Hawaiian wet-season regional climate variability associated with the El Niño and Pacific Meridional Mode
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2024
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This dissertation aims to delve into the intricate interactions between the El Niño-Southern Oscillation (ENSO) and the Pacific Meridional Mode (PMM), examining their impact on the wet-season regional climate in Hawaii.While previous studies have offered a comprehensive understanding of the influence of canonical El Niño events on Hawaii's climate, the first part of this research focuses on the nuanced interplay within the large-scale atmospheric circulation of the North Pacific during two specific types of El Niño events – eastern Pacific (EP) and central Pacific (CP). The analysis reveals distinctive patterns associated with EP and CP El Niño winters, influencing Hawaii's rainfall and temperature variability. EP El Niño events, characterized by active convective heating in the equatorial EP, create unfavorable conditions for winter rainfall in Hawaii. This is manifested through the enhanced Hadley circulation, anomalous sinking motion, and the eastward shift of the subtropical jet stream in the North Pacific, resulting in drier climates. Conversely, CP El Niño events, featuring weaker and westward-shift equatorial ocean warming and variable sinking motion near Hawaii, lead to a larger variation in rainfall patterns.
Additionally, the second part of this dissertation focuses on the PMM’s atmospheric features and their impact on Hawaiian rainfall variability in the preceding winter and concurrent spring. Positive PMM in winter is linked to anomalous surface westerly winds, inducing an east-west rainfall dipole pattern in the Hawaiian Islands. In spring, (+) PMM enhances moisture and rising motion over the tropical North EP, leading to a widespread rainfall in the state. Our assessment shows that moderate daily rainfall intensity over the windward sides and extreme rainfall events over lee sides are elevated in winter in (+)PMM. In contrast, springtime windward sides see a decrease in light daily rainfall frequency and an increase in moderate daily rainfall frequency. For the leeward side in spring, a substantial higher occurrence of heavy/extreme rainfall events is found in (+) PMM. Through the multiple linear regression, ENSO dominates the Hawaiian rainfall pattern in winter, whereas PMM plays a more critical role in spring.
The final section of this dissertation delves into the oceanic dynamical processes underlying the evolution of PMM-related equatorial warming from 1950 to 2020. Emphasis is placed on identifying the dominant factors contributing to PMM-related equatorial warming in both the prior and posterior winter, as these factors play a crucial role in modulating ENSO diversity and intensity. The preexisting PMM-related equatorial warming in the prior winter is linked to anomalous positive vertical temperature gradients generated by convergence and descending motions via Western North Pacific Oscillation-related anomalous westerlies and Ekman transports. Moreover, the extensive PMM-related El Niño-like equatorial warming in the following winter is associated with meridional temperature advection, thermocline feedback, and Bjerknes feedback. The study also identifies interdecadal changes in PMM-related equatorial warming and examines potential influences of the Interdecadal Pacific Oscillation (IPO) and global warming effects on this interdecadal variation.
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Atmospheric sciences, ENSO, Hawaii, PMM, Regional Climate
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111 pages
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