Pacific Atmospheric Rivers: A Sign of Change
Kyle Petroziello, Meteorologist
The Pacific Ocean Basin currently resides in a La Niña Climate Pattern. La Niña is referred to as the “cold phase” of the ENSO (El Niño/Southern Oscillation) cycle. It occurs when the sea-surface temperatures of the Equatorial Pacific are below average. ENSO is an oscillating warming and cooling pattern change of the tropical Pacific Ocean that affects global circulation.
In a normal La Niña winter, there are two main weather features over the Northeast Pacific that drive the usual weather pattern. A semi-permanent blocking high pressure is found between Hawaii and the Aleutian Islands. The Polar Jet Stream takes on a wave-like characteristic as it flows north of this high, then south into the Pacific Northwest U.S. while remaining amplified. This setup produces occasional gales that track similarly to the jet stream and remain north of the ridge before tracking SW’ward into the Pacific Northwest U.S., while the East-Central Pacific remains quieter.
La Niña is expected to gradually weaken over the next several months, with these dominant features losing their influence. This is right on par for the expected ENSO cycle. The historical interval between La Niña and its warmer water counterpart – El Niño – occur every 2 to 7 years, and we are now approaching two and a half years of a prevailing La Niña (26 of the last 28 months). A shift to an ENSO-neutral pattern (the happy medium between these two) is expected to develop into this upcoming spring.
A glaring indicator of this soon-to-be changing of the guard has been the unprecedented Northeast Pacific weather pattern for much this past month. This break in the pattern has instead seen persistent upper-air troughing across the Gulf of Alaska while the Polar Jet Stream retreats far to the north and the strong Pacific Jet Stream has taken over. It has become an immensely straight-line zonal feature, extending across the entire Pacific from Japan to California along 30N-40N.
This change caused January 2023 to feature an unprecedented series of successive Atmospheric Rivers flowing into the U.S. West Coast, with historic rainfall. Atmospheric Rivers are essentially narrow columns of water vapor (moisture) that originate over the tropics and flow like rivers aloft over long distances. Consecutive strong gales, under the influence of the persistent Pacific Jet Stream, have propagated E’ward in a conga-line fashion across the S’rn mid-latitudes of the Pacific, only a few days apart. These strong gales pull moisture from the tropics as Atmospheric Rivers, producing a firehose-like release of tropical moisture upon the U.S. West Coast. A notable example of this was the notorious “Bomb Cyclone” within the first week of January that approached California, depicted below.
Figure 1 shows impressive Visible Satellite Imagery of the January 04th Bomb Cyclone with notations
Figure 2 shows Winds, Seas, and Pressure associated with the January 04th Bomb Cyclone (source: www.seaweather.net)
In Figure 1, the moisture plume transported water nearly 2,000 nautical miles from the Hawaiian Islands to just offshore California at the time. In Figure 2, the Winds, Seas, and Pressure map from SeaWeather of the same system indicated sustained winds over 50 knots and swells in excess of 10 meters produced just south of the center of the gale. These are overlaid in the notated visible satellite imagery in Figure 1 as well.
The more S’rn tracks of these gales within the Pacific mid-latitudes have allowed large swells to propagate much further south, through the entire Baja Peninsula and towards the west coast of Mexico at times, than usual for a La Niña winter. This has posed considerable challenges for potential Baja and West Coast transits this past month. Though, this month of January is considered an extreme pattern and these strong S’rn gale tracks are unlikely to persist as frequently in the many months ahead. However, it is an indicator of a more active Northeast Pacific climate pattern expected through the next several months as La Niña phases out, sustaining challenges for N’ward transits into the warmer spring and summer months.
WRI meteorologists are ready to assist you with any West Coast hops in this active pattern. Our SeaWeather website (www.seaweather.net) also offers many features to help examine impacts from this changing pattern on your potential voyage in the months ahead.
