This study demonstrates that water vapor transport and precipitation are largely modulated by the intensity of the subtropical jet, transient eddies, and the location of wave breaking events during the different phases of ENSO. Clear differences are found in the potential vorticity (PV), meteorological fields, and trajectory pathways between the two different phases. Rossby wave breaking events have cyclonic and anticyclonic regimes, with associated differences in the frequency of occurrence and the dynamic response. During La Niña, there is a relatively weak subtropical jet allowing PV to intrude into lower latitudes over the western United States. This induces a large amount of moisture transport inland ahead of the PV intrusions, as well as northward transport to the west of a surface anticyclone. During El Niño, the subtropical jet is relatively strong and is associated with an enhanced cyclonic wave breaking. This is accompanied by a time-mean surface cyclone, which brings zonal moisture transport to the western United States. In both (El Niño and La Niña) phases, there is a high correlation (>0.3–0.7) between upper-level PV at 250 hPa and precipitation over the west coast of the United States with a time lag of 0–1 days. Vertically integrated water vapor fluxes during El Niño are up to 70 kg m−1 s−1 larger than those during La Niña along the west coast of the United States. The zonal and meridional moist static energy flux resembles wave vapor transport patterns, suggesting that they are closely controlled by the large-scale flows and location of wave breaking events during the different phase of ENSO.

Regional-scale projections of climate change signals due to increases in atmospheric CO2 are generated for the western United States using a regional climate model (RCM) nested within two global scenarios from a GCM. The downscaled control climate improved the local accuracy of the GCM results substantially. The downscaled control climate is reasonably close to the results of an 8-yr regional climate hindcast using the same RCM nested within the NCEP–NCAR reanalysis, despite wet biases in high-elevation regions along the Pacific coast. The downscaled near-surface temperature signal ranges from 3 to 5 K in the western United States. The projected warming signals generally increase with increasing elevation, consistent with earlier studies for the Swiss Alps and the northwestern United States. In addition to the snow–albedo feedback, seasonal variations of the low-level flow and soil moisture appear to play important roles in the spatial pattern of warming signals. Projected changes in precipitatio... Abstract Regional-scale projections of climate change signals due to increases in atmospheric CO2 are generated for the western United States using a regional climate model (RCM) nested within two global scenarios from a GCM. The downscaled control climate improved the local accuracy of the GCM results substantially. The downscaled control climate is reasonably close to the results of an 8-yr regional climate hindcast using the same RCM nested within the NCEP–NCAR reanalysis, despite wet biases in high-elevation regions along the Pacific coast. The downscaled near-surface temperature signal ranges from 3 to 5 K in the western United States. The projected warming signals generally increase with increasing elevation, consistent with earlier studies for the Swiss Alps and the northwestern United States. In addition to the snow–albedo feedback, seasonal variations of the low-level flow and soil moisture appear to play important roles in the spatial pattern of warming signals. Projected changes in precipitatio...

Precipitation related to atmospheric rivers (ARs) is critical in shaping climate and extreme hydrologic events in the mid‐latitude coastal regions, making it a recent topic of intense research. This study has analysed the AR climatology, AR‐precipitation characteristics and their regional variations in East Asia (EA) for 1979–2015 using an AR inventory based on vertically‐integrated water‐vapour transport and AR geometry. The AR frequency over EA varies seasonally with highest (lowest) frequency in summer (winter). The AR frequency peaks in the coastal regions of the mid‐latitude western North Pacific (WNP), then decreases rapidly away from the region towards higher/lower latitudes and inlands. ARs are related to 30–60% of the annual precipitation in EA with maximum in summer; secondary peaks occur in spring in eastern China, Korea and southern Japan, and in winter for the southern China coast. Calculating the AR precipitation on days of partial AR occurrences from daily precipitation data is a key source of uncertainties in the AR‐precipitation fraction in this study. The AR days are substantially more related to heavy precipitation than non‐AR days for all seasons and regions. Seasonal cycle of AR frequency over EA and the WNP follows the northward march of the main rainband from spring to mid‐summer, but is decoupled from it from late summer to winter as the North Pacific storm track migrates into the Eastern North Pacific. The annual cycle of AR frequency and AR precipitation varies regionally; Japan and southern China adjacent to WNP show high (low) frequency of weak‐ (heavy‐) precipitating ARs in early spring (late fall) with highly frequent heavy‐precipitating ARs in summer while the mid‐latitude coastal regions, eastern China and Korea, show low frequency of heavy‐precipitating ARs in early fall with highly frequent heavy‐precipitating ARs in mid‐summer.