Abstract
A particular episode of blocking that occurred in the North Atlantic during December 1978 is analyzed from NMC gridpoint data using isentropic analysis. The advantages of this analysis technique are outlined, specifically with regard to isentropic potential vorticity. Analysis of the Montgomery streamfunction on the 315 K isentropic surface is preferable to the more traditional analysis of geopotential height on the 300 mb surface for describing the evolution of the block. The sea level pressure pattern shows several intense cyclonic storms upstream of the block prior to its development. During its mature stage, the block exhibits little tilt with height, has a warm core throughout most of the troposphere and a high tropopause. The block also significantly affected the passage of fronts through the blocking region.
The potential vorticity analysis is shown to be a useful framework for understanding several characteristics of the block. The blocking region is characterized by a broad, uniform area of low potential vorticity—a fact well illustrated by an EOF analysis. The evolution of the two components of potential vorticity, namely, the absolute vorticity and the pseudodensity (a measure of the inverse of stability), is also investigated. Isentropic trajectories are used to determine the origin of the air in the blocking region and show that the low potential vorticity air characterizing the block originated from well south of the block. Its advection into the block is a reflection of the flow around a strong, synoptic-scale storm off the coast of North America.
