Saline Indian Ocean waters invaded the South Atlantic thermocline during glacial termination II

Abstract and Figures

Salty and warm Indian Ocean waters enter the South Atlanticvia the Agulhas leakage, south of Africa. Model simulations andproxy evidence of Agulhas leakage strengthening during glacial terminationsled to the hypothesis that it was an important modulator ofthe Atlantic Ocean circulation. Yet, the fate of the leakage salinity andtemperature anomalies remains undocumented beyond the southerntip of Africa. Downstream of the leakage, new paleoceanographicevidence from the central Walvis Ridge (southeast Atlantic) showsthat salinity increased at the thermocline, and less so at the surface,during glacial termination II. Thermocline salinity change coincidedwith higher frequency of Agulhas rings passage at the core locationand with salinity maxima in the Agulhas leakage area, suggesting thatleakage waters were incorporated in the Atlantic circulation throughthe thermocline. Hydrographic changes at the Walvis Ridge and inthe leakage area display a distinct two-step structure, with a reversalat ca. 134 ka. This matched a wet interlude within the East Asia weakmonsoon interval of termination II, and a short-lived North Atlanticwarming. Such concurrence points to a Bølling-Allerød-like recoveryof the Atlantic circulation amidst termination II, with a northwardshift of the Intertropical Convergence Zone and Southern Hemispherewesterlies, and attendant curtailment of the interocean connectionsouth of Africa.
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Saline Indian Ocean waters invaded the South Atlantic thermocline
during glacial termination II
Paolo Scussolini1*, Gianluca Marino2,3, Geert-Jan A. Brummer1,4, and Frank J.C. Peeters1
1Earth and Climate Group, Vrije Universiteit, Amsterdam 1081HV, Netherlands
2Institut de Ciència i Tecnologia Ambientals, Univesitat Autònoma de Barcelona, Bellaterra 08193, Spain
3Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia
Department of Geology and Chemical Oceanography, NIOZ Royal Netherlands Institute for Sea Research, Den Burg NL-1790AB,
Salty and warm Indian Ocean waters enter the South Atlan-
tic via the Agulhas leakage, south of Africa. Model simulations and
proxy evidence of Agulhas leakage strengthening during glacial ter-
minations led to the hypothesis that it was an important modulator of
the Atlantic Ocean circulation. Yet, the fate of the leakage salinity and
temperature anomalies remains undocumented beyond the southern
tip of Africa. Downstream of the leakage, new paleoceanographic
evidence from the central Walvis Ridge (southeast Atlantic) shows
that salinity increased at the thermocline, and less so at the surface,
during glacial termination II. Thermocline salinity change coincided
with higher frequency of Agulhas rings passage at the core location
and with salinity maxima in the Agulhas leakage area, suggesting that
leakage waters were incorporated in the Atlantic circulation through
the thermocline. Hydrographic changes at the Walvis Ridge and in
the leakage area display a distinct two-step structure, with a reversal
at ca. 134 ka. This matched a wet interlude within the East Asia weak
monsoon interval of termination II, and a short-lived North Atlantic
warming. Such concurrence points to a Bølling-Allerød–like recovery
of the Atlantic circulation amidst termination II, with a northward
shift of the Intertropical Convergence Zone and Southern Hemi-
sphere westerlies, and attendant curtailment of the interocean con-
nection south of Africa.
The South Atlantic subtropical gyre is a main component of the
Atlantic meridional overturning circulation (AMOC), acting as a con-
duit for the warm and saline upper waters that flow northward across the
equator (Gordon, 1986; Garzoli and Matano, 2011). Most of those waters
originate in the subtropical Indian Ocean and are transported by the Agul-
has Current to the southern tip of Africa, where they leak into the South
Atlantic (Agulhas leakage, AL; Fig. 1A), increasing its heat and salinity
(Gordon et al., 1992; Biastoch et al., 2009; Beal et al., 2011). Numerical
models point to a correlation of AL intensity with North Atlantic salinity
and with AMOC strength (Biastoch et al., 2009; Biastoch and Böning,
2013). Advancing on indications by Gordon et al. (1992), others suggested
a role for the AL in Pleistocene glacial terminations (e.g., Weijer et al.,
2002; Knorr and Lohmann, 2007). Paleoceanographic reconstructions
reveal that AL maxima were a persistent feature of terminations (Peeters
et al., 2004), whereby AL intensifications resulted from deglacial reor-
ganizations of the Southern Hemisphere westerlies (Toggweiler et al.,
2006), due either to their southern shift (Sijp and England, 2009; Bias-
toch et al., 2009; Biastoch and Böning, 2013), or to the redistribution of
momentum associated with their intensification (Durgadoo et al., 2013).
The so-called AL hypothesis contends that enhanced salt input from the
AL into the upper branch of the AMOC may have helped to overcome the
glacial density stratification that impeded deep convection in the northern
North Atlantic (Knorr and Lohmann, 2007), thereby nudging the AMOC
*Current address: IVM (Institute for Environmental Studies), Vrije Univer-
siteit, Amsterdam 1081HV, Netherlands.
GEOLOGY, February 2015; v. 43; no. 2; p. 1–4; Data Repository item 2015054
Published online XX Month 2014
© 2014 Geological Society of America. For permission to copy, contact
Sea-surface height anomaly (cm)
95 105 115 125 135 145 155 165
Age (ka)
Temp(Mg/Ca) (°C)
G. ruber s.l.
G. truncatulinoides sin.
MIS 5e
Ocean Data View
Figure 1. A: Map of the Agulhas system, with the location of Walvis
Ridge core 64PE-174P13 and other cores discussed in the text. Con-
toured sea-surface height anomalies (seasonal climatological mean; indicate the transmission of Agulhas rings
into the Atlantic Ocean. B–D: Geochemical results from core 64PE-
174P13 for Globigerinoides ruber (s.l.—sensu lato) and Globorotalia
truncatulinoides (sin—sinistral). MIS—marine isotope stage. B: Cal-
cite d18O (VPDB—Vienna Peedee belemnite). C: Mg/Ca-derived tem-
peratures (Temp). D: Ice-volume corrected seawater d18O (d18Osw-ivc).
