The entrainment and homogenization of tracers within the cyclonic gulf stream recirculation gyre

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1987The various distributions of tracer associated with the Northern Recirculation Gyre of the Gulf Stream (NRG) are studied to try to obtain information about the flow. An advective-diffusive numerical model is implemented to aid in the investigation. The model is composed of a gyre adjacent to a boundary current in which a source of tracer is specified at the upstream edge of the current. This set up attempts to simulate the lateral transfer of properties from the Deep Western Boundary Current (DWBC) to the NRG in the region where the two flows are in close contact west of the Grand Banks. The results of the model are analyzed in some detail. Tracer is entrained into the gyre as a plume which extends from the boundary current and spirals across streamlines toward the gyre center. The maintenance of the spiral during spin-up and its relationship to the occurrence of homogenization at steady state is examined. An asymmetry in the spiral exists due to the ellipticity of the gyre, which also effects homogenization. The anomalous properties that are fluxed into the NRG include salt, oxygen, and freon. These particular tracers are independent from each other, the former two because they are characterized by different vertical profiles in the deep layer. This results in a decay of oxygen but not salt, due to the presence of vertical mixing as discussed by Hogg et al. (1986, Deep-Sea Research, 33, 1139-1165). Their analysis is expanded upon here. The effect of vertical mixing on the gyre/boundary current system is examined within the context of the numerical model. Results are applied to recently collected water sample data from the region which leads to an estimate of the lateral and vertical eddy diffusion coefficients and an estimate of the amount of oxygen in the NRG that has diffused from the DWBC. The accumulation of freon within the NRG is considered in addition to salt and oxygen. Appreciable levels of freon have been present in the ocean only since 1950, and the atmospheric source functions have been increasing steadily since then. A simple overflow model is presented of the manner in which freon may be stirred in the Norwegian-Greenland basin prior to overflowing and entering the DWBC. Once in the boundary current the concentrations are diluted by way of mixing with surrounding water. Two different schemes are considered in which the immediate surrounding water accumulates a substantial amount of freon as time progresses. These models suggest that the freon-11:freon-12 ratio may not be a conserved quantity for the water in the core of the DWBC. It is found that the level of freon in the NRG is barely above the existing background level.This work was supported by the office of Naval Research through contracts N00014-76-C-0197 and N00014-84-C-0134, NR 083-400; and N00014-82-C-0019 and N00014-85-C-0001, NR 083-004, and the National Science Foundation through grant 0CE82-14925

Shelfbreak circulation in the Alaskan Beaufort Sea : mean structure and variability

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C04024, doi:10.1029/2003JC001912.Historical hydrographic and current meter data are used to investigate the properties and circulation at the shelf edge of the Alaskan Beaufort Sea. Thirty-three individual cross-sections, spanning the time period 1950 to 1987, are combined in a topographical framework to produce mean vertical hydrographic sections, as well as a section of mean absolute geostrophic velocity referenced using the current meter data. This reveals the presence of a narrow (order 20 km) eastward current, referred to as the Beaufort shelfbreak jet. The jet has three distinct seasonal configurations: In late-spring to late-summer, cold, winter-transformed Bering water is advected in a subsurface current; from mid-summer to early fall a surface intensified current advects predominantly Bering summer water; and from mid-fall to mid-spring, under easterly winds, the jet transports upwelled Atlantic water. The volume transport of the jet represents a significant fraction of the inflowing transport through Bering Strait. While the characteristics and flow of the winter-transformed Bering water vary interannually, this water mass ventilates predominantly the upper halocline.This work was supported by the Office of Naval Research under contract N00014-98- 1-0046

