Thomas Leif N.

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Role of shelfbreak upwelling in the formation of a massive under-ice bloom in the Chukchi Sea

( 2014-02) Spall, Michael A. ; Pickart, Robert S. ; Brugler, Eric T. ; Moore, G. W. K. ; Thomas, Leif N. ; Arrigo, Kevin R.

In the summer of 2011, an oceanographic survey carried out by the Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment (ICESCAPE) program revealed the presence of a massive phytoplankton bloom under the ice near the shelfbreak in the central Chukchi Sea. For most of the month preceding the measurements there were relatively strong easterly winds, providing upwelling favorable conditions along the shelfbreak. Analysis of similar hydrographic data from summer 2002, in which there were no persistent easterly winds, found no evidence of upwelling near the shelfbreak. A two-dimensional ocean circulation model is used to show that sufficiently strong winds can result not only in upwelling of high nutrient water from offshore onto the shelf, but it can also transport the water out of the bottom boundary layer into the surface Ekman layer at the shelf edge. The extent of upwelling is determined by the degree of overlap between the surface Ekman layer and the bottom boundary layer on the outer shelf. Once in the Ekman layer, this high nutrient water is further transported to the surface through mechanical mixing driven by the surface stress. Two model tracers, a nutrient tracer and a chlorophyll tracer, reveal distributions very similar to that observed in the data. These results suggest that the biomass maximum near the shelfbreak during the massive bloom in summer 2011 resulted from an enhanced supply of nutrients upwelled from the halocline seaward of the shelf. The decade long trend in summertime surface winds suggest that easterly winds in this region are increasing in strength and that such bloom events will become more common.
Rapid generation of upwelling at a shelf break caused by buoyancy shutdown

(American Meteorological Society, 2015-01) Benthuysen, Jessica A. ; Thomas, Leif N. ; Lentz, Steven J.

Model analyses of an alongshelf flow over a continental shelf and slope reveal upwelling near the shelf break. A stratified, initially uniform, alongshelf flow undergoes a rapid adjustment with notable differences onshore and offshore of the shelf break. Over the shelf, a bottom boundary layer and an offshore bottom Ekman transport develop within an inertial period. Over the slope, the bottom offshore transport is reduced from the shelf’s bottom transport by two processes. First, advection of buoyancy downslope induces vertical mixing, destratifying, and thickening the bottom boundary layer. The downward-tilting isopycnals reduce the geostrophic speed near the bottom. The reduced bottom stress weakens the offshore Ekman transport, a process known as buoyancy shutdown of the Ekman transport. Second, the thickening bottom boundary layer and weakening near-bottom speeds are balanced by an upslope ageostrophic transport. The convergence in the bottom transport induces adiabatic upwelling offshore of the shelf break. For a time period after the initial adjustment, scalings are identified for the upwelling speed and the length scale over which it occurs. Numerical experiments are used to test the scalings for a range of initial speeds and stratifications. Upwelling occurs within an inertial period, reaching values of up to 10 m day−1 within 2 to 7 km offshore of the shelf break. Upwelling drives an interior secondary circulation that accelerates the alongshelf flow over the slope, forming a shelfbreak jet. The model results are compared with upwelling estimates from other models and observations near the Middle Atlantic Bight shelf break.
Rapid mixing and exchange of deep-ocean waters in an abyssal boundary current.

(National Academy of Sciences, 2019-07-02) Naveira Garabato, Alberto C. ; Frajka-Williams, Eleanor E. ; Spingys, Carl P. ; Legg, Sonya ; Polzin, Kurt L. ; Forryan, Alexander ; Abrahamsen, E. Povl ; Buckingham, Christian E. ; Griffies, Stephen M. ; McPhail, Stephen D. ; Nicholls, Keith W. ; Thomas, Leif N. ; Meredith, Michael P.

