Atmospheric Response to Tropical Thermal Forcing in Real Data Integrations

Abstract

Evidence is presented for a highly regular seasonal rearrangement of the long-wave pattern over North America from the winter to the spring. The change involves a reversal of the trough-ridge pattern between the two seasons wherein the anticyclone observed in the winter climatology over and west of the Rocky Mountains reverses to a spring trough, and the winter lee cyclone changes to a spring ridge over the eastern United States. These changes, which represent a westward shift of the long-wave pattern from winter to spring, are accompanied by westward shifts of the subtropical jet core from the east to the west coast, and of the tropical rain maxima from the Amazon Basin to the tropical East Pacific Ocean. We investigate the possibility that these westward displacements are dynamically related in a series of ten-day integrations of a general circulation model over four separate ensembles, each consisting of ten cases, during the winter and spring. The data for these ensembles is taken from the Global Weather Experiment, and is used to initialize the NCAR general circulation model. This model maintains the winter-spring patterns observed in the western hemisphere when run in a control mode. The tropical precipitation distribution around the Eastern Pacific and the Amazon Basin is then modified to produce a winterlike rainfall pattern for the spring ensemble. The resulting extratropical changes in the forecast wind and height field resemble the observed winter pattern, rather than the spring distribution of the control. When the tropical precipitation of the winter cases is modified to simulate the distribution observed in spring, the resulting experiments produce forecast changes that resemble the extratropical pattern of spring, although the apparent response is not as great as the observed seasonal changes. The statistical reliability of the winter experiments appears to be higher than that of the spring experiments.