Baroclinic and deep barotropic eddy variability - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, C12069, doi:10.1029/2010JC006236, 2010

Observations of mesoscale eddies in the South Atlantic Cape Basin: Baroclinic and deep barotropic eddy variability S. Baker‐Yeboah,1 D. A. Byrne,2 and D. R. Watts3 Received 4 March 2010; revised 10 September 2010; accepted 23 September 2010; published 29 December 2010.

[1] Anticyclones and cyclones in the eastern South Atlantic are characterized based on data collected during January 2003 to March 2005, along a Jason 1 satellite altimeter ground track, as part of the Agulhas South Atlantic Thermohaline Transport Experiment. Large and small cyclones and anticyclones were ubiquitous in the deep ocean of the eastern South Atlantic, as well as in the upper ocean. Eddy structures jointly corotating in the upper and deep water column were common; most of the time (94%) these were not axially aligned as they copropagated. The Agulhas rings and cyclones that populate the region generally carry both a steric component (baroclinic) and a mass loading component (deep barotropic structure). Average translation speeds were 7.5 cm s−1 for baroclinic eddies and twice as fast for barotropic eddies, irrespective of polarity. Translation speeds were higher than advection by the mean background flow field. In addition, large mixed baroclinic‐barotropic rings crashed into the Agulhas Ridge and nearby seamounts and split into two or more parts. Some ring parts were also observed to fuse together. Deep cyclones, as well as interactions with topography, were observed to play a role in the fission process of Agulhas rings. These processes can increase the population of Agulhas rings and their remnant eddies, which took three pathways from the Agulhas and into the Cape Basin: (1) a deep pathway between the continental slope and Erica Seamount, (2) a shallower pathway over or near the Agulhas Ridge and Schmitt‐Otto Seamount, and (3) a deep seaward pathway around the Agulhas Ridge. Citation: Baker‐Yeboah, S., D. A. Byrne, and D. R. Watts (2010), Observations of mesoscale eddies in the South Atlantic Cape Basin: Baroclinic and deep barotropic eddy variability, J. Geophys. Res., 115, C12069, doi:10.1029/2010JC006236.

1. Introduction [2] Mesoscale eddy processes play an important role in the Atlantic meridional overturning circulation [Gordon, 1985; Biastoch et al., 2008], and improved understanding of their structure and dynamics is needed. Energetic anticyclones and cyclones dominate the circulation in the eastern South Atlantic Ocean. Part of this ocean region is called the Cape Cauldron [Boebel et al., 2003] due to the rich eddy field. During their lifetime in the southeastern South Atlantic, Agulhas rings were traditionally thought to be embedded in a sluggish, advecting mean flow, the Benguela Current [Olson and Evans, 1986; McDonagh et al., 1999]. However, the more recent view of the Benguela Current in the Cape Basin consists of a field of closely packed eddies [Richardson et al., 2003; Richardson and Garzoli, 2003; Boebel et al., 2003; Schmid et al., 2003] as opposed to the earlier view of a broad 1 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA. 2 National Oceanographic Data Center, National Oceanic and Atmospheric Administration, Silver Spring, Maryland, USA. 3 Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA.

Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2010JC006236

laminar flow with a few embedded eddies. Eddy energy is much larger than that of the mean flow field in this region [Boebel et al., 2003] and contributes to both the transient field and the mean circulation [Matano and Beier, 2003]. A model study by Treguier et al. [2003] and observations by Byrne et al. [1995] and Schouten et al. [2000] convey a turbulent path rings take in the Cape Basin compared to the more streamlined path they traverse in the Benguela Extension. [3] Agulhas rings detach from the Agulhas Retroflection in the vicinity of 15°E to 21°E and 37°30′S to 40°S [Lutjeharms and Ballegooyen, 1988; Lutjeharms and Valentine, 1988] and may form within the Retroflection region due to a mixed baroclinic‐barotropic instability [Chassignet and Boudra, 1988]. They carry cores of warm and salty Indian Ocean waters into the South Atlantic Ocean [Gordon, 1985; Byrne et al., 1995] and influence water mass variability in the Cape Basin region, where interocean exchange takes place [Gordon, 1985; Duncombe Rae, 1991; Duncombe Rae et al., 1996]. These large (∼200–300 km) rings dominate the western portion of the Benguela Current [Garzoli and Gordon, 1996; Richardson et al., 2003], and the region along which they pass is called the “eddy corridor” [Garzoli and Gordon, 1996; Goni et al., 1997], which is broad. The upper ocean structure of Agulhas rings has been described in a number of studies [Olson and Evans, 1986; Arhan et al., 1999; McDonagh et al.,

