| Currently,climate change has become one of the common challenges faced by all mankind.The most intuitive manifestation of climate change is global warming,and the continuous warming will have far-reaching effects on the marine ecological environment.The presence of upwelling makes the exchange of upper and lower waters frequent,and more bottom nutrients can be brought to the sea surface to promote the growth of phytoplankton,thus forming an upwelling fishing ground.The sea area in northern Zhejiang Province is close to the mouth of the Yangtze River,and the input of land-based runoff and the supplementation of biogenic elements brought by upwelling make it a famous fishing ground of the Yangtze River mouth and Zhoushan fishing ground.As an important fishing area in China,it is more sensitive to the change of hydrodynamic environment brought by climate change,and the change of hydrodynamic environment will then directly affect the ecological environment and the occurrence of fishery resources in the fishing grounds.The existing studies on the interannual variation of upwelling characteristics in northern Zhejiang are relatively simple,mostly considering single factors such as wind field to calculate the upwelling index,while the formation of upwelling in northern Zhejiang is driven and regulated by multiple factors such as wind stress,topography and background circulation.Therefore,this paper focuses on the spatial and temporal changes of multiple dynamical environmental factors affecting the strength of upwelling under the background of global warming,analyzes the changing characteristics of the dynamical environment of upwelling in northern Zhejiang and its influence mechanism,and explores the possible change trend of future upwelling as the focus and innovation point of the study.This paper analyzes the spatial and temporal variability of summer sea surface temperature in the northern Zhejiang Sea through historical data from 1990 to the present.The summer sea surface temperature in the northern Zhejiang waters has shown an increasing trend in the past three decades,and after 2010,there are more likely to be abnormally high years with the average summer sea surface temperature around 27 °C.The results of the jump analysis show that there is a significant warming jump in the summer sea surface temperature around 2016.The results of the jump analysis show that there is a significant warming jump in the summer sea surface temperature around 2016,and the average temperature increases by about 1 °C,and the spring warming is an important reason for the warming of seawater in summer.The change of sea air heat flux explains the rapid warming of seawater in spring.The decrease of cloudiness,the enhancement of short-wave radiation and the increase of net heat flux at the sea surface in spring make the upper seawater store more heat and provide a higher initial value for the continued warming of seawater in summer.The results of the analysis of the spatial and temporal variation characteristics of the July monsoon in the northern Zhejiang Sea indicate that the summer wind in the northern Zhejiang Sea has a tendency to weaken since 1990.The influence of Kuroshio,Taiwan warm current and Yangtze River runoff are mainly considered for the open boundary conditions,in which the Kuroshio and Yangtze River runoff tend to increase in summer,while the Taiwan warm current shows the opposite characteristics.Based on the characteristics and patterns of interannual variation of the above environmental factors,a numerical model before and after the temperature jump was constructed to conduct a comparative study.The numerical simulation results reproduced that the sea surface temperature increased by about 1 °C before and after the temperature jump,and the surface-bottom temperature difference and buoyancy frequency in the study area decreased.From the analysis of the typical sections of the Yangtze River estuary and Zhoushan fishing grounds,the isotherm and the isosaline have been raised more obviously after the temperature jump,and the vertical flow velocity of the upwelling position in the section of the Yangtze River estuary has increased,while the upwelling range in the section of Zhoushan fishing grounds has increased,and the flow velocity on the shoreward side has increased,while the flow velocity on the seaward side has slightly decreased.To analyze the effects of various dynamical factors on the interdecadal variability of the upwelling characteristics in northern Zhejiang,four sets of sensitivity experiments were set up with the post-jump arithmetic as control experiments: changes in shelf circulation flux,changes in Yangtze River runoff flux,changes in sea-air heat flux and changes in wind field intensity.The results show that the change of shelf circulation flux(increase of Kuroshio flux and decrease of Taiwan warm current flux)under the change of climate state makes the vertical stratification of upwelling area along the north Zhejiang coast enhanced and the strength of upwelling is suppressed.The increase of Yangtze River runoff increases the extension of the low saline water tongue of Yangtze River freshwater,which makes the density stratification of the upper layer of seawater in the upwelling zone stronger and the bottom water less likely to be lifted upward;the most obvious effect of the increase of heat flux is to increase the surface-bottom temperature difference,which makes the stratification of seawater stronger and unfavorable to the lifting of the bottom water;the decrease of summer wind stress in the north Zhejiang sea will weaken the extension of the low saline water tongue of Yangtze River freshwater,which makes the stratification of the upper layer of seawater in the upwelling zone The reduction of the summer wind stress in the northern Zhejiang area will weaken the expansion of the low-salt water tongue of the Yangtze River,thus weakening the stratification of the upper layer of seawater in the upwelling area,and at the same time strengthening the creep of the lower layer of seawater to the shore,making the coastal upwelling stronger.In general,the changes in the inland shelf circulation and the Yangtze River runoff in the past 30 years under the background of temperature rise do not have a significant impact on the upwelling zone in the northern Zhejiang waters.The main changes come from the strengthening of the stratification of seawater brought by the temperature rise and the weakening of the stratification of the upper layer of seawater in the upwelling zone caused by the reduction of wind stress and the increase of the bottomto-shore advection transport.On this basis,this paper analyzes the changes of upwelling in northern Zhejiang under the two most likely emission scenarios around 2050 using the BCC-CSM2-MR(submitted by the National Climate Center)climate model data coupled with the ocean numerical model of CMIP6 program.The upwelling is suppressed more obviously under the high emission scenario.In this paper,we analyze the changes of summer sea surface temperature in the upwelling area of northern Zhejiang Province in recent decades,and divide the two periods before and after the sea surface temperature jump,and simulate the changes of coastal upwelling in northern Zhejiang Province before and after the temperature jump by establishing a high-resolution numerical model and analyzing the main influence mechanism.In summary,global warming will have a significant impact on the hydrodynamic conditions in the upwelling zone along the north coast of Zhejiang.In recent years,marine heat waves,seawater hypoxia,and red tides have occurred frequently in the coastal waters of northern Zhejiang Province,and climate change and the resulting changes in the hydrodynamic conditions of the sea are among the causes of such marine disasters.The research in this paper can provide a reference for the future formulation of ecological environment and fishery resources protection policies in the Yangtze River estuary and Zhoushan fishing grounds. |
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