| Abstract: | In this paper,we find the optimal precursors which can cause double-gyre regime transitions based on conditional nonlinear optimal perturbation(CNOP)method with Regional Ocean Modeling System(ROMS).Firstly,we simulate the multiple-equilibria regimes of double-gyre circulation under different viscosity coefficient and obtain the bifurcation diagram,then choose two equilibrium states(called jet-up state and jet-down state)as reference states respectively,propose Principal Component Analysisbased Simulated Annealing(PCASA)algorithm to solve CNOP-type initial perturbations which can induce double-gyre regime transitions between jet-up state and j et-down state.PCASA algorithm is an adj oint-free method which searches optimal solution randomly in the whole solution space.In addition,we investigate CNOP-type initial perturbations how to evolve with time.The results show:(1)the CNOP-type perturbations present a two-cell structure,and gradually evolves into a three-cell structure at predictive time;(2)by superimpo sing CNOP-type perturbations on the j et-up state and integrating ROMS,double-gyre circulation transfers from jet-up state to jet-down state,and vice versa,and random initial perturbations don't cause the transitions,which means CNOP-type perturbations are the optimal precursors of double-gyre regime transitions;(3)by analyzing the transition process of double-gyre regime transitions,we find that CNOPtype initial perturbations obtain energy from the background state through both barotropic and baroclinic instabilities,and barotropic instability contributes more significantly to the fast-growth of the perturbations.The optimal precursors and the dynamic mechanism of double-gyre regime transitions revealed in this paper have an important significance to enhance the predictability of double-gyre circulation. |
