hainan Island was studied based on observations taken during two upwelling seasons. The observations were made in August 2007 and July 2008, respectively. We found that, in principle, similar structure of sea surface temperature and bottom temperature prevailed in both observational periods, providing evidence that upwelling events occur frequently during the summer monsoon along the eastern Hainan shelf. Based on a simple momentum balance theory, we studied the balances between momentum fluxes, wind stress, and bottom stress. The results showed that the Burger number is S ≈ 1, indicating that the cross-shelf momentum flux divergence was balanced by the wind stress and the onshore return flow occurred in the interior of the water column. Hence, a conceptual model of the upwelling structure was built for further understanding of upwelling events. In addition, it was also observed that variations in the strength of upwelling are controlled by storm events, i.e., strong northerly winds change the structure of the thermocline on the shelf significantly. The strong mixing caused by wind reduces the strength of the thermocline, in particular in coastal seas. Based on our conceptual model, a frontal zone between mixed coastal water and offshore water develops which destabilizing the water column and hence decreases the upwelling strength. Freshwaters from the two main rivers in the Wenchang Bay are confined to the coastal area less than 20–30 m deep, as confirmed by our water mass analysis. Freshwater discharge stabilized the water column, inhibiting the upwelling as shown by the potential energy calculation. Consequently, estuarine water only inhibits the upwelling in the near coastal area. Therefore, it can be concluded that estuarine water does not have a significant impact on upwelling strength on the shelf.
KeywordsHainan Upwelling dynamics Observational data Thermocline
Continental shelves exhibiting wind-driven coastal upwelling have been studied for decades because of high biologically productivity in these areas (Pedlosky 1978; Lentz 1992; Smith 1995; Allen et al. 1995; Federiuk and Allen 1995; Gan and Allen 2002). There exists a strong upwelling along the northern South China Sea (SCS) shelf break, which has a strong influence on shelf circulation (Jing et al. 2009). Guo et al. (1998) estimated the location and magnitude of the maximum vertical velocity using both a diagnostic model and observations at eastern Hainan shelf. However, the water volume transported to the shelf induced by upwelling events has not been estimated. Recently, there have been some studies on upwelling around Hainan Island (Lü et al. 2008; Su and Pohlmann 2009). Nevertheless, the dynamics of the upwelling, such as upwelling structure, the influence of wind events, and response of estuarine water, is not yet fully understood.
There have been relatively few hydrographic surveys on the northern SCS shelf (Su 2004). In summer, upwelling and cold eddies, both associated with increased nutrient levels and high primary production, have been confirmed only by the modeling studies of Ning et al. (2004). Therefore, it seems appropriate to analyze the shelf response to upwelling by combining our observational data with information from other available datasets. Indeed, the most important effect of upwelling is its capability of bringing nutrient-rich waters from the deep ocean to the shelf and estuaries. Hence, we aim to understand the dynamical changes in shelf circulation due to upwelling. In this context, a conceptual model of upwelling induced circulation has been developed and to provide an estimate of the flux from offshore to the shelf area.
The eastern Hainan shelf is relatively narrow, which has an influence of the permanent stratification in this area. When a wind-driven upwelling is acting on a stratified ocean, the development of a jet depends on the strength of the stratification, and the coastal upwelling is confined to an area 30 km off the coast (O’Brien and Hurlburt 1972; Allen 1973). The interaction between wind-driven upwelling and stratification could also occur on different spatial scales. Monteiro and Largier (1999) found that, in the southern Benguela system, wind drives upwelling raises bottom water, which strengthens stratification on a regional scale; conversely, on the local bay scale, wind drives vertical mixing which weakens stratification. Therefore, the knowledge of the dynamics of the upwelling system off eastern Hainan could also help to understand other similar upwelling systems.
On the eastern Hainan shelf, we can find two estuaries and several lagoons enclosed by coral reefs, which has a typical tropical geographical characteristics. The study of the estuaries response to upwelling may provide a reference base for similar type along the tropical coasts (Ye 1988). Thus, we aim at three major topics in this study: (1) the balance between momentum flux divergence, wind stress, and bottom stress based on existing theory; (2) the impacts of storms on the strength of upwelling; and (3) the influence of estuaries, including coastal zone, on upwelling.
2 Data and methods
2.1 Cruise data
2.2 Relevant data
Sea surface temperature (SST) data are provided by the Operational SST and Sea Ice Analysis (OSTIA) system. OSTIA uses satellite SST data, provided by international agencies via the Global Ocean Data Assimilation Experiment High-Resolution SST Pilot Project regional/global task sharing framework, which is available online (http://ghrsst-pp.metoffice.com/pages/latest_analysis/ostia.html).
