Decadal Changes in Multiscale Water Vapor Transport and Atmospheric River Associated with the Pacific Decadal Oscillation and the North Pacific Gyre Oscillation

This study investigates the features of atmospheric circulation and moisture transport associated with two modes of decadal variability in the North Pacific: the Pacific decadal oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO), with emphasis on the multiscale water vapor transport and atmospheric river (AR) over the North Pacific region. During the positive phase of PDO, the geopotential height anomaly at 500-hPa exhibits a Pacific-North American-like pattern. During the positive phase of NPGO, the geopotential height anomaly at 500 hPa features a dipole pattern with a negative anomaly north of 40 degrees N and a positive anomaly south of 40 degrees N over the North Pacific. Associated with the positive PDO phase, the ocean-to-land moisture transport is enhanced between 25 degrees and 35 degrees N and reduced over the northeastern Pacific (25 degrees-62 degrees N, 180 degrees-110 degrees W) for the time-mean integrated vapor transport (IVT). The synoptic poleward transport is suppressed north of 40 degrees N and enhanced south of 40 degrees N. In the positive NPGO phase, the zonal moisture transport is intensified south of 20 degrees N and between 40 degrees and 50 degrees N for the time-mean IVT and weakened over the west coast of North America for the low-frequency (10-100 days) IVT. The synoptic poleward transport is suppressed south of 30 degrees N. The eastern part of the North Pacific AR belt moves southward during positive PDO as the entire North Pacific AR belt shifts slightly northward during positive NPGO. An investigation of AR anomalies during a period over which the PDO and NPGO coexist demonstrates that the AR frequency over the North American western coastal regions is significantly influenced by the conjunction of the PDO and NPGO modes.