Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Chuanmin Hu, Ph.D.

Committee Member

Dennis J. McGillicuddy, Jr., Ph.D.

Committee Member

Yonggang Liu, Ph.D.

Committee Member

Vassiliki H. Kourafalou, Ph.D.

Committee Member

Brian B. Barnes, Ph.D.

Keywords

Gulf of Mexico, ocean eddies, ocean fronts, remote sensing, Straits of Florida

Abstract

Ocean eddies and fronts in the Gulf of Mexico (GoM) and the Straits of Florida (SoF) play important roles in ocean physics, biology, and ecology. These two ubiquitous physical features have been studied worldwide for decades, yet many questions remain unanswered or poorly studied for the GoM and SoF. This study aims to improve the understanding of ocean eddies and frontal zones in the GoM and SoF using satellite remote sensing, in situ observations and model outputs, as well as state-of-the-art algorithms, with the following objectives: 1) analyze the three-dimensional (3-D) statistical characteristics and thermohaline structure of mesoscale eddies in the GoM, and understand the 3-D bio-optical, physical, and chemical properties of a characteristic eddy in the GoM (i.e., Loop Current eddy or LCE); 2) quantify the spatial distribution, seasonal and interannual variability of cyclonic eddies (CEs) in the SoF, and understand the evolution and 3-D physical and biochemical properties of a characteristic cyclonic eddy (CE) in the SoF (i.e., Tortugas eddy, the largest CE in the SoF); 3) investigate the long-term dynamics, spatial patterns, seasonal and interannual variations of ocean thermal and color frontal zones in the GoM, as well as the mechanisms behind such variations; and 4) to explore the biological and ecological implications of ocean eddies and fronts in the GoM and SoF.

Many studies have been conducted on the ocean eddies in the GoM over the past decades, yet our understanding of their 3-D characteristics is still limited. For example, how are the mesoscale eddies distributed at depth? Are their spatial distributions different from those at the sea surface? What are their size distributions and lifespans? What is the composite thermohaline structure of these eddies? Chapter 2 aims to address these questions through combining model outputs from the global Hybrid Coordinate Ocean Model (HYCOM) with the angular momentum eddy detection and tracking algorithm (AMEDA). Specifically, the AMEDA was applied to the 3-D daily global HYCOM ocean current velocity data (0-800 m) between 1997 and 2010 to detect and track eddies, from which an eddy dataset including eddy location, polarity, size, intensity, boundary, occurrence frequency, and lifetime was generated. The spatial distributions and other statistical characteristics (e.g., rotational speed) of the detected eddies were elucidated via a series of statistical analyses of this dataset. For example, the results indicate that CEs have a high occurrence in the Loop Current (LC) region (especially the eastern branch of the LC), northern GoM (between 25 oN and 28 oN), and the Bay of Campeche between 1997 and 2010. In addition, CEs have a similar occurrence at the selected four representative layers (10, 100, 300, and 600 m). Regarding anticyclonic eddies (AEs), they have a high occurrence in the northwestern GoM, central GoM (between 23 oN and 25 oN), and northeastern GoM (i.e., an area north of the LC region). As with the detected CEs, the detected AEs also have similar occurrence at the selected four layers (10, 100, 300, and 600 m). More importantly, in this study, the composite thermohaline structure of the detected eddies was studied. Both CEs and AEs were found to play an important role in heat and salt transport both horizontally and vertically in the GoM. For example, the anomalies of surface water temperature and salinity show a distinct dipole structure for the AEs in the GoM, implying that the horizontal advection associated with these eddies has contributed to heat and salt transport in the surface layer of the GoM.

To better understand the 3-D properties of eddies in the GoM, I also combined in situ observations collected with multiple instruments (e.g., multi-sensor glider and CTD) and satellite measurements during August 2015 to study the 3-D bio-optical, physical, and chemical properties of a LCE (Chapter 3, Appendix A). This LCE was characterized with a surface radius of ~150 km and strong deepening of the isopycnals, and its influence extended to ~1400-1500 m. This eddy was found to have distinct bio-optical, physical, and chemical properties from the background water at both surface and depth. Specifically, strong contrasts were found in the bio-optical (beam-C attenuation, chlorophyll-a concentration, particulate backscattering, and absorption of particulate and dissolved matters), physical (density, temperature, and buoyancy frequency), and chemical properties (salinity and dissolved oxygen concentration) across the eddy core, eddy edge, and background waters as well as in their vertical distributions.

CEs in the adjacent SoF have been known to play an important role in the transport, retention, and successful recruitment of fish larvae. To study the spatial distribution, seasonal and interannual variability of both submesoscale and mesoscale CEs in the SoF, I generated daily Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite chlorophyll-a products at 500-m spatial resolution during 2002–2018. In addition, I developed algorithms to detect, quantify, and statistically analyze the CEs in the SoF, from which a complete data record of CE occurrence between 2002-2018 was created and investigated (Chapter 4, Appendix B). This study demonstrates that novel satellite ocean color imagery enables detection and quantification of submesoscale CEs, and the results indicate that submesoscale CEs in the SoF (radius <15 km) have little or no seasonality with high occurrence frequency limited to 30–200‐m isobaths, while mesoscale CEs have strong seasonal variability with occurrence frequency decreasing from the Lower Keys to the Upper Keys.

