The effect of wind on the dispersal of a tropical small river plume

doi:10.1007/s11707-016-0628-6

Front. Earth Sci.

2018, Vol. 12

Issue (1) : 170-190 https://doi.org/10.1007/s11707-016-0628-6

RESEARCH ARTICLE

The effect of wind on the dispersal of a tropical small river plume

Junpeng ZHAO^1,², Wenping GONG^1,²(

), Jian SHEN³

¹. School of Marine Sciences, Sun Yat-Sen University, Guangzhou 510275, China
². Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Sun Yat-Sen University, Guangzhou 510275, China
³. Virginia Institute of Marine Science, The College of William and Mary, Gloucester Point, VA 23062, USA

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Abstract

Wanquan River is a small river located in Hainan, a tropical island in China. As the third largest river in Hainan, the river plume plays an important role in the regional terrigenous mass transport, coastal circulation, and the coral reef’s ecosystem. Studies have shown that wind forcings significantly influence river plume dynamics. In this study, wind effects on the dispersal of the river plume and freshwater transport were examined numerically using a calibrated, unstructured, finite volume numerical model (FVCOM). Both wind direction and magnitude were determined to influence plume dispersal. Northeasterly (downwelling-favorable) winds drove freshwater down-shelf while southeasterly (onshore) winds drove water up-shelf (in the sense of Kelvin wave propagation) , and were confined near the coast. Southwesterly (upwelling-favorable) and northwesterly (offshore) winds transport more freshwater offshore. The transport flux is decomposed into an advection, a vertical shear, and an oscillatory component. The advection flux dominates the freshwater transport in the coastal area and the vertical shear flux is dominant in the offshore area. For the upwelling-favorable wind, the freshwater transport becomes more controlled by the advection transport with an increase in wind stress, due to enhanced vertical mixing. The relative importance of wind forcing and buoyancy force was investigated. It was found that, when the Wedderburn number is larger than one, the plume was dominated by wind forcing, although the importance of wind varies in different parts of the plume. The water column stratification decreased as a whole under the prevailing southwesterly wind, with the exception of the up-shelf and offshore areas.

Keywords small river plume wind effect freshwater transport

Corresponding Author(s): Wenping GONG

Just Accepted Date: 25 November 2016 Online First Date: 24 March 2017 Issue Date: 23 January 2018

Cite this article:

Junpeng ZHAO,Wenping GONG,Jian SHEN. The effect of wind on the dispersal of a tropical small river plume[J]. Front. Earth Sci., 2018, 12(1): 170-190.

URL:

https://academic.hep.com.cn/fesci/EN/10.1007/s11707-016-0628-6
https://academic.hep.com.cn/fesci/EN/Y2018/V12/I1/170

Fig.1 Study area and the in-situ observation stations. In panel (b), the isobaths are included with an interval of 10 m. The filled squares represent the tidal gauge stations, and the filled triangles represent the shipboard survey stations in summer, 2012. The solid and dotted lines are the alongshore transect and cross-shelf transect, respectively. The filled cycles in panel (c) indicate the locations of underway stations, and the color denotes the surface salinity measured.

Fig.2 (a) The monthly measured discharge from 1998 to 2007 and (b) the monthly averaged discharge at the Jiaji hydrologic station.

Fig.3 The model domain and bathymetry (m).

Fig.4 Comparison of observed (dotted lines) and modeled (solid lines) water elevations at three tidal gauge locations.

Fig.5 Comparison of observed and modeled salinity profiles.

Tab.1 Table 1r, AAD, RMSE, and Skill Assessment Parameter (skill) for modeled elevation and surface salinity

Fig.6 Comparison of surface salinity from model simulation (blue lines) and underway observation (red lines) for the 12 sections.

Fig.7 Surface salinity and flow (a, c) and freshwater depth (b, d) for the initial condition and after 8 days of the simulation without wind. The white lines in the upper panel represent the 34.0-psu isohaline. The blue lines in (b) indicate the sections for calculating the Wedderburn number and PEA.

