Abstract

This paper is divided into two parts: the first describing an enhancement of the shipborne ADCP system and the second describing information about bottom currents that have been obtained therefrom on WOCE cruise A11 in the South Atlantic, Punta Arenas to Cape Town.

RRS Discovery has been fitted with a GPS3DF receiver and an array of antennas about its bridge, which determine the instantaneous attitude of the ship. The heading record from this instrument has been compared with the ship's gyrocompass throughout the 40-day cruise. Differences have been calculated and examined according to heading, speed, and movements. Underway differences are found insensitive to heading except in a southerly sector, but on station, differences exhibit a sinusoidal dependence on heading with range 3°. Steaming from a station led to transient behavior with recognizable effects for 1 hour. The results are generally consistent with earlier reports and are attributed to gyro error.

An improved heading measurement has a marked effect on currents determined from the ADCP when under way. The cruise data from this instrument has been reprocessed, replacing the observations of the gyrocompass with those from the GPS3DF unit. We describe and implement the procedure for dealing with two difficulties, namely, gaps in the data and misalignment of the two pieces of equipment. An estimate of the current error from known causes is found to be only 1 cm s⁻¹ for 4-h steaming or station averages.

A comparison of the ADCP cross-track component with geostrophic estimates referenced to the bottom for 91 stations shows excellent overall agreement, both for underway and station-pair measurements. Differences are principally due to transient ageostrophic currents, tides, and inertial oscillations, which are estimated at about 5 cm s⁻¹. Two rings in the Cape Basin and an intense vortex in the Argentine Basin exhibit significant centripetal contributions to the measured flow; when they are removed, the comparisons with geostrophy improve.

Differences, or cross-track bottom currents, are significantly above noise only in the Argentine Basin; three regions are identified there. In the Falkland Current strong barotropic components (16 to 40 cm s⁻¹ northward) augment the baroclinic transport fourfold to 50 Sv (Sv &equiv 10⁶ m³ s⁻¹), in accordance with the arguments of Peterson and the predictions of a numerical ocean model (FRAM). It follows that this will have a large impact on heat, salt, and tracer fluxes across the section, though these are not discussed here. The second region of significant bottom flow is in the confluence zone (Brazil and Falkland Current Extension), where intense surface circulations penetrate in attenuated form (+12 to −18 cm s⁻¹) to the seabed at 6000-m depth. Extremely turbid bottom mixed layers were detected there with transmittance values of 10% m⁻¹, verifying the existence of strong currents. The third region of significant bottom flows is contiguous with and east of the confluence zone, where a broad region of southerly flow is succeeded by a similar region of northerly flow (−15 to +8 cm s⁻¹). These lie over a shallow abyssal ridge, with axis along the cruise track (the Zapiola Drift), which exhibited mud waves throughout and was draped with a turbid bottom layer. An anticyclonic flow about the Zapiola Drift has been inferred from the morphology of the mud waves seen on its flanks, and this circulation is shown to be contemporary by two current meter records from the region. Our measurements appear to confirm this circulation and suggest that the strength exceeds 100 Sv. Is this a recirculation such as seen near the Gulf Stream extension? No evidence for it is found in numerical models and further observations of its existence are needed. The authors believe that the bottom currents over the deepest parts of the Argentine Basin are strong over a wide area.

The value of combining the GPS3DF and ADCP observations in order to identify regions of strong bottom flows is demonstrated.

Abstract

The most southerly WOCE one-time section in the South Atlantic, designated A11, was occupied in January 1993. The cruise track lay across the cool subantarctic zone of the Circumpolar Current in the west and the warm subtropical gyre in the east. In this paper estimates of the flux of heat, salt, oxygen, and other tracers across the section are presented. A brief description of the distribution of physical and chemical properties is followed by a determination of the flux in the surface Ekman layer. Direct measurements of current shear made from a shipborne ADCP and estimates from cruise wind data and climatological data all yield a flux of 5 ± 1 Sv (Sv = 10⁶ m³ s⁻¹) equatorward. When combined with geostrophic estimates relative to a level near 3500 dbar, or the bottom where shallower, an initial guess for the flow field is derived.

This initial guess is combined with absolute currents derived from the ADCP when the ship is underway. Unrealistic aspects of the circulation are found, and we conclude that the ageostrophic “noise” of the ADCP measurements is to blame. A second and preferred flow field is derived using an inverse analysis supplemented by a few estimates of total current transports. Climatological transports in the Falkland Current are estimated at about 45 Sv equatorward, while on the eastern margin a poleward transport of about 5 Sv is assumed. The equatorward flux of bottom water is taken from a recent determination to be 6 Sv.

