Flierl Glenn R.

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Flierl

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Glenn R.

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Role of residual overturning for the sensitivity of Southern Ocean isopycnal slopes to changes in wind forcing

(American Meteorological Society, 2019-10-30) Youngs, Madeleine K. ; Flierl, Glenn R. ; Ferrari, Raffaele

The Antarctic Circumpolar Current plays a central role in the ventilation of heat and carbon in the global ocean. In particular, the isopycnal slopes determine where each water mass outcrops and thus how the ocean interacts with the atmosphere. The region-integrated isopycnal slopes have been suggested to be eddy saturated, that is, stay relatively constant as the wind forcing changes, but whether or not the flow is saturated in realistic present day and future parameter regimes is unknown. This study analyzes an idealized two-layer quasigeostrophic channel model forced by a wind stress and a residual overturning generated by a mass flux across the interface between the two layers, with and without a blocking ridge. The sign and strength of the residual overturning set which way the isopycnal slopes change with the wind forcing, leading to an increase in slope with an increase in wind forcing for a positive overturning and a decrease in slope for a negative overturning, following the usual conventions; this behavior is caused by the dominant standing meander weakening as the wind stress weakens causing the isopycnal slopes to become more sensitive to changes in the wind stress and converge with the slopes of a flat-bottomed simulation. Eddy saturation only appears once the wind forcing passes a critical level. These results show that theories for saturation must have both topography and residual overturning in order to be complete and provide a framework for understanding how the isopycnal slopes in the Southern Ocean may change in response to future changes in wind forcing.
A new mechanism for the generation of quasi-zonal jets in the ocean

(American Geophysical Union, 2012-05-16) Wang, Jinbo ; Spall, Michael A. ; Flierl, Glenn R. ; Malanotte-Rizzoli, Paola

A simple barotropic quasi-geostrophic model is used to demonstrate that instabilities radiated from an unstable eastern boundary current can generate zonal striations in the ocean interior with realistic wavelengths and amplitudes. Nonlinear transfer of energy from the more unstable trapped modes is important for radiating modes to overcome friction. The dynamics shown here are generic enough to point to the eastern boundary current as a likely source of the observed striations extending from oceanic eastern boundaries.
Diversity of growth rates maximizes phytoplankton productivity in an eddying ocean

(American Geophysical Union, 2022-01-20) Freilich, Mara ; Flierl, Glenn R. ; Mahadevan, Amala

In the subtropical gyres, phytoplankton rely on eddies for transporting nutrients from depth to the euphotic zone. But, what controls the rate of nutrient supply for new production? We show that vertical nutrient flux both depends on the vertical motion within the eddying flow and varies nonlinearly with the phytoplankton growth rate. Flux is maximized when the growth rate matches the inverse of the decorrelation timescale for vertical motion. Using a three-dimensional ocean model and a linear nutrient uptake model, we find that phytoplankton productivity is maximized for a growth rate of 1/3 day−1, which corresponds to the timescale of submesoscale dynamics. Variability in the frequency of vertical motion across different physical features of the flow favors phytoplankton production with different growth rates. Such a growth-transport feedback can generate diversity in the phytoplankton community structure at submesoscales and higher net productivity in the presence of community diversity.
Basin-width dependence of northern deep convection

(American Geophysical Union, 2020-07-08) Youngs, Madeleine K. ; Ferrari, Raffaele ; Flierl, Glenn R.

