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Amy S. Bower

Abstract

Recent observations of fluid parcel pathways in the Gulf Stream using isopycnal RAFOS floats revealed a striking pattern of cross-stream and vertical motion associated with meanders (Bower and Rossby 1989). In an attempt to explain the observed pattern, a two-dimensional kinematic model of a meandering jet has been developed which enables examination of the relationship between streamfunction patterns and fluid parcel trajectories. The streamfunction fields are displayed in a reference frame moving with the wave pattern so motions of fluid parcels relative to the jet can be seen more easily.

The results suggest that the observed pattern of cross-stream motion results primarily from the downstream phase propagation of meanders. The model successfully reproduces several of the most distinctive features of the float observations: 1 ) entrainment of fluid into the Gulf Stream occurs at the leading edges of meander extrema while detrainment takes place at the trailing edges; 2) exchange between the Gulf Stream and its surroundings increases with a) increasing depth, b) increasing meander amplitude, and c) increasing wave phase speed.

Transport calculations from the model streamfunction fields indicate that for typical phase speeds (10 km d−1) and amplitudes (50 km), roughly 90% of the fluid in the surface layers of the Gulf Stream flows downstream in the jet while 10% continuously recirculates into the surroundings. In the deep main thermocline, where downstream speeds are less, only about 40% of the fluid is retained in the jet and 60% is trapped in the recirculating cells. It is concluded that this simple kinematic mechanism could lead to cross-stream mixing of fluid parcels, especially in the deeper layers of the Gulf Stream.

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Amy S. Bower

Abstract

Trajectories of 37 isopycnal RAFOS floats launched in the Gulf Stream off Cape Hatteras have been analyzed to examine the dynamics of meanders from a Lagrangian viewpoint. Using the float data in conjunction with information on the structure of horizontal velocity shear from the PEGASUS study (Halkin and Rossby), variations in planetary, curvature, and shear vorticity have been estimated along the float trajectories. Changes in fluid layer thickness were then inferred assuming potential vorticity is conserved following the floats.

This analysis shows that curvature vorticity changes are typically 10%–20% of f (planetary vorticity) as fluid parcels travel between meander troughs and crests. Lateral shear changes on the order of 20%–30% of f are common as parcels move laterally relative to the jet axis between meander extrema. Although changes in these two terms are usually of opposite sign and tend to compensate, significant layer thickness changes do occur, with some parcels exhibiting 30% changes in thickness over several days.

Sixty-one estimates of horizontal divergence were made from the average time rate of change of layer thickness between meander extrema. The magnitude of horizontal divergence [O(.01 f)] was found to be a strong function of temperature, clearly decreasing with decreasing temperature through the main thermocline.

Even more striking was the dependence of the sign of horizontal divergence on cross-stream position. On the anticyclonic side of the stream, divergence (convergence) was indicated downstream of a trough (crest). On the cyclonic side, convergence (divergence) was present downstream of a trough (crest). These results are discussed in relation to the free inertial jet model of the Gulf Stream developed by Robinson and Niiler. Implications for quasi-geostrophic theory are examined and it is found that this approximation may not be appropriate for use in the Gulf Stream.

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Amy S. Bower, Laurence Armi, and Isabel Ambar

Abstract

Mediterranean eddies (meddies) play an important role in maintaining the temperature and salinity distributions in the North Atlantic, but relatively little is known about their early life histories, including where, how often, and by what mechanism they form. A major field program, called A Mediterranean Undercurrent Seeding Experiment, has been carried out to directly observe meddy formation and the spreading pathways of Mediterranean Water into the North Atlantic. Between May 1993 and March 1994, 49 RAFOS floats were deployed sequentially in the Mediterranean Undercurrent south of Portugal and tracked acoustically for up to 11 months. The float deployments were accompanied by high-resolution XBT sections across the undercurrent.

