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Coastal upwelling on the far eastern Agulhas Bank associated with large meanders in the Agulhas Current

Authors:
  • South African Environmental Observation Network - Elwandle Node
  • South African Environmental Observation Network (SAEON)

Abstract and Figures

Six large solitary meanders in the Agulhas Current, so-called Natal Pulses, propagated down the eastern coast of South Africa between 2009 and 2011. Their influence on the coastal waters between Port Alfred and Algoa Bay, on the far eastern Agulhas Bank, was measured by thermistor strings moored at 30–80 m bottom depths and two current metres (30 m bottom depth) located at both sides of Algoa Bay. During all events active upwelling lasting 1–3 weeks was observed over the inner shelf and in Algoa Bay. During upwelling the isotherms ascended at an average rate of 1.8 m day−1 as the cold bottom layer increased in thickness to 40–60 m, although upwelled water did not break the surface in all cases. Cold water remained in the area for a further 2–3 weeks. During three Natal Pulses the water temperatures at the outer moorings initially increased as the plume of the leading edge (crest) of the meander moved onshore. During one Natal Pulse upwelling was recorded before the warm water plume impacted the moorings. At the onset of upwelling currents switched to the southwest in the case of Bird Island and southward at the Cape Recife inner-bay site and reached a maximum speed of 80 cm s−1. During all Natal Pulses cold bottom water (10–12 °C) flooded over the 80 m bottom depth moorings as the crest of the meander moved onshore, but also around the same time the core of the Agulhas Current began to move offshore. In all cases upwelling was wide-spread.
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Research papers
Coastal upwelling on the far eastern Agulhas Bank associated with
large meanders in the Agulhas Current
W.S. Goschen
a,d,
n
, T.G. Bornman
b,d
, S.H.P. Deyzel
b,d
, E.H. Schumann
c,d
a
South African Environmental Observation Network, SAEON Marine Offshore Egagasini Node, Private Bag X2, Roggebaai 8012, South Africa
b
South African Environmental Observation Network, SAEON Marine Coastal Elwandle Node, Private Bay Bag 1015, Grahamstown 6140, South Africa
c
Department of Geosciences, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth 6031, South Africa
d
Institute for Coastal and Marine Research, Nelson Mandela Metropolitan University, P.O. Box 77000, Port Elizabeth 6031, South Africa
article info
Article history:
Received 17 November 2014
Received in revised form
6 March 2015
Accepted 6 April 2015
Available online 7 April 2015
Keywords:
Agulhas Current
Natal Pulse
Upwelling
Coastal
Agulhas Bank
Algoa Bay
abstract
Six large solitary meanders in the Agulhas Current, so-called Natal Pulses, propagated down the eastern
coast of South Africa between 2009 and 2011. Their inuence on the coastal waters between Port Alfred
and Algoa Bay, on the far eastern Agulhas Bank, was measured by thermistor strings moored at 3080 m
bottom depths and two current metres (30 m bottom depth) located at both sides of Algoa Bay. During all
events active upwelling lasting 13 weeks was observed over the inner shelf and in Algoa Bay. During
upwelling the isotherms ascended at an average rate of 1.8 m day
1
as the cold bottom layer increased in
thickness to 4060 m, although upwelled water did not break the surface in all cases. Cold water re-
mained in the area for a further 23 weeks. During three Natal Pulses the water temperatures at the
outer moorings initially increased as the plume of the leading edge (crest) of the meander moved on-
shore. During one Natal Pulse upwelling was recorded before the warm water plume impacted the
moorings. At the onset of upwelling currents switched to the southwest in the case of Bird Island and
southward at the Cape Recife inner-bay site and reached a maximum speed of 80 cm s
1
. During all Natal
Pulses cold bottom water (1012 °C) ooded over the 80 m bottom depth moorings as the crest of the
meander moved onshore, but also around the same time the core of the Agulhas Current began to move
offshore. In all cases upwelling was wide-spread.
&2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Meanders in the Agulhas Current (Lutjeharms, 2006) with an
offshore extent of 30300 km, propagate south-westwards along
the east coast of South Africa (Fig. 1). They generally form off
KwaZulu-Natal and to the north, but they may also form down-
stream of Algoa Bay (Lutjeharms et al., 1989). Very large solitary
meanders, with amplitudes of 100300 km (Gründlingh, 1979), are
termed Natal Pulses (Lutjeharms and Roberts, 1988). Natal Pulses
occur on average 1.61.7 times per year (Rouault and Penven,
2011;Krug et al., 2014), but their number can range from zero to
56 per year (Bryden et al., 2005;Krug and Tournadre, 2012;Krug
et al., 2014). The meanders generally travel at speeds of between
10 and 20 km day
1
, although speeds of 465 km day
1
have
been recorded (Lutjeharms et al., 1989;Rouault and Penven, 2011;
Krug et al., 2014). With a residence time of 65 days on average and
impacting on the eastern Agulhas Bank 110 days per year (Krug
et al., 2014), the dynamics and structure of the coastal ocean is
inuenced substantially by Natal Pulses.
During such a meander, including a Natal Pulse, the Agulhas
Current ows around an inshore cold core cyclonic eddy (Lutje-
harms et al., 1989,2003). Cyclonic eddies on the inshore side of the
Agulhas Current have an upward doming of isotherms in their
centre with upwelling of water on the inshore side and these move
cold water onto the shelf and subsequently closer to the coast
(Lutjeharms and Roberts, 1988;Lutjeharms et al., 1989,2003).
Bryden et al. (2005) described very cold waters near the shelf
accompanied by large upwelling velocities during Natal Pulses.
The cold water consists of South Indian Central Water, which
Lutjeharms et al. (2000) found to lie deeper than 40 0 m along the
continental slope off Durban, whereas off Port Elizabeth it is on the
shelf at 150 m depth. The upwelled water domes up over the shelf
break and frequently breaks the surface along the inshore
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/csr
Continental Shelf Research
http://dx.doi.org/10.1016/j.csr.2015.04.004
0278-4343/&2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
n
Corresponding author at: South African Environmental Observation Network,
SAEON Marine Offshore Egagasini Node, Private Bag X2, Roggebaai 8012, South
Africa.
E-mail addresses: wayne@saeon.ac.za (W.S. Goschen),
tommy@saeon.ac.za (T.G. Bornman), shaun@saeon.ac.za (S.H.P. Deyzel),
eckarts@mweb.co.za (E.H. Schumann).
Continental Shelf Research 101 (2015) 3446
boundary of the Agulhas Current (Lutjeharms et al., 2000). This
shelf-edge upwelling is found close to the coast near Mbashe,
north of East London, but lies further offshore towards the south as
the shelf- edge and Agulhas Current begin to diverge from the
coast (Schumann, 1987).
The presence of this cold water over the shelf is due to a
number of mechanisms. The dynamic requirement that vertical
current shear requires a horizontal density gradient (the so-called
thermal windequation) means that isotherms slope upwards on
the inner boundary of the Current. Cold water is also forced up-
wards in the bottom boundary layer through the process of Ekman
veering (Schumann, 1986;1987), while divergence of isobaths as
the shelf widens southwards of Port Alfred will contribute to this
upwelling (Gill and Schumann, 1979).
Natal Pulses have in the past been associated with a decrease in
near-shore and coastal temperatures in the Algoa Bay region, lo-
cated on the far eastern Agulhas Bank. Upwelling of cold water
along the coast adjacent to a Natal Pulse, to the north of Algoa Bay,
was noted by Lutjeharms and Roberts (1988).Lutjeharms and
Roberts (1988) did however not discuss the signicance of the
coastal upwelling visible in several of their thermal infrared sa-
tellite images of Natal Pulses. Goschen and Schumann (1988) de-
scribed how cold water upwelled in Algoa Bay during the passing
of a Natal Pulse. Enhanced upwelling off the southern shorelines of
the prominent capes of the South and Eastern Cape during large
meanders of the Agulhas Current (Natal Pulses) was also men-
tioned by Schumann et al. (1988). Although their paper describes
wind-driven upwelling at the capes, they also showed extensive
cold water entering Algoa Bay from the north, which was asso-
ciated with a large meander. In addition, they described an initial
inshore inux of warm surface water at the leading crest of the
meander followed by coastal upwelling opposite the main bight of
the meander. More recently, Roberts (2010) observed an event
where cold water associated with a Natal Pulse reached the
northern shoreline of Algoa Bay.
