The sensitivity of Peruvian stratocumulus to the large-scale environment

Marine stratocumulus decks play an important role in the climate systems 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.

One area of particularly persistent stratocumulus is located off the western coast of South America. The present study examines the sensitivity of those stratocumulus decks to two large-scale factors, namely South American orography and prescribed sea surface temperature (SST) anomalies. This is done in the context of an uncoupled atmospheric general circulation model capable of producing a realistic simulation of planetary boundary layer (PBL) clouds.

In one set of experiments, the influence of South American orography on Peruvian stratocumulus is investigated. The removal of orography results in a substantial decrease of stratocumulus incidence off the coast of Peru, and the strongest impact is found in local winter. Inspection of the lower tropospheric stability (LTS) reveals that stability is enhanced when orography is present. An explanation for this is found in the blocking effect of the Andes. As the midlatitude westerlies impinge on the western slope of the mountains they are, in part, diverted equatorward along the South American coast. This is accompanied by a decrease in planetary vorticity. Since changes in relative vorticity are small in comparison, equatorward traveling parcels are compressed in accordance with conservation of potential vorticity. The result is a very stable layer in the lower troposphere, which is conducive to the persistence of stratocumulus. This presents a mechanism different from the one found in a companion study of the stratocumulus off the Namibian coast. There, the orographic impact on stratocumulus is mediated mostly through horizontal warm air advection above the PBL.

In another set of experiments, several SST anomaly patterns are prescribed in the Peruvian stratus region and its vicinity. A cold SST anomaly that is constant in time results in a year-round increase in LTS compared to the control experiment. Stratocumulus incidence, on the other hand, increases only during the first half of the year, when the cold anomaly opposes the annual cycle of SST. When the sign of the SST anomaly is switched, a year-round reduction of LTS is obtained. Analogously to the cold anomaly experiment, this leads to a decrease in stratocumulus incidence only during the second half of the year. In a further experiment, the annual cycle of SST is removed from the Peruvian stratus region and its vicinity. Remarkably, this does not lead to an elimination of the annual cycle of stratocumulus in the region. Rather, stratocumulus incidence achieves a minimum in austral winter. This is associated with low wintertime temperatures above the PBL and the accompanying decrease in LTS.

The results of this study suggest that both adjacent orography and underlying SST are crucial components in creating a stable environment that can foster persistent stratocumulus. It is found that orography increases stability over the depth of the entire lower troposphere, while SST anomalies mostly affect only the PBL.