Roles of External Forcing and Internal Variability in Precipitation Changes of a Sub‐Region of the U.S. Mid‐Atlantic During 1979–2019

• Published in: Journal of geophysical research. Atmospheres 2022-11, Vol.127 (22), p.n/a
• Main Authors: Dong, Lu, Leung, L. Ruby
• Format: Article
• Language: English
• Subjects: Anomalies; Atmospheric forcing; Circulation; Climate; Climate change; Climate models; Drying; ENVIRONMENTAL SCIENCES; External pressure; Geophysics; Intercomparison; Low pressure; Mean precipitation; Modelling; Moisture; Moisture effects; North Atlantic Oscillation; Ocean-atmosphere system; Oceans; Precipitation; Pressure anomalies; Regional climates; Simulation; Summer; Summer precipitation; Time; Uncertainty; Variability; Wetting; Winter; Winter precipitation
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2022JD037493

Observations of the seasonal mean precipitation averaged over a sub‐region of the U.S. mid‐Atlantic show significant increases in summer but no changes in winter during 1979–2019. Clarifying the drivers of these observed changes is important for understanding and interpreting future projections of regional precipitation by climate models. This study examines the relative effects of external forcing and internal variability in the observed multi‐decadal changes using the historical and RCP8.5/SSP585 simulations from 37 models in the Coupled Model Intercomparison Project phase 5 (CMIP5), 35 models in CMIP6, as well as large ensemble simulations from 6 models, each including more than 20 members. During 1979–2019, enhanced warming over land relative to ocean due to external forcing induces low pressure anomalies over land that contribute to the wetting summer. While similar wetting also occurs during winter, such effect is offset by the drying associated with the positive phase of the North Atlantic Oscillation (NAO) in recent decades. Internal variability produces large uncertainty in the multi‐decadal precipitation changes simulated by climate models through changes in large‐scale circulation associated with the NAO in winter and the North Atlantic Subtropical High in summer. These multi‐decadal relationships between precipitation and large‐scale circulation under internal variability also manifest on inter‐annual timescale. These relationships highlight enhanced moisture transport from the Gulf of Mexico and North Atlantic in winter and near the mid‐Atlantic coast in summer that modulates precipitation change in the study region. Plain Language Summary The mid‐Atlantic region of the U.S. receives more precipitation in summer than winter. However, observations during 1979–2019 show that summer precipitation has increased significantly while the area‐averaged winter precipitation has not changed. This study investigates the drivers of the seasonal mean precipitation changes using observations and simulations. We find that enhanced warming over land relative to the ocean induced by external forcing contributes to the summer wetting by inducing low pressure anomalies over land. The same wetting mechanism also occurs in winter, but it is offset by the drying related to the positive phase of the North Atlantic Oscillation (NAO) in the past decades. Internal variability induces large uncertainty in simulating the precipitation changes as winter and summer precipi