A visual inspection of satellite images (SST and chlorophyll)
over this period showed that the upwelling extends further off-
shore and lasts much longer than wind-driven upwelling (Schu-
mann et al., 1982;Goschen et al., 2012), while it also appeared that
nutrients were introduced into coastal waters over a broad area.
Inevitably, there may also be occasions when the two processes
overlap. The aim of this paper is to describe the coastal upwelling
between Algoa Bay and Port Alfred, on the far eastern Agulhas
Bank, associated with the passage of Natal Pulses. The contribution
of wind to upwelling is largely ignored.
2. Methods and measurements
Measurements for this study were made by an array of un-
derwater temperature recorders (UTRs) and two acoustic Doppler
current prolers (ADCPs) located between Cape Recife and Port
Alfred (Fig. 1). The shallow UTR moorings were deployed in 30 m
bottom depth and the deeper UTRs in 6080 m bottom depth.
Temperatures were measured at one hourly interval by Onset
Hobo Pro V2 Water Temperature Loggers (given accuracy of
70.2 °C). The UTRs on each anchor/rope/buoy mooring were tied
at 10 m depth intervals from the bottom up until 10 m below the
surface. An extra UTR was added at 15 m depth below the surface
for UTRs deployed in 30 m depth. Currents were measured by
Teledyne RDI Express 600 kHz ADCPs placed in a frame xed to
the sea oor, with the height of the sensors about 1 m above the
bottom and sampling at 20 min intervals. The ADCPs were moored
in 30 m bottom depth and were set up with 59 bins each with a
length of 0.5 m and had 121 pings per ensemble with 7 pings s
1
.
Hourly wind data was provided by the South African Weather
Service (SAWS) coastal weather stations located at Bird Island and
Port Elizabeth airport. MODIS sea surface temperature (SST) and
surface chlorophyll concentrations (chl-a, OC3 Algorithm) images
with a temporal resolution of 1 day were downloaded from the
Fig. 1. A map of Algoa Bay showing the location of underwater temperature recorders (UTR) and two acoustic Doppler current prolers (ADCP) used in this study. The UTRs
are each marked by crosses and three letters. A coastal weather station was located on Bird Island and wind data was also available from Port Elizabeth airport. The core of
the Agulhas Current is generally located offshore the 200 m isobath.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 3446 35
Marine Remote Sensing Unit website (http://www.afro-sea.org.za).
In order to eliminate short term variability of less than about
30 h in the temperature data, the measured one hourly values
were rst low-pass ltered to daily values by a standard cosine-
Lanczos lter with 97 weights and a quarter power point (qpp) of
0.031 cycles per hour (cph). The measured ADCP data (20 min
intervals) were initially ltered to one hourly values by a cosine-
Lanczos lter with 18 weights and a qpp of 0.53496 cph before
further analysis. The currents were also then further low-pass
ltered by the cosine-Lanczos lter with 97 weights and a quarter
power point (qpp) of 0.031 cycles per hour (cph) to eliminate tidal,
inertial currents and other shorter period oscillations.
In the context of Natal Pulses, the initiation of coastal upwelling
is dened here to be when bottom layer water decreases in tem-
perature and increases in thickness over a period of 7 days or
greater. It is further assumed to be ended when the temperature of
the bottom layers start to increase and its thickness decrease over
a period of 7 days or greater. Active upwelling is dened as the
period between the start and end of upwelling. Seven days were
chosen because wind-driven upwelling in the study area uctuates
at periods between 2 and 7 days (Schumann et al., 1991;Schu-
mann and Martin, 1991), so any drop in temperature over a period
of greater than 7 days could probably be attributed to Agulhas
Current inuences, if features of the Agulhas Current such as a
Natal Pulse were present offshore during that time.
Of interest was the onset of upwelling in relation to the dis-
tance of the Agulhas Current from the shoreline, as this was con-
sidered relevant to understanding the mechanisms behind the
upwelling. If cold water emerged at the coastline opposite the
leading edge of the meander, a possible cause of the upwelling
could be encroachment of the Agulhas Current upon the shoreline
(e.g. Condie, 1995;Roughan and Middleton, 2002,2004 for the
East Australian Current). If emergence of cold water at the coast
occurred in the trough or opposite the trailing edge of the
meander, it was possible that upwelling was caused by the di-
vergence of the Agulhas Current from the coastline (e.g. Tsugawa
and Hasumi, 2010).
Thus, in order to investigate when upwelling was initiated in
relation to the Agulhas Current's position offshore of Algoa Bay,
the core, edge, plume and lament (see Lutjeharms et al., 1989,
Lutjeharms, 2006) of the Agulhas Current were identied from SST
images and their distances measured from the northern shoreline
in mid Algoa Bay (Sundays River). The particular measurement line
was chosen because the inuence of the Agulhas Current on the
waters inside Algoa Bay, where the majority of the instruments
were located, was of interest. The method was similar to that used
by Goschen and Schumann (1990). The core of the Agulhas Current
was identied on the SST images as the position of maximum sea
surface temperature of the Agulhas Current, the edge of the
Agulhas Current was identied as the maximum sea surface
temperature gradient on the inshore thermal front of the Agulhas
Current, a plume (Lutjeharms et al., 1989)was identied as the
inshore maximum temperature sea surface gradient of the cyclo-
nic eddy or shear-edge eddy on the inshore boundary of the
Agulhas Current, and a lament was identied as the inshore
maximum temperature gradient of Agulhas Current water that
appeared to be attached to plumes and had penetrated into the
coastal waters (see Fig. 4). Distances were estimated by working
out the scale of the images and then measuring the distance from
the shoreline to the features in an across-shelf direction. It must be
emphasised that these were rough measurements that gave ap-
proximate distances and were thus highly subjective. However, it
was considered an appropriate method for this study since of in-
terest was only a rough indication of how close Agulhas Current
features were from the shoreline. Errors in measurements were
estimated to be in the order of 1015 km, as were those given by
Goschen and Schumann (1990).
Fig. 4 also illustrates the three types of upwelling that have
been documented for this region: wind-driven upwelling off the
capes (Schumann et al., 1982;Schumann, 1999;Goschen et al.,
2012), upwelling along the inshore edge of Agulhas Current (Lut-
jeharms et al., 2000) and upwelling in the core of a cyclonic eddy
(Lutjeharms et al., 1989).
Temperature sections perpendicular to the shoreline were
constructed from mooring timeseries data at a time of 12:00 h on
selected dates. The Port Alfred section comprised data from Port
Alfred Inner (PAI) and Port Alfred Offshore (PAO), the Bird Island
section comprised data from Woody Cape Inner (WCI), Bird Island
Outer (BIT) and Bird Island Offshore (BIO) while the Algoa Bay
Middle section comprised data from Sundays River Inner (SRI),
Algoa Bay Central (ABC) and Algoa Bay Mouth (ABM).
3. Results
Sea temperatures through much of the water column (1080 m
depths) from the Algoa Bay UTR moorings over a period of four
years (20092012) are shown in Fig. 3. The data was low-passed
ltered which allowed for an illustration of longer-period (days to
weeks) changes in temperatures that could have been caused by
uctuating (27 days) coastal winds (Schumann et al., 1991;
Schumann and Martin, 1991;Schumann, 1999), Agulhas Current-
driven upwelling (as noted by Schumann et al., 1988) and Agulhas
Current warm water intrusions into Algoa Bay as observed by
Goschen and Schumann (1994).
