Hydrodynamics between Africa and Antarctica during Austral Summer of 2008 and 2009: Results of the IPY Project

Alvarinho J. Luis, S. M. Pednekar

National Centre for Antarctic and Ocean Research (NCAOR), Earth System Science Organization, Ministry of Earth Sciences, Government of India, New Delhi, India.

DOI: 10.4236/ijg.2013.42046                

Abstract

A hydrographic section between Cape Town,South AfricaandIndiaBay,Antarcticawas sampled by deploying expendable CTD probes during the austral summer of 2008 and 2009. In 2009, the Agulhas Retroflection (AR) Front was displaced southward by 1.5° latitude, while the northern and southern Polar Front meandered southward by 1° and 1.4° latitude, respectively, relative to their positions in 2008. Geostrophic transport, relative to1000 m, indicates that Antarctic Circumpolar Current (ACC) flow decreased by 2.5 Sv in 2009 compared to that in 2008. The anticyclones which are detached from the AR facilitates a transport to the southeast Atlantic Ocean ranging between 8 and 12 Sv. Nearly 50% of the ACC transport is confined to the 100 - 500 m layer. A comparison of water mass distribution for 2008 and 2009 suggests that Mode Water distribution was restricted to 42.5°S and 41.9°S, and the Antarctic Surface Water ex°tended to 57.6°S and 46.6°S, respectivel°. In 2009, the along track Heat Content (HC) and Salt Content (SC) for the upper750 mof the water column decreased each by 1% compared to those in 2008. In the ACC domain, the HC and SC dip by 36% and 40% in 2009, respectively. The HC and SC associated with Agulhas Retroflection Front increase in 2009 by 1% and 2%, respectively, due to an enhanced Agulhas transport of warm and saline water from the tropics by 2%.

Keywords

Component; Formatting; Style; Styling; Insert

1. Introduction

The ocean region between Africa and Antarctica is characterized by three regimes: the subtropical region to the north of 40˚S - 42˚S; the circumpolar region between 40S˚ - 42˚S and 55˚S - 57˚S, embedded with fronts of the Antarctic Circumpolar Current (ACC); and the eastern limb of the Weddell Sea gyre. Within the subtropical region, the Agulhas Current (AC) constitutes the western limb of the Subtropical gyre of the South Indian Ocean (IO). South of Africa, the AC curls back upon itself and flows into the South IO as the Agulhas Return Current (ARC) along the Subtropical Convergence and continues to flow eastward as the South Indian Ocean Current [1]. After the Agulhas Retroflection (AR), the AC facilitates a transport of Indian Ocean Central Water of 10 Sv (1 Sv = 10⁶ m³·s⁻¹) to the Atlantic, which is a key link to the global thermohaline circulation [2-4]. In the global conveyor system, the Subtropical IO also exports Antarctic Bottom Water (AABW) and diluted North Atlantic Deep Water (NADW) at the deeper depths to the Atlantic Ocean. Down south, the eastward flowing ACC accomplishes an inter-ocean exchange of colder water with Atlantic, Indian and Pacific Oceans. The hydrological frontal zone in the south IO plays a significant role as a source for atmospheric CO₂ during austral summer, which contrasts with the majority of the regions which represents CO₂ sink [5]. Knowledge of the ocean fronts is thus important in determining the atmospheric CO₂ balance of the region, primary productivity, water mass distributions, circulation, etc. Further south, the Weddell Sea circulation exhibits a cyclonic pattern, with the western limb having higher transport than the eastern counterpart by ~5 Sv [6]. The circulation enables transfer of heat and salt from ACC to the Antarctic continental shelves, where deep and bottom waters are formed [7].