Convection penetrates to the ocean bottom in the North Atlantic but not in the North Pacific. This study examines the role of basin width in shutting down high‐latitude ocean convection. Deep convection is triggered by polar cooling, but it is opposed by precipitation. A two‐layer analytical model illustrates that the overturning circulation acts to mitigate the effect of precipitation by advecting salty, dense water from subtropical latitudes to polar latitudes. The nonlinear dependence of the overturning strength on basin width makes it more efficient in a narrow basin, resulting in a convection shutdown at a stronger freshwater forcing. These predictions are confirmed by simulations with a general circulation model configured with a single closed basin to the north and a reentrant channel to the south. This suggests that basin width may play a role in suppressing convection in the North Pacific but not in the North Atlantic.
The generation of Rossby waves and wake eddies by small islands

(Sears Foundation for Marine Research, 2018-03) Musgrave, Ruth C. ; Flierl, Glenn R. ; Peacock, Thomas

The influence of small islands on zonal geostrophic currents is examined in a two-layer configuration. An analytic solution for steady quasigeostrophic flow is derived under the assumption of no upstream influence and is validated numerically in a time-dependent quasigeostrophic model. Under these conditions solutions are the sum of two eigenmodes, which are either arrested Rossby waves or evanescent depending on background flow conditions (layer speeds, stratification, and latitude). In contrast to homogeneous flows, arrested Rossby waves in two layers can occur even when the depth mean flow is westward and can be generated both to the east and west of the island. A third blocking mode may play a role in general, altering the meridional structure of the zonal flow upstream and downstream of the island. The influence of the quasigeostrophic modes on submesoscale island wake eddies is considered in a two-layer primitive equation model with no-slip boundary conditions at the island. Wake eddy formation is inhibited in the presence of an arrested Rossby wave, though the overall drag is similar.
Interdisciplinary study of warm core ring physics, chemistry, and biology

(Woods Hole Oceanographic Institution, 1981-03) Kester, Dana R. ; Flierl, Glenn R. ; McCarthy, James J. ; Schink, David R. ; Wiebe, Peter H. ; Joyce, Terrence M. ; Backus, Richard H. ; Cowles, Timothy J.

We are conducting an interdisciplinary study of the structure and dynamics of Gulf Stream \Warm Core Rings by a time series investigation of selected rings. This program consists of highly integrated components which include physical, chemical, and biological investigation and modeling studies. These components are designed to provide information on the structure of rings and exchange mechanisms at ring boundaries, on their marine chemistry, and on the environmental controls of biological activity of selected constituents associated with Warm Core Rings. This research is being conducted by approximately two dozen investigators from thirteen marine institutions. An interdisciplinary program of the scope proposed is required in order to understand the interdependence among biological, chemical, and physical processes in the ocean. This study of the structure and evolution of Warm Core Rings will enhance the understanding of fundamental oceanic processes and the role of rings in the region where they occur.
Cross-shelf and out-of-bay transport driven by an open-ocean current

(American Meteorological Society, 2011-11-01) Zhang, Yu ; Pedlosky, Joseph ; Flierl, Glenn R.

This paper studies the interaction of an Antarctic Circumpolar Current (ACC)–like wind-driven channel flow with a continental slope and a flat-bottomed bay-shaped shelf near the channel’s southern boundary. Interaction between the model ACC and the topography in the second layer induces local changes of the potential vorticity (PV) flux, which further causes the formation of a first-layer PV front near the base of the topography. Located between the ACC and the first-layer slope, the newly formed PV front is constantly perturbed by the ACC and in turn forces the first-layer slope with its own variability in an intermittent but persistent way. The volume transport of the slope water across the first-layer slope edge is mostly directly driven by eddies and meanders of the new front, and its magnitude is similar to the maximum Ekman transport in the channel. Near the bay’s opening, the effect of the topographic waves, excited by offshore variability, dominates the cross-isobath exchange and induces a mean clockwise shelf circulation. The waves’ propagation is only toward the west and tends to be blocked by the bay’s western boundary in the narrow-shelf region. The ensuing wave–coast interaction amplifies the wave amplitude and the cross-shelf transport. Because the interaction only occurs near the western boundary, the shelf water in the west of the bay is more readily carried offshore than that in the east and the mean shelf circulation is also intensified along the bay’s western boundary.
The nonlinear dynamics of time-dependent subcritical baroclinic currents