In Fig. 3 seasonal variability is evident, although perhaps
masked by the complexity inherent in the system. The seasons,
marked at the top of Fig. 3,aredened as summer from December
to February, autumn from March to May, winter from June to
August and spring from September to November. During winter
and spring, water with temperatures of between 16 and 18 °C
generally permeated the bay and the water column was generally
well mixed, as was found by Schumann and Beekman (1984) and
Swart and Largier (1987) over the Agulhas Bank and Schumann
et al. (2005) in the western sector of the Algoa Bay. However,
warmer (1922 °C) and cooler (1015 °C) water was found in Algoa
Bay during these seasons which may be attributed to Agulhas
Current inuences (discussed below). A typical example of the
water column during winterspring of 2009 is marked as A on
Fig. 3 when water of 1618 °C predominated in the bay over the
(approx.) 4 month period from June to September. Other years at
the other moorings showed similar well-mixed structure of the
water column. During summerautumn there was greater varia-
bility in temperatures with warm water periods (1922 °C) inter-
sected by cold water (1012 °C) events. The warming of the surface
water during summerautumn was caused by an increase in solar
ux at this time of year which contributes to a strong thermocline
above a stratied water column (Schumann and Beekman, 1984;
Largier and Swart, 1987;Swart and Largier, 1987;Schumann et al.,
2005).
However, it is clear that there was great variability in both cold
bottom water and warm surface waters over periods of days and
weeks throughout the year that did not conform to the seasonal
trend. Over these long periods, cold bottom water of between
10 °C and 16 °C periodically reached the surface (B and C on Fig. 3).
These long periods also showed that warm surface waters of be-
tween 18 °C and 22 °C (and higher) were recorded in Algoa Bay,
even at sites close to the shoreline such as St. Croix (SCI in Fig. 1),
for example D in Fig. 3. During events of this nature generally the
whole water column was affected by both cooling and warming
events.
A visual inspection of satellite images (SST and chl-a) over this
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 344636
period showed that the warm water events were generally caused
by surface plumes or laments from the Agulhas Current entering
the bay. Some cold water events were linked to long periods
(several days) of prevalent easterly component winds, but some
also occurred during Natal Pulses and other large meanders, or
during a combination of both. In this study we focus on the role of
Natal Pulses in driving upwelling in the coastal zone of the eastern
Agulhas Bank and largely ignore the wind-driven effects.
The six Natal Pulses chosen for this study are shown as SST
images in Fig. 4. The rst Natal Pulse propagated past Algoa Bay
during December 2009, three others occurred during April/May,
June/July and August/September 2010, and the other two occurred
during January/February and November 2011. The shape and size
of the Natal Pulses were different in each case, although during
December 2009 (Fig. 4a) and April 2010 (Fig. 4b) they appear si-
milar on the selected images. In three of the images (Fig. 4a, d, e) it
is clear that cold water (approximately 1213 °C), shown as shades
of blue, had broken the surface in patches in Algoa Bay, St Francis
Bay and over the inner shelf. In these images water with tem-
peratures lower than the shelf water is visible in the central area
or near to the shoreline of the bays. These pockets of cooler waters
observed near the coast are indicative of upwelling. The surface
expressions had a different signature to wind-driven upwelling,
which show upwelling anchored to the southern sides of the capes
and extending westward into the bays in the general shape of a
triangle (Schumann et al., 1982,1988). However, cold surface
water during the December 2009 Natal Pulse (Fig. 4a) appears to
be a combination of wind-driven and Agulhas Current-driven
upwelling, as is conrmed by the strong north-easterly component
winds that were active during that period (not show). During the
event of January 2011 (Fig. 4e) the cooler surface water appears to
have originated from upwelling along the inshore edge of the
Agulhas Current.
The six Natal Pulses were also evident in the chl-a images
shown in Fig. 5 where high surface chl-a concentrations in the
nutrient rich waters coastal zone during Natal Pulses is apparent.
During all six Natal Pulses chl-a concentrations of
2.050.0 mg m
3
were found over a large area from north-east of
Port Alfred to west of St. Francis Bay, with a possible exception
being the large meander of November 2011 when there was cloud
cover to the north-east of Algoa Bay. Of interest is that during four
Natal Pulses (Fig. 5a, b, d, e) there were areas of lower chl-a con-
centrations (about 0.5 mg m
3
), signifying shelf water, found be-
tween high concentrations along the inshore boundary of the
Agulhas Current and high concentrations in the bays and along the
coastline. The inshore cyclonic eddies also elevated the chl-a to
2.020.0 mg m
3
in their core.
Shown in Fig. 6 are low-pass ltered temperatures from the
Algoa Bay Mouth mooring (ABM in Fig. 1) during the six Natal
Pulses. The periods over which each plot were drawn covered the
time during which the Natal Pulses were offshore of Algoa Bay,
from the leading edge of the meanders to the trailing edge (de-
ned in Fig. 2). The other deeper (6080 m bottom depth) moor-
ings, such as offshore Port Alfred (PAO), Bird Island (BIO) and Algoa
Bay Central (ABC) showed similar variations in temperature over
time, but for clarity only the Algoa Bay Mouth mooring is illu-
strated here. Fig. 6 shows that during all events, cold water
(o14 °C, shown as dark blue and shades of purple) moved over
the moorings and then began to subside a week or more later. At
ABM and the other deep moorings the cold water was rst re-
corded by the mooring's bottom sensors at depths of 70 m, in 80 m
bottom depth. This layer of cold bottom water then increased in
thickness to between 40 and 60 m off the bottom. During April
2010 the cold bottom water temperature was o10 °C, but
Fig. 2. A schematic of a Natal Pulse offshore Algoa Bay. The leading edge (crest) and trailing edge of the meander are marked. Illustrated is the sea surface expression of
wind-driven upwelling that takes place off the capes of the Eastern Cape. Upwelling along the Agulhas Current inshore edge is illustrated. Shown is a line perpendicular to
the coastline off Sundays River (middle Algoa Bay) along which the distances to Agulhas Current related features were measured.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 3446 37
generally the bottom layer temperature was 1014 °C. During two
of the Natal Pulses (Fig. 6a, e) water with temperatures o14 °C
reached the surface (here taken as 10 m depth).
Despite the lack of satellite images showing cold water on the
surface during the April/May and June/July 2010 Natal Pulses
(Fig. 4b and c respectively), Fig. 6b and c shows that upwelling did
occur, although the cold water did not reach the surface. During
the Natal Pulses that occurred during mid-summer, such as the
December 2009 (Fig. 6a) and January/February 2011 events
(Fig. 6e), the water column was already cool at around 1214 °C.
However, during the other Natal Pulses, water of even lower
temperature (1012 °C) penetrated into Algoa Bay along the bot-
tom and eventually broke the surface. The August/September 2010
(Fig. 6d) event showed an intrusion of cold water, but the up-
welling was weak and bottom layer did not thicken substantially.
This event was intercepted during a cruise described by Jackson
et al. (2012) who found no evidence of upwelling, although their
cruise transects off Algoa Bay intersected the trailing edge of the
Natal Pulse and could have missed the cold water intrusion.
During the Natal Pulses of April/May 2010, June/July 2010 and
November 2011, warm surface water (plume) intrusions into Algoa
Bay occurred before upwelling (Fig. 6b, c, f). During the April/May
2010 event, cold water was also upwelled along the inshore edge
of the plume, before the inuence of plume water was evident at
the moorings. From satellite imagery (Fig. 4) these warm water
intrusions (laments) were identied as originating from the
plume at the crest of the Natal Pulse. In the majority of cases, the
inuence of a Natal Pulse was rst recorded by a warm surface
intrusion of Agulhas Current water, if it had penetrated that far
inshore to be recorded by the moorings. The warm water then
owed past the area toward the south-west, followed by an in-
trusion of cold water which led to prolonged active upwelling over
13 weeks, while it took 23 weeks for the cold water to disperse
after active upwelling had stopped.
