Bipolar Seesaw Research Articles

Competing climate feedbacks of ice sheet freshwater discharge in a warming world

Freshwater discharge from ice sheets induces surface atmospheric cooling and subsurface ocean warming, which are associated with negative and positive feedbacks respectively. However, uncertainties persist regarding these feedbacks’ relative strength and combined effect. Here we assess associated feedbacks in a coupled ice sheet-climate model, and show that for the Antarctic Ice Sheet the positive feedback dominates in moderate future warming scenarios and in the early stage of ice sheet retreat, but is overwhelmed by the negative feedback in intensive warming scenarios when the West Antarctic Ice Sheet undergoes catastrophic collapse. The Atlantic Meridional Overturning Circulation is affected by freshwater discharge from both the Greenland and the Antarctic ice sheets and, as an interhemispheric teleconnection bridge, exacerbates the opposing ice sheet’s retreat via the Bipolar Seesaw. These results highlight the crucial role of ice sheet-climate interactions via freshwater flux in future ice sheet retreat and associated sea-level rise.

Coupled atmosphere-ocean response of the southwest Pacific to deglacial changes in Atlantic meridional overturning circulation

The last glacial termination was characterised by millennial-scale episodes of warming and cooling that appear offset between the hemispheres. It has been proposed that this bi-polar seesaw is the result of climate system feedbacks. A key debate, which remains unresolved, concerns the relative roles of the atmosphere and oceans in transmitting these climate responses between the hemispheres. In this study we present quantitative climate proxy data to show that air temperatures in New Zealand, as recorded by mountain glaciers, tracked millennial-scale warming and cooling of local surface temperatures of the adjacent Tasman Sea throughout the last glacial termination. Both realms were dominated by warming between 18 ka and 12 ka, interrupted by a multi-centennial to millennial-scale cooling event centred on 14 ka, coincident with the Antarctic Cold Reversal. Reconciling our climate proxy evidence with a transient climate model simulation of the glacial termination, we find that the timing and amplitude of temperature changes are consistent with changing Atlantic meridional overturning circulation (AMOC). The southwest Pacific region displays a particularly sensitive response to AMOC intensity changes, despite its far-field situation from the North Atlantic. This sensitivity represents the combined impact of fast-acting oceanic teleconnections and regional atmosphere-ocean response associated with changes to the southern westerly winds. Our findings highlight that recent hypotheses promoting the role of southern westerlies as a critical component of deglaciation may be complementary to, rather than competitive with, the bipolar seesaw paradigm.

Anti-phase glacier fluctuations on the millennial-scale on the southern Tibetan Plateau and New Zealand during the last glacial period

The sparsity of high-resolution 10Be chronologies in the Northern Hemisphere makes it difficult to determine whether the global fluctuations in glacier extent during the last glacial occurred simultaneously. This limits our understanding of the past mechanisms driving global climate changes. We compiled and reanalyzed 476 10Be exposure ages from the southern Tibetan Plateau for the last glacial and compared them with coeval 10Be ages from the Southern Alps. The results indicate an anti-phase relationship between the glacier expansions in these two regions on the millennial-scale during the last glacial period. Comparison with a glacier-derived temperature reconstruction suggests that maintaining a stable glacier state in both regions required a similar cooling magnitude. We speculate that variability of the Atlantic Meridional Overturning Circulation, which induced poleward/equatorward shifts in atmospheric and oceanic circulation, was the primary driving mechanism of this anti-phase relationship. Our findings provide glacial geological evidence to support the bipolar seesaw hypothesis.

