Effects of the estimated inversion strength and aerosols on monthly variations of subtropical marine stratocumulus to the west of Africa

Abstract

Satellite and reanalysis data over the 2003–2022 period are used to study how the monthly variations of marine stratocumulus (MSc) to the west of Canary Islands and Namibia are influenced by thermodynamic variables and aerosols. Although the estimated inversion strength (EIS) has been thought to be the main predictor of low-cloud cover (LCC), the seasonal cycles of EIS and LCC differ significantly in the Canary MSc region. In linear regression models, adding dust aerosol (DA) optical depth as a predictor improves the prediction of LCC across all seasons and almost everywhere in the region compared to using EIS alone. In boreal summer, large amount of DAs are transported from the Sahara to the Canary MSc region. Analysis of the regression models shows that DAs contribute more than EIS to the Canary summer LCC maximum, reducing the error by up to 11 %. Biomass-burning aerosols (BBAs) also improve the prediction of LCC in the Canary MSc region, but less so than DAs, suggesting that BBAs are less effective as cloud condensation nucleii. Other known thermodynamic predictors of LCC (sea surface temperature, horizontal surface temperature advection, near-surface wind speed, and free-tropospheric relative humidity and vertical velocity) also improve the prediction, but much less effectively than the aerosols. In the Namibia MSc region, there are virtually no DAs but BBAs are abundant. There, EIS alone captures well the seasonal cycle of LCC. In this region, the effect of BBAs on LCC is hidden behind that of EIS, potentially because BBA optical depth and EIS coincidentally have similar seasonal cycles.