The latest of the wind-measuring satellites, NASA’s Quick Scatterometer (QuikSCAT), was launched into orbit on 19 June 1999. The data are available online at >http://podaac.jpl.nasa.gov/quikscat. We use its daily averaged wind vector close to the observational area to detect the wind situation during the months of 2007 and 2008 when our campaigns were conducted.
The precipitation data in stations of Haikou and Qionghai were used to obtain the information of river discharge difference in both years. The precipitation in Haikou is related to river discharge from the Wenchang River, while that in Qionghai represents the river discharge from the Wanquan River. The data are available online (Climate Center, Utah State University; http://climate.usurf.usu.edu/products/data.php?tab=gsod).
3.1 Thermocline variation at different wind stages
However, there are still visible differences between these two observational records. Temperatures in 2007 were colder than that in 2008, from which we could speculate that the typhoon brought cold water from the northern SCS shelf to the observational area. Besides the difference in SST, the salinity in the Wanquan estuary (section T4) is much lower in 2007, which definitely relates to the typhoon event (Fig. 4c). According to the wind strength and direction during the month of observation, we can define the upwelling event in August 2007 as post-storm stage and that in July 2008 as a pre-storm stage. According to this obvious difference in meteorological forcing, we can speculate that a stronger mixing after the strong wind event or a changed large scale advection field is responsible for the observed deviations.
A thermal front has been detected by satellite observations during summer off the northeastern Hainan coast (Hu et al. 2003). Fronts normally form natural barriers to the water transport (Wolanski and Hammer 1988) and, thus, decrease the horizontal and in turn vertical transport of the upwelled water masses. It has been shown that frontal zones could also be regions where the deep water is upwelled into the surface layer (Savidge 1976; James 1978; Lü et al. 2006). Nevertheless, the generation of the front under the post-storm stage off the northeastern Hainan coast is a temporary phenomenon, which is gradually developing, and thus does not support the prerequisite condition of a frontal induced upwelling.
In 2007, the temperature distribution at section T1 shows stratification at 20 m depth at the near coastal station and at 35 m depth at the offshore station, respectively, if we define the position of maximum vertical temperature gradient as the location of the thermocline (Fig. 5a). We noted the location of the thermocline in all sections in Fig. 5. Therefore, the thermocline was elevated by 15 m from the offshore station toward the nearshore station at section T1. We calculate thermocline uplift at all sections and the biggest difference of 25 m was found at section T1 between the two observations. In addition, the average uplift of the thermocline was 15 m in 2007 and 20 m in 2008. From this calculation, we could conclude that the uplift of the thermocline decreases under post-storm conditions.
From the observations, it is seen that the structure of the upwelling, i.e., the onshore site of the southern section T5 is the coldest, do not change from the pre-storm to the post-storm period. However, strong winds influence the stability of stratification and the uplift of cold water from deeper zones due to wind mixing causes an enhanced energy input. Consequently, the strength of the thermocline decreases by 0.2°C/m. This disturbance by the typhoon event is followed by a reequilibrium which also has an influence on water masses outside the shelf break, which in turn have an influence on the coastal environment.
3.2 Cross-shelf momentum balance
A simple theory of wind-driven coastal upwelling related to the structure of the cross-shelf circulation was developed by Lentz and Chapman (2004). The theory predicts that the magnitude of the cross-shelf momentum flux divergence relative to the wind stress depends on the Burger number S = α N/f, where α is the bottom slope, N is the buoyancy frequency, and f is the Coriolis parameter. For S ≪ 1, the cross-shelf momentum flux divergence is small, the bottom stress balances the wind stress, and the onshore return flow occurs primarily in the bottom boundary layer. For S ≈ 1 or larger (strong stratification), the cross-shelf momentum flux divergence balances the wind stress, the bottom stress is small, and the onshore return flow returns in the interior (Lentz and Chapman 2004).
Parameters at section T2 (Fig. 1) on Eastern Hainan shelf derived from observations: α is the bottom slope, N is the buoyancy frequency, f is the Coriolis frequency, and S = α N/f is the Burger number
α (10 − 3)
N (10 − 3 s − 1)
f (10 − 4 s − 1)
Based on the theory of Lentz and Chapman (2004), we can draw a schematic of the circulation pattern along a cross-shelf section off the eastern Hainan coast (Fig. 6). In a study of a comparable weak upwelling system, i.e., upwelling off Oregon, Huyer (1983) found that the offshore Ekman transport is carried by the surface Ekman layer, and the onshore return flow occurs through the quasi-geostrophic interior. Our studies lead to the conclusion that the onshore transport required to balance the wind-driven offshore transport occurs mainly in the interior. Using this analysis, we can estimate the upwelling water fluxes even if we have only a basic understanding of the structure of the cross-shelf circulation, and it is not necessary to have a detailed knowledge of the flow in the bottom boundary layer.