(American Meteorological Society, 2007-04) Flierl, Glenn R. ; Pedlosky, Joseph

The nonlinear dynamics of baroclinically unstable waves in a time-dependent zonal shear flow is considered in the framework of the two-layer Phillips model on the beta plane. In most cases considered in this study the amplitude of the shear is well below the critical value of the steady shear version of the model. Nevertheless, the time-dependent problem in which the shear oscillates periodically is unstable, and the unstable waves grow to substantial amplitudes, in some cases with strongly nonlinear and turbulent characteristics. For very small values of the shear amplitude in the presence of dissipation an analytical, asymptotic theory predicts a self-sustained wave whose amplitude undergoes a nonlinear oscillation whose period is amplitude dependent. There is a sensitive amplitude dependence of the wave on the frequency of the oscillating shear when the shear amplitude is small. This behavior is also found in a truncated model of the dynamics, and that model is used to examine larger shear amplitudes. When there is a mean value of the shear in addition to the oscillating component, but such that the total shear is still subcritical, the resulting nonlinear states exhibit a rectified horizontal buoyancy flux with a nonzero time average as a result of the instability of the oscillating shear. For higher, still subcritical, values of the shear, a symmetry breaking is detected in which a second cross-stream mode is generated through an instability of the unstable wave although this second mode would by itself be stable on the basic time-dependent current. For shear values that are substantially subcritical but of order of the critical shear, calculations with a full quasigeostrophic numerical model reveal a turbulent flow generated by the instability. If the beta effect is disregarded, the inviscid, linear problem is formally stable. However, calculations show that a small degree of nonlinearity is enough to destabilize the flow, leading to large amplitude vacillations and turbulence. When the most unstable wave is not the longest wave in the system, a cascade up scale to longer waves is observed. Indeed, this classically subcritical flow shows most of the qualitative character of a strongly supercritical flow. This result supports previous suggestions of the important role of background time dependence in maintaining the atmospheric and oceanic synoptic eddy field.
Reconstructing the ocean's interior from surface data

(American Meteorological Society, 2013-08) Wang, Jinbo ; Flierl, Glenn R. ; LaCasce, Joseph H. ; McClean, Julie L. ; Mahadevan, Amala

A new method is proposed for extrapolating subsurface velocity and density fields from sea surface density and sea surface height (SSH). In this, the surface density is linked to the subsurface fields via the surface quasigeostrophic (SQG) formalism, as proposed in several recent papers. The subsurface field is augmented by the addition of the barotropic and first baroclinic modes, whose amplitudes are determined by matching to the sea surface height (pressure), after subtracting the SQG contribution. An additional constraint is that the bottom pressure anomaly vanishes. The method is tested for three regions in the North Atlantic using data from a high-resolution numerical simulation. The decomposition yields strikingly realistic subsurface fields. It is particularly successful in energetic regions like the Gulf Stream extension and at high latitudes where the mixed layer is deep, but it also works in less energetic eastern subtropics. The demonstration highlights the possibility of reconstructing three-dimensional oceanic flows using a combination of satellite fields, for example, sea surface temperature (SST) and SSH, and sparse (or climatological) estimates of the regional depth-resolved density. The method could be further elaborated to integrate additional subsurface information, such as mooring measurements.
Spatial variability of movement, structure, and formation of Warm Core Rings in the Northwest Atlantic Slope Sea

(American Geophysical Union, 2022-08-16) Silver, Adrienne M. ; Gangopadhyay, Avijit ; Gawarkiewicz, Glen G. ; Andres, Magdalena ; Flierl, Glenn R. ; Clark, Jenifer