Long-term variability of the western tropical Atlantic sea surface temperature driven by greenhouse gases and AMOC

The long-term orbital-scale sea surface temperature (SST) variability in the tropics is thought to be mainly driven by greenhouse gases (GHG) forcing. However, few studies have investigated the drivers of such variability in the tropical Atlantic. Given the evidence of orbital-scale changes in Atlantic Meridional Overturning Circulation (AMOC) strength, one can hypothesize that AMOC variability also modulates the long-term tropical Atlantic SST through the bipolar seesaw mechanism. According to this mechanism, under weak [strong] AMOC conditions, the Southern Hemisphere is expected to warm up [cool down], while the Northern Hemisphere cools down [warms up]. Here, we investigate the long-term SST variability of the western tropical South Atlantic (WTSA), i.e., along the main pathway of the upper AMOC branch towards the equator, using a new 300 thousand years (kyr)-long Mg/Ca-based SST record. Our SST record shows glacial-interglacial variability superimposed by four remarkable long-term warm events during the three recorded glacial periods. These glacial warm events occurred between ca. 280–260, 160–143, 75–60, and 40–24 ka before present. Our results support the notion that atmospheric GHG plays a leading role in modulating the glacial-interglacial SST variability in the WTSA. However, it does not explain the occurrence of glacial warm events. Our study supports that the glacial warm events were caused by an orbital-scale bipolar seesaw mechanism operating in the Atlantic due to changes in the AMOC strength. These warm events may have been amplified by annual mean insolation driven by obliquity. Finally, we suggest that the long-term bipolar seesaw warmed the western tropical (South) Atlantic during the MIS 5/4 transition when the Earth's climate was cooling off.

Subsurface warming in the tropical Atlantic for the last 3 deglaciations: Insights from organic molecular proxies

During glacial terminations, the role of cross-equatorial currents in the northward spreading of warm and salty waters is essential to the reach of the following interglacial state. In the western tropical Atlantic, where the interhemispheric North Brazil Current develops, most of the existing temperature records focused on the last deglaciation. Here, using a marine sediment core covering the last 305 kyr before present (BP), we present two records of the surface (mixed layer) and the subsurface (upper thermocline) temperatures based on organic proxies UK’37 and TEXH86, respectively. During the last three glacial terminations, these records exhibit large temperature increases especially in the subsurface layer. We suggest the combined influences of the thermal bipolar seesaw and the Agulhas Leakage, acting together to warm the interior South Atlantic, to explain the strong warming of the tropical western Atlantic thermocline (+5 °C) during these periods of reduced Atlantic meridional overturning circulation (AMOC). In addition, we propose an enhanced cross-equatorial gyre to elucidate the synchronous deglacial warming of the western and eastern sides of the tropical Atlantic. The resumption of the AMOC at the onset of the following interglacial leads to the release of heat stored in thermocline waters mainly in the whole South Atlantic, as highlighted by the sharp upper ocean cooling recorded at the end of the deglacial interval. The combination of this temperature record and previously published ones in the western tropical Atlantic confirms the strong sensitivity of the upper ocean to AMOC changes in this region.

Millennial-scale changes in the Asian monsoon during the MIS12 period as recorded by a Chinese stalagmite

Marine Oxygen Isotope Stage 12 (MIS 12), the last glacial period before the Mid-Brunhes event (MBE), is considered the coldest glacial period during the past several glacial/interglacial cycles and is important because it can help better understand the cyclicity and mechanism of the millennial-scale variability of a glacial period. However, owing to the lack of precisely dated and high-temporal-resolution records, the millennial-scale variability during this period remains unclear. A 230Th/U dated stalagmite from Luoshui Cave, Hubei Province, Central China, was analyzed for stable oxygen isotopes (δ18O). An ∼120-yr-resolution δ18O profile is presented to clarify the Asian summer monsoon (ASM) history between 464 and 420.6 kyr BP (Before present, where present = 1950 CE), which corresponds to the interval from the MIS 12 to the early part of the MIS 11. The MIS 12 monsoon generally followed the insolation differences on June 21 between 30°N and 30°S and was interrupted by ten Chinese interstadial events (CISs). At least nine prominent weak monsoon intervals (WMIs) were identified during MIS 12 and displayed a strong coupling with the northern high-latitude climate and Antarctic temperature through the bipolar seesaw model, suggesting a substantial relationship with the bipolar region. This indicates that millennial-scale variability in the ASM region is an inherent feature of the MBE and has a global teleconnection. Additionally, in contrast to the EPICA DOME C3 (EDC 3) timescale, the new absolutely dated speleothem record confirms the Antarctic Ice Core Chronology 2012 (AICC 2012) at MIS 12, a period with a large difference between the two ice record geochronologies over the past 800 kyr.

