Predictability associated with nonlinear regimes in an atmospheric model
Abstract
Numerous previous studies addressed mid-latitude atmospheric flow patterns, or regimes, that persist for periods of time exceeding a few days (typical lifetimes of weather systems). In this paper, output of an extensively studied and realistic three-layer quasi-geostrophic (QG3) atmospheric model is analyzed to examine previously hypothesized and anticipated relationship between regimes and predictability. Here, the authors defined the regimes as the regions of the model phase subspace characterized by excess probability of persistence relative to a benchmark linear empirical model (EMR), for geographically two-dimensional, as well as zonally averaged flow patterns. The regimes identified here match those found in previous studies and correspond to the opposite phases of the largely zonally symmetric Arctic Oscillation (AO+ and AO-), and to the more regionally identified regimes reflecting the positive phase of the North Atlantic Oscillation (NAO+). It is found that predictable regimes are connected to particular "regime precursor" regions of the model's phase space. Packets of trajectories originating from these regions "flow" into regime regions following mean phase-space velocities, and are characterized by anomalously low spreading rates. The nonlinear component of mean phase-space velocity in predictable-regime regions counteracts the linear EMR-based tendency of trajectories toward climatology and contributes to maintenance of the regime anomalies. Low spreading rates of trajectories in these regions are due to a combination of low local stochastic diffusivity along the trajectory pathways and convergence of the nonlinear component of mean phase-space velocities there. In contrast, unpredictable regimes, while being also maintained by the nonlinear phase-space velocities counteracting linear tendency toward climatology, are not connected to well-defined precursor regions and exhibit high spreading rates. In this model, the predictable regimes AO+ and AO- are largely zonally symmetric and insensitive to the metric used to identify the regimes, while the unpredictable, regionally intensified regime NAO+ in the two-dimensional space is not directly associated with its counterpart defined using zonal metric. These results provide a unifying framework for previously proposed theories and interpretations of weather regimes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMNG32B..04K
- Keywords:
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- 4400 NONLINEAR GEOPHYSICS