Vertical error bars are propagated. VSMOW—Vienna standard mean
ocean water.
as doi:10.1130/G36238.1Geology, published online on 5 January 2015
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from a shallower and potentially slower glacial mode to a stronger inter-
glacial one. During terminations, however, the AMOC underwent major
reductions (McManus et al., 2004; Denton et al., 2010). This appears to
contradict the link between AL strengthening and AMOC intensification
as derived from modern climate models (Biastoch and Böning, 2013), and
as formulated in the AL hypothesis. Lack of data tracking the Atlantic
propagation of salinity anomalies coupled with past AL intensifications
(Marino et al., 2013) casts further doubts on the AL hypothesis.
Here we document the incorporation of AL heat and salt anoma-
lies into the South Atlantic subtropical gyre during glacial termination II
(T-II), which featured the most prominent intensification in AL volume of
the past 640 k.y. (Caley et al., 2014).
Core 64PE-174P13 (29°45.7S, 2°24.1E, 2912 m water depth) is
located on the central Walvis Ridge, in the eastern sector of the South
Atlantic gyre (Fig. 1A), directly in the path of present and past Agulhas
rings (Gordon et al., 1992; Scussolini et al., 2013). We generated stable
isotope (d18O) and trace element (Mg/Ca) time series from two planktic
foraminiferal species, Globigerinoides ruber (sensu lato), which dwells
close to the surface (within the upper ~100 m), and Globorotalia trun-
catulinoides (sinistral), which dwells in the subsurface (within the per-
manent thermocline at ~500 m, herein referred to as the thermocline) (cf.
Scussolini and Peeters, 2013). The d18O measurements were conducted as
reported in Scussolini and Peeters (2013). Mg/Ca ratios were analyzed by
inductively coupled plasma–optical emission spectrometry at the VU Uni-
versity Amsterdam (Netherlands) on cleaned foraminiferal samples (cf.
Barker et al., 2003). We converted Mg/Ca to calcification temperatures
using the calibration of Anand et al. (2003) for G. ruber and by Regen-
berg et al. (2009) for G. truncatulinoides. Paired Mg/Ca-d18O and coeval
sea-level records yield ice-volume corrected seawater d18O (d18Osw-ivc), a
proxy for local salinity changes (e.g., Marino et al., 2013). The chronol-
ogy adopted here for the 165–95 ka interval is consistent with that of the
230Th-dated speleothems (Cheng et al., 2009; Barker et al., 2011). See the
GSA Data Repository1 for details about the methods.
The d18O values of G. ruber and G. truncatulinoides from core
64PE-174P13 display distinct decreases during T-II that are synchronous
but different in magnitude: ~1.5‰ in G. truncatulinoides and ~1.1‰ in
G. ruber (Fig. 1B; Table DR4 in the Data Repository). The Mg/Ca tem-
peratures feature millennial-scale variations superimposed upon a longer
term glacial-interglacial warming (Fig. 1C). Surface temperatures exhibit
suborbital oscillations of ~1 °C during glacial marine isotope stage 6
and two distinct warming shifts of up to 2.5 °C between ca. 142 ka and
126 ka, interspersed with an ~3 k.y. return to near-glacial values. Glacial
thermocline temperatures show larger fluctuations (to 3 °C) and a two-
phase warming (2–4 °C) at ca. 142 ka and 130 ka. The d18Osw-ivc (Fig. 1D)
increased at both surface (~0.7‰) and thermocline (~1.2‰) from ca. 140
ka to 142 ka, decreased gradually from 125 ka, and remained at relatively
low levels afterward.
Agulhas Leakage Control on the Walvis Ridge Salinity
Several lines of evidence indicate that multimillennial changes,
starting ca. 142 ka, in upper ocean temperature and d18Osw-ivc (hence
salinity) in the central Walvis Ridge reflected the hydrographic response
of the South Atlantic subtropical gyre to the AL strengthening of T-II
(Figs. 2B–2F). First, the thermocline d18Osw-ivc anomaly, which is ~70%
larger than the simultaneous d18Osw-ivc change at surface, correlates with
the enhanced propagation of Agulhas rings to the Walvis Ridge, as
inferred from a proxy registered in the same core and foraminiferal spe-
cies (G. truncatulinoides) (Fig. 2C; Table DR2; Scussolini et al., 2013).
An increase in the advection of Indian Ocean saline waters transported
by anticyclonic Agulhas rings would depress the South Atlantic isoha-
lines (van Aken et al., 2003) and increase the salt content of the sub-
100110 120 130 140 150 160
Age (ka)
NE Brazil speleothem
growth Phases
East Asia
Sea Surface
Temp. (°C)
100110 120130 140 150 160
Agulhas rings proxy
Agulhas leakage (%)
Antarctic temp. (°C)
Cape Basin record
Agulhas Bank
Walvis Ridge
Walvis Ridge
Agulhas Bank
Walvis Ridge
Agulhas Bank
Walvis Ridge
1GSA Data Repository item 2015054, method details; details on the com-
parison of the South Atlantic reversals with records from East Asia, Brazil and
the North Atlantic; and datasets, is available online at
/ft2015.htm, or on request from or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
Figure 2. South Atlantic records of upper ocean hydrography com-
pared with Antarctic temperatures and time series of tropical pre-
cipitation across glacial termination II (T-II). MIS—marine isotope
stage; VSMOW—Vienna standard mean ocean water; WMI—weak
monsoon interval. A: Antarctic temperature (temp.) from European
Project for Ice Coring in Antarctica (EPICA) Dome C (Jouzel et al.,
2007) on the speleothem (230Th) chronology of Barker et al. (2011).
B: Agulhas leakage (AL) fauna record, proxy for AL intensity, from
the Cape Basin (core GeoB3603–2; Peeters et al., 2004; the age
model was adapted, see the Data Repository [see footnote 1]). C:
Thermocline (Thermocl.) d18Osw-ivc (sea water, ice-volume corrected;
proxy for salinity) from Walvis Ridge core 64PE-174P13 (this study),
compared to the proxy for Agulhas rings in the same core (Scusso-
lini et al., 2013). D, E: Surface and thermocline d18Osw-ivc from Walvis
Ridge (this study), compared to surface d18Osw-ivc from Agulhas Bank
(core MD96–2080; Marino et al., 2013). F: Surface temperatures from
Walvis Ridge (this study) and from Agulhas Bank (MD96–2080; Ma-
rino et al., 2013). G: East Asia speleothems from the Sanbao cave,
China (Wang et al., 2008; Cheng et al., 2009), and growth phases
from northeastern Brazil speleothems (Wang et al., 2004; 1s error
bars). White bar indicates South Atlantic reversals (SAR) during T-II.
