The Evolution and Time Scales of the Atmospheric Response to Tropical Diabatic Heating Events.

Simulations of tropical-midlatitude interactions have shown tropical heating anomalies to provoke significant responses in the midlatitude circulation on time scales shorter than those of current operational forecasts. In order to determine the impact of these tropical forcings, a series of 36-day real data global forecasts are used to examine the evolution and time scales of the geopotential height and zonal wind response to both realistic and theoretical configurations of imposed tropical diabatic heating. The principal goals are to determine: (1) the time scale at which the transient response of real data integrations with a general circulation model begins to resemble the perpetual season response found in the same model for long -term climate integrations; (2) the sensitivity of extended range forecasts to the presence or absence of time oscillations in the tropical heating; (3) the ways in which the responses forced in real data forecasts vary with the geographic location of the tropical heating anomaly. Remote responses to anomalous heating placed in the eastern Pacific are first evident between days 5 to 10. The transition from a local, short-term type of response, to a large-scale response pattern similar to that seen in perpetual climate runs occurs after approximately 3 -15 days into the integration. The comparison of time-dependent forcings to steady forcings which share the same time-averaged value shows the responses at individual times in the integrations to be quite different between the two treatments. However, time averages of these extended range predictions produce rather similar response patterns. A westward displacement of the tropical heating anomaly into the central Pacific Ocean results in an enhanced midlatitude response. A similar enhancement occurs for an eastward displacement of the heating anomaly into the central Tropical Atlantic Ocean. A heating anomaly at this location is particularly effective in accelerating the subtropical jet stream in the North Atlantic Ocean, thereby shifting the storm track ordinarily found around the east coast of North America toward the Atlantic Ocean. The different heating configurations employed in this study indicate that the presence of moderate to strong tropical forcings in the western hemisphere have a systematic effect upon the flow in the middle and high latitudes after approximately 15 days. Strong correlations have been shown between forecast skill in the Pacific/North American (PNA) region with the signed amplitude of the PNA mode. Thus, in an operational setting, the presence of a moderate to strong disturbance in the equatorial western hemisphere may provide the stimulation of the PNA mode sufficient to result in periods of increased midlatitude forecast skill.