Session J33: Geophysical Fluid Dynamics: Oceanography II

Disentangling the Oceanic General Circulation

The coupling between scales ranging from $O(10^4)$ km down to $O(1)$ mm presents a major difficulty in understanding, modeling, and predicting oceanic circulation and mixing, where our constraints on the energy budget suffer from large uncertainties. To address this problem, we are working toward mapping out a Lorenz Energy Cycle concurrently in (i) scale, (ii) space, and (iii) time. We have developed a coarse-graining framework on the sphere that is more versatile and powerful than the classical `mean-eddy' decomposition. Coarse-graining has a rigorous mathematical foundation and is closely related to well-established physics techniques, including macroscopic electromagnetism, renormalization group, and large eddy simulation. Moreover, unlike the classical decomposition, coarse-graining is consistent with the parameterization requirements of coarse-resolution climate simulations. I will focus on three applications using satellite and model data: the first measurement of the global oceanic energy spectrum, the energy cascade, and the killing of eddies by wind.

    

Quasi-Objective Eddy Visualization from Sparse Drifter Data

Lagrangian eddies, generally referred to as elliptic Lagrangian coherent structures (LCS) in the dynamical systems literature, are material objects that trap and transport floating particles over large distances in the ocean in a coherent fashion. In order to expand our understanding of the transport of marine tracers, we need to accurately and reliably track the evolution of vortical flow structures. Drifter trajectories represent a valuable but sparse source of information for this purpose. We employ a recently developed single-trajectory Lagrangian diagnostic tool, the trajectory rotation average (TRA), to visualize oceanic vortices (or eddies) from sparse drifter data in a quasi-objective fashion. We apply the TRA to two drifter data sets that cover various oceanographic scales: the Grand Lagrangian Deployment (GLAD) and the Global Drifter Program (GDP). Based on the TRA, we develop a general algorithm that extracts approximate eddy boundaries. We find that the TRA outperforms other available single-trajectory-based eddy detection methodologies on sparse drifter data and identifies eddies on scales that are unresolved by satellite-altimetry.   

Buoyancy transport induced by baroclinic turbulence

Baroclinic instability of zonal ocean currents like the Antarctic Circumpolar Current induces a turbulent flow that redistributes buoyancy across the current. The small scales of the turbulent flow are hardly resolved in global climate models, which calls for a physically-based parameterization of the turbulent transport. Based on a hierarchy of idealized models, I will report on a recent scaling theory for the magnitude of the meridional buoyancy flux, before discussing the three-dimensional structure of eddy-induced buoyancy transport. The theory leads to a quantitative parameterization providing the meridional temperature profile in terms of the large-scale external forcing in an idealized model of a zonal ocean current.   

Seasonal Evolution of Environmental Indicators in Narragansett Bay

Estuarine environments are transition regions in which freshwater output from rivers mixes with ocean currents, thereby providing organic matter to the upper ocean. In New England, estuaries are a key part of the state economy. Nonetheless, research on the seasonal covariation of marine environmental proxies and their link to anthropogenic climate change remains to be studied. In this talk, we use multi-satellite data from NASA's Landsat program from 1984 to 2021 in Narragansett Bay and its neighboring Mt. Hope Bay. We evaluate the dominant interannual spatial and temporal patterns in sea surface temperature (SST), chlorophyll-a (Chl), and sea surface salinity (SSS) via an Extended Empirical Orthogonal Function (EEOF) decomposition. Leveraging this framework, we analyze the seasonal and decadal variability and covariance of SST, Chl, and SSS within the estuary. Going forward, our goal is to provide a robust analysis of the evolution of environmental indicators to inform policy-making in Narragansett Bay.   