The study of southwestern IO is important because of various reasons related to oceanographic and meteorological features. Among the world oceans, about 67% of the total water volume with a temperature from −2˚C and 2˚C is found in the southwest IO which lies in immediately downstream of the Weddell Sea, where most of this water is formed [8]. The region exchanges a large amount of heat with the atmosphere, and it receives this heat largely from the warm (16˚C - 26˚C) and saline AC water (35.5 psu) which gets trapped in the AR. The south Indian subtropical gyre is unique in that most of the water recirculates in the western and central parts basin [9]. The warming of the Southern Ocean (SO) in the mid-latitude over the past decades [10], due to the austral summertime strengthening of the circumpolar westeries and weakening of the mid-latitude westerlies from stratosphere to the surface, have forced the southward shift and spin-up of the subtropical gyres [11], thereby advecting more warm water and increasing the ocean heat content southward [12]. The merging of fronts is particularly dramatic in the southwest Indian Ocean, where the Subantarctic Front (SAF) and Polar Front (PF) of the ACC and the Subtropical Front (STF) and ARC are all in close proximity and together produce some of the largest temperature and salinity gradients in the world Ocean [13-15].

Under the project “Monitoring of the Upper Ocean Circulation, Transport and Water Masses between Africa and Antarctica,” the authors deploy expendable conductivity-temperature-depth (XCTD) along the ship track: Cape Town—Prydz Bay—India Bay—Cape Town during austral summer, by taking advantage of the ice-class ships chartered under the Indian Scientific Expedition to Antarctica. The analysis of the XCTD data [16] collected along Cape Town—India Bay (Section-1) and Cape Town—Prydz Bay (Section-2) during the first week of January and mid-March 2008, respectively, indicate that the STF, SAF and PF exhibited double frontal structures, whose meridional meandering is governed by bottom topography through planetary vorticity. We compared the frontal position relative to that of [14,17]. The southern PF (PF1) exhibited a southward shift in its position by 3.5˚ latitude on Section-1. A northward meander of the southern STF (SSTF) and the northern and southern SAF (SAF1 and SAF2) by 2˚ - 3.5˚ and their near-merger facilitated an enhanced baroclinic transport of 12 Sv in the upper 1000 m, just north of Crozet Island. Three anticyclones that detached from the AR were effective in transporting 17 Sv into the southeast Atlantic. The baroclinic transport contributed by the AC and its retroflection across Section-2 amounted to 17.6 Sv. These results, along with the inferences from the literature, suggest that the ACC and its fronts undergo temporal and spatial variability, both in horizontal and vertical domains; so there is a need to compare year-to-year hydrodynamics.

The aims of this study are to compare 1) the positions of the hydrological fronts; 2) the spatiotemporal variability of the geostrophic transport associated with the individual fronts; and 3) the net baroclinic transport across the quasi-meridional hydrographic section occupied during 2008 and 2009. We also compare geostrophic transport with that estimated from satellite and identify the water masses and compare their zonal extent during austral summer of 2008 and 2009, along a nearoverlapping ship track from Cape Town to India Bay, Antarctica. The purpose of this case study is to quantify the changes observed in two successive years in terms of spatial front meandering, heat and salt content, geostrophic transport, etc., which would serve as benchmark for future studies. The results of this work cannot be extrapolated to infer interannual variability. Though hydrodynamics are compared along a section occupied in January 2008 and March 2009, we assume that the thermohaline changes are negligible between January and March. In fact, other researchers have compared year-to-year frontal dynamics within a season (summer or winter) irrespective of months of the season [14,17].

2. Data and Methods

Vertical profiles of temperature and salinity in the upper 1 km of the ocean were recorded by deploying XCTD probes (type: XCTD-3; temperature/conductivity precision: ±0.02˚C/±0.03 mS·cm⁻¹; depth accuracy: ±2% of the depth) along the ship track (Figure 1). The hydrographic stations were spaced roughly 30 - 32 nautical miles apart. The section from south of Cape Town (34.62˚S, 18.15˚E) to India Bay, Antarctica (69˚S,

Figure 1. Map of the study area overlaid with the positions of the XCTD stations. The stations from the 2008 (2009) survey are shown as black (white) bullets. The bathymetry following [64] is shown in the background, and the schematic of the circulation of Agulhas Current System is superimposed. PEI: Prince Edward Island.