The distances of the core, edge, plume and lament from the
shoreline in the middle of Algoa Bay (Sundays River) over a period
when the six Natal Pulses were passing Algoa Bay are shown as
coloured dots on Fig. 6. The dots for the core and plume have been
joined by black and orange lines, respectively. Fig. 6 shows that
before the Natal Pulses reached the measuring line through the
middle of Algoa Bay, the core of the Agulhas Current lay south-
eastward of the shelf break, generally between 100 and 200 km off
the Algoa Bay shoreline at Sundays River (or 50100 km off Cape
Recife). During this time the inshore edge of the Agulhas Current
was always found closer to the shore, near the shelf break, and
generally there were no shear edge eddies and plumes along its
inshore boundary. As time progressed, the core of the Agulhas
Current moved further offshore, as did the edge of the current
while at the same time the crest of the meander moved over the
shelf and closer to the shoreline. Fig. 2 provides an illustration of
how this could be accomplished. With this the plumes and la-
ments could have reached the shoreline along the middle section
of Algoa Bay, off Sundays River, as occurred in June 2010 and No-
vember 2011. After a few weeks the core and edge of the Agulhas
Current returned to the shelf edge and the plume and lament
moved away, out of Algoa Bay, as the Natal Pulse propagated
downstream.
During all six Natal Pulses studied, upwelling was observed to
begin at the 80 m bottom depth moorings soon after the core of
the Agulhas Current began to move offshore (Fig. 6). This was also
around the time when the plume at the crest of the meander or
Agulhas Current laments began to move onshore. Since cold
water reached the Algoa Bay mooring at the same time as the crest
of the meander moved closer to shore and when the core of the
meander moved further offshore, it is difcult to ascertain from
these data what caused the upwelling; encroachment of the
Agulhas Current meander leading edge upon the coastline or di-
vergence of the Agulhas Current core from the coastline. The time
delay between the initiation of upwelling over the shelf and the
recording of cold water by the 80 m depth moorings is unknown,
since no measurements were made further out over the shelf.
Fig. 3. Depth/time plots of sea temperatures over a 4 year period, January 2009 to
December 2012, recorded at the moorings located in Algoa Bay at (a) Bird Island
Inner (BII), (b) Woody Cape (WCI), (c) Algoa Bay Central (ABC), (d) Algoa Bay Mouth
(ABM), (e) St Croix (SCI) and (f) Cape Recife (CRI). Seasons are marked at the top.
Selected warming and cooling events are marked AE. Lost data are shown as
blanks.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 344638
The extent of upwelling in the coastal zone and bays of the far
eastern Agulhas Bank during a Natal Pulse was wide-spread. An
example is given by the event of April/May 2010 (Fig. 7) where the
water column was initially well mixed at about 19 °C. As the ef-
fects of the Natal Pulse was felt, the temperature at the offshore
mooring dropped by up to 9 °C while those at the inner mooring
(30 m bottom depth) dropped by 68°C. Other Natal Pulses
showed similar wide-spread inux of cold water, although the
extent was especially wide-spread during summer upwelling
season when cold water was already near the shoreline, as in the
example during the December 2009 (Fig. 8). The inshore extent
and thickness of the cold bottom layer varied in each case and at
times the cold water did not break the surface. The December
2009 Natal Pulse was intercepted over the central Agulhas Bank by
Krug et al. (2014), in 250 m water depth about 115 km from the
shore; they only noted an increase in surface temperature.
Fig. 4. NOAA/MODIS satellite images of sea surface temperatures (SST) offshore Algoa Bay showing six Natal Pulses in the Agulhas Current between 2009 and 2011.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 3446 39
The progression of the Natal Pulse past Algoa Bay and inuence
of the Agulhas Current on the water structures is illustrated by
sections made during the June/July 2010 event (Fig. 9). The event
occurred during winter, out of the usual wind-driven upwelling
season and implies that any major, sudden and long lasting change
in temperature of the water column or currents over period of
days was due to Agulhas Current inuences. As with the April/May
2010 event, the water was generally well mixed before the event,
however during this event there was rst a warming as the plume/
lament penetrated towards the shore. This is seen as water with
temperatures 419 °C reached to within 10 km of the shoreline.
Cold water of o13 °C then began to ow along the bottom toward
shore and eventually the whole column became more stratied
and cooler. In Algoa Bay the thermocline was found at around
30 m depth on 28 July 2010 (Fig. 9d), near the end of the active
upwelling phase.
Fig. 5. NOAA/MODIS satellite images of surface Chlorophyll Concentrations (OC3 Algorithm) offshore Algoa Bay during six Natal Pulses in the Agulhas Current.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 344640
Isotherms were extracted from the UTR moorings timeseries
data during the six Natal Pulses and their rate of accent during the
active phase of upwelling calculated using linear regression. Ta-
ble 1 shows the rate of upward displacement of selected isotherms
from the Algoa Bay Mouth (ABM) mooring. The average time of
active upwelling was 17 days, with a minimum of 7 days and a
maximum of 24 days. The average upward movement of the iso-
therms was 30 m, with a minimum of 14 m and a maximum of
42 m, giving the average rate of upward displacement of isotherms
equal to 1.8 m day
1
(2.1 10
5
ms
1
). The estimated error in
calculations was 0.8 m day
1
.
The principal axes at 10 m depth at Cape Recife and Bird Island
during ve Natal Pulses were calculated using the method de-
scribed by Kundu and Allen (1976) and plotted in Fig. 10. The
currents were then oriented to the principal axes direction of 58/
238°true north for Bird Island and 178/358°true north for Cape
Recife. The principal axis direction were aligned with the local
isobaths (as was found by Goschen et al., 2012) so this gave the
current speed component parallel to the isobaths (and the
shoreline) at the location of the moorings. The mean direction of
currents was towards the south-west at Bird Island and towards
the south at Cape Recife, out of Algoa Bay. Currents were measured
at Bird Island during the rst four Natal Pulses but unfortunately
current data at Cape Recife were only available during the De-
cember 2009 event and at the start of the January 2011 event.
The relationship between along-shore and across-shore cur-
rents and temperatures is shown in Fig. 11. It is evident that there
was great variability in the system, even at periods greater than
30 h to which the data was low-pass ltered. However, it is dis-
cernible that at Bird Island the long-shore currents made an
abrupt switch to the south-westward near the start of the active
upwelling phase, on 1415 December 2009 (Fig. 10a), 2425 April
(Fig. 10b), 2728 June 2010 (Fig. 10c) and 3031 August (Fig. 10d).
These are marked as arrows on Fig. 11. At Bird Island the currents
reached a maximum speed of 80 cm s
1
during those events. At
Cape Recife southward currents also reached a maximum speed of
80 cm s
1
on 1415 December 2009 and 2122 January 201. Since
these maximum occurred near the start of the active upwelling
period, it is likely that the Natal Pulse had an inuence on the
coastal currents. Jackson et al. (2012) and Porri et al. (2014) also
found that the Natal Pulse of September 2010 enhanced westward
ow out of the bays of the eastern Agulhas Bank. The cross-shore
currents were at speeds o20 cm s
1
and highly variable with no
discernible correlation to other parameters.
4. Discussion
Six large solitary meanders (Natal Pulses) were chosen to in-
vestigate the role of the Agulhas Current in causing the emergence
of cold water in Algoa Bay, a large open log-spiral bay on the far
eastern Agulhas Bank, and towards the north off Port Alfred. It was
found that during all Natal Pulses cold bottom water ooded into
the entire study area, up until at least the 30 m isobath. These
results were conrmed by satellite imagery which showed that the
surface signature of the upwelled water was often observed along
the bay's shoreline and in the centre of the bays, and not anchored
to the bay's capes as in the case of wind-driven upwelling (Schu-
mann et al., 1982,1988).