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BAKER‐YEBOAH ET AL.: OBSERVATIONS OF MESOSCALE EDDIES

1999; van Aken et al., 2003]. An Agulhas ring can have a homogeneous core at 100–550 dbar [McDonagh and Heywood, 1999; Arhan et al., 1999] or a stratified core [Arhan et al., 1999]. In the Retroflection region, a newly formed ring can have “atypical size” and mixed baroclinic‐ barotropic structure [van Aken et al., 2003], reaching the bottom near 5000 m. Although it is typical to consider only waters in the upper ocean above the 10°C isotherm [Olson et al., 1985; Olson and Evans, 1986] to model rings, some Agulhas rings can transport waters below the thermocline [Lutjeharms, 1996] down to the 3.5°C isotherm [McDonagh et al., 1999]. Indeed, van Aken et al. [2003] suggest that a “typical” Agulhas ring may not exist, and this inference is supported by the work presented herein. [4] Byrne et al. [1995] tracked over 20 Agulhas eddies from maps of anomalous sea surface height data and showed that anticyclones translated WNW across the South Atlantic propelled by the mean flow and internal dynamics. Some Agulhas rings do not propagate northwestward through the South Atlantic but coalesce with the Agulhas Retroflection [Boebel et al., 2003]. Eddy tracks are influenced by the Walvis Ridge topography [Byrne et al., 1995], with slowed translation over areas of steep relief. A follow‐on study [Kamenkovich et al., 1996] showed that baroclinic eddies become depth‐compensated (no motion in the lower layer) as they cross the Walvis Ridge system, while barotropic eddies could not cross. This suggests that a mixed baroclinic‐barotropic eddy can lose its deep barotropic structure in the vicinity of a ridge. The shedding rate of Agulhas rings is four to six per year [Byrne et al., 1995; Duncombe Rae et al., 1996; Goni et al., 1997], and these rings are estimated to contribute a minimum of 5 Sv [Byrne et al., 1995] to the Indian‐South Atlantic water mass transfer. Translation speeds can range from 5 to 15 km d−1 [Byrne et al., 1995], which are generally higher than those based on theoretical speed from Nof [1981] and Flierl [1984a] as presented by Olson and Evans [1986]. The mean background flow based on Olson and Evans [1986], Chassignet et al. [1990], and McDonagh et al. [1999] is thought to play a major role in such high translation speeds, which is not supported herein. [5] Agulhas rings can split, and this process may be more common than previously thought. An analytical study by Nof [1991] suggested that under the conservation of integrated angular momentum, only eddies of cyclonic polarity had sufficiently large total angular momentum to split (“fission”). However, Drijfhout [2003] countered that vertical coupling such as baroclinic instability allows eddies of both polarity to split. Arhan et al. [1999] used hydrographic sections to describe three thermocline‐intensified Agulhas rings, two of which were previously one large ring, before it encountered the Erica seamount and split. Such splitting may be common, as Agulhas rings encounter the Agulhas ridge and nearby seamounts on their way into the Cape Basin. In addition to breaking apart shortly after formation in the Agulhas Basin, Agulhas rings can break apart within the Cape Basin. Schouten et al. [2000] suggest that about one third of the rings break apart and decay in the Cape Basin, while the rest continue to translate through the Cape Basin and cross the Walvis Ridge. They also suggest that splitting can enhance mixing of Indian Ocean water from the ring into the southern Atlantic. As shown by Richardson et al. [2003] and Boebel et al. [2003], both anticyclones and

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cyclones dominate the velocity field in the Cape Basin off Cape Town, South Africa. In addition to bottom topographic features, cyclones can play a role in the fission of an Agulhas ring, as presented herein. [6] Aside from large Agulhas rings, slightly smaller cyclones, 120 km in diameter, are known to translate across the path of Agulhas rings [Boebel et al., 2003] and to have a generally southwestward translation [Morrow et al., 2004]. Shown herein, cyclones can have a strong northward or southward component to their translation velocity, which can make continuous identification of a particular cyclone over time difficult when using the widely spaced Jason 1 altimeter ground track data. Using in situ data from PIES instruments in conjunction with satellite data, we will show that in the Cape Basin, cyclones tend to have scales similar to those of Agulhas rings and tend to be nearly equal in number as suggested by Schmid et al. [2003], rather than outnumbering Agulhas rings by factor of 3:2 as reported by Boebel et al. [2003]. In addition to Agulhas rings, which can play an important role in the meridional overturning circulation, cyclones in the South Atlantic are also significant in the thermohaline circulation as they can alter the trajectories and characteristics of Agulhas rings, although their role has previously not been explored. [7] This work examines baroclinic and barotropic spatial and temporal scales and amplitudes in surface height (SSH) variability along the Agulhas ring eddy corridor using data presented by Baker‐Yeboah et al. [2009], referred to as BWB: a line of 12 pressure sensor equipped inverted echo sounders (PIES), three deep current meters, and one tall conductivity‐ temperature mooring, deployed along a Jason 1 satellite altimeter ground track southwest of South Africa for 16 January 2003 to 29 March 2005 (Figure 1). Contributions to SSH anomaly from changes in mass in the water column (barotropic) and from purely steric changes (baroclinic) can be determined from data recorded by a pressure sensor equipped inverted echo sounder (PIES). As noted by BWB, a deep reference pressure (4500 dbar) is used for the baroclinic variations. This convention, suited to baroclinic currents that carry transport [Fofonoff, 1962], differs from the dynamical mode convention that uses a middepth reference. Additional process studies related to results presented herein are given by Baker‐Yeboah et al. [2010] and S. Baker‐ Yeboah et al. (Dipoles and the propagation of Agulhas eddies: The effects of laterally and vertically coupled vortices, submitted to Journal of Physical Oceanography, 2010), referred to as BFSZ and BFW, respectively.

2. Thermocline (Baroclinic) and Deep Mass Load (Barotropic) Eddy Structures [8] The e‐folding lateral correlation scales from PIES baroclinic and barotropic time series data [Baker‐Yeboah, 2008] reveal large (r = 200 km) lateral correlation scales in the barotropic SSH data, which were nearly twice the size of those of the baroclinic SSH data (r = 110 km). Temporal e‐folding correlation scales for baroclinic SSH (17.2 days) and barotropic SSH (16.7 days) data were similar. Although spatial correlation scales from the barotropic time series were twice the size of the baroclinic scales, both sets of scales were larger than the baroclinic Rossby radius of deformation (Rd = 30 km, computed from CTD stratification data for the phase

