Simulation and Projection of Circulations Associated with Atmospheric Rivers along the North American Northeast Coast

摘要

Torrential rainfall occurring along the North American northeast coast (NANC) in summer and autumn is accompanied by strong atmospheric rivers (ARs), which efficiently transport abundant moisture along a narrow-stretched path associated with a low pressure system. In this study, an autodetection method was used to identify ARs that reached the NANC, based on the 6-hourly data of the ERA-Interim reanalysis conducted by the European Centre for Medium-Range Weather Forecasts, in summer and autumn from 1979 to 2016. Stronger ARs tended to occur in the eastern flank of a cyclonic anomaly that covered the entire North American east coast from Florida to Newfoundland, with a positive precipitation anomaly over the NANC. The cyclonic anomalies and precipitation in autumn were stronger but less frequent than those in summer. Cyclonic anomalies were parts of westward-tilting wavelike circulation perturbations moving into North America from the extratropical North Pacific and moving continuously eastward, reaching the east coast in approximately five days. The Geophysical Fluid Dynamics Laboratory (GFDL) High-Resolution Atmospheric Model (HiRAM), which realistically simulates the occurrence frequency and key characteristics of ARs in current climatic conditions, was used to project the AR activity and corresponding circulations in the future warmer climate under the representative concentration pathway 8.5 scenario. The HiRAM that was driven by sea surface temperature changes projected an overall increase in the occurrence of stronger ARs in both summer and autumn and the precipitation strength in autumn along the NANC by the end of the twenty-first century. This projected enhancement was contributed to by two processes-a smaller contribution was from the weakened basin-scale North Atlantic anticyclone but with higher moisture content, and a larger contribution was from the enhancement in anomalous circulation during AR events with integrated vapor transport exceeding the 75th percentile. These results suggest that the influence of strong ARs on the NANC may increase in the warmer future due to the combination of increased water vapor in the large-scale environment (thermodynamic effect) and enhanced anomalous circulations (dynamic effect). The AR-associated circulations in autumn were also projected to have a stronger tropical connection in the warmer future.