On the energetics of the Gulf Stream at 73W

From September 1980 through May 1983 a series of nineteen sections of velocity profiles were obtained across the Gulf Stream 200 km northeast of Cape Hatteras. By decomposing the velocity and temperature observations into mean and fluctuating fields in two coordinate systems, geographic (or Eulerian) and \u27stream\u27 coordinates, it is shown that at least ⅔ of the eddy kinetic and potential energy is caused by the meandering of a well defined baroclinic front with a structure that is nearly independent of space and time. It is also shown that more than 95% of the kinetic energy of the front can be accounted for by a barotropic and a baroclinic mode with near equipartition between the two.The cross-stream baroclinic, barotropic, and pressure-work terms in the eddy energy production equation are estimated to determine what processes contribute to the rapid growth of meandering after the current leaves the coast. In order of importance, the cross-stream average of the baroclinic conversion term is a factor three larger than the other two. The cross-stream averaged production of eddy energy is, however, clearly too large to be consistent with the observed rate of growth of the meander envelope since it would lead to a doubling of eddy kinetic and potential energy in only 2.1 days or 50 km following the mean flow. It is shown that in the case of the baroclinic conversion term the large cross-stream covariances 〈u′T′〉 have a simple geometric interpretation in terms of meander growth (and decay). They represent a down (or up) gradient heat flux that is not actually participating in the conversion processes suggesting that the baroclinic production terms are nearly horizontally nondivergent. Similarly, the pressure-work terms must be very nearly horizontally nondivergent (geostrophy). Thus, estimates of energy conversion rates are bound to be greatly exaggerated unless both horizontal components are included. Furthermore, conclusions about the relative importance of the cross-stream conversion terms to the production of eddy energy depend upon their horizontal divergence being in the same proportions, a very unsatisfactory assumption.A simple kinematic model is used to show that the amount of energy needed to support meander growth is quite small. It is clear that to determine these rates experimentally puts great stress on conventional measurement procedures and suggests that alternative approaches such as paying more attention to boundary or flux conditions might be more rewarding in future studies

An oceanic vorticity meter

An acoustical method of determining the relative vorticity of oceanic motions is advanced. By transmitting signals in opposite directions around a closed loop, for example a 3 km triangle, one can obtain directly the circulation, which by virtue of Stoke\u27s theorem is equivalent to the average vorticity of the enclosed fluid times the area...

On the structure and distribution of thin anticyclonic lenses in the southeast Pacific Ocean

The velocity structure of an anticyclonic lens in the southeast Pacific centered at 300–400 m depth was scanned in detail with a 75 kHz acoustic Doppler current profiler on the RV Atlantis during a February 2010 transit along 20° S. Embedded in the main thermocline with a vertical span of ∼500 m, the lens had an overall diameter of ∼150 km with a peak speed of 0.33 m s–1 at ∼35 km radius. Knowledge of the velocity field permitted determination of energetics and vorticity properties in detail. The ratio of potential to kinetic energy was ∼14, and the relative vorticity of the lens core was –0.7 times the local Coriolis parameter. Using a dynamically estimated density field, the potential vorticity of the lens\u27s core was a factor of 6 less than the surrounding waters, the only source of which could be equatorial subsurface water. The lens had an estimated age of ∼4 years depending on where it was formed off the Peru-Chile coast. A search for similar lenses in the Argo database showed that they could be found throughout the southeast Pacific. Emanating from the southern Chile continental slope, their thickness-to-width aspect ratio decreased as they drifted west and equatorward. The inferred very slow rate of decay suggests that these rapidly spinning disks must be quite stable, but the thinning population suggests that they rarely get to live their full life cycle; more likely, they are torn apart by collision with other lenses or energetic mesoscale events

