Synoptic and satellite-derived cloud property variations for the southeast Pacific stratocumulus region associated with changes in coastal satellite-derived cloud droplet number concentrations (<i>N</i><sub><i>d</i></sub>) are explored. MAX and MIN <i>N</i><sub><i>d</i></sub> composites are defined by the top and bottom terciles of daily area-mean <i>N</i><sub><i>d</i></sub> values over the Arica Bight, the region with the largest mean oceanic <i>N</i><sub><i>d</i></sub>, for the five October months of 2001, 2005, 2006, 2007 and 2008. The ability of the satellite retrievals to capture composite differences is assessed with ship-based data. <i>N</i><sub><i>d</i></sub> and ship-based accumulation mode aerosol concentrations (<i>N</i><sub><i>a</i></sub>) correlate well (<i>r</i> = 0.65), with a best-fit aerosol activation value <span style="border-bottom: 1px solid #000; vertical-align: 50%; font-size: .7em; color: #000;"><i>d</i>ln <i>N</i><sub><i>d</i></sub></span><span style="margin-left: -2.7em; margin-right: 0.5em; vertical-align: -45%; font-size: .7em; color: #000;"><i>d</i>ln <i>N</i><sub><i>a</i></sub></span> of 0.56 for pixels with <i>N</i><sub><i>d</i></sub>>50 cm<sup>−3</sup>. The adiabatically-derived MODIS cloud depths also correlate well with the ship-based cloud depths (<i>r</i>=0.7), though are consistently higher (mean bias of almost 60 m). The MAX-<i>N</i><sub>d</sub> composite is characterized by a weaker subtropical anticyclone and weaker winds both at the surface and the lower free troposphere than the MIN-<i>N</i><sub><i>d</i></sub> composite. The MAX-<i>N</i><sub>d</sub> composite clouds over the Arica Bight are thinner than the MIN-<i>N</i><sub>d</sub> composite clouds, have lower cloud tops, lower near-coastal cloud albedos, and occur below warmer and drier free tropospheres (as deduced from radiosondes and NCEP Reanalysis). CloudSat radar reflectivities indicate little near-coastal precipitation. The co-occurrence of more boundary-layer aerosol/higher <i>N</i><sub><i>d</i></sub> within a more stable atmosphere suggests a boundary layer source for the aerosol, rather than the free troposphere. <br><br> The MAX-<i>N</i><sub><i>d</i></sub> composite cloud thinning extends offshore to 80° W, with lower cloud top heights out to 95° W. At 85° W, the top-of-atmosphere shortwave fluxes are significantly higher (~50%) for the MAX-<i>N</i><sub>d</sub> composite, with thicker, lower clouds and higher cloud fractions than for the MIN-<i>N</i><sub>d</sub> composite. The change in <i>N</i><sub><i>d</i></sub> at this location is small (though positive), suggesting that the MAX-MIN <i>N</i><sub>d</sub> composite differences in radiative properties primarily reflects synoptic changes. Circulation anomalies and a one-point spatial correlation map reveal a weakening of the 850 hPa southerly winds decreases the free tropospheric cold temperature advection. The resulting increase in the static stability along 85° W is highly correlated to the increased cloud fraction, despite accompanying weaker free tropospheric subsidence
