Twentieth century droughts and wet periods in Auckland (New Zealand) and their relationship to ENSO

In early 1994 the city of Auckland (New Zealand) was in the midst of a major water supply crisis. El Niño was blamed, and it is now a widely held view that Auckland's droughts and wet periods are predominantly a consequence of the El Niño–southern oscillation (ENSO) phenomenon. The scientific literature is consistent with a significant influence of ENSO on New Zealand's climate, but is less convincing that it is the dominant forcing factor for Auckland's droughts and wet periods. To investigate this, we constructed a seasonal history of 20th century droughts and wet periods and compared it with a seasonal time series of ENSO activity based on the southern oscillation index. Our drought/wetness index was derived from daily modelling of soil water content and used the climatological potential evaporation method advocated by Fowler (2002. Journal of Hydrology 256: 248–263) for century‐scale soil water modelling. In deriving this index we identified a seasonal bias issue associated with soil‐water‐based methods and developed a simple index calculation method to circumvent the problem. Comparison of the ENSO and drought/wetness indices does not support the contention that droughts and wet periods in Auckland are primarily associated with ENSO. Relationships are weak and inconsistent (especially between La Niñas and wet periods), and decadal‐scale variation in ENSO activity is not coupled with changes in the frequency and magnitude of droughts and wet periods. Many droughts and wet periods do not coincide with ENSO events, and many ENSO events have little impact. These findings were confirmed by a mean sea‐level pressure compositing analysis. This linked droughts and wet periods with synoptic‐scale atmospheric circulation patterns that show no significant relationship to the pressure anomaly patterns associated with ENSO events. We conclude that ENSO has been overrated as a cause of Auckland's droughts and wet periods, and that it would be appropriate to focus more attention on identifying and investigating other plausible forcing factors.