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Ming Liu and T. Rossby

Abstract

In September–October 1988 and in April 1989 two 4-week cruises were organized to study the structure and dynamics of Gulf Stream meanders. The first one focused on an anticyclonic crest, and the second one on a cyclonic trough. One objective of this program was to conduct a high-resolution study of the upper-ocean velocity and vorticity field using a CTD and a continuously profiling acoustic Doppler current profiler (ADCP).

The cross-stream sections of velocity exhibit the typical pattern of a single velocity maximum with a narrow zone of cyclonic shear and a broad region of anticyclonic shear. The cyclonic velocity shear is larger and extends to greater depths in a crest than in a trough and can exceed 120% of the Coriolis parameter (f) at depths as great as 300 m. The vertically integrated transport to 250 m appears to be more symmetric in troughs than in crests. The computed sea level difference across the stream is about 0.46 m greater in the trough than the crest after seasonal correction and “normalization” to the same surface transport. The surface velocity vectors are divergent upstream and convergent downstream of a crest, consistent with upwelling and shingle formation, and entrainment/downwelling, respectively.

By combining the CTD observations of density with the velocity data, the cross-stream structure of the potential vorticity field and its components can be mapped. It is found that shear vorticity greatly enhances −(f/ρ)dρ/dz on the cyclonic side and weakens it to a distinct minimum in the center of the current at the crest. Although the database is not so extensive for the trough, the evidence suggests that the opposite is true: cyclonic shear is weakened and anticyclonic shear is strengthened resulting in a less asymmetric velocity and transport distribution. There was considerable “marbling” of the potential vorticity field by both the stratification and the shear vorticity field, but there is no evidence to suggest that the two fields are correlated in such a way as to reduce the marbling to less than what the component fields individually contribute.

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T. Rossby and E. Gottlieb

An overview of the first 4.5 years of operation of a program to monitor the structure and variability of the Gulf Stream (GS) is presented. A container vessel that operates on a weekly schedule between Port Elizabeth, New Jersey, and Hamilton, Bermuda, is equipped with a 150-kHz narrowband acoustic Doppler current profiler to measure currents from the surface to ~300 m depth. A major objective of the multiyear program is to study the annual cycle and interannual variations in velocity structure and transport by the GS. In this survey the focus is on the transport and lateral structure of the current at 52-m depth.

The velocity maximum is constant at 2.07 ± 0.24 m s−1 (4 kt) with a seasonal range of ~0.1 m s−1 . Seasonal and interannual variations in total transport are observed but appear to be limited to the edges of the current, apparently reflecting low-frequency variations in the intensity of the recirculating waters adjacent to the stream. The transport by the central core of the current, defined as those waters moving at 1 m s−1 or faster, equals 0.9 × 105 m2 s−1 , has no seasonal signal, and is constant to within a few percent when averaged in half-year intervals. If the central core of the current is viewed as “insolated” from the effects of meandering, this result implies substantial stability to the large-scale wind-driven and thermohaline circulations during the observation program. Variations in poleward heat transport probably originate less in the GS and more from changing heat loss patterns at higher latitudes.

Other issues concerning the potential vorticity field and energy conversion rates are also discussed. This ongoing program illustrates the role commercially operated vessels can play in making repeat observations of the velocity structure (and other parameters) of the ocean on a regular basis.

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Daniel Halkin and T. Rossby

Abstract

Between September 1980 and May 1983, 16 sections of temperature and velocity were obtained with the Pegasus instrument along a transect crossing the Gulf Stream at 73°W. The mean temperature and velocity fields of the upper 2000 meters were calculated. The transport above 2000 meters calculated from the sections had a mean of 87.8 (± 17.3) × 106 m3 s−1. A comparison with mass flux measurements obtained by previous investigators at Cape Fear indicated that the addition to the transport of the Stream in the region was uniform with depth above 800 meters. The mean inflow measured at the Pegasus line was found to be uniform above 800 meters and to decrese substantially below this depth. The increase in transport at the line estimated from the mean cross-stream velocity field was 15.4 (± 5.8) × 106 m3 s−1 per 100 km downstream distance. The temporal fluctuation of the Pegasus transport measurements was consistent with an annual cycle with a maximum in April. The mean eddy kinetic energy at the surface was 1500 cm2 s−2, of which 1000 cm2 s−2 was due to the meandering of the current. The velocity structure of the front was remarkably constant: the eddy kinetic energy attributable to structural changes of the front was SW 500 cm2 s−2, only three times as great as midocean values.

