South Atlantic Ocean Deep Circulation Printer-friendly Version Interactive Discussion Climate of the past the Impacts of Meltwater Pulse-1a in the South Atlantic Ocean Deep Circulation since the Last Glacial Maximum South Atlantic Ocean Deep Circulation Printer-friendly Version Interactive Discussio

Discussions This discussion paper is/has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP if available. Abstract Since 21 000 yr ago, the oceans have received large amounts of freshwater in pulses coming from the melting ice sheets. A specific event, known as meltwater pulse 1A (MWP-1A), has been identified in sea-level and temperature proxy records as responsible for the increase of ∼ 20 m in sea level in less than 500 yr. Although its origin and 5 timing are still under discussion, MWP-1A seems to have had a significant impact on several components of the climatic system. The present work aims to elucidate these impacts on the water mass distribution of the South Atlantic Ocean through the analysis of a transient simulation of the climate evolution from the Last Glacial Maximum to Present Day using a state-of-art CGCM, the National Center for Atmospheric Research 10 Community Climate System Model version 3 (NCAR CCSM3). Results show that the freshwater discharge associated with the timing of MWP-1A was crucial to establish the present thermohaline structure associated with the North Atlantic Deep Water, marking the transition between a shallower and a deeper Atlantic Meridional Overturning Circulation.


Introduction
The last deglaciation, triggered by increasing summer insolation in the Northern Hemisphere (NH) nearly 21 thousand years ago (ka), was marked by several meltwater discharge episodes resulting from ice sheet retraction.A major meltwater discharge registered on sea level records (Fairbanks, 1989;Deschamps et al., 2012) occurred around 14 ka resulting from massive loss of ice from the ice sheets into the ocean (Weaver et al., 2003;Peltier and Fairbanks, 2006;Hanebuth et al., 2009), which was responsible for ∼ 20 m of sea level rise in less than 500 yr.This event is known as meltwater pulse 1A (MWP-1A).
The origins of MWP-1A has been discussed in the last decades and is still subject related to its Northern Hemisphere source.It is suggested by several authors that this freshwater input came solely from the Laurentide Ice Sheet (LIS) over North America, ruling out any contribution from the Southern Hemisphere (SH) (Fairbanks, 1989;Peltier, 1994Peltier, , 2005;;Peltier and Fairbanks, 2006;Bentley et al., 2010;Mackintosh et al., 2011;Anderson et al., 2013).On the other hand, several studies support the idea of a significant freshwater contribution from the Antarctic Ice Sheet (AIS) (Clark et al., 1996(Clark et al., , 2002;;Kienast et al., 2003;Weaver et al., 2003;McManus et al., 2004;Bassett et al., 2005Bassett et al., , 2007;;Carlson, 2009;Stenni et al., 2010;Bethke et al., 2012;Carlson et al., 2012;Deschamps et al., 2012;Gregoire et al., 2012).In fact, some of these later studies discuss that the idea of MWP-1A generated only in the Northern Hemisphere would be inconsistent with proxy data evidence.MWP-1A originating only from the Northern Hemisphere would not be physically plausible due to the necessary melting rates needed to account for the sea level rise records (Clark et al., 1996;McManus et al., 2004;Stanford et al., 2006;Praetorius et al., 2008).The modeling study of Carlson et al. (2012) attributes about 37 % of the LIS mass loss contribution to MWP-1A, which suggests that a significant fraction of its origin could come from the AIS.Seidov et al. (2001) and Weaver et al. (2003) suggested that meltwater discharges near the location of deep water formation in one hemisphere may lead to the strengthening of the ocean circulation in the other hemisphere.The main players on the lower and upper limbs of the Atlantic Meridional Overturning Circulation (AMOC) are the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW).So, when freshwater enters the North Atlantic, for example, the NADW formation rate decreases.In order to maintain the deep water input into the world ocean, more AABW is produced, which would lead to a shallower AMOC in the North Atlantic.This scenario is what is believed to have happened at the Last Glacial Maximum (LGM, ∼ 21 ka) when the deep Atlantic was flooded with extremely salty AABW (Marchitto et al., 2002;Curry and Oppo, 2005) and the AMOC was shallower than today (Lippold et al., 2012;Zhang et al., 2012).According to chemical proxy evidence, the AABW water does fill most of the Atlantic basin and a NH-origin water, the Glacial North Atlantic Intermediate Figures
In this study, these aspects of Atlantic Ocean's deep circulation are evaluated within the results of a transient simulation of the last 21 000 yr, which prescribes several meltwater pulses as identified by sea level records (including a MWP-1A with 75 % of its freshwater coming from AIS).This simulation is an extension for the Holocene of the DGL-A experiment described in Liu et al. (2009).It is our hypothesis that the present day circulation pattern of the South Atlantic Ocean was established approximately at the onset of the Holocene (11 ka).Therefore we will investigate the impacts of the freshwater discharge associated with MWP1-A, assuming contributions from both Hemispheres, on the structure of the NADW in South Atlantic Ocean.

