The Unusual Warmth of March 2012: Some Possible Causes

Using the National Centers for Atmospheric Research – National Centers for Environmental Prediction (NCAR/NCEP) reanalyses and the 20th Century Reanalysis project, the warm March of 2012 will be analyzed for possible causes of the anomalous warmth over the Eastern United States. This event will also be compared to an event that was likely of similar magnitude during December 1889. During the winter of 2012, there was relatively little blocking, especially over the North Pacific. This was reflective of a more zonal jet stream which is characteristic of a strong positive Arctic Oscillation – type flow regime. Over North America, the winter was warm and dry leading up to March 2012. Then a strong ridging event developed, which set the stage for an anomalously warm and dry March. Here we propose that surface forcing was important in setting the stage for the development of ridging, which further amplified the surface conditions. Thus, the anomalous warmth was driven by processes considered to be more prevalent in the summer. Similar conditions persisted during the months leading up to December 1889. Both years were considered to be in a weak La Nina phase in the Pacific.