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surface, thus explaining the prominent salinification of the thermocline
during T-II. Second, the thermocline waters at the Walvis Ridge, con-
trary to their counterpart from the Agulhas Plateau (core MD02–2588;
Ziegler et al., 2013), lacked the carbon isotope minimum (Fig. DR2)
that fingerprinted deglacial Subantarctic Mode Water (Spero and Lea,
2002). This indicates that the Walvis Ridge subsurface hydrography was
not controlled by fresh and low-d13C Subantarctic Mode Water, but by
saltier and more nutrient-depleted AL waters. Third, both surface and
thermocline d18Osw-ivc from the Walvis Ridge covaried with their surface
counterpart from the Agulhas Bank (Marino et al., 2013), where warm
and saline Indian Ocean waters leak into the South Atlantic (Beal et
al., 2011) (Figs. 2D–2F; Table DR2). Lastly, the upper ocean d18Osw-ivc
maxima of T-II at the Walvis Ridge coincided, within chronological
uncertainties (Table DR1), with maximum transport of AL planktic fora-
minifera to the South Atlantic (Peeters et al., 2004) (Fig. 2B).
Marino et al. (2013) found that an increase of saline waters trans-
port via the AL during T-II coincided with the Heinrich event 11 in the
North Atlantic and with the profound interhemispheric reorganization of
the ocean-atmosphere system coupled with a weakening of the AMOC
(Stocker and Johnsen, 2003; Lee et al., 2011). Likewise the temperature
relationship between bipolar ice cores across the last glacial cycle (Stocker
and Johnsen, 2003), the Agulhas salt leakage developed in response to
North Atlantic cooling and peaked prior to (or in step with) North Atlantic
warming (Marino et al., 2013). Our Walvis Ridge thermocline d18Osw-ivc
record indicates a strong similarity in timing and magnitude with the
Agulhas Bank surface d18Osw-ivc over the entire 160–100 ka interval. One
example is the glacial salinification at 152–147 ka, also tied to a North
Atlantic cold phase (Marino et al., 2013). This suggests that the efficient
transmission of the AL salt to the thermocline was a persistent millennial-
scale feature of the South Atlantic gyre circulation and the interhemi-
spheric climate links.
Although salinification of the wider South Atlantic may have
stemmed from salt retention in this sector of the basin, due to deglacial
AMOC slowdown (Lohmann, 2003), the surface d18Osw-ivc anomaly at
the Walvis Ridge during T-II is smaller in amplitude than its counterpart
from the Agulhas Bank (Figs. 2D and 2E). Consequently, enhanced AL
was likely a more important driver of the salinity increase at the Walvis
Ridge, at least during Heinrich event 11. Accordingly, the surface salinity
anomaly associated with the AL would attenuate downstream in the South
Atlantic, as observed in the data.
Modern AL and rings propagating in the South Atlantic are character-
ized by elevated heat and salinity at the thermocline (Arhan et al., 2011).
Still, at the Walvis Ridge, thermocline temperature mirrors the proxy for
Agulhas rings (Scussolini et al., 2013) less closely than the d18Osw-ivc proxy,
indicating that salinity was a more persistent property of the AL waters.
This, along with the lower correlation between the surface temperature
profiles at the Walvis Ridge and Agulhas Bank (Fig. 2F), suggests that
the AL heat dissipated faster than the salinity anomaly in the Cape Basin,
probably due to convection and air-sea interaction (Arhan et al., 2011;
Beal et al., 2011). Hence, cooling would reduce buoyancy, and sink the
AL waters in the South Atlantic to the thermocline.
South Atlantic Evidence for a Climate Reversal During T-II
Our new Walvis Ridge records reveal a brief, mid-termination return
to low surface temperature and d18Osw-ivc values (Figs. 2D and 2F) inter-
jecting the long-term temperature and salinity increases of T-II. This mil-
lennial-scale, relatively cold and fresh episode corresponded, within age
uncertainties (Table DR1), to analogous temperature and d18Osw-ivc drops
centered ca. 134 ka in the AL area (Marino et al., 2013). Although the
uncertainties in the South Atlantic chronology are relatively large (see
the Data Repository), such timing appears to agree with the brief inter-
ruption of the weak monsoon interval of T-II in East Asia (Fig. 2G; see
also Fig. DR3, Table DR3; see the Data Repository), and with a distinct
warm phase in the wider North Atlantic region (Barker et al., 2011; Mar-
trat et al., 2014) (Figs. DR3F and DR3G). It has been proposed that both
the South Atlantic (Barker et al., 2009) and the AL (Marino et al., 2013)
responded to interhemispheric reorganizations of the ocean-atmosphere
system on a millennial scale. The South Atlantic reversal punctuating T-II
likely reflected the effect of a short-lived reduction in Indian-to-Atlantic
heat and salt transport. This reversal may have arisen from: (1) a north-
ward shift of the tropical and subtropical wind fields and weakening of the
subtropical wind fields. This could have affected the mesoscale processes
at the oceanic fronts south of Africa (Biastoch et al., 2009; Durgadoo et
al. 2013), in turn reducing the AL (Peeters et al., 2004), and is further con-
sistent with the more northern position of the Intertropical Convergence
Zone inferred from Chinese speleothems (Wang et al., 2008; Cheng et al.,
2009). (2) A transient resumption of the AMOC may have restored the
piracy of heat from the upper layers of the South Atlantic (Crowley, 1992)
and contributed to the decrease in upper ocean salinity.
As the AMOC and the latitudinal arrangement of atmospheric cir-
culation appear mutually dependent (Sijp and England, 2009; Toggwei-
ler, 2009; Frierson et al., 2013), both mechanisms together may account
for the South Atlantic reversals of T-II. This reinforces the analogy with
events amid termination I (Martrat et al., 2014), when cold conditions in
the South Atlantic corresponded to the Bølling-Allerød interval (Barker
et al., 2009).
