Sinking of the Eastern North Atlantic Stratocumulus: Implications for the Macaronesian Cloud Forest

Abstract The existence of Macaronesian cloud forests is closely linked to cloud immersion, a phenomenon associated with the regional subtropical stratocumulus (Sc). Any change in the Sc is likely to have a significant impact on these ecosystems. This study examined the Sc height in the Northeast Atlantic off the African coast, the Central Macaronesian region, over the last two decades, using a dataset of sounding and surface meteorological data from Madeira and Tenerife. A Mann–Kendall analysis revealed a significant downward trend in Sc boundary heights at both sites, with Madeira experiencing more than twice the decrease observed in Tenerife. The decrease in cloud top height, measured using trade wind inversion base as a proxy, is consistent with the observed increase in subsidence at the 850 hPa level, concomitant with the documented intensification of the Azores High. The observed reduction in the cloud base height, calculated using the lifting condensation level as a proxy, can be attributed to an increase in the moisture content of the air, in line with the observed changes in the regional and local sea surface temperature. However, the potential impact of changes in the Azores High, as evidenced by the observed trend in surface pressure at Central Macaronesia, on regional moisture transport patterns cannot be disregarded. Among the many impacts that this lowering of Sc may have on the climatic and environmental conditions of the region, one of the most obvious is the influence of these changes on the altitudinal distribution of regional cloud forests, which may be forced to migrate to lower altitudes as cloud immersion patterns are disrupted. Graphical Abstract This study analyses the evolution of stratocumulus height in the eastern North Atlantic over the Central Macaronesian region. The region is subject to the influence of subsidence associated with the descending branch of the Hadley circulation cell, which creates a thermal inversion that limits the vertical development of the cloud. This has a limited effect on the development of ecosystems such as cloud forests, which depend on cloud immersion for their survival. Our findings reveal a sinking of the Sc in this region, particularly in the northern part surrounding Madeira. The decrease in the top of the Sc is associated with an increase in regional subsidence, which is linked to the strengthening of the Hadley cell circulation. The decrease at the base of the stratocumulus is explained by the increase in surface moisture, which correlates well with the observed increase in SST. The most obvious consequence of this is on the altitudinal distribution of regional cloud forests, which will be forced to migrate to lower altitudes.