12.97˚E) was surveyed during the first week of January 2008 (Figure 1, black closed circles) and re-occupied in the third week of March 2009 (Figure 1, white closed circles). We did not apply fall rate correction because a comparison of XCTD-3 and Sea Bird CTD profiles revealed that the former is consistent with temperature and salinity accuracy specified by the manufacturer [18], and the fall rate for XCTD showed no systematic bias in the fall rate equation provided by the manufacturer [19]. The temperature profiles were quality controlled by following the standard [20]; the reader is referred to [16] for complete details. High frequency noise in the salinity profiles was mitigated by applying a median filter with a 15-m window [21].

We used ocean temperature and salinity as criteria to identify the locations of fronts since these properties can vary as a result of gradual modification of the adjacent water masses by air-sea interaction and cross-frontal mixing [14]. We located the central position of the front by using these values at axial locations at a given depth, e.g., temperature at surface (θ₀) or at 200 m (θ₂₀₀) and salinity at surface (S₀) or at 200 m (S₂₀₀) [22]. Several surface criteria, such as surface temperature and its gradient [23] and surface salinity have been proposed to identify frontal structures, but these can vary with seasons and geographical location. The temperature and salinity criteria employed to identify fronts and water masses is summarized elsewhere [16]. The geostrophic transport (T_sv) across a pair of XCTD stations was estimated, relative to the deepest common level (1000 db), by using:

where f is the Coriolis parameter (s^–1) at a mean latitude, dz is the depth interval (m) and ΔΦ is the geopotential anomaly (m²·s^–2) between an adjacent station pair.

We also used the “Maps of Absolute Dynamic Topograpy (MADT)” with 1/3˚ × 1/3˚ resolution produced by merging TOPEX/Poseidon, JASON-1, ERS-1/2 and Envisat altimeters [24]. The MADT is the sum of the sea level anomaly data and a mean dynamic topography (Rio05-Combined Mean Dynamic Topography (CMDT)). The CMDT is a combined product using the in-situ measurements (hydrographic and surface drifter data), altimetry data and the EIGEN-GRACE 03S Geoid. The CMDT is computed over a seven year period (1993-1999). Since the ACC is characterized by fine-scale structures and variability, we used “up-to-date” absolute dynamic topography data and absolute geostrophic velocity components. The details on the mapping methods and different corrections applied to these fields are available elsewhere [25]. Using the altimetry-based geostrophic velocity components, sea surface convergence was computed to infer upwelling and downwelling signatures in the thermohaline structures. Surface convergence (divergence) indicates downwelling (upwelling). An examination of the vertical thermohaline structure suggests that it is inappropriate to assume the level-of-no-motion at 1000 db to estimate volume transport from XCTD data. So, we have also estimated transport, referenced to ocean geoid, from altimeter (MADT) data by using:

∆SSHwhere g is gravity (m·s^–2), D is the thickness of the water column (m), and ∆SSH is the sea surface height difference (m). It has been demonstrated that ∆SSH across the Kuroshio can be used as a proxy for the full depth transport [26]. The geostrophic currents were estimated from MADT using geostrophic relations.

To gain insights into the background conditions that existed during the period of sampling, we used sea surface temperature (T_s, version 5) derived from the Advanced Microwave Scanning Radiometer (AMSRE) flown on Aqua satellite [27]. The AMSRE instrument scans conically with a swath width of 1450 km at an incidence angle of 55˚. It has field-of-views of 26 × 16 km and 14 × 10 km with its 18.7 and 36.5 GHz channels, respectively, due to which the measurements are recorded in consistent cloud cover conditions. The ocean heat content (HC, J·m⁻²) for vertical oceanic column was computed by using

where ρ is the seawater density at surface (kg·m^─3) and C_p is the specific heat of seawater at constant pressure at surface (J·kg⁻¹·K⁻¹), dz is the thickness of ocean layer with temperature mean T (˚K). Similarly, the salt content

(, kg·m⁻²) was estimated from salinity profiles (with S in psu).