Nine meddy formation events were observed in the float trajectories, six near Cape St. Vincent, at the southwestern corner of the Iberian Peninsula, and three near the Estremadura Promontory, along the western Portuguese continental slope. Meddy formation thus occurs where the continental slope turns sharply to the right (when facing in the downstream direction of the undercurrent). After conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent, the authors have estimated a meddy formation rate of 15–20 meddies per year. The timescale for meddy formation at Cape St. Vincent was found to be 3–7 days, shorter than previous estimates based on the volume of larger meddies. Meddies were observed to form most frequently when the speed of the Mediterranean Undercurrent was relatively fast.

The meddy formation process at Cape St. Vincent resembles the conceptual model of E. A. D’Asaro, whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer (with negative relative vorticity) at a sharp corner. Comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent. This confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices, by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean, may not be the principle mechanism at work in the formation of meddies at Cape St. Vincent.

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Amy S. Bower and Nelson G. Hogg

Abstract

Highly energetic velocity fluctuations associated with topographic Rossby waves are frequently observed over the continental slope and rise off the United States and Canadian east coast. It has been suggested that the energy source for these waves could be eastward-propagating Gulf Stream meanders, which can couple to the westward-propagating Rossby waves if the meander shape is time dependent. In this study, a historical archive of all available current meter data from the western North Atlantic has been examined for evidence of energy radiation from the Gulf Stream via barotropic/topographic Rossby waves. Maps of abyssal eddy kinetic energy (EKE) and Reynolds stress were constructed for four frequency bands (including motions with periods between 256 and 7.8 days) to examine distributions of these quantities over the largest possible geographical area.

Maximum eddy kinetic energy is observed along the mean axis of the Gulf Stream at low frequencies (50–250 days), but this maximum shifts north and west with increasing frequency. The westward shift may be due to the fact that the phase speed of Gulf Stream meanders decreases with increasing downstream distance. The northward shift of maximum EKE to a position over the continental slope and rise is discussed in terms of the refraction and convergence of barotropic Rossby wave energy rays emanating from the region of the Gulf Stream.

The Reynolds stress maps show strong evidence of onshore energy radiation north of the Gulf Stream over a large geographical area and at all frequencies considered. The velocity components are found to be statistically coherent and 18O° out of phase at many locations when viewed in a coordinate system aligned with the local ambient potential vorticity gradient. Energy radiation away from the Stream to the south, although expected, is not apparent in the observations, perhaps due to the dominance of other eddy-generating mechanisms there such as baroclinic instability of the westward recirculation.

Large-scale features of the maps compare poorly with similar maps generated from a stochastic wave radiation model, and it is suggested that such models need to include more realistic forcing and basin geometry before detailed model-data comparisons can be made.

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Lagrangian Observations of the Deep Western Boundary Current in the North Atlantic Ocean.

Part II: The Gulf Stream–Deep Western Boundary Current Crossover

Amy S. Bower and Heather D. Hunt

Abstract

In this study, the authors analyze the trajectories of 18 RAFOS floats, launched in the deep western boundary current (DWBC) between the Grand Banks and Cape Hatteras to investigate the kinematics and dynamics in the region where the DWBC crosses under the Gulf Stream, near 36°N (the “crossover region”). Floats deployed in the chlorofluorocarbon (CFC) maximum associated with upper Labrador Sea Water (depth ∼800 m) illustrate the entrainment process of this water mass into the Gulf Stream. The behavior of the floats (and fluid parcels) in the crossover region is strongly dependent on the meandering of the Gulf Stream. When the stream is close to its mean position, fluid parcels entrained from the upper DWBC travel along the northern edge of the stream. When a meander trough is present downstream of the entrainment location, DWBC fluid parcels cross the Gulf Stream and sometimes are expelled on the south side. This represents a previously unrecognized mechanism for transporting upper Labrador Sea Water properties across the Gulf Stream. Floats deployed in the DWBC near the deep CFC maximum that identifies overflow water from the Nordic seas (depth ∼3000 m) show a bifurcation in fluid parcel trajectories in the crossover region: fluid parcels that intersect the stream farther west tend to cross more directly and smoothly under the stream, while fluid parcels that hit the stream farther east exhibit more eddy motion and are more likely to be diverted into the interior along the Gulf Stream path. The deep float observations also reveal directly that the deep DWBC crosses under the Gulf Stream while conserving potential vorticity by sliding down the continental slope, as first conceptualized in a steady, two-layer model of the crossover. While potential vorticity is conserved along the deep float tracks on the short timescales associated with crossing under the Gulf Stream (up to a month), potential vorticity decreases over the longer timescales required for fluid parcels to transit the entire crossover region (several months to a year), consistent with what would be expected from eddy mixing.