When the leading edge (crest) of a Natal Pulse meander was
opposite Algoa Bay, during some events the water column in the
bay rst increased in temperature as a warm water lament/
plume (originating from the meander's crest) penetrated over the
shelf and moved towards the shore. During some Natal Pulses the
plume/lament did not reach the moorings. By comparing in situ
measurements with satellite imagery it was found that upwelling
Fig. 6. Depth/time plots of temperatures recorded at the Algoa Bay Mouth (ABM)
mooring located near the mouth of Algoa Bay, approximately midway between
Cape Recife and Cape Padrone, during six Natal Pulses. Also shown are plots of the
distances over time from Sundays River (mid north-eastern shoreline of Algoa Bay)
to features of the Agulhas Current identied as the Agulhas Current core (Core), the
inshore edge of the Agulhas Current (Edge), the plume of the leading crest of the
meander (Plume) and Water (lament or surface water that appeared to originate
from the plume).
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 3446 41
in Algoa Bay also began when the crest of the Natal Pulse meander
began to move onshore, but because of the dynamics of the Natal
Pulse, also when the core of the Agulhas Current began to move
offshore. The thickness of the bottom upwelled layer expanded
over time, to 4060 m at the outer moorings, as the isotherms
ascended at an average rate of 1.8 m day
1
and the warm water
plume moved out of the bay. Active upwelling then lasted between
1 and 3 weeks until the time when the Agulhas Current core began
to move onshore again. This was when the Natal Pulse started to
propagate downstream toward the southwest. In addition, as the
leading edge of a Natal Pulse passed Algoa Bay, it was found, that
the current strength increased in the coastal zone toward the
southwest, or towards the south in the western sector of Algoa
Bay. This would result in a net ow of water offshore from the
coast. Krug et al. (2014) also noted that the strongest current and
temperature anomalies on the central Agulhas Bank were asso-
ciated with the leading edge of Natal Pulses.
Cold water on the inshore edge of the Agulhas Current was
Fig. 7. Sea temperatures recorded at thermistor string moorings between Port Alfred and Cape Recife during the Natal Pulse of April/May 2010. Different colour lines reect
the temperatures at the indicated depths: black for 10 m, blue for 20 m and red for 30 m. (For interpretation of the references to colour in this gure legend, the reader is
referred to the web version of this article.)
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 344642
clearly visible during some Natal Pulses. Moreover, due to the close
proximity of the coastline to the shelf edge in this region, this cold
water and high chl-a concentrations originating from shelf-edge
upwelling were also found in Algoa Bay during Natal Pulses. In the
absence of coastal wind-driven upwelling, one source of the cold
water in the coastal zones and bays of the eastern Agulhas Bank is
likely to originate from the upwelling along the inshore edge of
the Agulhas Current.
Although not measured as part of this study it is expected that
Natal Pulses will stimulate active autochthonous primary pro-
duction, by upwelling nutrient-rich deep waters into the euphotic
zone at the coast and along the inshore edge of the Agulhas Cur-
rent. The resultant phytoplankton response (using chl-a as a proxy
for biomass) will signicantly increase productivity within the
coastal zone (Brown 1992) and this new production is largely in
the form of microphytoplankton (diatoms). Natal Pulses can in-
uence the bays for sufcient time to allow phytoplankton to re-
spond and positively inuence the higher trophic levels. However,
the southward currents will ultimately result in plankton biomass
loss from the bays through alongshore and offshore advection, as
was found by Porri et al. (2014).
Meanders in the Agulhas Current ow around large inshore
cyclonic eddies (Lutjeharms et al., 1989). These eddies have an
upward doming of isotherms in their centre with upwelling of
water on the inshore side (Lutjeharms et al., 1989), similar to ed-
dies in the Brazil Current (Compos et al., 1999;Castelão et al.,
Fig. 8. Depth/time plots of sea temperatures during the Natal Pulse of December 2009. The temperature scale is shown on (f) Algoa Bay Mouth.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 3446 43
2004). These also contribute to bringing cold water onto the shelf,
and subsequently closer to the coast. In this study the dynamics
and inuence of cyclonic eddies along the inshore boundary of the
Agulhas Current were not investigated, since the moorings were
located too close to the coast. However, eddies off Algoa Bay ap-
pear to be constrained by the shelf edge and lie seaward of the
plume on the leading edge of the meander. They may contribute to
the upliftment of cold water onto the shelf (Chapman and Largier,
Fig. 9. Temperature sections perpendicular to the coastline off Port Alfred, Bird Island and Algoa Bay Middle showing isotherms at noon (12:00) on (a) 12, (b) 18, (c) 23 and
(d) 28 of the June 2010 during a Natal Pulse. The underwater temperature recorders on vertical strings are marked on the section as dots.
Table 1
Estimated rate of upward displacement of isotherms at the Algoa Bay Mouth (ABM)
moorings during the six Natal Pulses.
Natal pulse Isotherm
(°C)
Time up-
wards
(days)
Upward dis-
placement (m)
Rate of up-
lift
(m day
1
)
December 2009 13 7 14 2.1
12 9 36 4.0
April/May 2010 13 12 33 2.8
12 12 41 3 .4
June/July 2010 16 21 27 1.3
15 24 30 1.3
August/Septem-
ber 2010
15 23 26 1.1
14 20 42 0.2
January 2011 13 22 19 0.7
November 2011 16 18 40 2.1
15 15 17 1.1
Average 14 17 30 1.8
Max 16 24 42 4.0
Min 12 7 14 0.2
Fig. 10. Principal axes direction and magnitude for the Bird Island and Cape Recife
currents at 10 m depth over a one year period (20092010). Bird Island currents
were aligned on an axis of 58.5/238.5°N and Cape Recife currents to an axis of
178.2/358.2°N, parallel to the local isobaths.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 344644
1989), supplying more cold water for coastal upwelling, but do not
appear to have played a measurable role near the coastline and in
the bays of the far eastern Agulhas Bank during the Natal Pulses
studied here.
Other major western boundary currents, such as the Gulf
Stream (Blanton et al., 1981;McClain et al., 1984;Savidge and
Bane, 2001;Hyun and He, 2010) and the Kuroshio (Sugimoto et al.,
1988;Ito et al., 1995)inuence their coastal waters in a similar
fashion. In the southern hemisphere, the Brazil Current and the
East Australian Current are western boundary currents not unlike
the Agulhas Current, although smaller in scale. Compos et al.
(1999) and Castelão et al. (2004) observed and modelled how the
leading edge of cyclonic meanders along the inshore edge of the
Brazil Current are responsible for pumping bottom waters onto
shelf, which leads to enhanced coastal upwelling during wind-
driven events. With the East Australian Current, McClean-Padman
and Padman (1991) found that about half the major upwelling
events were identied with wind forcing, while the remainder
were attributed to cross-shelf advection associated with anti-cy-
clonic mesocale eddies. However, Roughan and Middleton (2002,
2004) suggested that the encroachment of the East Australian
Current upon the coast could be the major contributor to coastal
upwelling. They found that during encroachment the southward
currents accelerate, enhancing the onshore Ekman pumping
through the bottom boundary layers that may decrease the bottom
temperatures by up to 5 °C.
The present study shows that during Natal Pulses and large
meanders in the Agulhas Current, strong south-westward currents
may be initiated in the shallows (30 m bottom depth) opposite the
leading edge of the meander, and these currents coincided with
the onset and progression of upwelling. Bottom temperatures may
drop by as much as 9 °C. From this it is inferred that the onshore
movement of colder bottom water from the shelf edge is Agulhas
Current driven, probably accentuated in the bottom boundary
layer by the increase in current speed and diverging isobaths in
the measurement area. This is not inconsistent with the
Fig. 11. Along-shore (black lines) and across-shore (grey lines) currents at depths of 10 m measured by the Bird Island (ad) and Cape Recife (e, f) ADCPs during selected
Natal Pulses. Temperatures measured throughout the water column at the same sites over the same period are shown as background. Currents were measured at both Bird
Island (a) and Cape Recife (e) during the Natal Pulse of December 2009, but only at Cape Recife during January 2011 (f). Along-shore currents are shown as negative towards
south and southwest. Across-shore currents are positive toward the west and northwest (towards land).
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 3446 45
hypotheses of Roughan and Middleton (2004).
