Noise-Induced Transitions in Anisotropic Two-Dimensional Turbulence

Two-dimensional (2D) turbulence features an inverse energy cascade that produces large-scale flow structures such as hurricane-like large-scale vortices (LSVs) and jets. We investigate the dynamics of such large-scale structures using extensive direct numerical simulations of stochastically forced 2D turbulence in a periodic rectangular domain. LSVs form in the system when the aspect ratio δ≈1 and unidirectional jets arise at δ≳1.1. At intermediate δ, noise-induced transitions occur between LSVs and jets. We collect detailed statistics on the lifetimes of these structures, revealing an approximately exponential dependence of the mean lifetime on δ. We also analyze how the lifetimes are impacted by varying the Reynolds number and the scale separation between forcing scale and domain size. The system is found to traverse the same path in Fourier space when transitioning from LSVs to jets and vice versa. In highly elongated domains (δ≫1), an array of jets forms, where the number of jets tends to increase with δ, albeit irregularly. Noise-induced transitions also occur between different numbers of jets. The profiles of the jets in this system are sinusoidal, in contrast with jets on the beta plane. Our findings shed new light on the dynamics of LSVs and jets in anisotropic turbulence.