Gulf Stream Warm Core Rings (WCRs) have important influences on the New England Shelf and marine ecosystems. A 10-year (2011–2020) WCR dataset that tracks weekly WCR locations and surface areas is used here to identify the rings' path and characterize their movement between 55 and 75°W. The WCR dataset reveals a very narrow band between 66 and 71°W along which rings travel almost due west along ∼39°N across isobaths – the “Ring Corridor.” Then, west of the corridor, the mean path turns southwestward, paralleling the shelfbreak. The average ring translation speed along the mean path is 5.9 cm s−1. Long-lived rings (lifespan >150 days) tend to occupy the region west of the New England Seamount Chain (NESC) whereas short-lived rings (lifespan <150 days) tend to be more broadly distributed. WCR vertical structures, analyzed using available Argo float profiles indicate that rings that are formed to the west of the NESC have shallower thermoclines than those formed to the east. This tendency may be due to different WCR formation processes that are observed to occur along different sections of the Gulf Stream. WCRs formed to the east of the NESC tend to form from a pinch-off mechanism incorporating cores of Sargasso Sea water and a perimeter of Gulf Stream water. WCRs that form to the west of the NESC, form from a process called an aneurysm. WCRs formed through aneurysms comprise water mostly from the northern half of the Gulf Stream and are smaller than the classic pinch-off rings.
The baroclinic adjustment of time-dependent shear flows

(American Meteorological Society, 2010-08) Poulin, Francis J. ; Flierl, Glenn R. ; Pedlosky, Joseph

Motivated by the fact that time-dependent currents are ubiquitous in the ocean, this work studies the two-layer Phillips model on the beta plane with baroclinic shear flows that are steady, periodic, or aperiodic in time to understand their nonlinear evolution better. When a linearly unstable basic state is slightly perturbed, the primary wave grows exponentially until nonlinear advection adjusts the growth. Even though for long time scales these nearly two-dimensional motions predominantly cascade energy to large scales, for relatively short times the wave–mean flow and wave–wave interactions cascade energy to smaller horizontal length scales. The authors demonstrate that the manner through which these mechanisms excite the harmonics depends significantly on the characteristics of the basic state. Time-dependent basic states can excite harmonics very rapidly in comparison to steady basic states. Moreover, in all the simulations of aperiodic baroclinic shear flows, the barotropic component of the primary wave continues to grow after the adjustment by the nonlinearities. Furthermore, the authors find that the correction to the zonal mean flow can be much larger when the basic state is aperiodic compared to the periodic or steady limits. Finally, even though time-dependent baroclinic shear on an f plane is linearly stable, the authors show that perturbations can grow algebraically in the linear regime because of the erratic variations in the aperiodic flow. Subsequently, baroclinicity adjusts the growing wave and creates a final state that is more energetic than the nonlinear adjustment of any of the unstable steady baroclinic shears that are considered.
Grazing behavior and winter phytoplankton accumulation

(European Geosciences Union, 2021-10-18) Freilich, Mara ; Mignot, Alexandre ; Flierl, Glenn R. ; Ferrari, Raffaele

Recent observations have shown that phytoplankton biomass increases in the North Atlantic during winter, even when the mixed layer is deepening and light is limited. Current theories suggest that this is due to a release from grazing pressure. Here we demonstrate that the often-used grazing models that are linear at low phytoplankton concentration do not allow for a wintertime increase in phytoplankton biomass. However, mathematical formulations of grazing as a function of phytoplankton concentration that are quadratic at low concentrations (or more generally decrease faster than linearly as phytoplankton concentration decreases) can reproduce the fall to spring transition in phytoplankton, including wintertime biomass accumulation. We illustrate this point with a minimal model for the annual cycle of North Atlantic phytoplankton designed to simulate phytoplankton concentration as observed by BioGeoChemical-Argo (BGC-Argo) floats in the North Atlantic. This analysis provides a mathematical framework for assessing hypotheses of phytoplankton bloom formation.
Rotating convection : 1995 Summer Study Program in Geophysical Fluid Dynamics

(Woods Hole Oceanographic Institution, 1996-07) Salmon, Rick ; Flierl, Glenn R. ; Campbell, Lee Anne

The 1995 program in Geophysical Fluid Dynamics addressed "Rotating Convection," with particular emphasis on high-Rayleigh-number convection and on convection in the ocean.
Bio-physical models of oceanic population dynamics : 1994 summer study program in geophysical fluid dynamics

(Woods Hole Oceanographic Institution, 1997-11) Flierl, Glenn R. ; Olson, Donald B.

