Mesoscale bio-physical interactions between the Agulhas Current and Agulhas Bank, South AfricaJennifer M Jackson1,2, Luc Rainville1, Michael J Roberts3,Christopher D McQuaid4, Francesca Porri4, Jonathan Durgadoo5, and Arne Biastoch5

1 Applied Physics Laboratory, University of Washington, Seattle USA 2 Department of Oceanography, University of Cape Town, Cape Town, South Africa 3 Department of Environmental Affairs, Cape Town, South Africa4 Rhodes University, Grahamstown, South Africa 5 Helmholtz Centre for Ocean Research, GEOMAR, Kiel, Germany

1. Introduction

- The Agulhas Bank is a continental shelf that extends south between Port Alfred andCape Town- The Agulhas Bank is very productive and is an important commercial fishing ground - Due to the widening of the continental shelf, meanders in the Agulhas Current often form at the eastern edge of the Agulhas Bank- Solitary meanders, also known as Natal Pulses, are thought to form off the northeast coast of South Africa and propagate downstream at a speed of about 20 km/day - Natal Pulses occur about 1.6 (Rouault and Penven, 2011) to 6 (Lutjeharms and Roberts, 1988) times per year and their frequency is thought to vary on interannual timescales (Krug and Tournadre, 2012)- Despite the importance of the Agulhas Bank, little is known about how mesoscale physical mechanisms influence both phytoplankton biomass and larval dispersal

2. Data and Methods

- A dedicated winter cruise from 31 August - 22 September 2010 allowed the jointcollection of physical (CTD and ADCP), chemical (nutrients), and biological(fluorometer, transmissometer, and mussel larval abundance) data. See Figure 1 for station locations.- Results from the cruise were enhanced with satellite (SST from remote sensingsystems, the absolute dynamic height from AVISO, and chlorophyll a from MODIS aqua) and meteorological (wind speed and directions from the NCEP/NCAR reanalysis) data.

Figure 1: The top panel is the SST from 2 September, the beginning of the cruise. Grey lines mark the bathymetry at 1000m intervals. Black isolines are the absolute dynamical height. Note the solitary meander (Natal Pulse; NP) and Agulhas Current meander (ACM) offshore of sections 1-6. The bottom panel is 3-daycomposite of chlorophyll a in logarithmic value. !"#$%&&'()#!)#*+#,

##-./0# ##12/0# ##11/0# ##13/0# ##14/0# ##1./0###*./5#

##*6/5#

##*4/5#

##*+/5#

##*3/5#

##**/5#

5000

4000

40002000

-22#7,#8 -

##-./0# ##12/0# ##11/0# ##13/0# ##14/0# ##1./0###*./5#

##*6/5#

##*4/5#

##*+/5#

##*3/5#

##**/5#

1000

2000

4000

9"#$%&&'()#!)#-2+#,

-+

-3-*

-1 -- -2: . 6

4+

-1*3

-+-3 -*

-1 -- -2:

-1*3

+6 4.

!!"#$%! !!&'$%! !!&&$%! !!&($%! !!&)$%! !!&#$%!

!!*+$,!

!!*)$,!

!!*-$,!

!!**$,!

1000

30003000

!

!./!0$1!"'!$2!345$6$74855!9!:;1!; */

!!*($,!

!!"#$%! !!&'$%! !!&&$%! !!&($%! !!&)$%! !!&#$%!

!!*+$,!

!!*)$,!

!!*-$,!

!!*($,!

!!**$,!

!

!

'<(

'<&

'<&

'

'

'

'

'

'<&

'<&

'<&

'<&

'<&

'<(

'<(

'<(

'<(' )'<)

'<)

'<)'<#

'<#

9/!,=9!>?6293=!@=;7=69@?6=!:°A/

"*

"(

"-

")

"+

"#

"B

&'

&"

&&

&*

'<"

'<*

"

*<"

"'

&'

NPACM

#

#

"-

"*"(

"& "" "'

B"&

*(-

)+

"-

"*"(

"& "" "'

B +& "

*(-

)

Fluorescence [10−6 g L−1]

−37−36.5−36−35.5−35−34.5

0

50

100

150

200

250

300

0

1

2

3

4

5

0

20

40

60

80

10010016°C15.815.615.415.2 16°C15.815.615.415.2

0

1

2

3

40

20

40

60

80

0

50

100

150

200

250

300−37−36.5−36−35.5−35−34.5

Meriodional velocity [m s−1]

−0.2

−0.1

0

0.1

0.2

Time (hh) on Sept 13th Time (hh) on Sept 17th121416182022242628h14 16 18 20 22h

Time (hh) on Sept 13th Time (hh) on Sept 17th121416182022242628h14 16 18 20 22h

Fl

Fluor.

velocity

Station 5Temperature

Station 6Temperature

pres

sure

Latitude (°N)

Pres

sure

(d

b)Pr

essu

re

(db)

!"#$$%"#&'()*

&

&

12

14

1616

+,-.+,-/+,-,+,-++,-0+,-1

/2

122

1/2

022

0/2

+22 2-/

1

1-/

!"#$$%"#&'()*

&

&

12

14

16

18

20

+,-3/+,-3+,-./+,-.+,-//+,-/+,-,/+,-,+,-+/+,-++,-0/

/2

122

1/2

022

0/2

+22 2-/

1

1-/

45676%(#&'°8*

!"#$$%"#&'()*

&

&

12

14

16

18

2012

+,-9+,-:+,-3+,-.+,-/+,-,+,-++,-0

/2

122

1/2

022

0/2

+22 2-/

1

1-/

5*

)*

;*

For more information please see: Jackson, J.M., Rainville, L., Roberts, M.J., McQuaid, C.D., and J.R.E. Lutjeharms, 2012, Continental Shelf Research, in press.

