Interaction of dipole eddies with the western continental slope of the Mozambique Channel

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Abstract

Sea Level Anomaly (SLA) data were used to track a southward propagating eddy dipole along the western slope of the Mozambique Channel over some 6 months. In April 2005, this dipole (with the cyclone to the south) was close to the continental slope off southern Mozambique. The contact zone between the contra-rotating vortices and the slope was surveyed by ship using onboard (S-)ADCP and CTD lines. The data showed strong (>1.4 m s−1) southward (geostrophic) currents over the slope adjacent to the anticyclone with horizontal divergence over the shelf edge. Significant slope upwelling between the dipole and the shelf was evident, concomitant with enhanced nutrient and chlorophyll levels enriching shelf near-surface waters. Satellite observations depicted a 300 km long surface chlorophyll filament extending offshore in the frontal zone between the contra-rotating vortices. A satellite-tracked drifter deployed at the coastal base of this filament confirmed the offshore advection of chlorophyll-enriched shelf water, which ultimately wrapped around the cyclone and filling its centre. The slope upwelling was also clearly evident in hourly temperature data collected by a recorder deployed on a nearby reef (Zambia Reef) in a depth of 18 m. According to the SLA data, the dipole took several weeks to pass Zambia Reef causing prolonged bouts of upwelling that finally ceased when it left the continental slope and moved southwards into the open ocean. Further analysis showed that lone anticyclones and cyclones against the Mozambique continental shelf also induce slope upwelling as a result of horizontal divergence created by the radial circulation of the vortex. In the case of cyclones, the divergence occurs north of the contact zone. Overall, this case study confirms that eddies moving southwards along the western side of the Mozambique Channel are the main mechanism for pumping nutrients into the otherwise oligotrophic surface waters, and moreover, provide a vigorous mechanism for shelf–open ocean exchange.

Introduction

The Mozambique Channel is dominated by a field of anticyclonic and cyclonic mesoscale eddies (Schouten et al., 2003). Tracks in satellite altimetry (Fig. 1, insert) and models show the anticyclones to migrate southward through the narrow Mozambique Channel entrance and then typically along the western slope with speeds ranging from 4.5 to 10 km day−1 depending on latitude (Biastoch and Krauss, 1999, Schouten et al., 2003, De Ruijter et al., 2005, Lutjeharms, 2006, Halo et al., 2013). This spatially biased behaviour has been emphasised using maps of sea surface height (SSH) variability as well as SVP (Surface Velocity Programme) surface drifters (Hancke et al., 2013). Model outputs such as time-averaged sea level skewness (Backeberg et al., 2009) and satellite-derived vorticity (Ridderinkhof et al., 2013) further illustrate this distinct “anticyclone corridor” that leads to the source of the Agulhas Current (Fig. 1). Schouten et al. (2003) and Ridderinkhof and de Ruijter (2003) estimate about four anticyclones are generated per year. However, the dominant frequency of variability in SLA data is found to decrease poleward in the Channel with a peak period of 55 days in the north and 90 days in the south. This infers a reduction from 7 to 4 anticyclones with dissipation and merging, both of which are commonly observed in the altimetry field (Schouten et al., 2003, Lutjeharms, 2006). On average, anticyclone energy expressed by SSH is found to increase between 12° and 24°S (i.e., 15–42 cm in SLA; Schouten et al., 2003). This is in agreement with the expected increase in eddy intensity due to the latitudinal displacement and the planetary vorticity gradient (Schouten, 2003). Palastanga et al. (2006) showed that a connexion exists between mesoscale eddy activity around Madagascar and Indian Ocean dipole (IOD) events.

