Spring-summer atmospheric circulation and boundary layer processes modulating aerosol and cloud characteristics over the coastal Northeast Pacific during EPCAPE

Observations collected during the ARM Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE) for the period spanning April to September 2023 (spring and summer) at Scripps Pier in La Jolla, California (32.8669° N, 117.2569° W) were used to investigate the regional-scale atmospheric factors that control the variability of predominant marine low clouds and aerosols in the coastal boundary layer. Using Self-Organizing Maps (SOMs) applied to ERA5 sea level pressure and near-surface wind fields, we classify the synoptic evolution of the subtropical anticyclone into 9 regimes. This regime categorization highlights the following dominant features in Northeast Pacific: (1) regimes that capture the evolution of the anticyclone in terms of magnitude (strong vs. weak) and location (coastal vs. offshore), with their corresponding transitions in boundary layer wind strengthening and large-scale subsidence (regimes: 1–3 vs. 6–8), (2) a second dominant feature characterized by an anticyclone with its core at the northwestern edge of the domain, resulting in modest sea level pressure changes near Scripps Pier (regimes 6–8), (3) patterns with a weakened subtropical anticyclone south of Scripps Pier and a midlatitude cyclone further north (regimes 4 and 5), and (4) an additional regime that captures anomalies that minimally depart from the climatological mean (regime 9). GOES-18 satellite cloud retrievals reveal that regimes associated with an anticyclone core closer to the EPCAPE coastal site (regimes 1–4) is associated with reduced low-cloud fraction, shallower clouds, and low liquid water path (LWP) along the shoreline; whereas regimes with a west/north-westward-displaced anticyclone combined with a weak coastal inland low-pressure feature (regimes 6–8) that support extensive stratocumulus with higher LWP and elevated cloud tops. Variability in the aerosol distribution is also controlled by the synoptic scale processes. Regimes with a relatively weak anticyclone centered adjacent to the Scripps Pier (regime 4) generally feature the highest concentrations of particles with diameter ≤ 0.1 µm, associated with a stable boundary layer and stagnation under weak winds. Regimes with anticyclone strengthening farther offshore (regimes 6–8) have relatively lower aerosol concentrations. The partial inconsistency in the evolution of the cloud droplet number concentration ( N _d ) and aerosol concentration indicates that the variability in boundary layer turbulence is a critical component that explains the aerosol activation into N _d .