Observed and Projected Ocean Wind Speed Trends and Marine Boundary Layer Clouds

An uncharted feedback mechanism of marine low clouds to anthropogenic climate forcing is their response to wind speed. This mechanism proceeds, inter alia, via the effect of wind speed on the surface fluxes of heat, moisture, horizontal momentum (shear), and sea spray aerosol. Satellite observations show a general trend towards higher ocean surface wind speeds in the period 1991-2008, which have increased by at least 5-10 %, depending on region. This decadal trend is not necessarily related to anthropogenic climate forcing, but could arise from internal variability of the climate system. Climate simulations project ocean surface wind speed trends for the 21st century in the range of -10% to 10% at the locations of large marine stratocumulus decks, and an increase in excess of 10% in the Southern Ocean in response to anthropogenic climate forcing. This presentation investigates radiative forcing by marine stratocumulus clouds in response to changes in wind speed. A particular focus is placed on the response of cloud-top entrainment to wind speed and its effect on cloud properties. Results of cloud-system-resolving simulations of marine stratocumulus clouds are presented, and the effect of wind speed on cloud radiative forcing is quantified.