Abstract. Cloud drop number concentrations (Nd) over the western North Atlantic Ocean (WNAO) are generally highest during the winter (DJF) and lowest in summer (JJA), in contrast to aerosol proxy variables (aerosol optical depth, aerosol index, surface aerosol mass concentrations, surface cloud condensation nuclei (CCN) concentrations) that generally peak in spring (MAM) and JJA with minima in DJF. Using aircraft, satellite remote sensing, ground-based in situ measurement data, and reanalysis data, we characterize factors explaining the divergent seasonal cycles and furthermore probe into factors influencing Nd on seasonal timescales. The results can be summarized well by features most pronounced in DJF, including features associated with cold-air outbreak (CAO) conditions such as enhanced values of CAO index, planetary boundary layer height (PBLH), low-level liquid cloud fraction, and cloud-top height, in addition to winds aligned with continental outflow. Data sorted into high- and low-Nd days in each season, especially in DJF, revealed that all of these conditions were enhanced on the high-Nd days, including reduced sea level pressure and stronger wind speeds. Although aerosols may be more abundant in MAM and JJA, the conditions needed to activate those particles into cloud droplets are weaker than in colder months, which is demonstrated by calculations of the strongest (weakest) aerosol indirect effects in DJF (JJA) based on comparing Nd to perturbations in four different aerosol proxy variables (total and sulfate aerosol optical depth, aerosol index, surface mass concentration of sulfate). We used three machine learning models and up to 14 input variables to infer about most influential factors related to Nd for DJF and JJA, with the best performance obtained with gradient-boosted regression tree (GBRT) analysis. The model results indicated that cloud fraction was the most important input variable, followed by some combination (depending on season) of CAO index and surface mass concentrations of sulfate and organic carbon. Future work is recommended to further understand aspects uncovered here such as impacts of free tropospheric aerosol entrainment on clouds, degree of boundary layer coupling, wet scavenging, and giant CCN effects on aerosol–Nd relationships, updraft velocity, and vertical structure of cloud properties such as adiabaticity that impact the satellite estimation of Nd.

Decades of atmospheric research have focused on the Western North Atlantic Ocean (WNAO) region because of its unique location that offers accessibility for airborne and ship measurements, gradients in important atmospheric parameters, and a range of meteorological regimes leading to diverse conditions that are poorly understood. This work reviews these scientific investigations for the WNAO region, including the East Coast of North America and the island of Bermuda. Over 50 field campaigns and long‐term monitoring programs, in addition to 715 peer‐reviewed publications between 1946 and 2019, have provided a firm foundation of knowledge for these areas. Of particular importance in this region has been extensive work at the island of Bermuda that is host to important time series records of oceanic and atmospheric variables. Our review categorizes WNAO atmospheric research into eight major categories, with some studies fitting into multiple categories (relative %): aerosols (25%); gases (24%); development/validation of techniques, models, and retrievals (18%); meteorology and transport (9%); air‐sea interactions (8%); clouds/storms (8%); atmospheric deposition (7%); and aerosol‐cloud interactions (2%). Recommendations for future research are provided in the categories highlighted above.

The Western North Atlantic Ocean (WNAO) region represents a complex climate system that comprises a wide range of spatiotemporal scale phenomena: mesoscale continental convection and tropical cyclones, synoptic-scale processes (e.g., frontogenesis), and interannual climate variability (e.g., North Atlantic Oscillation). The region is influenced by the Gulf Stream current system, which gives rise to sharp spatial gradients in sea surface temperature (SST) and is responsible for significant ocean-atmosphere interactions (Small et al., 2008). In this regard, interactions between SST, surface air temperature, and winds yield strong turbulent fluxes that regulate the evolution of the atmospheric boundary layer and the regional atmospheric circulation (Nakamura et al., 2008). The WNAO climate is strongly controlled by the semipermanent North Atlantic Anticyclone, which modulates the air flow patterns and aerosol transport from North America and Africa. The diverse atmospheric and oceanic processes over WNAO are responsible for a variety of cloud morphological types: (i) stratiform boundary-layer clouds preferentially in winter and spring, (ii) shallow cumulus over the ocean during the warm season, and (iii) and deep convective and cirrus clouds associated with fronts, continental convection, and tropical cyclones. Noteworthy is the intense convective

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