The \u3cem\u3eOleander\u3c/em\u3e Project: Monitoring the Variability of the Gulf Stream and Adjacent Waters between New Jersey and Bermuda

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

A Simple Method for Measuring Deep Convection

The glass-pipe technology used for RAFOS floats is applied to the monitoring of convection in deep mixed layers. The velocity of a vertical current is estimated from the relationship between the drag force exerted on a float by the vertical current and the buoyancy force induced by the float\u27s resultant displacement from hydrostatic equilibrium. Tests conducted in the winters of 1990 and 1991 in the 18°C waters of the northwestern Sargasso Sea reveal definite convective events. Vertical velocities of both upwelling and downwelling plumes are estimated to approach maxima nearing 0.05 m s−1, with durations of up to 2 h. One float that crossed the Gulf Stream and entered the Newfoundland Basin showed evidence of very active vertical currents in the near-surface waters with maximum velocities greater than 0.09 m s−1

The RAFOS System

The RAFOS float is a small neutrally buoyant subsurface order, which, like its big brother the SOFAR float, uses the deep sound (or SOFAR) channel to determine its position as a function of time. Whereas the SOFAR float transmits to moored receivers, the ∼12 kg glass pipe RAFOS float listens for accurately timed signals from moored sound sources to determine its position. The acoustic signal detection and norm of data are all handled by a CMOS microprocessor in the float. The data are recovered at the end of its mission when the float surface and telemeters its memory contents to Systeme Argos, a satellite-borne platform location and data collection system. Just a few sound sources provide navigation for an arbitrary number of floats

Operating an Acoustic Doppler Current Profiler aboard a Container Vessel

Since October 1992 an acoustic Doppler current profiler (ADCP) has been in near-continuous operation on board a 118-m-long container vessel, the container motor vessel Oleander, which operates on a weekly schedule between Port Elizabeth, New Jersey, and Hamilton, Bermuda. The ADCP collects information on currents from the surface to depths as great as 404 m depending on zooplankton concentrations, ship’s speed, sea state conditions, and the ship’s load factor. The southbound transits provide more and better data because the ship is loaded and rides deeper resulting in less bubble formation and entrainment underneath the vessel. Installation and operation of an ADCP on a cargo ship has involved a number of factors not typical of research vessels. Providing a data acquisition system that could operate on its own without assistance from the ship’s officers and that could recover from problems was the first issue. Isolating and removing electrical transients from the ship’s electrical system was extremely challenging. The presence of bubbles underneath the vessel due to variable draft and in heavy weather conditions significantly limits the performance of the ADCP. These difficulties not withstanding, the system is working well and is delivering good data on the southbound legs in most weather conditions and on the northbound legs under more favorable weather conditions. Starting in 1995, differential and attitudinal global positioning system enhancements have made significant improvements to navigational accuracy and ship’s heading data

Wavenumber Spectrum in the Gulf Stream from Shipboard ADCP Observations and Comparison with Altimetry Measurements

The wavenumber spectra for velocity and temperature in the Gulf Stream region are calculated from a decade (1994–2004) of shipboard acoustic Doppler current profiler (ADCP) measurements taken as part of the Oleander Project. The velocity and temperature spectra have comparable magnitude, in terms of the kinetic and potential energy, and both indicate a k−3 slope in the mesoscales. In contrast, the corresponding velocity spectrum determined from satellite altimetry sea surface heights yields a significantly higher energy level and a k−2 slope. The discrepancy between altimeter-derived and directly measured velocity spectra suggests that altimetric velocity probably is contaminated by noise in sea surface height measurement. Also, the k−3 slope, which appears to be in agreement with two-dimensional quasigeostrophic turbulence theory, does not support the contemporary surface quasigeostrophic theory. These results highlight large gaps in the current understanding of the nature of surface geostrophic turbulence

Analysis of Lagrangian Potential Vorticity Balance and Lateral Displacement of Water Parcels in Gulf Stream Meanders

The balance of potential vorticity components following fluid parcel motion in Gulf Stream meanders was studied using RAFOS float data from the SYNOP Experiment. By introducing curvature dependent variations to the velocity and density fields, the authors relaxed the rigid field assumption used in earlier studies and examined closely 61 floats in the upper layers (13°–16°C) of the main thermocline. Float trajectories were segmented according to transition from crest to trough and trough to crest, and grouped by their positions relative to the current center. A total of 154 segments were collected to estimate the horizontal divergence and the mean lateral displacement of parcels under two distinct regimes: growing and decaying meanders

Towards a Lagrangian Description of the Gulf Stream

Downstream velocity relative to the axis of the Gulf Stream is examined through the use of data from SOFAR floats. The speed calculated from the position of the floats along constant pressure surfaces is expressed in terms of a transformed cross-stream coordinate given by temperature, which is telemetered from the floats. The result is a distribution of downstream velocity unaffected by meanders from Cape Hatteras to 46°W. The speed at 700 m is about 75 cm s−1 west of 57°W and decreases sharply to 40 cm s−1 to the east. In the deep water from 1300 to 2200 m, the core speed is 35 cm s−1 between 65° and 50°W, if it is present. The flow in the Gulf Stream may be disturbed by local processes, which are frequently observed in satellite imagery. Examples am shingles, ring formation and meanders. Although SOFAR floats are quasi-Lagrangian (isobaric) devices, the float data can give a Lagrangian description of the Gulf Stream. Above the main thermocline, a current coinciding with the tilting isotherms from Cape Hatteras to 46°W implies that water is efficiently transported downstream. In the deep ocean, water is accelerated by the surface Stream off Cape Hatteras and is at times transported downstream by the deep flow thus formed. The New England Seamounts can block this deep flow. There is little evidence of a deep current and thus, water transport east of the Seamounts

Measuring Mean Velocities with POGO

POGO is a simple technique for measuring water transport between the surface and some preselected depth. Equipped with a 12-kHz pinger for tracking and range measurement, a xenon flasher for nighttime relocation, and a VHF beacon for daytime recovery, it has been used over 200 times in the Gulf Stream to measure volume transport and to provide a reference velocity (transport) for geostrophic calculations from pairs of hydrographic stations. This note gives a brief technical description of POGO and how it is used. Loran C was used for navigation in this study, but with the advent of the Global Positioning System (GPS), POGO can be used worldwide

The Average Distribution of Volume Transport and Potential Vorticity with Temperature at Three Sections across the Gulf Stream

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 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

The Gulf Stream - Barrier or Blender?

The Gulf Stream ’60 hydrographic survey has been used to examine the distribution of water properties across the Gulf Stream as a function of potential density. This survey covered a half million square miles of Slope, Gulf Stream and Sargasso Sea Waters in the western North Atlantic. Quantities plotted as a function of density are acceleration potential, potential temperature, dissolved oxygen and potential vorticity. The transition from Sargasso Sea Water to Slope Water in the upper thermocline (σe \u3c 27.1) is sharp and coincides closely with the dynamical boundary of the Gulf Stream, defined by the gradient of acceleration potential. This indicates that water mass exchanges across the Gulf Stream-Slope Water front are limited at these levels. Below the 27.1 σe surface, the gradient of acceleration potential still reveals the position of the Stream, but there is no coincident water man boundary. This and the uniformity of potential vorticity across the Stream suggest that the deep property fields are being efficiently homogenized by mesoscale exchanges across the Gulf Stream. A cross-frontal eddy diffusivity of K11 =2.5×106 cm2 s−1 estimated from oxygen flux calculations agrees well with previously published values for frontal regimes