-EPO, -AO, +PNA And -NAO Blocking Signatures Affecting North America; Revised, Edited, Thursday, November 4, 2021

[Larry Cosgrove]

-EPO, -AO, +PNA And -NAO Blocking Signatures Affecting North America

 

Height anomalies above 534dcm at 500MB fall into the following classifications:

 

Eastern Pacific Oscillation (EPO): negative phase favors blocking in Alaska and its adjacent Gulf waters, or immediately off of the western shoreline of North America.

 

Arctic Oscillation (AO): negative phase points toward the presence of higher than average atmospheric heights in the vicinity of the North Pole or above the Arctic Circle.

 

Pacific North America or Positive/Negative Anomaly (PNA): positive signature is indicated when ridging develops -mostly- above 40 N Latitude and west of 100 W Longitude (i.e. Pacific Northwest, western Canada including Prairie Provinces and the Yukon Territory).

 

North Atlantic Oscillation (NAO): negative character is shown when upper ridging is present in an area characterized as all of eastern Canada generally along and below the Arctic Circle, the northern Great Lakes region, northern New England, most of Greenland and Iceland and the northern Atlantic Ocean above 45 N Latitude. The designation does NOT include the British Isles or Scandinavia.

 

The following classifications of types of blocking ridges (with storms which typically accompany the block) is meant to speed recognition of coming changes in temperature and precipitation during the near and medium ranges. Often, numerical models and ensembles will show signs of these developments as much as 10 days in advance of occurrence. Keep in mind that you should ALWAYS balance an operational model forecast (or lack thereof) of an extreme blocking pattern against its variant members before calling for the development of an extreme in apparent weather.

 

Omega Blocks

 

Perhaps the best known of blocking signatures, the wind and height contour fields are distributed in a manner mimicking the Greek letter Omega. The ridge is abutted by two storms on either side; one impulse in an exit fashion while the other disturbance undercuts the positive height anomaly. Because of the closed nature of the anticyclonic circulation, Omega blocking in winter months can result in extremely high surface pressures (especially when a dense snowpack is present). The ridging may last a very long time, as the warmer values aloft only gradually cool when shortwaves (or other cyclonic systems) pass south of the core of the ridge center.

 

Thumb Projection Blocks

 

Unlike a Rex or Omega block, this type of signature does not necessarily have a closed height contour at its core. The ridge serves to shift incoming energy to the north (having the effect of pumping up atmospheric heights) while providing an impetus for digging to the right of the ridge axis. Until the ridging breaks down (usually by erosion on its right side, but sometimes by a high-speed jet stream maximum smashing through from the west), potential for cold and precipitation increases on either side of the anticyclonic curvature. Keep in mind that a true elongated "thumb" block must have its core heights above 45 N Latitude for the ridge to serve as a detour to inbound shortwaves. Otherwise, energy will quickly pass over top of the positive height anomaly and the threat for cold advection will be seriously diminished.

 

Rex Blocks

 

As defined by Professor James Rex (in Tellus, 1950), this type of block, at its simplest, consists of a higher latitude ridge that is positioned directly north of a closed cyclonic circulation. But you can break down this formation of anticyclone into four recognizable patterns, listed below:

 

1) Classic This blocking signature produces a vivid "figure 8" arrangement in the height contour field. Starting with warm advection aloft toward the northeast, a shortwave to the right of the building ridge tunnels or digs in a southwestward manner. This pattern is noted for bringing unseasonable cold and storminess to the western U.S., and can also result in shortwaves being ejected out of the resultant closed low. These impulses, traveling eastward into a shallow layer of colder air in the Great Plains, can trigger overrunning well in advance of the main storm, which can take up to five days before beginning to move out from underneath the high center (usually when a kicker collapses the right quadrants of the anticyclone).

 

2) Lateral Sometimes, when a blocking signature extends in an west-to-east manner (often through merger with another positive height anomaly), the resulting stretched appearance aids in building surface pressures over a wide west to east area. Also, the block helps to maintain an eastward trajectory and prevent recurvature. Often I find this blocking signature builds pronounced thermal boundaries with well-defined precipitation type banding (say from snow to sleet to freezing rain to rain, and then convective showers).

 

3) Split Flow (El Nino Associated) This configuration results when the jet stream splits into two parts, where the southern branch is as strong, or stronger, than its higher latitude counterpart. Prevalent during El Nino years (especially of the moderate to strong variety), this split (or "smile") signature is a true block. Ridge centers often appear above 45 N Latitude, and act to keep storms in the subtropical or lower polar flow from phasing with the traditional polar westerlies. When this height contour variation arises, cPk and cA regimes do NOT migrate southward from Canada. Snow only occurs in the strongest storms through dynamic cooling processes, cold sector convection, or in nocturnal patterns.

 

4) In-Situ In cases where a strong closed low separates from a high-velocity Arctic or Polar jet stream, two impressive baroclinic zones may set up in the corridor between the storm and the fast upper flow. Supporting anticyclonic curvature aloft, and imparting this to low level windfields, the resultant flat ridge can be deemed a block where 500MB heights exceed 534dcm. The presence of the ridging slows the forward motion of the disturbance while at the same time acting to hold colder air in place.

"Dual Blocking" Configuration

 

When two blocking signatures are present (with some combination of a -EPO, +PNA, -NAO or -AO phase), teleconnections favor much lower than normal height presentations over a latitude far below climatological means. If you review the winter of 1976-77, for example, the +PNA/-NAO configuration was associated with a brutally cold cAk regime and deep vortex that at one point (December of that season) was centered over Lake Erie! So if there are two (rarely more) positive height anomalies in the same theater, forecasts of negative temperature deviations are in order, with some dependency on snow cover, SSTs of water bodies, and admixture with ImP, mT or mT regimes.

 

Summary

 

While blocking patterns in upper atmospheric wind and height fields can occur at any time of the year, the presence of such signatures in the lower sun/colder phase often signifies two synoptic episodes: greater amplification of connective trough complexes and drainage of mA, cA and cPk regimes. The above categories can be easily determined by reviewing current or forecast upper level wind and height data.