ORCID

https://orcid.org/0009-0005-3659-9566

Date of Award

Spring 2026

Language

English

Embargo Period

2-6-2026

Document Type

Master's Thesis

Degree Name

Master of Science (MS)

College/School/Department

Department of Atmospheric and Environmental Sciences

Program

Atmospheric Science

First Advisor

Oliver Timm

Committee Members

Mathias Vuille

Keywords

Precipitation, Extreme events, Temperature, Specific Humidity, Vertical Motion, Total Column Water Vapor

Subject Categories

Atmospheric Sciences | Climate

Abstract

The Hawaiian Islands experience extreme maxima in precipitation that vary spatially, by season, and in response to different teleconnection patterns. Due to the islands’ unique geography, they are not well-resolved by global climate models, necessitating a focused assessment to identify  climatological trends. In this study, a probability-based analysis was conducted on regionally averaged thermodynamic variables relevant to the formation of extreme events. The primary objective was to evaluate whether changes in the occurrence of extreme conditions have occurred and to understand how the El Niño-Southern Oscillation (ENSO) influences these occurrence rates.

Hourly data from ERA5 spanning 1980-2019 were resampled to obtain daily maxima for specific humidity at 850 hPa, vertical motion (omega) at 500 hPa, total column water vapor (TCWV), and 2-meter temperature and daily totals for precipitation. To assess the incidence rates of extreme events, the ERA5 data were divided into two periods: 1980-1999 and 2000-2019. The fraction of attributable risk (FAR), a parameter that compares the relative risk of event occurrence, was employed to analyze the incidence rates for the two periods, with a focus on the upper quantiles of the distribution. Confidence intervals (CIs) were estimated using a 1000-member bootstrapping approach.

To assess seasonal and interannual variability, FAR values were calculated for both wet (October-April) and dry (May-September) seasons, as well as for El Niño versus La Niña conditions. Results from the ERA5 data were compared with a 14-member multi-model ensemble (MME) of CMIP6 models, which include daily historical data from 1970-2009 encompassing specific humidity at 850 hPa, omega at 500 hPa, total precipitation, and maximum surface temperature.

The analysis revealed that the FAR displays statistically significant trends in daily maximum temperature extremes during both the wet and dry seasons. Omega consistently exhibited negative FAR values across both seasons, while specific humidity, TCWV, and precipitation showed bipolar FAR values depending on the season: these three variables exhibited negative FAR values during the wet season, which shifted to positive values in the dry season. The lack of proportionality between the extreme precipitation FAR values and those of the associated thermodynamic variables may indicate a greater influence of specific storm types’ frequencies rather than merely the presence of favorable background conditions for extreme precipitation events.

In future research, the FAR method should be applied to local precipitation data and downscaled general circulation models to evaluate historical changes in extreme weather events and assess the relative risk associated with leading modes of climate variability.

License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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