Project Overview

Summary:

     Dr. Yonggang Liu at the University of South Florida’s College of Marine Science was awarded an RFP-VI grant at $709,456 to conduct the RFP-VI project titled, “Effects of Mesoscale Eddies on Three-Dimensional Oil Dispersion: Data Integration, Interpretation and Implications for Oil Spill Models”. The project consisted of 1 other institution (University of Delaware); 1 principal investigator (Liu); 1 Project Director (Dr. Xinfeng Liang); 2 co-PIs (Drs. Yun Li and Robert Weisberg); 3 PhD students (Minghai Huang, Li Pan, and Yang Zhang); and 2 research staff members (Fanglou Liao and Yingli Zhu).

     After the Deepwater Horizon Oil Spill, the scientific community immediately realized that a system for tracking the discharged petroleum and associated contaminants, both at the surface and at depth, was needed for effective mitigation efforts. In the past years, largely within the framework of GoMRI, many efforts have been devoted to this, and significant progress has been made in tracking the petroleum via numerical models and satellite remote sensing resources. However, accurate oil tracking remains challenging. Among the challenges, tracking the subsurface hydrocarbons and other materials is particularly difficult, we, therefore, need to understand better the physical processes that are essential for the dispersal of subsurface materials, and to implement these understanding into oil tracking models. Also, approaching the end of GoMRI, it seems to be the right time to evaluate the existing numerical models in presenting the dispersal of subsurface materials in the Gulf of Mexico (GoM).

     Among many oceanic physical processes that affect the dispersal of subsurface materials, mesoscale eddies likely play an essential yet overlooked role. Limited studies in the GoM and other regions of the global ocean show that mesoscale eddies can significantly influence the deep-ocean subinertial flows (horizontal dispersal), particularly near large topography. Also, these eddy-influenced deep-ocean currents, mainly subinertial, likely contribute to driving, dissipating and modulating internal waves, and consequently are expected to affect diapycnal mixing (vertical dispersal). Mesoscale eddies can thus affect both the horizontal and vertical dispersal of subsurface materials. Examining the likely impacts of mesoscale eddies on the subsurface low-frequency currents as well as vertical mixing can potentially improve our understanding of the three-dimensional dispersal of subsurface materials in the GoM, including the spilled oil and applied chemical dispersants after Deepwater Horizon Oil Spill.

     The major objective of this proposal is to analyze, understand and quantify the roles of mesoscale eddies in the three-dimensional dispersal of subsurface materials. This will be achieved through three specific goals: 1) Characterize and quantify the subsurface impacts of mesoscale eddies in the GoM by synthesizing and analyzing multiple streams of data sets (e.g., satellite altimetry, historical in-situ deep-ocean current measurements); 2) Evaluate the existing state-of-the-art GoM circulation/oil tracking models (a synthesis of existing products within previous and ongoing research effort during RFP I to V) by focusing on the relationships between surface mesoscale features and subsurface currents and vertical mixing (if available); 3) Produce a semi-empirical formula to present the subsurface impacts of mesoscale eddies; conduct sensitivity experiments showing the impacts of this new formula in modifying the trajectory predictions with the existing GoM models.

     The results gained from this study have numerous potential scientific and societal impact. Along with improving oil spill tracking capabilities in the GoM, the proposed study is also expected to yield useful information for harmful algae bloom prediction and tracking, fisheries ecology, dispersal of geochemical tracers, sediment transport, and so forth.

Research Highlights

     Dr. Liu’s research, which included 3 outreach products and activities, resulted in 6 peer-reviewed publications and 18 scientific conference presentations to date, and 7 datasets submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are available to the public.  Significant outcomes of their research (all related to GoMRI Research Theme 1) are highlighted below.