Bio-Physical Models of Oceanic Population Dynamics was the central theme of the 1994 summer program in Geophysical Fluid Dynamics (GFD) at the Woods Hole Oceanographic Institution. This unusual topic brought together mathematical population biologists and geophysical fluid dynamicists and provided a new synthesis of ideas and methods for coupling these two broad and diverse fields. Lectures and seminars given by staff and visitors addressed the nature of physical controls on free-drafting or active swimming organisms in the oceans, the biological responses of marine populations to their physical environment and their own internal states, and the dynamics of coupled biophysical processes on marine populations. This volume includes write-ups of the principal lectures, abstracts of some seminars, and the reports of the fellows' research projects.
Parametric instability in oscillatory shear flows

(Cambridge University Press, 2003-04-28) Poulin, Francis J. ; Flierl, Glenn R. ; Pedlosky, Joseph

In this article we investigate time-periodic shear flows in the context of the two-dimensional vorticity equation, which may be applied to describe certain large-scale atmospheric and oceanic flows. The linear stability analyses of both discrete and continuous profiles demonstrate that parametric instability can arise even in this simple model: the oscillations can stabilize (destabilize) an otherwise unstable (stable) shear flow, as in Mathieu's equation (Stoker 1950). Nonlinear simulations of the continuous oscillatory basic state support the predictions from linear theory and, in addition, illustrate the evolution of the instability process and thereby show the structure of the vortices that emerge. The discovery of parametric instability in this model suggests that this mechanism can occur in geophysical shear flows and provides an additional means through which turbulent mixing can be generated in large-scale flows.
Dynamics of the outer planets : 1992 Summer Study Program in Geophysical Fluid Dynamics

(Woods Hole Oceanographic Institution, 1992) Flierl, Glenn R. ; Ingersoll, Andrew P. ; Yano, Jun-Ichi ; Ewing-DeRemer, Barbara

The topic this summer was "The Dynamics of the Outer Planets." Andrew Ingersoll gave an excellent review of the current understanding of the strcture of the atmospheres of Jupiter, Neptune, Saturn, and Uranus. He presented the flow structures inferred from the information gathered by the Voyager probes and other observations. The models of the circulations of the interior and of the weather layer - the jets and vortices that we see in the images - were discussed. Jun-Ichi Yano gave further discussions on vortex dynamics in the lab, analytical, and numerical models as applied to the outer planets. Finally, Andy returned with a discussion of thin atmospheres (some so thin that they disappear at night) and new approaches to the dynamics of the interiors. These lectures provided a thorough background in both the data and the theory. As usual, we had talks (or what are sometimes called interactive seminars!) from many visitors during the summer, some directly related to the main topic and others covering other new research in geophysical fluid dynamics. From these, the fellows and staff found new aras for collaborative research and new ideas which they may explore after the summer. Finally, the summer was completed with talks from the fellows on their individual research during the summer. These reports reflect the thought and energy that went into learning new topics and formulating new problems. We look forward to seeing fuller versions of these in journal articles. We gratefully acknowledge the support of the National Science Foundation and the Office of Naval Research. The assistance of Jake Peirson and Barbara Ewing-DeRemer, made the summer, once again, pleasant and easy for all.
The influence of convection on large-scale circulations : 1988 Summer Study Program in Geophysical Fluid Dynamics

(Woods Hole Oceanographic Institution, 1989-07) Flierl, Glenn R. ; Goulart, Pamela J.

