Cloud‐SST feedback in southeastern tropical Atlantic anomalous events

Abstract

Using satellite‐based cloud measurements for 1984–2004, the interannual variability of the low‐level cloud cover over the tropical Atlantic Ocean in austral winter (June–July–August, JJA) is examined. It is found that the leading pattern of the low‐cloud anomalies in this season is a modulation of the climatological center of the cloud cover over the southeastern tropical Atlantic Ocean off the Angola and Namibia coasts. The fluctuation of cloud amount there occurs on both interannual and longer timescales. The relationship between this low‐cloud anomalous pattern and basinwide ocean‐atmosphere anomalies is studied through a composite analysis based on the objectively selected major low‐cloud deficit and excess years. For the composites we intentionally use data sets mainly based on satellite measurements for the past one to two decades to minimize the potential influences of the bias in the model‐based ocean‐atmosphere analyses. The composites show that the JJA anomalous cloud pattern is strongly influenced by the sea surface temperature (SST) anomalies of the equatorial and southeastern tropical Atlantic Ocean in the previous summer. The anomalous surface warm events off the southwestern coast of Africa near 15°S in January and February are usually initiated dynamically by remote forcing from the westerly wind anomalies over the western equatorial Atlantic, preceding the cloud anomalies. During the next few months, the warm water is spread into the southeastern tropical Atlantic Ocean and is conducive to deficit low‐cloud cover in subsequent JJA. The reduced low‐cloud cover in turn forces positive SST tendency in a larger area of the southeastern Atlantic Ocean by changing the amount of the local solar radiation reaching the sea surface. In June and July, this process moves the major center of the SST anomalies away from the coast and closer to the equator when the coastal process weakens. The low cloud‐radiation‐SST feedback also plays a role in the slow westward expansion of the SST anomalies in late austral winter and spring. Overall, the influence of the cloud fluctuation is an important component in the evolution of the southeastern tropical Atlantic anomalous events.