Processes controlling the mean tropical Pacific precipitation pattern. Part II: The SPCZ and the southeast Pacific dry zone

Abstract

The nature of the South Pacific convergence zone (SPCZ) is addressed by focusing on the dry (and cool) zone bounded by it and the coast of South America through numerical experiments. As shown in a companion paper, this dry zone is due, to a large extent, to orographically forced subsidence. Here it is shown that the northwestward expansion of this dry zone can be explained by advection of low moist static energy by the trade winds. These results provide an explanation of the geometry of the western edge of the dry zone and, therefore, of the eastern edge of the adjacent SPCZ. Sea surface temperature underneath the SPCZ is enhanced by relatively high near-surface humidity through evaporative processes, which feeds back into its organization. However, in this model, this feedback is not critical for the existence of the SPCZ. The subsidence associated with the ITCZ in the North Hemisphere negatively affects the precipitation rate in the SPCZ. It was also found that the sensitivity of the forced response is largest for peak orographic heights below 3000 m, which indicates that the exact representation of the Andes in numerical models might not be as critical as that of lower orography such as that in southern Africa.