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Figure 1. The ASTTEX array off of South Africa in the Cape Basin; asterisks are PIES sites and diamonds are current meter mooring sites. Mean deep currents from current meters (gray vectors), and upper ocean geostrophic currents relative to 4500 dbar from PIES data (thick black vectors) and from RIO05 1993–1999 mean dynamic topography (thin black vectors), as summarized in Table 2. speed of long baroclinic gravity waves), and both were far smaller than the barotropic Rossby radius of deformation (2000 km). Spatial correlation scales were used to optimally interpolate the PIES data along the ASTTEX line (Figure 1) onto a 20 km grid to analyze the upper and deep ocean mesoscale eddy variability, which explain the large correlation scales. [9] The circulation in the southeastern South Atlantic contains a rich field of eddies and remnant parts that were observed during this experiment to break away from eddies, illustrated in the time‐space plot of the total SSH anomaly from AVISO along‐track Jason 1 data (Figure 2a) and the total SSH anomaly from PIES data (Figure 2b). These data are well correlated [Baker‐Yeboah, 2008; S. Baker‐Yeboah et al., Sea surface height variability in the eastern South Atlantic from satellite and in situ measurements: A comparative study, submitted to Journal of Geophysical Research, 2010]. There is more complexity than meets the eye in the eddy‐rich field of SSH data; although the SSH features in altimeter and PIES data (Figure 2) are consistent, closer examination using PIES data reveals a rich field of anticyclones and cyclones of variable structure: thermocline (baroclinic, Figure 3a) and deep mass load bottom pressure (barotropic, Figure 3b) eddy signals in the water column that

may or may not move in concert. A distinction is made between the two fields of eddies to emphasize the two signals, which may not always be vertically aligned and can present separate regions of closed streamlines; anticyclonic highs and cyclonic lows in the baroclinic field are labeled as A and C events, respectively, for those SSH anomalies that exceeded ∣±10∣ cm; anticyclonic highs and cyclonic lows in the barotropic field are labeled as H and L events for those SSH anomalies that exceeded ∣±5∣ cm. The array was continuously populated with anticyclones and cyclones in the deep and upper ocean: a total of ∼25 A events and ∼30 C events and, strikingly, 22 medium (150 km) to large (≥200 km) scale H events and at least 32 L events. [10] High along‐track spatial resolution (6.7 km) satellite data provide a better resolved spatial signal, while the hourly PIES along‐track array provides a better resolved temporal signal. For example, the cyclone during October 2003, between PIES Site 1 (31.957°S) and Site 2 (33.419°S) in the satellite data, does not appear in the PIES data (see Figure 2) because there was no inverted echo sounder in the gap between PIES Site 1 and Site 2. Most of the region between sites 1 and 2 of the baroclinic SSH data is thus shaded (Figure 3a, gray lines). This is a region where anticylones and cyclones on the scale of 130 km in diameter can transit

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Figure 2. Time‐space plot of total SSH anomaly along the ASTTEX line (Figure 1) for (a) AVISO delayed‐time data and (b) PIES 3 day low‐pass filtered (baroclinic plus barotropic) data. The abscissa extends along the ASTTEX array starting near the southernmost site, site 12. Solid red patches in Figure 2a represent regions of no satellite data. the array unmeasured and would affect flux calculations. The October 2003 cyclone moved on and off the array as it translated SW away from the slope region toward the deep ocean during November to December 2003 (events C12,

C14, and L13 in PIES data, Figure 3). On the other hand, high temporal resolution of PIES hourly data captures the center of the eddies that cross the array. For example, the same cyclone during October 2003 had different scales in

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Figure 3. Time‐space plots of PIES 3‐day low‐pass filtered (a) baroclinic and (b) barotropic (mass load) SSH variability along the ASTTEX line (Figure 1). Shaded region denotes no baroclinic SSH data collected. The abscissa extends along the ASTTEX array starting near the southernmost site, site 12. Anticyclonic highs and cyclonic lows in the baroclinic field are labeled as A and C events, respectively, for those SSH anomalies that exceeded ∣±10∣ cm; anticyclonic highs and cyclonic lows in the barotropic field are labeled as H and L events for those SSH anomalies that exceeded ∣±5∣ cm.

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the baroclinic (C12 and C14, same eddy) and barotropic (L13) signals. Together, these data give new insight on the characteristics of the eddy structures. 2.1. Anticyclones [11] Eighteen anticyclones were tracked back to the Agulhas Retroflection source region, using a threshold SSH anomaly value of 15 cm in sequential AVISO maps of satellite data, and their scales and amplitudes in altimeter data can be characterized by the two components sampled in the PIES data. Tracks near the array are shown in Figures 4a and 4b. East‐west scales Lx were slightly larger than north– south scales Ly (Figure 4c); the typical value of Ly/Lx was 0.7 ± 0.28. Some events had a meridional elongation due to nonlinear interaction with a cyclone or with the continental slope (see BFW and BFSZ). For example, a NW (NE), W (E), to SW (SE) oriented dipole (BFW) could give an eddy meridional elongation. [12] Anticyclones in the PIES baroclinic data (Figure 3a, A events) had extrema that ranged from 16 to 54 cm with a mean of 29.7 cm and spatial scales from 82 to 229 km in diameter with a mean of 131 km (a mean radius around 2Rd), excluding A05 and A19, which had diameters (>10Rd) of 393 and 310 km, respectively (both had multicores in the horizontal plane rather than single cores). A events took 7–84 days to cross the array, with a mean of 23 days (±4 days); large events A05 and A19 took 84 and 74 days to cross. [13] Anticyclones in the PIES barotropic data (Figure 3b, H events) had extrema that ranged from 6 to 24 cm with a mean of 13.2 cm and spatial scales ranged from 73 to 286 km in diameter with a mean of 146 km (a mean radius around 2.5Rd), excluding H04 and H15, which had diameters of 336 (>10Rd) and 53 km (15 cm s−1) and large (R > 3Rd) events. Geostrophic velocities of barotropic H events ranged between

