Between December 2022 and early January 2023, several atmospheric rivers (ARs) drenched the state of California, which caused major floods, closed roads, and loss of power, in addition to resupplying in part California's reservoirs (Fox Weather). According to the National Oceanic and Atmospheric Administration (NOAA), downtown San Francisco received 17.64 inches of rain, with California's maximum total rain in Honeydew, California (Humboldt County) receiving 47.74 inches of rain from December 26th, 2022 to January 17th, 2023. As a comparison, the 1981 to 2010 average annual rainfall for California is about 22 inches, and the average December rainfall is about 3.75 inches (NOAA and Western Regional Climate Center).
Heavy precipitation and flooding on land can have an impact on the coastal ocean via runoff. The unusually large amount of freshwater discharged into the ocean can form a plume that can be traced using satellite measurements of Sea Surface Salinity (SSS). For example, the NASA Soil Moisture Active Passive (SMAP) instrument was used to detect the unusually fresh SSS associated with the atmospheric rivers that affected the western part of the U.S. in December 2022 and January 2023. The figure above shows the persistence of fresh anomalies (deviation from normal) along the western coast of the U.S. few days to few weeks following the atmospheric rivers. Unusual presence of fresh waters, highly concentrated in terrestrial organic and inorganic matter, can have a detrimental impact on ocean ecosystems due to the significant perturbation of salinity and the terrestrial matter intrusion carried from land via runoff that can threaten marine life. Therefore, impacts of runoff from heavy land precipitation into the ocean should be monitored.
The end of February 2023 is seeing another pulse of heavy rainfall from a cold northern system with Downtown Los Angeles receiving 4.49 inches of rain and the mountain communities up to 93 inches of snow (NOAA).