The exact mechanism(s) of anticyclone generation in the Mozambique Channel has been a topic of considerable interest. Suggested mechanisms have included barotropic instabilities of the South Equatorial Current (SEC) north of Madagascar (Quadfasel and Swallow, 1986, Schott et al., 1988, Biastoch and Krauss, 1999; Fig. 1), incoming Rossby waves travelling westward across the Indian Ocean around 12°S at a frequency of about four per year (Schouten et al., 2002), and eddy shedding as a result of strong currents flowing through the Channel narrows at 17°S (Ridderinkhof and de Ruijter, 2003). Additionally, LaCasce and Isachsen (2007) demonstrated discontinuity in the Sverdrup stream function at the northern tip of Madagascar and the occurrence of a westward jet which is barotropically unstable leading to vortices. Harlander et al. (2009) observed a southward current along Madagascar as a precursor to the formation of an anticyclone in the Channel narrows which then propagated westward and was related to a Rossby normal mode in the Channel. Recently, Backeberg and Reason (2010) using current and vorticity fields from a model (HYCOM) and altimetry, showed anticyclones form in the Channel narrows some 20 weeks following a westward transport pulse in the SEC. At 13 weeks a positive vorticity anomaly initiated at the northern tip of Madagascar reached the Mozambique coast where it intensifed the poleward slope current. They argue that the conservation of potential vorticity generates additional anticyclonic flow curvature which further serves to intensify the positive vorticity anomaly leading to the formation of an anticyclonic eddy. A poleward moving eddy will then experience an increasingly negative ƒ (Coriolis parameter) and thus, to conserve potential vorticity, its relative vorticity (ζ) must become increasingly positive (anticyclonic). The magnitude of ƒ and ζ increases by about 40% between 14°S and 16°S. Interestingly, based on the model and altimetry data, Halo et al. (2013) also identified the westernside of Madagascar as a generation site for anticyclonic activity.

The origin, and in fact existence of cyclonic eddies (negative anomalies) in the Mozambique Channel, has also been contentious, especially on the western side. While early observations carried out during the ACSEX programme in 2000 confirmed the presence of anticyclonic eddies here, vessel-mounted (S-)ADCP current measurements and XBT lines in the vicinity of negative SSH anomalies during the cruise showed little sign of cyclonic dynamic features (De Ruijter et al., 2000, De Ruijter et al., 2002, Schouten et al., 2003). This led to the view that negative SSH anomalies were perhaps a consequence of the dominant anticyclones on the western flank of the Mozambique Channel leaving a signal in the mean SSH data field and hence the fabrication of negative anomalies in their absence. Their existence however, has now been shown to be certain as not only do cyclones occur in models in this region (e.g. Biastoch and Krauss, 1999, Backeberg et al., 2009, Halo et al., 2013), but have also been observed in situ using S-ADCP by Ternon et al. (2013) and Roberts et al. (2008), both in the “anticyclone corridor” between positive anomalies and elsewhere in the Channel. Unlike anticyclones, Halo et al. (2013), using both model and altimetry data, showed that cyclones in the channel have little spatial preference and therefore occur everywhere.

Schouten et al. (2003) and De Ruijter et al. (2004) suggested that cyclones in the Mozambique Channel originate from the southwestern edge of Madagascar and not within the Channel. Observations of the SLA field, however, show some to also form in the northwestern bight of Madagascar at ~15°S, sometimes when a positive anomaly is generated to the north. This is supported in the 6 year average of vorticity data shown in Fig. 1 (i.e., area of negative ζ). As seen in Fig. 2, the northern cyclones can be comparable in size and energy to anticyclones, move southward through the Channel narrows and intensify in a similar manner as positive anomalies between 16°S and 18°S. They are often seen to move down the western flank of the Channel forming dipoles and tripoles with anticyclones (e.g. Fig. 2e). From model data, Halo et al. (2013) showed cyclones to be more frequent than anticyclones, to also form in the central Channel (possibly a consequence of ringlet formation at the periphery of the more energetic anticyclones; Wang, 1992, Nof, 1993), and tend to be smaller with shorter life spans and lower amplitudes. Certainly observations using SLA imagery show cyclones to be more fickle than their counterparts, seldom lasting complete propagation through the Channel. Models on the other hand show dipoles to be much more robust (Biastoch and Krauss, 1999, Halo et al., 2013).

Quartly and Srokosz (2004) were the first to examine eddy activity in the Mozambique Channel using satellite-derived ocean colour. Confined to the southern part, they noted a number of large circular features (anticyclonic eddies) on the western side about 200 km in diameter, with low chlorophyll concentrations (CC) in their centres. These intermittently propagated poleward along the western edge of the Channel with long (~500 km) filaments of surface chlorophyll extending offshore between eddies. Later Tew-Kai and Marsac (2009) investigated the coupling between sea surface CC and the physical environment using statistical models. From satellite SST, altimetry and chlorophyll they found surface CC variance in the northern and southern regions of the Channel to be strongly driven by seasonality, with highest levels in winter (August–September), but less so in the central region between 16°S and 24°S. Here they observed an east – west gradient in chlorophyll increasing from Madagascar to Mozambique, suggesting drivers other than the seasonal cycle being involved in the spatial distribution of CC. Apart from noting higher levels of CC in the shelf regions (>1 mg m−3) in contrast to the less productive open Channel (average=0.15 mg m−3), they too remarked on the CC field on the western side being well marked in spatial features, with low CC in anticyclones and high CC in cyclones. They suggested that the observed chlorophyll filaments emanating from the Mozambique shelf are created at the periphery of anticyclones and that there is also offshore transport of shelf production from the Sofala Bank (19–22°S), the latter possibly encouraged by nutrient loading from riverine runoff.