3. Results and Discussion

3.1. Satellite Observations

Figure 1 shows the topographic features of the study area overlaid with locations of the XCTD stations. The generalized schematic of the AC system superimposed on Figure 1 was constructed by referring to the MADT and T_s fields. It observed that the subtropical gyral circulation in the southwestern IO is stronger than that in the other two oceans. It is also noted that the gyre is asymmetric in nature because the circulation in the western limb is stronger because most of the water recirculates in the western and central parts (up to 70˚E) of the ocean basin [9]. At the AR, the anticyclonic eddies which are detached from the current are carried into the Atlantic Ocean, are effective in promoting an exchange of salt and mass. Figures 2(a) and (b) depicts the MADT, superimposed with altimetry-based geostrophic currents. The outline of AC, AR and ARC are traced out following the 2.4-dym contour (Figures 2(a) and (b)). The AMSRE-based T_s maps for 2008 (left panel) and 2009 (right panel) are portrayed on Figures 2(c) and (d), where the AC flow can be roughly represented by the 22˚C contour in 2008 and by 23˚C in 2009 map. Since the sampling from Cape Town to India Bay was completed in about 10 days, Figures 2(a)-(d) represents a 10 day average for the survey period.

In 2008, the AC extends up to 40˚S, while its southernmost limit in 2009 is traced to 39˚S (Figures 2(a) and (b)). The AC retroflects onto itself at about 22.5˚E, 40˚S in 2008 and at 24˚E, 39˚S in 2009, curves northeastwards and continues to flow into the southwest IO as ARC in a convoluted pattern [28] which is dominated by a number of anticyclones. The ARC is characterized by an elevated surface topography (~2.7 dyn m, Figures 2(a) and (b). Along the southeast coast, the upwelling-favorable

Figure 2. (a) and (b) represent absolute dynamic topography overlaid with geostrophic velocity components (displayed at every third grid for clarity); (c) and (d) sea surface temperature (˚C) field derived from the advanced microwave scanning radiometer onboard NASA’s Aqua satellite. The XCTD stations are overlaid as black bullets; (e) MADT; and (f) AMSRE-based SST for 2008 and 2009 along the track.

wind-stress curl in 2009 promotes a low-SST over a large area depicted by the 24˚C contour, which is marked by a relief in dynamic topography (<1.8 dyn m). We note that the isotherms in 2009 representing SST of 22˚C to 24˚C are displaced southward off the Port Elizabeth due to the Natal Pulse (NP). It is a solitary meander initiated by a baroclinic instability at the Natal Bight (just north of Durban), when the intensity of the landward border of the current exceeds a certain threshold [29] (Figure 2(d)). The NP in the Natal Bight area (Figure 1) occurs at irregular intervals with a periodicity ranging from 50 to 240 days [30]. The NPs, which are generated at the Natal Bight by anticyclonic eddies (see white arrow pointing to the eddy on Figure 2(b)), move downstream along the AC at speeds about 20 km·day⁻¹ [31] and widen the AC to about 300 km seaward of its usual location. The NP is also known to cause major disruptions to the AC flow, including retroflections off Port Elizabeth, which inhibits the flow of water to the AR region, thereby providing less water for inter-ocean transfers. A number of warm and cold-core eddies are shed from the AR and ARC due to fluctuations in the Rossby waves on the ARC. During 2008 an anticyclone with a 130-km diameter (represented by letter A) was identified with its core at 17˚E, 38.5˚S in the MADT map. It is characterized by a dynamic topography of 2.7 dyn·m, Ts of 20˚C - 21˚C, and rotational velocity of 1.5 m·s⁻¹. In 2009, we detected an ovalshaped anticyclone (represented by letter B) with a diameter of 220 km and oriented in the northeast-southwest direction in the MADT map. Its core is located at 16.3˚E, 41˚S and it is characterized by surface dynamic topography of 2.7 dyn·m, Ts of 22˚C, and radial velocity of 1.2 m·s⁻¹. These eddies spawn across the Subtropical Convergence and are responsible for a substantial meridional heat flux into the SO [32], and their genesis is linked to natural fluctuations in the current or to the adsorption of deepsea eddies into the current. In the climate change scenario, marked by altered wind stress curl over the south IO, it is plausible that the threshold for the triggering of a NP will occur more frequently. A sharp SST gradient in the contours marks the northern extent of the ACC which extends from 42.8˚S to 51˚S, marked by a SST range of 3˚C - 12˚C in 2008 and from 43.8˚S to 52.8˚S with a SST range of 2˚C - 9˚C in 2009.

