Extreme Compound and Seesaw Hydrometeorological Events in New Zealand: An Initial Assessment

Abstract

Attention is increasingly being turned toward land atmosphere interactions within the wider hydrological cycle when investigating extreme hydrometeorological events. This is particularly the case with the identification of compound and seesaw events. To do so, accurate soil moisture data are essential. Here, soil moisture from three reanalysis products (ERA5-Land, BARRA and ERA5) is compared to station observations from 12 sites across New Zealand, for an average timespan of 18 years. Soil moisture data from all three reanalyses are used to investigate land-atmosphere coupling with gridded (observational) precipitation and temperature. This enables compound (co-occurrence of hot and dry) and seesaw (rapid transitions from dry to wet) events to be identified and examined. No best performing reanalysis data set for soil moisture is evident (median Pearson's r range: 0.78–0.81). All reanalyses successfully capture the seasonal and residual component of soil moisture, but not the observed soil moisture trends at each location. Strong coupling between soil moisture and temperature occurs in all three reanalyses across the predominately energy-limited regions of the lower North Island and entire South Island. Consequently, these regions reveal a high frequency of compound event occurrence and potential shifts in land states to a water limited phase during compound months. A series of seesaw events is also detected for the first time in New Zealand (terminating approximately one-fifth of drought events), with a high frequency of seesaw event occurrence detected in previously identified areas of atmospheric river (AR) activity.