2 and 27 cm s−1 and a mean of 8 cm s−1 ± 2 cm s−1. About 18% of the 22 H events had strong SSH anomalies (>20 cm) and high rotational speeds. Geostrophic velocities from SSH gradients of barotropic L events ranged between 2 and 13 cm s−1, with a mean of 6 cm s−1 ± 0.5 cm s−1. About 15% of the 32 L events had strong SSH anomalies. Deep currents from current meter moorings revealed similar rotation speeds of deep eddies of cyclonic and anticyclonic rotation (section 3.2). [28] Translation speeds of the barotropic anticyclones (H events) and cyclones (L events) were generally higher than those of baroclinic events (Figure 8). One can compare the barotropic eddy speeds to the short, rather than long, barotropic Rossby wave speed. Using an eddy radius of R = 100 km to approximate k, the short barotropic Rossby wave speed is −18.5 cm s−1. The along‐track component is −8.3 cm s−1 and the cross‐track component is −16.5 cm s−1. Total translation speeds measured at the ASTTEX line for both H and L events ranged from 7 to 28 cm s−1 with an average of 15 cm s−1, which was close to the short barotropic wave speed at the eddy scale. The ratio of rotational speeds to translational speeds was around or slightly less than one (Figure 9). [29] The strikingly fast propagating barotropic eddies were not simply advected by a deep mean current, since the deep mean current was weak to nonexistent in the basin and toward the southeast as observed in the current meter data (see section 3.2 and Figure 1). Also, the mean baroclinic current runs counter to the propagation tendency of most of

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Figure 8

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Figure 9. Ratio of geostrophic rotation speed to translation speed of PIES baroclinic and barotropic anticyclones and cyclones; a measure of nonlinearity (Vg/Ul > 1) of PIES events. Dashed line shows ratio of Vg/Ul = 1. the barotropic eddies (see section 3.2 on mean currents). Nof [2008] suggests a rapid meridional migration of barotropic eddies: a 11.6 cm s−1 equatorward speed for anticyclones like Agulhas rings and slightly lower poleward speeds for cyclones, due to the meridional induced, b force and opposing form, drag on the moving eddy. These forces are important in understanding the large translation speeds of H and L events. In addition, high speeds are related to eddy‐ eddy interactions and the topographic b effect, since barotropic eddies contribute to the nonlinear effects associated with high translation speeds of the baroclinic eddies (addressed further by BFSZ and BFW). 3.2. Currents [30] The spatial trend in the time mean flow was not reflected in the translation of anticyclonic eddies. Figure 1

(black vectors) shows the mean geostrophic currents from PIES mean baroclinic SSH data over the 27 month period of observations: a WNW flow (north of 36°S) of 8 cm s−1 ± 6 cm s−1 and an ESE flow (south of 36°S) of 5 cm s−1 ± 6 cm s−1 along the eddy corridor, which were similar to currents relative to 1500 m from RIO05 1993–1999 mean dynamic topography (see Table 2). Long‐term mean currents (1993–2007) show a similar pattern: a WNW flow (north of 36°S) of 7 cm s−1 and ESE flow (south of 36°S) of 4 cm s−1 (D. A. Byrne and D. L. Witter, manuscript in preparation, 2010). The east‐southeastward flow counters the northwestward propagation trajectory of Agulhas eddies but is more consistent with the trajectories of many cyclones (Figure 5). However, the time‐mean SSH topography is partially due to the repeated passage of Agulhas eddies over the sites (Figures 3 and 4), indicated by the high standard

Figure 8. Average speeds from PIES data for baroclinic anticyclones (row 1) and cyclones (row 2), barotropic anticyclones (row 3), and cyclones (row 4). For A and C events, the black dash lines in along‐track (−0.9 cm s−1) and in cross‐track (∣−1.8∣ cm s−1) plots are components of the first baroclinic Rossby wave speed c = 2 cm s−1 (black dot‐ dashed in total velocity plots). For H and L events, the gray dash lines in along‐track (−8.3 cm s−1) and in cross‐track (∣−16.5∣ cm s−1) plots are components of the wave speed c = b R2 = 18.5 cm s−1 (gray dot‐dashed in total velocity plots) for R = 100 km. Gray regions show uncertainty in speeds (±1.2 cm s−1 in along track, ±4 cm s−1 cross track, and 3.6 cm s−1 absolute velocity). 13 of 20

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Figure 10. Ratio of translation speeds (U) from satellite altimeter data and the linear Rossby wave speed (c = −2 cm s−1), a measure of nonlinearity for (top) anticyclones and (botom) cyclones. error of the mean, ±6 cm s−1. Since the eddies moved differently from the mean flow, additional processes must contribute substantially to their motion. [31] Mean deep currents (Figure 1, gray vectors, and Table 2, CMM2, CTM, and CMM3) were weak to nonexistent, except along the continental slope, and were influenced if not driven by eddies (Figure 11). Mean deep currents from bottom moored current meters were generally weak: 2–3 cm s−1 in the Cape Basin along the eddy corridor. These speeds are similar to those reported in the Benguela Extension [Richardson and Garzoli, 2003], and these low speeds again suggest that additional processes must con-

tribute to the large translation speeds of deep barotropic eddies. A deep (2930 m), southeastward flowing (poleward) undercurrent with a mean speed of 7 cm s−1 ± 1 cm s−1 was measured in the current meter record near 32.66°S, 15.42°E, between sites 1 and 2 (Figure 11, CMM1). [32] Time‐varying deep currents reveal deep eddies of cyclonic and anticyclonic rotation with velocities up to ±33 cm s−1 (Figure 11, CMM2, CTM, and CMM3). Mean speeds over bimonthly periods ranged up to 30.3 cm s−1, which is representative of the mesoscale variability observed in the South Atlantic Cape Basin region. The poleward undercurrent was influenced by the formation of cyclones