The overturning circulation of the global ocean is critically shaped by deep-ocean mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topography and the rate at which well-mixed boundary waters are exchanged with the stratified ocean interior. Here, we use innovative observations of a major branch of the overturning circulation—an abyssal boundary current in the Southern Ocean—to identify a previously undocumented mixing mechanism, by which deep-ocean waters are efficiently laundered through intensified near-boundary turbulence and boundary–interior exchange. The linchpin of the mechanism is the generation of submesoscale dynamical instabilities by the flow of deep-ocean waters along a steep topographic boundary. As the conditions conducive to this mode of mixing are common to many abyssal boundary currents, our findings highlight an imperative for its representation in models of oceanic overturning.
The LatMix summer campaign : submesoscale stirring in the upper ocean

(American Meteorological Society, 2015-08) Shcherbina, Andrey Y. ; Sundermeyer, Miles A. ; Kunze, Eric ; D'Asaro, Eric A. ; Badin, Gualtiero ; Birch, Daniel ; Brunner-Suzuki, Anne-Marie E. G. ; Callies, Joern ; Cervantes, Brandy T. Kuebel ; Claret, Mariona ; Concannon, Brian ; Early, Jeffrey ; Ferrari, Raffaele ; Goodman, Louis ; Harcourt, Ramsey R. ; Klymak, Jody M. ; Lee, Craig M. ; Lelong, M.-Pascale ; Levine, Murray D. ; Lien, Ren-Chieh ; Mahadevan, Amala ; McWilliams, James C. ; Molemaker, M. Jeroen ; Mukherjee, Sonaljit ; Nash, Jonathan D. ; Ozgokmen, Tamay M. ; Pierce, Stephen D. ; Ramachandran, Sanjiv ; Samelson, Roger M. ; Sanford, Thomas B. ; Shearman, R. Kipp ; Skyllingstad, Eric D. ; Smith, K. Shafer ; Tandon, Amit ; Taylor, John R. ; Terray, Eugene A. ; Thomas, Leif N. ; Ledwell, James R.

Lateral stirring is a basic oceanographic phenomenon affecting the distribution of physical, chemical, and biological fields. Eddy stirring at scales on the order of 100 km (the mesoscale) is fairly well understood and explicitly represented in modern eddy-resolving numerical models of global ocean circulation. The same cannot be said for smaller-scale stirring processes. Here, the authors describe a major oceanographic field experiment aimed at observing and understanding the processes responsible for stirring at scales of 0.1–10 km. Stirring processes of varying intensity were studied in the Sargasso Sea eddy field approximately 250 km southeast of Cape Hatteras. Lateral variability of water-mass properties, the distribution of microscale turbulence, and the evolution of several patches of inert dye were studied with an array of shipboard, autonomous, and airborne instruments. Observations were made at two sites, characterized by weak and moderate background mesoscale straining, to contrast different regimes of lateral stirring. Analyses to date suggest that, in both cases, the lateral dispersion of natural and deliberately released tracers was O(1) m2 s–1 as found elsewhere, which is faster than might be expected from traditional shear dispersion by persistent mesoscale flow and linear internal waves. These findings point to the possible importance of kilometer-scale stirring by submesoscale eddies and nonlinear internal-wave processes or the need to modify the traditional shear-dispersion paradigm to include higher-order effects. A unique aspect of the Scalable Lateral Mixing and Coherent Turbulence (LatMix) field experiment is the combination of direct measurements of dye dispersion with the concurrent multiscale hydrographic and turbulence observations, enabling evaluation of the underlying mechanisms responsible for the observed dispersion at a new level.
Subduction on the northern and southern flanks of the Gulf Stream

(American Meteorological Society, 2010-02) Thomas, Leif N. ; Joyce, Terrence M.

Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream’s warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (64°–70°W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature–salinity relation. From the western-to-easternmost sections, where the intrusion was observed, the depth of the intrusion’s salinity minimum descended by 90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10–40 m day−1, a range of values consistent with the intrusion’s observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.
Nonlinear stratified spindown over a slope

(Cambridge University Press, 2013-06-05) Benthuysen, Jessica A. ; Thomas, Leif N.

