Thursday, March 13, 2014

Temperatures during Northern Hemisphere Winter 2013/14

The Northern Hemisphere's meteorological winter (Dec–Feb) is now in the bag.  The National Climatic Data Center served up its data for February today, allowing us to take a look at what winter brought the United States and Utah.

On a national scale, temperatures were well below average in the upper midwest (aren't winters in Minnesota nasty enough already!) and below average across most of the US east of the Rockies except near the coast where temperatures were near average.  In our part of the world, we ended up near or just above average.  In the valleys of northern Utah, that largely reflects below average temperatures in December, near average in January, and above average in February.

Source: NCDC

The image below shows the rank of this past winter relative to all 118 years since 1895.  A good swath of the upper midwest observed a top-10 coldest winter.  On the other hand, most of California basked in record warmth.  Flights from Minneapolis to SoCal were likely in high demand.

Source: NCDC
These temperatures largely reflect a large-scale 500-mb (upper-level) pattern that featured strong ridging over the northeast Pacific Ocean and a deep trough over most of eastern Canada and the eastern U.S.
Source: ESRL
Compared to climatology, this pattern features anomalously high 500-mb heights over most of the arctic extending down over the northeast Pacific and anomalously low 500-mb heights from north-central North America across the north Atlantic to western Europe.

Such a pattern essentially opens up the eastern U.S. to air originating over the high latitudes, resulting in a remarkably cold winter.  Over and east of "the pond", however, the pattern favored a strong and active Atlantic storm track that brought record rains to the UK.  

The global numbers for February haven't been released yet, but January was the fourth warmest and December the third warmest since 1880, so it is likely that this Dec–Feb period will rate near or in the top 5 warmest on record.  The fact that the globally averaged temperature was so high, yet much of the upper midwest was quite cold, illustrates the power of the large-scale circulation to have a strong impact on local and regional temperatures.

Addendum: 14 Mar 2014:

A day after writing this post, the global temperature rankings came out and the Dec–Feb period was only the 11th warmest on record.  Warm, but not in the top–5 as I suspected above.  Nevertheless the global temperature was 0.97ºF above the 20th century average, so this past winter still provides a good example of how you can get a strong regional cold anomaly even in a warming climate.  

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