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Olga T. Sato and T. Rossby

Abstract

Historical hydrographic sections are used to investigate the seasonal and interannual variability in the meridional heat flux at 36°N in the North Atlantic. The data consist of ten transatlantic sections and sections from four sectors, which combined, cross the entire basin. These sectors are the slope water, the Gulf Stream, the Sargasso Sea, and the midocean. The data from the first three sectors actually come from sections that span all three regions, but their properties are examined individually. To improve estimates of the Gulf Stream contribution to the total heat flux, a tangent hyperbolic model of the current’s temperature field is used to retain its structure in the temperature flux integrations even when only a few stations are available. The technique removes biases due to undersampling that averages about 0.3 PW.

The temperature flux of the upper layer is estimated for the four sectors plus the climatologically forced Ekman layer. The annual mean is 1.4 ± 0.3 PW with a range of 0.6 ± 0.1 PW. The zero net mass flux across the transect can be accomplished by assuming that in the deep layer an equivalent amount of water to that estimated for the upper layer flows in the southward direction presumably via the deep western boundary current. The temperature flux of the deep layer, with its mean temperature of 2.3°C, has an annual mean of −0.20 ± 0.06 PW and a range of 0.05 ± 0.02 PW. The net annual mean of the meridional heat flux is 1.2 ± 0.3 PW and a range of 0.6 ± 0.1 PW. Its phase is dominated by the annual cycle of the Ekman temperature flux.

The heat flux residual is examined for evidence of long-term change in the poleward heat flux. While the database is very limited for a conclusive statement, it appears that the residual for the pentads 1935–39, 1970–74, and 1975–79 agreed to within 0.1 PW. The tightness of these estimates in the presence of a 0.6 PW annual range makes it clear how important it is to know the latter accurately before statements about long-term change can be made. To date, most individual transoceanic sections were taken during the summer and spring. The standard deviation of the heat flux estimates is 0.3 PW, much of it due to eddy variability, making it essential to obtain repeat sections preferably with a uniform distribution throughout the year.

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K. L. Schultz and T. Rossby

Abstract

The spatial distribution of static stability in the Gulf Stream was studied using cross-and downstream XBT sections taken between Cape Hatteras and 70°W.

The cross-stream sections revealed large variations in static stability, even over small spatial scales (∼10 km). In the downstream direction, following isopycnal Swallow floats, changes occur more gradually and are clearly correlated with the curvature of the float trajectory. With curvature effects removed, the residual down-stream or Lagrangian variance is just 15% of that in the cross-stream direction.

The variability of the cross-stream sections which sample different water parcels, compared to that of the downstream sections which follow a single parcel, suggests that the balance of the constituent terms of potential vorticity varies from one fluid parcel to another, reflecting their different origins.

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A. S. Bower and T. Rossby

Abstract

A unique set of Lagrangian observations has recently been collected in the Gulf Stream using the newly developed isopycnal RAFOS float. Between January, 1984 and October, 1985, thirty-seven of these drifters were launched in the main thermocline of the current off Cape Hatteras and lacked acoustically downstream for 30 or 45 days. Temperature and pressure were also recorded along each float trajectory. The isopycnal capability of this drifter allows it to follow fluid parcel pathways quite accurately along the sloping density surfaces of the Gulf Stream.

The RAFOS drifters revealed a striking pattern of vertical and cross-stream motion in Gulf Stream meanders. Floats were consistently observed to upwell (downwell) and move onshore (offshore) as they approached anticyclonic (cyclonic) meander crests (troughs). The rms vertical velocity in the center of the stream was observed to be 0.08 cm s−1 on the 12°C surface. No mean vertical motion was detected in the main thermocline of the Gulf Stream between 70° and 55°W. Using a model of the mean cross-stream thermal structure of the current, rms cross-stream velocities were estimated to be 8–10 cm s−1.

This meander-induced circulation represents an important mechanism for cross-frontal exchange. When the Gulf Stream is meandering, fluid parcels from the center of the current are brought to the edges and often escape completely. In fact, 60% of the floats launched at Cape Hatteras escaped from the Gulf Stream at least once before reaching 65°W. These losses were not evenly distributed in the vertical; retention was greater in the upper thermocline (11–16°C) than in the lower thermocline (7°–11°C). Comparison of goat trajectories with infrared satellite imagery shows that the meander-induced cross-frontal fluid exchange is enhanced by ring—current interaction and time evolution of the Gulf Stream path.

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Arthur J. Mariano and T. Rossby

Abstract

The terms in the Lagrangian potential vorticity equation are estimated by three different methods using clusters of SOFAR floats in the main (700 m) and lower (1300 m) thermocline of the POLYMODE region. The Lagrangian stretching term, which is the most difficult to observe, has been estimated in the main thermocline using a combination of SOFAR float and hydrographic data. The stretching term may be estimated below the main thermocline from float trajectories and knowing the topographic gradient.

