Lisa M. Beal

On the role of the Agulhas system in ocean circulation and climate

The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.

Continuous, Array-Based Estimates of Atlantic Ocean Heat Transport at 26.5°N

Continuous estimates of the oceanic meridional heat transport in the Atlantic are derived from the Rapid Climate Change–Meridional Overturning Circulation (MOC) and Heatflux Array (RAPID–MOCHA) observing system deployed along 26.5°N, for the period from April 2004 to October 2007. The basinwide meridional heat transport (MHT) is derived by combining temperature transports (relative to a common reference) from 1) the Gulf Stream in the Straits of Florida; 2) the western boundary region offshore of Abaco, Bahamas; 3) the Ekman layer [derived from Quick Scatterometer (QuikSCAT) wind stresses]; and 4) the interior ocean monitored by “endpoint” dynamic height moorings. The interior eddy heat transport arising from spatial covariance of the velocity and temperature fields is estimated independently from repeat hydrographic and expendable bathythermograph (XBT) sections and can also be approximated by the array. The results for the 3.5 yr of data thus far available show a mean MHT of 1.33 ± 0.40 PW for 10-day-averaged estimates, on which time scale a basinwide mass balance can be reasonably assumed. The associated MOC strength and variability is 18.5 ± 4.9 Sv (1 Sv ≡ 106 m3 s−1). The continuous heat transport estimates range from a minimum of 0.2 to a maximum of 2.5 PW, with approximately half of the variance caused by Ekman transport changes and half caused by changes in the geostrophic circulation. The data suggest a seasonal cycle of the MHT with a maximum in summer (July–September) and minimum in late winter (March–April), with an annual range of 0.6 PW. A breakdown of the MHT into “overturning” and “gyre” components shows that the overturning component carries 88% of the total heat transport. The overall uncertainty of the annual mean MHT for the 3.5-yr record is 0.14 PW or about 10% of the mean value.

Spreading of Red Sea overflow waters in the Indian Ocean

As a result of its remarkably high salinity and despite its small volume input, remnants of Red Sea Water (RSW) have been identified in the Agulhas Current, over 6000 km distant of their source. This provided the motivation to investigate the long-term mean spreading of RSW throughout the Indian Ocean, using a comprehensive set of observations, taken from the National Oceanographic Data Center archives and from the World Ocean Circulation Experiment Hydrographic Program for the Indian Ocean. After emerging from the Gulf of Aden into the Arabian Sea, RSW spreads predominantly southwestward along the African continental slope, as indicated by strongly inclined isohalines across the Arabian Sea. There is some monsoon variability, so that during the winter monsoon there is more RSW present in the Gulf of Aden and an intensification of southward spreading along the western boundary, between 12° and 5°N. Elsewhere the intermediate depth salinity field of the Indian Ocean appears relatively stationary. Between 5° and 10°S, in the region of the South Equatorial Current, isohalines of the RSW layer become quasi-zonal across the width of the Indian Ocean, only dipping southward toward the western boundary west of 50°E. South of here, there is a strong tongue of RSW spreading southward through the Mozambique Channel and into the Agulhas Current. These conclusions concur with previous localized investigations of intermediate water properties. Using a simple mixing model, the percentage of RSW throughout the Indian Ocean was quantified. It was found that the flux of salt into the Gulf of Aden from the Red Sea is similar to that estimated to cross 32°S in the Agulhas Current. This result implies that all the RSW which is mixed into the interior of the Indian Ocean may eventually be exported at the western boundary. Furthermore, it implies that RSW is the dominant component of the salt budget for the intermediate layer and that input from the Indonesian Seas and via diapycnic processes are small.

Observations of an Agulhas Undercurrent

An Agulhas Undercurrent has been directly measured on two occasions using a new acoustic current profiling technique (Lowered Acoustic Doppler Current Profiler, LADCP). The core of the Undercurrent is centred around a depth of 1200 m, against the continental slope and directly below the surface core of the south-westward flowing Agulhas Current. Maximum velocities of 30 cm s−1 to the north-east are observed in the Undercurrent, and its volume transport is 6 Sv (1 Sv = 106 m3 s−1), approximately one-tenth that of the Agulhas Current.

Role of the Agulhas Current in Indian Ocean circulation and associated heat and freshwater fluxes

A reduced estimate of Agulhas Current transport provides the motivation to examine the sensitivity of Indian Ocean circulation and meridional heat transport to the strength of the western boundary current. The new transport estimate is 70 Sv, much smaller than the previous value of 85 Sv. Consideration of three case studies for a large, medium and small Agulhas Current transport demonstrate that the divergence of heat transport over the Indian Ocean north of 32°S has a sensitivity of 0.08 PW per 10 Sv of Agulhas transport, and freshwater convergence has a sensitivity of 0.03×109 kg s−1 per 10 Sv of transport. Moreover, a smaller Agulhas Current leads to a better silica balance and a smaller meridional overturning circulation for the Indian Ocean. The mean Agulhas Current transport estimated from time-series current meter measurements is used to constrain the geostrophic transport in the western boundary region in order to re-evaluate the circulation, heat and freshwater transports across 32°S. The Indonesian Throughflow is taken to be 12 Sv at an average temperature of 18°C. The constrained circulation exhibits a vertical–meridional circulation with a net northward flow below 2000 dbar of 10.1 Sv. The heat transport divergence is estimated to be 0.66 PW, the freshwater convergence to be 0.54×109 kg s−1, and the silica convergence to be 335 kmol s−1. Meridional transports are separated into barotropic, baroclinic and horizontal components, with each component conserving mass. The barotropic component is strongly dependent on the estimated size of the Indonesian Throughflow. Surprisingly, the baroclinic component depends principally on the large-scale density distribution and is nearly invariant to the size of the overturning circulation. The horizontal heat and freshwater flux components are strongly influenced by the size of the Agulhas Current because it is warmer and saltier than the mid-ocean. The horizontal fluxes of heat and salt penetrate down to 1500 m depth, suggesting that warm and salty Red Sea Water may be involved in converting the intermediate and upper deep waters which enter the Indian Ocean from the Southern Ocean into warmer and saltier waters before they exit in the Agulhas Current.