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Amy S. Bower and Heather D. Hunt

Abstract

Twenty-six RAFOS floats were deployed in the deep western boundary current (DWBC) of the North Atlantic Ocean between the Grand Banks and Cape Hatteras in 1994–95 and tracked acoustically for up to two years. Half of the floats were launched in the upper chlorofluorocarbon (CFC) maximum associated with upper Labrador Sea Water (∼800 m), and the other half near the deep CFC maximum that identifies the overflow water from the Nordic seas (∼3000 m). The float observations reveal the large-scale pathways of these recently ventilated water masses in the subtropics. The shallow float tracks show directly that upper Labrador Sea Water is diverted away from the western boundary and into the interior at the location where the DWBC encounters the Gulf Stream near 36°N (the “crossover region”), consistent with previous hydrographic studies. East of the crossover region, only one upper Labrador Sea Water float out of seven (∼15%) “permanently” crossed to the south side of the stream in two years, caught in a cold core ring formation event. The other shallow floats recirculated north of the Gulf Stream, apparently confined by the mean potential vorticity gradient aligned with the stream. The deep floats closely followed the topography to the crossover region, then revealed a bifurcation in fluid parcel pathways. One branch continues equatorward along the western boundary, and the other turns first eastward along the Gulf Stream path, then southward. The deep float pathways, including the bifurcation in the crossover region, can be explained in terms of the deep potential vorticity distribution. Comparison of the float results with results from recent modeling studies suggests that the deep flow is strongly influenced by both the depth of the main pycnocline and bottom depth. The effective spreading rates of upper Labrador Sea Water and overflow water estimated directly from the float data, southward at 0.6 ± 0.2 cm s−1 and 1.4 ± 0.4 cm s−1, respectively, agree well with tracer-derived spreading rates. Mean velocities in the DWBC, equatorward at 2–4 cm s−1 (upper Labrador Sea Water) and 4–5 cm s−1 (overflow water), are consistent with other in situ measurements. One deep float drifted almost 4000 km along the western boundary in two years, revealing a “fast track” for the spreading of overflow water in the DWBC. These observations emphasize the importance of the crossover region in the spreading and mixing of recently ventilated water masses, addressed in Part II of this study.

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Amy S. Bower and M. Susan Lozier

Abstract

The trajectories of 95 isopycnal floats deployed in the Gulf Stream in the last decade have shown that a substantial amount of particle exchange takes place between the Gulf Stream and the surrounding fluid at the level of the main thermocline. This exchange is suggestive of significant cross-stream eddy mixing, but in order to accurately interpret the float exchange in terms of property exchange the location of float deployment was assessed relative to the strong potential vorticity front associated with the Gulf Stream. The basic result of this analysis is that most of the observed float exchange is not representative of cross-frontal exchange. At the level where a strong potential vorticity front is present, some fluid particles escape from the jet, but most of them stay on the same side of the front. In the deep main thermocline, significant particle exchange is observed between the Gulf Stream and fluid on both sides of the jet, but this exchange is indicative of particles circulating in a relatively homogeneous pool of potential vorticity and thus does not signify a cross-stream property flux. These characteristics of particle exchange in the Gulf Stream are found to be generally compatible with the results from a study of particle behavior in a quasigeostrophic eddy-resolving GCM.

