(Blog Written by Donald Sutherland)
While large portions of North America continue to suffer through extreme and even historic heat, a partial analysis based on the emerging El Niño (the ENSO Region 3.4 anomaly has now reached +0.6°C) suggests that blocking will be key to both Fall 2012 and Winter 2012-13 outcomes. El Niño events that followed either neutral or La Niña ENSO winters and had warm summers in North America were 1953, 1957, 1986, 1991, and 2002. ENSO-Teleconnection analogs have persistently been highlighting 1953, 1991, and 2002 for the summer. The 2009 analog had faded and Summer 2012 is on course to be notably warmer than the 2009 case.
The following are the summer anomalies for the above noted cases:
Below are composite temperature anomalies for the fall and winter outcomes depending on the presence/absence of blockiness:
This partial analysis does not take into consideration the ongoing observed warming that remains underway. Both fall and winter outcomes have become warmer across the northern U.S. and all of Canada between the 1971-2000 and 1981-2010 base periods. The lone exception is a portion of the Southeast that has grown slightly cooler for the winter months.
The partial analysis also does not take into consideration the abnormal Arctic warmth. June 2012 had Arctic anomalies that were warmer than all of the above cases. June 1953 offered perhaps the closest match. Were the excessive warmth to persist in the Arctic, the actual fall/winter outcomes would probably be somewhat warmer than denoted by the maps and the expanse of warm anomalies would likely be larger/cool anomalies would likely be smaller. Those are details left for later.
For now, the important point is that neither the fall nor winter outcome is a proverbial slamdunk on either the warm or cold side. A lot will depend on whether blocking develops and then predominates.