3.1 Results - Observation of a Natal Pulse and Agulhas Current Meander

- Satellite data show that there was a Natal Pulse on the southern end of sections 1-3 and an Agulhas Current meander at the southern end of sections 4-6 (Figure 1). - The ADCP data show that the current was northeastward at the edge of the cyclonic Natal Pulse (Figure 2)- The ADCP data show that the current was northward and southwestward on the shelf within the Agulhas Current meander (Figure 2)- These results suggest that a Natal Pulse can cause the Agulhas Current to cross the continental shelf as an Agulhas Current meander

Figure 2: Curent speed and direction at 35m from the shipboard ADCP. The current was westward or southwestward except where circled in grey.

Figure 3: Vertical sections of beam attenuation (m-1) across a) section 4, b) section 5, and c) section 6. High beam attenuation values represents high particle concentrations. Black lines indicate the observed temperature

Figure 4: Vertical view of section 12 of a) fluorescence withtemperature contour lines where the 15ºC isotherm is highlighted in bold white. Panel b) shows the meridionalvelocity with temperature contour lines. A large storm occurred on 14 September so sampling of stations 1-5 took place 5 days before stations 6-16.

4. Internal waves

- The depth of the thermocline varied near the shelf break along section 12 (Figure 4)- The meridional velocity was baroclinic near the shelf-break- The periodicity of the vertical displacements suggest the passage ofinternal waves with a wave height of about 50m and a wavelengthof about 50 km- Fluorescence (and larval data - see Porri et al., 2012) data show that the phytoplankton were concentrated at the crest of the internal waves, suggesting the crests are biological hotspots

Figure 5: A snapshot from the high-resolution nested oceanmodel, INALT01, shows that Natal Pulses and their associatedAgulhas Current meander can be modeled. Here the blue lineis the 0.15 cm isoline of sea surface height.

5. Model Results

- The high-resolution nested ocean model INALT01 (Biastoch et al.,2009; Durgadoo and Biastoch, 2012) was used to study Natal Pulses (Figures 5 and 6)- Similar to Rouault and Penven (2011), the model showed that therewere about 1-2 Natal Pulses per year (Figure 6)- Agulhas Current meanders were defined as when the core of theAgulhas Current moved inshore of its mean position- An Agulhas Current meander almost always preceded a NatalPulse. - Agulhas Current meanders occur more frequently than NatalPulses

Figure 6: Results from INALT01 show the prevalence of Natal Pulses (red) and Agulhas Current meanders (blue) from 1999-2007. A Natal Pulse was defined as when the core of the Agulhas Current was at least 10km offshore of its mean position.

6. Future Research

- Mechanisms that form internal waves, and the aggregation of phytoplankton at their crest, are important areas of future research - Agulhas Current meanders, and the observed cross-shelf transport, likely influences local ecosystems and fisheries- An ACEP III project from 2012-2015 will study the small-scale bio-physical dynamics of larvaldispersal in Algoa and St Francis Bays; thisproject will hopefully give insight into Natal Pulses - We plan to submit a proposal to NSF to study the physics of Natal Pulses and Agulhas Current meanders and their biological impact on the eastern Agulhas Bank - please talk to us if you are interested in being involved!

References:Biastoch, A., L. Beal, J.R.E. Lutjeharms, and T.G.D. Casal, (2009), Variability and coherence of the Agulhas undercurrent in a high-resolution ocean general circulation model, Journal of Physical Oceanography, 39, 2417-2435.Durgadoo, J.V., and A. Biastoch, (2012), The Agulhas System as a key region of the global oceanic circulation, in W.E. Nagel et al. (Eds.): High Performance Computing in Science and Engineering ‘12, Spring, in press.Krug, M., and J. Tournadre, (2012), Satellite observations of an annual cycle in the Agulhas Current, Geophysical Research Letters, 38, L15607, doi:10.1029/2012GL052335.Lutjeharms, J.R.E., and H.R. Roberts, (1988), The Natal Pulse: an extreme transient on the Agulhas Current, JGR-Oceans, 93(C1), 631-645.Rouault, M., and P. Penven, (2011), New Perspective of Natal Pulses from satellite observations, JGR-Oceans, 116, C07013 doi:10.1029/JC006866.

3.2 Results - The biological influence of a Natal Pulse

- Beam attenuation (calculated from the transmissometer data) show that particle concentrations increased from sections 4 - 6 asthe Agulhas Current moved offshore (Figure 3). - Fluorescence (not shown) and larval abundance (Porri et al., in review) data also showed increased concentrations at the shelfbreak of section 6.- These results suggest that during a Natal Pulse-driven Agulhas Current meander, particles are entrained into the Agulhas Current and transported offshore.

Clearwater

Clearwater

Particlerichwater

Section 4

Section 5

Section 6