Table 2. Along‐Track (u) and Cross‐Track (v) Mean Currents in the Deep Ocean Along the ASTTEX Array From Current Meters and Mean Surface Geostrophic Currents (Vg) Between PIES Sites 12 and 6 and Sites 6 and 1a

Depth (m) u (cm s−1) v (cm s−1) Vg PIES (cm s−1) Vg RIO (cm s−1)

CMM3 37.4160°S, 12.5469°E

CTM 34.8270°S, 14.1708°E

CMM2 33.7721°S, 14.7880°E

CMM1 32.6639°S, 15.4204°E

Vg 12_6 37.6084°S, 12.3899°E

Vg 6_1 33.9663°S, 14.6389°E

5060 2.73 1.05

4640 −0.096 −0.79

4215 2.73 −1.35

2930 3.68 −5.80

surface

surface

−4.5 −8.5

8 6

a See vectors in Figure 1. Vg (PIES) relative to 4500 dbar from ASTTEX PIES 27 month baroclinic mean SSH data and Vg (RIO) relative to 1500 m from RIO05 1993–1999 mean dynamic topography are similar. CMM, current meter mooring; CTM, conductivity and temperature current meter mooring. Current meters are Aanderaa RCM8.

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Figure 11. Deep currents from current meters CMM1 (at 2930 m near 32.6639°S, 15.4204°E), CMM2 (at 4215 m near 33.7721°S, 14.7880°E), CTM (at 4640 m near 34.8270°S, 14.1708°E), and CMM3 (at 5060 m near 37.4160°S, 12.5469°E), subsampled every 12 h. CTM, current and temperature mooring. along the continental slope (BFSZ) that propagate through the array (Figure 3), such that cyclones along the slope coincided with pulses in the poleward undercurrent. For example, cyclones formed near the slope causing higher velocities there (Figure 11, CMM1) during July 2003 (C07C09, Figure 3), September 2003 to January 2004 (C00, which evolved into C12C14[L13]), and January to February 2005 (C30[L30and L32]).

4. Eddy Pathways [33] Sequential maps of satellite data along with PIES in situ data suggest specific pathways of deep‐reaching eddies relative to bottom topography, which may explain some of the differences in the deep structure of the observed eddies. Out of the 18 rings that crossed the ASTTEX array, 22% traversed the Agulhas Ridge and the Schmitt‐Otto Seamount (a pathway of shallower than 2000 m), indicating their lack of an abyssal deep structure. The remaining rings did not encounter the Agulhas Ridge but traversed a nearly flat bottom east of the ridge. After skirting the ridge, about five had tracks overlying or slightly to the west of the Schmitt‐ Otto Seamount. Deeper‐reaching ring structures traversed

the nearly flat bottom pathway, along the eastern side of the Erica seamount (A14, A17, and part of A05) and eventually split, providing additional deep reaching structures (A16 from A14, A19 from A17, and H04 from A05). H04 was one of the three (H04, A16 and A22) most barotropic rings. Both split rings A16 (a case study in BFW) and A19 (a case study in BFSZ) interacted with the continental slope and formed a cyclonic partner (BFSZ). [34] Four main pathways for anticyclones from the Agulhas Basin during 2002–2005 (Figure 4d) were identified from the ASTTEX array and altimeter data: (1) a deep pathway between the continental slope and Erica Seamount, (2) a shallower pathway over or near the Agulhas Ridge and Schmitt‐Otto Seamount, (3) a deep seaward pathway around the Agulhas Ridge, and (4) a southeastward pathway back into the Agulhas Retroflection. After traversing pathways 1–3 from the Agulhas Basin into the Cape Basin, tracks of anticyclones were more variable: toward the NW in the upper and deep ocean along the middle and eastern portion of the ASTTEX array, NW and WNW along the western portion of the array but primarily in the upper ocean (Figure 3), and W and SW south of the array. These variable trajectories were associated with nearby cyclones, addressed further by BFW.

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[35] Between the array and the Retroflection region, tracks of cyclones demarcate a counterclockwise path (Figure 5d). Cyclones are known to form (1) within the Cape Basin along the African shelf [Boebel et al., 2003], (2) inshore of the Agulhas current along the eastern margin of South Africa [Penven et al., 2001], and (3) in the sub‐Antarctic region south of Africa [Boebel et al., 2003]. Cyclones can also form in association with Agulhas rings near the slope (BFSZ). Cyclones that encountered the eastern part of the array (east of 13°E) appeared to come from the slope region, taking on (1) a NW or W trajectory when formed near slope farther south of the array (west of the Agulhas Bank) and (2) a SW trajectory when formed near the slope at the eastern end of the array. For example, during November 2003 to February 2004 (Figure 3), mixed baroclinic‐barotropic cyclones (C12C14 with L13 and C30 with L30 and L32) entered the array from the shelf region (see BFSZ for case study) and altered the pathway of an Agulhas ring along the eddy corridor by forcing the ring toward the WSW via dipole advection (see BFW). Cyclones that crossed the western part of the array (west of 36°S, 13.4°E) had tracks from the SE and appeared to form south of Africa or along the eastern margin of South Africa as described by Lutjeharms et al. [2003] and Boebel et al. [2003]. These cyclones propagated (3) NW and often turned back to cross the array a second time, translating (4) SE toward the Agulhas Ridge. Also, cyclones appear to form and merge south of the array near the continental slope, westof the Agulhas Bank, between 36 and 38°S (Figure 5). Together, these cyclones suggest a counterclockwise cross‐basin circulation pattern, between the Agulhas and Cape Basins. This pattern can support interocean transport between the Indian, South Atlantic, and Southern oceans. [36] The deep pressure and current meter data also suggest a counterclockwise circulation in the deep ocean associated with the eddies: NW from the Retroflection region and along the eastern side of the Erica Seamount WSW across the eddy corridor, and ESE back into the Agulhas Basin west of the Schmitt‐Otto Seamount. The deep eddy pathways are consistent with the schematic circulation for intermediate depths derived from float data by Boebel et al. [2003]. The deep poleward flow along the continental slope is also consistent with this flow pattern. Furthermore, cyclones provide a pathway away from the slope, WSW across the eddy corridor, and ESE into the Agulhas Basin along the western portion of the array. Thus, in the form of eddies, deep water can be advected away from the slope, westward and then southeastward out of the Cape Basin. However, the geostrophic currents around the barotropic (deep mass load) eddies were usually less than their translation speeds (Figure 9), while at the same time most of these eddies were part of a nonlinear process of corotating joint eddy dynamics (see section 5). Further studies on the role of such eddies in deep water pathways are needed.

