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Abstract:

© 2016 Royal Meteorological Society.A two-dimensional regime diagram for geostrophic turbulence in the ocean is constructed by plotting observation-based estimates of the non-dimensional eddy length-scale against a nonlinearity parameter equal to the ratio of the root-mean-square eddy velocity and baroclinic Rossby phase speed. Two estimates of the eddy length-scale are compared: the equivalent eddy radius inferred from the area enclosed by contours of sea-surface height, and the 'unsuppressed' mixing length, based on an estimate of the eddy diffusivity with mean flow effects removed. For weak nonlinearity, as found in the Tropics, the mixing length mostly corresponds to the stability threshold for baroclinic instability whereas the eddy radius corresponds to the Rhines scale; it is suggested that this mismatch is indicative of the inverse energy cascade that occurs at low latitudes in the ocean and the zonal elongation of eddies. At larger values of nonlinearity, as found at mid- and high latitudes, the eddy length-scales are much shorter than the stability threshold, within a factor of 2.5 of the Rossby deformation radius.

Abstract:

© 2015 Royal Meteorological Society. We investigate the predictability of the thermally driven rotating annulus, a laboratory experiment used to study the dynamics of planetary atmospheres under controlled and reproducible conditions. Our approach is to apply the same principles used to predict the atmosphere in operational weather forecasting. We build a forecasting system for the annulus using the analysis correction method for data assimilation, the breeding method for ensemble generation, and the Met Office/Oxford Rotating Annulus Laboratory Simulation as the forecast model. The system forecasts the annulus in steady (2S), amplitude vacillating (3AV), and structurally vacillating (3SV) flow regimes, verifying the forecasts against laboratory data. The results show that a range of flow regimes from this experiment can be accurately predicted. Forecasts in the steady wave flow regime perform well, and are predictable until the end of the available data. Forecasts in the amplitude and structural vacillation flow regimes lose quality and skill by a combination of wave drift and wavenumber transition. Amplitude vacillation is predictable up to several hundred seconds ahead, and structural vacillation is predictable for a few hundred seconds. The wavenumber transitions are partly explained by hysteresis in the rotating annulus experiment and model.