The topic for the 30th Geophysical Fluid Dynamics Summer Program was "The Influence of Convection on Large- Scale Motions." The principal lecturer, Kerry Emanual, presented the essential elements of dry convection, the thermodynamics of moist air, and moist convection (both precipitating and not). Building upon this excellent summary of the fundamentals of atmospheric convection, he then discussed its role in the dynamics of squall lines, tropical cyclones, and the 40-day tropical wave. Moist convection has not been discussed in recent GFD seminars, and Kerry's lectures provided a superb introduction to the topic for both Fellows and Staff. Other meteorologists discussed recent theoretical and observational research on atmospheric convecting systems. Convectively-driven flows in the ocean were described by Peter Killworth; Ed Spiegel and Christian Elphick reviewed the theory of nonlinear convection. These views of convection in other systems lent valuable perspectives on the atmospheric problem.
Shelf circulation and cross-shelf transport out of a bay driven by eddies from an open-ocean current. Part I : interaction between a barotropic vortex and a steplike topography

(American Meteorological Society, 2011-05) Zhang, Yu ; Pedlosky, Joseph ; Flierl, Glenn R.

This paper examines interaction between a barotropic point vortex and a steplike topography with a bay-shaped shelf. The interaction is governed by two mechanisms: propagation of topographic Rossby waves and advection by the forcing vortex. Topographic waves are supported by the potential vorticity (PV) jump across the topography and propagate along the step only in one direction, having higher PV on the right. Near one side boundary of the bay, which is in the wave propagation direction and has a narrow shelf, waves are blocked by the boundary, inducing strong out-of-bay transport in the form of detached crests. The wave–boundary interaction as well as out-of-bay transport is strengthened as the minimum shelf width is decreased. The two control mechanisms are related differently in anticyclone- and cyclone-induced interactions. In anticyclone-induced interactions, the PV front deformations are moved in opposite directions by the point vortex and topographic waves; a topographic cyclone forms out of the balance between the two opposing mechanisms and is advected by the forcing vortex into the deep ocean. In cyclone-induced interactions, the PV front deformations are moved in the same direction by the two mechanisms; a topographic cyclone forms out of the wave–boundary interaction but is confined to the coast. Therefore, anticyclonic vortices are more capable of driving water off the topography. The anticyclone-induced transport is enhanced for smaller vortex–step distance or smaller topography when the vortex advection is relatively strong compared to the wave propagation mechanism.
2014 program to study : climate physics and dynamics

(Woods Hole Oceanographic Institution, 2015-09) Provenzale, Antonello ; Ferrari, Raffaele ; Flierl, Glenn R.

From the Preface: The 2014 Geophysical Fluid Dynamics Summer Study Program started on June 16th, with the topic of Climate Physics and Dynamics. The topic proved very timely and attracted an unprecedented number of applications from brilliant students. Professors Kerry Emanuel (MIT) and Geoff Vallis (Exeter) gave the principal lectures. They began with the simplest energy balance models and then included adjustment of the vertical profiles by convection (dry and moist). Kerry delved more deeply into convection and the processes found in "cloud-permitting" models, including island effects and the spontaneous formation of clusters surrounded by dry regions. Geoff discussed the larger-scale dynamics of the atmosphere and oceans, including the transports by eddies and the thermohaline circulation.
Mesoscale eddies and Trichodesmium spp. distributions in the southwestern North Atlantic

(John Wiley & Sons, 2015-06-08) Olson, Elise M. B. ; McGillicuddy, Dennis J. ; Flierl, Glenn R. ; Davis, Cabell S. ; Dyhrman, Sonya T. ; Waterbury, John B.

Correlations of Trichodesmium colony abundance with the eddy field emerged in two segments of Video Plankton Recorder observations made in the southwestern North Atlantic during fall 2010 and spring 2011. In fall 2010, local maxima in abundance were observed in cyclones. We hypothesized surface Ekman transport convergence as a mechanism for trapping buoyant colonies in cyclones. Idealized models supported the potential of this process to influence the distribution of buoyant colonies over time scales of several months. In spring 2011, the highest vertically integrated colony abundances were observed in anticyclones. These peaks in abundance correlated with anomalously fresh water, suggesting riverine input as a driver of the relationship. These contrasting results in cyclones and anticyclones highlight distinct mechanisms by which mesoscale eddies can influence the abundance and distribution of Trichodesmium populations of the southwestern North Atlantic.

Flierl Glenn R.

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