Analysis of Variability in Atmospheric River Characteristics in New Zealand from 1940 through 2023

Atmospheric rivers (ARs) are long, narrow corridors of concentrated atmospheric moisture that transport water vapor from the tropics to the middle latitudes where they can drive extreme precipitation and flooding. They have become especially important for New Zealand where ARs are critical for their water resources but also cause societal impacts. This study examines long-term changes in AR frequency, seasonality and strength in New Zealand across a pre-satellite era (1940–1978), satellite era (1979–2023), and modern climate reference period (1991–2020) to account for shifts in observational capability following the advent of satellite data in 1979. ERA5 reanalysis was used to create a regionally tailored atmospheric river detection technique (ARDT) to identify and track ARs across the three periods. The ARDT classified ARs using the 85th and 95th percentiles of integrated vapor transport (IVT) to isolate moderate and extreme ARs and assess their sensitivity across the three periods. Mann-Kendall tests were produced to analyze trends in AR strength and size across the three periods, while Mann-Whitney U tests compared AR strength across the three periods. Additional analyses looked at the contribution of extreme landfalling ARs to high-intensity 6-hourly in-situ precipitation recorded at two high-fidelity stations in New Zealand. Results indicate that extreme ARs are showing the most consistent increasing trends across all three periods, especially the satellite and reference period. Mann-Whitney U tests exhibit statistically significant differences primarily between the pre-satellite and reference period and extreme landfalling ARs account for a disproportionate share of the most intense precipitation events. These findings suggest that extreme ARs provide the most informative lens for understanding future AR impacts in New Zealand and, in the context of independent projections showing future increases in AR-induced extreme precipitation, highlight the importance of resilience planning and adaptive decision-making.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Geography

Department dc:contributor.department

Geography

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2026

Author dc:creator

• Holgerson, Ryan

Chair dc:contributor.committeechair

• Ramseyer, Craig A.

Committee members dc:contributor.committeemember

• Bukvic, Anamaria
• Ellis, Andrew

• Atmospheric River
• Data Assimilation
• IVT
• ERA5
• Mann-Kendall
• Pre-Satellite
• Satellite
• New Zealand

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

en

Dc Identifier Other

vt_gsexam:45551

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/140763