Data and methods
The results of a paleoclimate transient simulation using the Community Climate System Model version 3 (CCSM3) are used in this study.CCSM is a global coupled model developed by the National Center for Atmospheric Research (NCAR).The simulation experiment analyzed is referred to as TraCE-21K.It was run from 21 ka to 0 ka (He, 2011) and it is the extended version of the DGL-A experiment discussed in Liu et al. (2009).It should be noted that the model air temperature, when compared to the GISP2 ice core reconstructions (Alley, 2000) show very good agreement (Liu et al., 2009).A brief description of TraCE-21K setup is given as follows.
The oceanic component has 25 vertical levels and a spatial resolution of 3.6 Joos and Spahni (2008).The coastlines and ice sheets volume variability followed ICE-5G database (Peltier, 2004).Several meltwater schemes were tested in sensitivity experiments (He, 2011), but just the ones that resulted in similar values compared to proxy data were chosen for TraCE-21K.He (2011) derived the meltwater forcing based on the records of sea level rise (Clark et al., 2002;Peltier, 2004) and geological indicators of ice sheet retreat and meltwater discharge (Licciardi et al., 1999;Clark et al., 2001Clark et al., , 2002;;Clark and Mix, 2002;Carlson, 2009;Carlson et al., 2012;Obbink et al., 2010).The rates of meltwater discharge in the model are shown in Fig. 1.
In TraCE-21K, MWP-1A is prescribed from 14.35 to 13.85 ka leading to the abrupt inversion within the BA period: the Older Dryas event (Stanford et al., 2006;Liu et al., 2009).However, this timing may be late according to recent findings that constrain MWP-1A between 14.65 and 14.31 ka (Deschamps et al., 2012), making it simultaneous with the Bolling warming, although the causality relations are still under debate.The meridional heat transport through a section of the Atlantic basin was calculated as in Bryan (1962): Where Q t is the meridional heat transport, c p is the sensible heat of seawater, ρ is seawater density, v is the meridional velocity and θ is the potential temperature.The meridional salt transport was calculated in the same way, simply substituting c p θ by S, where S is the salinity of seawater.
Here, we define the NADW as being a tongue-like signal of maximum salinity around 2500 m depth in vertical profiles across all longitudes spanning the Atlantic basin extending from the northern high latitudes into the Southern Ocean.The thermohaline signature of NADW was analyzed for the South Atlantic (0-30 • S), where this water mass is well defined.

Results and discussion
The ocean model results are examined with respect to the AMOC through meridional heat and salt transport time series.In contrast with the other oceans, where the heat is generally transported from the equator to the poles, the meridional heat transport in the South Atlantic is directed northward.This happens because the upper branch of the AMOC carries warm surface layer waters northward to compensate the great amount of heat loss in the Nordic Seas thus making the South Atlantic circulation studies instrumental to fully understand Earth's climate system.The heat transport anomaly (with respect to the mean) for the South Atlantic (0-30 Negative meridional heat transport anomalies are associated with the Northern Hemisphere cooling relative to the Heinrich 1 (H1) and Younger Dryas (YD) events, while temperature increases in the Southern Hemisphere as in Liu et al. (2009).After 11 ka, the heat transport values increase slowly towards the mean.From 11 ka to 0 ka, the northward heat transport experiences a long period with little variability.After the mid-Holocene (∼ 6 ka) there is a noticeable stable equilibrium in the heat transport between Northern and Southern Hemisphere, at the same time that the meltwater pulses are shutdown in the simulation.The relationship between TraCE-21K meridional heat transport with the meridional salt transport anomalies (where the mean is −7.58 × 106 kg s −1 ) (Fig. 2, green line) shows an out-of-phase behaviour between the two time series.This is best observed through the dispersion diagram in Fig. 3.It shows that the correlations between heat and salt transport are high before and after the BA warming period (circles with a correlation coefficient (r) of −0.99, and triangles with r = −0.93,respectively).During the transition (from approximately 14.7 ka to 12.9 ka) the correlation falls (but is still significant; Fig. 3 The different slopes of the circles and triangles clusters in Fig. 3 indicate that, overall, the transport exhibit two regimes: a large heat transport and small salt transport before BA and a small heat transport and large salt transport especially after YD (greenish to reddish tones).It is interesting to notice that heat transport is higher at LGM than at modern times, although the temperatures today are far superior than the glacial ones.
