Rypina
Irina I.
Rypina
Irina I.
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ArticleExploring interannual variability in potential spawning habitat for Atlantic bluefin tuna in the Slope Sea(Elsevier, 20210117) Rypina, Irina I. ; Dotzel, Michael M. ; Pratt, Lawrence J. ; Hernandez, Christina M. ; Llopiz, Joel K.The Slope Sea in the Northwest Atlantic Ocean, located between the Gulf Stream and the continental shelf of the Northeast United States, is a recentlydocumented possible major spawning ground for Atlantic bluefin tuna (Thunnus thynnus). Larval surveys and a habitat modeling study have shown that suitable spawning habitat occurs in the Slope Sea, but the degree to which this habitat varies interannually is an open question. Here, we perform a decadelong (2009–2018) numerical modeling analysis, with simulated larvae released uniformly throughout the Slope Sea, to investigate the interannual variability in the water temperature and circulation criteria deemed necessary for successful spawning. We also quantify the influence of Gulf Stream meanders and overshoot events on larval retention and their effect on habitat suitability rates throughout the Slope Sea, defined as the percentage of simulated larvae released at a given location that satisfy criteria related to water temperature and retention near nursery habitat. Average environmental oceanographic conditions over the decade are most favorable in the western part of the Slope Sea, specifically in the Slope Gyre and away from the immediate vicinity of the Gulf Stream. Variability in domain and summertimeaveraged yearly spawning habitat suitability rates is up to 25% of the mean decadalaveraged values. Yearly habitat suitability correlates strongly with the Gulf Stream overshoot but does not correlate well with other oceanographic variables or indices, so an overshoot index can be used as a sole oceanographic proxy for predicting yearly bluefin spawning habitat suitability in the Slope Sea. Selective spawning can weaken the correlation between habitat suitability and Gulf Stream overshoot. Effort should be put towards collecting observational data against which we could validate our findings.

ArticleChaotic advection in a steady, threedimensional, Ekmandriven eddy(Cambridge University Press, 20131205) Pratt, Lawrence J. ; Rypina, Irina I. ; Ozgokmen, Tamay M. ; Wang, P. ; Childs, H. ; Bebieva, Y.We investigate and quantify stirring due to chaotic advection within a steady, threedimensional, Ekmandriven, rotating cylinder flow. The flow field has vertical overturning and horizontal swirling motion, and is an idealization of motion observed in some ocean eddies. The flow is characterized by strong background rotation, and we explore variations in Ekman and Rossby numbers, E and Ro, over ranges appropriate for the ocean mesoscale and submesoscale. A highresolution spectral element model is used in conjunction with linear analytical theory, weakly nonlinear resonance analysis and a kinematic model in order to map out the barriers, manifolds, resonance layers and other objects that provide a template for chaotic stirring. As expected, chaos arises when a radially symmetric background state is perturbed by a symmetrybreaking disturbance. In the background state, each trajectory lives on a torus and some of the latter survive the perturbation and act as barriers to chaotic transport, a result consistent with an extension of the KAM theorem for threedimensional, volumepreserving flow. For shallow eddies, where E is O(1), the flow is dominated by thin resonant layers sandwiched between KAMtype barriers, and the stirring rate is weak. On the other hand, eddies with moderately small E experience thicker resonant layers, widerspread chaos and much more rapid stirring. This trend reverses for sufficiently small E, corresponding to deep eddies, where the vertical rigidity imposed by strong rotation limits the stirring. The bulk stirring rate, estimated from a passive tracer release, confirms the nonmonotonic variation in stirring rate with E. This result is shown to be consistent with linear Ekman layer theory in conjunction with a resonant width calculation and the Taylor–Proudman theorem. The theory is able to roughly predict the value of E at which stirring is maximum. For large disturbances, the stirring rate becomes monotonic over the range of Ekman numbers explored. We also explore variation in the eddy aspect ratio.

ArticleThe influence of an eddy in the success rates and distributions of passively advected or actively swimming biological organisms crossing the continental slope(American Meteorological Society, 20200629) Rypina, Irina I. ; Pratt, Lawrence J. ; Entner, Samuel ; Anderson, Amanda ; Cherian, Deepak A.The Lagrangian characteristics of the surface flow field arising when an idealized, anticyclonic, mesoscale, isolated deepocean eddy collides with continental slope and shelf topography are explored. In addition to fluid parcel trajectories, we consider the trajectories of biological organisms that are able to navigate and swim, and for which shallow water is a destination. Of particular interest is the movement of organisms initially located in the offshore eddy, the manner in which the eddy influences the ability of the organisms to reach the shelf break, and the spatial and temporal distributions of organisms that do so. For nonswimmers or very slow swimmers, the organisms arrive at the shelf break in distinct pulses, with different pulses occurring at different locations along the shelf break. This phenomenon is closely related to the episodic formation of trailing vortices that are formed after the eddy collides with the continental slope, turns, and travels parallel to the coast. Analysis based on finitetime Lyapunov exponents reveals initial locations of all successful trajectories reaching the shoreline, and provides maps of the transport pathways showing that much of the crossshelfbreak transport occurs in the lee of the eddy as it moves parallel to the shore. The same analysis shows that the onshore transport is interrupted after a trailing vortex detaches. As the swimming speeds are increased, the organisms are influenced less by the eddy and tend to show up en mass and in a single pulse.