New paleoceanographic data from the Walvis Ridge document that
enhanced Agulhas leakage raised the salinity and heat content of surface
and, more prominently, of thermocline layers of the South Atlantic gyre
during T-II. AL waters seem to have sunk upon entrance in the South
Atlantic. The salinification of the South Atlantic did not seem to entail
the resumption of the AMOC envisaged by the AL hypothesis. Recon-
structions of thermocline salinity further downstream should determine
whether the salinity anomalies also impacted the North Atlantic, or were
retained in the South Atlantic, possibly by the severing of the oceanic bot-
tleneck of the North Brazil Current, produced by the deglacial southern
shift in the easterlies’ position (Wilson et al., 2011).
The T-II increase of surface temperature and salinity at the Walvis
Ridge and in the AL area occurred in two steps, with a return to cold and
fresh conditions ca. 134 ka. Comparison with records of Asian monsoon
and of North Atlantic climate ascribes the reversal to a transient resump-
tion of the AMOC, and attendant reduction of heat and salt transfer via the
AL, potentially mediated by rearrangements of the Southern Hemisphere
westerlies and associated oceanic fronts.
This work was funded by the European Commission FP7 Marie-Curie ITN
grant agreement 238512, GATEWAYS project. Marino acknowledges support
from the Universitat Autonoma de Barcelona (postdoctoral research grant PS-688-
01/08) and from the Australian Laureate Fellowship project FL120100050 (to
E.J. Rohling). Core 64PE-174P13 was taken as part of the MARE-program
funded by the Netherlands Organisation of Scientific Research. We thank Sanne
Vogels and Laura Rodríguez-Sanz for advice on Mg/Ca cleaning, and John Visser
for assistance with the inductively coupled plasma–optical emission spectrometry
at the Vrije Universiteit Amsterdam. Comments from J.R. Toggweiler and two
reviewers greatly improved this paper.
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Manuscript received 2 September 2014
Revised manuscript received 22 November 2014
Manuscript accepted 25 November 2014
Printed in USA
as doi:10.1130/G36238.1Geology, published online on 5 January 2015
doi: 10.1130/G36238.1
published online 5 January 2015;Geology
Paolo Scussolini, Gianluca Marino, Geert-Jan A. Brummer and Frank J.C. Peeters
glacial termination II
Saline Indian Ocean waters invaded the South Atlantic thermocline during
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as doi:10.1130/G36238.1Geology, published online on 5 January 2015
... For example, during the transition from the penultimate glacial (i.e., Marine Isotope Stage, MIS 6) to the Last Interglacial (MIS 5e), records from the eastern South Atlantic on Mg/Ca-derived sea surface temperature (SST) and ice-volume-corrected seawater δ 18 O (δ 18 O IVC-SW , as a proxy for relative changes in ocean salinity) suggest the AL intensified from as early as ca. 142 ka BP (Scussolini et al., 2015). However, some deep-water convection in high latitudes of the North Atlantic apparently did not strengthen until late into glacial Termination II (TII), ca. ...
... NBC/NBUC: North Brazil Current/Undercurrent; BC: Brazil Current; MC: Malvinas Current; BMC: Brazil-Malvinas Confluence; SAC: South Atlantic Current; AL: Agulhas Leakage. The yellow dots mark the positions of core GL-1090 (this study) and other relevant records discussed in the text: 64PE-174P13 (Scussolini et al., 2015), ODP Site 1063 and 983 (Deaney et al., 2017), ODP Site 1058 (Bahr et al., 2013), MD03-2664(Irvalı et al., 2016, NEAP-18K (Hall et al., 1998), and IODP Site U1304 (Hodell et al., 2009). mid-Atlantic ridge along the South Atlantic Current (Garzoli & Matano, 2011). ...
... To reconstruct propagation of AL signal across the South Atlantic subtropical gyre via its salt signature, we compared the surface (Santos et al., 2017b) and thermocline (this study) δ 18 O IVC-SW records of core GL-1090 with surface and thermocline records from core 64PE-174P13 of the Walvis Ridge (eastern South Atlantic) (Scussolini et al., 2015) (Figure 4). Figure 4A and B). During the period between 160-142 ka BP, when the AL was weaker (Scussolini et al., 2015), similarity of surface δ 18 O IVC-SW records between both sites is low, suggesting that the surface salinities of Santos Basin and Walvis Ridge evolved independently, so they were likely controlled by independent processes prior to 142 ka BP. ...
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Intensification of the Agulhas Leakage (AL) during glacial terminations has long been proposed as a necessary mechanism for reverting the Atlantic Meridional Overturning Circulation (AMOC) to its interglacial mode. However, lack of records showing the downstream evolution of AL signal and substantial temporal differences between AL intensification and resumption of deep‐water convection have cast doubt on the importance of this mechanism to the AMOC. Here, we analyze a combination of new and previously published data relating to Mg/Ca‐derived temperatures and ice volume‐corrected seawater δ18O records (δ18OIVC‐SW, as a proxy for relative changes in ocean salinity), which demonstrate propagation of AL signal via surface and thermocline waters to the western South Atlantic (Santos Basin) during Termination II and the early Last Interglacial. The saline AL waters were temporally stored in the upper subtropical South Atlantic until they were abruptly released in two stages into the North Atlantic via surface and thermocline waters at ca. 129 and 123 ka BP, respectively. Accounting for age model uncertainties, these two stages are coeval with the resumption of convection in the Labrador and Nordic seas during the Last Interglacial. We propose a mechanism whereby both active AL and a favorable ocean‐atmosphere configuration in the tropical Atlantic were required to allow flux of AL waters into the North Atlantic, where they then contributed to enhancing the AMOC during the Last Interglacial period. Our results provide a framework that connects AL strengthening to the AMOC intensifications that followed glaciations.
... However, the IOSTUW may play an important role in modulating the sea level change in the southeastern Indian Ocean, as has been suggested by Llovel and Lee (2015) and Huang et al. (2020) who found that that this region experiences a large halosteric contribution to sea level change. Additionally, the IOSTUW may invade the South Atlantic Ocean via Agulhas leakage (Scussolini et al. 2015) and play an important role in interoceanic salinity exchange. ...
