Weather Regimes in the Pacific from a GCM

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Abstract
Weather regimes have been sought by examining the 500-mb streamfunction of the UGAMP GCM run for 10 yr at T42 resolution with perpetual January forcing. Five-day low-pass EOFs provide a low-order phase space in which to study dynamical aspects of the variability. The PNA pattern shows up as the first EOF over the Northern Hemisphere representing 12% of the variance, rising to 18.5% for Pacific-area-only EOFs. Within the phase space of three to five EOFs, two local minima of the area-averaged ψ tendency (based on rotational velocity advection) are found. These two flow patterns both have a smaller implied tendency than the climatology and lie in the ±PNA regions of the phase space. It is suggested that these patterns may be acting as “fixed points” within the atmospheric attractor, encouraging persistent flows and the formation of weather regimes. These dynamical attracting points are compared with a more conventional means of identifying weather regimes using a statistical maximum likelihood analysis of all model states during the 10-yr GCM run. This analysis also indicates two preferred classes, separate from the climatology, in the ±PNA regions of phase space. These classes tend to be nearer the climatology than the dynamical states but have similar appearance otherwise. Finally the role of low-frequency transients are examined to improve the dynamical interpretation of the regime centers. The method is first demonstrated for the extended Lorenz model of Molteni et al. The fixed points of the GCM attractor are assumed to be steady solutions to the 500-mb vorticity equation in the absence of contributions from transient eddies. The eddy contributions to the climatological vorticity budget are first determined, and then the deviations from the climatology that could provide similar contributions to the budget are found. Again two states in the ±PNA regions of phase space are found to satisfy the above conditions. The authors speculate that the attractors themselves are determined by the large-scale steady effects of topography and land-sea contrasts.

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