Abstract
Extreme hydrometeorological events cause significant multi-sector impacts, and are most readily witnessed at the interface between the land and atmosphere. Two examples of such events are compounding hot and dry conditions, and seesaw (whiplash) events, defined as heavy rainfall or flooding following dry conditions. This thesis provides one of the first investigations of multivariate extreme hydrometeorological events for New Zealand. European ReAnalysis 5th Generation Land Component (ERA5-Land) data are utilised, with soil moisture representation comparable to observations. Copula-based methodologies are contrasted with the more common coincident / consecutive approaches for event detection. Representation of compound events using the vine copula (Standardised Multivariate Index (SMI)) reveals a higher occurrence compared to coincident methodologies, with a median occurrence of 65 events in comparison to the coincident Standardised Soil Moisture Index (SSMI)/Standardised Temperature Index (STI) (47 events) and coincident Standardised Precipitation Index (SPI)/STI (34 events). Representation of seesaw events using the bivariate copula (Standardised Bi-Copula Index (SBI)) reveals an average occurrence of 62 events across New Zealand, in close agreement to consecutive SSMI (57 events), but less than the consecutive SPI (79 events).
Persistent High Pressure (PHP) systems were identified using a atmospheric blocking detection algorithm. PHP systems are associated with a 70% increase in compound hot and dry days country wide. Strong regional variation in the expression of compound events is shown to be dependent on the positioning of PHP systems, with PHP systems originating to the north west of the country resulting in a 50% to 100% increased risk of experiencing a hot day, a 0% to 50% increased risk of experiencing a dry day, and a 150% to 250% increased risk of experiencing a compound hot and dry day across the east coast of the South Island and much of the North Island. Meanwhile, phases 4 through 6 of the Madden Julian Oscillation (MJO) indicate significant increases in compound hot and dry days (ranging from 6% to 49%) during their phase expression.
Storm types associated with whiplash events are also investigated, which shows Extra Tropical Cyclone (ETC) events as a determinant of whiplash behaviour (driver of 53% of whiplash events) for New Zealand, with the highest number of whiplash events occurring on the west coast of the South Island. Atmospheric River (AR) activity drives 48% and 70% of whiplash events across the west and north of the North Island, respectively. Only 16% of whiplash events on the west coast of the South Island are induced by AR behaviour, with ETC-induced whiplash (68%) being dominant. MJO connections are dominant for the east coast of the North Island, with a 59% increase in whiplash occurrence during phases 2, 4, 5 and 6.
Collectivity, the findings document the occurrence, characteristics and driving forces behind both compounding hot and dry conditions, and seesaw / whiplash events for the first time across New Zealand. Future work should continue to unravel the ocean-
atmosphere-land connection behind these events, including how climate change may impact these connections.