Mechanisms of Interannual Variability of the Southwest United States Summer Rainfall Maximum

Abstract

The mid-summer rainfall singularity of the Southwest United States (principally Arizona) exhibits marked variations on interannual and decadal time scales. Examination of the synoptic mechanisms involved in these variations is undertaken here. In particular, associations between the rainfall, the dominant latitude of the summertime mid-tropospheric subtropical ridge (STR) over the southwest United States, and the sea surface temperatures (SSTs) of the equatorial and North Pacific region are documented. The analysis utilizes a composite approach for sets of extreme years chosen on the basis of the rainfall and circulation anomalies. It is found that northward (southward) displaced seasonal STR is associated with wetter (drier) summers in Arizona. Further, these extremes have tended to follow winters characterized by positive (negative) phases of the Pacific-North America (PNA) teleconnection pattern. The latter association arises, at least in part from the ?memory? imparted to the atmosphere by the accompanying anomalies of Pacific SSTs. However, during the summer season, more localized anomalies of SST appear important for Arizona rainfall variations. In wet (but not dry) summers, an enhanced longitudinal gradient of SST exists between the west coast of the United States, Baja California, and the Gulf of California. This is accompanied by a steeper gradient of lower tropospheric heights (and implied stronger geostrophic flow) and also a reversal in both the total (850?500 mb) and partial (850?700 mb) thickness gradients across the region compared with dry summers. These results seem to confirm the importance of lower-level southwesterly flow for moisture transport into the deserts. Recent decadal variations in the singularity involve particularly runs of wetter (drier) summers in the 1950s (1970s). Preliminary analysis of these variations for years that were non-ENSO suggests that they may result from the operation of mechanisms similar to those attending the interannual variability of Arizona summer rainfall (viz., the STR and Pacific SSTs). A contributory mechanism in the longer-term trend of STR between these decades appears to be a change in the tropical?extratropical gradient of Pacific SSTs during the summer and antecedent spring. The gradient evidently strengthened during the period, helping to explain the shift to more frequent southward displacements of STR over the Southwest and, accordingly, reduced summer rainfall in Arizona.