High-frequency variability in the North Icelandic Jet

Author Posting. © The Authors, 2018. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 76 (2018): 47-62, doi:10.1357/002224018824845910.We describe the high-frequency variability in the North Icelandic Jet (NIJ) on the Iceland Slope using data from the densely instrumented Kögur mooring array deployed upstream of the Denmark Strait sill from September 2011 to July 2012. Significant sub-8-day variability is ubiquitous in all moorings from the Iceland slope with a dominant period of 3.6 days. We attribute this variability to topographic Rossby waves on the Iceland slope with a wavelength of 62 ± 3 km and a phase velocity of 17.3 ± 0.8 km/day−1 directed downslope (−9◦ relative to true-north). We test the theoretical dispersion relation for these waves against our observations and find good agreement between the predicted and measured direction of phase propagation.We additionally calculate a theoretical group velocity of 36 km day−1 directed almost directly up-slope (106◦ relative to true-north) that agrees well with the propagation speed and direction of observed energy pulses. We use an inverse wave tracing model to show that this wave energy is generated locally, offshore of the array, and does not emanate from the upstream or downstream directions along the Iceland slope. It is hypothesized that either the meandering Separated East Greenland Current located seaward of the NIJ or intermittent aspiration of dense water into the Denmark Strait Overflow are the drivers of the topographic waves.This work was supported by National Science Foundation grants OCE-1433958 (BH), OCE-0959381 (BH and RP) and OCE-1558742 (RP)

Structure of an inertial deep western boundary current

An inertial model of the deep western boundary current (DWBC) is presented where the cross-stream distribution of potential vorticity varies in a realistic fashion. The case of uniform potential vorticity, which has been solved earlier, is included for comparison. The potential vorticity distribution used in the model is obtained from a hydrographic density section across the North Atlantic DWBC. The model solutions using this distribution differ significantly from the uniform potential vorticity case. Most notably the current is wider and weaker with substantially reduced relative vorticity, more indicative of observed DWBCs. The addition of an exponential continental slope leads to a further constraint on the existence of the current. Finally, it is demonstrated how a topographic ridge can partially block the DWBC and give rise to recirculation of the deepest water, reminiscent of the deep flow near the Southeast Newfoundland Rise in the North Atlantic

Impact of Labrador Sea convection on the North Atlantic meridional overturning circulation

Author Posting. © American Meteorological Society, 2007. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 37 (2007): 2207-2227, doi:10.1175/jpo3178.1.The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1 Sv (1 Sv ≡ 106 m3 s−1), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea.This work was funded by the National Science Foundation through Grants OCE-0450658 (RP) and OCE-024978 (MS)

Deep Western Boundary Current variability at Cape Hatteras

Data from an array of inverted echo sounders and bottom current meters off Cape Hatteras, where the Gulf Stream and Deep Western Boundary Current (DWBC) cross each other, are analyzed to investigate the deep components of flow. While the mean flow is to the southwest with the DWBC, the observed temporal variability is dominated by energetic 40 day topographic Rossby waves. By optimally weighting the individual deep current meter measurements, the deep flow is averaged across the wavelength of the 40 day wave, thereby reducing the wave signal and revealing variations of the spatially averaged DWBC. The DWBC fluctuations are found to be oriented more along the isobaths than the wave motions (which have an essential cross-isobath component). Lateral path displacements of the upper layer Gulf Stream, as measured by the inverted echo sounders, are correlated with deep velocity and temperature fluctuations at specific sites, which can be understood in terms of deep Gulf Stream influence. Cross-slope flow of the spatially averaged DWBC is found to vary with changes in angle of the Gulf Stream path in a manner consistent with simple dynamics

Using the Inverted Echo Sounder to Measure Vertical Profiles of Gulf Stream Temperature and Geostrophic Velocity

The methodology for converting the travel time measurement of the inverted echo sounder (IES) into an amplitude of the first baroclinic dynamical mode, A1, is presented. For a Gulf Stream IES record the so-generated A1(t) time series is used to compute a vertical profile of first mode temperature versus time by perturbing a basic state temperature profile. The basic state is constructed by averaging together historical CTD data collected near the IES site. Similarly the first mode amplitudes are used to perturb a basic state dynamic height profile, and, using neighboring IESs, a profile of alongstream geostrophic velocity is obtained at the same location. The resulting IES-derived temperatures and velocities compare favorably to independent current meter results, exhibiting most of the variability observed in both the current meter temperature and alongstream velocity

