Leading Modes of Wintertime North Pacific Atmospheric Rivers and Their Drivers

North Pacific Atmospheric Rivers (ARs) are affected by many climate modes, including the El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). However, such impacts are often examined individually and outside of the context of ARs' own variability. It remains an open question as to the relative importance of each climate mode in affecting the inter-annual variability of AR occurrences. This work proposes an answer to the above inquiry, via a rotated Empirical Orthogonal Function (EOF) analysis on wintertime AR occurrences. Four major modes of AR occurrence variability are identified: The first mode features a southeast-northwest shift of the AR tracks, and is associated with the pair of the second leading EOFs of the North Pacific atmospheric circulation and sea surface temperature (SST). The second mode features a large scale suppression of North Pacific ARs and an enhancement west of the California coasts. This is a decadal variability associated with PDO. The third mode features meridionally contracted AR tracks over the Eastern Pacific. This is associated with the leading EOFs of the North Pacific atmosphere/SST, and the inter-annual component of PDO. These three modes each contributes similar to 19% to the total variances. The fourth mode, with a 12.7% variance contribution, carries the complementary information of the non-linear El Nino - La Nina transition, and features two slanted anomaly bands over boundaries of the basin. Key atmospheric/oceanic driving forces associated with each mode are examined, and certain climate modes exhibit promising predictive power to wintertime AR occurrences.