Observations and modeling of the impacts of ocean currents on surface waves with an emphasis on near-inertial currents

Abstract

Surface gravity waves play an important role in mediating the exchange of momentum, heat, and gases between the ocean and the atmosphere. While surface wave properties in the open ocean are predominantly determined by atmospheric winds, surface currents have been shown to play a role in modifying surface wave properties. In the past few decades significant progress has been made towards a more complete understanding of the mechanisms and impacts of how surface currents modify surface waves, both through both observational and numerical modeling approaches. In certain conditions, currents have been found to play a significant role in modifying wave heights and wave slopes, however most of this analysis has been limited to considering tidal and steady currents, with little attention focused on how currents varying at the inertial frequency modify waves. This study is focused on addressing some of the gaps in understanding surrounding how inertial currents modify surface waves; expanding observational tools that could be used to study these processes, interrogating modeling experiments to investigate how these currents might modify waves, and analyzing observational records to see how these effects manifest in the real ocean. This work is structured in three primary chapters, focused on observational method development, idealized modeling, and observational analysis, respectively. The first research chapter of this dissertation (Chapter 2) evaluates the ability of EMAPEX profiling floats, instruments long used to measure ocean current profiles, to measure 1D surface wave spectra and bulk wave properties. A recently developed algorithm to measure surface waves from 1Hz measurements of ocean current variability by EM-APEX floats is applied to a deployment of 6 EM-APEX floats and compared against simultaneous measurements by two other wave measuring platforms deployed nearby. This work finds that measurements of both bulk wave properties and wave spectra by EM-APEX floats arein good agreement with measurements from more established wave measuring platforms, and that these floats are capable of measuring wave spectra over a range of swell to windwave frequencies, potentially making them a useful tool in future wave-current interaction studies. The second research chapter of this dissertation (Chapter 3) targets improving understanding of how the time varying component of near-inertial currents impacts surface waves through a series of idealized models. This work demonstrates how the relationship between the spatial structure of inertial currents, the local inertial period, and wave group speeds plays a significant role in determining how inertial currents modify waves. By comparing the impacts on surface waves between simulations with currents varying inertially and steady currents with the same geometry, this work demonstrates that inertial currents can result in notably different patterns of spatial and temporal wave variability than steady currents. Finally, the third research chapter (Chapter 4) investigates how near-inertial current impacts on surface waves manifest in historical observations from two locations in the North Pacific. By analyzing frequency spectra from time series of winds, waves and currents at these locations, this work demonstrates that inertial currents play an important role in modifying both significant wave heights and wave slopes. Additionally, this work demonstrates that the relationships between inertial currents and the modification of wave properties differs from what would be expected for steady currents, in a manner that is qualitatively consistent with the result from Chapter 3.