ArticleObserving and quantifying ocean flow properties using drifters with drogues at different depths(American Meteorological Society, 20210716) Rypina, Irina I. ; Getscher, Timothy ; Pratt, Lawrence J. ; Mourre, BaptisteThis paper presents analyses of drifters with drogues at different depths—1, 10, 30, and 50 m—that were deployed in the Mediterranean Sea to investigate frontal subduction and upwelling. Drifter trajectories were used to estimate divergence, vorticity, vertical velocity, and finitesize Lyapunov exponents (FTLEs) and to investigate the balance of terms in the vorticity equation. The divergence and vorticity are O(f) and change sign along trajectories. Vertical velocity is O(1 mm s−1), increases with depth, indicates predominant upwelling with isolated downwelling events, and sometimes changes sign between 1 and 50 m. Vortex stretching is one of the significant terms, but not the only one, in the vorticity balance. Twodimensional FTLEs are 2 × 10−5 s−1 after 1 day, 2 times as large as in a 400mresolution numerical model. Threedimensional FTLEs are 50% larger than 2D FTLEs and are dominated by the vertical shear of horizontal velocity. Bootstrapping suggests uncertainty levels of ~10% of the timemean absolute values for divergence and vorticity. Analysis of simulated drifters in a model suggests that drifterbased estimates of divergence and vorticity are close to the Eulerian model estimates, except when drifters get aligned into long filaments. Drifterbased vertical velocity is close to the Eulerian model estimates at 1 m but differs at deeper depths. The errors in the vertical velocity are largely due to the lateral separation between drifters at different depths and are partially due to only measuring at four depths. Overall, this paper demonstrates how drifters, heretofore restricted to 2D nearsurface observations, can be used to learn about 3D flow properties throughout the upper layer of the water column.

ArticleInvestigating the connection between complexity of isolated trajectories and Lagrangian coherent structures(Copernicus Publications on behalf of the European Geosciences Union and the American Geophysical Union, 20111215) Rypina, Irina I. ; Scott, S. E. ; Pratt, Lawrence J. ; Brown, Michael G.It is argued that the complexity of fluid particle trajectories provides the basis for a new method, referred to as the Complexity Method (CM), for estimation of Lagrangian coherent structures in aperiodic flows that are measured over finite time intervals. The basic principles of the CM are explained and the CM is tested in a variety of examples, both idealized and realistic, and in different reference frames. Two measures of complexity are explored in detail: the correlation dimension of trajectory, and a new measure – the ergodicity defect. Both measures yield structures that strongly resemble Lagrangian coherent structures in all of the examples considered. Since the CM uses properties of individual trajectories, and not separation rates between closely spaced trajectories, it may have advantages for the analysis of ocean float and drifter data sets in which trajectories are typically widely and nonuniformly spaced.

ArticleNearsurface transport pathways in the north Atlantic Ocean : looking for throughput from the subtropical to the subpolar gyre(American Meteorological Society, 201105) Rypina, Irina I. ; Pratt, Lawrence J. ; Lozier, M. SusanMotivated by discrepancies between Eulerian transport estimates and the behavior of Lagrangian surface drifters, nearsurface transport pathways and processes in the North Atlantic are studied using a combination of data, altimetric surface heights, statistical analysis of trajectories, and dynamical systems techniques. Particular attention is paid to the issue of the subtropicaltosubpolar intergyre fluid exchange. The velocity field used in this study is composed of a steady drifterderived background flow, upon which a timedependent altimeterbased perturbation is superimposed. This analysis suggests that most of the fluid entering the subpolar gyre from the subtropical gyre within two years comes from a narrow region lying inshore of the Gulf Stream core, whereas fluid on the offshore side of the Gulf Stream is largely prevented from doing so by the Gulf Stream core, which acts as a strong transport barrier, in agreement with past studies. The transport barrier near the Gulf Stream core is robust and persistent from 1992 until 2008. The qualitative behavior is found to be largely independent of the Ekman drift.