5. Conclusions
This study provides evidence that coastal upwelling may be
driven (or at least enhanced) by Natal Pulses of the Agulhas Cur-
rent. However, it is probable that wind-driven dynamics operate
on circulation patterns and water temperature structures already
created by Agulhas Current forcing that will often be the nal
component in lifting the cold bottom water to the surface at the
coast. Upwelling along the inshore boundary of the Agulhas Cur-
rent appears to be an important contributor to the cold water.
Further investigation needs to be done on the role that the
Agulhas Current has in driving coastal upwelling. In particular,
instruments need to be deployed across the shelf off the eastern
Agulhas Bank, at least to the edge of the Agulhas Current. Ship
based measurements are needed in this region, in order to de-
termine the spatial extent and dynamics of shelf-edge upwelling
and its inuence on the coast. The role of bottom topography and
shelf-edge upwelling need more investigation. The bottom
boundary layer in the Algoa Bay regions needs to be quantied and
understood.
Acknowledgements
The authors appreciate the diligent work of Sean Bailey and
technical crew from SAEON Elwandle Coastal Node and the South
African Institute for Aquatic Biodiversity in setting up the instru-
ments, handling the boats, diving and maintaining the numerous
deployments. Thanks to the South African Weather Service for the
wind data and the Marine Remote Sensing Unit (http://www.afro-
sea.org.za) and NOAA for the MODIS satellite images. The South
African Department of Science and Technology and the National
Research Foundation are thanked for providing funding. Ocean
Data View is acknowledged for their software (Schlitzer, 2014).
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W.S. Goschen et al. / Continental Shelf Research 101 (2015) 344646
... The Agulhas Current influences coastal regions by forcing cold, nutrient-rich water onto the continental shelf (known as upwelling) or through eddies (Bryden et al. 2005). These meanders or eddies are usually recorded near Durban, South Africa (where the continental shelf is narrow) and travel further south along the Agulhas Shelf to Port Alfred and Algoa Bay, where they are known as Natal Pulses (Goschen et al. 2015). Upwelling is driven by the topographical forcing of the current in conjunction with wind stress (Lutjeharms 2006). ...
... The Agulhas Current waters are oligotrophic and when the current intrudes onto the continental shelf it is hypothesized to suppress primary productivity (Schumann et al. 2005). However, the Agulhas Current can also enhance primary productivity on the shelf through nutrient-rich upwellings (Probyn et al. 1994;Demarcq et al. 2003), which have been recorded around regions such as Richards Bay (Lamont et al. 2014;Lamont and Barlow 2015) and between Port Alfred and Algoa Bay (Goschen et al. 2015). Changes in productivity have a knock-on effect up the food chain and ultimately influence commercially important fish species (Krug et al. 2014). ...
... Fortunato et al. (2012) argued that the mixing between different habitats, in their case driven by upwelling, creates a vertical gradient in the bacterial communities. Mixing is particularly evident between the coast and the Agulhas Current system where the shearing zone responsible for upwelling is a highly turbulent mixed area (Goschen et al. 2015), which may explain this transition. ...
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Context: Harmonic summation and amplification by winds blowing contrary to currents are known contributions to rogue waves, but the causes of the observed wave steepness, asymmetric form, and non-breaking are poorly understood. The potential effect of bathymetric and meteorological features has not been addressed. Method: Vortex theory was applied qualitatively to the weather and ocean conditions of the Agulhas region. Results: Rogue wave formation is attributed to: (1) Wind lee vortices cause steepening of wave leeward face, and suppresses wave breaking. (2) Boundary layer vortices from the meteorological cold front transfer energy to the wind lee vortices thereby enhancing their wave sharpening effect. (3) Agulhas current boundary layer vortices interact with water lee vortices to accelerate a jet of water between them, thereby steepening the wave and enhancing the preceding trough. (4) Bathymetric topology, especially a canyon on the continental slope, generates a vortex in the Agulhas current. This vortex is detached from the canyon by prising of the coastal downwelling current (induced by the meteorological cold front), and combines with the water lee vortex to heighten the wave. (5) Jetting arises when the canyon vortex and the Agulhas current boundary layer vortices pass each other, thereby accentuating wave height, steepness, and asymmetry. Conclusions: The novel contribution is the provision of a mechanism for rogue wave formation, using vortex theory, that conceptually integrates wave formation, Agulhas sea currents, bathymetric features including submarine canyons, and meteorological cold front weather systems.
... This continental configuration has led to many studies of the influence of the Agulhas Current (AC) on the meridional overturning circulation (e.g., Beal et al., 2011;Biastoch et al., 2008;De Ruijter et al., 1999;Weijer et al., 1999), but surprisingly few about the contiguous coastal regions. In fact, although observations have shown vigorous exchanges between the AC and the African shelves (Goschen et al., 2015;Jackson et al., 2012;Krug, Tournadre, & Dufois, 2014;Lutjeharms, 2006), the dynamical processes connecting these distinct realms remain largely unexplored. There are few observational studies of the AB circulation, most of which are severely limited by the sparsity of the field data (Largier et al., 1992;Lutjeharms et al., 1989). ...
... Local upwelling cells spread along the coast, farther from the coastline there are weak and highly variable currents (0.25-0.5 m/s; Boyd et al., 1992), which advect a tongue of cold waters generating a feature known as the "Cold Ridge" (Boyd & Shillington, 1994;. Variability of the shelf circulation is highly influenced by the onshore advection of open ocean waters (Chapman & Largier, 1989;Goschen et al., 2015;Krug, Tournadre, & Dufois, 2014, Krug, Cipollini, & Dufois, 2014Lutjeharms et al., 2003). Particularly during the transit of the socalled Natal Pulses, which are meanders formed farther north, in the KwaZulu-Natal Bight. ...
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This modeling study analyzes the circulation over the Agulhas Bank (AB). It is suggested that the time mean circulation over the bank is primarily driven by the inflow of shelf waters from the northeastern region, and not by local forcing as previously postulated. Seasonal variations of the circulation and temperature and salinity fields are highly correlated with the atmospheric forcing. Currents shift inshore during the winter, returning to its original position during summer. The equatorward flow in the western AB, which includes a deep, previously unreported, countercurrent, strengthens during spring and summer and wanes during fall and winter. Tracer diagnostics and Eulerian mass balances reveal very energetics mass exchanges between the eastern AB and the Agulhas Current (AC). The AB Bight is the preferential site for these exchanges. Lagrangian diagnostic show 0.45 Sv of deep open‐ocean waters entrained into the bottom layer of the shelf. Cross‐shelf exchanges produce significant water mass transformations. Tides play an unexpectedly significant role on the AB circulation. Preliminary considerations suggest that shelf/open‐ocean interactions could have a significant impact on water mass conversions within the AC.
... While seasonal longshore wind stress induces Ekman transport and pumping in the surface mixed layer, the deep~1 m/s Agulhas current on the southeast coast of South Africa can also induce upwelling [16][17][18]. Cyclonic shear draws water off the shelf, independent of wind forcing, and intensifies when the Agulhas current meanders shoreward [19][20][21]. ...
... Coupled with the coastal lows are continental shelf waves that pulse the upwelling [15]. While seasonal longshore wind stress induces Ekman transport and pumping in the surface mixed layer, the deep ~1 m/s Agulhas current on the southeast coast of South Africa can also induce upwelling [16][17][18]. Cyclonic shear draws water off the shelf, independent of wind forcing, and intensifies when the Agulhas current meanders shoreward [19][20][21]. ...
