UC San Diego

Satellite altimetry has revolutionized our understanding of upper-ocean circulation dynamics and sea level change. However, there is limited understanding of sub-mesoscale to mesoscale ocean dynamics because they are too small to be detected globally with today's technology. Mesoscale ocean activities are associated with eddy kinetic energy generation and dissipation, and play an important role in the dynamics of ocean energy transfer and mixing \citep{ferrari2009ocean} and local and global budgets of heat and carbon \citep{morrow2019global}. The Surface Water and Ocean Topography (SWOT) satellite altimetry mission, launched in December 2022, will provide an opportunity to refine the measured sea surface height resolution down to 15 km, allowing for direct global observations of mesoscale oceanic activity. This new capability motivates investigations of submesoscale to mesoscale oceanic activities, and the separation between balanced geostrophic flows and unbalanced wave motions. This thesis uses the ICESat-2 photon height data and radar altimetry data collected over the past thirty years to study small-scale ocean surface variability, its influencing factors, and seasonality. The results in Chapter 2 show that an ICESat-2 single track can recover the marine geoid at wavelengths $>$ 20 km, which is similar to the best radar altimeter data. However, the wavelength and propagation direction of surface gravity waves are sometimes well resolved by using a combination of the strong and weak beams, which are separated by 90 m. In Chapter 3, we identify the most important factors that influence mesoscale ocean variability to be the distance to the nearest thermocline boundary, significant wave height, mean dynamic topography gradient, and M2 tidal speed. Nonetheless, some regions such as the Amazon outflow cannot be predicted by the model, suggesting that these regions are governed by local processes not represented in the input features. In Chapter 4 we found that the high-latitude Northern Hemisphere and the south Indian Ocean are associated with large annual cycles at mesoscales. The variability is higher in local wintertime, except for a few regions such as the Bay of Bengal, which shows high variability in the boreal spring and fall.