Reconstructing the South Pacific Convergence Zone Position during the Presatellite Era: A La Niña Case Study

Abstract

ixty-four southwest Pacific island meteorological stations were used to develop ENSO-related austral warm season (November?April) rainfall climatologies for 1961?90. Historical data for the same station set were then examined for 1955/56, one of the strongest La Niñas of the twentieth century. The pattern for that event was typical of a well-coupled protracted La Niña, with above-normal rainfall (>150%) observed for 17 stations in the case study.Tropical cyclone tracks were also used to illustrate the proximal effects of historical storms on station rainfall. Several stations that were located along the northern edge of the tropical cyclone swarm, but south of a region of anomalously low rainfall, were selected as possible candidates having been close to the mean South Pacific convergence zone (SPCZ) location. Linear interpolations between those stations highlighted a likely SPCZ position for the 1955/56 event.The reconstructed SPCZ location indicated a position southwest of normal, consistent with many La Niñas observed during the satellite era. The 1955/56 SPCZ reconstruction compared favorably with NOAA Cooperative Institute for Research in Environmental Sciences (CIRES) twentieth-century reanalysis (20CR) positions of maximum atmospheric water content and omega (time derivative of pressure that illustrates vertical velocity, and a proxy for convection and SPCZ location) at 500 hPa. Comparisons between Global Precipitation Climatology Project (GPCP) warm season rainfall composite plots for several satellite era La Niñas and one well-coupled La Niña event that occurred during 2010/11 to omega at 500 hPa and maximum rainfall amounts suggest that the 1955/56 reconstruction from in situ rainfall measurements captured the SPCZ location. These findings suggest that use of the 20CR could help to identify hemispheric-scale atmospheric circulation features like the SPCZ and improve understanding of ENSO dynamics prior to the satellite era.