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In this study, the Indian Ocean Subtropical Underwater (IOSTUW) was investigated as a subsurface salinity maximum using Argo floats (2000–20) for the first time. It has mean salinity, potential temperature, and potential density values of 35.54 ± 0.29 psu, 17.91° ± 1.66°C, and 25.56 ± 0.35 kg m ⁻³ , respectively, and mainly extends between 10° and 30°S along the isopycnal surface in the subtropical south Indian Ocean. The annual subduction rate of the IOSTUW during the period of 2004–19 was investigated based on a gridded Argo dataset. The results revealed a mean value of 4.39 Sv (1 Sv ≡ 10 ⁶ m ³ s ⁻¹ ) with an interannual variability that is closely related to the southern annular mode (SAM). The variation in the annual subduction rate of the IOSTUW is dominated by the lateral induction term, which largely depends on the winter mixed layer depth (MLD) in the sea surface salinity (SSS) maximum region. The anomalies of winter MLD are primarily determined by SAM-related air–sea heat flux and zonal wind anomalies through modulation of the buoyancy. As a result, the annual subduction rate of the IOSTUW generally increased when the SAM index showed negative anomalies and decreased when the SAM index showed positive anomalies. Exceptional cases occurred when the wind anomaly within the SSS maximum region was weak or was dominated by its meridional component. Significance Statement The Indian Ocean Subtropical Underwater (IOSTUW) is characterized as a subsurface salinity maximum in the subtropical south Indian Ocean, and it has potential contribution to regional sea level change and interoceanic salinity exchange. The purpose of this work is to give a more precise description of the IOSTUW and to better understand the formation process of this water mass. Our results shine new light on hydrological characteristics of the IOSTUW based on an observational dataset collected during the past two decades. In addition, the formation of the IOSTUW was found to be closely related to the leading mode of variability in the atmospheric circulation of the Southern Hemisphere though air–sea heat flux and surface wind anomaly.
... The δ 18 O sw-ivc increases by ∼1.2 during Termination II (Fig. 5e). This salinity effect was possibly boosted by an increase in South Atlantic thermocline salinity at the beginning of Termination II (Fig. 5e, f) (Ballalai et al., 2019), probably due to increases in the advection of saline waters from the Indian Ocean to the South Atlantic thermocline (Scussolini et al., 2015). A decoupling between δ 18 O and temperature is also noticed during Termination I and the Holocene (Fig. 2c), which could possibly indicate a recurring feature of glacial terminations, though a further investigation would be required. ...
The upper ocean circulation in the western tropical Atlantic (WTA) is responsible for the northward cross-equatorial heat transport as part of the Atlantic Meridional Overturning Circulation (AMOC). This cross-equatorial transport is influenced by the thermocline circulation and stratification. Although seasonal thermocline stratification in the WTA is precession-driven, the existence of an orbital pacemaker of changes in the entire WTA upper ocean stratification, which comprises the main thermocline, remains elusive. Here, we present a 300 ka-long record of the WTA upper ocean stratification and main thermocline temperature based on oxygen isotopes (δ 18 O) and Mg/Ca of planktonic foraminifera. Our δ 18 O record between Globigerinoides ruber and Globorotalia truncatulinoides, representing upper ocean stratification, shows a robust precession pacing, where strong stratification was linked to high summer insolation in the Northern Hemisphere (precession minima). Mg/Ca-based temperatures support that stratification is dominated by changes in thermocline temperature. We present a new mechanism to explain changes in WTA stratification, where during the Northern Hemisphere summer insolation maxima, the Intertropical Convergence Zone shifts northward, developing a negative wind stress curl anomaly in the tropical Atlantic. This, in turn, pulls the main thermocline up and pushes the South Atlantic Subtropical Gyre southwards, increasing the stratification to the north of the gyre. This mechanism is supported by experiments performed with the Community Earth System Model (CESM1.2). Finally, we hypothesize that the precession-driven WTA stratification may affect the cross-equatorial flow into the North Atlantic.
... The interest of the palaeoclimate community in Agulhas leakage arose from the finding that peak Agulhas leakage occurred during glacial terminations 8 and plausibly aided the AMOC to shift to its full-strength interglacial mode 9,10 . This hypothesis builds on a variety of records from within the Agulhas leakage pathway, which inferred fluctuations in the strength of Agulhas leakage over the late Pleistocene epoch based on a variety of faunal and geochemical proxy reconstructions 8,[11][12][13][14][15][16][17][18] . These findings are further reinforced by a numerical model study 19 indicating that the strength of the Agulhas leakage varied by ~10 Sv between glacial and interglacial periods. ...
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The interocean transfer of thermocline water between the Indian and the Atlantic Oceans known as ‘Agulhas leakage’ is of global significance as it influences the Atlantic Meridional Overturning Circulation (AMOC) on different time scales. Variability in the Agulhas Current regime is key in shaping hydroclimate on the adjacent coastal areas of the African continent today as well as during past climates. However, the lack of long, continuous records from the proximal Agulhas Current region dating beyond the last glacial cycle prevents elucidation of its role in regional and wider global climate changes. This is the first continuous record of hydrographic variability (SST; δ¹⁸Osw) from the Agulhas Current core region spanning the past 270,000 years. The data set is analytical sound and provides a solid age model. As such, it can be used by paleoclimate scientists, archaeologists, and climate modelers to evaluate, for example, linkages between the Agulhas Current system and AMOC dynamics, as well as connections between ocean heat transport and Southern African climate change in the past and its impact on human evolution.
... The forward modelling exercise therefore indicates that bioturbation is a possible alternative mechanism for the variance peak, but also indicates that the conclusions are sensitive to the parameterization. Forward modelling cannot disprove enhanced Agulhas leakage as the source of increased IFA variance across the MIS 5-6 transition (Marine Isotope Stage), and there is other evidence for increased leakage such as the tight coupling between the Agulhas rings proxy and the δ 18 O of G. truncatulinoides Scussolini et al. (2015). However, given that bioturbation depths as low as 3 cm still produce a quite visible variance peak, we argue that bioturbation is at least a plausible mechanism behind some of the change in variance over the MIS 5-6 transition. ...