ArticleEddyinduced particle dispersion in the nearsurface North Atlantic(American Meteorological Society, 201212) Rypina, Irina I. ; Kamenkovich, Igor V. ; Berloff, Pavel S. ; Pratt, Lawrence J.This study investigates the anisotropic properties of the eddyinduced material transport in the nearsurface North Atlantic from two independent datasets, one simulated from the sea surface height altimetry and one derived from realocean surface drifters, and systematically examines the interactions between the mean and eddyinduced material transport in the region. The Lagrangian particle dispersion, which is widely used to characterize the eddyinduced tracer fluxes, is quantified by constructing the “spreading ellipses.” The analysis consistently demonstrates that this dispersion is spatially inhomogeneous and strongly anisotropic. The spreading is larger and more anisotropic in the subtropical than in the subpolar gyre, and the largest ellipses occur in the Gulf Stream vicinity. Even at times longer than half a year, the spreading exhibits significant nondiffusive behavior in some parts of the domain. The eddies in this study are defined as deviations from the longterm timemean. The contributions from the climatological annual cycle, interannual, and subannual (shorter than one year) variability are investigated, and the latter is shown to have the strongest effect on the anisotropy of particle spreading. The influence of the mean advection on the eddyinduced particle spreading is investigated using the “eddyfollowingfulltrajectories” technique and is found to be significant. The role of the Ekman advection is, however, secondary. The pronounced anisotropy of particle dispersion is expected to have important implications for distributing oceanic tracers, and for parameterizing eddyinduced tracer transfer in noneddyresolving models.

ArticleConnection between encounter volume and diffusivity in geophysical flows(Copernicus Publications on behalf of the European Geosciences Union, 20180404) Rypina, Irina I. ; Llewellyn Smith, Stefan ; Pratt, Lawrence J.Trajectory encounter volume – the volume of fluid that passes close to a reference fluid parcel over some time interval – has been recently introduced as a measure of mixing potential of a flow. Diffusivity is the most commonly used characteristic of turbulent diffusion. We derive the analytical relationship between the encounter volume and diffusivity under the assumption of an isotropic random walk, i.e., diffusive motion, in one and two dimensions. We apply the derived formulas to produce maps of encounter volume and the corresponding diffusivity in the Gulf Stream region of the North Atlantic based on satellite altimetry, and discuss the mixing properties of Gulf Stream rings. Advantages offered by the derived formula for estimating diffusivity from oceanographic data are discussed, as well as applications to other disciplines.

ArticleCompetition between chaotic advection and diffusion: Stirring and mixing in a 3D eddy model.(European Geosciences Union, 20190405) Brett, Genevieve ; Pratt, Lawrence J. ; Rypina, Irina I. ; Wang, PengThe importance of chaotic advection relative to turbulent diffusion is investigated in an idealized model of a 3D swirling and overturning ocean eddy. Various measures of stirring and mixing are examined in order to determine when and where chaotic advection is relevant. Turbulent diffusion is alternatively represented by (1) an explicit, observationbased, scaledependent diffusivity, (2) stochastic noise, added to a deterministic velocity field, or (3) explicit and implicit diffusion in a spectral numerical model of the Navier–Stokes equations. Lagrangian chaos in our model occurs only within distinct regions of the eddy, including a large chaotic “sea” that fills much of the volume near the perimeter and central axis of the eddy and much smaller “resonant” bands. The size and distribution of these regions depend on factors such as the degree of axial asymmetry of the eddy and the Ekman number. The relative importance of chaotic advection and turbulent diffusion within the chaotic regions is quantified using three measures: the Lagrangian Batchelor scale, the rate of dispersal of closely spaced fluid parcels, and the Nakamura effective diffusivity. The role of chaotic advection in the stirring of a passive tracer is generally found to be most important within the larger chaotic seas, at intermediate times, with small diffusivities, and for eddies with strong asymmetry. In contrast, in thin chaotic regions, turbulent diffusion at oceanographically relevant rates is at least as important as chaotic advection. Future work should address anisotropic and spatially varying representations of turbulent diffusion for more realistic models.

ArticleChaotic advection in an archipelago(American Meteorological Society, 201009) Rypina, Irina I. ; Pratt, Lawrence J. ; Pullen, Julie ; Levin, Julia C. ; Gordon, Arnold L.Techniques from dynamical systems theory have been applied to study horizontal stirring of fluid in the Philippine Archipelago. The authors’ analysis is based on velocity fields produced by two highresolution (3 and 6 km) numerical models. Particular attention is paid to identifying robust surface flow patterns and associating them with dominant Lagrangian coherent structures (LCSs). A recurrent winddriven dipole in the lee of the coastline is considered in detail. The associated LCSs form a template for stirring, exchange, and biological transport in and around the dipole. Chaotic advection is argued to provide a relevant framework for interpreting mesoscale horizontal stirring processes in an archipelago as a whole. Implications for the formation of filaments, the production of tracer variance, and the scale at which stirring leads to mixing are discussed in connection with an observed temperature record.

