What are atmospheric rivers?

One constant in the world of weather is that things seem to always be on the move, whether it’s up, down, or horizontal. In scientific terminology the words describing movement would be cycle, transport, or advection of some meteorological variable (e.g., temperature, wind, and moisture). An agitated planet certainly makes for a challenging environment to predict the comings and goings of a myriad of interesting weather phenomena. Weather stems from systematic chaos.

Bringing the topic at hand back into focus, I wanted to talk a bit how the intense concentration of moisture offers both opportunity and risk. The moisture concentration I am referring to is what are known as atmospheric rivers, or “AR” for short. There are specific criteria for the identification of an AR, but the gist is a long and narrow fetch of anomalously wet atmosphere, in terms of water vapor content, moving at a fast clip by prevailing winds in the lower to mid altitudes of the troposphere. Weather satellites tuned to monitor atmospheric water vapor content can capture the true breadth of ARs, with tropical low latitude origins, extending hundreds of miles across the vast Pacific Ocean that become entrained by the polar jet stream steering midlatitude cyclones west to east across the Northern Hemisphere. The origin proximity to Hawaii is why ARs are sometimes called the “Pineapple Express”.

California and the rest of the West Coast of the United States are no strangers to this peculiar atmospheric circumstance. Chances are if you have lived in this region, the AR phenomenon has been communicated to you at some point, likely as a heightened flooding risk. The "opportunity" comes as massive amounts of water vapor are forced to precipitate out over the mountains for several days at a time. This is excellent for replenishing municipal water supplies and providing soil moisture for agriculture and regional plant life. The "risk" is that for the handful of AR events expected in any given year on the West Coast, a majority balance of annual precipitation can be realized! Anytime such an average is dictated by extremes, planning for both flood control and drought resilience can be a challenging to manage.

Photo Credit: Jonny William Malloy