Past Atlantic Meridional Overturning Circulation Research Articles

Evolution of Atlantic Meridional Overturning Circulation since the last glaciation: model simulations and relevance to present and future.

The Atlantic Meridional Overturning Circulation (AMOC) and the associated water masses have changed dramatically during the glacial-interglacial cycle. Here, I review some recent progress in the modelling of the AMOC and water masses since the Last Glacial Maximum and discuss the relevance of these past AMOC studies to the present and future AMOC study. Recent studies suggested that Atlantic water masses were constrained by carbon isotopes (δ13C) and neodymium isotopes (εNd), while the strength of the AMOC better was constrained by protactinium/thorium ratio (231Pa/230Th) and the spatial gradient of calcite oxygen isotopes (δ18Oc). In spite of the shallower AMOC at the glacial period, its intensity did not differ substantially from the present because of the cancellation of opposite responses to the rising CO2 and the retreating ice sheet. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Weakened AMOC related to cooling and atmospheric circulation shifts in the last interglacial Eastern Mediterranean

There is limited understanding of temperature and atmospheric circulation changes that accompany an Atlantic Meridional Overturning Circulation (AMOC) slowdown beyond the North Atlantic realm. A Peqi’in Cave (Israel) speleothem dated to the last interglacial period (LIG), 129–116 thousand years ago (ka), together with a large modern rainfall monitoring dataset, serve as the base for investigating past AMOC slowdown effects on the Eastern Mediterranean. Here, we reconstruct LIG temperatures and rainfall source using organic proxies (TEX86) and fluid inclusion water d-excess. The TEX86 data show a stepwise cooling from 19.8 ± 0.2° (ca. 128–126 ka) to 16.5 ± 0.6 °C (ca. 124–123 ka), while d-excess values decrease abruptly (ca. 126 ka). The d-excess shift suggests that rainfall was derived from more zonal Mediterranean air flow during the weakened AMOC interval. Decreasing rainfall d-excess trends over the last 25 years raise the question whether similar atmospheric circulation changes are also occurring today.

Links between precipitation patterns over eastern tropical South America and productivity in the western tropical South Atlantic Ocean during the last deglacial

The possibility of a collapse (or marked weakening) of the Atlantic Meridional Overturning Circulation (AMOC) within a few centuries boosted the interest of the scientific community on the responses of different compartments of the climate system to past AMOC collapses (or marked weakening). The most recent examples of such periods are Heinrich Stadial 1 (HS1,18-15 ka BP; near-collapsed AMOC) and the Younger Dryas (YD, 12.9-11.7 ka BP; markedly weakened AMOC). Here, we present data from elemental ratios, spectral reflectance, organic biomarkers, sea surface temperatures and planktonic foraminiferal assemblage from two marine sediment cores recovered from the western tropical South Atlantic Ocean spanning the last ca. 20 kyr. The recovered sediments record the responses of eastern tropical South American hydroclimate, western tropical South Atlantic productivity and sea surface temperatures to changes in AMOC strength associated with HS1 and the YD. Our data show that both periods were characterized by wetter conditions over eastern tropical South America associated with increased continental runoff compared to the Last Glacial Maximum. High sea surface temperatures during HS1 and the YD are in line with marked AMOC decreases that led to heat accumulation in the surface layer of the western South Atlantic Ocean, facilitating increased precipitation over eastern tropical South America. Our data demonstrate for the first time that intense runoff during HS1, and hence nutrient transfer to the ocean, affected the pelagic ecosystem, likely increasing primary productivity in the otherwise oligotrophic western tropical South Atlantic Ocean. Our study demonstrates a tight coupling between marine and terrestrial processes in the eastern South American realm during periods of strong climate instability.