• By combining and analyzing the surface satellite measurements and deep-ocean current measurements, we found that the upper and deep layers of the GoM are coupled in a few different ways. First, coupling between the surface and bottom currents in the Gulf of Mexico is more frequent over the Loop Current region than other regions. Second, the surface-bottom coupling is likely associated with the surface mesoscale variability that develops along the Loop Current. Third, a relatively small percent of surface mesoscale eddies have direct impacts on the near-bottom currents at the same geographic location.

• By examining the statistical properties of the surface mesoscale eddies in the GoM, we found that mesoscale eddies in the GoM have similar distributions of lifetime, radius and propagation speed to the global eddies, but much larger amplitudes and maximum rotational speeds. Both cyclonic and anticyclonic eddies are concentrated in the deep water around the Loop Current and the western GoM (at about -95W). The largest mean amplitude and mean rotational speed of surface eddies appear in the Loop Current region. Moreover, eddies are more likely generated along the Florida escarpment and the Bay of Campeche, and terminated along the western boundary of GoM.

• By collecting and analyzing a large number of moored current observations, we examined the characteristics of near-inertial currents in the deep GoM. We found that the energy density at the near-inertial frequency generally decreases from the surface to about 1200 m and then remains a relatively small value below. In the Loop Current region and the northeastern GOM, however, high near-inertial energy appears in both the upper (above 800 m depth) and the bottom ocean (below 1000 m depth and within 200 m above the bottom). Although downward energy propagation from the upper layer to the deep ocean is common, upward propagation in the deep ocean is also observed, consistent with bottom generation of near-inertial oscillations.

• By analyzing various satellite measurements and numerical simulations, we explored the roles of mesoscale eddies in connecting the tropical Atlantic Ocean and the GoM. We identified a few clear cases in which mesoscale eddies originated around the North Brazil Current can propagate through Caribbean Sea and eventually squeeze into the GoM and consequently affect the Loop Currents. In addition, those eddies transport heat and freshwater, and affect the heat and freshwater contents of the GoM. 

• Satellite-derived sea surface temperature (SST) products are widely used in analyzing mesoscale eddies and other oceanographic features. By comparing with the moored observations on the West Florida Shelf, we found that most of the SST products did not show the rapid SST drop (about 4°C within 1 day) in response to the passage of Hurricane Irma. This finding has important implications to oceanographic and meteorological studies that relied on satellite data. The limitations of the popular satellite products call for additional coastal ocean observations as well as proper inclusion of the real-time observations in satellite-derived products. This finding has been published in Liu et al. (2018).

• By analyzing the 20 yr GoM HYCOM hindcast simulations using an advanced machine learning technique (Self-Organizing Map), we characterized the ocean circulation patterns in the western GoM, and examined how they were influenced by Loop Current Eddies (LCEs). The influence of LCEs on the upper slope of the western Gulf was found to be along the 200 m isobath, a zone under constant pressure by strong currents from impacting eddies. The frequency of LCE impact oscillating around four to five months with an estimated ring life-span of one year. These findings have been published in Meza-Padilla et al. (2019).

• We introduced a statistical framework for the simulation of ocean current patterns based on the autoregressive logistic regression models, and apply it to the GoM Loop Current. The statistical model was forced by three autoregressive terms, the wind stress curl in the GoM and in the Caribbean Sea, and the sea level pressure anomalies over the North Atlantic. The model reproduced the inter-annual and intra-annual variability of the original time series, showing notable fitting capacity. A point-by-point comparison between the actual and simulated pattern series confirmed the capability of the model in analyzing the evolution of ocean current patterns. The predictive skill of the model is up to 3 months. These results have been published in Chiri et al. (2019).

• Using novel satellite ocean color data products and modified algorithms, we investigated both mescoscale and submescoscale eddies in the Florida Straits. While mesoscale eddies show strong seasonality with occurrence frequency decreasing from Lower Keys to Upper Keys, submesoscale eddies show little or no seasonality with high occurrence frequency restricted to 30–200?m isobaths. The number of mesoscale eddies decreases exponentially in size, but submesoscale eddies show a normal distribution in size. These findings are significant in filling our knowledge gap in submesoscale eddies in this physically and ecologically important region as it encompasses world?renowned coral reefs, seagrasses, and fisheries.  These findings have been published in Zhang et al. (2019).

