Influence of Agulhas forcing of Holocene climate change in South Africa’s southern Cape

Abstract

This paper analyses a series of high-quality continuous records from southeastern Africa to study the spatiotemporal pat-terning of Holocene hydroclimatic anomalies in the region. Results indicate dominant frequencies of variability at millen-nial time scales, and a series of anomalies broadly common to all records. Of particular interest, data from the southern Cape coast exhibit periods of wetter/drier conditions that are out of phase with the sites less than 150 km away in the adjacent interior, but in phase with sites in tropical regions over 1000 km to the northeast. To explain such spatial patterns and gradients, we propose that the Agulhas Current may be a critical vector by which tropical climatic signals are propagated along the littoral zone, exerting a dominant, highly localized influence on near-coastal environmental conditions. Limitations in the data available do not allow for a detailed examination of the climatic dynamics related to these phenomena, but this paper highlights a series of avenues for future research to clarify the spatial extent and stability of the patterns observed.

Full text

Quaternary Research (2018), 1–7
Copyright © University of Washington. Published by Cambridge University Press, 2018.
doi:10.1017/qua.2018.57
Influence of Agulhas forcing of Holocene climate change in South
Africa’s southern Cape
Brian M. Chase
a
*, Lynne J. Quick
b,c
a
Centre National de la Recherche Scientifique, UMR 5554, Institut des Sciences de l’Evolution-Montpellier, Université Montpellier, 34095 Montpellier,
Cedex 5, France
b
Department of Environmental and Geographical Science, University of Cape Town, South Lane, Upper Campus, 7701 Rondebosch, South Africa
c
Centre for Coastal Palaeoscience, Nelson Mandela University, Port Elizabeth, Eastern Cape 6031, South Africa
(RECEIVED February 23, 2018; ACCEPTED May 14, 2018)
Abstract
This paper analyses a series of high-quality continuous records from southeastern Africa to study the spatiotemporal pat-
terning of Holocene hydroclimatic anomalies in the region. Results indicate dominant frequencies of variability at millen-
nial time scales, and a series of anomalies broadly common to all records. Of particular interest, data from the southern
Cape coast exhibit periods of wetter/drier conditions that are out of phase with the sites less than 150 km away in the adja-
cent interior, but in phase with sites in tropical regions over 1000 km to the northeast. To explain such spatial patterns and
gradients, we propose that the Agulhas Current may be a critical vector by which tropical climatic signals are propagated
along the littoral zone, exerting a dominant, highly localized influence on near-coastal environmental conditions. Limita-
tions in the data available do not allow for a detailed examination of the climatic dynamics related to these phenomena,
but this paper highlights a series of avenues for future research to clarify the spatial extent and stability of the patterns
observed.
Keywords: Southern Africa; Palaeoclimate; Holocene; Agulhas Current
INTRODUCTION
Southern African climatic variability is determined by the
influence of two primary circulation systems: (1) the tropical
easterlies, which advect moisture to the continent from the
Indian Ocean, and (2) the southern westerlies and associated
storm track (Tyson and Preston-Whyte, 2000). Precipitation
across most of the subcontinent is related to the tropical
easterlies and falls primarily in the summer months (the
“summer rainfall zone”[SRZ]; sensu Chase and Meadows,
2007), when evaporation and convective potential are high-
est. In contrast, the southwestern Cape receives the bulk of its
rainfall in the winter months (the “winter rainfall zone”
[WRZ]), when the westerly storm track shifts equatorward.
This seasonal dynamic has been widely applied as a model to
understand and explore Quaternary climatic dynamics, with a
coeval inverse relationship being proposed wherein summer
(winter) rainfall systems are relatively more invigorated
during interglacial (glacial) periods (van Zinderen Bakker,
1976; Cockcroft et al., 1987; Chase and Meadows, 2007).
In recent years, focused efforts to test and refine this model
have shown that, while it may be applicable in broad terms
(Chase et al., 2017), there exists a much greater degree of
spatiotemporal complexity than was previously predicted.
Evidence indicates, for example, that the eastern SRZ cannot
be treated as a homogeneous region, as there exist at least two
(southern-central and northern) subregions (Chevalier and
Chase, 2015), and that climatic variability across much of the
interior is driven by the interaction between temperate and
tropical systems, rather than by either system in isolation
(Chase et al., 2017). It has also been suggested that the
influence of the tropical easterlies has at times been a key
determinant of climatic variability not only in the SRZ, but in
parts of the modern WRZ as well (Chase et al., 2015b), and a
comparison of records from sites along the interface between
the SRZ and WRZ seem to suggest that strong dipoles may
exist over very short distances (<100 km) (Chase et al.,
2015a, 2015b).
In this paper, we apply a selection of recent data sets to
explore the spatiotemporal distribution of millennial-scale
hydroclimatic anomalies during the Holocene in the southern
*Corresponding author at: Centre National de la Recherche Scientifique,
UMR 5554, Institut des Sciences de l’Evolution-Montpellier, Université
Montpellier, Bat.22, CC061, Place Eugène Bataillon, 34095 Montpellier,
Cedex 5, France. E-mail address: brian.chase@um2.fr (B.M. Chase).
1
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Cape of South Africa. This region receives precipitation from
both tropical and temperate systems (in what is referred to
here as the “aseasonal rainfall zone”[ARZ]; cf. “YRZ”
definition of Chase and Meadows, 2007) and is further
influenced by the warm Agulhas Current, which may induce
either localised precipitation (Jury et al., 1993) or may con-
tribute to the development of larger regional convective
systems (Tyson and Preston-Whyte, 2000). As such, the
southern Cape is recognised as being a climatically complex
region, and while it is currently one of the most mesic regions
in South Africa, it may also be one of the most sensitive to
change as a result of perturbations in any one of the elements
that comprise its climatic system.
SITE SELECTION
For this study, we consider sites from the central and eastern
SRZ and both the interior and coastal zones of the southern
Cape (Fig. 1). As we focus on identifying millennial-scale
hydroclimatic anomalies, we have selected only those
records that (1) are continuous, (2) have sampling intervals
that are a consistently less than 500 yr, and (3) can be related
with reasonable certainty to changes in precipitation and/or
aridity. Underpinning our analyses at the regional scale are
the northern and southern-central SRZ (N-SRZ and SC-SRZ,
respectively) summer precipitation reconstructions of Che-
valier and Chase (2015) (Fig. 2a and e). These reconstruc-
tions (1) include data from the best-resolved fossil pollen
records from eastern southern Africa, (2) analyse the data
using the CREST software (Chevalier et al., 2014) to obtain
quantified estimates of past precipitation, and (3) allow for
the identification the N-SRZ and SC-SRZ as two largely
distinct regions based on differences observed between the
records considered (see Chevalier and Chase [2015] for more
details). To explore the extent of the climatic anomalies
observed in these regional reconstructions and the influence
of the dominant climatic systems, we consider four additional
records (Fig. 1). The first, a δD record recovered from a
marine core off the coast from the Zambezi River mouth
(Schefuß et al., 2011), is of relatively low resolution, but as
an indicator of rainfall amount/intensity it provides informa-
tion regarding the northern extent of the patterns observed.
Along the southeastern African coastal margin, Holocene
pollen records have been recovered from Lake Eteza (Neu-
mann et al., 2010) and the Mfabeni Peatland (Finch and Hill,
2008), but the resolution of these records is too low (sampling
intervals exceed 500 yr in some portions of the records) for
the level of analysis undertaken in this study. We instead use
the δ
13
C record from the Mfabeni Peatland, which provides a
higher-resolution measure of C
3
(primarily trees and shrubs)
versus C
4
(primarily tropical drought-adapted grasses)
vegetation and has been interpreted as relating to general
changes in humidity during the Holocene (Baker et al., 2014).
While general, this index of vegetation change is similar to
the interpretive basis of the Mfabeni and Lake Eteza pollen
records, and the patterns of change observed in the records
are similar. From the southern Cape coast, the high-resolution
pollen record from Eilandvlei provides valuable insight into
regional hydroclimatic conditions, with changes in afrotempe-
rate taxa being strongly linked to changes in humidity (Quick
et al., 2018). Inland of Eilandvlei, 130 km to the northwest in
the Groot Swartberg mountains, rock hyrax middens from
Seweweekspoort have provided a detailed δ
15
Nrecord,
reflecting changes in hydroclimate over the last 22,000yr
(Chase et al., 2013, 2017). While other records do exist from
this region of southern Africa—such as those recovered from
Groenvlei (Martin, 1968) and Rietvlei-Stillbaai (Quick et al.,
2015)—and results do share similarities with the Eilandvlei
record (see Quick et al., 2018), their continuity, resolution,
chronologic control, and/or significance in terms of hydrocli-
matic variability limit their suitability for this study (see Deacon
and Lancaster [1988], Chase and Meadows [2007], and Scott
et al. [2012] for further information).
18
18
20
20
22
24
26
16
12
16
18
12
WRZ
ARZ
SRZ
RI
TW
WK
TV
MF
EV
ZB
FL
SW
BL
EQ
BR ET
10°E 15°E 20°E 25°E 30°E 35°E 40°E
45°S
40°S
35°S
30°S
25°S
20°S
15°S
A
g
u
l
h
a
s
C
u
r
r
e
n
t
Figure 1. Map of southern Africa showing sea-surface temperature
isolines (°C), the extent of the southern African winter rainfall zone
(WRZ), aseasonal rainfall zone (ARZ), and summer rainfall zone
(SRZ) (sensu Chase and Meadows, 2007), and the location of the
sites considered in this study. They are: the palaeoenvironmental
sites used for the reconstruction (Chevalier and Chase, 2015) of
northern summer rainfall zone climates (Tate Vondo [TV; Scott,
1987a], Wonderkrater [WK; Scott, 1982], Tswaing Crater [TW;
Scott, 1999; Metwally et al., 2014], and Rietvlei [RI; Scott and
Vogel, 1983]); southern-central summer rainfall zone climates
(Braamhoek [BR; Norström et al., 2009], Florisbad [FL; Scott and
Nyakale, 2002], Equus Cave [EQ; Scott, 1987b], and Blydefontein
[BL; Scott et al., 2005] according to Chevalier and Chase [2015]);
the GeoB9307-3 marine core (ZB; Schefuß et al., 2011); Mfabeni
Peatland (MF; Baker et al., 2014); Lake Eteza (ET; Neumann et al.,
2010); Eilandvlei (EV; Quick et al., 2018); and Seweweekspoort
(SW; Chase et al., 2017). Sites are colour coded to reflect
similarities in climatic variability phasing, and regions (as defined
by these findings) are described by shaded areas. (For interpretation
of the references to color in this figure legend, the reader is referred
to the web version of this article.)
2B.M. Chase and L.J. Quick
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0
Age (cal ka BP)
25810134679
0
Age (cal ka BP)
25810134679
Seweweekspoort
composite δ15N (‰)
2
1
0
3
4
5
Frequency (years)
1600
400
100
200
(f)
800
6.75-6.75
SC-SRZ PWetQ
(detrended, normalised mm)
-2
0
1
-1
Frequency (years)
1600
400
100
200
(e)
800
4.5-4.5
Eilandvlei afrotemperate forest
pollen (%)
4
8
12
16
Frequency (years)
1600
400
100
200
(d)
800
7.6-6.6
-1
Mfabeni δ13C (‰, normalised)
0
1
Frequency (years)
1600
400
100
200
(c)
800
4.5-4.5
-1
Zambezi Basin δD
(‰, detrended, normalised)
0
2
1
Frequency (years)
1600
400
100
200
(b)
800
2.75-2.75
N-SRZ PWetQ
(detrended, normalised mm)
-2
0
1
-1
Frequency (years)
1600
400
100
200
(a)
800
4-4.5
2
Frequency (years)
1600
400
100
200
(a)
800
3.7-6.3
Frequency (years)
1600
400
100
200
(b)
800
2.7-7.3
Frequency (years)
1600
400
100
200
(c)
800
4.3-5.7
Frequency (years)
1600
400
100
200
(d)
800
5.7-4.5
Frequency (years)
1600
400
100
200
(e)
800
4.4-5.7
Frequency (years)
1600
400
100
200
(f)
800
5.5-4.5
1
Figure 2. (color online) Comparison of the records considered for this study: (a) the northern summer rainfall zone (N-SRZ) and (e)
southern-central summer rainfall zone (SC-SRZ) summer precipitation stacks (Chevalier and Chase, 2015), (b) the δD record from the
GeoB9307-3 marine core (Schefuß et al., 2011), (c) the δ
13
C record from the Mfabeni Peatland (Baker et al., 2014), (d) the Eilandvlei
afrotemperate forest pollen record (Quick et al., 2018), and (f) the Seweweekspoort δ
15
N composite record (Chase et al., 2017). Each
record has been analysed using continuous Morlet wavelet transforms. Panel 1 shows the local wavelet power spectrum for each record,
with black lines indicating the cone of influence to show where boundary effects are present and the regions of greater than 95%
confidence using a white-noise model. Panel 2 highlights the real-value signal power at different time scales. In this paper, we are
considering the common millennial-scale variability identified at >1600 yr frequencies, which highlights an antiphase relationship between
tropical and coastal sites (panels a–d) and those from the interior (panels e and f). Each record has been oriented according to
interpretations of aridity, with “wetter”conditions towards the top of the graph and with higher real-value signal power.
Influence of Agulhas forcing of Holocene climate change in South Africa’s southern Cape 3
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DEFINING MILLENNIAL-SCALE CLIMATIC
VARIABILITY IN THE STUDY REGION
As we are concerned here with the mechanistic relationships
between sites/regions, we focus primarily on the direction
and timing of change rather than amplitude. To highlight this
clearly, we have removed low-frequency multimillennial/
orbital-scale trends (cubic polynomial) from the SRZ stacks
and the Zambezi Basin record, as they are dominated by a
signal linked to direct insolation forcing during the Holocene
(Schefuß et al., 2011; Chevalier and Chase, 2015). For this
study, we have also removed the pollen records from the
coastal sites of Lake Eteza and the Mfabeni Peatland from the
SC-SRZ stack, as defined by Chevalier and Chase (2015), to
more clearly differentiate between coastal and interior sig-
nals. Chronologies for all records presented were established
by the original authors using Bacon (Blaauw and Christen,
2011) and the SHCal13 (Hogg et al., 2013) or Marine13
(Reimer et al., 2013) calibration data. We have updated the
Mfabeni Peatland and Zambezi Basin records using these
same calibration data to ensure comparability.
To identify patterns of millennial-scale climatic variability
within the Holocene, we have used continuous Morlet wavelet
transforms (Torrence and Compo, 1998) (Fig. 2). Before
analysis, each record was linearly interpolated to a common
100 yr resolution for analytical purposes, but the differing
sampling resolutions limit the scope of robust comparisons
to lower, millennial-scale frequencies. While the variable
sampling resolutions result in the appearance of significant
bands at different frequencies, we focus here on the common
bands of high significance in the 1600 to 2400yr frequencies
to distil millennial-scale variability (Fig. 2, panel 1).
Real-value wavelets show positive or negative oscillations in
the data, and strength of the anomalies at these frequencies
(Fig. 2, panel 2). These results indicate that common cycles of
variability are identifiable in records from across the study
region, and anomalies can be seen to manifest between
approximately 0–1.2 cal ka BP, 1.2–3 cal ka BP, 3–5.2 cal ka
BP, 5.2–7.2 cal ka BP, and 7.2–9.3 cal ka BP. These signals
have been isolated in Figure 3, which plots the average signal
strength and sign for the 1600 to 2400yr frequencies for each
record across the Holocene.
DISCUSSION
When considered together, records from the study region
indicate a strong antiphase relationship between the N-SRZ
and the SC-SRZ over millennial time scales (Figs. 2a and e,
and 3), and the pattern exhibited in the SC-SRZ extends well
into the ARZ. It has been proposed that this latter phenom-
enon may be determined by the interaction between tempe-
rate and tropical systems to create composite synoptic
systems such as tropical–temperate troughs (Chase et al.,
2017).
Of particular interest is the strong dipole that has been
revealed to exist within the ARZ between Eilandvlei (Quick
et al., 2018) on the southern Cape coast and Seweweekspoort
(Chase et al., 2013, 2017) (Fig. 2). Considering their position
and proximity, it would be expected that they share similar
climatic histories. Instead, for most of the Holocene, the
progression of hydroclimatic anomalies are strongly opposed
between the sites. The differences between these sites are
unlikely to be related to Seweweekspoort’s position closer to
the WRZ, as a similarly antiphase relationship can be
observed between the Eilandvlei record and the SC-SRZ
stack (Figs. 2 and 3).
We propose that the similarities in phasing between
Eilandvlei and the N-SRZ are the result of the transport of
warm waters along the east coast via the Agulhas Current,
which, by modifying the surface heat flux and the onshore flow
of moist air (Jury et al., 1993, 1997), effectively propagates a
tropical climate signal along its zone of influence. As sug-
gested earlier, the influence of this vector is apparently both
strong and highly localized. While its resolution precludes it
from the methods of analysis employed here, analyses of the
Lake Eteza pollen record indicate distinct similarities with the
Eilandvlei data, supporting this finding (Fig. 4).
The consideration of the interior-coastal dipole may thus
be critical when either (1) extrapolating results from coastal
sites to understand broader palaeoenvironmental conditions
across the subcontinent or (2) drawing on data from non-
coastal sites to establish a context for the rich archaeological
sites of the southern Cape coastal region (Klein, 1975; Hen-
shilwood et al., 2002; Marean, 2010). However, at this stage,
the observation of this phenomenon is restricted to those
few records of sufficient resolution and palaeoclimatic
0
Age (cal ka BP)
25810134679
Tropical and coastal signal strength
(average of 1600-2400 yr frequencies)
0
1
2
-1
-2
N-SRZ
Zambezi
Mfabeni
Eilandvlei
r2=0.923
r2=0.870
r2=0.782
r2=0.625
(a)
Interior and temperate signal strength
(average of 1600-2400 yr frequencies)
0
1
2
-1
-2
SC-SRZ
Seweweekspoort
(b)
Figure 3. (color online) Comparison of millennial-scale signals
(1600–2400 yr frequencies) from (a) the northern summer rainfall
zone (N-SRZ; Chevalier and Chase, 2015), Zambezi Basin
(Schefuß et al., 2011), Mfabeni Peatland (Baker et al., 2014), and
Eilandvlei (Quick et al., 2018); and (b) the southern-central
summer rainfall zone (SC-SRZ; Chevalier and Chase, 2015) and
Seweweekspoort (Chase et al., 2017). In panel a, correlations are
calculated relative to the N-SRZ signal, which is considered to be
most representative of tropical variability in the region.
4B.M. Chase and L.J. Quick
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significance that have been recovered from the southern Cape
coastal region. Thus, several key questions await the recov-
ery of suitable records:
1. What is the spatial extent of the dominant Agulhas
influence? Similarities between records from Mfabeni
(Baker et al., 2014) and Eilandvlei (Quick et al., 2018)
(Figs. 2, 3, and 4) may indicate that the Agulhas has a
strong influence along the whole of the southeastern
South African coast, but confirmation will require the
recovery of further records from the region between
these sites. Further, it may be predicted that the signal
observed at Eilandvlei will diminish to the west of
Eilandvlei, towards the zone of Agulhas Current retro-
flection and the cooler waters of the Benguela Current
(Lutjeharms and Van Ballegooyen, 1988; Jury et al.,
1993), but better-resolved records from the southwestern
Cape coast will be required to test this hypothesis.
2. Is the dominance of the Agulhas Current restricted to
specific climate states? Even within the Holocene there
are some indications that the localized influence of the
Agulhas Current and the establishment of the coastal-
interior dipole may be restricted to specific global
boundary conditions. As an example, the strong relation-
ship between the Eilandvlei and N-SRZ records at
millennial time scales appears to break down over the
last 2000 yr (Fig. 3). Recent work has shown that the
dominant drivers of millennial-scale climate change
across the interior may have changed significantly as
global climates evolved from glacial to interglacial states
(Chase et al., 2017). Considering changing spatial
relationships in the earliest Holocene and the last
millennium (Fig. 2), it may be that the coastal-interior
dipole did not exist during the last glacial period, and it
may not exist in the future. Here again, more records
from the region are required to study these possibilities.
3. How might changes in sea-surface temperatures versus
changes in the position, strength, and/or extent of
Agulhas Current flow combine or counteract each other
to modulate the degree of Agulhas influence on
terrestrial environments? To some degree, a positive
relationship exists between the temperature of the
Agulhas Current and the strength and extent of its flow
along the southern Cape coast. During glacial periods,
the source regions for the Agulhas Current were cooler
(Bard et al., 1997; Sonzogni et al., 1998; Caley et al.,
2011) and the westward flow of the Agulhas may have
been restricted as the subtropical front shifted equator-
ward (Rau et al., 2002; Peeters et al., 2004; Bard and
Rickaby, 2009). At finer spatiotemporal scales, however,
the relationship between temperature and flow, and the
influence on the climates of the subcontinent may be
significantly more complex (Cohen and Tyson, 1995),
and it remains to be seen to what extent changes in the
Agulhas Current’s sea-surface temperatures may be
applied as a proxy for coastal-zone humidity during
periods not covered by the available records. To date, no
sea-surface temperature records of sufficient resolution
have been recovered from the region to adequately
explore these dynamics.
CONCLUSIONS
Recent data from South Africa highlight the existence of a
strong dipole in hydroclimatic conditions between interior
and coastal regions of the southern Cape during the Holo-
cene. The coastal signal appears to be highly localized, and
observations of its spatial extent suggest that the Agulhas
Current is a strong determinant of Holocene coastal climatic
variability, operating to propagate climate change signals
from the tropics to the southern Cape coast. Consideration of
this mechanism is critical for understanding the extent to
which data from interior or coastal sites may be extrapolated
to understand or predict change in other regions of the sub-
continent. Research targeting coastal and marine sites with
the goal of obtaining high-resolution hydroclimatic and sea-
surface temperature records extending into the last glacial
period will allow for a clearer understanding of the stability
and dynamics of the relationship observed during the Holo-
cene. This is ISEM contribution no. 2018-112.
ACKNOWLEDGMENTS
This study was funded in part by the European Research Council
under the European Union’s Seventh Framework Programme (FP7/
2007-2013)/ERC Starting Grant HYRAX, grant agreement no.
258657, and by the German Federal Ministry of Education and
Research through the collaborative project Regional Archives for
Integrated Investigations, which is embedded in the international
research program Science Partnership for the Assessment of Com-
plex Earth System Processes. We would like to thank Nicholas
Lancaster, Tyler Faith, and Claude Hillaire-Marcel for their thought-
provoking comments.
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0
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0
Age (cal ka BP)
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Lake Eteza PCA1
4
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