The most recent remote sensing study of the chlorophyll field in the Mozambique Channel is that by Omta et al. (2009) mainly aimed at demonstrating the inconsistent ratio between CC and algal biomass (i.e., the Chl-a:C ratio is not constant). Nonetheless, they too showed the chlorophyll field in the channel to have a pronounced seasonal cycle with maximal concentrations in austral winter (typically ~0.2 mg m−3) and minimal in austral spring–summer (typically ~0.1 mg m−3). But at times in winter they observed phytoplankton blooms in localised patches with surface CC ranging between 0.3 and 0.7 mg m−3. In contrast, very low CC (<0.2 mg m−3) characterises most of the region in spring, except in shelf waters along the Mozambican coast where they hypothesise upwelling occurs between 20°S and 23°S. With overlays of SLA data, they noted that some of the eddies on the western side of the channel, while able to generate localised phytoplankton blooms through presumably enhanced upwelling of deeper nutrient-rich water, also appear to entrain coastal waters high in chlorophyll that are then swept around them during their southward progression.

There are few in situ observations of the eddy field in the Mozambique Channel. Early research cruises are reported in Harris (1972), Saetra and da Silva (1984), Nehring et al. (1987), Donguy and Piton (1991), and De Ruijter et al. (2002) and were mostly concerned with understanding the general circulation in the Mozambique Channel, especially the character of the then perceived Mozambique Current. Of these early cruises, only data and results presented by Nehring et al. (1987) managed to capture the first impressions of an eddy–slope interaction, although unwittingly, they were unaware of the eddy field surrounding their observations. In particular they observed a strong (250 cm s−1) southward jet along the northern Mozambique shelf between 13°S and 15°S, which according to the dynamic topography, appears to be driven by an adjacent offshore anticyclone. The jet detached from the shelf south of 15°S (shelf edge recedes westward at this point) and flowed around a deep depression (cyclonic eddy) close to the slope and centred directly off Angoche. Horizontal maps of temperature and salinity data at 50 m, clearly show slope upwelling in the centre of this depression, concomitantly with elevated nutrient levels. Water column concentrations of chlorophyll a within 0–75 m matched this pattern with the highest values in the cyclonic eddy. The authors suggested that this cyclone is driven by the then perceived southward flowing Mozambique Current and is therefore a semi-permanent lee-driven feature. More recently, Swart et al. (2010) presented in situ hydrographic and nutrient data from the ACSEX I cruise (De Ruijter et al., 2000, De Ruijter et al., 2002) which profiled two warm anticyclonic eddies on the western side of the channel at 20°S and 24°S to the bottom. LADCP velocities showed these vortices to reach the bottom ~3000 m with surface velocities of >0.5 m s−1 (De Ruijter et al., 2002). In both cases, the thermohaline and nutricline structure sloped upwards towards the shelf, but this was more marked at 24°S, with the isopleths lifting over 200 m relative to the eddy centre in the upper 1000 m. Sloping was evident even at depths >1000 m.

Malauene et al. (2014) have further investigated shelf edge upwelling off northern Mozambique using satellite SST and chlorophyll, together with in situ underwater temperature recorder (UTR) data at a site at 18 m off Mozambique Island (15°S). In this study, the appearance of upwelling was reflected between 15°S and 18°S by slightly enhanced surface CCs, cooler SSTs, and the upward sloping of inshore isotherms near the shelf observed during the ASCLME cruises in December 2008 and August 2009. The upward sloping structure was accompanied by increased subsurface chlorophyll. The UTR time series showed coastal upwelling to be prominent from August to March (i.e., austral summer). Periods of increased upwelling were observed every two months, with shorter period upwelling events at 8–30 days. Coupling satellite, wind and UTR data indicated that the seasonal upwelling was predominately driven by northeasterly monsoon winds, while the same analysis with SLA data suggested that passing eddies played only a limited role in driving upwelling off the northern Mozambique shelf, the latter possibly indicated by the 2 monthly signal in the wavelet analysis.

