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Characterization of Non-El Niño Induced Dry Conditions across the U.S. Affiliated Pacific Islands.

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Title:Characterization of Non-El Niño Induced Dry Conditions across the U.S. Affiliated Pacific Islands.
Authors:Ludert, Alejandro T.
Contributors:Atmospheric Sciences (department)
Keywords:Paci c Islands
El Ni~no
La Ni~na
Date Issued:Dec 2017
Publisher:University of Hawaiʻi at Mānoa
Abstract:The U.S. A liated Paci c Islands (USAPIs), located in the Western Paci c, have limited
water resources making them very susceptible to severe drought conditions. The annual
cycle and ENSO response of rainfall di ers between USAPIs north of 7 N and those to the
south. Southern stations show a canonical negative correlation between dry season (December
to May) rainfall and ENSO. Northern stations, on the other hand, show little correlation
with ENSO if the three super El Ni~nos are excluded. Instead, they exhibit two distinct
rainfall regimes, the Canonical regime, and a Non-Canonical regime, in which the dry season
rainfall is positively correlated with ENSO. Non-canonical years pose an important forecasting
challenge. Cool Dry events are of particular interest because they have coincided
with several emergency and disaster-level droughts across the Northern USAPIs. Composite
analysis of the Canonical and Non-Canonical regimes show stark di erences between dry season
atmospheric and SST patterns. Compared to Canonical composites, the Non-Canonical
composites show clear and previously undescribed anomaly patterns during the dry season.
In Cool Dry events, circulation anomalies over the Western Paci c are anticyclonic, with a
band of anomalous dry conditions extending from the central Paci c towards Micronesia that
causes unexpected droughts in the Northern USAPIs. Canonical Cool Wet events, on the
other hand, show cyclonic West Paci c circulation anomalies and a La Ni~na like horseshoe
rainfall pattern over the Paci c Basin. Non-canonical Cool events also show SST anomalies
narrowly constrained near the dateline, while Canonical Cool events show their largest
anomaly magnitude east of the dateline. Both Non-Canonical and Canonical Cool events
show negative rainfall and Western Paci c anticyclonic anomalies before the onset of the
Dec-May dry season. In Non-Canonical events, these anomalies persist throughout the dry
season, while for Canonical events they shift, rapidly becoming positive rainfall and cyclonic
circulation anomalies during the dry season. SST anomalies also evolve di erently, with
Non-Canonical Cool events showing anomalies that extend eastward from the central Paci c
rather than intensifying in place over the eastern Paci c. The features are similar and opposite
for Canonical and Non-CanonicalWarm events. Di erences in the evolution of anomalies
suggest that the physical mechanisms governing the Non-Canonical and Canonical ENSO
regimes are distinct. These di erences have been leveraged to develop a novel 2-tier forecasting
methodology that combines logistic and linear regression to forecast the Dec-May dry
season Standardized Precipitation Index in the Northern USAPIs. This 2-tier methodology
achieves signi cant improvement in the forecast of Dec-May rainfall anomalies as compared
to a benchmark forecast. This type of improved forecasts will help provide local governments
and decision makers with guidance for mitigation and relief during Non-Canonical events.
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2017.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Atmospheric Sciences

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