The heat flux estimate across the section is found to be robust and to have twice the value reported recently. The flux is equatorward with magnitude of 0.5 ± 0.1 PW and occurs entirely within the subtropical gyre. A net equatorward transport or 1 0 Sv of upper thermocline water, which takes place near the eastern margin, has also been inferred. This pair of estimates identifies the warm water path as the route for the upper-ocean replacement of NADW. The flux of salt, for zero volume transport across the section, is found to be 0 ± 5 Sv psu (1 Sv psu ≈ 10⁶ kg s⁻¹) and when interpreted as a freshwater flux is quite at variance with estimates based on the global freshwater balance. Oxygen and other nutrients have fluxes generally differing in sign from previous determinations. Sensitivity of the results to assumptions made is considered.

Abstract

A box inverse of the World Ocean Circulation Experiment A10 (30°S) and A11 (nominally 45°S) sections in the South Atlantic Ocean was undertaken. The authors find a heat flux across A10 of 0.22 ± 0.08 PW, consistent with previous studies, and a heat flux of 0.43 ± 0.08 PW across A11. The A11 heat flux is lower than some previous analyses of this section but implies a plausible oceanic heat convergence (heat loss to the atmosphere) of 0.21 ± 0.10 PW. The difference is principally due to adding a cyclonic component to the circulation in the Cape Basin. As compared with the solution of other studies, the anticyclonic circulation in the surface and intermediate water of the subtropical gyre is weakened. The circulation of the deep water is cyclonic rather than anticyclonic; this is in better agreement with previously published circulation schemes based on examination of water properties. A southward freshwater flux of 0.7 Sv (1 Sv ≡ 10⁶ m³ s⁻¹) at A11, consistent with previous inverse studies, is still inconsistent with the net Atlantic evaporation inferred from integrated surface climatologies. Results suggest a small gain of freshwater (0.2 ± 0.1 Sv) between the sections.

Abstract

The early twenty-first century’s warming trend of the full-depth global ocean is calculated by combining the analysis of Argo (top 2000 m) and repeat hydrography into a blended full-depth observing system. The surface-to-bottom temperature change over the last decade of sustained observation is equivalent to a heat uptake of 0.71 ± 0.09 W m⁻² applied over the surface of Earth, 90% of it being found above 2000-m depth. The authors decompose the temperature trend pointwise into changes in isopycnal depth (heave) and temperature changes along an isopycnal (spiciness) to describe the mechanisms controlling the variability. The heave component dominates the global heat content increase, with the largest trends found in the Southern Hemisphere’s extratropics (0–2000 m) highlighting a volumetric increase of subtropical mode waters. Significant heave-related warming is also found in the deep North Atlantic and Southern Oceans (2000–4000 m), reflecting a potential decrease in deep water mass renewal rates. The spiciness component shows its strongest contribution at intermediate levels (700–2000 m), with striking localized warming signals in regions of intense vertical mixing (North Atlantic and Southern Oceans). Finally, the agreement between the independent Argo and repeat hydrography temperature changes at 2000 m provides an overall good confidence in the blended heat content evaluation on global and ocean scales but also highlights basin-scale discrepancies between the two independent estimates. Those mismatches are largest in those basins with the largest heave signature (Southern Ocean) and reflect both the temporal and spatial sparseness of the hydrography sampling.

Abstract

Subsurface ocean currents can be estimated from the positions of drifting profiling floats that are being widely deployed for the international Argo program. The calculation of subsurface velocity depends on how the trajectory of the float while on the surface is treated. The following three aspects of the calculation of drift velocities are addressed: the accurate determination of surfacing and dive times, a new method for extrapolating surface and dive positions from the set of discrete Argos position fixes, and a discussion of the errors in the method. In the new method described herein, the mean drift velocity and the phase and amplitude of inertial motions are derived explicitly from a least squares fit to the set of Argos position fixes for each surface cycle separately. The new method differs from previous methods that include prior assumptions about the statistics of inertial motions. It is concluded that the endpoints of the subsurface trajectory can be estimated with accuracy better than 1.7 km (East Sea/Sea of Japan) and 0.8 km (Indian Ocean). All errors, combined with the error that results from geostrophic shear and extrapolation, should result in individual subsurface velocity estimates with uncertainty of the order of 0.2 cm s⁻¹.

Abstract

A time series of the physical and biogeochemical properties of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the Drake Passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage. Both water masses have experienced substantial variability on interannual to interdecadal time scales. SAMW is formed by winter overturning on the equatorward flank of the Antarctic Circumpolar Current (ACC) in and to the west of the Drake Passage. Its interannual variability is primarily driven by variations in wintertime air–sea turbulent heat fluxes and net evaporation modulated by the El Niño–Southern Oscillation (ENSO). Despite their spatial proximity, the AAIW in the Drake Passage has a very different source than that of the SAMW because it is ventilated by the northward subduction of Winter Water originating in the Bellingshausen Sea. Changes in AAIW are mainly forced by variability in Winter Water properties resulting from fluctuations in wintertime air–sea turbulent heat fluxes and spring sea ice melting, both of which are linked to predominantly ENSO-driven variations in the intensity of meridional winds to the west of the Antarctic Peninsula. A prominent exception to the prevalent modes of SAMW and AAIW formation occurred in 1998, when strong wind forcing associated with constructive interference between ENSO and the southern annular mode (SAM) triggered a transitory shift to an Ekman-dominated mode of SAMW ventilation and a 1–2-yr shutdown of AAIW production.

