基于四维变分同化方法的南海中尺度涡后报实验

THE MESOSCALE EDDY HINDCAST EXPERIMENT FOR THE SOUTH CHINA SEA BASED ON 4D-VAR METHOD

海洋中尺度涡在本质上是属于满足准地转平衡的大尺度运动,因此理论上,其在短时间内的运动将主要受到准地转平衡关系的约束,而外部强迫场的影响在短期内不会明显改变其运动特征。基于上述思想,我们提出了一种基于四维变分同化初始场的中尺度涡旋预报方案。为了检验该方案的可行性,本文使用区域海洋模式(regional ocean modeling system,ROMS)和其内建的增量强约束四维变分同化(incremental strong constraint four dimensional variational,I4D-Var)模块,建立了一个南海海洋同化模拟系统。首先,通过I4D-Var方法将AVISO卫星高度计资料同化到海洋数值模拟中,获得了理想的中尺度涡同化模拟结果。同化、模式模拟和观测三者的中尺度涡统计结果表明,该同化系统模拟的南海中尺度涡的路径、半径、海表高度异常和振幅等特征信息与AVISO(Archiving Validation and Interpolation of Satellite Oceanographic Data)观测结果高度吻合,同时在深度上的分析表明,涡旋对应的温度、盐度和密度均得到有效的调整。然后,将该同化系统的模拟结果做为初始场,对某一特定时段的南海中尺度涡进行了后报模拟和结果的定量化分析。通过比较后报模拟与观测资料中对应涡旋的海表面高度异常(sea surface height anomalies,SSHA)相关系数、涡心差距和半径绝对误差,证明该方案的中尺度涡后报时效至少可达10 d以上。后报实验结果验证了该中尺度涡预报方案的可行性,从而为中尺度涡的预报提供一定的理论基础和可行性方案。

Ocean mesoscale eddy is essentially a large scale motion satisfying quasi geostrophic equilibrium.Theoretically,its motion will be mainly constrained by the quasi geostrophic equilibrium relationship in a short time,while the influence of external forcing field will not change its motion characteristics significantly.Therefore,a prediction scheme of mesoscale eddy was proposed based on the initial field of four-dimensional variational assimilation.To test the feasibility of the scheme,a regional ocean model system(ROMS)and its built-in the primal formulation of incremental strong constraint four dimensional variational(I4D-Var)module were used to establish a marine assimilation simulation system for the South China Sea(SCS).First,AVISO altimeter data were assimilated into the ocean numerical simulation by the I4D-Var method,and the ideal mesoscale eddy assimilation simulation results were obtained.The statistical results of assimilation,model simulation,and observation show that the path,radius,sea surface height anomaly and amplitude of the mesoscale eddies simulated by the assimilation system are in good agreement with those observed by AVISO.Meanwhile,the depth analysis shows that the temperature,salinity,and density of eddies could be effectively adjusted.Secondly,the simulation results of the assimilation system were used as the initial field to simulate and quantitatively analyze the mesoscale eddies in the SCS in a certain period.By comparing the SSHA(sea surface height anomaly)correlation coefficient,eddy center distance,and radius absolute error of the corresponding eddies in the post prediction simulation and observation data,the post prediction time of mesoscale eddies in this scheme reached at least 10 days.The results of the post prediction experiments verified the feasibility of the proposed scheme,which provides a theoretical basis and a feasible scheme for the prediction of mesoscale eddies.