Study On Coupling Of Biochemistry And Physical Processes In The Upper Ocean During Typhoon

Typhoon,as a powerful weather process,not only affects the dynamic processes such as ocean upwelling and vertical mixing,but also changes the ocean biogeochemical and physical coupling processes,thus causing strong biological response.A deep understanding of ocean response to typhoon can not only improve the ability of typhoon prediction,but also deepen our understanding of local and global air-sea interaction.Many international cooperative research programs that have been implemented so far,such as "Ocean Observation System and Forecasting Experimental Research-THORPEX" and "Pacific and Asian Regional Seas Research-T-PARC",all point out that clarifying the dynamic process and biological environment changes caused by typhoons is the most challenging key to accurately understand the response of the upper ocean to typhoons.At present,because it is extremely difficult to observe during typhoon,the research on the response of the upper ocean during typhoon mostly depends on remote sensing data.Therefore,the research on ocean dynamic process and biological environment is still at the qualitative level,and the response and modulation of the upper ocean to typhoons with different characteristics are still unclear.In view of this situation,based on multi-source data(satellite,Argo and mooring observation)and model research,this paper synthetically analyzes and quantitatively evaluates the spatial and temporal distribution characteristics of physical and biochemical elements in Asia tropical marginal seas(Northwest Pacific Ocean: WNPO,South China Sea:SCS,Bay of Bengal: Bo B,Arabian Sea: AS)under the influence of typhoons,and discusses several typical typhoon events.The main conclusions of this study are as follows:(1)Within the typhoon path 0～100 km(100～300 km),the upper ocean is dominated by cooling(surface cooling,subsurface warming).The amplitude is skewed to the right.The cooling(warming)inside the mixed layer(bottom)is dominated by vertical mixing and upwelling and the air-sea heat flux(vertical mixing),while the cooling in the thermocline is dominated by upwelling.The salinity of WNPO,SCS and Bo B(AS)increases(decreases)with the depth,resulting in the increase(decreases)of sea surface salinity(SSS)after typhoons.The increased SSS in Bo B is the most significant.The greater vertical salinity gradients gradient,the greater abnormal salinity value.The nutricline in the AS and Bo B are located in the mixed layer,which is prone to ecological processes,resulting in a larger increase in chlorophyll-a(Chl-a)concentration than that of SCS and WNPO.The Chl-a anomaly caused by strong and fast(strong and slow)typhoon shows right bias(symmetry).The increased Chl-a in WNPO is more the result of the change of Chl-a structure caused by physical processes.The typhoons in WNPO increase(decrease)dissolved oxygen(DO)concentration in the surface(subsurface)because the vertical mixing and upwelling transport the high(low)concentration DO water body in the subsurface to the surface(subsurface)layer,thus forming a positive-negative structure.Since DO concentration in the AS and Bo B decreases rapidly with depth,the DO exhibits a negative-positive structure after the strong tropical cyclones.(2)The linear regression equations between SST and Chl-a and wind speed,moving speed,sea level anomaly(SLA)and mixed layer depth(MLD)under different types of typhoons are established,which provides a quantitative basis for evaluating the relationship between physics and biological response.Severe cooling and high concentration of Chl-a are mainly distributed in the sea area with MLD < 30 m,and the SLA can be used as an indicative parameter to represent the change characteristics of Chl-a and SST.SLA and MLD contribute more to the increased Chl-a than typhoon characteristics.The results of 1-D Price Weller Pinkel model show that the cooling magnitude of cyclonic eddy caused by the same typhoon is twice that of anticyclonic eddy,and only strong and slow typhoon caused long-term strong shear instability,and the accompanying turbulent mixing and upwelling process transported cold rich-nutrients water.(3)After the passage of three sequential typhoons(Haitang,Nesat and Nalgae),the low temperature area in the northwestern SCS lasted for 38 days,and two phytoplankton blooms appeared.The upwelling caused by typhoon is the main cause of the first bloom,and the second bloom is the result of strong near inertial turbulent mixing caused by Nesat and Nalgae.After the sequential tropical cyclones Kyarr and Maha in AS,two cooling processes and two short-period Chl-a increases occurred.At first,the rapid increase(decrease)of Chl-a in the surface(subsurface)is mainly due to the redistribution of physical processes after Kyarr,while the more obvious increase of Chl-a in the upper ocean is due to the net growth of phytoplankton.Upwelling(vertical mixing)is the primary driver of nutrient resupply near the path(right).(4)Typhoon Linfa triggered two phytoplankton declines in the northern SCS.One phytoplankton decline(3-fold reduction)in the continental shelf region can probably be attribute to the limited nutrient supply induced by the decayed coastal northeastern current and onshore Ekman transport(OET)and Kuroshio intrusion water,as well as the uplifted subsurface’s low Chl-a driven by vertical mixing and upwelling.Another phytoplankton decline(3.5-fold reduction)in the eastern Leizhou Peninsula-coastal region is probably caused by OET and a decayed coastal northern current.Annular tropical storm(TS)Nakri caused long-term cooling and bloom.The results of the temperature diagnostic equation show that the cooling mechanism is mainly mixed entrainment and vertical advection,rather than horizontal advection,and the contribution of mixing term is greater than(equal to)advection term for strong(weak)TS,effectively increasing the nutrient supply.