Fig.8 The surface salinity overlain with velocity (upper panels) and freshwater depth (lower panels) after 8 days of the simulation under the 5 m·s^–1 southwesterly (a, e), northeasterly (b, f), northwesterly (c, g), and southeasterly (d, h) winds. The white lines in the upper panel represent the 34.0-psu isohaline.

Fig.9 The 5 m·s^–1 wind-induced vertical salinity profile and current field for cross-shelf section (left panels) and alongshore section (right panels) after 8 days of the simulation with southwesterly (a, b), northeasterly (c, d), northwesterly (e, f), and southeasterly (g, h) winds. The distance in the cross-shelf section is from mouth to offshore, and the distance in the alongshore section is from south to north. The vertical velocity is scaled by 500 m·s^–1. Filled contours indicate salinity and the contours superimposed on the shading indicate alongshore velocity (in cross-shelf sections, positive down-shelf) or cross-shelf velocity (in alongshore sections, positive offshore).

Fig.10 The surface salinity overlain with velocity (upper panels) and freshwater depth (lower panels) after 8 days of the simulation under a southwesterly wind with 1 (a, d), 3 (b, e), and 10 m·s^–1 (c, f) of speed. The corresponding results with 5 m·s^–1 are shown in Figs. 8(a) and 8(e).

Fig.11 The southwesterly wind-induced vertical salinity profiles and current field for the cross-shelf section (left panels) and alongshore section (right panels) after 8 days of the simulation with no wind (a, b), 1 m·s^–1 (c, d), 3 m·s^–1 (e, f), and 10 m·s^–1 (g, h) wind speed. The corresponding results with 5 m·s^–1 are shown in Figs. 9(a) and 9(b). The distance in the cross-shelf section is from the mouth to offshore, and the distance in the alongshore section is from the south to the north. The vertical velocity is scaled by 500 m·s^–1. Filled contours indicate salinity and the contours superimposed on the shading indicate alongshore velocity (in cross-shelf sections, positive down-shelf) or cross-shelf velocity (in alongshore sections, positive offshore).

Fig.12 Vertical integrated freshwater transport under 5 m·s^–1 wind in southwesterly (a, e, i, m), northeasterly (b, f, j, n), northwesterly (c, g, k, o) and southeasterly (d, h, l, p) directions. The panels in the first, second, third, and fourth rows are total freshwater flux, the advection transport, vertical shear transport, and oscillatory transport, respectively.

Fig.13 Vertically integrated freshwater transport under no-wind (a, e, i, m), 1 m·s^–1 (b, f, j, n), 3 m·s^–1 (c, g, k, o), and 10 m·s^–1 (d, h, l, p) under a southwesterly wind. The panels in the first, second, third, and fourth rows are total freshwater flux, the advection transport, vertical shear transport, and oscillatory transport, respectively. The corresponding results with 5 m·s^–1 are shown in Figs. 12(a), 12(e), 12(i), 12(m).

Tab.2 Wedderburn number under different wind velocity

Fig.14 Schematic diagrams showing the influence of the wind straining and mixing on the stratification of the plume under the southwesterly (a), northeasterly (b), northwesterly (c), and southeasterly (d) winds at a speed of 5 m·s^–1. Arrows indicate the directions of buoyancy-driven surface flows in different areas of the plume. Symbols at two sides of the arrows indicate the effect of wind straining (red) and wind mixing (black). Plus (+) indicates an increase in stratification and minus (?) indicates a decrease.

Tab.3 Non-dimensional PEA at different parts of the plume in different wind conditions. The positions of the sections are indicated in Fig. 7

Fig.15 The correlation between Wedderburn number and non-dimensional PEA.

Fig.16 Simulated surface plume and current structures on 24 August 2012 under real (a) and no-tide (b) conditions. The white lines represent the 34.0-psu isohaline.

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