Perturbation pressure depth as a tool for analysis of tropical-extratropical interactions

Once considered as separate regimes, interactions between the tropics and extratropics have been increasingly highlighted as an important driver of global weather and climate processes. Many of these interactions are manifest as tropical convective outflow deposited on the anticyclonic shear side of the mid-latitude jet stream. Ease of identification of the principals in such interactions has been biased toward the mid-latitude perspective, as objective tracking of tropical convective outflow is difficult.

Under the assumption that the mass exhausted from deep convective towers will remain near an isentropic level corresponding to the value of its equivalent potential temperature prior to ascent, we propose that convective outflow can be reasonably well tracked on the synoptic scale by identifying the perturbation pressure depth in appropriate isentropic layers. The excess mass manifest in these perturbation pressure depths serves to decrease the static stability of the given layer, increases the height and slope of the tropopause, and provides a direct analytical means of quantifying the perturbation vertical shear in the inflated layer. High-resolution data from NCEP's Climate Forecast System Reanalysis are used to calculate a 32-year climatology of pressure depth in a series of isentropic layers spanning the upper troposphere and lower stratosphere. Perturbation pressure depth is computed for a given time by subtracting this climatology from the GFS analysis at that time.

Cases of high-impact weather that demonstrate the utility of this analysis method in assessing the nature of tropical-extratropical interactions will be presented. Particular emphasis will be placed on characterizing remote forcing on the mid-latitude waveguide as well as on the tropical-extratropical interactions influencing synoptic-scale intraseasonal variations of the north Pacific jet.