Estimating dilation rate fields from sparse drifter data using Gaussian Process Regression

Dilation rate, the time average of divergence over a particle trajectory, reveals convergent and divergent features that persist over a finite time interval. Direct measurements of dilation rate in the ocean are difficult due to the lack of accurate high-resolution velocity fields. Drifter swarms can be used to estimate dilation rates. However, these estimates are sparse and subject to errors as a result of the relative position of drifters within a swarm, limiting the ability to locate regions of strongest convergence and divergence. Adding more drifters to densely cover the space could address these issues, but this is rarely possible. Alternatively, we can synthetically generate drifter trajectories if a velocity field can be estimated. In our study, we use Gaussian Process Regression to obtain velocity fields from sparse drifter data to generate synthetic trajectories and subsequently, estimate dilation rates. A detailed error analysis is performed for various flow features on an analytic system before testing the method on a realistic data-assimilative model of the Western Mediterranean Sea. A parametric study of the effect of spatial and temporal resolution of drifter data on the dilation rate estimates is also performed.   

Magnetic signature of vertically migrating aggregations in the ocean

The transport of heat and solutes by vertically migrating aggregations of plankton has long been explored as a potentially important source of ocean mixing. However, direct evidence of enhanced mixing due to these migrations remains challenging to obtain and inconclusive. These shortcomings are due to the limitations of current measurement techniques, i.e., velocimetry techniques, which require a priori knowledge of the precise aggregation location and typically trigger animal avoidance behavior from introducing instrumentation into the migration. Here, we develop a new approach to overcome these longstanding limitations by leveraging advancements in modern magnetometry to detect the flow-induced magnetic fields that naturally arise from seawater as it moves through the Earth's geomagnetic field. We derive quantitative predictions showing that these flow-induced magnetic fields in the vicinity of migrating aggregations have a strength proportional to the integrated fluid transport due to the migration. Importantly these magnetic signatures are potentially detectable remotely at a significant distance far from the aggregation and region of moving fluid with emerging quantum-enhanced magnetometry techniques such as Nitrogen-Vacancy centers in diamond. These results provide a new, testable framework for quantifying the significance of fluid transport in the ocean due to swimming organisms.

    

A new framework to characterize Arctic Ocean eddies leveraging Lagrangian observations of sea ice

Ocean eddies spanning small-to-moderate length scales (1 to 100 km) are an essential element of the Arctic system, given their role in transporting heat and nutrients and changing freshwater budgets. Advances in satellite altimetry have allowed the detection of coherent structures within the upper bound of the energetic scale range (>50 km). However, detecting eddies under sea ice is not possible using conventional satellite altimetry. In this talk, we outline a new framework to identify upper-ocean eddies from Lagrangian statistics of sea ice plates or floes. First, we leverage our recently developed automatic ice floe tracker algorithm to retrieve Lagrangian ice floe measurements from satellite observations. Then, we implement our newly developed data-assimilation method to recover missed observations resulting from the susceptibility of optical imagery to atmospheric noise. Finally, we exploit the strong coupling between the motion of free-drifting ice floes and ocean eddies with surface expression to retrieve ocean eddy characteristics. We focus our study on the Beaufort Gyre, intending to evaluate the interannual variability of the eddy field in this region within the past two decades.   

Investigating the role of abyssal stratification in the propagation of vorticity throughout the water column: a Mediterranean example (Ionian Sea)

Analysis of in situ data from the Ionian Sea revealed the presence of variability with tidal periodicity in the deep layer, suggesting that the deep variability is actually related to the whole water column. The observations were made from 1999 to 2003, during the years of adjustment to the Eastern Mediterranean Transient, a major climate event that occurred at the end of the 80s. During the observation period, the Ionian deep layers were characterized by the presence of a stable water mass, the Ionian Abyssal Water, whose presence could be a key condition for catching such variability in the deep and for studying the role of the stratification on the propagation of the perturbation throughout the water column. The observed mean structure of the stratification suggests that a 4-layer scheme should be sufficient to have a realistic yet simple representation. To study how much and under which conditions a vorticity input can propagate, a quasi-geostrophic equation has been considered, with 4 coupled layers of arbitrary thickness and density, simulated with a custom-designed algorithm. This case study aims to give more insight into how energy stored by the deep layers can be released along the entire water column, contributing to the climate variability of the Mediterranean Sea.

