In the wake of much discussion about atmospheric blocking in Fall 2012, Team BirdCast returns to the scene of the crime, so to speak. The Climate Prediction Center, among others, is forecasting a period of negative North Atlantic Oscillation (NAO) indices in the coming weeks. These negative values correspond to building high pressure over Greenland and Iceland, and high pressure in that region means substantial easterly flow across the North Atlantic. In 2012 (and other years) this pattern of strongly negative NAO values translated, ornithologically speaking, into numerous European vagrants in northeastern North America. Will this blocking phenology build to a “lapwing wind?” BirdCast will follow this pattern and update this post as events warrant.
The image directly below shows a Northern Hemisphere view of geopotential heights, with warmer colors indicating higher geopotential height values and areas of higher pressure (and therefore blocking). Low values in blue indicate anomalously colder air (troughs) whereas high value in orange (ridges) indicates anomalously warmer air (warm air expands producing higher heights). Notice the forecast for the end of the first week in October, in which orange colors expand near Greenland.
The geopotential height forecast is mirrored in the plots of NAO index forecasts below (see this article for details on how these values are calculated). These plots suggest that early October will begin a period of negative NAO, in which high pressure stabilizes near Greenland and Iceland. When this happens, winds across the North Atlantic become generally easterly, as winds circulate clockwise around high pressure (anti-cyclones).
This expansion of high pressure in this area is one of the characteristics of a Greenland block. Those of you in the Northeast US probably remember Greenland block discussions from last fall as Hurricane and post-tropical cyclone Sandy slammed into New Jersey. Whether this Greenland intensifies to forecast levels, and beyond, remains to be seen, but we will be watching!