| Marine subtropical stratocumulus clouds play an important role in the climate of the tropical Pacific and Atlantic. It is therefore important to understand the processes governing their formation, maintenance and destruction. Even though much progress has been made in this regard, the simulation of these clouds in coupled ocean-atmosphere models remains a difficult problem.; In order to gain insight into the large-scale factors controlling stratocumulus, we examine the impact of orography and prescribed SST anomalies on the persistence and annual cycle of these clouds. This is done in the context of an uncoupled atmospheric general circulation model that realistically simulates stratocumulus.; In two sets of experiments we isolate the influence of African and South American orography, respectively. In both cases, the removal of orography results in a decrease of stratocumulus incidence off the respective coasts of Namibia and Peru, and the strongest impact is found in local winter. Temperatures at 700 mb are enhanced when orography is present, albeit for fundamentally different reasons in the two regions. In the African case, the relatively low orography induces anti-cyclonic circulation centered over Southern Africa, which is associated with warm advection over the Namibian stratus region. In the South American case, on the other hand, the steep and high orography effectively blocks trade easterlies and midlatitude westerlies. The latter are diverted equatorward along the coast of South America. Above the PBL, the air sinks along the downward sloping isentropes as it moves equatorward, leading to subsidence warming, and resulting in an increase in lower tropospheric stability. At the surface level, blocking occurs in both cases, and is associated with cold advection. In each region, the combination of above processes increases lower tropospheric stability which is conducive to stratocumulus persistence.; Stratocumulus off the Namibian coast are very sensitive to prescribed warm and cold SST anomalies, but physical processes act to keep incidence between 20% and 80%. When the annual cycle of SST is removed from the region, overlying stratocumulus retain a marked seasonality, with reduced incidence during southern winter. This emphasizes the importance of above-PBL temperatures in the annual cycle of Namibian stratocumulus. |
