Towards the dynamical convergence on the jet stream in aquaplanet AGCMs

A systematic equatorward bias of the jet position is identified in several aquaplanet AGCMs, with the coarser horizontal resolution producing more equatorward eddy-driven jet and stronger jet intensity in the upper troposphere. As the grid mesh of the models becomes finer than 50km, the jet position and intensity show signs of convergence. The mechanism for this convergence behavior is investigated using a hybrid Eulerian-Lagriangian finite-amplitude wave activity budget developed for the upper tropospheric absolute vorticity. The result suggests that the poleward shift of the eddy-driven jet with higher resolution can be attributed to the smaller effective diffusivity in the midlatitude that allows more wave activity to survive the dissipation and to reach the subtropical critical latitude for wave breaking. The enhanced subtropical wave breaking and associated irreversible vorticity mixing act to maintain a more poleward peak of the vorticity gradient, and thus a more poleward jet.

In accordance with the asymptotic behavior of effective diffusivity of Batchelor turbulence in large Peclet number limit, the upper tropospheric effective diffusivity of these aquaplanet AGCMs displays signs of convergence in the midlatitude towards a value of approximately 107 m2 s-1 for the ∇2 diffusion. This provides a dynamical underpinning for the convergence of the jet stream observed in these AGCMs.