Synoptic Systems Interacting with the Rocky Mountain Barrier: Observations and Theories

Abstract

Using a composite analysis for strong sea level pressure perturbations off the west coast of North America, the evolution of large-amplitude synoptic systems upstream of the Rocky Mountains is investigated for the winter season. Corresponding previous analyses are refined by avoiding multiple counting of events and extended by including potential vorticity, vertical motion, and deformation in the analysis. Cyclonic and anticyclonic anomalies behave similarly, with weak local extrema forming in the lee of the mountain range southeast of the parent systems. However, neither the geopotential anomaly nor the associated potential vorticity anomaly cross the mountain range. Nevertheless, these anomalies contribute to the sea level pressure anomaly in the lee. For both positive and negative anomalies, potential vorticity exhibits a bipolar structure with lobes over the reference point and over the Cordillera, respectively. The relevance of several theories describing the interaction between synoptic systems and mountains are discussed in the light of these findings. It is important to note that these findings differ considerably from results reported in an earlier study. Key differences are the previously reported passage of a wave train over the reference point and the movement of the anomalies over the Rocky Mountains. Both features are absent in the current analysis. However, these features can be recovered if a 6-day high-pass filter is applied before the events are selected or if the analysis is applied to predominantly zonal flow situations.