Hurricane Sandy’s epic, devastating, and predicted jog to the Northwest just prior to her New Jersey landfall was driven, largely, by a “rex block” or “Greenland block” – serious high pressure over the North Atlantic that, in effect, blocked the passage of the gigantic circulation of Sandy and forced her inland over New Jersey and Pennsylvania as she worked her way north from the Caribbean. Looking into the past, the October pattern of blocking highs over the North Atlantic can be a harbinger of additional, similar blockages to come during the winter months (see this discussion for example).
Remember, these Greenland blocking patterns, associated with negative values of the North Atlantic Oscillation index (NAO), correspond to easterly flow across the Atlantic. This flow has the potential to bring European vagrants to North America. Thomas Alerstam (1993) discusses some elements of this in detail in his book, and recently, birders in the Northeastern US experienced this firsthand in October and November (for example, look here and here)! During the Sandy period, the NAO index was -2, indicating a strong Greenland block and strong easterlies over the North Atlantic. The resulting displacements of European species certainly included Northern Lapwing, and possibly also included Little Egret, and Black-headed Gull. If the pattern of easterly flow and negative NAOs reestablishes and/or strengthens in the winter 2012-2013, particularly in January and February, and if the United Kingdom has as cold and snowy a winter as forecast, the Northeast could once again be in for a European vagrant show. One might consider as possible a new incursion of European vagrants, including perhaps more Northern Lapwings and maybe even passerines like Fieldfare or Redwing.
We love to forecast bird movements, clearly, but forecasting three months in the future? Too extreme? Never! We will update this “Eastern Promises” feature numerous times in the coming months to see how the winter unfolds.
A Greenland block brings colder than average winter temperatures to the Eastern US and Europe, while creating easterly winds across the North Atlantic. The more negative the value for the North Atlantic Oscillation index, the stronger the Greenland high, the stronger the easterly flow.
