The responses of ocean surface boundary layers to abruptly turning winds, misaligned wind and waves, and diurnal heating
Date
2023
Authors
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Publisher
University of Delaware
Abstract
Turbulence within the ocean surface boundary layer (OSBL) controls the transport of heat, momentum, and dissolved and suspended matter between the atmosphere and the ocean, significantly influencing weather and climate dynamics. Langmuir turbulence (LT), driven by non-breaking surface gravity waves through the Craik-Leibovich mechanism, has recently been recognized as an important contribution to the OSBL turbulence for local and global oceans. Until now, most of our conceptual framework for understanding OSBL turbulence assumes constant and aligned wind and waves. Heat fluxes from the atmosphere are often neglected or set constant. Observations, however, indicate that the typical ocean environments are usually more complex and characterized by transient winds, waves, and heat fluxes. Additionally, wind and waves are often misaligned due to swell waves and rapidly changing winds. Investigating the influences of these complicated but common ocean conditions on the OSBL is the aim of this dissertation. This dissertation utilizes observations and numerical simulations, particularly a turbulence-resolving large eddy simulation (LES) model, to understand the OSBL dynamics for a range of commonly observed conditions focusing on the role of LT during those processes. ☐ The work of this dissertation includes four parts. Motivated by observation, the first part investigates the OSBL response to abruptly turning winds based on LES experiments. Our results indicate that OSBL turbulence significantly weakens after the wind turns, although the wind stress remains constant. This work suggests that transient wind plays a key role in generating non-equilibrium OSBL turbulence. After the wind turns, the LES results indicate that LT weakens because wind turning results in wind-wave misalignment. The second part utilizes coastal observation to further demonstrate the influences of the wind-wave misalignments on LT. We find that the short coastal wind fetches limit the development of the offshore wind wave components, resulting in persistent wind-wave misalignments. By analyzing LT measurements and LES results, those misalignments are confirmed to lead to weaker LT. This work investigates the connection between wind fetch effects and coastal LT activities, improving our understanding of coastal dynamics. The last two parts focus on the characteristics and dynamics of the diurnal warm layer (DWL) caused by transient diurnal heating. The third part develops a scaling of the DWL depths based on LES results, encompassing the effects of wind, wave, and surface heating. This scaling successfully predicts the DWL depths around the diurnal heating peak. By comparing the scaling and DWL observations, this work provides observational evidence that DWL depths depend on LT strength. The fourth part employs a detailed LES analysis of the OSBL turbulent response for a diurnal cycle. The LES results indicate that the transient turbulence is important during the DWL evolution and can generally not be scaled by constant surface forcing. Once the DWL forms, turbulence production and dissipation enhance for shear-driven turbulence as momentum is trapped within DWL. Conversely, LT results in a relatively mixed DWL because Stokes drift shear production is less sensitive to the evolution of DWL. This part also compares simple one-dimensional column models in which turbulence is parameterized with LES results and indicates that the LT effects can be implicitly tuned in those models. Overall, this dissertation suggests that investigating the OSBL responses to transient forcing conditions and misaligned wind and waves is important for better understanding OSBL dynamics, helping improve our understanding and prediction of ocean-atmosphere models.
Description
Keywords
Air-sea interaction, Langmuir turbulence, Large eddy simulation, Ocean surface boundary layer, Turbulence mixing, Diurnal heating, Winds