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Lisa M. Beal

Abstract

Recent observations taken at a number of latitudes in the Agulhas Current reveal that the water mass structure on either side of its dynamical core is distinctly different. Moreover, interleaving of these distinct water masses is observed at over 80% of the stations occupied in the current, particularly within the subsurface density layer between tropical surface water and subtropical surface water masses, and within the intermediate layer between the Antarctic Intermediate Water and Red Sea water masses. Direct velocity measurements allow for a comparison between the characteristic vertical length scales of the Agulhas intrusions and those of velocity perturbations found throughout the current. It is found that the interleaving scales match those of the velocity perturbations, which are manifest as high-wavenumber vertical shear layers and are identified as near-inertial oscillations. Furthermore, the properties of the intrusions indicate that double diffusion is not an important process in their development: they are generally not associated with a density anomaly, their slope and thickness fall outside the predicted maxima for instability, and a strong horizontal shear field acts to separate water parcels more quickly than intrusions would be able to grow by double-diffusive processes. Instead, the position, thickness, and slope of Agulhas intrusions relative to the background salinity and density field suggest that they are forced by rotating inertial velocities, with subsequent growth possibly driven by small-scale baroclinic instabilities. However, not all the evidence points conclusively toward advectively driven intrusions. For instance, there is a discrepancy between the observed salinity anomaly amplitude and the predicted inertial displacement given the background salinity gradient, which deserves further examination. Hence, there is a future need for more pointed observations and perhaps the development of an analytical or numerical model to understand the exact nature of Agulhas intrusions.

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Lisa M. Beal

Abstract

A 550-day record of Agulhas Undercurrent transport between March 2003 and August 2004 is constructed from five deep moorings placed on the continental shelf off South Africa at nominally 32°S. The vertical and lateral scales of the undercurrent are estimated to be 2000 m and 40 km, respectively, using the average of seven direct velocity sections, predominantly taken in austral autumn over a 10-yr period from 1995 to 2005. Peak speeds in the undercurrent are some of the greatest ever seen at depth: over 90 cm s−1 at 1400 m. The undercurrent has a transport of 4.2 ± 5.2 Sv (1 Sv = 1 × 106 m3 s−1), in close agreement with a previous estimate from a single current meter record during 1995 of 4.2 ± 2.9 Sv. Records below 1800 m, within the North Atlantic Deep Water (NADW) layer of the undercurrent, show a higher level of coherence and less variance than those at intermediate depths. On average, 2.3 ± 3.0 Sv of NADW is carried northeastward within the undercurrent, an amount similar to that estimated previously by analyses of deep water mass characteristics around South Africa.

Short-term variability in the undercurrent peaks at the semidiurnal period, at a local shear-adjusted inertial period (21.6 h), and at 4.5, 6.5, and 9.5 days. The latter may be associated with topographic Rossby waves, although no evidence for enhanced onshore velocities was found at these periods. The variability of the undercurrent is highly topographically controlled, strongly aligned in the along-stream direction, with significant variance in cross-stream velocity only at 2-day periods or less and isotropic variance only at the (effective) inertial period. For the mesoscale, the deeper current meters within the NADW layer all exhibit broad peaks at 50–60 days, which matches the periodicity of solitary meanders of the Agulhas Current (so-called Natal pulses) presented previously in the literature. The results of this study demonstrate that these meanders are highly barotropic in nature.

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Shane Elipot and Lisa M. Beal

Abstract

Of the interannual variance of the Agulhas Current transport, 29% can be linearly related to six modes of Southern Hemisphere atmospheric variability. Agulhas Current transport is quantified by a 24-yr proxy constructed using satellite altimetry and in situ data, while atmospheric variability is represented by two reanalysis products. The two leading modes of atmospheric variability, each explaining 5% of the variance of the Agulhas Current, can be described as a tropical Indo-Pacific mode, strongly correlated to ENSO, and a subtropical–subpolar mode, strongly correlated with the SAM. ENSO alone can explain 11.5% of Agulhas transport variance, yet SAM alone has no significant correlation. The remaining four atmospheric modes are not related to common climate indices and together they explain 19% of Agulhas variance, describing decadal oscillations. In previous studies using reanalyses and climate models it has been suggested that the Agulhas Current is intensifying in response to a strengthening and poleward shift of the westerlies, expressed by a positive trend in the SAM. Here, the authors find that, given its apparent weak sensitivity to the SAM, the increase in SAM over the past 24 years does not lead to a detectable trend in Agulhas Current transport.

