Sensitivity analysis of atmospheric low-frequency variability

The sensitivity of the Pacific-North-American (PNA) pattern and the North-Atlantic Oscillation (NAG) pattern to small perturbations is investigated using adjoint techniques. Analysis is performed on integrations of a quasi-geostrophic (QG) model whose internal low-frequency variability is realistic. Specifically, the sensitivity of the QG-model PNA and NAO patterns have been made from a large set of five-day integrations of the adjoint of the QG-model, linearized about time-varying basic-state solutions of the model. The sensitivity perturbations are initially located well upstream of the pattern of low-frequency variability and propagate downstream by wave dynamics, gaining energy through both barotropic and baroclinic processes. For the PNA pattern, it was found that variations in sensitivity were associated with fluctuations in the strength of the jet across the North Pacific. For the NAO pattern, it was found that fluctuations in sensitivity were correlated with the strength of the PNA pattern, the more sensitive NAO basic states having negative PNA index. This analysis has identified fairly distinct paths in phase space associated with predictable and unpredictable transitions, qualitatively similar to that found in the three-component Lorenz model.