- The focus of this dissertation is the analysis of submesoscale and finescale features measured at oceanic fronts and the role they play in the transport of heat and salt. Two different geographical areas were used to analyze this transport: one, an area on the western side of the North Atlantic subtropical gyre where the Gulf Stream (GS) creates a constant front with the cold waters from the north; and two, an area on the western side of the North Pacific subtropical gyre, where the warm and salty Kuroshio current occasionally intrudes into the fresh and colder South China Sea through Luzon Strait. These are areas of interest due to the constant presence of mesoscale instabilities, meanders, and intrusions, which produce smaller-scale features that have shown to be important for the transport of momentum, heat, and salt. These submesoscale features have been observed using satellite data and were analyzed in model outputs throughout the last 20 years. However, they were barely surveyed in-situ at finer resolutions. Advancements in technology have created faster profiling instruments and autonomous underwater vehicles, which make in-situ measurements possible. Using these types of observations, we investigated the lateral transport of heat and salt in the two ocean fronts.
In Chapter 2, using high-resolution measurements of the Gulf Stream North Wall,
temperature, salinity, and velocity properties, we analyzed the horizontal and vertical structure of streamers observed in sea surface temperature based on satellite data during winter 2012. The goal of this study was to determine whether streamers at the edge of the front affect the overall heat and salt content of the mixed layer. We estimated along-isopycnal diffusivities of 130-220 m2s−1 and 90-170 m2s−1 that would account for the observed temporal changes in temperature and salinity, respectively. These values are consistent with other studies that assert that lateral mixing is required for the production of 18◦ subtropical mode water.
In chapter 3, we investigate the variability at the boundary the Kuroshio Branch Current (KBC) which seasonally carries warm and salty Pacific-origin water into the South China Sea (SCS), forming a strong front with a complex temperature-salinity structure to the west of the Luzon Strait. Small scale interleaving features (< 10km) observed during the 2014 winter monsoon season were characterized and used to assess the local strength of interleaving using the finescale parameters of Turner angle, Tu, and Richardson number, Ri, and compare these values to the observed thermal variance dissipation measurements. These calculations showed that both mechanisms were associated with the small-scale features and that both contributed to the observed high values of thermal dissipation. Both mechanisms contribute to the mixing between the two distinct water masses, although the have different vertical flux associated with the interleavings. Regions unstable to both had a KT = 5.7 × 10−4m2s−1 and regions only stable to shear 1.7 × 10−5m2s−1 which represent a difference of ∼ 40 W/m2 vertical heat flux.
This work contributes to the understanding of the role that small-scale features have in the net transport of heat and salt across oceanic fronts associated with western boundary currents.