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Climate reconstructions based on proxy records recovered from marine sediments, such as alkenone records or geochemical parameters measured on foraminifera, play an important role in our understanding of the climate system. They provide information about the state of the ocean ranging back hundreds to millions of years and form the backbone of paleo-oceanography. However, there are many sources of uncertainty associated with the signal recovered from sediment-archived proxies. These include seasonal or depth-habitat biases in the recorded signal; a frequency-dependent reduction in the amplitude of the recorded signal due to bioturbation of the sediment; aliasing of high-frequency climate variation onto a nominally annual, decadal, or centennial resolution signal; and additional sample processing and measurement error introduced when the proxy signal is recovered. Here we present a forward model for sediment-archived proxies that jointly models the above processes so that the magnitude of their separate and combined effects can be investigated. Applications include the interpretation and analysis of uncertainty in existing proxy records, parameter sensitivity analysis to optimize future studies, and the generation of pseudo-proxy records that can be used to test reconstruction methods. We provide examples, such as the simulation of individual foraminifera records, that demonstrate the usefulness of the forward model for paleoclimate studies. The model is implemented as an open-source R package, sedproxy, to which we welcome collaborative contributions. We hope that use of sedproxy will contribute to a better understanding of both the limitations and potential of marine sediment proxies to inform researchers about earth's past climate.
The morphological evolution was investigated in the tropical Neogene planktonic foraminiferal lineage Globorotalia menardii, G. limbata and G. multicamerata during the past 8 million years at ODP Hole 806C (Ontong-Java Plateau, western equatorial Pacific). This research is an extension of several similar studies since 2007 from the Caribbean Sea, the tropical Atlantic and the Eastern Equatorial Pacific. The goal is to find empirical and quantitative confirmation for morphological speciation – splitting or phyletic gradualism or accelerated evolution – in planktonic foraminifera with the above lineage as model objects. The present study from ODP Hole 806C serves as a test to discriminate between these evolutionary scenarios. In the western equatorial Pacific warm and stable environments prevailed back to Pliocene times, and potential influences of Northern Hemisphere Glaciation are thought to bear less severely on shell size evolution than in the Atlantic Ocean. A slow and gradual pattern of shell size increase is therefore expected in the western tropical Pacific, in contrast to the intermittent rapid menardiform shell size increase during periods of intensified formation of warm water eddies in the southern to tropical Atlantic. For this study a total of 11,101 specimens from 37 stratigraphic levels extending over the past 8 million years were morphometrically investigated thanks to the AMOR robot for imaging and microfossil orientation. Of those, 6080 specimens comprise the G. menardii–limbata-multicamerata plexus. Special attention was given to trends of spiral height (δX) versus axial length (δY) in keel view, for which bivariate contour- and volume-density diagrams were constructed to visualize speciation and evolutionary trends. The investigation at Hole 806C showed, that G. menardii evolved in a more gradual manner than in the Atlantic. Contour plots of δX versus δY reveal modest bimodality between 3.18 Ma – 2.55 Ma with a dominant branch consisting of smaller G. menardii (δX<∼300 μm) persisting until the Late Quaternary, and a less dominant branch of larger G. menardii (δX>∼300 μm) until 2.63 Ma. There is evidence for cladogenesis – splitting with subsequent morphological divergence in the Late Pliocene G. menardii-limbata-multicamerata lineage, and which may be linked to changes in the thermocline. Due to the general scarcity of G. multicamerata at Site 806, the divergence was less clearly expressed than originally expected. Up-section, bimodality vanished but G. menardii populations shifted towards extra large shells between 2.19-1.95 Ma. The morphological evolution of Pacific menardiform globorotalids contrasts the one observed in the Atlantic. This inter-oceanic asymmetry may indicate possible long-distance dispersal of G. menardii, at least during intermittent phases. For plankton biostratigraphy this implies, that correlating events represent more often than previously thought large scale environmental perturbations with local morphological ecophenotypic adaptations and nuances.
The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today¹; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled². An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50° E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to ‘escape’ into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the ‘southern escape’ and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the ‘100-kyr world’, in which glacial–interglacial cycles occur at roughly 100,000-year periods.
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The responses of Asian monsoon subsystems to both hemispheric climate forcing and external orbital forcing are currently issues of vigorous debate. The Indian summer monsoon is the dominant monsoon subsystem in terms of energy flux, constituting one of Earth’s most dynamic expressions of ocean–atmosphere interactions. Yet, the Indian summer monsoon is grossly under-represented in Asian monsoon palaeoclimate records. Here, we present high-resolution records of Indian summer monsoon-induced rainfall and fluvial runoff recovered in a sediment core from the Bay of Bengal across Termination II, 139–127 thousand years ago, including coupled measurements of the oxygen isotopic composition and Mg/Ca, Mn/Ca, Nd/Ca and U/Ca ratios in surface-ocean-dwelling foraminifera. Our data reveal a millennial-scale transient strengthening of the Asian monsoon that punctuates Termination II associated with an oscillation of the bipolar seesaw. The progression of deglacial warming across Termination II emerges first in the Southern Hemisphere, then the tropics in tandem with Indian summer monsoon strengthening, and finally the Northern Hemisphere. We therefore suggest that the Indian summer monsoon was a conduit for conveying Southern Hemisphere latent heat northwards, thereby promoting subsequent Northern Hemisphere deglaciation. During deglacial warming at Termination II, about 130,000 years ago, the Indian summer monsoon helped convey heat northwards as deglaciation progressed from the Southern to the Northern Hemisphere, according to sediment records from the Bay of Bengal.
