Atmospheric Imprint of a Cold Sea Patch in the North-western Tropical Atlantic

This work investigates the influence of a persistent mesoscale (O(100 km)) cold patch of Sea Surface Temperature (SST) on the atmospheric boundary layer exploiting a wide range of in-situ, ship-based, and satellite remote sensing observations collected in the northwestern tropical Atlantic during the EUREC4A campaign. The in-situ observations collected on the R/V M. S. Merian include data from the ship-borne Thermosalinograph and the atmospheric mast. The ship-based W-band radar, the Atmospheric Raman Temperature and Humidity Sounder (ARTHUS), and wind lidars recorded profiles of horizontal and vertical wind speed, temperature, water vapor mixing ratio, turbulent heat fluxes, low-level cumulus clouds properties, and rainfall continuously in the marine boundary layer. In addition, remote sensing products from the GOES-East geostationary satellite, the Metop-A ASCAT scatterometer, the MUR and the ESA-CCI SST datasets have also been used to detect cloud properties and skin SST. The SST patch is roughly 1.5 K cooler than the surrounding waters. Above it, the mean horizontal wind slows down, and the vertical wind fluctuations are reduced. Scatterometer wind speed data highlight a surface wind convergence along the front of the SST cold patch, where relatively strong SST gradients (O(0.01 K/km)), are observed along the surface wind direction. Over the cold patch, the cloud fraction decreases, with fewer clouds forming because of weaker latent heat flux and more robust air column stability, resulting in a net rainfall shutdown. The observed atmospheric response agrees with the so-called Downward Momentum Mixing (DMM) mechanism, which affects the atmospheric dynamics at these spatio-temporal scales but has rarely been observed at such a level of detail.