Onset of unsteady horizontal convection in rectangle tank at Pr=1Pr=1

The horizontal convection within a rectangle tank is numerically simulated. The flow is found to be unsteady at high Rayleigh numbers. There is a Hopf bifurcation of $Ra$ from steady solutions to periodic solutions, and the critical Rayleigh number $Ra_c$ is obtained as $Ra_c=5.5377\times 10^8$ for the middle plume forcing at $Pr=1$, which is much larger than the formerly obtained value. Besides, the unstable perturbations are always generated from the central jet, which implies that the onset of instability is due to velocity shear (shear instability) other than thermally dynamics (thermal instability). Finally, Paparella and Young's [J. Fluid Mech. 466 (2002) 205] first hypotheses about the destabilization of the flow is numerically proved, i.e. the middle plume forcing can lead to a destabilization of the flow.Comment: 4pages, 6 figures, extension of Chin. Phys. Lett. 2008, 25(6), in pres

The near-surface velocity and potential vorticity structure of the Gulf Stream

Using Acoustic Doppler Current Profiler and XBT data between 1992 and 1999 from a container vessel that crosses the Gulf Stream twice weekly near 70W, we examine the near-surface velocity, thermal and vorticity structure of the current. These data come from an ongoing sampling program that has as its overall objective to measure the currents between New York and Bermuda to provide a high-quality database for studies of variability and long-term trends in the region. These Gulf Stream sections, when averaged in natural or stream coordinates, exhibit a remarkable double-exponential structure. The scale-widths of the lateral shear north and south of the velocity maximum, 20 and 34 km respectively, agree well with estimates of the radius of deformation from simple modal analysis (19 and 34 km, respectively). Significantly, the entire Eulerian mean field of the Gulf Stream and over 80% of the eddy kinetic energy can be accounted for in terms of shift and rotation of this simple double exponential structure. The remainder of this variability can be accounted for rather effectively in terms of a limited number of empirical modes. The first and most energetic mode consists of a \u27rocking\u27 mode such that the velocity increases on the concave side of meander extrema. The second EOF mode which measures changes in shear on the anticyclonic side, increases as expected when the stream shifts to the south and vice versa to the north. These two account for nearly half of the remaining variability of the Gulf Stream and adjacent waters (26 and 21%, respectively). These modes notwithstanding, the stiffness of the Gulf Stream is striking. With the help of concurrent XBTs and historical hydrography, we show that the double-exponential velocity pattern is consistent with a uniformity of potential vorticity between the Gulf Stream and recirculating gyres to either side, but not across the velocity maximum where it undergoes nearly a factor 5 change in ~ 20 km. The ambient eddy field is sufficiently energetic to maintain the uniformity to either side but much too weak to break down the front. Interestingly, the potential vorticity evinces a slight minimum south of the velocity maximum that appears to be robust. Unlike other locations along the path of the Gulf Stream, specifically the Pegasus line at 73W and the SYNOP array at 68-69W, the current loses water to the north at this site (with no evident gain or loss to the south). Further, at this location the u-v covariances to both sides of the Gulf Stream suggest a conversion of kinetic energy from the eddy to mean flow. We interpret this as a geometric result of the downstream decrease in meandering approaching the Oleander line. It appears that patterns of in- and outflow and energetics can be quite site specific, reflecting, we think, preferred states or patterns of the meandering of the Gulf Stream

On the variability of Gulf Stream transport from seasonal to decadal timescales

Given the Gulf Stream\u27s central role in the North Atlantic\u27s wind-driven and meridional overturning circulation (MOC), there is considerable interest in measuring mass and heat flux to sufficient accuracy that their variability can be quantified with some degree of confidence. Here we combine high-resolution direct measurements of upper ocean transport from the last 17 years of Oleander ADCP data with previously published estimates of baroclinic transport to examine Gulf Stream transport variability over the last 80 years just downstream of where the current separates from the U.S. east coast.By far the greatest source of variability occurs on short time scales related to the meandering of the current and energetic eddy field to either side such that the inherent uncertainty of a single transport estimate is ∼15% with respect to an annual mean. The annual cycle of layer transport at 55-m depth has a maximum increase of 4.3% of the mean in September while the annual cycle at 205 m reaches a maximum of only 1.5% in July. A running low-pass filter indicates transport variations of only a few percent of the mean on inter-annual and longer time scales although swings as large as 10–12% over a few years can occur. The length of the time series now reveals a significant correlation between the NAO index and near-surface transport in the Gulf Stream. No significant trend in transport can be detected from either the last 17 years of directly measured surface currents, or from hydrographic sections starting in the 1930\u27s. It follows therefore that the upper branch of the MOC, the other major component of Gulf Stream transport at the Oleander line, must have been quite stable over the last 80 years

What can hydrography tell Us about the strength of the Nordic Seas MOC over the last 70 to 100 years?

The flow of warm water into the Nordic Seas plays an important role for the mild climate of central and northern Europe. Here we estimate the stability of this flow thanks to the extensive hydrographic record that dates back to the early 1900s. Using all casts in two areas with little mean flow just south and north of the Greenland‐Scotland Ridge that bracket the two main inflow branches, we find a well‐defined approximately ±0.5 Sv volume transport (and a corresponding ±30 TW heat flux) variation in synchrony with the Atlantic multidecadal variability that peaked most recently around 2010 and is now trending down. No evidence is found for a long‐term trend in transport over the last 70 to 100 years

KAPEX: an international experiment to study deep water movement around southern Africa

Over the past decade it has been demonstrated unequivocally that the exchange of water masses between the Indian and the Atlantic oceans, south of Africa, is a key component of the global thermohaline circulation cell. This thermohaline circulation plays a large part in controlling global climate. Numerous South African and international studies have now been completed on these water exchanges, but most have concentrated their efforts on the upper water masses. We descriptionbe here an extensive and exciting new venture, called KAPEX, to investigate the movement of water at intermediate depths using sophisticated Lagrangian floats. This research programme is a collaborative effort between research groups from three countries and constitutes the geographically largest oceanographic experiment ever carried out in South African waters. Detailed information on the aims as well as the progress of KAPEX is available on the Internet on the home page http://triton.sea.uct.ac.z

Drifters in the Gulf Stream

In the past, the Gulf Stream has frequently been viewed as essentially a barrier between the Slope and Sargasso seas. On the other hand, surface drifters have often been observed to leave the stream to the south. To gain a better understanding of surface flow of the Gulf Stream, we used drifter trajectory data to study their drift east and patterns of loss from the current. Two sets of drifter data were used, one from the 1995–1999 Georges Bank GLOBEC program and the other from the Global Drifter Program. We also made use of atmospheric wind stress estimates from a reanalysis data set to evaluate the effect of wind forcing on their movements. Without fail, all drifters that enter into the stream eventually detrain out of the current to the south, indicating significant cross-frontal transport at the surface. A first explanation of these detrainments relies on the Ekman drift to the south due to the westerly winds over the study area. However, the exits to the south are not uniformly distributed, but aggregate in three areas where the meandering is particularly sharp: the New England Seamount Chain, the Southeast Newfoundland Rise, and the 44°N trough in the North Atlantic Current. Although, intuitively, it would seem that the largest Ekman drift losses would occur in the winter, this study shows that the detrainment to the south occurs more effectively in the summer due to a minimum in the mixed layer depth

A quasi-Lagrangian study of mid-ocean variability using long range SOFAR floats

Twenty neutrally buoyant SOFAR floats were used in the Mid-Ocean Dynamics Experiment (MODE) to study the structure and variability of the deep ocean currents. The floats were clustered so that the pattern of motions could be resolved (mapping and pattern recognition). A number of float trajectories are shown and the very individual character of their signature is emphasized. Some floats remain nearly stationary for a year whereas others will cover hundreds of kilometers to the south or west in just a few months...