Wondering who stole our winter? It may have been a girl who periodically visits the tropical Pacific. Weak La Nina conditions have existed in the tropical Pacific throughout the 2018 winter, which may partially explain the missing precipitation in the Southern San Juan Mountains. I was curious if La Nina was also involved in the other major snow heists over the past 67 years.
Percent of Average Snow Water Equivalent in Colorado's Major Watershed (NRCS)
A background check of La Nina will lead you to her birthplace, the tropical Pacific. The temperature of the Earth’s largest ocean has a significant influence on global weather and climate, providing air masses with heat and moisture. These air masses are directed by the jet streams, which meander around the globe delivering stormy weather in their path.
Current equatorial Pacific sea surface temperature departure from normal (NOAA).
Like a pendulum, warm and cold water moves back and forth across the equatorial Pacific over the course of 2-7 years. Since 1951, ocean temperature records have documented this phenomenon known as the El Nino Southern Oscillation. When warm water pools off the equatorial coast of South America, and the 3-month average ocean temperatures in that region are at least 0.5 degrees Celsius above normal, El Nino conditions exist. Conversely, when cool water replaces the warm water, and the 3-month average ocean temperatures are at least 0.5 degrees Celsius below normal, La Nina conditions exist.
To answer my initial question (was La Nina involved in other past snow heists), I plotted the November through January (NDJ) Oceanic Nino Index (ONI) versus the February 1st snow water equivalent (SWE) measured at the Upper San Juan SNOTEL site on Wolf Creek Pass. The ONI records date back to 1951 so I was able to extend the snowpack records further into the past. By including snow course data (which dates back to 1935), 1977 stands out as another poor snow season. La Nina years are blue, neutral years are purple, and El Nino years are red.
Who knew little colored dots could contain so much information! I labeled the worst snow years, which appear at the bottom of the graph. 1996, 2000, 2006, and 2018 were all La Nina years, 1990 and 2002 were neutral years, and 1977 was an El Nino year. Based upon the graph, not every La Nina event corresponds with below average snow by February 1st. It interesting that 4 out of 7 really bad snow years occurred during La Nina events. However, to reiterate the poor correlation, last year was also a La Nina winter, but the Southern San Juans had above average snowpack.
To wrap-up this climate cold case, La Nina is not alone. La Nina events correspond to both below and above average snow years, which means there are some other anonymous accomplices that are involved this year’s snow heist. La Nina is just one weather variable that happens to be easier to recognize and has been on the climate watch list longer than the others. Either La Nina has multiple personalities or she brings or leaves behind different accomplices each time she visits.
This year has fit the profile of a “normal” La Nina weather pattern. The diagram below is a generalized La Nina weather pattern. Notice the blocking high pressure system in the Pacific and the dip in the Polar Jet Stream to the east of the Rockies, which has been responsible for the snow in Texas. Colorado has been located on the warm, dry side of the Polar Jet Stream, and the next two graphics confirm that.
Generalized La Nino Pattern for North America (NOAA).
Departure from the average January temperatures (NCEI)
Departure from the average January precipitation (NCEI)
The Boulder/Denver National Weather Service posted an excellent explanation of La Nina’s typical influence on Colorado as well as some great graphics. Here are two of their graphics that depict the jet stream during typical mid and late La Nina winters. For those of us who live in the South San Juans, let’s hope they are wrong!
Mid Winter La Nina Jet Stream Patterns (Boulder NWS)
Late Winter La Nina Jet Stream Patterns (Boulder NWS)