Resonant oscillations in the Hawai`ian archipelago and tropical instability vortices and their fronts, frontal instabilities, and cross-frontal differences

Benjamin, Lindsey

Resonant oscillations in the Hawai`ian archipelago and tropical instability vortices and their fronts, frontal instabilities, and cross-frontal differences

Files

Benjamin_hawii_0085A_11489.pdf (11.44 MB)

Date

2022

Authors

Benjamin, Lindsey

Advisor

Flament, Pierre

Department

Oceanography

Abstract

This dissertation consists of two unrelated parts: an analysis of resonance modes from tsunamis and potential meteotsunamis, and an analysis of tropical instability vortices (TIVs) and fronts. In the first part, the resonant response of tsunamis and possible meteotsunamis is examined. The 2011 Tohoku tsunami described from surface currents in high-frequency Doppler radio (HFDR) data and model simulations has two modes over Penguin Bank: a stronger mode with one larger and stronger antinode on the southern part of the bank and a weaker, smaller antinode of opposite polarity on the northern part with 43-min oscillations, and a weaker mode with two relatively equal antinodes of opposite polarity situated in a north-south fashion on the bank with oscillations with periods between 15 and 30 min. Resonance modes depend on local features of bathymetry and coastlines, not the excitation force; other seismic tsunamis as well as meteotsunamis, or long-period waves caused by atmospheric pressure anomalies interacting resonantly with the ocean surface, would be expected to excite the same modes. A search in 29-mo of data using the 2011 Tohoku tsunami modes as a spatial filter not only did not detect any likely meteotsunami events, but it failed to detect two other, weaker, seismic tsunamis that occurred. The HFDR used was not optimally positioned to detect currents on Penguin Bank, and the inverse relationship between time step width and velocity resolution in all HFDRs means this instrument could only detect stronger currents in the resonance modes. In response, it is recommended that Penguin Bank be instrumented with five moorings, each with an upward-looking ADCP and a bottom pressure sensor, at locations chosen based on modeled resonance modes that would allow in-situ detection of resonance mode oscillations; also, another HFDR could be placed in a more optimal position to detect currents on Penguin Bank. Additionally, changes to the currently-installed HFDR and modeling of meteotsunamis in the Hawai`ian Islands is recommended. In the second part, the fronts, frontal instabilities, and cross-frontal differences in TIVs are examined. TIVs are 500-km diameter anticyclones with Rossby number ∼ − 1 on the North Equatorial Front that swirl colder, upwelled equatorial waters northward on their western flanks and advect warmer surface water of ITCZ-origin to the south on their eastern flanks. This swirling creates a cusp of colder water that extends northward of the mean meridional position of the North Equatorial Front with two roughly north-south fronts separating water of different temperatures, salinities, and densities: the leading front on the western side of the cusp separates warm, fresh, less-dense water to the west from the cold, salty, more-dense water in the cusp to the east, while the trailing front on the eastern side of the cusp separates cold cusp water to the west from warmer water to the east. The fronts are rotated and deformed by the swirling currents and simultaneously develop waves, cusps, and breaks due to shear current instabilities. The orientation of TIV fronts, which changes by advection of swirling large-scale currents, means that winds generally support frontogenesis on the trailing front, but either oppose frontogenesis or have littleimpact on leading fronts. Temperature effects on the wind, including changes in wind speed and drag coefficient, typically explain surface roughness differences across leading fronts, but some of the trailing fronts have large enough differences in currents that can overpower that effect. Currents around sub-mesoscale fronts within TIVs evolve as the fronts are advected. TIV fronts and frontal instabilities should be modeled to determine specifically which shear current instability is present and the amount of energy and heat involved. Wind estimates derived from SAR can be significantly altered by a difference in the currents across the front. SAR can be used to observe sub-mesoscale fronts and frontal instabilities.

Keywords

Physical oceanography, meteotsunamis, resonance oscillations, sub-mesoscale fronts, synthetic aperture radar, tropical instability vortices, tsunamis

URI

https://hdl.handle.net/10125/103914

Extent

261 pages

Rights

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Collections

Ph.D. - Oceanography

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