DYNAMICS AND INFLUENCE OF THE BREAKAWAY EDDY ON THE EAST COAST OF Lisa Guastella 1,2 , Mike Roberts 1,3,4 [email protected] Durban breakaway eddy downstream propagation

1 Bayworld Centre for Research and Education, 5 Riesling Rd, Constantia, 7806, South Africa 2Oeanography Department, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa Introduction : 3Oceans & Coasts, Dept of Environmental Affairs, Private Bag X2, Roggebaai, 8012, Cape Town, South Africa 4 (a) (b) The Durban eddy is a lee-trapped, semi-permanent, cold core cyclonic eddy Dept Ichthyology & Fisheries Science, Rhodes University, South Africa that occurs south of Durban (29.87°S, 31.03°E) at the southern end of the Table 1: Durban Eddy statistics for the study area obtained from in-depth KZN Bight and inshore of the Agulhas Current off the east coast of South analysis of current meter data and satellite imagery Africa (Fig. 1). It is driven by the Agulhas Current which moves offshore of a % of time eddy present 55% regressing shelf edge south of Durban. The eddy draws cold water upward, which is nutrient-rich compared to surrounding waters, facilitating biological No. of eddies p.a. 22.5 (almost 2 per month) production. Eddy occurrence is sporadic and inshore currents are Ave eddy lifespan 8.6 days (sd=4.5 days) characterised by frequent current reversals associated with eddy spin-up Min eddy lifespan 3 days followed by downstream propagation and dissipation of the eddy. Roberts et Max eddy lifespan 19 days al. (2010) has shown that short-term current reversals (2-3 days) observed in Ave No. of days between eddy events 7.7 days (sd=10.6 days) ADCP data measured off Port Edward (31.08°S, 30.22°S), some 150 km Average eddy size 40-50 km wide, 60-90 km long south of Durban (refer Fig. 3), corresponds with downstream propagation of Time taken to do one rotation 2.5 – 3.5 days the Durban breakaway eddy. The downstream influence of the southward Figure 5: MODIS chlorophyll-a images for (a) 12 Sepember 2009, showing an eddy off Durban Ave current speed v (Northward) 29 cm/s propagating breakaway eddy on the Agulhas current system is investigated. and breakaway eddy off Port Edward, and (b) 20 September 2009 showing evidence of the Max current speeds (NE’ward) 135 cm/s breakaway eddy now off the Kei River mouth,while the Durban eddy remained off Durban. Ave current speed v (Southward) 57 cm/s Max current speeds v (SW’ward) 185 cm/s (a) (d)

Delagoa Bight Example of eddy lifecycle

KZN Bight RICHARDS nt re BAY ur C 29°S as lh TUGELA gu A

DURBAN A gu 30°S lh a s n R (b) io e (e) ct tu Durban le rn of tr eddy Re Durban breakaway eddy PORT 31°S EDWARD

30°E 31°E 32°E (c) (f)

Figure 1: Location of the study area at the southern extent of the KZN Bight off the coast of KwaZulu-

Figure 6: Moored ADCP currents for 16 to 21 September 2009, with direction at (a) Cape Morgan (including Study Methods bottom temperature (80 m)), (b) East London and (c) Fish River, near Port Alfred and correponding velocity Cold Eddy flattens th th filament & moves in (d) to (f). Eddy propagates past Cape Morgan on 18 and 19 as indicated by shift to NW and then NE • Deployment of moored ADCPs (refer Figs 2 & 5) down coast and decrease in bottom temperature and current speed (d). A weaker signal is indicated at (b) East • 2 Ship surveys London, with the current reversal only evident in the bottom half of the water column (<40m), although a decrease in speed was evident throughout (e). At (c) Fish RIver (Port Alfred), eddy signal was only evident

• Analysis of satellite imagery mg/m 3 as a reversal to NW near the bottom 20m with a decrease in current speeds (f) in the lower 20-40m. The eddy propagates downstream, ultimately dissipating into lateral waves in the inshore boundary of the • Deployment of satellite-tracked drifters Figure 3: Life cycle and downstream progression of a Durban Eddy during 6- Agulhas Current near Port Alfred (33.60°S, 26.90°E), a further 600 km south of Durban. Downstream 22 February 2010. The eddy remains semi-stationary from 6 th to 15 th after propagation speeds of the dissipating eddy are slower in the north between Durban and Sezela (approx which it breaks away and propagates downstream. A previous breakaway 7km/day) but increase as the eddy breaks away from the Durban area and migrates southwards between eddy is indicated in the south off Port Shepstone on the 6 th . Sezela and Cape Morgan. A propagation speed of approximately 28km/day was recorded between Port Edward and Cape Morgan, which is higher than that estimated for Natal Pulses, e.g. 10-20 km/day, Lutjeharms (2006). A maximum downstream propagation speed of 35 km/day has been recorded. (a) (b)

Conclusion:

(c) Durban Eddy waters and associated biota are transported downstream into the Agulhas Current both by leakage from the outer rim of the eddy into the inshore boundary of the Agulhas Current (refer Fig. 4) and through downstream propagation of the eddy itself (refer Figs 3, 5 & 6). The Durban break-away eddy is a transient, southward propagating, mechanism for the upslope movement of cold water rich in nutrients, which may well be capable of stimulating primary production. Thus the Durban cyclonic Eddy is an important source of nutrients and associated biota to the Agulhas Current downstream of Durban and plays an important role in its ecosystem functioning, providing the nutrient source for primary production.

References: Figure 2: S-ADCP near-surface current field for 24 to 28 March 2009 superimposed on a chlorophyll a image for 25 March 2009. The inset currents showing more detail Roberts, M.J., van der Lingen, C.D., Whittle, C and M van den Berg, 2010: Shelf currents, lee-trapped and transient eddies on the inshore boundary of the Agulhas Current, South Africa: their relevance to the KwaZulu-Natal , Afr. J Mar Sci 32(2): 423-447. for the Durban Eddy are for the top 3 bins at 19 m (red), 27 m (green) and 35 m Figure 4: Satellite derived (a) SST (°C) and (b) Chl-a (mg/m 3) showing a Lutjeharms JRE. 2006: The Agulhas Current. Berlin Heidelberg: Springer-Verlag. (blue). White dots indicate the positions of ACEP moorings as follows: DI = Durban breakaway eddy off Sezela and (c) moored ADCP currents and bottom inshore, DO = Durban offshore, DT = Durban thermistor, S = Sezela, T = Tugela, RM temperature (27m) off Sezela indicating a current reversal to NE and Acknowledgements: Marcel van den Berg, Bradley Blows and the Captains & crews of the FRS Algoa and FRS = Richards Bay mid-shelf and RO = Richards Bay offshore. Black dots indicate other slight decrease in temperature as the eddy passes through between 28 Africana are warmly thanked for their assistance with mooring deployment and recovery and sampling. Lisa Hancke is current meters located at M = Mkomazi and A = . April and 1 May. The hashed vertical line represents 12 am on 1 May. acknowledged for assistance with the satellite-tracked drifters. The MRSU and NASA are acknowledged for the satellite imagery.