Why has it been so warm?

I think it is important to introduce the main game players here. The first would be the El Nino Southern Oscillation (ENSO). For those of us new to the climate world, ENSO is a sea surface temperature anomaly in the Pacific Ocean induced by both 1)Oceanic currents 2)Atmospheric wave patterns. Shown below is the Multivariate ENSO Index (MEI), which takes into account several variables for the ENSO phase. Red = El Nino Blue = La Nina. Neither pattern has a significant climatological change on AVERAGE temperatures; indeed there have been very cold La Nina winters (1959-1960) and very warm La Nina winters (2011-2012). A La Nina allows for a more meridional flow though, which translates to a wider range of temperatures.

Remember the 2009-2010 Armageddon winter; it may have been very cold and snowy, but it was CONSISTENTLY cold and snowy. The winter after (2010-2011) is remembered for it's abrupt start and it's even more abrupt finish.

We get to our second player, the Pacific Decadal Oscillation. As its name suggests, it exists on a multi-year scale. Shown below is an image from Wikipedia depicting a positive (warm phase) of the PDO. This is a sea surface temperature anomaly. A negative phase of the PDO would be very conducive for warmer weather across the Southeast. Note that both the 2010-2011 and 2011-2012 winters were a negative phase PDO. But 2010-2011 was a far cry from the near record low of 2011-2012 winter.

Dec - Feb 1980 - 2010 PDO correlation to 500mb height levels

The negative event of 2010 - 2011 was weak enough to be overpowered by another variable, the Arctic Oscillation, thus giving us a very cold start despite the unfavorable conditions in the Pacific. But in 2011-2012, the PDO was much stronger and limited the number of cold air outbreaks we received. Switching gears away from the 2011-2012 winter, we go to this summer.

Dec - Feb 1980 - 2010 PDO correlated to temperatures.

This is why we never stood a chance for a cold 2011-2012 winter.

Summertime PDO correlated to geopotential height. Note that we are still within a negative phase of the PDO.

Summertime PDO correlated to temperatures. Note that we are still within a negative phase of the PDO.

Summertime PDO correlated to zonal winds. This actually fits what we have seen so far: a lack of a subtropical jet influence over the Southeast and a stronger zonal flow over the northern tier of the United States.

 The equalizing factor. We have been moving towards a weak El Nino (started off with a La Nina spring). The zonal flow into the Pacific Northwest and over the northern tier of the United States is weakened during an El Nino.

 Put the -PDO with an El Nino and the effects start to cancel out. However, since there is a lag for when an El Nino develops and when it influences our weather, we are unlikely to see its impacts until the second half of summer. Conversely, since we have been in a long trending -PDO, we are already seeing those effects now.

And believe it or not, this summer could be far hotter. We haven't seen much intrusion from the Bermuda High to our east. If we were, we would see a much hotter and moisture loaded air mass. Instead, the Midwest and the Northeast are taking the bulk of the heat. We can thank the -NAO for keeping the Bermuda High at bay.



Verdict: Despite a hot start to our summer, it will become a little cooler and much rainier for the second half.

SNOWFALL
Winter snowfall actually has some ramifications over spring time and to a lesser extent, summertime conditions as well. With a lack of snowfall during the winter, temperature cold sectors are not as able to get established. That's why it flat out never stayed cold across the United States this winter. There was a lack of a snowpack to speak of. Below shows the snowfall anomalies from December 1st, 2011 to March 1, 2012. Red = deficit, Blue = surplus.

Snow coverage anomalies from 2011 - 2012.

 Speaking of a tale for two winters, Europe was struggling to get ANY snowfall for the first half of the winter, only to get hammered the second half.

2010-2011 snow coverage December 1st - April 30th

2011-2012 snow coverage December 1st - April 30th

Ouch. That last animation is just painful to look at.

There is something I have been saying since January, and it's that this winter was very good for the Arctic. This was indeed true, as there was a higher amount of sea ice over the Arctic since at least 2007 and was near the normal for 1979-2000 mean. However, there was a large degree of ice retreat in June. The less ice over the Arctic, the lower the albedo (reflectivity), meaning more energy from the sun is absorbed at the surface rather than reflected. Meaning...warmer temperatures!!!



And this is getting into unfamiliar territory, but we were at a sunspot minimum for the 2009-2010 winter. I am not that familiar with this area, but from my understanding a sunspot minimum means a less solar energy reaching Earth. We are near the crest of a sunspot maximum now.