Low-frequency variabilities for widely different basic flows

Abstract

The internal dynamics associated low-frequency variabilities (LFVs) with timescales between 7 days and 31 days, relevant to the potential predictability of monthly means, are investigated. Using observed data and a set of general circulation model produced data, it is shown that the LFV characteristics are distinct for widely different basic flows: much smaller variability for north Pacific cyclonic basic flows than anti-cyclonic basic flows. Their preferred development locations are also distinct. The dynamical processes leading to this large difference is examined in the light of the local barotropic energy conversion between basic flow and low-frequency components. Over the eastern North Pacific where the LFVs are primarily located, the energy conversion decays more from the low-frequency disturbances into the cyclonic basic flows, while much more extraction of LFV energy from the anti-cyclonic basic flows takes place. The internal LFVs considered here are closely related to the climate noise pertaining to the prediction of monthly means. A dynamical link between the phase of El Ni±o/Southern Oscillation (ENSO) and the potential predictability of monthly means can therefore be established: during the warm phase of ENSO when the mean north Pacific flow is usually cyclonic, the potential predictability of the monthly means can be expected to be higher than during the cool phase, due to the substantially different magnitudes of unpredictable noise being generated through barotropic instability of these different zonally varying basic flows.