4.1 The response of the estuaries to the typhoon and precipitation events and how estuaries affect upwelling
4.2 Estimation the vertical transport
Finally, we aim at a qualitative estimation of the water mass transported into the coastal area by an upwelling event. There are several methods to estimate the upwelling strength. A common way to obtain a rough estimate of the amount of upwelled water and nutrients transported into the surface layer by upwelling is to multiply the average increase of nutrient concentration by the volume of the affected water mass (Lips et al. 2009). To consider a quasi-equilibrium upwelling state for this estimation, we choose a stronger upwelling event (i.e., the upwelling in July 2008), since the upwelling favorable southerly wind dominated the entire month before the observation. The upwelling is controlled by intermittent wind direction variation, and the development of the upwelling in this region takes about 1 month, as Su and Pohlmann (2009) concluded from model studies. The given value for upwelling fluxes thus provides a good estimation for an upwelling event. The QuikSCAT wind stress in July 2008 along the eastern Hainan coast is used to calculate the Ekman transport (ME). ME along the T2 section is about 2,000 kg m − 1 s − 1. The width of the upwelling region (B) is about 60 km (comparable to the horizontal scale used in the next paragraph). The vertical compensation water velocity (w) is w = ME / ρ ·B, where ρ is water density. Therefore, the resulting vertical velocity according to classic Ekman theory is about 2.8 m/day. The upwelling at eastern Hainan is associated with a strong stratification, which leads to the consequence that the transport rates estimated by the classic Ekman theory accounting only for the barotropic conditions may lead to an overestimation. Due to a positive stability of the water column, additional energy is needed to uplift the respective water mass. For this reason, it is not possible to define a simple balance as for the Benguela upwelling system and the upwelling system Off Oregon, where Ekman suction induced by wind stress curl was the major contributor to the total upward velocity (Halpern 1976; Fennel 1999).
Here, we use a schematic plot to describe our conceptual model to roughly estimate the vertical water fluxes (Fig. 6). Upwelling develops when interior water breaks a front and thermocline reaches the coastal area. Therefore, upwelled water volume could be roughly estimated by vertical distance of the thermocline raising from the offshore to the coast (shaded area in Fig. 6). Of course, the error of this estimation might be relatively large, since the strength and duration of the wind cannot be fully accounted for. Taking section T2 as a reference section, the thickness of the upper mixed layer near the shelf break station is about 35 m, whereas in the estuary it is about 15 m. The distance from the station furthest offshore to the estuary is about 60 km. Thus, the uplifted water along one section of unit width amounts to 600,000 m2. The coastline of the observation area is about 100 km long. Therefore, the total amount of uplifted water by upwelling is equal to approximately 60 km3. The development of the upwelling in this region takes about 1 month Su and Pohlmann (2009). Therefore, a simple calculation of the uplift rate of the thermocline is 0.67 m/day (20 m/30 day), the magnitude is comparable to the calculation of Ekman suction off Peru (Halpern 2002). The upwelling flux is about 2 km3/day, which is a minimum value of a strong upwelling in this region. From this, we can estimate the total nutrient transport by knowing the nutrient concentrations of the sub-thermocline water mass off the shelf break.
The response of the eastern Hainan shelf to upwelling events is studied by means of cruise data combined with SST and SSH data from 2007 and 2008. Based on hydrodynamical observations, we found that the distribution of upwelling centers along eastern Hainan Island is relatively stable, whereas strong winds lead to high instabilities of stratification on the shelf. Some conclusions can be drawn from a basic analysis: (1) We calculate the Burger number to be S ≈ 1, indicating that the cross-shelf momentum flux divergence balances the wind stress and the onshore return flow occurs in the interior of the water column, which agrees nicely with our results of current measurements. (2) The strong mixing caused by typhoons reduces the strength of the thermocline, particularly in the coastal area. The front between the mixed coastal water and offshore water is established, which in turn weakens upwelling strength. (3) The freshwaters from the two main rivers in the Wenchang Bay are confined to the coastal area inside the 20–30 m isobath, as confirmed by water mass analysis. Therefore, estuarine waters do not have a significant impact on the upwelling.
We thank Dr. Daoru Wang for the preparation for the cruises, Dr. Daniela Unger for the information of precipitation data, Dr. Xiaopei Lin for the valuable discussion, and Kieran O’Driscoll for revising the paper. This work was performed in the frame of the project “Land-Sea Interactions along Coastal Ecosystems of Tropical China: Hainan (LANCET)” under grant number BMBF-03F0457B and BMBF-03F0620B. We are grateful to two anonymous reviewers for their constructive comments, which improved the manuscript.
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