Extreme Compound and Seesaw Hydrometeorological Events in New Zealand:
An Initial Assessment
Abstract
Attention is increasingly being turned towards an investigation of
extreme hydrometeorological events within the context of land-atmosphere
coupling in the wider hydrological cycle, particularly with respect to
the identification of compound and seesaw events. To examine these
events, accurate soil moisture data are essential. Here, soil moisture
from three reanalysis products (ERA5-Land, BARRA and ERA5) are compared
to station observations from 12 sites across New Zealand for an average
timespan of 18 years. Soil moisture data from all three reanalyses were
subsequently used to investigate land-atmosphere coupling with gridded
(observational) precipitation and temperature. Finally, compound
(co-occurrence of hot and dry) and seesaw (transitions from dry to wet)
periods were identified and examined. No best performing reanalysis
dataset for soil moisture is evident (min r = 0.78, max r = 0.80). All
datasets 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
across the predominately energy-limited regions of the lower North
Island and entire South Island. Consequently, these regions reveal a
high frequency of compound period occurrence and potential shifts in
land states to a water limited phase during compound months. A series of
seesaw events are also detected for the first time in New Zealand
(terminating an average of 17% of droughts), with particularly high
frequency of seesaw event occurrence detected in previously identified
areas of atmospheric river (AR) activity, indicating the likely wider
significance of ARs for drought termination.