Tuesday, September 3, 2013

A Tale of Two Summers: 1993 vs. 2013

In the past 20 years, Salt Lake City has experienced its coldest summer on record (1993, based on the record beginning in 1928) and its hottest (2013), illustrating the remarkable amount of variability that exists within the Earth's climate system.  Both 1993 and 2013 are clear outlier years, with 1993 having an average temperature of 68.7ºF and 2013 having an average temperature of 80.7ºF.
Source: NCDC
The extreme cold of 1993 and the extreme warmth of 2013 reflect the juxtaposition of several factors.  First, lets place the two summers within a long-term context.  In terms of the July global average temperature (I'm using July since the numbers for August 2013 are not in yet, precluding a summer-long analysis at this time), 2013 ranks as the 5th warmest of all time, which reflects the long-term warming trend.  In contrast, July 1993 was one of the coolest years in the past 25 years, a consequence of the eruption of Mt. Pinotubo in June 1991, which spewed planet cooling aerosols into the stratosphere, resulting in a marked cooling effect during 1992 and 1993.  

Source: NCDC
That's all fine and dandy, but extremes like those in 1993 and 2013 require anomalous large-scale circulations.  On average, the summertime circulation over the Northern Hemisphere features a circumpolar vortex with the jet in the high latitudes (i.e., over southern Canada and northern Europe).  Over North America, a weak upper-level trough lies along the U.S. west coast with a broad upper-level ridge over the central United States, placing Utah in broad southwesterly flow.
In 1993, however, the circumpolar vortex was weaker and the jet was displaced southward, which reflects a greater exchange of air between the polar region and the midlatitudes.  In addition, a wave pattern known as the Pacific-North American (PNA) pattern setup in its negative phase over the north Pacific and western North America, putting a deep upper-level trough over the inland western United States.  
As a result, most (but not all) the polar region experienced a near average or warmer than average summer, whereas much of the midlatitudes were cooler than average.  With a deep trough over ther interior west, Utah and the adjoining northwest US interior experienced the coldest temperatures relative to average in the Northern Hemisphere (average here is based on 1981–2010).  
In 2013, the July climate was about 0.7ºF warmer than 1993 and all aspects of the large-scale circulation came together to give us a hot summer.  First, the circumpolar vortex was much stronger and farther north in 2013 than 1998, with cooler air bottled up in the polar region.  Second, the ridge that is climatologically located over the central U.S. shifted west and amplified.  In fact, stronger ridging than average was a common feature throughout the subtropics this summer. 
This led to near or below average temperatures in most of the polar region (note the cool colors extending across Greenland and the northwest passage area), but above average temperatures across most of the high latitudes including Utah.  
So, several factors came together in 1993 to give us an unusually cold summer.  The first was the eruption of Mt. Pinotubo, which dropped temperatures globally.  The second was a weaker-than normal circumpolar vortex, which enabled cooler air to spread more frequently from the high to the low latitudes.  The third was a strong negative phase of the PNA pattern, which put persistent troughing over the interior west.  

In 2013, we see several factors coming together in a way to give us a hot summer.  We're dealing with a warmer climate system, but more importantly is a stronger circumpolar vortex, which keeps cooler air bottled up in the polar region, and a strong upper-level ridge that shifts westward and amplifies relative to its climatological position and intensity. 

2013 was a very hot year in Salt Lake City, but only relative to the climate of the recent past (i.e, 20th century).  If we compare it to the projected climate of the late 21st century, it doesn't seem so bad.  


  1. That point about Arctic temperatures in 2013 will be quite relevant when we end up seeing stories about 2013 Arctic sea ice extent being around 6th place after setting a record minimum last year.

    1. Yes, the slower melt this year is at least partly related to the pattern.

  2. July 1993, in fact, was a record cool month over the West since records have been kept.

    In fact, the month strikingly resembles Utah’s record cold winter month of January 1937 in at least four respects:

    1) extremely cool temperatures (as low as four standard deviations below the mean) over the western contiguous US
    2) extremely hot temperatures over the eastern US
    3) extremely warm temperatures over Alaska (on 13 July 1993, Barrow recorded its highest-ever temperature of 79˚F, whilst in January 1937 Fairbanks exceeded 32˚F on nine days, a total exceeded only in the freakish January of 1981)
    4) extreme rain and floods over part of the Mississippi Basin (the Ohio Basin in January 1937; the Upper Mississippi in July 1993)