Seven years of data collected at the Atmospheric Radiation Measurement (ARM)’s Eastern North Atlantic (ENA) site are analyzed to understand the controls of Cloud Condensation Nuclei (CCN) concentrations in the region. Day-night differences in the aerosol data as segregated by wind direction demonstrated the aerosol observations to be impacted by local emissions when the wind direction (wdir) is between 90° and 310° (measured clockwise from the North where air is coming from). Data collected during marine conditions (wdir<90° or wdir>310°) showed the CCN concentrations to be higher in the summer months as compared to the winter months. CCN budget analysis revealed advection and precipitation scavenging being primarily responsible for modulating the CCN concentrations at the site on monthly timescales, with rain rates driving the precipitation scavenging term. High (greater than 75th percentile) and low (lower than 25th percentile) CCN events were identified for each month to characterize the sub-monthly variability of CCN concentrations. Low CCN events had deeper clouds, stronger rain rates, and lower free-tropospheric aerosol mass at the ENA site as compared to the high CCN events. Analysis of satellite data of air-parcels 48 hours prior to their arrival at the ENA site demonstrated the air parcels during low CCN events to encounter higher cloudiness, stronger rain rates, and higher cloud top heights as compared to the high CCN events. The results presented herein provide key constraints for model evaluation studies and climatological studies conducted at the ENA site.

Deep convection occurs periodically in the Gulf of Lion, driven by the seasonal atmospheric change and Mistral winds. To determine the variability and drivers of the seasonal and Mistral forcing, 20 years of ocean simulations were run. Two sets of simulations were performed: a control set, forced by unfiltered atmospheric forcing, and a seasonal set, forced by filtered forcing. The filtered forcing retained the seasonal aspects but removed the high frequency phenomena. Assuming the Mistral acts primarily in the high frequency, comparing the two sets allows for distinguishing the effects of the Mistral on the ocean response. During the preconditioning phase, the seasonal forcing was found to be the main destratifying process, removing on average 45.7% of the stratification, versus the 28.0% removed by the Mistral. Despite this difference, at the time of deep convection, both the seasonal and Mistral forcing each triggered deep convection in roughly half of the events. Larger sensible and latent heat fluxes were found in the seasonal forcing of the years with deep convection, acting as the main drivers (removing 0.17 m2s-2 and 0.43 m2s-2 of stratification, respectively). They are themselves driven by increased wind speeds, believed to be the low frequency signal of the Mistral, as more Mistral events occur during winters with deep convection (34.3% versus 28.6%). The evolution of the seasonal forcing in a changing climate may have a significant effect on the future deep convection cycle of the Gulf of Lion.