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Coastal upwelling that cools sea temperatures and nutrifies the euphotic layer is the focus of this research, motivated by how these processes benefit the marine ecosystem. Here, atmosphere–ocean reanalysis fields and satellite radiance data are employed to link South African coastal upwelling with nearshore winds and currents in the 2000–2021 period. Temporal behavior is quantified in three regimes—Benguela, transition, and Agulhas—to distinguish the influence of offshore transport, vertical pumping, and dynamic uplift. These three mechanisms of coastal upwelling are compared to reveal a leading role for cyclonic wind vorticity. Daily time series at west, south, and east coast sites exhibit pulsing of upwelling-favorable winds during summer. Over the western shelf, horizontal transport and vertical motion are in phase. The south and east shelf experience greater cyclonic wind vorticity in late winter, due to land breezes under the Mascarene high. Ekman transport and pumping are out of phase there, but dynamic uplift is sustained by cyclonic shear from the shelf-edge Agulhas current. Temporal analysis of longshore wind stress and cyclonic vorticity determined that vertical motion of ~5 m/day is pulsed at 4- to 11-day intervals due to passing marine high/coastal low-pressure cells. Height sections reveal that 15 m/s low-level wind jets diminish rapidly inshore due to topographic shearing by South Africa’s convex mountainous coastline. Mean maps of potential wind vorticity show a concentration around capes and at nighttime, due to land breezes. Air–land–sea coupling and frequent coastal lows leave a cyclonic footprint on the coast of South Africa that benefits marine productivity, especially during dry spells with a strengthened subtropical atmospheric ridge. This work has, for the first time, revealed that South Africa is uniquely endowed with three overlapping mechanisms that sustain upwelling along the entire coastline. Amongst those, cyclonic potential vorticity prevails due to the frequent passage of coastal lows that initiate downslope airflows. No other coastal upwelling zone exhibits such a persistent feature.
... Persistent upwelling occurs at the headwaters of the current near Port Elizabeth, South Africa, while wind-driven instances of upwelling occur in areas where the shape of the coastline allows for it (Lutjeharms, 2007;Schumann, 1982). The Agulhas Current also experiences a phenomenon known as Natal Pulses, which are large meanders beginning in the Natal Bight that then flow southwest along the coast and create cold water eddies close to the coast that cause upwelling (Goschen et al., 2015;van Leeuwen et al., 2000). These pulses occur between 1 and 6 times per year and can influence the ocean climate of the Agulhas Bank for up to 110 days (Krug et al., 2014). ...
... From these observations, we conclude that while primary productivity is greater at site U1479 throughout our record, terrigenous input is greater in the Natal Bight on the same timescale. This is consistent with modern observations and paleorecords of productivity and continental sediment influx for both core sites (Goschen et al., 2015;Gunn et al., 2020;Hood et al., 2017;Krug et al., 2014;Lutjeharms et al., 2000;Meyer et al., 2002;Vinayachandran et al., 2021;Wells et al., 2024). ...
Thesis
The Agulhas Current is one of the strongest currents in the southern hemisphere and is responsible for the transport of waters along the southeastern coast of Africa. The sediment load of the Agulhas Current and its interactions with other water masses in the region are essential to primary productivity along Africa’s southern coast. Despite this importance, there are still many discrepancies in our understanding of how sediment burial and nutrient variability have been impacted by regional and global changes in climate. The focus of this research is to explore the connections between climate and elemental burial from the Pleistocene through the Holocene, with an emphasis on phosphorus and metal burial over the last 1.1 mya. Two ocean sediment cores from the International Ocean Discovery Program (IODP) Expedition 361 were selected based on their proximity to the Agulhas Current. Site U1474 lies within the flow of the current, while Site U1479 lies within the Agulhas Ring Corridor, outside of its direct influence. Different downcore elemental concentrations were collected using a combination of X-Ray Fluorescence (XRF) and UV-Visible Spectroscopy. Site U1474 displayed strong dependence on southeastern African hydroclimate changes and orbitial eccentricity. Site U1479 also displayed dependence on these processes, in addition to changes in the strength of Agulhas Leakage and glacial-interglacial variability. Additionally, we explore the teleconnections between both core sites and how processes upstream can affect productivity and terrigenous burial dynamics downstream.
... In addition to managing potential physiological stressors (Heupel and Simpfendorfer 2008;Dowd et al. 2010;Tunnah et al. 2016), movement within specific salinity ranges could also serve as a strategy for predator avoidance or maximizing prey availability (Heupel and Hueter 2002;Ubeda, Simpfendorfer, and Heupel 2009;Pillans et al. 2020). Across the regional species distribution of S. zygaena, the Agulhas Current has been shown to significantly shape oceanographic conditions along the south-east coast of South Africa (Lutjeharms et al. 2001;Popova et al. 2016;Rouault, Pohl, and Penven 2010), with prominent upwelling observed at the northern end of the Natal Bight (Lutjeharms, Valentine, and Van Ballegooyen 2000) and across the Agulhas Bank (Lutjeharms et al. 1996;Goschen et al. 2015). These upwelling processes transport nutrient-rich water to the surface, significantly affecting productivity and nutrient concentrations in the region (Lutjeharms et al. 1996;Lutjeharms, Cooper, and Roberts 2000;Barlow et al. 2010). ...
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Globally, hammerhead sharks have experienced severe declines owing to continued overexploitation and anthropogenic change. The smooth hammerhead shark Sphyrna zygaena remains understudied compared to other members of the family Sphyrnidae. Despite its vulnerable status, a comprehensive understanding of its genetic landscape remains lacking in many regions worldwide. The present study aimed to conduct a fine‐scale genomic assessment of Sphyrna zygaena within the highly dynamic marine environment of South Africa's coastline, using thousands of single nucleotide polymorphisms (SNPs) derived from restriction site‐associated DNA sequencing (3RAD). A combination of differentiation‐based outlier detection methods and genotype‐environment association (GEA) analysis was employed in Sphyrna zygaena. Subsequent assessments of putatively adaptive loci revealed a distinctive south to east genetic cline. Among these, notable correlations between adaptive variation and sea‐surface dissolved oxygen and salinity were evident. Conversely, analysis of 111,243 neutral SNP markers revealed a lack of regional population differentiation, a finding that remained consistent across various analytical approaches. These results provide evidence for the presence of differential selection pressures within a limited spatial range, despite high gene flow implied by the selectively neutral dataset. This study offers notable insights regarding the potential impacts of genomic variation in response to fluctuating environmental conditions in the circumglobally distributed Sphyrna zygaena.
... Macroalgae, particularly the red seaweed Plocamium corallorhiza and other fleshy species dominate the shallow rocky areas ( Fig. 1 detailed in Edworthy et al., 2022). Cape Recife is influenced by upwelling and associated algal bloom events (Goschen et al., 2015;Goschen and Schumann, 2011), as well as anthropogenic inputs from wastewater treatment works (Lemley et al., 2019). The macroalgal reef around Cape Recife is an important nursery habitat for juvenile sparids, including Diplodus capensis (Beckley, 1985;Beckley and Buxton, 1989;James et al., 2024;Mkhize et al., 2024). ...
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Fishes have shown varying responses to the decline in seawater pH associated with ocean acidification. Coastal marine species inhabit characteristically dynamic environments which requires physiological adaptation to variability, including fluctuations in pH and associated carbonate chemistry parameters. Our study assessed the response of the early life stages (postflexion) of a common coastal fish species (Diplodus capensis) that is found in coastal nearshore and estuarine habitats along the South African coastline. We assessed their metabolic and behavioural response to a range of pH conditions which covered a high pH (8.02), the lowest pH that they are naturally exposed to (7.75) as well as extremely low pH levels (∼7.75–7.27) exceeding their current range of exposure, which may occur with future coastal acidification. Our findings suggest that this species is metabolically tolerant of acute low pH conditions (down to 7.27 pH) showing no changes in either routine or active metabolic rates. Although our study identified a slight increase in swimming activity in D. capensis larvae exposed to low pH, there was no change in feeding activity. These results suggest that this species may have the physiological capacity to withstand the present and future high pH variability associated with its environments, in the absence of other stressors and ecological interactions. However, the increased swimming activity at low pH may translate into downstream ecological impacts, for which the mechanisms need to be assessed with further research.