A 1.5-million-year record of orbital and millennial climate variability in the North Atlantic

Abstract. Climate during the last glacial period was marked by abrupt instability on millennial timescales that included large swings of temperature in and around Greenland (Daansgard–Oeschger events) and smaller, more gradual changes in Antarctica (AIM events). Less is known about the existence and nature of similar variability during older glacial periods, especially during the early Pleistocene when glacial cycles were dominantly occurring at 41 kyr intervals compared to the much longer and deeper glaciations of the more recent period. Here, we report a continuous millennially resolved record of stable isotopes of planktic and benthic foraminifera at IODP Site U1385 (the “Shackleton Site”) from the southwestern Iberian margin for the last 1.5 million years, which includes the Middle Pleistocene Transition (MPT). Our results demonstrate that millennial climate variability (MCV) was a persistent feature of glacial climate, both before and after the MPT. Prior to 1.2 Ma in the early Pleistocene, the amplitude of MCV was modulated by the 41 kyr obliquity cycle and increased when axial tilt dropped below 23.5∘ and benthic δ18O exceeded ∼3.8 ‰ (corrected to Uvigerina), indicating a threshold response to orbital forcing. Afterwards, MCV became focused mainly on the transitions into and out of glacial states (i.e. inceptions and terminations) and during times of intermediate ice volume. After 1.2 Ma, obliquity continued to play a role in modulating the amplitude of MCV, especially during times of glacial inceptions, which are always associated with declining obliquity. A non-linear role for obliquity is also indicated by the appearance of multiples (82, 123 kyr) and combination tones (28 kyr) of the 41 kyr cycle. Near the end of the MPT (∼0.65 Ma), obliquity modulation of MCV amplitude wanes as quasi-periodic 100 kyr and precession power increase, coinciding with the growth of oversized ice sheets on North America and the appearance of Heinrich layers in North Atlantic sediments. Whereas the planktic δ18O of Site U1385 shows a strong resemblance to Greenland temperature and atmospheric methane (i.e. Northern Hemisphere climate), millennial changes in benthic δ18O closely follow the temperature history of Antarctica for the past 800 kyr. The phasing of millennial planktic and benthic δ18O variation is similar to that observed for MIS 3 throughout much of the record, which has been suggested to mimic the signature of the bipolar seesaw – i.e. an interhemispheric asymmetry between the timing of cooling in Antarctica and warming in Greenland. The Iberian margin isotopic record suggests that bipolar asymmetry was a robust feature of interhemispheric glacial climate variations for at least the past 1.5 Ma despite changing glacial boundary conditions. A strong correlation exists between millennial increases in planktic δ18O (cooling) and decreases in benthic δ13C, indicating that millennial variations in North Atlantic surface temperature are mirrored by changes in deep-water circulation and remineralization of carbon in the abyssal ocean. We find strong evidence that climate variability on millennial and orbital scales is coupled across different timescales and interacts in both directions, which may be important for linking internal climate dynamics and external astronomical forcing.

A new view on abrupt climate changes and the bipolar seesaw based on paleotemperatures from Iberian Margin sediments