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Shane Elipot and Lisa M. Beal

Abstract

The Agulhas Current intermittently undergoes dramatic offshore excursions from its mean path because of the downstream passage of mesoscale solitary meanders or Natal pulses. New observations and analyses are presented of the variability of the current and its meanders using mooring observations from the Agulhas Current Time-Series Experiment (ACT) near 34°S. Using a new rotary EOF method, mesoscale meanders and smaller-scale meanders are differentiated and each captured in a single mode of variance. During mesoscale meanders, an onshore cyclonic circulation and an offshore anticyclonic circulation act together to displace the jet offshore, leading to sudden and strong positive conversion of kinetic energy from the mean flow to the meander via nonlinear interactions. Smaller meanders are principally represented by a single cyclonic circulation spanning the entire jet that acts to displace the jet without extracting kinetic energy from the mean flow. Synthesizing in situ observations with altimeter data leads to an account of the number of mesoscale meanders at 34°S: 1.6 yr−1 on average, in agreement with a recent analysis by and significantly less than previously understood. The links between meanders and the arrival of Mozambique Channel eddies or Madagascar dipoles at the western boundary upstream are found to be robust in the 20-yr altimeter record. Yet, only a small fraction of anomalies arriving at the western boundary result in meanders, and of those, two-thirds can be related to ring shedding. Most Agulhas rings are shed independently of meanders.

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Clément Rousset and Lisa M. Beal

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.

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Lisa M. Beal, Shane Elipot, Adam Houk, and Greta M. Leber

Abstract

The volume transport of the Agulhas Current was measured over a 3-yr period by an array of seven current meter moorings and four current- and pressure-recording inverted echo sounders (CPIES) deployed at 34°S. CPIES extended the array farther offshore in order to capture, for the first time, the full Agulhas Current during meander events. Transports derived from CPIES are well correlated with overlapping current meter transports (0.89). The Eulerian mean current is 219 km wide and 3000 m deep, with peak surface speeds of 1.8 m s−1 and a weak northward undercurrent on the continental slope below 1200 m. A new algorithm to capture the western boundary jet transport at each time step T is defined as the poleward transport out to the first maximum of the vertically integrated velocity beyond the half-width of the mean jet. The mean transport of the Agulhas Current jet, so defined, is −84 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) with a standard error of 2 Sv. Sampling and instrumental errors are explicitly estimated and amount to an additional 9 Sv. A more traditional estimate, based on net transport integrated to a fixed distance offshore T box, gives a mean transport of −77 ± 5 Sv. This transport is 10 Sv greater than an equivalent transport at 32°S, corresponding to a latitudinal increase equal to that predicted by Sverdrup dynamics. The time series of T and T box show important differences during solitary meander events and at longer time scales. In terms of an annual cycle, the Agulhas Current appears strongest during austral summer, a similar phase to the Gulf Stream and Kuroshio.

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Greta M. Leber, Lisa M. Beal, and Shane Elipot

Abstract

Strong upwelling events inshore of the Agulhas Current close to 33.5°S are investigated. These events are important to the exchange of shelf and slope waters, potentially enhancing primary productivity and advecting larvae offshore. Using hydrographic observations, this study shows that a wind-driven upwelling event and a current-driven upwelling event can each advect central waters more than 130 m upward, resulting in a maximum 9°C cooling at 50-m depth over the continental shelf and surface cooling greater than 4°C. The authors use satellite data to assess the frequency and forcing mechanisms of similar cold events from January 2003 through December 2011, defining cold events as days when the sea surface temperature (SST) anomaly is significantly correlated with a local current or wind forcing. The authors identify 47 events with an average length of 2.2 days and SST anomaly of −1.6°C, corresponding to an average 13 days of surface cold events along the Agulhas Current front per year. This study uses combined EOF analysis to characterize these cold events based on four highly correlated forcing mechanisms: alongshore wind speed, wind stress curl, current meandering, and current speed over the slope. The authors find that meanders act in combination with upwelling-favorable winds to force the strongest cold events, while upwelling-favorable winds alone, possibly primed by Ekman veering, force weaker cold events. Most significantly, it is found that the frontal curvature of warm Agulhas Current meanders couples with the atmosphere to drive local wind stress curl anomalies that reinforce upwelling.

