Causes of Late Twentieth Century Trends in New Zealand Precipitation

Late twentieth Century trends in New Zealand precipitation are examined using observations and reanalysis data for the period 1960–2004. One of the aims of this study is to investigate the link between these trends and recent changes in the large-scale atmospheric circulation in the Southern Hemisphere. The contributions from changes in Southern Hemisphere climate modes, particularly the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM), are quantified. Increasingly drier conditions across the North Island of New Zealand and eastern regions of the South Island are, in large part, consistent with recent trends in ENSO and the SAM. However, prominent increases in rainfall along the west coast of the South Island are of opposite polarity to what would be expected under a positively trending SAM scenario. This increased rainfall appears to be associated with a local reorganization in the atmospheric circulation, with strengthened westerly flow across the South Island and weakening of the subpolar westerlies further south. In contrast, with SAM trending toward a more positive phase, one would expect an increased meridional sea level pressure gradient and a strengthened westerly wind field centered at 60◦S, with winds weakening at 35◦S. On a large scale it appears that while the high latitude component of observed climate trends is consistent with an increasingly positive phase of the SAM, the mid-latitude trends are not as well explained. This is particularly true over the Pacific sector and the New Zealand region.