Tortugas eddies, which often remain quasi-stationary near the Dry Tortugas, are the largest eddies along the Florida Current (FC) in the SoF. In Chapter 5 (Appendix C), satellite altimetry and ocean color measurements, shipborne acoustic Doppler current profiler (ADCP) observations and Argo profiling float records, together with high-resolution outputs from the global HYCOM were combined to investigate a long-lasting Tortugas eddy. Specifically, its entire evolution and 3-D physical and biochemical properties were investigated. In addition to this, its interaction with the LC/FC system was also investigated from the perspective of energy transfer. The results show that this Tortugas eddy was characterized by lower temperatures and higher chlorophyll-a concentrations than surrounding waters, with a radius of ~50 km and a penetration depth of ~800 m. This eddy was found to form along the eastern edge of the LC and kept quasi-stationary off the Dry Tortugas for about three months until becoming much smaller because of its interaction with the topography and the FC. The analysis, based on global HYCOM outputs, clearly indicates that the baroclinic and barotropic instabilities of the mean flow (i.e., LC/FC system) greatly modulated the potential and kinetic energy of the eddy, which also contributed to the eddy’s growth and evolution. This type of long-lasting Tortugas eddy has been found to have biophysical connectivity implications. For example, it is responsible for the transport of fish larvae in the SoF.

In addition to ocean eddies, ocean fronts are also very important because they are often the locations of strong biological and physical activities that influence ocean ecosystem. In Chapter 6 (Appendix D), daily ocean color and SST imagery at 9-km resolution, collected by MODIS/Aqua during 2002–2019 were used to study the spatial and temporal patterns of ocean frontal zones in the GoM. A gradient-based front detection algorithm was applied to daily ocean color and SST imagery to detect frontal features, from which a comprehensive dataset of ocean frontal zones (including time series and climatology) was established. In this study, seasonal and interannual variations of ocean color and thermal frontal activities were revealed, and the underlying mechanisms behind such variations were investigated. Specifically, both ocean color and thermal frontal zones had prominent seasonality, while their seasonal variability differed. Major persistent frontal zones were mainly found in coastal waters within the 130-m isobath, and typically associated with wind-driven Ekman transport, upwelling/downwelling, river discharge, ocean currents, and topography.

The findings on ocean eddies and frontal zones have significant implications on ocean biology and ecology. For example, ocean frontal features identified in certain months were found to coincide with a well-known fishing ground on the West Florida Shelf (i.e., the Florida Middle Grounds), and slicks of pelagic Sargassum were found to align well with detected frontal features (Chapter 6, Appendix D). Furthermore, Chapter 7 focuses on the effects of GoM ocean eddies and surface currents on the transport and spatial distribution of pelagic Sargassum, and explores the origins and transport pathways of large amounts of pelagic Sargassum in the GoM. These were achieved through analyzing the satellite-derived Sargassum distributions in the context of ocean surface currents and eddies. Satellite observations of Sargassum areal density and altimetry-based ocean surface currents during 2000-2022 were used in this study. The results suggest that large amounts of Sargassum in the GoM can either originate from the northwestern GoM or be a result of physical transport from the northwestern Caribbean Sea. For the former, Sargassum of the northwestern GoM can be transported to the eastern GoM via ocean currents and eddies, eventually entering the Sargasso Sea. For the latter, Sargassum of the northwestern Caribbean Sea can be transported in three different directions within the GoM, with eastward and northward transport controlled by the LC system, and westward transport driven by the westward extension of the LC system, the propagation or relay of the LCEs, the wind-driven ocean currents on the Campeche Bank with or without eddies, and the westward currents associated with eddies together with westward coastal currents in the northern GoM. Overall, the spatial distribution patterns of pelagic Sargassum in the GoM has been significantly influenced by the LC system.

In summary, through integration of satellite remote sensing, in situ observations and numerical model outputs, and state-of-the-art algorithms, this dissertation has worked on six connected research topics related to ocean eddies (e.g., eddy spatial distributions, thermohaline structure, and 3-D properties), ocean frontal zones (e.g., spatial distributions, seasonal and interannual variability, and the mechanisms behind such variability), and their biological and ecological implications (e.g., the effects of ocean eddies on the transport and spatial distributions of pelagic Sargassum). The findings of this dissertation provide a better understanding of ocean eddies and frontal zones in the GoM and SoF, and the outcomes are expected to make a broad impact on future studies of the GoM ocean dynamics and bio-physical interactions.

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