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Timothy W. Barker
and
John D. Horel

Abstract

The persistence of the planetary-scale circulation in a perpetual January experiment of the NCAR Community Climate Model is investigated. Pattern correlations between maps of 500 mb geopotential height are used to identify periods in which the large scale flow remains quasi-stationary for periods of a week or longer. Thirty- one distinct periods are dominated by quasi-stationary flow patterns encompassing 22% of the model experiment. The time between quasi-stationary periods is typically longer than their duration. On the basis of subjective similarities among some of these events, we classified many of them into four distinct types. The characteristics of the model quasi-stationary regimes are contrasted with those observed and those found in simpler models.

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John D. Horel
and
Carlos R. Mechoso

Abstract

The persistence of the planetary-scale simulation over the North Pacific Ocean is investigated during 18 Northern Hemisphere winters from 1965/66 to 1982/83. Quasi-stationary flow patterns dominate 20 periods during the 6 El Niño winters. In contrast, 29 such periods are observed during the remaining 12 winters. Nearly all of the quasi-stationary episodes during El Niño winters exhibit negative 500 mb geopotential height anomalies in the Gulf of Alaska-Aleutian Island region. During the other 12 winters, episodes characterized by positive height anomalies in that region occur as frequently as those exhibiting negative height anomalies.

The observed persistence of the planetary circulation is contrasted to that simulated by the UCLA general circulation model. Ten winters of model output are analyzed: during five winters, sea surface temperatures (SSTs) are prescribed to evolve through their climatological seasonal cycle while during the other five winters, SST anomalies corresponding to idealized or observed El Niño conditions are added to the climatological field. The model atmosphere has less intraseasonal variability, and quasi-stationary events are less frequent than observed. However, the model is successful in simulating the observed preponderance of quasi-stationary regimes which exhibit below-normal 500 mb geopotential height anomalies in the Gulf of Alaska during winters with positive SST anomalies in the equatorial Pacific. The evolution of the model's quasi-stationary events suggests that they result directly from dynamical processes in midlatitudes, but their characteristics are apparently affected by SST conditions.

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John D. Horel
,
Andrea N. Hahmann
, and
John E. Geisler

Abstract

Outgoing longwave radiation (OLR) is used to describe the annual cycle of convection that resides over the Amazon Basin during austral summer and over Central America and the adjacent waters of the Pacific during austral winter. The preferred locations of the convective activity during the wet season in the respective hemispheres are determined, and the beginning and ending of these seasons is specified. The onset of the wet season over Amazonia usually occurs within a single month, while the onset of the wet season over Central America typically requires from one to three months. The annual cycle of convective activity in this regime is shown to exhibit a seasonal regularity and degree of symmetry with respect to the equator which exceeds those characterizing the other two annually varying regimes in the tropical belt. Analyses produced by the European Centre for Medium-range Weather Forecasts (ECMWF) are superimposed upon OLR fields to illustrate features of the atmospheric circulation in the vicinity of the tropical Americas that are associated with the annual cycle of convection. The onset and demise of the wet season in the Amazon Basin are further described by means of composites of these data. It is found that the Bolivian high inferred from the ECMWF data develops rapidly during the onset transition in a manner that is temporally and spatially consistent with the distribution of OLR.

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John D. Horel
,
Judith B. Pechmann
,
John E. Geisler
, and
Andrea N. Hahmann

Abstract

Numerical simulations of the atmospheric circulation over tropical South America are performed with a regional model developed at The Pennsylvania State University and the National Center for Atmospheric Research and commonly referred to as the MM4. The authors focus on a 5-day period beginning at 1200 UTC 27 February 1990. The observed circulation is evaluated in terms of initialized analyses of standard meteorological variables from the National Meteorological Center, outgoing longwave radiation from polar orbiting satellites, and surface observations. The NMC analyses are also used to specify the initial conditions, as well as provide the lateral boundary conditions, for the 5-day simulations.

The impacts on the simulated circulation of major changes to the standard MM4 are assessed. When an improved treatment of radiative processes is included, excessive rainfall develops over the Andes Mountains and over the Amazon Basin. The excessive rainfall is concentrated in “gridpoint” storms that are not eliminated when the surface physical parameterizations are improved. Modifications to the treatment of the vertical transport of moisture are required to diminish the excessive rainfall. Even with these and other changes included in the model, the simulated basin-averaged rainfall continues to exhibit unrealistic features. The improved, though still imperfect, model simulations are used to diagnose the temporal and spatial evolution of the circulation with an emphasis on equatorial-subtropical interactions.

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