The composite temporal evolution of the anomalous 300 mb streamfunction field and each term in the vorticity equation is examined for both phases of the PNA (Pacific North America) pattern. For this purpose, NCEP-NCAR Reanalysis data is used, and the PNA is identified using a rotated principal component analysis.
The covariance between each term in the vorticity equation and the anomalous streamfunction field is calculated for all lags relative to the onset day. Such calculations reveal that linear processes dominate during most of the growth and the decay of the PNA anomalies. The anomaly growth and maintenance is primarily due to the interaction between the PNA anomaly and the climatological stationary eddies, and the decay of the anomaly arises from dispersion through divergence and advection. The lifetime of the PNA is found to be prolonged several days by driving from high-frequency (<10 day) eddies. The forcing of the anomaly by incoherent low-frequency transient eddies also contributes secondarily to the growth and decay of the PNA anomalies.
A series of barotropic model calculations is performed to isolate the role of each forcing term on the evolution of the PNA anomalies. These calculations reveal that although the amplitude of the incoherent low-frequency transient eddy forcing is small, the structural changes induced by this forcing on the PNA anomaly account for the dispersive decay of the anomaly.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics