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

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 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.

Full text

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 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 re-
mained 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 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 (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.
&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 30–300 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 100–300 km (Gründlingh, 1979), are
termed Natal Pulses (Lutjeharms and Roberts, 1988). Natal Pulses
occur on average 1.6–1.7 times per year (Rouault and Penven,
2011;Krug et al., 2014), but their number can range from zero to
5–6 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
influenced substantially by Natal Pulses.
During such a meander, including a Natal Pulse, the Agulhas
Current flows 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) 34–46

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 wind’equation) 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 significance 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 influx 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 profilers (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 60–80 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 fixed to
the sea floor, 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 profilers (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) 34–46 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 first low-pass filtered to daily values by a standard cosine-
Lanczos filter 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 filtered to one hourly values by a cosine-
Lanczos filter with 18 weights and a qpp of 0.53496 cph before
further analysis. The currents were also then further low-pass
filtered by the cosine-Lanczos filter 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 defined 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 defined as the
period between the start and end of upwelling. Seven days were
chosen because wind-driven upwelling in the study area fluctuates
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 influences, 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 filament (see Lutjeharms et al., 1989,
Lutjeharms, 2006) of the Agulhas Current were identified 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 influence 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 identified on the SST images as the position of maximum sea
surface temperature of the Agulhas Current, the edge of the
Agulhas Current was identified as the maximum sea surface
temperature gradient on the inshore thermal front of the Agulhas
Current, a plume (Lutjeharms et al., 1989)was identified 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 filament was identified 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 10–15 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 (10–80 m
depths) from the Algoa Bay UTR moorings over a period of four
years (2009–2012) are shown in Fig. 3. The data was low-passed
filtered which allowed for an illustration of longer-period (days to
weeks) changes in temperatures that could have been caused by
fluctuating (2–7 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,aredefined 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 (19–22 °C) and cooler (10–15 °C) water was found in Algoa
Bay during these seasons which may be attributed to Agulhas
Current influences (discussed below). A typical example of the
water column during winter–spring of 2009 is marked as A on
Fig. 3 when water of 16–18 °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 summer–autumn there was greater varia-
bility in temperatures with warm water periods (19–22 °C) inter-
sected by cold water (10–12 °C) events. The warming of the surface
water during summer–autumn was caused by an increase in solar
flux at this time of year which contributes to a strong thermocline
above a stratified 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) 34–4636

period showed that the warm water events were generally caused
by surface plumes or filaments 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 first 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 12–13 °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 confirmed 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
2.0–50.0 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
2.0–20.0 mg m
3
in their core.
Shown in Fig. 6 are low-pass filtered 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-
fined in Fig. 2). The other deeper (60–80 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 first 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) 34–46 37

generally the bottom layer temperature was 10–14 °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 12–14 °C.
However, during the other Natal Pulses, water of even lower
temperature (10–12 °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 influence of plume water was evident at
the moorings. From satellite imagery (Fig. 4) these warm water
intrusions (filaments) were identified as originating from the
plume at the crest of the Natal Pulse. In the majority of cases, the
influence of a Natal Pulse was first 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
flowed past the area toward the south-west, followed by an in-
trusion of cold water which led to prolonged active upwelling over
1–3 weeks, while it took 2–3 weeks for the cold water to disperse
after active upwelling had stopped.
The distances of the core, edge, plume and filament 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 50–100 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 fila-
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 filament
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 filaments 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 difficult 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 A–E. Lost data are shown as
blanks.
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 34–4638

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 6–8°C. Other Natal Pulses
showed similar wide-spread influx 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) 34–46 39

The progression of the Natal Pulse past Algoa Bay and influence
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 influences. As with the April/May
2010 event, the water was generally well mixed before the event,
however during this event there was first a warming as the plume/
filament 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 flow along the bottom toward
shore and eventually the whole column became more stratified
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) 34–4640

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 five 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 first 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 filtered. 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 14–15 December 2009 (Fig. 10a), 24–25 April
(Fig. 10b), 27–28 June 2010 (Fig. 10c) and 30–31 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 14–15 December 2009 and 21–22 January 201. Since
these maximum occurred near the start of the active upwelling
period, it is likely that the Natal Pulse had an influence on the
coastal currents. Jackson et al. (2012) and Porri et al. (2014) also
found that the Natal Pulse of September 2010 enhanced westward
flow 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 flooded into
the entire study area, up until at least the 30 m isobath. These
results were confirmed 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 first increased in temperature as a warm water filament/
plume (originating from the meander's crest) penetrated over the
shelf and moved towards the shore. During some Natal Pulses the
plume/filament 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 identified 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 (filament or surface water that appeared to originate
from the plume).
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 34–46 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 40–60 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 flow 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 reflect
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 figure legend, the reader is
referred to the web version of this article.)
W.S. Goschen et al. / Continental Shelf Research 101 (2015) 34–4642

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 significantly increase productivity within the
coastal zone (Brown 1992) and this new production is largely in
the form of microphytoplankton (diatoms). Natal Pulses can in-
fluence the bays for sufficient time to allow phytoplankton to re-
spond and positively influence 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 flow 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) 34–46 43

2004). These also contribute to bringing cold water onto the shelf,
and subsequently closer to the coast. In this study the dynamics
and influence 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 (2009–2010). 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) 34–4644

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)influence 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 identified 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 (a–d) 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) 34–46 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 final
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 influence 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 quantified 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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