The last glacial cycle provides the opportunity to investigate large changes in the Atlantic Meridional Overturning Circulation (AMOC) beyond the small fluctuations evidenced from direct measurements. Paleotemperature records from Greenland and the North Atlantic show an abrupt variability, called Dansgaard-Oeschger (DO) events, which is associated with abrupt changes of the AMOC. These DO events also have Southern Hemisphere counterparts via the thermal bipolar seesaw, a concept describing the meridional heat transport leading to asynchronous temperature changes between both hemispheres. However, temperature records from the North Atlantic show more pronounced DO cooling events during massive releases of icebergs known as Heinrich (H) events, contrary to ice-core-based temperature records from Greenland. Here, we present high-resolution temperature records from the Iberian Margin and a Bipolar Seesaw Index to discriminate DO cooling events with and without H events. We show that the thermal bipolar seesaw model generates synthetic Southern Hemisphere temperature records that best resemble Antarctic temperature records when using temperature records from the Iberian Margin as inputs. Our data-model comparison emphasizes the role of the thermal bipolar seesaw in the abrupt temperature variability of both hemispheres with a clear enhancement during DO cooling events with H events, implying a relationship that is more complex than a simple flip-flop between two climate states linked to a tipping point threshold.

A Walker switch mechanism driving millennial-scale climate variability

<p>A key goal of paleoclimate science is to identify the source of millennial-scale climate fluctuations. Although long-term changes in the Earth��s position relative to the Sun are the driver of climate change on timescales of �� 19 thousand years (ka), it is still incompletely understood what causes rapid sub-orbital changes in climate that were common during glacial periods. Here, we provide a continuous, precisely dated speleothem record from southeastern Alaska that spans the last 13.5 ka. Despite its location in the high latitudes of the Northern Hemisphere, this record shows an equatorial Pacific climate pattern during the end of the last deglaciation and Holocene, and is at odds with North Atlantic climate patterns, calling into question the bipolar seesaw mechanism for the Pacific realm. Because of this, we propose a new mechanism to account for sub-orbitally forced climate changes. The mechanism, termed the Walker switch, is forced by insolation and results in rapid zonal sea-surface temperature changes in the equatorial Pacific. The climatic effects of the Walker switch are propagated across the globe, including the high northern latitudes.</p>

Coupled atmosphere-ice-ocean dynamics during Heinrich Stadial 2

Our understanding of climate dynamics during millennial-scale events is incomplete, partially due to the lack of their precise phase analyses under various boundary conditions. Here we present nine speleothem oxygen-isotope records from mid-to-low-latitude monsoon regimes with sub-centennial age precision and multi-annual resolution, spanning the Heinrich Stadial 2 (HS2) — a millennial-scale event that occurred at the Last Glacial Maximum. Our data suggests that the Greenland and Antarctic ice-core chronologies require +320- and +400-year adjustments, respectively, supported by extant volcanic evidence and radiocarbon ages. Our chronological framework shows a synchronous HS2 onset globally. Our records precisely characterize a centennial-scale abrupt “tropical atmospheric seesaw” superimposed on the conventional “bipolar seesaw” at the beginning of HS2, implying a unique response/feedback from low-latitude hydroclimate. Together with our observation of an early South American monsoon shift at the HS2 termination, we suggest a more active role of low-latitude hydroclimate dynamics underlying millennial events than previously thought.

Heinrich summers

Millennial-scale climate oscillations of the last ice age registered in Greenland and Antarctic ice cores did not always vary in unison. A striking example is that the strongest Antarctic warming episodes occurred during Heinrich episodes in the North Atlantic region. Although the bipolar seesaw affords a possible explanation for such anti-phasing, it does not account for the equally striking observation that climate varied in unison between the hemispheres about half the time. Such phasing differences suggest the need for an alternative hypothesis in which the polar regions at times responded in unison to common forcing, and at other times left the impression of a bipolar seesaw. We posit that this impression arose from the effect of warmer-than-usual summers on continental ice sheets adjacent to the North Atlantic Ocean during each Heinrich episode. The relatively warm Heinrich summers produced discharges of meltwater and icebergs of sufficient volume to stimulate very cold winter conditions from widespread sea ice on a freshened ocean surface. The intervals between Heinrich episodes featured relaxation of sea-ice-induced winter severity from reduced summertime influx of meltwater and icebergs, indicating relatively cooler summer conditions. It is postulated that the causative variations in freshwater fluxes were driven by a climate signal most evident in Antarctic ice cores but also recognized in other paleoclimate records in both polar hemispheres. We suggest that this widespread signal arose from changes in the latitude and strength of the austral westerlies and the resulting effect on the western Pacific tropical warm pool, a mechanism dubbed the Zealandia Switch.