The 700 m mesoscale balance is either that of beta and vortex stretching balancing the material time derivative of relative vorticity (dζ/dt), or that of beta and dζ/dt balancing vortex stretching. The mean 700 m balance is the former type. The Lagrangian potential vorticity balances indicate internal convergences and divergences at 700 m. The mean and most of the synoptic 1300 m balances are achieved by beta and vortex stretching of equal magnitude balancing dζ/dt. This is because topographic and planetary beta are of the same magnitude. Equivalently, the vertical velocity induced by eddy flow over topography in this area is dynamically important.

This study emphasizes that the use of float cluster trajectories to obtain time series of the terms in the potential vorticity equation from a Lagrangian viewpoint is a powerful diagnostic tool for the study of ocean dynamics.

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K. Schultz Tokos and T. Rossby

Abstract

Two surveys of the absolute velocity field of an eddy of Mediterranean Water (meddy) in the Eastern North Atlantic were conducted one year apart in 1984 and 1985. Two velocity regimes were revealed. Within the radius of maximum velocity, the meddy rotated anticyclonically as a solid body with a depth-dependent rotation period near 6 days at its mid-depth (1000 m). One year later the radius of the core had decreased by one third. The rotation rate of the lens also decreased, except at its mid-depth where there was a small but perceptible increase.

There was a sharp (5 km or less) transition between the core and the outer region where the velocity decayed exponentially with radius. A strong potential vorticity front, due to the abrupt change in sign of horizontal shear, kept the core isolated from the outer region. Potential vorticity was nearly constant within the upper confines of the core over the study period, whereas, there was a notable increase in potential vorticity in the lower portion of the core due to erosion from underneath. Although there were significant azimuthal velocities beyond the transition, the potential vorticity was nearly that of the background field. The horizontal uniformity of the potential vorticity field suggests free exchange along isopycnal surfaces.

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T. Rossby, J. Price, and D. Webb

Abstract

Up to forty neutrally buoyant floats (20 at 700 m, 20 at 1300 m) were used in the POLYMODE Local Dynamics Experiment (LDE) to provide a quasi-Lagrangian description of the structure and evolution of the mesoscale eddy field in a limited region characterized by higher kinetic energy levels than those obtained in MODE. This paper is an overview and summary of the data collected.

The temporal development of the two-level float array is presented in a sequence of maps, each spanning five days. In these one readily notices an “oscillation” of floats at 1300 m in a NE-SW direction before the cluster breaks apart. At 700 m the float cluster subdivides much more rapidly. The first setting of 700-m floats drifts to the west; the second group, launched two months later, goes far to the east.

Ensemble averages as a function of time of the floats at 1300 m reveal great sensitivity to the “oscillatory” velocity field while the array is “tight” or coherent the corresponding 700-in averages, although noisy, show clearly the westward and eastward motion of the first and second clusters respectively. Grand averages for μ, υ (cm s−1), and kinetic energy (ergs gm−1) are (−1.8, 0., 80) and (−1.6, 0., 34) for the shallow and deep floats.

The spatial correlation functions show 1arge scales of coherence. The zero-crossing of the transverse-velocity correlation function is about 100 km for both the 700- and 1300-m floats compared to 55 km for the 1500-m goats in MODE. The longitudinal correlation scale is also much larger.

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K. D. Leaman, E. Johns, and T. Rossby

Abstract

Average cross sections of downstream velocity and temperature, obtained using PEGASUS current profiles at three locations along the Gulf Stream, have been partitioned into 2.5°C temperature intervals to examine the distribution of transport increase versus temperature between the two southern sections (27° and 29°N) and off Cape Hatters(73°W). Between 27° and 29°N the total transport of the Florida Current over the sections increased only by about 3 × 106 m3 s−1 (3 Sv) but the current broadens by about 50%. By Cape Hatteras, the transport has increased nearly three-fold to 93.7 Sv, of which two-thirds of the increase is contained in the 19.5°–17.0°C (“18°”) layer and in water colder than the 7°C “still” temperature found at 27°N.

Cross-stream distributions of layer transport, potential vorticity, and thickness are estimated. At each section, the 10 × 10−7 m−1 s−1 contour tends to be a boundary (independent of temperature) between the region of relatively uniform layer potential vorticity on the anticyclonic (offshore) side of the current and an area with high lateral potential vorticity gradients on the cyclonic (onshore) side. In the colder (<7°C) waters off Cape Hatteras, layer potential vorticity also tends to be uniform at ∼5 × 10−7 m−1 s−1. Layer potential vorticity in the 18° layer is quite uniform with minimum values ∼3.5 × 10−7 m−1 s−1 at 27° and 29°N and somewhat less off Cape Hatteras, which is close to where 18°C water is formed in the wintertime. At Cape Hatteras this same layer shows a peak in transport/unit width at the point where the layer begins to thin as one moves into the Gulf Stream core from the southeast. A simple model based on conservation of layer potential vorticity is proposed to describe this transport structure.

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