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Amy S. Bower and Nelson G. Hogg

Abstract

Two years of direct current and temperature observations from an army of 13 current meter moorings deployed near 55°W as part of the SYNOP (Synoptic Ocean Prediction) Experiment have been used to explore the spatial and temporal variability of the Gulf Stream from three points of view. In the geographic reference frame, mean eastward velocities were observed from the surface to 4000 m. There was no evidence of westward flow south of the eastward jet, suggesting that the Worthington recirculation gyre was located south of the array during this time period. Westward flow was observed north of the jet only at 4000 m, where it had a magnitude comparable to the mean Gulf Stream (5–10 em s−1). These data also indicate that the mean eastward jet is much more vertically aligned than was depicted in an earlier picture constructed from noncontemporaneous observations. In the Lagrangian, or streamwise, reference frame, it was found that, at the thermocline level, the width of the “average synoptic” Gulf Stream and the velocity structure remain virtually unchanged between Cape Hatteras (73°W) and 55°W, in spite of large amplitude meandering. The barotropic velocity component of the average synoptic stream increases fivefold over this distance, and the baroclinic component weakens. The northern recirculation appears more clearly in the stream coordinate frame as a 130-km wide barotropic flow with peak westward velocities of about 8 cm s−1. South of the stream, there was no evidence of westward flow, even in the stream coordinate system. Finally, a consideration of eddy-mean flow interactions in terms of the eddy energy equations shows that at the thermocline level, there were no significant cross-gradient fluxes of heat or momentum, supporting the notion that 55°W is at a maximum in eddy energy. At 4000 m, there was some indication of upgradient heat and momentum fluxes in the Gulf Stream, consistent with decreasing eddy energy following the moan flow to the east. These results point to the region between 55°W and the Tail of the Grand Banks (50°W) as the site of eddy energy decay in the Gulf Stream system.

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Amy S. Bower and Wilken-Jon von Appen

Abstract

Recent studies have indicated that the North Atlantic Ocean subpolar gyre circulation undergoes significant interannual-to-decadal changes in response to variability in atmospheric forcing. There are also observations, however, suggesting that the southern limb of the subpolar gyre, namely, the eastward-flowing North Atlantic Current (NAC), may be quasi-locked to particular latitudes in the central North Atlantic by fracture zones (gaps) in the Mid-Atlantic Ridge. This could constrain the current’s ability to respond to variability in forcing. In the present study, subsurface float trajectories at 100–1000 m collected during 1997–99 and satellite-derived surface geostrophic velocities from 1992 to 2006 are used to provide an improved description of the detailed pathways of the NAC over the ridge and their relationship to bathymetry. Both the float and satellite observations indicate that in 1997–99, the northern branch of the NAC was split into two branches as it crossed the ridge, one quasi-locked to the Charlie–Gibbs Fracture Zone (CGFZ; 52°–53°N) and the other to the Faraday Fracture Zone (50°–51°N). The longer satellite time series shows, however, that this pattern did not persist outside the float sampling period and that other branching modes persisted for one or more years, including an approximately 12-month time period in 2002–03 when the strongest eastward flow over the ridge was at ∼49°N. Schott et al. showed how northward excursions of the NAC can temporarily block the westward flow of the Iceland–Scotland Overflow Water through the CGFZ. From the 13-yr time series of surface geostrophic velocity, it is estimated that such blocking may occur on average 6% of the time, although estimates for any given 12-month period range from 0% to 35%.

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M. Susan Lozier, Timothy J. Bold, and Amy S. Bower

Abstract

A kinematic model is developed to examine the relationship between meander propagation and Lagrangian pressure change within a meandering jet. Basically, the model equates changes in pressure along the path of a water parcel with the cross-stream motion of a parcel in a reference frame moving with the meander. The model is tested by combining isopycnal float data from the Gulf Stream with contemporaneous meander phase speed observations from satellite infrared images. Time series of pressure changes along individual float trajectories show a qualitative trend for the amplitude of pressure changes to generally increase in response to large phase speeds. However, the model suggests that the pressure change following a fluid parcel is related to the vector difference between the velocity and phase speed vectors, not just the magnitude of the phase speed. This is confirmed by the data analysis, which shows that Lagrangian pressure changes are more highly correlated with cross-stream flow when both the zonal and meridional components of the meander propagation are included in the kinematic model. Approximately 90% of the variability associated with the floats’ pressure changes can he accounted for by cross-stream flow using this kinematic formulation.

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