5. Remarks on Eddy Structure [37] The observed baroclinic and deep barotropic eddy structures (Figure 3) suggest variable, deep‐reaching Agulhas rings and cyclonic eddies exist in the Cape Basin and that the dynamics of their interactions with each other (BFW) and with topography (BFSZ) can alter their scales and speeds. In

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this work, the baroclinic component of geostrophic velocity is defined as motion relative to 4500 dbar (as given by BWB). Using this definition, the baroclinic transport is not identical to zero and in fact can be substantial. These baroclinic velocity fields have depth‐dependent magnitude, and their direction is depth‐independent. In other words, the sheared flow has coherent structure: aligned in the same direction at all depths. Below 4500 dbar, vertical shear in the geostrophic velocity field is assumed to be negligible. Independently, a combination of information from deep current meters and lateral pressure gradients (derived from bottom pressure sensors) is used to determine near‐bottom velocities and their pressure stream function. These deep velocities provide a reference for the baroclinic fields; they are added independent of depth and are defined in this work as the barotropic component of geostrophic velocity. The sum of the two components is the absolute geostrophic velocity. Further details are given by BWB and Donohue et al. [2010]. [38] The “typical” description of Agulhas eddies as “equivalent barotropic” has been avoided in this work because this term can have different meanings. Meteorologists tend to use it for structures with baroclinic flow aligned with depth and summed to barotropic flow along and possibly crossing it, the case described above. More restrictive conventions apply the term to flow in one direction which does not reverse with height but which has substantial shear. In the two‐layer context, it is used for flows with no correlated deep signal [see Flierl, 1984b]. In the continuous stratification context, this could correspond to vanishing velocity at the bottom. Pressure centers in PIES baroclinic and barotropic fields may coincide such that their surrounding isobars parallel each other. In other cases, centers may be offset laterally to produce a vertically tilted structure, or they may be unrelated. An active deep layer can alter the dynamics [see Polvani, 1991], in particular, deep‐ocean closed regions of potential vorticity. [39] As illustrated in Figure 12, eddy structures were generally not equivalent barotropic, as their barotropic signal contributes to total SSH anomaly. The baroclinic field (Figure 3, A and C events) is depicted by the overlay contours and the barotropic field (Figure 3, H and L events) is represented by the underlying contour. Often, deep pressure signals are a precursor of upper eddies with spatial offsets around 80–150 km and temporal offsets around 10–30 days. On the basis of the combination of PIES data and maps of altimeter SSH anomaly data, all deep eddies appear to accompany upper ocean eddies, with an offset in space and time, indicative of a vertical tilt in the combined vertical structure of the eddy field. These combined observations reveal an upper ocean region of closed streamlines with its center offset from the center of a deep ocean region of closed streamlines; strong enough encircling currents in upper or deep eddies may enclose cores of potential vorticity. [40] About 44% of the Agulhas anticyclones exhibited no barotropic signal (Figure 3) and other Agulhas rings appeared to be losing their barotropic structure as they transited the ATTTEX array (Figure 12). Although three anticyclonic pairs (A14 and H16, A16 and H18, and A22 and H22, Figure 3) were vertically aligned to within a horizontal distance Dc < R, where Dc is the lateral distance between the baroclinic and barotropic eddy centers (Figure 12) and R is the mean radius for the two eddies, four were more mis-

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Figure 12. PIES SSH baroclinic (overlay contours of ±5, 10, 15, 30, and 45 cm) and barotropic (lower layer map in full color) variability along the ASTTEX array. (Figure 3a was superimposed on Figure 3b.)

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aligned (A05 and H04, A05 and H07, A17 and H19, A19 and H20), within the range R ≤ Dc < 1.5R. [41] Nearly all tracked cyclones (87%) had deep barotropic signatures: Only smaller cyclones C08 and C22 lacked a deep barotropic structure. Baroclinic cyclones had less of a tilt with their nearby barotropic structure than did anticyclones; most of the cyclonic eddies had Dc < 0.5R. Differences between Dc for anticylones and cyclones may be related to their different points of origin, as discussed in section 6. [42] As illustrated in Figures 2, 3, and 12, the deep barotropic eddies were not compensated for in the upper layer. When baroclinic and barotropic eddies jointly crossed the ASTTEX array (Figure 12), their SSH contours translated together (copropagation). These signals are confirmed in satellite along‐track data (Figure 2 and section 3.1) and maps of satellite data. Baroclinic eddies displace the thermocline, which compensates the pressure gradients from their sea surface height displacement, causing them to weaken with increasing depth. Barotropic eddies cause an isobaric displacement throughout the water column and a displacement in sea surface height. As noted in section 3.1, these barotropic eddies were comparably strong with respect to the baroclinic eddies. [43] Some baroclinic and barotropic eddy pairs (corotating) remained coupled as they copropagated across the array, and some did not. Their deep structure may evolve to lose the accompanying baroclinic steric component and propagate separately as a depth‐independent eddy. This loss decreases their formerly combined SSH signal, with examples provided by satellite maps of eddy pairs C12C14‐and‐L13 and A16‐ and‐H18 that dropped below 25 cm after they crossed the array. While crossing the array, their SSH signatures were 27.5 cm and 25 cm (52.5 cm total) and 16 cm and 24 cm (40 cm total). The large decrease sometimes observed in total SSH anomaly for these eddies does not necessarily imply immediate dissipation, contrary to the conclusion reached by Schouten et al. [2000], but may imply decoupling of the baroclinic and barotropic signals.