Nonlinear stratified spindown of an along-isobath current over an insulated slope is shown to develop asymmetries in the vertical circulation and vertical relative vorticity field. During spindown, cyclonic vorticity is weakened to a greater extent than anticyclonic vorticity near the boundary because of buoyancy advection. As a consequence, Ekman pumping is weakened over Ekman suction. Momentum advection can weaken Ekman pumping and strengthen Ekman suction. Time-dependent feedback between the geostrophic flow and the frictional secondary circulation induces asymmetry in cyclonic and anticyclonic vorticity away from the boundary. Buoyancy advection over a slope can modify the secondary circulation such that anticyclonic vorticity decays faster than cyclonic vorticity outside the boundary layer. In contrast, momentum advection can cause cyclonic vorticity to spin down faster than anticyclonic vorticity. A scaling and analytical solutions are derived for when buoyancy advection over a slope can have a more significant impact than momentum advection on these asymmetries. In order to test this scaling and analytical solutions, numerical experiments are run in which both buoyancy and momentum advection are active. These solutions are contrasted with homogeneous or stratified spindown over a flat bottom, in which momentum advection controls the asymmetries. These results are applied to ocean currents over continental shelves and slopes.
The CLIMODE field campaign : observing the cycle of convection and restratification over the Gulf Stream

(American Meteorological Society, 2009-09) Marshall, John C. ; Ferrari, Raffaele ; Forget, Gael ; Andersson, A. ; Bates, Nicholas R. ; Dewar, William K. ; Doney, Scott C. ; Fratantoni, David M. ; Joyce, Terrence M. ; Straneo, Fiamma ; Toole, John M. ; Weller, Robert A. ; Edson, James B. ; Gregg, M. C. ; Kelly, Kathryn A. ; Lozier, M. Susan ; Palter, Jaime B. ; Lumpkin, Rick ; Samelson, Roger M. ; Skyllingstad, Eric D. ; Silverthorne, Katherine E. ; Talley, Lynne D. ; Thomas, Leif N.

A major oceanographic field experiment is described, which is designed to observe, quantify, and understand the creation and dispersal of weakly stratified fluid known as “mode water” in the region of the Gulf Stream. Formed in the wintertime by convection driven by the most intense air–sea fluxes observed anywhere over the globe, the role of mode waters in the general circulation of the subtropical gyre and its biogeo-chemical cycles is also addressed. The experiment is known as the CLIVAR Mode Water Dynamic Experiment (CLIMODE). Here we review the scientific objectives of the experiment and present some preliminary results.
Wintertime observations of Subtropical Mode Water formation within the Gulf Stream

(American Geophysical Union, 2009-01-28) Joyce, Terrence M. ; Thomas, Leif N. ; Bahr, Frank B.

We study the structure of Subtropical Mode Water (STMW) within the eastward-flowing Gulf Stream as it forms during strong winter cooling. Shipboard observations using SeaSoar and ADCP reveal that while active mixing by gravitational instabilities is common, large vertical and lateral shears of the Gulf Stream play a central role in determination of the modes of active mixing. Evidence is presented that low static stability and large vertical shear can combine to cause slantwise convection/symmetric instabilities, while the large anticyclonic shears to the south of the Gulf Stream core can cause low absolute vorticity and precondition the Ertel potential vorticity to be small and more susceptible to instabilities. The area of active mixing driven by surface forcing in the presences of shear occupies a swath 50–90 km wide immediately south of the Gulf Stream core at the northern edge of the Sargasso Sea.
Moored observations of bottom-intensified motions in the deep Canada Basin, Arctic Ocean

(Sears Foundation for Marine Research, 2010-05-01) Timmermans, Mary-Louise ; Rainville, Luc ; Thomas, Leif N. ; Proshutinsky, Andrey

In the deep Canada Basin, below the sill depth (about 2400 m) of the Alpha-Mendeleyev Ridge, potential temperature and salinity first increase with depth, then remain uniform from about 2600 m to the bottom (approximately 3500 m). Year-long moored measurements of temperature, salinity and pressure in these deep and homogeneous bottom waters reveal significant vertical excursions with periods of about 50 days. The observed isopycnal displacements have amplitudes up to 100 m at the top boundary of the bottom layer; moored profiler measurements in the intermediate water column indicate that the amplitudes of these vertical displacements decay toward the surface over a scale of about 1000 m. The subinertial excursions are consistent with a bottom-trapped topographic Rossby wave. Given the magnitude of the bottom slope in the vicinity of the mooring, the observed vertical velocities correspond to only weak (about 1 cm s−1) cross-slope horizontal velocities. The generation mechanism for the waves remains an open question.
A near-inertial mode observed within a Gulf Stream warm-core ring

(John Wiley & Sons, 2013-04-08) Joyce, Terrence M. ; Toole, John M. ; Klein, Patrice ; Thomas, Leif N.

Layering of ocean velocity “fine structure” has been coherently observed across the entire extent of a Gulf Stream warm-core ring using a shipboard acoustic Doppler current profiler system in September 2009 and independently sampled as the ring transited a moored array. Lines of constant velocity phase generally followed isopycnals as they deepened within the ring center. We also observed a clear separation of the vertical structure of the flows associated with the ring (of order 0.5 m/s) with the shorter (200 m) and less energetic (~0.2 m/s) flows of the velocity fine structure, which was further observed to rotate clockwise with increasing depth, consistent with downward propagating near-inertial waves (NIWs). Observations are consistent with a ring-scale NIW packet, probably wind forced, that shows enhanced NIW energy within the sloping pycnocline at depths of 300–700 m. Evidence of wind-forced NIWs within anticylonic eddies in a numerical simulation shows some similar features to our observations, which we try to understand physically with basic WKB-type wave/current dynamics along the lines of previously published work and a new calculation of NIW trapping within an isolated, baroclinic vortex.
Formation of intrathermocline eddies at ocean fronts by wind-driven destruction of potential vorticity

( 2008-02) Thomas, Leif N.

A mechanism for the generation of intrathermocline eddies (ITEs) at wind-forced fronts is examined using a high resolution numerical simulation. Favorable conditions for ITE formation result at fronts forced by “down-front” winds, i.e. winds blowing in the direction of the frontal jet. Down-front winds exert frictional forces that reduce the potential vorticity (PV) within the surface boundary in the frontal outcrop, providing a source for the low-PV water that is the materia prima of ITEs. Meandering of the front drives vertical motions that subduct the low-PV water into the pycnocline, pooling it into the coherent anticyclonic vortex of a submesoscale ITE. As the fluid is subducted along the outcropping frontal isopycnal, the low-PV water, which at the surface is associated with strongly baroclinic flow, re-expresses itself as water with nearly zero absolute vorticity. This generation of strong anticyclonic vorticity results from the tilting of the horizontal vorticity of the frontal jet, not from vortex squashing. During the formation of the ITE, high-PV water from the pycnocline is upwelled alongside the subducting low-PV surface water. The positive correlation between the ITE’s velocity and PV fields results in an upward, along-isopycnal eddy PV flux that scales with the surface frictional PV flux driven by the wind. The relationship between the eddy and wind-induced frictional PV flux is nonlocal in time, as the eddy PV flux persists long after the wind forcing is shut off. The ITE’s PV flux affects the large-scale flow by driving an eddy-induced transport or bolus velocity down the outcropping isopycnal layer with a magnitude that scales with the Ekman velocity.
Asymmetries in vertical vorticity and vertical velocity arising during nonlinear homogeneous spindown

( 2012-05-21) Benthuysen, Jessica A. ; Thomas, Leif N.

During the spindown of a uni-directional, geostrophic current in a homogeneous fluid, asymmetry arises in the vertical velocity and vertical relative vorticity fields. A closed-form, time-dependent solution valid to order Rossby number is derived to explore these asymmetries. Momentum advection in the interior and the Ekman layer leads to competing tendencies in the vertical vorticity’s evolution. In the interior, momentum advection hastens spindown in cyclonic regions. In the Ekman layer, momentum advection weakens Ekman pumping over Ekman suction and thus tends to slow the spindown of cyclonic vorticity. It is shown that the former effect dominates, and hence cyclonic vorticity decays faster than anticyclonic vorticity.
Friction, frontogenesis, and the stratification of the surface mixed layer

(American Meteorological Society, 2008-11) Thomas, Leif N. ; Ferrari, Raffaele

The generation and destruction of stratification in the surface mixed layer of the ocean is understood to result from vertical turbulent transport of buoyancy and momentum driven by air–sea fluxes and stresses. In this paper, it is shown that the magnitude and penetration of vertical fluxes are strongly modified by horizontal gradients in buoyancy and momentum. A classic example is the strong restratification resulting from frontogenesis in regions of confluent flow. Frictional forces acting on a baroclinic current either imposed externally by a wind stress or caused by the spindown of the current itself also modify the stratification by driving Ekman flows that differentially advect density. Ekman flow induced during spindown always tends to restratify the fluid, while wind-driven Ekman currents will restratify or destratify the mixed layer if the wind stress has a component up or down front (i.e., directed against or with the geostrophic shear), respectively. Scalings are constructed for the relative importance of friction versus frontogenesis in the restratification of the mixed layer and are tested using numerical experiments of mixed layer fronts forced by both winds and a strain field. The scalings suggest and the numerical experiments confirm that for wind stress magnitudes, mixed layer depths, and cross-front density gradients typical of the ocean, wind-induced friction often dominates frontogenesis in the modification of the stratification of the upper ocean. The experiments reveal that wind-induced destratification is weaker in magnitude than restratification because the stratification generated by up-front winds confines the turbulent stress to a depth shallower than the Ekman layer, which enhances the frictional force, Ekman flow, and differential advection of density. Frictional destratification is further reduced over restratification because the stress associated with the geostrophic shear at the surface tends to compensate a down-front wind stress.
Downfront winds over buoyant coastal plumes

(American Meteorological Society, 2016-10-07) Spall, Michael A. ; Thomas, Leif N.

Downfront, or downwelling favorable, winds are commonly found over buoyant coastal plumes. It is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport, spatial extent, and stability of the plumes. In the present study, the interaction of the Ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets. The approach makes use of an idealized numerical model and scaling theory. It is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near-zero average potential vorticity q. For weak to moderate winds, the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratification. However, for very strong winds the wind stress overwhelms the geostrophic stress and leads to a reduction in q, which is balanced by the vertical mixing term. In the absence of winds, the geostrophic stress dominates mixing and the flow rapidly restratifies. Nonlinearity, extremes of relative vorticity and vertical velocity, and mixing are all enhanced by the presence of a coast. Scaling estimates developed for the eddy buoyancy flux, the surface potential vorticity flux, and the diapycnal mixing rate compare well with results diagnosed from a series of numerical model calculations.
A warm jet in a cold ocean

(Nature Research, 2021-04-23) MacKinnon, Jennifer A. ; Simmons, Harper L. ; Hargrove, John ; Thomson, Jim ; Peacock, Thomas ; Alford, Matthew H. ; Barton, Benjamin I. ; Boury, Samuel ; Brenner, Samuel D. ; Couto, Nicole ; Danielson, Seth L. ; Fine, Elizabeth C. ; Graber, Hans C. ; Guthrie, John D. ; Hopkins, Joanne E. ; Jayne, Steven R. ; Jeon, Chanhyung ; Klenz, Thilo ; Lee, Craig M. ; Lenn, Yueng-Djern ; Lucas, Andrew J. ; Lund, Björn ; Mahaffey, Claire ; Norman, Louisa ; Rainville, Luc ; Smith, Madison M. ; Thomas, Leif N. ; Torres-Valdes, Sinhue ; Wood, Kevin R.

Unprecedented quantities of heat are entering the Pacific sector of the Arctic Ocean through Bering Strait, particularly during summer months. Though some heat is lost to the atmosphere during autumn cooling, a significant fraction of the incoming warm, salty water subducts (dives beneath) below a cooler fresher layer of near-surface water, subsequently extending hundreds of kilometers into the Beaufort Gyre. Upward turbulent mixing of these sub-surface pockets of heat is likely accelerating sea ice melt in the region. This Pacific-origin water brings both heat and unique biogeochemical properties, contributing to a changing Arctic ecosystem. However, our ability to understand or forecast the role of this incoming water mass has been hampered by lack of understanding of the physical processes controlling subduction and evolution of this this warm water. Crucially, the processes seen here occur at small horizontal scales not resolved by regional forecast models or climate simulations; new parameterizations must be developed that accurately represent the physics. Here we present novel high resolution observations showing the detailed process of subduction and initial evolution of warm Pacific-origin water in the southern Beaufort Gyre.

Thomas Leif N.

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