The interdecadal evolutions of SAMW and AAIW in the Drake Passage are distinct and driven by different processes. SAMW warmed (by ∼0.3°C) and salinified (by ∼0.04) during the 1970s and experienced the reverse trends between 1990 and 2005, when the coldest and freshest SAMW on record was observed. In contrast, AAIW underwent a net freshening (by ∼0.05) between the 1970s and the twenty-first century. Although the reversing changes in SAMW were chiefly forced by a ∼30-yr oscillation in regional air–sea turbulent heat fluxes and precipitation associated with the interdecadal Pacific oscillation, with a SAM-driven intensification of the Ekman supply of Antarctic surface waters from the south contributing significantly too, the freshening of AAIW was linked to the extreme climate change that occurred to the west of the Antarctic Peninsula in recent decades. There, a freshening of the Winter Water ventilating AAIW was brought about by increased precipitation and a retreat of the winter sea ice edge, which were seemingly forced by an interdecadal trend in the SAM and regional positive feedbacks in the air–sea ice coupled climate system. All in all, these findings highlight the role of the major modes of Southern Hemisphere climate variability in driving the evolution of SAMW and AAIW in the Drake Passage region and the wider South Atlantic and suggest that these modes may have contributed significantly to the hemispheric-scale changes undergone by those waters in recent decades.

Abstract

In May and June 2005, a transatlantic hydrographic section along 36°N was occupied. A velocity field is calculated using inverse methods. The derived 36°N circulation has an overturning transport (maximum in the overturning streamfunction) of 16.6 Sv (1 Sv ≡ 10⁶ m³ s⁻¹) at 1070 m. The heat transport across the section, 1.14 ± 0.12 PW, is partitioned into overturning and horizontal heat transports of 0.75 and 0.39 PW, respectively. The horizontal heat flux is set by variability at the gyre rather than by mesoscale. The freshwater flux across the section is 1.55 ± 0.18 Sv southward based on a 0.8-Sv flow from the Pacific through the Bering Strait at a salinity of 32.5 psu. The oceanic divergence of freshwater implies a net input of freshwater to the ocean of 0.75 Sv over the North Atlantic and Arctic between 36°N and the Bering Strait. Most (85%) of the recently ventilated upper North Atlantic Deep Water (water originating in the Labrador Sea) transport across the section occurs in the deep western boundary current rather than being associated with an interior pathway to the west of the mid-Atlantic ridge.

Abstract

A significant change in properties of the thermocline is observed across the whole Indian Ocean 32°S section between 1987 and 2002. This change represents a reversal of the pre-1987 freshening and decreasing oxygen concentrations of the upper thermocline that had been interpreted as a fingerprint of anthropogenic climate change. The thermocline at the western end of the section (40°–70°E) is occupied by a single variety of mode water with a potential temperature of around 13°C. The thermocline at the eastern end of the 32°S section is occupied by mode waters with a range of properties cooling from ∼11°C at 80°E to ∼9°C near the Australian coast. The change in θ–S properties between 1987 and 2002 is zonally coherent east of 80°E, with a maximum change on isopycnals at 11.6°C. Ages derived from helium–tritium data imply that the mode waters at all longitudes take about the same time to reach 32°S from their respective ventilation sites. Dissolved oxygen concentration changes imply that all of the mode water reached the section ∼20% faster in 2002 than in 1987.

Abstract

Northward ocean heat transport at 26°N in the Atlantic Ocean has been measured since 2004. The ocean heat transport is large—approximately 1.25 PW, and on interannual time scales it exhibits surprisingly large temporal variability. There has been a long-term reduction in ocean heat transport of 0.17 PW from 1.32 PW before 2009 to 1.15 PW after 2009 (2009–16) on an annual average basis associated with a 2.5-Sv (1 Sv ≡ 10⁶ m³ s⁻¹) drop in the Atlantic meridional overturning circulation (AMOC). The reduction in the AMOC has cooled and freshened the upper ocean north of 26°N over an area following the offshore edge of the Gulf Stream/North Atlantic Current from the Bahamas to Iceland. Cooling peaks south of Iceland where surface temperatures are as much as 2°C cooler in 2016 than they were in 2008. Heat uptake by the atmosphere appears to have been affected particularly along the path of the North Atlantic Current. For the reduction in ocean heat transport, changes in ocean heat content account for about one-quarter of the long-term reduction in ocean heat transport while reduced heat uptake by the atmosphere appears to account for the remainder of the change in ocean heat transport.