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Erik van Sebille, Lisa M. Beal, and William E. Johns

Abstract

The advective transit time of temperature–salinity anomalies from the Agulhas region to the regions of deep convection in the North Atlantic Ocean is an important time scale in climate, because it has been linked to variability in the Atlantic meridional overturning circulation. Studying this transit time scale is difficult, because most observational and high-resolution model data are too short for assessment of the global circulation on decadal to centennial time scales. Here, results are presented from a technique to obtain thousands of “supertrajectories” of any required length using a Monte Carlo simulation. These supertrajectories allow analysis of the circulation patterns and time scales based on Lagrangian data: in this case, observational surface drifter trajectories from the Global Drifter Program and Lagrangian data from the high-resolution OGCM for the Earth Simulator (OFES). The observational supertrajectories can only be used to study the two-dimensional (2D) surface flow, whereas the numerical supertrajectories can be used to study the full three-dimensional circulation. Results for the surface circulation indicate that the supertrajectories starting in the Agulhas Current and ending in the North Atlantic take at least 4 yr and most complete the journey in 30–40 yr. This time scale is, largely because of convergence and subduction in the subtropical gyres, longer than the 10–25 yr it takes the 3D numerical supertrajectories to complete the journey.

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Yu Cheng, Lisa M. Beal, Ben P. Kirtman, and Dian Putrasahan

Abstract

We investigate the interannual variability of Agulhas leakage in an ocean-eddy-resolving coupled simulation and characterize its influence on regional climate. Many observational leakage estimates are based on the study of Agulhas rings, whereas recent model studies suggest that rings and eddies carry less than half of leakage transport. While leakage variability is dominated by eddies at seasonal time scales, the noneddy leakage transport is likely to be constrained by large-scale forcing at longer time scales. To investigate this, leakage transport is quantified using an offline Lagrangian particle tracking approach. We decompose the velocity field into eddying and large-scale fields and then recreate a number of total velocity fields by modifying the eddying component to assess the dependence of leakage variability on the eddies. We find that the resulting leakage time series show strong coherence at periods longer than 1000 days and that 50% of the variance at interannual time scales is linked to the smoothed, large-scale field. As shown previously in ocean models, we find Agulhas leakage variability to be related to a meridional shift and/or strengthening of the westerlies. High leakage periods are associated with east–west contrasting patterns of sea surface temperature, surface heat fluxes, and convective rainfall, with positive anomalies over the retroflection region and negative anomalies within the Indian Ocean to the east. High leakage periods are also related to reduced inland convective rainfall over southeastern Africa in austral summer.

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Dian Putrasahan, Ben P. Kirtman, and Lisa M. Beal

Abstract

The Agulhas leakage transports warm and saline water from the Indian Ocean into the South Atlantic Ocean, forming part of the upper returning arm of the meridional overturning circulation, which can influence climate. Ocean–atmosphere interactions and the strength of Agulhas leakage control sea surface temperature (SST) in the Agulhas leakage corridor, which may in turn affect regional climate variability. In a high-resolution run of the Community Climate System Model (version 3.5; CCSM3.5), it is found that the interannual variability of Agulhas leakage SST is linked to El Niño–Southern Oscillation (ENSO). Anomalous wind stress curl over the south Indian Ocean associated with ENSO excites westward-propagating oceanic Rossby waves that initiate southwestward-propagating anomalies along the coast of Africa. It takes approximately 2 years for this signal to reach the southern tip of South Africa and enter the South Atlantic, where it accounts for 20%–30% of the interannual SSH variability in the Agulhas leakage region. The authors find a similar propagation of anomalies with satellite observations. A similar ENSO cycle along with Rossby wave adjustment is detected in an analogous low-resolution CCSM3.5 run. However, the signal does not propagate all the way along the boundary to affect Agulhas leakage SST. Hence, it is found that high-resolution coupled climate models are necessary to resolve the tropical–subtropical oceanic teleconnection between ENSO and Agulhas leakage SST.

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