2025 Theses Doctoral

Observations and Physical Modeling of the Near-Surface Ocean: Fundamental Insights into Solar Heating, Diurnal Warming, Precipitation, and Ocean-Ice Heat Flux

Witte, Carson Riggs

The interaction between ocean and atmosphere sits at the heart of the climate system, and the empirical parameterizations of air-sea fluxes required to couple models of the two media together typically rest on the assumption that the upper ocean is homogenized by turbulent mixing. However, there are a number of globally relevant phenomena that modify the density structure of the surface ocean at vertical scales of just a few meters, affecting the coupling between the ocean and atmosphere. Because of their limited vertical scale and intermittent temporal occurrence, the effects of near-surface processes can be challenging to represent accurately in both fundamental physical theory and computational modeling, and must be accounted for when making in-situ and remote sensing measurements of the ocean surface.

The four chapters presented herein address diverse near-surface phenomena – sea ice, precipitation, phytoplankton, and diurnal warming – through a consistent philosophy of using comprehensive observational datasets from above and below the air-sea interface to interrogate and improve upon our theoretical understanding of the processes at play. In each case, the comparison between observations and theoretical modeling reveals the strengths and limitations of the current models and motivates new modifications. Specifically, this work provides observation-based improvements to the accuracy of theoretical frameworks for the ocean-ice heat transfercoefficient, ocean skin temperature during precipitation, solar heating in the ocean’s upper meters in the presence of variable phytoplankton concentrations, and diurnal warm layer response to changes in wind forcing. Crucially, the proposed modifications avoid introducing unnecessary or prohibitive increases in complexity, so that, where appropriate, they may be readily implemented into the current generation of global climate models.

In Chapter 1, we present oceanographic and atmospheric time series from a heavily instrumented “ice-tethered observatory” located on landfast ice above the river outflow channel in front of Kotzebue, Alaska. This observing station was deployed as part of the Ikaaġvik Sikukun (Iñupiaq for “Ice Bridges”) project, in which hypotheses and subsequent observational programs were co-produced in partnership with an Indigenous Elder advisory council in Kotzebue. The measurements allow us to quantify the heat budget of the ice above the outflow channel, and identify the ocean as the primary source of heat contributing to thinning of the ice, while also revealing a fundamental limitation of the current approach to calculating ocean-ice heat fluxes from bulk properties.

In Chapter 2, we present radiometric observations of ocean skin temperature, near-surface (5cm) temperature from a towed thermistor, and bulk atmospheric and oceanic variables, for 69 rain events observed over the course of 4 months in the Indian Ocean as part of the DYNAMO project. We test a state-of-the-art prognostic model developed by Bellenger et al. (2017) to predict ocean skin temperature in the presence of rain, and demonstrate a physically motivated modification to the model that improves its performance with increasing rain rate. We also characterize the vertical skin-bulk temperature gradient induced by rain and find that it levels off at high rain rates, suggestive of a transition in skin-layer physics that has been previously hypothesized in the literature.

In Chapter 3, we identify a need for a parameterization that is accurate in the upper meters and contains an explicitly spectral dependence on the concentration of biogenic material, while maintaining the computational simplicity of the parameterizations currently in use. To address this, we assemble simple, observationally-validated physical modeling tools for the key controls on ocean radiant heating, and simplify them into a parameterization that fulfills this need. We then use observations from 64 spectroradiometer depth casts across 6 cruises in diverse water bodies, 13 surface hyperspectral radiometer deployments, and 2 UAV flights to probe the accuracy and uncertainty associated with the new parameterization. We conclude with a novel case study using the parameterization to demonstrate the impact of chlorophyll concentration on the structure of diurnal warm layers.

In Chapter 4, we present co-located measurements of vertical temperature and turbulence structures in large DWLs made from a lagrangian float featuring a robotic lead screw T/S profiler and pulse-to-pulse coherent ADCP, yielding particularly revealing observations of the DWL response to variability in wind and solar forcing at sub-hourly timescales. Comparison of these observations with several upper ocean models reveals the importance of the solar heating parameterization developed in Chapter 3, and suggests a modification to the critical bulk Richardson number currently employed in the K-Profile Parameterization. Comparison to a simple scaling for DWL evolution highlights both the scaling’s potential and its limitations, and a new extension to the scaling is developed to remedy its inaccuracy in cases of wind decrease.