Most recently, Kolasinski et al. (2012) published in situ C and N stable isotope data sampled from a slope-bound dipole near Beira (20°S) with the intent to study eddy induced transport processes affecting sources and sinks of organic matter, and moreover, to test the hypothesis that eddies near the slope entrain coastal production offshore. A combination of natural isotope tracers, elemental composition and physico-chemical variables allowed two sources of organic matter within the eddy dipole to be distinguished. Near the surface, coastal POM was entrained at the anticyclonic boundary and transferred into the eddy, where it was downwelled into deeper layers with decomposition greatly affecting its composition. In contrast, cyclonic water promoted new production by upwelling nutrient-rich deep waters into the euphotic zone. This production circulated through the mixed layer of the cyclone and was possibly advected into the boundary. Fmax and surface waters at the boundary shared characteristics with the cyclone and anticyclone, respectively, as the result of both the import of phytoplankton biomass of cyclonic origin occurring at Fmax and entrainment of coastal biological material at the surface. The authors suggested that the continuous migration of mesoscale eddies along the shelf plays an important ecological role both in enhancing pelagic production and transporting coastal production offshore.

Whilst these satellite and direct observations provided sufficient evidence that dipole eddies along the northern sectors of the Mozambique shelf are associated with a form of slope upwelling, and that the surface chlorophyll filaments observed in satellite images comprise mainly of shelf waters drawn offshore by the vortices, they do not reveal the mechanism(s) involved or whether such upwelling similarly occurs along the central and southern regions of the Mozambique shelf. The objective of this study therefore was to utilise ship, satellite, and mooring (UTR) data to study the interaction between a dipole eddy and the continental slope in order to elucidate some of the processes involved in upwelling off southern Mozambique.

Section snippets

Data and methods

In situ data was acquired during the April 2005 ACEP cruise (4 April–2 May) onboard the RV Algoa. A survey of the dipole contact zone between Zambia Reef and Ponta Zavora occurred between 11 and 16 April. Since the focus was on shelf and slope oceanography, the CTD lines were limited to an offshore distance of 160 km (Fig. 3, Fig. 4. Temperature, salinity, dissolved oxygen and fluorescence profile data were acquired using a Seabird SBE 911 CTD to a maximum depth of 1000 m. Water samples were

Tracks and behaviour of eddies and dipoles during 2004/2005: a case study

Fig. 2 shows selected SLA maps of the eddy field in the Mozambique Channel for an 8 month (August 2004–April 2005) time series that tracks an anticyclone C and a cyclone B pair (dipole) from early stages of formation off the northwestern tip of Madagascar in late August 2004 (Fig. 2a) until they leave the channel at 24°S in late April 2005 (Fig. 2h). Positive anomaly C moved westerly across the northern reaches of the Channel to the African continent (Fig. 2b) and then southerly through the

Should a western boundary–eddy interaction produce slope upwelling?

In essence, mesoscale eddies are observed to generate enhanced productivity via two mechanisms. The one is by vertical “eddy pumping” of nutrients from deep waters into the shallower euphotic zone (e.g. McGillicuddy et al., 1998) and the other by transport of nutrients and phytoplankton offshore from coastal, frontal and upwelling areas (e.g. Crawford et al., 2005). The mechanics, vertical nutrient transport and biological response to eddy pumping in the interior of mesoscale anticyclones and

Conclusions

In situ ship and mooring data have demonstrated that when dipole eddies (with the cyclone in the south) are in contact with the western shelf of the Mozambique Channel, slope upwelling is induced as a result of horizontal (and vertical) divergence at the shoreward leading (southern) edge of an anticyclone. The upwelled water moves onto the shallow shelf (50 m) but does not generally reach the surface. There is a resulting elevation in the subsurface chlorophyll maximum against the slope, and an

Acknowledgements

This work was carried out under the auspices of the ACEP (African Coelacanth Ecosystem Project), ASCLME (Agulhas Somali Current Large Marine Ecosystem) and WIOMSA (West Indian Ocean Marine Science Association). The officers and crew of the RV Algoa (South Africa) are thanked for their enthusiastic participation in the surveying of the eddy dipole off southern Mozambique. Finally we thank the three reviewers for their constructive comments and contributions which ultimately improved the paper.

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