Carbon Fluxes during Dansgaard–Oeschger Events as Simulated by an Earth System Model

Abstract The Community Earth System Model with marine and terrestrial biogeochemistry is configured to simulate glacial climate. The integration shows transitions from warm to cold states—interstadials to stadials—and back. The amplitude of the associated Greenland and Antarctica temperature changes and the atmospheric CO2 signal are consistent with ice-core reconstructions, and so are the time lags between termination of a stadial, Antarctic temperature reversal, and the decline of the atmospheric CO2 concentration (for brevity’s sake simply referred to as CO2 from here on). The present model results stand out because the transitions occur spontaneously (without forcing changes like hosing) and because they reproduce the observed features above in a configuration that uses the same parameterizations as climate simulations for the present day (i.e., no retuning has been done). During stadials, precipitation shifts lead to reduced growth on land, which dominates the CO2 increase; the ocean acts as a minor carbon sink during the stadials. After the end of the stadials, however, the sudden reversal of the stadial anomalies in temperature, wind, and precipitation turns the ocean into a carbon source, which accounts for the continued rise of CO2 for several hundred years into the interstadial. The simulations also provide a novel possible interpretation for the observed correlation between CO2 and Antarctic temperature: rather than both being controlled by Southern Ocean processes, they are both controlled by the North Atlantic Ocean, and most of the extra CO2 may not be of Southern Hemisphere origin. If the stadials are prolonged through North Atlantic hosing, the upper ocean comes to an equilibrium, and the CO2 response is dominated by a single process: reduced export production in the North Atlantic as result of the collapsed overturning circulation. This is in contrast to the unforced simulation where the net ocean carbon flux anomaly is the sum of several regional responses of both signs and similar magnitudes. Reducing the aeolian iron deposition by half, to account for the observed reduction of Southern Hemisphere dust fluxes during stadials, reduces biological productivity and export production so that the Southern Ocean emerges as an important carbon source, at least for the three centuries up until a new equilibrium for the upper ocean is reached. Significance Statement The last ice age featured millennial-scale oscillations in CO2 and temperature, the latter being anticorrelated between Greenland and Antarctica. This is often referred to as the bipolar seesaw. Here, an Earth system model is described that reproduces these signals, and its results are used to explain the observed correlation between temperature and CO2. In contrast to previous idealized studies it is found that all ocean basins and the land each contribute equally to the CO2 signal.

One‐To‐One Coupling Between Southern Ocean Productivity and Antarctica Climate

AbstractThe Southern Ocean is supposed to play a crucial role in influencing atmospheric CO2 concentrations. However, the dynamic relationships among the rate of Southern Ocean upwelling, atmospheric CO2 concentration, and Antarctic climate at millennial timescales remain unclear. Here, we present high‐resolution color reflectance component b* and natural gamma radiation from the southern Scotia Sea sector of the Southern Ocean to reconstruct productivity over the past 160 ka. We find that these two independent productivity proxies, reflecting the signal of nutrient supply in this region, captured millennial‐scale upwelling that covaried in timing with Heinrich Stadials and Antarctic warming events, supporting the bipolar seesaw mechanism. The one‐to‐one coupling of variability between productivity and atmospheric CO2 concentrations reveals that the upwelling is closely linked to atmospheric CO2 and climate change.

The Roles of Orbital and Meltwater Climate Forcings on the Southern Ocean Dynamics during the Last Deglaciation

The last deglacial climate evolution, from 19 to 9 thousand years before the present, represents the vital role of feedback in the Earth’s climate system. The Southern Ocean played a fundamental role by exchanging nutrients and carbon-rich deep ocean water with the surface during the last deglaciation. This study employs a fully coupled Earth system model to investigate the evolution of Southern Ocean dynamics and the roles of changes in orbital and meltwater forcings during the last deglaciation. The simulation supports that the Southern Ocean upwelling was primarily driven by windstress. The results show that the melting and formation of Antarctic sea ice feedback influenced Southern Ocean surface buoyancy flux. The increase in Antarctic sea ice melt-induced freshwater flux resulted in a steepened north-south surface salinity gradient in the Southern Ocean, which enhanced the upwelling. The single-forcing experiments indicate that the deglacial changes in orbital insolation influenced the Southern Ocean upwelling. The experiments also highlight the dominant role of Northern Hemisphere meltwater discharge in the upper and lower branch of the Meridional Overturning Circulation. Furthermore, orbital forcing shows lesser deglacial Antarctic sea ice retreat than the Northern Hemisphere meltwater forcing, which follows the bipolar seesaw mechanism.

Toward Reconciling Radiocarbon Production Rates With Carbon Cycle Changes of the Last 55,000 Years

AbstractSince it is currently not understood how changes in 14C production rate (Q), and in the carbon cycle, can be combined to explain the reconstructed atmospheric Δ14C record, we discuss possible reasons for this knowledge gap. Reviewing the literature, we exclude that changes in the content of atoms in the atmosphere, which produce cosmogenic 14C after being hit by galactic cosmic rays, might be responsible for parts of the observed differences. When combining Q with carbon cycle changes, one needs to understand the changes in the atmospheric 14C inventory, which are partially counterintuitive. For example, during the Last Glacial Maximum, Δ14C was ∼400‰ higher compared with preindustrial times, but the 14C inventory was 10% smaller. Some pronounced changes in atmospheric Δ14C do not correspond to any significant changes in the atmospheric 14C inventory, since CO2 was changing simultaneously. Using two conceptually different models (BICYCLE‐SE and LSG‐OGCM), we derive hypothetical Qs by forcing the models with identical atmospheric CO2 and Δ14C data. Results are compared with the most recent data‐based estimates of Q derived from cosmogenic isotopes. Millennial‐scale climate change connected to the bipolar seesaw is missing in the applied models, which might explain some, but probably not all, of the apparent model‐data disagreement in Q. Furthermore, Q based on either data from marine sediments or ice cores contains offsets, suggesting an interpretation deficit in the current data‐based approaches.

Deglacial patterns of South Pacific overturning inferred from 231Pa and 230Th

The millennial-scale variability of the Atlantic Meridional Overturning Circulation (AMOC) is well documented for the last glacial termination and beyond. Despite its importance for the climate system, the evolution of the South Pacific overturning circulation (SPOC) is by far less well understood. A recently published study highlights the potential applicability of the 231Pa/230Th-proxy in the Pacific. Here, we present five sedimentary down-core profiles of 231Pa/230Th-ratios measured on a depth transect from the Pacific sector of the Southern Ocean to test this hypothesis using downcore records. Our data are consistent with an increase in SPOC as early as 20 ka that peaked during Heinrich Stadial 1. The timing indicates that the SPOC did not simply react to AMOC changes via the bipolar seesaw but were triggered via Southern Hemisphere processes.

A 125-ka record of northern South American precipitation and the role of high-to-low latitude teleconnections

The occurrence of rapid last glacial temperature shifts in both Greenland ice and subpolar marine sediment cores, so-called Dansgaard-Oeschger (D-O) cycles, is evidence of a tight regional climate connectivity in these northern latitudes. By contrast, processes behind high-to-low-latitude teleconnections during the D-O cycles are less well understood, due to imprecisions in cross-dating marine and ice core records and a general lack of solid multi-proxy records from the tropical region. Here we aim to study the response of the tropical ocean-atmosphere system to D-O variability by using a sediment core from the southern Caribbean Sea, located under the direct influence of the Intertropical Convergence Zone (ITCZ) and the Atlantic meridional overturning circulation (AMOC). During D-O cycles, abrupt shifts in the deposition of fine-grained terrigenous material sourced from the Orinoco and Amazon rivers are recognized. These sedimentary changes were associated with fluctuations in both the ITCZ location and the AMOC strength through their respective influence on the hydroclimate of northern South America and strength of ocean currents delivering fluvial material to the study site. Stable oxygen isotope data further suggest increase in the upper ocean temperatures in the southeastern Caribbean Sea during stadial events. As these warming trends correlate with Antarctic temperatures, they seem to contain imprints of the bipolar seesaw and AMOC variability, demonstrating that our core location was influenced by processes prevalent in both hemispheres.

Diatom based reconstruction of climate evolution through the Last Glacial Maximum to Holocene in the Cosmonaut Sea, East Antarctica

Diatom composition has been studied in two cores retrieved from the Cosmonaut Sea, East Antarctica, during the 36th Chinese Antarctic Research Expedition. The resulting diatom data are compared to other records from the Southern Ocean and the deglaciation history of the Antarctic ice sheet to reveal the climatic response of the ice-proximal environment to the melting of the ice sheet from the Last Glacial Maximum (LGM) to Holocene. The LGM is marked by low productivity and extensive sea ice cover that resulted in poor preservation of diatom frustules. The last deglacial warming process is facilitated by the “bipolar seesaw” effect. The environment in the Cosmonaut Sea is primarily influenced by the melting of the West Antarctic Ice Sheet (WAIS) during the last deglaciation, while the East Antarctic Ice Sheet (EAIS) remained relatively stable. Maximum sea ice retreat and southward intrusion of the warm Circumpolar Deep Water facilitate the melting of the EAIS during the early Holocene. The developing cavity under the WAIS is suggested to be responsible for the mid-late Holocene cooling in the Cosmonaut Sea. This indicates that the deglaciation of the WAIS is a primary control of environmental changes in the Cosmonaut Sea, despite its ice proximal location adjacent to the EAIS.

SST Changes in the Indian Sector of the Southern Ocean and Their Teleconnection With the Indian Monsoon During the Last Glacial Period

AbstractThe last glacial period was marked by interhemispheric coupling of abrupt climate changes in the northern and the southern high latitude regions through the “bipolar seesaw” mechanism. However, the spatial footprint of the bipolar seesaw in the mid‐latitude Indian sector of the Southern Ocean and its likely transmission into the tropical region is not well understood. Here, we have reconstructed the first multi‐centennial record of past SST variability in the Indian sector of the Southern Ocean spanning parts of MIS‐3, the last deglaciation, and the early to mid‐Holocene period using Mg/Ca of G. bulloides. We find that SST increased by around 4°C between the last glacial period and the early Holocene at the study site. Using this paleotemperature reconstruction and an independently available paleo‐simulation, we show that the mid‐latitude climate evolution in the Indian sector had signatures of both local‐scale changes and those emanating from the coupled bipolar influence. We further show that most of the millennial variance in Indian monsoon records during the period was linked to mid‐latitude SST variability in the Indian sector.

Different Trends in Antarctic Temperature and Atmospheric CO2 During the Last Glacial

AbstractWe analyze the past 67,000 years of climate using Antarctic ice‐core records to constrain the mechanisms involved in (a) the “bipolar seesaw” relationship between Greenland and Antarctic surface temperature variations, and (b) mechanisms of millennial‐scale atmospheric CO2 concentration variations. Specifically, we determine for each Greenland Stadial the rate of Antarctic temperature and atmospheric CO2 rise. We find that Antarctic warming rates significantly decrease as the climate cools during the glacial period, whereas the rate of atmospheric CO2 rise does not significantly change. Also, we find that the rates of Antarctic warming and atmospheric CO2 rise are both insensitive to whether a given stadial contains a Heinrich event. These results challenge the view that a single Southern‐Ocean‐based mechanism dominates the observed glacial variability in Antarctic temperature and atmospheric CO2. Instead, our results are consistent with an important contribution of low‐ and mid‐latitude processes to millennial‐scale atmospheric CO2 changes.