The Sources and Mixing Characteristics of the Agulhas Current

Recent observations taken at four principal latitudes in the Agulhas Current show that the watermass properties on either side of its dynamical core are significantly different. Inshore of its velocity core are found waters of predominantly Arabian Sea, Red Sea, and equatorial Indian Ocean origin, while offshore waters are generally from the Atlantic Ocean, the Southern Ocean, and the southeast Indian Ocean. For the most part, the inshore waters approach the Agulhas Current through the Mozambique Channel, while those offshore are circulated within the southern Indian Ocean subtropical gyre before joining the current. These disparate water masses remain distinct during their 1000-km journeys along the South African continental slope, despite the convergence, extreme velocity shears, and high eddy kinetic energies found within the Agulhas Current. Both potential vorticity conservation and kinematic arguments are discussed as potential inhibitors of along-isopycnal mixing. It is concluded that a high cross-stream gradient of potential vorticity is the dominant mechanism for watermass separation near the surface, while the kinematic steering of water particles by the current is dominant at intermediate depths, where cross-stream potential vorticity is homogeneous. Hence, three lateral mixing regimes for the Agulhas Current are suggested. The surface and thermocline waters are always inhibited from mixing, by the presence of both a strong, cross-frontal potential vorticity gradient and kinematic steering. At intermediate depths mixing is inhibited by steering alone, and thus in this regime periodic mixing is expected during meander events (such as Natal pulses), when the steering level will rise and allow cross-frontal exchange. Below the steering level in the deep waters, there is a regime of free lateral mixing. The deep waters of the Agulhas Current are homogeneous in the cross-stream sense, being from the same North Atlantic source, and their salinity steadily (and rather rapidly) decreases to the north. Here, it is suggested that mixing must be dominated by vertical processes and a large vertical mixing coefficient of order 10 cm 2 s 1 is estimated.

Variability and coherence of the Agulhas Undercurrent in a High-resolution Ocean General Circulation Model

Abstract The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50–70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north.

Observations of the Florida and Yucatan Currents from a Caribbean Cruise Ship

Abstract The Yucatan and Florida Currents represent the majority of the warm-water return path of the global thermohaline circulation through the tropical/subtropical North Atlantic Ocean. Their transports are quantified and compared by analyzing velocity data collected aboard the cruise ship Explorer of the Seas. From 157 crossings between May 2001 and May 2006, the mean transport of the Florida Current at 26°N was estimated to be 30.8 ± 3.2 Sv (1 Sv ≡ 106 m3 s−1), with seasonal amplitude of 2.9 Sv. Upstream, the Yucatan Current was estimated from 90 crossings to be 30.3 ± 5 Sv, with seasonal amplitude of 2.7 Sv. These two currents are shown to be linked at seasonal time scales. Hence, contrary to former results, it was found that transports through the Florida Straits and the Yucatan Channel are similar, with the implication that only small inflows occur through minor channels between them.

Comparison of three velocity sections of the Agulhas Current and Agulhas Undercurrent

Lowered acoustic Doppler current profiles (LADCP) from an early March 1995 cruise across the Agulhas Current show a swift, narrow undercurrent flowing northeast along the continental slope. Neither this Agulhas Undercurrent nor the adjacent deep extension of the Agulhas Current are evident from measurements of water properties alone, and their absence from the conventional referencing of geostrophic current estimates biases net southward transport estimates high by several sverdrups. Here we refine the original transport calculation by removing barotropic tides and by estimating instrumental and sampling errors. Two additional LADCP sections, from cruises in late March and June 1995, also show the undercurrent and the deep extension of the Agulhas. Differences in the current structure are evident. The Agulhas Current extends throughout the water column in March, but extends only to 2300 m depth in June. Additionally, the current extends further offshore in March. Of the three available LADCP sections, only those from early March and June have sufficient sampling to calculate the net southward transport of the Agulhas Current and Undercurrent. The two estimates, 78±3 and 76±2 Sv, are nearly identical. Consideration of water properties on density surfaces shows that although the undercurrent carries intermediate water with Red Sea Water influence northward, the bulk of this water mass is flowing southward, away from its source, in the Agulhas Current.

The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing

AbstractTwo decades of drifter and satellite data allow the authors to describe the seasonal evolution of the surface circulation of the Arabian Sea, which reverses annually with the Indian monsoon winds. This study finds several features that advance current understanding. Most significantly, northward flow appears along the length of the western boundary, together with a weak anticyclone at 6°N (a precursor to the Great Whirl) as early as March or April, one or two months before the southwest monsoon winds. This circulation is driven by planetary waves, which are initiated by wind curl forcing during the previous southwest monsoon, leading the authors to speculate that there is an oceanic mechanism through which one monsoon may precondition the next. Second, the authors find that the eastward South Equatorial Counter Current (SECC) is present year-round, fed by the northward East African Coastal Current (EACC). During the southwest monsoon the EACC overshoots the equator and splits, feeding both northwa...