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Shyh-Chin Chen
and
Daniel R. Cayan

Abstract

Behavior of regional precipitation and temperature over the West Coast of the United States was examined in a long perpetual winter simulation from a simplified global general circulation model. The model, a simplified version of the U.S. National Weather Service global operational forecast model, was run over a series of 568 winters, complete with geopotential, precipitation, and near-surface temperature. In spite of the fixed climatologica boundary conditions, the simulated winter-mean precipitation and temperature anomalies have a fairly realistic low-frequency regional variability. Both synoptic-scale events and seasonal average behavior are produced quite realistically by the model. Like observations, the regional surface variations can be related to the large-scale low-frequency circulation.

Four regional temperature/precipitation extreme—namely, warm/dry, cool/wet, cool/dry, and warm/wet—can be identified from the simulated winter-mean time series over the West Coast. Associated with these four regional extremes, model Northern Hemisphere 500-rnb height composites exhibit distinct planetary-scale circulation patterns. An empirical orthogonal function analysis further reveals that the first and third modes of the 500-mb height anomalies am primary contributors to these four regional extremes. The first mode largely governs the regional temperature variation, whereas the third mode largely determines the precipitation variation.

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Warren B. White
and
Shyh-Chin Chen

Abstract

Tropospheric temperature and vorticity budgets for the Antarctic Circumpolar Wave (ACW) are diagnosed utilizing the National Centers for Environment Prediction–National Center for Atmospheric Research reanalysis datasets from 1983 to 1992, focusing on the eastern Atlantic, Indian, and western and central Pacific sectors of the Southern Ocean where remote forcing from the Tropics has been observed to be weak. There, warm sea surface temperature (SST) anomalies are found in the ACW propagating eastward together with anomalous upward latent heat flux, positive precipitation, low-level convergence, upper-level divergence, midlevel ascent, and poleward surface wind. Diagnosing the anomalous temperature budget finds SST-induced latent heat flux instigating anomalous mid- to upper-level diabatic heating and low-level diabatic cooling in the absence of significant eddy heat flux divergence. This diabatic heating profile is balanced by a combination of vertical and horizontal heat advection, giving rise to anomalous ascent and poleward wind throughout the column. The thermodynamics of this deep diabatic heating scenario are different from those of Palmer and Sun. An intrinsic feedback from atmosphere to ocean is indicated by reduced sensible-plus-latent heat flux displaced 45° to 90° of phase to the east of warm SST anomalies, yielding an anomalous SST warming tendency that contributes both to eastward phase propagation and amplitude maintenance of the ACW. Diagnosing the anomalous potential vorticity budget finds the vertical gradient of anomalous diabatic heating, negative over most of the column, balanced by the anomalous advection of planetary vorticity, the mean advection of anomalous relative vorticity, and net vortex tube advection, together yielding a poleward equivalently barotropic wind response to warm SST anomalies. This deep diabatic heating scenario is contrasted against the remote forcing scenario in the eastern Pacific and western Atlantic sectors of the Southern Ocean where remote forcing associated with the El Niño–Southern Oscillation (ENSO) in the Tropics can now be seen to drive the SST tendency in the ACW.

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