Animation of atmospheric river event, December 2014. (CREDIT: NOAA/ESRL Physical Sciences Division)

In December 2014, many Americans watched news coverage of torrential downpours in California. The torrential downpours, still occurring when this post was written, came at a time when California had been in the midst of the worst drought in its history. (Ironically NOAA claimed that California’s drought was not man-made but naturally caused that same month.) Where is the rain coming from? Did the skies suddenly open up to release a bounty of needed rain over California? If not what brought this unexpected (yet needed) deluge?

For those unfamiliar with meteorology, the rain storms in California seem like some cosmic form of deus ex machina. However, it’s a bit more complicated than that. It was the Pineapple Express that brought this much needed moisture to the U.S. west coast. Moreover, it was the moisture brought, and continued to be brought, by the Pineapple Express, culminating in those rain storms that California desperately needed. What exactly is the Pineapple Express? Where does the Pineapple Express originate? These are questions that can best be answered by examining information provided by the University Corporation for Atmospheric Research (UCAR) and National Oceanic & Atmospheric Administration (NOAA).

The University Corporation for Atmospheric Research explains that the term Pineapple Express is “[a]n informal name for the flow of low- and mid-level moist air, driven by the subtropical jet stream, that sometimes extends from the region around Hawaii (hence “pineapple”).” NOAA’s description of the Pineapple Express meteorological phenomenon adds to this rather vague yet convoluted UCAR definition. NOAA describes the Pineapple Express as “a type of strong AR [Atmospheric River] that can hit the U.S. west coast.” What exactly is an Atmospheric River (AR)? Again, we must delve deeper into the field of meteorology to find our answers. Nevertheless, NOAA provides an excellent (and brief) definition of this fascinating meteorological phenomenon:

“Atmospheric Rivers (AR) are relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. While ARs come in many shapes and sizes, those that contain the largest amounts of water vapor, the strongest winds, and stall over watersheds vulnerable to flooding, can create extreme rainfall and floods. These events can disrupt travel, induce mud slides, and cause catastrophic damage to life and property. However, not all ARs cause damage – most are weak, and simply provide beneficial rain or snow that is crucial to [the] water supply.”

[A more detailed explanation of ARs can be found here.]

Interesting facts concerning Atmospheric Rivers (from NOAA):

• On average, about 30-50% of annual precipitation in the west coast states occurs in just a few AR events, thus contributing to water supply.
• A strong AR transports an amount of water vapor roughly equivalent to 7.5–15 times the average flow of liquid water at the mouth of the Mississippi River.
• In the strongest cases ARs can create major flooding when they make land-fall and stall over an area.
• On average ARs are 400-600 km wide.
• ARs are a primary feature in the entire global water cycle, and are tied closely to both water supply and flood risks, particularly in the Western U.S.
• ARs move with the weather and are present somewhere on the earth at any given time.