6. Discussion [44] The well‐resolved barotropic signatures associated with both anticyclonic and cyclonic eddies in the Cape Basin add a new element to understanding the regional dynamics. The barotropic structure tends to lead the baroclinic structure and may or may not remain coupled. In most cases of deep‐reaching barotropic eddies that crossed or encountered the array with a baroclinic eddy of same sign vorticity, the deep pressure signals cross the line before the upper eddies by approximately 15–30 days and were offset toward the west, leading the baroclinic eddy signal by a distance of less than 0.5R for cyclones and R to 1.5R for anticyclones, indicating a vertical tilt (spatiotemporal offset of eddy centers in the water column; see section 5) in the combined eddy structure. The velocity structure of ring ASTRID presented by van Aken et al. [2003] also shows a slight tilt with the deep structure leading the upper ocean structure, which was also against the background current shear. The newly formed ring ASTRID had corotating upper and deep structure and van Aken et al. [2003] suggested that ring ASTRID was “atypical.” New results presented herein

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suggest that the structure of the ring ASTRID was not at all unusual for the region. [45] Barotropic dynamics [Olson and Evans, 1986; Chassignet et al., 1990; Dewar and Gailliard, 1994; Clement and Gordon, 1995] is an equally important factor in the evolution and behavior of Agulhas rings and cyclones. The existence of strong barotropic anticyclones was anticipated based on results from Olson and Evans [1986], Clement and Gordon [1995], and Chassignet et al. [1990]. Using satellite‐tracked drifters, Olson and Evans [1986] observed rotational velocities in two newly shed Agulhas rings that were well in excess of those indicated by the geostrophic balance. In addition, they noted that the rings’ translation changed with bottom topography, the rings reversed course on encountering seamounts, and changed direction when encountering the continental slope. On the basis of these observations, Olson and Evans [1986] hypothesized a coherent barotropic mode superimposed in the ring interior. The SSH anomaly associated with the barotropic mode was estimated at ∼20 cm (D. Olson, personal communication, 1994). Clement and Gordon [1995], using a combination of ADCP and CTD data, also observed rotational velocities well in excess of geostrophic ones in two Agulhas rings surveyed in 1992–1993. They posited a “significant barotropic component” near the ring center. This excess velocity was also 2 or 3 times what could be accounted for by the cyclostrophic balance, or up to 15–25 cm s−1. Finally, Chassignet et al. [1990] found barotropic rotational velocities of ∼15 cm s−1 in numerical model data of Agulhas rings. These previous suggestions of important barotropic dynamics are confirmed and complemented by the barotropic data collected during ASTTEX (section 3.1). As presented by BFSZ, an Agulhas anticyclone can interact with the continental slope and this process can generate strong cyclonic eddies of mixed baroclinic and barotropic structure. In addition, the horizontal structures of the rings (and cyclones) are found to be far more complex than previously surmised, suggesting that different physical processes govern the water mass structure of these eddies. [46] Previous studies have shown that Agulhas rings have high translation speeds [Byrne et al., 1995; Goni et al., 1997; Garzoli et al., 1999], and speeds reported herein are consistent with these previous studies. Complementary to previous studies, our work shows that the barotropic structure can enhance translation speeds. Nof [2008] showed that the barotropic component gives eddies like Agulhas rings a very fast (10 km d−1) meridional pull and cyclones a slightly lower poleward meridional pull. No attempt was made to fit the observed eddy speed into linear Rossby wave theory; the nonlinear dynamics of these eddies is apparent. Recent studies by BWF and BFSZ show that a “dipole tendency” of mutual advection by the counterrotating eddies in addition to a deep barotropic coupled eddy are both important in understanding the observed high translation speeds and evolution of the eddies. Both anticyclones and cyclones in the Cape Basin play a significant role in the Agulhas leakage at thermocline and intermediate levels as shown herein via the abundance of deep‐reaching eddy structures, which is in agreement with previous studies by Giulivi and Gordon [2006] and Richardson [2007]. Fission and fusion processes as well as eddy‐eddy interactions are thought to play major roles in understanding the variable thermocline structure of Agulhas