This strengthens the hypothesis that the AMOC was stronger at 21 ka (Shin et al., 2003;Clauzet et al., 2007), which would explain the large quantity of heat being transported northward.Salt transport, on the other hand, was only consistently positive after the YD period.At glacial times the source of salt for the Atlantic was the salty AABW spanning almost the entire basin from its formation region up to high latitudes of the northern hemisphere.Figures 2 and 3 suggest that northern sources of salty water masses should appear after the YD.The mean potential temperature (θ) vs. salinity (S) for the TraCE-21K model results is shown in Fig. 4a averaged between 30 • S and 0 • from the LGM (blue dots) to 0 ka (red dots).It can be observed that there is a shift of the curve towards lower salinities throughout the entire water column, which is expected given all the freshwater entering the Atlantic Ocean in the last 21 000 yr.Not so obvious, however, are the changes in the shape of the θ-S curve for the different periods: the θ-S curve at 21 ka is representing only two deep water masses, while the modern curve (0 ka) presents the three main Atlantic Ocean deep water masses (AAIW, NADW and AABW, as indicated in Fig. 4a).The water type that specifies the NADW characteristics (salinity maximum at ∼ 2500 m) does not appear until the early Holocene.
The absence of the NADW during the glacial period is consistent with proxy records (Adkins et al., 2002).θ-S plots considering the same latitudinal interval for the South Atlantic, averaged for 21, 15, 14.1, 13 and 0 ka (before, during and after the MPW-1A) are shown in Fig. 4b.It is clear that the deep salinity maximum characteristic of the NADW within the South Atlantic starts to develop only after MWP-1A (θ-S diagram at 13 ka, pink curve).These changes become evident when examining the vertical profiles of salinity along the Atlantic Ocean for the different climatic periods (Fig. 5).m depth (Fig. 5f-g).Before MWP-1A, the water column was strongly stratified below 1500 m (Fig. 5a-c).
Figure 5 shows that the vertical salinity changes are not homogeneous.The impact of the freshwater discharge associated with the simulated MWP-1A does not only impact in reducing the entire water column salinity but also changes the water mass configurations along the Atlantic Ocean.At the LGM the deep ocean was very salty (Fig. 5a) when massive volumes of freshwater were stored in the ice sheets.At the LGM there was increased sea ice formation and associated brine rejection in the Southern Ocean (Shin et al., 2003).The sea ice formation was intensified largely by the lower atmospheric CO 2 concentration (Liu and Milliman, 2004).As a result, the glacial Atlantic Ocean was much more stratified than today (Adkins et al., 2002;Zhang et al., 2012).At about 19 ka, instability of northern ice sheets led to an abrupt discharge of icebergs in the North Atlantic, which is the signature of the H1 event (Denton et al., 2010).During this event, the meltwater input associated with icebergs led to a weaker convection in the north (He et al., 2013) and the total ocean heat transport to north is diminished (as shown in Fig. 2) consistent with Seidov et al. (2001).The main consequences are a decrease in air temperature in the NH and a rise in the SH (Fig. 1 in Liu et al., 2009) which is referred to as the bipolar seesaw.A freshening between ∼ 500-2500 m in the water column (Fig. 5b) is also observed.
When the H1 meltwater discharge was interrupted, the North Atlantic circulation was reinforced.The resulting strengthening of the AMOC and its overshoot possibly triggered the BA abrupt warming in the north at about 14.7 ka (Liu et al., 2009), represented by the abrupt increase of the heat transport anomaly as shown in Fig. 2 (purple line).The model MWP-1A (largest peak in Fig. 1) occurs mostly in the Southern Ocean between 14.35 and 13.85 ka.A freshwater source in the Southern Ocean will generate a salinity anomaly throughout the entire water column that will spread to all other ocean basins (Stouffer et al., 2007) mostly in the bottom layers.This can be observed in Fig. 5c-d.We also see in water type source.The water in this high salinity pocket has relative high temperatures (not shown), which maintains its density lower than that necessary for sinking.As a result, the formation of the deep water in the North Atlantic is not observed.The intrusion of freshwater in the North Atlantic during Heinrich 0 at Younger Dryas (around 12 ka) makes the salty pocket disappear through mixing at the same time that the salinity anomaly that originated from the freshwater input from the southern source for the model MWP-1A continues to spread over the Atlantic basin (Fig. 5e).As soon as the meltwater discharge from the North Atlantic is interrupted around 11.7 ka, the heat transport towards the north is re-established.At this time, the high salinity pocket centered around 30 • N (∼ 500 m) reappears in the vertical profile (Fig. 5f).The Southern Ocean deep and bottom waters at this moment are much fresher than at 13 ka due to mixing of MWP-1A discharge throughout the water column.The salinity barrier, which was responsible for the high stratification, is eroded and the dense water that was confined to the surface layers can finally sink.This is when the features of the modern NADW first appear in the deep ocean since the LGM (salty tongue at Fig. 5f) marking the birth of NADW as we know it today.We can compare this same structure with the one presented in Fig. 5g, which stands for 0 ka (1950).Also, Fig. 5g shows that the model succeeded in representing the three main modern water masses that are observed in the profile obtained with observed data from the World Ocean Circulation Experiment (WOCE) seen in Fig. 5h.The differences between the two profiles (Fig. 5g and h) can be explained by the low spatial resolution of the model.