Characteristics and dynamics of two major Greenland glacial fjords

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 119 (2014): 3767–3791, doi:10.1002/2013JC009786.The circulation regimes of two major outlet glacial fjords in southeastern Greenland, Sermilik Fjord (SF) and Kangerdlugssuaq Fjord (KF), are investigated using data collected in summer 2009. The two fjords show similar flow patterns, with a time-dependent, vertically sheared flow structure dominating over the background estuarine flow driven by buoyancy input. We show that this time-dependent flow is consistent with circulation induced by density interface fluctuations at the fjord mouth, often referred to as intermediary circulation. One difference between the fjords is that the hydrographic and velocity structure below a surface modified layer is found to be three layer in KF in summer, compared to two layer in SF. Outside each fjord, large-scale geostrophic currents dictate the stratification at the mouth, although the way in which these large-scale flows impinge on each fjord is distinct. Combining the observations with estimates from existing theories, we find the magnitudes of the estuarine (Qe) and intermediary (Qi) circulation and show that Qi >> Qe, although along-fjord winds can also be significant. We expect that the critical parameter determining Qi/Qe is the sill depth compared to the fjord depth, with shallower sills corresponding to weaker intermediary circulation. Finally, we discuss the implications of strong intermediary circulation on calculating heat transport to the glacier face and its potential feedbacks on the background circulation in these highly stratified estuaries.Funding for this work came from National Science Foundation OPP grant 0909373 and OCE grants 1130008 (D.A.S. and F.S.) and 0959381 (R.P.), and the WHOI Arctic Research Initiative (FS).2014-12-1

SYNOP Inlet Experiment: Bottom Current Meter Data Report for October 1987 to August 1990 Mooring Period

An array of five deep current meter moorings, each 100 m above the ocean bottom, was maintained along a line extending southeast from Cape Hatteras, NC for 34 months from October 1987 through August 1990. The transect line was part of the SYNOP (Synoptic Ocean Prediction experiment) Inlet Array, which also had a surrounding array of Inverted Echo Sounders. The line extended from approximately the 2300 m to the 3800 m isobath (roughly 100 km long), intended to measure currents in the Deep Western Boundary Current in the region where the Gulf Stream crosses over it. This report describes the array design and data processing, and presents the basic statistics for each record. The current and temperature records are also displayed in 40-hour low-passed time series plots

A continuous pathway for fresh water along the East Greenland shelf

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Foukal, N. P., Gelderloos, R., & Pickart, R. S. A continuous pathway for fresh water along the East Greenland shelf. Science Advances, 6(43), (2020): eabc4254, doi:10.1126/sciadv.abc4254.Export from the Arctic and meltwater from the Greenland Ice Sheet together form a southward-flowing coastal current along the East Greenland shelf. This current transports enough fresh water to substantially alter the large-scale circulation of the North Atlantic, yet the coastal current’s origin and fate are poorly known due to our lack of knowledge concerning its north-south connectivity. Here, we demonstrate how the current negotiates the complex topography of Denmark Strait using in situ data and output from an ocean circulation model. We determine that the coastal current north of the strait supplies half of the transport to the coastal current south of the strait, while the other half is sourced from offshore via the shelfbreak jet, with little input from the Greenland Ice Sheet. These results indicate that there is a continuous pathway for Arctic-sourced fresh water along the entire East Greenland shelf from Fram Strait to Cape Farewell.Funding for this work comes from the NSF under grant numbers OCE-1756361 and OCE-1558742 (N.P.F. and R.S.P.) and grant numbers OCE-1756863 and OAC-1835640 (R.G.)