• We studied the coastal ocean response to Hurricane Irma using both in situ data and a hindcast simulation by the West Florida Coastal Ocean Model (WFCOM). During the Irma passage, a negative storm surge followed by a positive surge occurred along the west Florida coast. The resulting ocean circulation patterns and water exchange pathways are revealed by simulated Lagrangian trajectories, which are widely used in oil spill tracking.  Also investigated is the memory of the coastal system. Whereas sea level and currents restored back to their normal fluctuations within a day, water temperature and salinity required several days to stabilize, and by virtue of heavy rainfall, excess freshwater input from rivers resulted in lower estuarine and nearshore salinities that lasted for several weeks. These results have been published in Liu et al. (2020).

• Using the multiscale window transform (MWT) and the theory of canonical energy transfer, we investigated the role of multiscale interactions and instabilities in the GoM Loop Current eddy (LCE) shedding. We found out that the canonical energy transfer between the background flow and the mesoscale windows played an important role in LCE shedding. Barotropic instability contributed to the generation and intensification of the mesoscale eddies over the eastern continental slope of the Campeche Bank. Baroclinic instability favored the growth of the mesoscale eddies that propagated downstream to the northeastern portion of the well-extended Loop Current, eventually causing the LCE shedding. These upper mesoscale eddies lose their kinetic energy back to the background Loop Current through inverse cascade processes in the neck region. The deep eddies obtained energy primarily from the upper layer through vertical pressure work and secondarily from baroclinic instability in the deep layer. In contrast, the canonical energy transfer between the mesoscale and the high-frequency frontal eddy windows accounted for only a small fraction in the mesoscale eddy energy balance, and this generally acted as a damping mechanism for the mesoscale eddies. These results have been published in Yang et al. (2020).

Peer-Reviewed Publications Derived From This Project

• Liu, Y., Weisberg, R.H., Law, J., Huang, B. (2018), Evaluation of satellite-derived SST products in identifying the rapid temperature drop on the West Florida Shelf associated with hurricane Irma, MTS Journal, 52(3), 43-50, https://doi.org/10.4031/MTSJ.52.3.7.
• Meza-Padilla, R., Enriquez, C., Liu, Y., Appendini, C. (2019), Ocean circulation in the western Gulf of Mexico using self-organizing maps, J. Geophys. Res. Oceans, 124, 4152-4167, https://doi.org/10.1029/2018JC014377.
• Chiri, H., Abascal, A.J., Castanedo, S., Antonio, J., Liu, Y., Weisberg, R.H., Medina, R. (2019), Statistical simulation of ocean current patterns using autoregressive logistic regression models: A case study in the Gulf of Mexico, Ocean Modelling, 136, 1-12, https://doi.org/10.1016/j.ocemod.2019.02.010.
• Zhang, Y., Hu, C., Liu, Y., Weisberg, R.H., Kourafalou, V. (2019), Submesoscale and mesoscale eddies in the Florida Straits: Observations from satellite ocean color measurements, Geophys. Res. Lett., 46, 13262-13270, https://doi.org/10.1029/2019GL083999.
• Liu, Y., Weisberg, R.H., Zheng, L. (2020), Impacts of hurricane Irma on the circulation and transport in Florida Bay and the Charlotte Harbor estuary, Estuaries and Coasts, 43, 1194-1216, https://doi.org/10.1007/s12237-019-00647-6.
• Yang, Y., Weisberg, R.H., Liu, Y., Liang, X.S. (2020), Instabilities and multiscale interactions underlying the Loop Current eddy shedding in the Gulf of Mexico, J. Phys. Oceanogr., 50(5), 1289-1317, https://doi.org/10.1175/JPO-D-19-0202.1.

 Proposal Abstract - RFP-VI PI Yonggang Liu & Xinfeng Liang