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O Workshop de Engenharia de Biossistemas (WEB) é um evento acadêmico que iniciou com foco no público interno, com o objetivo de divulgar os resultados dos projetos de pesquisa do Programa de Pós-Graduação em Engenharia de Biossistemas (PGEB). Ao longo das edições anteriores se expandiu para além da Universidade Federal Fluminense, englobando todos os interessados nos temas ligados à área das Ciências Ambientais. Nesta IV Edição, houve um passo na direção da Internacionalização, com convidados de outros países, além daqueles vindos de outros estados. A edição deste ano, teve como tema: “Ambiente, Agricultura e Tecnologias”, e ocorreu nos dias 06, 07 e 08 de novembro de 2018, em diferentes dependências do Campus da Praia Vermelha da Universidade Federal Fluminense (UFF). No total tiveram 220 inscrições (alunos de graduação e de pós-graduações), 7 palestras, 7 minicursos, 2 mesas redondas (uma sobre Zootecnia de Precisão e outra em Agricultura) 9 apresentações orais e 73 na forma de pôster. O WEB tem se consolidado como um dos principais eventos científicos da UFF, integrando alunos, sejam eles graduandos, pós-graduandos ou alumni, bem como professores das mais diversas áreas acadêmicas, da UFF e de outras instituições do estado do Rio de Janeiro e de outros estados do País. O evento mantém um ponto importante, qual seja a sua gratuidade, garantindo assim, uma maior participação num espaço plural de congregação do grande público acadêmico fluminense, nesta que é uma das mais importantes Universidades públicas brasileiras.
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A precisely ²³⁰ Th-dated stalagmite δ ¹³ C profile from Hulu Cave, China, is presented to characterize the frequency and pattern of millennial-scale Asian monsoon (AM) variability from 160.6 to 132.5 ka. Evidence for an antiphased relationship of the δ ¹³ C and δ ¹⁸ O on the millennial scale suggests that the δ ¹³ C is indicative of the local hydrological cycle associated with changes in AM strength. Owing to the δ ¹³ C responding to AM changes more sensitively than the δ ¹⁸ O, we could identify 15 strong AM events that correlate to cold intervals recorded in Antarctic ice cores within ²³⁰ Th dating uncertainty. This result supports a dynamic link of AM strength and southern hemispheric climates via the cross-equatorial airflows. Power spectrum analysis shows a predominant periodicity of 1.5–2.5 ka for the δ ¹³ C profile, similar to the Dansgaard-Oeschger frequency during the last glacial period. Moreover, the AM events are characterized by rapid transitions at the onset, suggesting that the observed millennial-scale AM variability is likely forced by northern high-latitude climates via north–south shifts of the Intertropical Convergence Zone associated with the bipolar seesaw mechanism. As evidence for a common mechanism for ice age terminations, a strong AM event (~134 ka) surrounding Termination II is analogous to the Bølling-Allerød warming interval.
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The Indian-Atlantic water exchange south of Africa (Agulhas leakage) is a key component of the global ocean circulation. No quantitative estimation of the paleo-Agulhas leakage exists. We quantify the variability in interocean exchange over the past 640,000 years, using planktic foraminiferal assemblage data from two marine sediment records to define an Agulhas leakage efficiency index. We confirm the validity of our new approach with a numerical ocean model that realistically simulates the modern Agulhas leakage changes. Our results suggest that, during the past several glacial-interglacial cycles, the Agulhas leakage varied by ~10 sverdrup and more during major climatic transitions. This lends strong credence to the hypothesis that modifications in the leakage played a key role in changing the overturning circulation to full strength mode. Our results are instrumental for validating and quantifying the contribution of the Indian-Atlantic water leakage to the global climate changes.
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Rainfall in the tropics is largely focused in a narrow zonal band near the Equator, known as the intertropical convergence zone. On average, substantially more rain falls just north of the Equator. This hemispheric asymmetry in tropical rainfall has been attributed to hemispheric asymmetries in ocean temperature induced by tropical landmasses. However, the ocean meridional overturning circulation also redistributes energy, by carrying heat northwards across the Equator. Here, we use satellite observations of the Earth's energy budget, atmospheric reanalyses and global climate model simulations to study tropical rainfall using a global energetic framework. We show that the meridional overturning circulation contributes significantly to the hemispheric asymmetry in tropical rainfall by transporting heat from the Southern Hemisphere to the Northern Hemisphere, and thereby pushing the tropical rain band north. This northward shift in tropical precipitation is seen in global climate model simulations when ocean heat transport is included, regardless of whether continents are present or not. If the strength of the meridional overturning circulation is reduced in the future as a result of global warming, as has been suggested, precipitation patterns in the tropics could change, with potential societal consequences.
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A maximum in the strength of Agulhas leakage has been registered at the interface between the Indian and South Atlantic oceans during glacial Termination II (T-II). This presumably transported the salt and heat necessary for maintaining the Atlantic circulation at rates similar to the present day. However, it was never shown whether these waters were effectively incorporated into the South Atlantic gyre, or whether they retroflected into the Indian and/or Southern oceans. To resolve this question, we investigate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic Basin than previously studied. Analysis of a 60 yr data set from the global-nested INALT01 model allows us to relate density perturbations at the depth of the thermocline to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of variability of individual Globorotalia truncatulinoides δ18O. We reveal high levels of pycnocline depth variability at the site, suggesting enhanced numbers of Agulhas rings moving into the South Atlantic Gyre around T-II. Our record closely follows the published quantifications of Agulhas leakage from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the South Atlantic circulation. This provides crucial support for the view of a prominent role of the Agulhas leakage in the shift from a glacial to an interglacial mode of the Atlantic circulation.
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Two eddies, one anticyclonic and the other cyclonic, intersected in the Subantarctic Zone south of South Africa during a hydrographic transect, are described using a large set of measurements including full depth hydrography, Acoustic Doppler Current Profiler velocities, biogeochemical tracers, air-sea fluxes and altimetric sea surface height. Both eddies have a subtropical origin. The anticyclone is an Agulhas ring with convected core water of ˜12°C, and swirl velocities of 1 m s-1. It was 9.5 months old when sampled and had crossed the Agulhas Ridge. Though sampled in summer, it was releasing ˜200 W m-2 (sensible plus latent heat flux) to the atmosphere. It was observed adjacent to the Subantarctic Front, illustrating the usual encounters of such structures with this front. The cyclone, marked by pronounced low oxygen and CFC anomalies revealing an origin at the continental slope, was 4.5 months old. It had swirl speeds of 0.3 m s-1, and was coupled with the anticyclone when observed. From their kinematics and water mass properties both structures were found to transport subtropical water down to ˜900 m, the water trapped below this depth being either from the northern Subantarctic Zone, or local water. The two structures illustrate the capacity of eddies in the region to transfer subtropical and alongslope water properties into the Subantarctic Zone.