    

Phytoplankton growth driven by stratified shear instabilities

Phytoplankton populations grow in the sunlit upper ocean where they often interact with strong gradients in buoyancy and velocity at the bottom of the well-mixed layer. We explore this interaction using direct numerical simulations of phytoplankton forced by two different stratified shear instabilities, the Kelvin-Helmholtz (KH) and Holmboe instabilities, with different mixing behaviors. The phytoplankton was parameterized as a reacting passive scalar using a simple linear model with a depth-dependent growth rate, mimicking the decay of sunlight with depth. We observe that KH instability causes the phytoplankton to become more evenly distributed, while Holmboe instability maintains a sharper gradient at the interface. Additionally, the non-uniform growth rate results in asymmetric phytoplankton distributions on either side of the interface. We further examine the effect of a Damköhler number, defined as the ratio of fluid flow and phytoplankton growth timescales, on the system. We discuss the importance of our results for developing a model of upper ocean phytoplankton growth that accounts for different stratified mixing behaviors.   

The Creation of 3D Zonal Flows, their Spacings, Rhines Scaling, Turbulence, and Inverse Cascades in Atmospheres and Oceans

The creation of large vortices and zonal flows by small-scale forcing on a β-plane has been examined numerically in 2D for over 30 years. Marcus (2000) showed in 2D that, at small length scales, the inverse cascade produces a k^-5/3 kinetic energy spectrum with vortices and, at larger scales, a k^-5 spectrum with east-west zonal flows. By mathematical tautology, zonal Fourier components (k) in the latter spectrum have RMS velocities proportional to βk^-2, meaning all components obey the Rhines relation. The largest size (zonal width) produced in our calculations was ~2πL_R (L_R is the Rossby deformation radius), which also obeys the Rhines relation (as k ≡ 2π/(2πL_R) is part of the k^-5 spectrum). Our 2D calculations were consistent with the interpretation that energy inverse cascades up to a length where potential energy dominates kinetic as, in the QG approximation, the ratio of the kinetic to potential energy is (kL_R)². Here, we use the 3D QG equation to ascertain if the 2D results for zonalization, Rhines relation, and inverse cascades are valid in 3D. Although the 3D QG equation does not formally have an L_R, we also determine whether the largest scale zonal flow (i.e., the terminus of the inverse cascade) occurs at the wave number where potential energy dominates kinetic.   

Momentum and pseudomomentum in a shallow water equation

A basic shallow water system with variable topography is analyzed from the point of view of a Lagrangian derivation of momentum, energy, and pseudomomentum balances. A two-dimensional action and associated momentum equation are derived. The latter is further manipulated to derive additional equations for energy and pseudomomentum. This revealed structure emphasizes broken symmetries in space and a reference configuration, and preserved symmetry in time.

    

Influence of wind and buoyancy on ocean circulation in the North Atlantic Ocean: insights from turbulence and convection-resolving simulations

In the Atlantic ocean, the Atlantic Meridional Overturning Circulation (AMOC) transports a huge volumes of water mass and regulates the global climate by transferring an estimated 1.25±0.25 petawatts of heat towards the North Pole. It is also a crucial mechanism in transporting nutrients and carbon dioxide. The Gulf Stream, a part of the North Atlantic gyre, broadly transports heat and large volumes of water northwards, impacting the climate and weather system of western Europe. However, it is still not clear the main driver of the global ocean circulation. Wind has long been regarded as the primary driver of the large scale ocean circulation. Surface buoyancy forcing has also been introduced as another parameter which is able to drive the ocean circulation. In the present work, we investigate the contribution of each wind and buoyancy in driving the meridional overturning and gyre circulations in the North Atlantic Ocean using direct numerical simulations of an idealised ocean concerning both wind and buoyancy forcing. We find out that the buoyancy forcing is the primary driver of the deep overturning circulation. It also contributes in driving the gyre circulation. Thus, weakening the surface buoyancy forcing can decrease the ocean circulation. This is the likely scenario which may happen due to the global warming and increasing the greenhouse effect. The global warming modifies the surface buoyancy forcing which can have significant impact on ocean circulation. It impacts the global climate system and stresses ecosystems.