Showing papers by "Isaac M. Held published in 1990"

The Stationary Wave Response to a Tropical SST Anomaly in an Idealized GCM

Abstract: The upper tropospheric stationary wave response to a tropical sea surface temperature (SST) anomaly is examined with an idealized general circulation model (GCM) as well as steady linear and nonlinear models. The control climate of the GCM is zonally symmetric; this symmetric climate is then perturbed by a dipolar SST anomaly centered at the equator. Two experiments, with anomaly amplitudes differing by a fact of two, have been conducted. The response is very linear in the amplitude of the SST anomaly. A steady, baroclinic model linearized about a zonally symmetric basic state simulates the GCM's stationary wave reasonably well when it is forced by anomalous heating as well as anomalous transients. When decomposing the GCMs flow into parts forced separately by heating and transients, tropical transients are found to play a dissipative role to first approximation, reducing the amplitude of the response to heating by a factor of two. The effects of extratropical transients are relatively weak. A st...

A Barotropic Model of the Interaction between the Hadley Cell and a Rossby Wave

Abstract: A barotropic model is described that is designed to study the interaction of the Hadley cell with a Rossby wave forced in midlatitudes by a stationary “topographic” source. The Hadley cell is driven by a mass source/sink that is partly fixed, representing solar heating, and partly dependent on the layer thickness, representing infrared cooling. The response of the mean zonal and meridional winds to infinitesimal wave forcing is analyzed in detail; then the forcing is gradually increased to examine the departures from linearity.

Orographic versus Thermal Forcing of Stationary Waves: The Importance of the Mean Low-Level Wind

Abstract: The amplitude of the linear, stationary response to low-level extratropical heating decreases as the magnitude of the low-level mean flow increases, while the amplitude of the orographically forced waves increases. As a result, linear theory predicts that the relative importance of thermal and orographic forcing for the extratropical stationary wave field is very sensitive to the magnitude of the zonal mean low-level winds. In the process of illustrating this sensitivity, we also show how the dependence of the orographic response on the low level winds can be distorted by a numerical σ-coordinate model.

The two-fifths and one-fifth rules for rossby wave breaking in the WKB limit

Abstract: A stationary Rossby wave, sinusoidal in longitude, is slowly switched on, and the meridional propagation of the resulting wave front through a shear flow is examined. Initially the flow is westerly everywhere and therefore free of critical layers. The transition from reversible to irreversible behavior as the wave amplitude is increased is described. It is shown that under slowly varying conditions in an inviscid quasi-linear model, a steady state is obtained if, and only if, the mean flow is decelerated by less than two-fifths of its initial value as a result of the passage of the wave front. If this passage causes a larger mean flow reduction, a pile-up of wave activity in the shear layer culminates in the generation of a critical layer, qualitatively as in Dunkerton's model of gravity wave–mean flow interaction. This qualitative picture is shown to be preserved in the quasi-linear model when the slowly varying assumption breaks down. Fully nonlinear calculations show that these quasi-linear re...