South of the continental shelf of South Africa, the AC retroreflects onto itself and curls westwards and flows eastward as the ARC [33]. The occurrence of AR over the western half of the Agulhas Plateau can be explained as follows. Baroclinic flows over smoothly varying topography tend to conserve angular momentum by following lines of constant potential vorticity approximated by PV = (f + ξ)/H, where f is planetary vorticity (s⁻¹), ξ is relative vorticity (s⁻¹), and H is water depth (m). In open ocean, f is much larger than ξ, so that the mean PV can be approximated by f/H. In SO, significant changes in the depth associated with mid-oceanic ridges and uneven bottom topography cause the ACC jets to deviate from the circumpolar lines of constant PV [34]. Having encountered a sharp gradient in the bottom topography (from ~500 m over the Agulhas Bank to ~5000 m, Figure 1), the AC progrades into the South Atlantic Ocean and retroflects back on itself. Here the exchange of Agulhas water between the South IO and South Atlantic Ocean is facilitated by Agulhas rings and to a lesser extent by Agulhas filaments [35]; the remainder of the AC water flows eastward in a series of steady state meanders of 700-km wavelength and amplitudes that decrease from 170 km in the first meander to 50 km in the following meanders. The vorticity balance of the meandering ARC speed axis has been analysed elsewhere [36] and it has been demonstrated that the balance between the beta term (meridional gradient of Coriolis force) and advection of curvature vorticity makes the ARC axis horizontally non-divergent.

We note that the AR region is associated with a very high eddy kinetic energy (EKE) [37], which makes it a “Cape Cauldron” characterized by turbulent stirring and mixing promoted at surface and intermediate depths [28], [29]. They demonstrated that the enhanced EKE field is for the most part composed of surface-intensified cyclonic and anticyclonic vortices from both the Indian and the Atlantic Oceans.

We compare the variability in MADT and SST (for 2008 and 2009) referring to Figures 2(e) and (f), respectively. North of 45˚S, the difference in MADT for 2009 is found to be higher than for 2008, which is due to the fact that the eddy was further north in 2008 and the observations were stopped due to technical reasons at about 38˚S, so the signature of the whole eddy is not captured. It is noted that the SST variation in 2009 is higher due to the presence of filaments of warm water (15˚C) protruding southward to ~45˚S (Figure 2(d)). With the background knowledge of these surface manifestations inferred from satellite observations, the features identified from the vertical temperature and salinity sections are discussed in the next subsection.

3.2. Hydrological Fronts and Thermohaline Characteristics

Figure 3 depicts surface gradients of T_s, S and σ_t, along with vertical distributions of XCTD-based temperature and salinity along the ship track. Surface convergence inferred from altimeter-based absolute geostrophic velocity components were used as guide in the interpretation of the changes and the nature of eddies reflected in the thermohaline structure (Figures 3(d) and (h). We used temperature and salinity as criteria for identification of

Conflicts of Interest

The authors declare no conflicts of interest.

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