Mean currents, eddies, and their interactions in the northwestern Atlantic Ocean and its marginal seas
Date
2024
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Western boundary currents (WBCs) are pivotal in redistributing global material and energy. In the northwestern Tropical Atlantic Ocean, these currents are characterized by the North Brazil Current (NBC), the Caribbean Current, the Loop Current (LC) in the Gulf of Mexico, and the Florida Current. The Loop Current is the dominant large-scale oceanic process in the Gulf of Mexico. Numerical circulation modeling and observational studies have been conducted to understand the Loop Current system behaviors in the Gulf of Mexico. However, the mechanism for variations of the LC system is still unsolved. One of the factors that may influence the LC is upstream eddies from within the Caribbean Sea. ☐ In the second part of the dissertation, we show that some mesoscale eddies originated in the tropical Atlantic Ocean can pass through the Caribbean Sea and eventually connect the Gulf of Mexico. These remotely generated eddies could be an important upstream factor affecting the behavior of the LC. In addition to advancing the understanding of the LC system, this study provides an explicit example showing eddies can serve as a route connecting regional seas and the open ocean. ☐ In addition to the long-distance connection, we also explore the seasonal variability of the eddy kinetic energy (EKE) and governing mechanisms using a time-dependent energetics framework based on the multiscale window transform. By analyzing six ocean reanalysis products, we find that barotropic instability is the controlling mechanism for the seasonal eddy variability in the most energetic NBC retroflection region. In the eastern NBCR area, seasonal EKE changes mirror the NBCR region, but nonlocal processes dictate their seasonality. Our findings underscore the intricate mechanisms influencing eddy variability in the northwestern tropical Atlantic and offer insights into how these mechanisms may shift under evolving background conditions. ☐ Aside from examining seasonal variabilities, we're also delving into the interannual changes in mean currents and mesoscale eddies in the Caribbean Sea and the Gulf Stream. We reveal a robust and coherent response of WBCs and associated eddies to El Niño-Southern Oscillation (ENSO) on interannual timescales by analyzing altimetry and reanalysis products. El Niño results in enhanced sea surface height (SSH) differences across the WBCs mainly through its role on offshore SSH and consequently stronger mean currents from the North Brazil to the Gulf Stream, and the opposite happened during La Niña. For the mean currents in the Gulf Stream, besides local forcing, upstream propagation from the Caribbean region generated by ENSO-related wind anomalies also contributes to the offshore SSH variation and is not likely driven by the Sverdrup adjustment in the western North Atlantic. The eddy kinetic energy displays a similar response to ENSO due to eddy-mean flow interaction governed by baroclinic instability. In addition to the leading mode of North Atlantic Oscillation (NAO) in the North Atlantic, our findings underscore the key role of ENSO in understanding the interannual variabilities of WBCs, particularly the Gulf Stream. This study highlights the inter-basin connection from the equatorial Pacific Ocean in modulating WBCs in the North Atlantic. ☐ Finally, alongside studying variations of eddy-mean flow interaction among different time scales, we estimate the eddy-induced heat and salt transport in the three subregions: the North Brazil, the Caribbean Sea, and the Gulf of Mexico. To achieve this goal, this study constructs the vertical structure of temperature (𝜃 ′ ), salinity (𝑆 ′ ), geostrophic current, and their anomalies induced by eddies by integrating eddy tracks and Argo floats data. Different 𝜃 ′ structures and two-layer and three-layer structures of 𝑆 ′ in the specific region are discussed. The Gulf of Mexico exhibits the strongest temperature, salinity, geostrophic current anomalies, and hence the heat/salt transport. ☐ This dissertation investigates the eddy-mean flow interaction across different time scales, ranging from seasonal to interannual, within the northwestern Tropical Atlantic Ocean. It emphasizes the influence of climate drivers, particularly ENSO, on the behavior of both mean currents and eddies in the Caribbean Sea and downstream regions. Our results suggest some predictability of the mean currents and eddies in the Caribbean Sea and its downstream, particularly during strong El Niño and La Niña events. Future research focusing on the long-term evolution of the ENSO-WBC connection over multiple decades could provide valuable insights.
Description
Keywords
Eddies, Eddy-mean flow interaction, ENSO, Instabilities, Mesoscale