Characteristics and influencing factors of frontal upwelling in the Yellow Sea in summer
Abstract: Frontal upwelling is an important phenomenon in summer in the Yellow Sea (YS) and plays an essential role in the distribution of nutrients and biological species. In this paper, a three-dimensional hydrodynamic model is applied to investigate the characteristics and influencing factors of frontal upwelling in the YS. The results show that the strength and distribution of frontal upwelling are largely dependent on the topography and bottom temperature fronts. The frontal upwelling in the YS is stronger and narrower near the eastern coast than near the western coast due to the steeper shelf slope. Moreover, external forcings, such as the meridional wind speed and air temperature in summer and the air temperature in the preceding winter and spring, have certain influences on the strength of frontal upwelling. An increase in air temperature in the previous winter and spring weakens the frontal upwelling in summer; in contrast, an increase in air temperature in summer strengthens the frontal upwelling. When the southerly wind in summer increases, the upwelling intensifies in the western YS and weakens in the eastern YS. The air temperature influences the strength of upwelling by changing the baroclinicity in the frontal region. Furthermore, the meridional wind speed in summer affects frontal upwelling via Ekman pumping.
Figure 1. Model domain with topography (a), and map of the study area (b). The red dashed box represents the location of the Yellow Sea and Bohai Sea. Coloured shading and grey solid lines denote bathymetry (in m). The black dots represent the locations of conductivity-temperature-depth (CTD) profiler stations from the cruise surveys in August 2015. The red star represents the H05 mooring station.
Figure 2. a. M2 co-tidal chart simulated by the model with homogeneous water. The solid and dashed lines denote the phase lag (°) and amplitude (cm), respectively. The phase lag was referenced to Beijing local time (UT + 8 h). The black dots denote locations of 77 tide gauges. b. Comparison between the modelled and observed amplitudes at 77 tidal gauges represented by the dots in a. c. Same as b but for the phase lag.
Figure 3. Comparisons between observed (solid lines) and modelled (dashed lines) results for meridional (a) and zonal (b) depth-mean tidal velocities at the H05 mooring station (shown in Fig. 1b).
Figure 5. Bottom temperature distribution of in situ observations (a) and monthly mean model results (b) in August 2015. The contour interval (CI) is 2°C. White dashed lines denote three cold cores of the Yellow Sea cold water mass. c. Distributions of thermocline strength in August 2015. Unit: °C/m with CI=0.5°C/m.
Figure 6. Temperature distributions of in situ observations (a, c) and monthly mean model results (b, d) in August 2015. The upper panels represent the 35°N section, and the lower panels represent the 36°N section. The contour interval is 2°C. The triangles in a and c denote the conductivity-temperature-depth stations.
Figure 7. Horizontal distributions of simulated bottom upwelling velocity (a); the dashed lines show two representative sections along 35°N (HK) and 37.2°N (CK); simulated bottom temperature fronts (BTFs) in August (b); the thick solid line denotes the location of the maximum temperature gradient along the belt of BTFs; S denotes the starting point of the solid line; A–H represent locations of maximum values in Fig. 6.
Figure 8. The distributions of bottom temperature fronts (BTFs) strength and upwelling velocity along the slope of the Yellow Sea (YS) (the thick solid line in Fig. 7b) in August. The green stars denote the maximum values in different regions. The locations of A–H are presented in Fig. 7b. The dashed line denotes the location of 124°E, which was selected as the divide between the western YS and eastern YS.
Figure 11. Distributions of temperature and u-w velocity along the 35°N section in the control run (a) and ExpA (b). The coloured shading and white solid lines denote temperature in °C, the white dashed lines denote cold cores of the Yellow Sea cold water mass. Vectors denote u-w velocity, and the vertical velocity has been multiplied by 1 000.
Table 1. Changes in external forcings in sensitivity experiments
Experiment Forcing conditions Expt1 air temperature in the previous winter increased by 20% Expt2 air temperature in spring increased by 20% Expt3 air temperature in summer increased by 20% Expw1 southerly wind in the previous winter increased by 20% Expw2 southerly wind in spring increased by 20% Expw3 southerly wind in summer increased by 20% Expr1 precipitation rate in the previous winter increased by 20% Expr2 precipitation rate in spring increased by 20% Expr3 precipitation rate in summer increased by 20% Note: The previous winter is defined as December to February, springis defined as March to May, and summer is defined as June to August.
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