Does the Madden–Julian Oscillation Influence Wintertime Atmospheric Rivers and Snowpack in the Sierra Nevada?

Abstract

The relationships between the Madden–Julian oscillation (MJO), activities of atmospheric rivers (ARs), and the resulting snowpack accumulation in the California Sierra Nevada, are analyzed based on 13 yr of observations for water years 1998–2010 inclusive. The AR activity, as measured by the number of high-impact ARs, mean per event snow water equivalent (SWE) changes, and the cumulative SWE changes, is shown to be significantly augmented when MJO convection is active over the far western tropical Pacific (phase 6 on the Wheeler–Hendon diagram). The timing of high-impact ARs (early- versus late-winter occurrences) also appears to be regulated by the MJO. Total snow accumulation in the Sierra Nevada (i.e., AR and non-AR accumulation combined) is most significantly increased when MJO convection is active over the eastern Indian Ocean (phase 3), and reduced when MJO convection is active over the Western Hemisphere (phase 8), with the magnitude of the daily anomaly being roughly half the cold-season mean daily snow accumulation over many snow sensor sites. The positive (negative) SWE anomaly is accompanied by a cold (warm) surface air temperature (SAT) anomaly and an onshore (offshore) water vapor flux anomaly. The contrasting SAT anomaly patterns associated with MJO phases 3 and 8, revealed by the in situ observations, are more realistically represented in the Atmospheric Infrared Sounder retrievals than in the European Centre for Medium-Range Weather Forecasts Interim reanalysis.