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

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Description

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, and sea spray aerosol. Satellite observations have shown a general trend towards higher ocean wind speeds in the period 1991-2008, with increases by at least 5-10 %, depending on region. This observed trend is not necessarily related to anthropogenic climate forcing, but could arise from decadal internal variability of the climate system. Climate simulations project ocean 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 the response of marine stratocumulus clouds to wind speed changes, and the associated effect on radiative forcing. Results of cloud-system-resolving simulations of marine stratocumulus clouds are presented. The effect of wind speed on cloud properties and radiative forcing is quantified and explained with a feedback mechanism in which wind speed generates shear at the inversion and thereby enhances entrainment.