Mechanism Of Phytoplankton Blooms Induced By Tropical Cyclones In The Southeastern Arabian Sea

Tropical cyclones are strong cyclonic vortices with warm structure occurring in the tropical ocean.When sea surface nutrients are scarce,tropical cyclones in the northern hemisphere can cause Phytoplankton blooms.The Arabian Sea,as a key area of the 21 st Century Maritime Silk Road,has not received much attention from scholars due to its relatively low frequency of tropical cyclones in the Bay of Bengal.In this thesis,14 cases of Phytoplankton blooms caused by tropical cyclones generated in the southeast Arabian Sea from 1998 to 2019 were analyzed,and 5 cases caused Phytoplankton blooms.Taking 1998 ARB 06,2004 ARB 01 and similar paths2002 ARB 01 and 2018 Luban as examples,this thesis discusses and analyzes them using reanalysis data,satellite data and measured data from Argo floats.The results showed that in the case of 1998 ARB 06 and 2004 ARB 01,it was found that the Phytoplankton blooms lasted for 22 days after the tropical cyclone ARB 06 passed over the southeast Arabian Sea.After tropical cyclone ARB 01,which circled the study area,passed over the southeastern Arabian Sea,a Phytoplankton blooms lasted for 10 days.Before the arrival of tropical cyclones,the deep nutrients in ARB 06 were prevented from ascending to the mixed layer mainly by the double thermocline,and the deep nutrients in ARB 01 were prevented by the single thermocline.After transit,the shallow thermocline disappears,the deep thermocline changes from 45.7 m to 76.5 m,and the deep thermocline changes from 40 m to 79.33 m in ARB01 case.The mixed layer deepens,and the upper ocean stratification weakens.According to the relative vorticity of ocean flow field before and after the passage of tropical cyclone,it is found that there exists an eddy pair in ARB 06.During the two-day inertia oscillation,the vorticity of the cyclonic eddy is stronger than that of the anticyclonic eddy,and lasts for more than 10 days,and then strengthens by 55 m with shallow strong upwelling.Analyzed the ARB 01 instance of Marine eddy on the relative vorticity,rossby number,level dynamics parameters,such as tensile strength,found in the type that jump a layer of three days of the cycle of staggered up and down the inertial oscillation of nutrient continuous rise in the mixed layer depth,upper ocean enhanced cold eddy mainly happens on the right side of the Box b,as a result,Strong vertical mixing and upwelling caused phytoplankton blooms to occur mainly on the right side of Box b.In addition,it was found that the vertical current shear caused by the oscillations caused by tropical cyclones not only enhanced the vertical mixing,but also played an important role in the distribution of phytoplankton blooms.After the passage of a tropical cyclone,there is strong upwelling and reduced stratification of the ocean,where nutrients from below the thermocline enter the upper layers of the ocean and trigger phytoplankton blooms.In studies of tropical cyclones with similar paths,Luban caused phytoplankton blooms as they passed over the southeastern Arabian Sea,but ARB 01 did not.By analyzing the response of sea-air environment change and related dynamic parameters,it is found that the weak vertical wind shear,the weakening of anticyclonic eddy,the enhancement of cyclonic eddy and the maintenance of long time can easily transport the nutrients below the thermocline to the ocean surface.