ArticleTrajectory encounter volume as a diagnostic of mixing potential in fluid flows(Copernicus Publications on behalf of the European Geosciences Union&the American Geophysical Union, 20170503) Rypina, Irina I. ; Pratt, Lawrence J.Fluid parcels can exchange water properties when coming into contact with each other, leading to mixing. The trajectory encounter mass and a related simplified quantity, the encounter volume, are introduced as a measure of the mixing potential of a flow. The encounter volume quantifies the volume of fluid that passes close to a reference trajectory over a finite time interval. Regions characterized by a low encounter volume, such as the cores of coherent eddies, have a low mixing potential, whereas turbulent or chaotic regions characterized by a large encounter volume have a high mixing potential. The encounter volume diagnostic is used to characterize the mixing potential in three flows of increasing complexity: the Duffing oscillator, the Bickley jet and the altimetrybased velocity in the Gulf Stream extension region. An additional example is presented in which the encounter volume is combined with the u approach of Pratt et al. (2016) to characterize the mixing potential for a specific tracer distribution in the Bickley jet flow. Analytical relationships are derived that connect the encounter volume to the shear and strain rates for linear shear and linear strain flows, respectively. It is shown that in both flows the encounter volume is proportional to time.

ArticleThe Western Alboran Gyre: an analysis of its properties and its exchange with surrounding water(American Meteorological Society, 20201118) Brett, Genevieve ; Pratt, Lawrence J. ; Rypina, Irina I. ; SánchezGarrido, José C.One of the largest and most persistent features in the Alboran Sea is the Western Alboran Gyre (WAG), an anticyclonic recirculation bounded by the Atlantic Jet (AJ) to the north and the Moroccan coast to the south. Eulerian budgets from several months of a highresolution model run are used to examine the exchange of water across the Eulerian WAG’s boundary and the processes affecting the salinity, temperature, and vorticity of the WAG. The volume transport across the sides of the WAG is found to be related to vertical isopycnal movement at the base of the gyre. Advection is found to drive a decay in the salinity minimum and anticyclonic vorticity of the Eulerian WAG. Given the large contributions of advection, a Lagrangian analysis is performed, revealing geometric aspects of the exchange that are hidden in an Eulerian view. In particular, stable and unstable manifolds identify a stirring region around the outer reaches of the gyre where water is exchanged with the WAG on a time scale of weeks. Its complement is an inner core that expands with depth and exchanges water with its surroundings on much longer time scales. The 3D evolution of one parcel, or lobe, of water as it enters the WAG is also described, identifying a general Lagrangian subduction pathway.

PreprintAssimilating Lagrangian data for parameter estimation in a multipleinlet system( 201704) Slivinski, Laura ; Pratt, Lawrence J. ; Rypina, Irina I. ; Orescanin, Mara M. ; Raubenheimer, Britt ; MacMahan, Jamie ; Elgar, SteveNumerical models of ocean circulation often depend on parameters that must be tuned to match either results from laboratory experiments or field observations. This study demonstrates that an initial, suboptimal estimate of a parameter in a model of a small bay can be improved by assimilating observations of trajectories of passive drifters. The parameter of interest is the Manning's n coefficient of friction in a small inlet of the bay, which had been tuned to match velocity observations from 2011. In 2013, the geometry of the inlet had changed, and the friction parameter was no longer optimal. Results from synthetic experiments demonstrate that assimilation of drifter trajectories improves the estimate of n, both when the drifters are located in the same region as the parameter of interest and when the drifters are located in a different region of the bay. Real drifter trajectories from field experiments in 2013 also are assimilated, and results are compared with velocity observations. When the real drifters are located away from the region of interest, the results depend on the time interval (with respect to the full available trajectories) over which assimilation is performed. When the drifters are in the same region as the parameter of interest, the value of n estimated with assimilation yields improved estimates of velocity throughout the bay.

ArticleInfluence of ocean circulation changes on the interannual variability of American eel larval dispersal(John Wiley & Sons, 20160624) Rypina, Irina I. ; Pratt, Lawrence J. ; Lozier, M. SusanAmerican eel (Anguilla rostrata) complete their life cycle by migrating from the east coast of North America to Sargasso Sea, where they spawn planktonic eggs and dye. Larvae that develop from eggs need to return to North American coastal waters within the first year of life and are influenced by the oceanic currents during this journey. A coupled physical–biological model is used to investigate the extent to which interannual changes in the ocean circulation affect the success rates of larvae in reaching coastal nursery habitats. Our results suggest that natural oceanic variability can lead to changes in larval success rates by a factor of 2. Interannual variation in success rates are strongly affected by the Gulf Stream inertial overshoot events, with the largest success in years with an inertial overshoot and the smallest in years with a straighter and more southern configuration of the Gulf Stream downstream of Cape Hatteras. The mean Gulf Stream length and latitude between 75W and 70W longitude can be used as proxies for characterizing the overshoot events and can be converted into success rates using linear regression.