Decadal Trend of Aerosol and Cloud Properties over the Western North Atlantic Ocean: Any Connection?

Both aerosols and clouds can influence atmospheric variability and Earth's radiative balance. Changes in aerosol loading can directly alter the radiative balance by scattering and absorption and indirectly via complex interactions with clouds. Past studies reveal a range of interaction strength and uncertainty in aerosol-cloud interactions, some of which arises from errors induced by spatial averaging in global studies as well as lax constraints on cloud water content. Few such studies have been reported on the timescale of a decade or longer.

In this work we investigate aerosol-cloud interactions over the western North Atlantic Ocean from 2000 to 2012. Monthly mean satellite observations of aerosol and cloud from NASA's MODIS instruments and atmospheric state from NASA's MERRA reanalysis are investigated. This domain and period are noteworthy due to significant changes that occur in aerosol loading: a -0.02 to -0.04 per decade trend in aerosol optical depth (AOD) is seen off the eastern U.S. coast in the mid-latitudes and is identified as anthropogenic in origin. Occurring concurrently are an increase in cloud effective radius (Re) and decrease in cloud optical thickness (COT), suggesting an anthropogenic impact on clouds in the region. To investigate, we employ a commonly used aerosol sensitivity parameter ACI = d(ln{C}) / d(ln{A}) constructed from binned scatterplots of aerosol-cloud (A-C) parameter space. When cloud water is constrained in the domain-wide analysis, sensitivities of ACI_Re < -0.10 and ACI_COT > 0.10 are found for 66% of observations, a set that covers cloud water path from 70 to 130 g/m². When analyzed in a more local context, preliminary results reveal a region along the eastern U.S. coast with sensitivities contrary to Twomey's theory (i.e. ACI_Re > 0 and ACI_COT < 0) while much of the remaining domain show sensitivity in agreement with Twomey's theory (with ACI_Re < 0 and ACI_COT > 0). Investigations into the cause of this coastal behavior are performed using the atmospheric state from MERRA reanalysis. Finally, we explore applications of principal component analysis to regional decadal studies of aerosol-cloud interactions and assess its utility in identifying independent proximate causes (e.g. aerosol vs. environment) leading to variability in cloud properties. This research provides an observational study of the aerosol-cloud interactions and the complex radiative effects in the context of air quality improvement policies.