On Air–Sea Interaction at the Mouth of the Gulf of California

Abstract

Surface flux, wind profiler, oceanic temperature and salinity, and atmospheric moisture, cloud, and wind observations gathered from the R/V Altair during the North American Monsoon Experiment (NAME) are presented. The vessel was positioned at the mouth of the Gulf of California halfway between La Paz and Mazatlan (?23.5°N, 108°W), from 7 July to 11 August 2004, with a break from 22 to 27 July. Experiment-mean findings include a net heat input from the atmosphere into the ocean of 70 W m?2. The dominant cooling was an experiment-mean latent heat flux of 108 W m?2, equivalent to an evaporation rate of 0.16 mm h?1. Total accumulated rainfall amounted to 42 mm. The oceanic mixed layer had a depth of approximately 20 m and both warmed and freshened during the experiment, despite a dominance of evaporation over local precipitation. The mean atmospheric boundary layer depth was approximately 410 m, deepening with time from an initial value of 350 m. The mean near-surface relative humidity was 66%, increasing to 73% at the top of the boundary layer. The rawinsondes documented an additional moist layer between 2- and 3-km altitude associated with a land?sea breeze, and a broad moist layer at 5?6 km associated with land-based convective outflow. The observational period included a strong gulf surge around 13 July associated with the onset of the summer monsoon in southern Arizona. During this surge, mean 1000?700-hPa winds reached 12 m s?1, net surface fluxes approached zero, and the atmosphere moistened significantly but little rainfall occurred. The experiment-mean wind diurnal cycle was dominated by mainland Mexico and consisted of a near-surface westerly sea breeze along with two easterly return flows, one at 2?3 km and another at 5?6 km. Each of these altitudes experienced nighttime cloudiness. The corresponding modulation of the radiative cloud forcing diurnal cycle provided a slight positive feedback upon the sea surface temperature. Two findings were notable. One was an advective warming of over 1°C in the oceanic mixed layer temperature associated with the 13 July surge. The second was the high nighttime cloud cover fraction at 5?6 km, dissipating during the day. These clouds appeared to be thin, stratiform, slightly supercooled liquid-phase clouds. The preference for the liquid phase increases the likelihood that the clouds can be advected farther from their source and thereby contribute to a higher-altitude horizontal moisture flux into the United States.