Increased Marine Stratocumulus Cloudiness due to Weakened Subsidence in the Subtropics: Possible Negative Feedback to Anthropogenic Warming

Large and persistent decks of marine stratocumulus clouds (MSC) exist under large-scale free-tropospheric subsidence over the eastern subtropical oceans. Over seasonal and inter-annual timescales, MSC fraction is positively correlated to pressure vertical velocity, such that enhanced subsidence tends to occur when MSC fraction is large. We hypothesize that this positive correlation is caused by subsidence-driven adiabatic warming above the boundary layer that generates a stronger capping inversion, which itself favors large MSC fraction, rather than any independent effect of subsidence. For the same strength of temperature inversion, we hypothesize that stronger subsidence actually promotes less MSC fraction. To investigate this hypothesis, we attempt to determine the direct effect of subsidence in forcing variations in MSC fraction over the seasonal cycle, using artifact-corrected International Satellite Cloud Climatology Project (ISCCP) low-level cloud fraction and meteorological data from reanalyses. To accomplish this, we plot MSC fraction as a function of estimated inversion strength (EIS) and vertical velocity at 700 hPa. The plots strongly suggest that stronger subsidence independent of EIS actually promotes less MSC fraction. To further investigate the role of subsidence, we perform a multi-linear least-squares regression, using EIS and vertical velocity as the predictors and MSC fraction as the predictand. The multi-linear regression indicates that subsidence is negatively related to MSC amount and is statistically significant. The results are also highly relevant to projections of MSC under greenhouse forcing, which predict that over the eastern subtropical oceans, EIS will increase while subsidence will decrease. The sign of the independent subsidence-MSC relationship determined in this study thus implies that MSC fraction may increase as subsidence weakens under greenhouse forcing, thereby partially offsetting anthropogenic warming since MSC have a cooling effect on the climate.