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rings presented herein and in the works of Garzoli et al. [1999] and Arhan et al. [1999] and the different water masses [Giulivi and Gordon, 2006] observed in anticyclones as well as cyclones in the Cape Basin (Indian Ocean, tropical and subtropical Atlantic water). They therefore contribute to our understanding the importance of the Agulhas leakage process for the South Atlantic Meridional Overturning Circulation; increased local leakage and mixing into the thermocline may occur near the topographic boundary between the Cape and Agulhas Basins at fission and fusion points closer to the Retroflection than the Vema Seamount or the Walvis Ridge. Moreover, dipolar eddy‐eddy interactions and barotropicity (as suggested by BFW) support advection of Agulhas water away from the Cape Basin and across the South Atlantic. [47] Strong cyclones in the barotropic data were not anticipated, although the thermocline eddy signatures were. Previous studies [Lutjeharms et al., 2003; Boebel et al., 2003] suggest that cyclones are generally smaller and weaker than anticyclones in this region, but this was not the case. Cyclones and anticyclones were observed to have comparable rotation and translation speeds. Nearly all tracked cyclones had corotating barotropic structure; only 12% (C08 and C22) lacked a barotropic structure. Baroclinic cyclones had less of a tilt with their nearby barotropic cyclonic partners than did anticyclones; most Agulhas rings appeared to be losing their barotropic structure as they transited the ASTTEX array (Figure 12). Cyclones that originate from the slope region (BFSZ) may not have had time to lose their deep barotropic structure. Such topographic interaction can also cause the deep eddy structure to intensify, which can explain the strikingly intense deep barotropic structures observed. The mixed baroclinic and barotropic structure, and nonlinear dynamics associated with upper and deep eddies are addressed further by Baker‐Yeboah [2008] and ongoing studies.

7. Summary and Conclusions [48] This work improved the characterization of mesoscale eddy variability in the Agulhas leakage and Cape Basin in terms of the baroclinic and barotropic components, using direct measurements by PIES instruments in conjunction with altimeter SSH data. Indeed, the barotropic eddies have a coherent SSH pattern as illustrated at the surface (observed in satellite data) and in the water column (observed in the PIES near bottom moored instruments). Strikingly, mesoscale, barotropic eddies in the deep pressure data were observed to play a role in the eddy dynamics; some of these acted in concert with baroclinic eddy signals but others did not. Traditional sampling methods such as CTD casts or single‐point time series are unable to resolve such rich eddy structures. The population of eddies revealed by the ASTTEX array was of baroclinic and mixed baroclinic‐barotropic structure, such that cyclones and Agulhas rings can have solely an upper ocean intensified structure, or can have a coupled corotating deep barotropic structure that may or may not remain coupled. Strong barotropic eddies had a dynamic range of −25 to 24 cm, similar to many of the baroclinic eddies. Rotational velocities were up to 70 cm s−1 ± 3 cm s−1 for baroclinic eddies and up to 27 cm s−1 ± 2 cm s−1 for deep barotropic eddies. Current meter data corroborated deep‐ ocean eddy currents from PIES data. For the first time,

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cyclones in the Cape Basin have been shown to have comparable scales and intensities to Agulhas rings, when viewed in terms of combined baroclinic and barotropic dynamics. [49] High translation speeds (7.5 cm s−1 for baroclinic eddies and twice as fast for barotropic eddies) were not due to advection by a uniform mean flow field in the Cape Basin, where the fate of Agulhas leakage is determined. The time mean flow countered the anticyclonic eddy field along the western portion of the observational array and was more consistent with the SW trajectory of cyclones along that portion of the array. In addition to the barotropic dynamics, translation speeds and directions of eddies appeared to be related to eddy‐eddy interactions in vertical and lateral structure, as addressed in further studies by BFSZ and BFW, who show that a “dipole tendency” of mutual advection by the counterrotating eddies in addition to a deep barotropic coupled eddy are both important. Also, a deep (near 2900 m) poleward undercurrent averaging 7 cm s−1 ± 1 cm s−1 was measured along the continental slope near 32.66°S. Strong currents around 20 cm s−1 along the continental slope appeared to be associated with eddies, particularly with large cyclones observed along the shoreward side of an Agulhas ring. [50] The structures of these eddies do not remain constant over time: Their water column structure and scales can vary due to (1) fission and fusion events, (2) loss of the barotropic component, and (3) interactions with the continental slope or other eddies. These processes contribute to our understanding the Agulhas‐leakage process important for the South Atlantic Meridional Overturning Circulation. Fission increased the number of anticyclones in the Cape Basin compared to the number that formed at the Retroflection, and fusion is thought to aid in diluting Agulhas water in the Cape Basin. Cyclones appear to form and merge south of the array near the continental slope, west of the Agulhas Bank, between 36°S and 38°S. Agulhas rings appear to form as mixed barotropic‐baroclinic eddies at the Agulhas Retroflection that are forced to split, such that typical Agulhas rings found in the Cape Basin were not spawned one by one from the Retroflection but split from large rings similar to ASTRID (or larger) that crash at a high speed into the Agulhas Ridge and nearby seamounts. [51] The region between the Cape and Agulhas Basins is one of strong topographic influence on Agulhas rings, engendering the fission of large mixed baroclinic‐barotropic rings from the Agulhas Retroflection and making fission more common than previously shown. Because of the distribution of ridges and seamounts between the two basins, three distinct pathways affect the structure of Agulhas rings: (1) the more flat‐bottomed route, from the Agulhas Basin, east of the Agulhas Ridge and Schmitt‐Otto Seamount, between the continental slope and Erica Seamount, (2) a shallower non‐flat‐bottomed route, over or near the Agulhas Ridge and Schmitt‐Otto Seamount, and (3) a deep seaward pathway around the Agulhas Ridge. Baroclinic rings traversed along the western portion of the array, having lost their barotropic structure before crossing the Schmitt‐Otto Seamount. Trajectories of mixed barotropic‐baroclinic cyclones and anticyclones suggest a counterclockwise circulation in the deep ocean between the Agulhas Ridge and the continental boundary of South Africa.

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[52] Acknowledgments. Support was provided by the National Science Foundation for grants OCE‐0095572 and OCE‐0099177. We gratefully acknowledge the efforts of Neal R. Pettigrew for collecting and providing current meter data. We thank Glenn R. Flierl and Carl Wunsch for editorial comments.

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