The TraCE-21 model results are consistent with the idea of a weaker NADW during YD, discussed by several studies based on proxy-data (Boyle, 1987;Hughen et al., 1998;McManus et al., 2004;Piotrowski et al., 2005;Praetorius et al., 2008;Roberts et al., 2010).became deeper (Lippold et al., 2012), meridional heat transport became more stable and the NADW becomes the main salt exporter across the Atlantic Ocean.Hence, the steep triangle cluster on Fig. 3 is explained: a smaller variability on the heat transport northwards and larger salt transport southwards in the Holocene.

Conclusions
This study examines the evolution of the South Atlantic Ocean salinity distribution relative to the NADW since the LGM, considering several meltwater pulses recorded in sea-level proxy data, through the analysis of the results of the first transient paleoclimate numerical simulation from the LGM to Present Day (PD).After evaluating the water mass distribution in the South Atlantic Ocean, it is concluded that the NADW, as we know today, was established at the onset of the Holocene (i.e.11 ka), playing an important role on the heat and salt transport stabilization during that period.Results show a salinity barrier generated at the LGM by Southern Ocean deep salty waters that prevents the deep NADW formation in its Northern Hemisphere origin.Zhang et al. (2012) discussed the importance of this stratification relative to the spatial configuration and strength of the AMOC.Therefore, much of the PD structure of the NADW occurs because of the impact of the freshwater discharge associated with the timing of the MWP-1A, which in this model is predominantly of Southern Ocean origin.This geographical source guarantees that the freshwater released spreads over the global ocean unlike the meltwater pulses from the North Atlantic, which are mostly constrained around their source region (Stouffer et al., 2007).In other words, if MWP-1A had occurred only within the northern ice sheets the resulting freshening would occur mostly at the surface, which hinders the NADW formation process.Furthermore, it is important to remember that these conclusions apply for the period starting at LGM, which means that a similar form of the PD NADW could have existed before.Using isotopic analysis, Frank et al. ( 2002 that a water mass of NH origin was present in the Southern Ocean between 14 and 3 millions of years ago, but was progressively reduced since then. Other studies point out that the source of Antarctic meltwater at ∼ 14 ka could be from the western side of the continental ice sheet (Weaver et al., 2003;Deschamps et al., 2012).Today, trends of reduced ice cover have been observed at the same location (Thomas et al., 2004;Jacobs et al., 2011;Joughin and Alley, 2011).Could this ice sheet melting change NADW properties again in the future?Introduction

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Full of controversial debate.Much of the controversy surrounding the origin of MWP-1A is Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | • longitude and variable latitude (finer resolution of ∼ 0.9 • in the equator).Initial conditions were established from LGM simulation results from Otto-Bliesner et al. (2006) and transient concentrations of greenhouse gases (CO 2 , CH 4 and N 2 O) were adopted from Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , crosses with r = −0.79).Discussion Paper | Discussion Paper | Discussion Paper | The NADWlike water mass tongue with high salinity values appears only from 11 ka around 2000 Discussion Paper | Discussion Paper | Discussion Paper | Fig. 5d a high salinity region centered at 30 • N confined within the upper layers.This is what is thought here as the precursor of the NADW Discussion Paper | Discussion Paper | Discussion Paper | Furthermore, through 231 Pa/ 230 Th ratio analysis, Negre et al. (2010) discuss that the modern Atlantic circulation was only established during the Holocene, which is also consistent with the evolution of the salinity-based water mass structure presented in Fig. 5.The establishment of the NADW as the main southward component of the AMOC preceded the change in the overturning circulation geometry.The AMOC Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |