T. Rossby

The Cape Cauldron: A regime of turbulent inter-ocean exchange

Combining in-situ Lagrangian intermediate depth velocity measurements from the KAPEX (Cape of Good Hope Experiments) float program with sea-surface height data, this study reviews the inter-ocean exchange mechanisms around southern Africa. In the southeastern Cape Basin, a highly energetic field of coexisting anticyclonic and cyclonic eddies is documented. Agulhas Rings of typically 200 km diameter are observed to merge, split, deform, and to reconnect to the Agulhas Retroflection. Concomitant, slightly smaller cyclones are observed to drift across the northwestward migration path of the Agulhas Rings. These cyclones, with typical diameters of 120 km, are formed within the Cape Basin along the African shelf, inshore of the Agulhas Current, and in the subantarctic region south of Africa. The data suggest the annual formation of 3–6 long-lived Agulhas Rings that eventually cross 5°E longitude, while approximately twice the number of rings occur in the southeastern Cape Basin. Within this region, cyclones outnumber anticyclones by a factor of 3:2. Both cyclones and anticyclones extend through the upper thermocline into the intermediate depth layer. Mean drifts of anticyclones are 3.8±1.2 cm s−1 to the northwest, while cyclones follow a west–southwestward route at 3.6±0.8 cm s−1. Transport estimates suggest that the intermediate depth layer in the southeastern Cape Basin is primarily supplied from the east (approximately 9 Sv), with minor direct inflow from the Atlantic to the west and south. Cyclone/anticyclone interaction is surmised to result in vigorous stirring and mixing processes in the southeastern Cape Basin, which necessitates a review of the traditional concept of Indo-Atlantic inter-ocean exchange. We propose to limit the concept of “isolated Agulhas Rings embedded in a sluggish Benguela Drift” to the northwestern Cape Basin and beyond, while linking this regime to the Agulhas Retroflection proper through a zone of turbulent stirring and mixing in the southeastern Cape Basin, named for the first time the “Cape Cauldron” hereinafter.

Slow variations in mean path of the Gulf Stream east of Cape Hatteras

The Gulf Stream between Cape Hatteras and the New England Seamounts varies in position over a wide range of time scales. We present evidence that interannual path changes may be governed by a time-varying outflow of waters from the Labrador Shelf region. When the shelf and Slope Waters south of New England turn cold and fresh, the stream frequently assumes a more southerly track and conversely when the waters are warm and saline. These changes in path and surface water properties most likely result from large variations in shelf transport and thus greater or less dilution of the warm saline Gulf Stream waters that continually leak from the current into the Slope Sea. These observations point to a thermohaline rather than winddriven mechanism for governing the path of the Gulf Stream after it leaves the coast.

Path and variability of the Agulhas Return Current

The combined analysis of hydrographic, kinematic, and dynamic data collected during the 1997–1999 KAPEX (CAPe of Good Hope EXperiments) reveals a quasi-stationary meandering pattern of the Agulhas Retroflection Current east and upstream of the Southwest-Indian Ridge. The current meanders between 38°S and 40°S in a spatially and temporally continuous fashion and has a core width of approximately Full-size image (<1 K) with an associated transport of Full-size image (<1 K) in the upper Full-size image (<1 K). Peak surface velocities decrease from Full-size image (<1 K) near the Agulhas Retroflection to Full-size image (<1 K) around 32°E. Meander troughs (northward extremes) are found predominantly near 26.8°E, 32.6°E and 38.9°E, while crests (southward extremes) are located with high probability near 29.7°E, 35.5°E and 42.9°E, resulting in a typical wavelength of Full-size image (<1 K). Cold eddies are shed along the northern boundary of the current from meander troughs into the recirculation regime between the Agulhas Current proper and the Agulhas Return Current. Strongest cyclonic eddies are preferably shed in austral autumn. The cyclonic eddies so formed propagate westward at an average phase-speed of Full-size image (<1 K), with, however, a variability of at least the same magnitude. Subsequently, the cyclones are absorbed by the next meander trough located upstream and to the west of the shedding trough.

The Oleander Project: Monitoring the Variability of the Gulf Stream and Adjacent Waters between New Jersey and Bermuda

Abstract 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, def...

Pathways of inflow and dispersion of warm waters in the Nordic seas

[1] In this study, we use 22 acoustically tracked RAFOS floats to examine the routes and spreading of warm North Atlantic waters entering the Norwegian Sea between Iceland and the Faroes. The majority of floats crossed the Iceland-Faroe Ridge at the eastern end where it is deepest. They joined the Iceland-Faroe Front, but rather than continue north with the outer branch of the Norwegian Atlantic Current into the Nordic seas, most of them jumped over to the inner branch, which continues the inflow through the Faroe-Shetland Channel northeast over the Voring Plateau toward the Lofoten Basin. Indeed, 17 floats, whether deployed near Iceland or the Faroes, did so; only 2 floats continued north along the outer branch. Despite the small numbers, these results highlight (1) the strong influence of topography on flow patterns, (2) the strong crossover of Iceland-Faroes waters to the inner branch, and (3) the rapid and structured spreading into the Nordic seas.

On gyre interactions

Abstract The principal meeting point of the subtropical and subpolar gyres of the North Atlantic is at the Tail of the Grand Banks where the two western boundary currents, the Gulf Stream and Labrador Current, join forces as the North Atlantic Current, which flows northeast almost 10° in latitude before turning east as the Subpolar Front, ultimately feeding the Labrador and Nordic Seas and the thermohaline overturning. After the Gulf Stream turns into the North Atlantic Current at the Grand Banks, its role shifts from a wind-driven current to a link in the large-scale thermohaline circulation. The processes governing this transition, in particular the continued transport north of mass and heat, are questions of considerable climatic importance. The North Atlantic Current is a very unusual western boundary current in that its mass transport decreases in the downstream direction. The mean path and annual shifting of the eastward flowing Gulf Stream is conjectured to result from a time-varying shelf-Slope Water overflow of waters from the Labrador shelf. As the volume transport increases in fall and deepens the Slope Water pycnocline, it forces the Gulf Stream south and deepens the Sargasso Sea thermocline as well. The timing of these steps governs the June maximum in baroclinic transport. There is some evidence that this ‘back-door’ gyre interaction may operate on interannual time scales as well. The question then arises whether the shelf-to-Slope Water Sea transport also plays a role in governing the separation of the Gulf Stream. The widely observed robustness of the width of the Gulf Stream appears to result from a tight balance between the release of available potential energy and the kinetic energy of the current. A broader current would release more energy than can be ‘disposed of’, while a narrower current requires more kinetic energy than is available to sustain it. It is shown that for plausible dissipation rates in the recirculation gyres, the amount of energy that needs to be expelled from the Gulf Stream is such a small fraction of that advected through as to be vitually undetectable, hence the stiffness of the current.

Seasonal and low frequency variations in dynamic height anomaly and transport of the Gulf Stream

Abstract One hundred and thirty pairs of hydrographic stations bracketing the Gulf Stream between Cape Hatteras and the New England Seamounts are used to study seasonal and low frequency changes in dynamic height and baroclinic transport. The data span a 56-year period from 1932 to 1988. The stations were selected from hydrographic sections as representative of conditions immediately outside the current on the Slope Water and Sargasso Sea sides. It is found that the dynamic height anomaly is maximum in the fall and it is almost entirely governed by the annual heating and cooling cycle. The baroclinic transport is calculated from the potential energy anomaly difference between stations located just outside of the northern and southern edges of the Gulf Stream. The annual cycle in the 0–2000 dbar baroclinic transport is maximum in early summer, with a peak to peak range of (8 ± 3) Sv. Low frequency changes in transport, after removing the annual cycle, arc difficult to discern with confidence due to eddy variability. However, there is evidence of a 6 Sv decrease in transport between the late 1950s and early 1970s.

Operating an Acoustic Doppler Current Profiler aboard a Container Vessel

Abstract 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 extr...

Seasonal and Low-Frequency Variability of the Meridional Heat Flux at 36°N in the North Atlantic

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 ±...

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

Given the Gulf Stream’s central role in the North Atlantic’s 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’s. 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.