Ocean-atmosphere feedbacks in two tropical atmospheric regimes: the inter-tropical convergence zone and the trade winds

Thesis (Ph.D.)--University of Washington, 2022Air-sea interactions are a critical component of the global climate system. Despite this, uncertainty remains in our understanding of air-sea interaction across spatial scales, particularly on small scales, which are not observable by remote sensing and often not explicitly represented in models due to computational limitations. This dissertation provides insight into the key processes that drive air-sea interaction in two tropical regions that are important for modulating synoptic to global-scale weather and climate: the inter-tropical convergence zone and the trade winds. The first chapter uses a series of model experiments based on observations from the rainy eastern tropical Pacific to evaluate the influence of preexisting ocean stratification and tropical rain modes on the upper ocean salinity response to rainfall, a process that modulates how freshwater is incorporated into the ocean structure. Differences in the timing of convective and stratiform components of rain events can modify the duration which surface salinity anomalies persist following rain for over two hours, while strong preexisting stratification can allow near-surface salinity anomalies produced by rain to persist for over seven hours longer compared to when rain falls on a well-mixed ocean. Similar differences in salinity structure can exist at deeper depths of up to 20 meters in the mixed layer. This work provides insight into the limitations of using low-resolution satellite rain observations in the context of physical oceanographic studies and examines a critical component of the global water cycle. The second and third chapters use observations from surface drifters and autonomous vehicles that measure the atmosphere and near-surface ocean in the tropical Atlantic trade wind region to evaluate the spatial variability of ocean surface waves and bulk air-sea fluxes. While previous research has mostly been limited to areas having particularly strong submesoscale activity, this work provides insight into the spatial variability present in a less energetic region representative of larger areas of the global ocean. Surface current variability in the trade winds influence wave slope and air-sea momentum flux due to changes in the relative wind speed and wave-current interactions. Wave-current interactions specifically modify momentum flux by as much as 10%. Across scales of tens of kilometers, air-sea sensible heat, latent heat, and upward buoyancy fluxes vary by 10, 50, and 10 watts per square meter. Sensible heat and upward buoyancy fluxes are significantly influenced by spatial sea surface temperature variability, while latent heat flux variability is primarily driven by changes in the atmosphere. The findings from this work could ultimately be used to guide the development of fully coupled atmosphere-wave-ocean models or quantify the limitations of using lower-resolution remote observations or models. Collectively, the following work serves to elucidate the physics of the dominant small-scale air-sea processes in two regions of the tropical ocean, quantify the influence of these processes on air-sea interaction and upper-ocean mixing, and suggest hypotheses on the implications of neglecting small-scale processes in regional or global studies of the coupled air-sea system