Sea surface temperatures (SSTs) recorded by alkenones and oxygen isotopes in the Alboran basin are used here to describe, at an unprecedented fine temporal resolution, the present interglaciation (PIG, initiated at 11.7 ka BP), the last interglaciation (LIG, onset approximately at 129 ka) and respective deglaciations. Similarities and dissimilarities in the progression of these periods are reviewed in comparison with ice cores and stalagmites. Cold spells coeval with the Heinrich events (H) described in the North Atlantic include multi-decadal scale oscillations not previously obvious (up to 4 °C in less than eight centuries within the stadials associated with H1 and H11, ca 133 ka and 17 ka respectively). These abrupt oscillations precede the accumulation of organic rich layers deposited when perihelion moves from alignment with NH spring equinox to the summer solstice, a reference for deglaciations. Events observed during the last deglaciation at 17 ka, 14.8 ka and 11.7 ka are reminiscent of events occurred during the penultimate deglaciation at ca 136 ka, 132 ka and 129 ka, respectively. The SST trend during the PIG is no more than 2 °C (from 20 °C to 18 °C; up to −0.2 °C/ka). The trend is steeper during the LIG, i.e. up to a 5 °C change from the early interglaciation to immediately before the glacial inception (from 23 °C to 18 °C; up to −0.4 °C/ka). Events are superimposed upon a long term trend towards colder SSTs, beginning with SST maxima followed by temperate periods until perihelion aligned with the NH autumn equinox (before ca 5.3 ka for the PIG and 121 ka for the LIG). A cold spell of around eight centuries at 2.8 ka during the PIG was possibly mimicked during the LIG at ca 118 ka by a SST fall of around 1 °C in a millennium. These events led interglacial SST to stabilise at around 18 °C. The glacial inception, barely evident at the beginning ca 115 ka (North Atlantic event C25, after perihelion passage in the NH winter solstice), culminated with a SST drop of at least 2 °C in two millennia (event C24, ca 111 ka). The Little Ice Age (0.7 ka) also occurred after the latest perihelion passage in the NH winter solstice and could be an example of how a glacial pre-inception event following an interglaciation might be.
work suggests that changes of the Southern Hemisphere (SH) winds led to an increase in Agulhas leakage and a corresponding salinification of the Atlantic. Climate model projections for the 21st century predict a progressive southward migration and intensification of the SH westerlies. The potential effects on the ocean circulation of such an anthropogenic trend in wind stress are studied here with a high-resolution ocean model forced by a step-function change in SH wind stress that involves a 7% increase in westerlies strength and a 2° shift in the zero wind stress curl. The model simulation suggests a rapid dynamic adjustment of Agulhas leakage by 4.5 Sv, about a third of its original value, after a few years. The change in leakage is reflected in a concomitant change in the transport of the South Atlantic subtropical gyre, but leads only to a small increase in the Atlantic Meridional Overturning Circulation (AMOC) of O(1 Sv) after three decades. A main effect of the increasing inflow of Indian Ocean waters with potential long-term ramifications for the AMOC is the salinification and densification of upper-thermocline waters in the South Atlantic, which extends into the North Atlantic within the first three decades.
A recent study found enhanced upwelling rates in the Southern Ocean during the last glacial termination that coincided with the deglacial warming in Antarctica and the rise in atmospheric CO2. They hypothesized that the intensification of Southern Hemisphere midlatitude westerlies, the presumed cause of the increased wind-driven upwelling, was triggered by an initial cooling within the glacial North Atlantic whose influence was then communicated to the southern midlatitudes through an atmospheric teleconnection. In this study, we explore the viability of the above hypothesis using a modeling strategy, focusing on the atmospheric teleconnection. In simulations where North Atlantic cooling was applied, the model Intertropical Convergence Zone shifted southward, and westerlies and wind stress over Southern Ocean increased by as much as 25%. While the perennial westerly anomalies occur over the entire Southern Ocean, they are strongest over the South Pacific during the austral winter. When the wind stress anomalies were applied to an Earth system model incorporating interactive marine biogeochemistry, atmospheric CO2 rises between 20 and 60 ppm, depending on the biological response. We thus confirm the viability of the proposed atmospheric teleconnection hypothesis. The teleconnection appears to involves two distinct steps: first, the North Atlantic cooling shifts the Intertropical Convergence Zone southward, weakening the southern branch of the Hadley circulation, and second, how the altered Hadley circulation in turn modifies the structure of midlatitude westerlies in the South Pacific, via the former's influence on the Southern Hemisphere subtropical jet. This study underscores the control of the Northern Hemisphere has on southern midlatitude westerlies, mediating by tropical circulation, in contrast to past paleoclimate hypotheses that the magnitude and position of the southern midlatitude westerlies was controlled by global mean temperature. Our results do not preclude other potential mechanisms for affecting Southern Ocean ventilation, in particular through oceanic pathways.
Twentieth century climate change has forced a poleward contraction of the Southern Hemisphere (SH) subpolar westerly winds. The implications of this wind shift for the ocean's thermohaline circulation (THC) is analyzed in models and, where available, observations. Substantial heat content anomalies can be linked to changes in the latitude and strength of the SH westerly winds. For example, the Southern Annular Mode projects onto sea surface temperature in a coordinated annular manner - with a conspiring of dynamic and thermodynamic processes yielding a strong SST signal. Subantarctic Mode Water (SAMW) change can be linked to fluctuations in the wind-driven Ekman transport of cool, low salinity water across the Subantarctic Front. Anomalies in air-sea heat fluxes and ice meltwater rates, in contrast, drive variability in Antarctic Surface Water, which is subducted along Antarctic Intermediate Water (AAIW) density layers. SAMW variations also spike T-S variability in AAIW, particularly in the southeast Pacific and southeast Indian Oceans. The location of zero wind stress curl in the SH controls the distribution of overturning in the Southern Ocean as well as in the North Pacific / North Atlantic. A southward wind shift can force a stronger Atlantic THC and enhanced stratification in the North Pacific, whereas a northward shift leads to a significantly reduced Atlantic THC and the development of vigorous sinking in the North Pacific. This is because the distribution of wind stress over the Southern Ocean influences the surface salinity contrast between the Pacific and Atlantic basins. Experiments with additional freshwater flux anomalies will also be presented. The implications of these findings for oceanic climate change are discussed.