... While such associations with a salinity gradient have been previously proposed in other marine species including the blue skate Dipturus batis (Delaval et al. 2022), the copper shark Carcharhinus brachyurus (Klein et al., 2024), and several dolphin species (Barcelo et al. 2022;Pratt et al. 2022), limited information exists regarding the relationships between environmental variables and the behavior of S. zygaena . Across the regional species distribution of S. zygaena , the Agulhas Current has been shown to significantly shape oceanographic conditions along the south-east coast of South Africa (Lutjeharms et al., 2001;Popova et al., 2016;Rouault et al., 2010), with prominent upwelling observed at the northern end of the Natal Bight (Lutjeharms et al., 2000a) and across the Agulhas Bank (Lutjeharms et al., 1996;Goschen et al., 2015). These upwelling processes transport nutrient-rich water to the surface, significantly affecting productivity and nutrient concentrations in the region (Lutjeharms et al., 1996(Lutjeharms et al., , 2000bBarlow et al., 2010). ...
Preprint
Globally, hammerhead sharks have experienced severe declines owing to continued overexploitation and anthropogenic change. The smooth hammerhead shark Sphyrna zygaena remains comparatively understudied compared to other members of the family Sphyrnidae, and despite its Vulnerable status, a comprehensive understanding of its genetic landscape remains lacking. The present study aimed to conduct a fine-scale genomic assessment of Sphyrna zygaena within the highly dynamic marine environment of South Africa’s coastline, using thousands of single nucleotide polymorphisms (SNPs) derived from restriction site-associated DNA sequencing (3RAD). A combination of differentiation-based outlier detection methods (OUTFlank and pcadapt) and Genotype-Environment Association (GEA) (Redundancy Analysis) analysis in Sphyrna zygaena were employed. Subsequent assessments of putatively adaptive loci revealed a distinctive south to east genetic cline. Amongst these, notable correlations between adaptive variation and sea-surface dissolved oxygen and salinity, in addition to spatial factors were evident. Conversely, analysis of 110, 965 neutral SNP markers revealed a lack of regional population differentiation, a finding that remained consistent across various analytical approaches, including an assessment of isolation-by-distance (IBD) and isolation-by-environment (IBE), genetic clustering analyses (LEA, fastSTRUCTURE, and find.clusters), and a discriminant analysis of principal components (DAPC). These results provide evidence for the presence of differential selection pressures within a limited spatial range, despite high gene flow implied by the selectively neutral dataset. This study offers notable insights regarding the potential impacts of genomic variation in response to fluctuating environmental conditions in the circumglobally distributed Sphyrna zygaena.
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In the austral winter of 1993, during a COROAS hydrographic cruise in the South Brazil Bight (SBB), an unexpected mass of cold and fresh water was observed on the continental shelf near 23°S. Subsequent analyses of different data sets suggested that the origin of that water was probably the Argentine continental shelf, near the Rio de la Plata mouth. In this article, a 13-year time series of AVHRR Sea Surface Temperature anomalies is analyzed to investigate the occurrence of this phenomenon in other periods. The results of these analyses and a good correlation with the Southern Oscillation Index suggest that the penetration of these waters into the SBB occurs in a frequency that may be associated with ENSO events.
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The intrusion of cold water can have marked effects on the biota, with consequences for top predators such as Cape gannets Sula capensis and jackass penguins Spheniscus demersus. The distribution of their prey can be altered. Observations of foraging patterns during upwelling events tie in closely with the changes in the physical environment. -from Authors
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The most marked yearly variability occurs between May, June and July, and September, October and November. Wind roses confirm these seasonal differences, while considerable spatial variability occurs with position around the bay. A strong diurnal signal due to land and sea breeze generation is evident at all the sites, while a marked seasonal change occurs in the nature of these breezes. The Sundays river station recorded the strongest diurnal variations, possibly due to the adjacent valley and extensive nearby sand dunes. -from Authors
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Satellite imagery combined with coastal measurements of wind and sea surface temperature has been used to investigate the upwelling. It is found that the prominent capes in the eastern half of the region are important in initiating the upwelling, and a simple theory is advanced to explain the observations.-from Authors
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Previous work off Natal showed that current flows are generally fairly strong on the narrow shelf regions inshore of the Agulhas Current. During August 1975, approximately hourly profiles of temperature, salinity and currents measured from a ship in 49 m of water over a period of six days permitted details of the bottom boundary layer (BBL) to be established. The height of the logarithmic layer was between 14 and 24 m, while Ekman veering took place over about 35 m. The BBL probably encompassed the whole water column, but processes at the sea surface also affected the conditions. Temperature and current data from a mooring in the vicinity compared favourably with the mean values obtained and indicated that conditions at the time were associated with an expected south-westward flow of the Agulhas Current. The results in the BBL, and an observed high-energy pulse, could be important in assessing mechanisms for sediment transport across the shelf.
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The city of Port Elizabeth lies in the western sector of Algoa Bay, and a number of projects have investigated the oceanic temperature, salinity and current structures in the vicinity. This paper analyses past results, and then incorporates measurements of salinity, sea temperatures and currents from projects over the years 1989 to 1998 in order to gain a better understanding of the oceanic variability in this region. Because of the limited inflow of freshwater from rivers, salinity variations from oceanic values were minimal, and it is surmised that the effects of freshwater influxes during floods are likely to be short-lived. The seasonal variation in temperature structures shows that intense thermoclines are established in summer in the deeper sections of the bay, with more isothermal conditions in winter. Marked variability occurred over spatial scales of kilometres, and over time scales of days. Average current speeds close to the coast were generally low, increasing farther offshore, though there was substantial variability. Moreover, in the lee of the harbour and close to the coast spatial scales were also small, and spectral analyses show that tidal and inertial currents were important at times. Driving forces were investigated, and it was found that wind is important in upwelling processes and in bringing colder water onshore from deeper regions of Algoa Bay and around Cape Recife. On the other hand, coastal-trapped waves do not appear to play a significant role. Sporadic intrusions of both warm and colder water were recorded at sites near the harbour, and are attributed to inputs of warm surface water from the Agulhas Current and the above-mentioned colder up-welled water. Such intrusions are probably important in flushing out accumulations of pollutants discharged to sea from Port Elizabeth. Dominant south-westward flow farther offshore at 40m depth also indicates that the Agulhas Current could be important in entraining water from the central areas of Algoa Bay.
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The Agulhas Current is one of the major currents of the world ocean. In the network of currents that connect the major ocean basins, it forms a key link. This linkage may be of critical importance in the way the ocean affects the earth's climate and climate change. Based on the research findings of 60 years, the author describes the origins of the Agulhas Current, its behaviour, its influence on the adjacent continental shelf, its effect on local weather as well as its role in linking the Indian and the Atlantic Ocean. Coverage includes the large-scale circulation of the current, reviews of its northern and southern flows, its retroflection and return flow. The text is well-illustrated and includes small asides on the history of research on the Agulhas Current. An exhaustive bibliography gives easy access to present knowledge on this important current system.
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A quasi-synoptic set of data, collected on the Agulhas Bank during the summer of 1985/86, is presented and discussed in terms of thermocline stability. The Bank is characterized by a continuous, strong thermocline which deepens towards the west. Typical values are extracted and Richardson numbers are calculated for various parts of the Bank. These are cautiously interpreted in terms of thermocline maintenance and vertical exchange potentials. The dipping thermocline reflects a conversion from advectively controlled structure in the east to an interaction between advective and atmospheric control in the west. Quasi-permanent structure gives way to seasonality towards the west.
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Cyclonic frontal eddies accompanying the sinuous meanders of the Agulhas Current along the south-eastern edge of the Agulhas Bank force both warm Subtropical Surface water and cold, less-saline Indian Ocean Central water onto the continental shelf. The advective juxtaposition of these water masses establishes an arealy extensive vertical thermocline of remarkable intensity over the shelf. Temperature differentials of 8–10°C over depth intervals of as little as 10 m are not uncommon. Several manifestations of upwelling, viz. coastal, midshelf and shelf-edge, are evident in the results of three CTD-surveys made since 1982. Advective maintenance of the thermocline appears to compete with turbulent breakdown, a seasonal change of dominance occurring in parts remote from Agulhas Current influence. The gravitationally unstable salinity distribution could facilitate double-diffusive diapycnal mixing as well as ease turbulent mixing with winter cooling of the surface mixed layer.