In-phase millennial-scale glacier changes in the tropics and North Atlantic regions during the Holocene

Based on new and published cosmic-ray exposure chronologies, we show that glacier extent in the tropical Andes and the north Atlantic regions (TANAR) varied in-phase on millennial timescales during the Holocene, distinct from other regions. Glaciers experienced an early Holocene maximum extent, followed by a strong mid-Holocene retreat and a re-advance in the late Holocene. We further explore the potential forcing of TANAR glacier variations using transient climate simulations. Since the Atlantic Meridional Overturning Circulation (AMOC) evolution is poorly represented in these transient simulations, we develop a semi-empirical model to estimate the “AMOC-corrected” temperature and precipitation footprint at regional scales. We show that variations in the AMOC strength during the Holocene are consistent with the observed glacier changes. Our findings highlight the need to better constrain past AMOC behavior, as it may be an important driver of TANAR glacier variations during the Holocene, superimposed on other forcing mechanisms.

Advances in reconstructing the AMOC using sea surface observations of salinity

The Atlantic meridional overturning circulation (AMOC) is one of the main drivers of climate variability at decadal and longer time scales. As there are no direct multi-decadal observations of this key circulation, the reconstruction of past AMOC variations is essential. This work presents a step forward in reconstructing the AMOC using climate models and time-varying surface nudging of salinity and temperature data, for which independent multi-decadal observed series are available. A number of nudging protocols are explored in a perfect model framework to best reproduce the AMOC variability accommodating to the characteristics of SST and SSS available products. As reference SST products with sufficient space and time coverage are available, we here choose to focus on the limitations associated to SSS products with the goal of providing protocols using independent salinity products. We consider a global gridded dataset and, additionally, a coarser SSS dataset restricted to the Atlantic and with a quite low spatial resolution (order of 10 degrees vs. 2 for the model grid). We show how, using the latter, we can improve the efficiency of the nudging on the AMOC reconstruction by adding a high-resolution annual cycle to the coarse resolution SSS product as well as a spatial downscaling to account for SSS gradient. The final protocol retained for the coarse SSS data is able to reconstruct a 100-year long AMOC period (average of 10.18 Sv and a standard deviation of 1.39 Sv), with a correlation of 0.76 to the target and a RMSE of 0.99 Sv. These values can be respectively compared to 0.85 and 0.75 Sv when using the global salinity surface observations. This work provides a first step towards understanding the limitations and prospects of historical AMOC reconstructions using different sea surface salinity datasets for the surface nudging.

Atlantic Meridional Overturning Circulation and δ13C Variability During the Last Interglacial

AbstractThe Atlantic Meridional Overturning Circulation (AMOC) is thought to be relatively vigorous and stable during Interglacial periods on multimillennial (equilibrium) timescales. However, recent proxy (δ13C benthic) reconstructions suggest that higher frequency variability in deep water circulation may have been common during some interglacial periods, including the Last Interglacial (LIG, 130–115 ka). The origin of these isotope variations and their implications for past AMOC remain poorly understood. Using iLOVECLIM, an Earth system model of intermediate complexity (EMIC) allowing the computation of and direct comparison to proxy reconstructions, we perform a transient experiment of the LIG (125–115 ka) forced only by boundary conditions of greenhouse gases and orbital forcings. The model simulates large centennial‐scale variations in the interior of the North Atlantic similar in timescale and amplitude to changes resolved by high‐resolution reconstructions from the LIG. In the model, these variations are caused by changes in the relative influence of North Atlantic Deep Water (NADW) and southern source water (SSW) and are closely linked to large (∼50%) changes in AMOC strength provoked by saline input and associated deep convection areas south of Greenland. We identify regions within the subpolar North Atlantic with different sensitivity and response to changes in preformed of NADW and to changes in NADW versus SSW influence, which is useful for proxy record interpretation. This could explain the relatively large δ13C gradient (∼0.4%0) observed at ∼3 km water depth in the subpolar North Atlantic at the inception of the last glacial.

Assessing the Ability of Zonal δ18O Contrast in Benthic Foraminifera to Reconstruct Deglacial Evolution of Atlantic Meridional Overturning Circulation

Abstractδ18O in foraminifera (δ18Oc) is a useful proxy for density, and the strength of the Atlantic Meridional Overturning Circulation (AMOC) can be reconstructed by the zonal density contrast in the Atlantic. However, whether the deglacial zonal δ18Oc contrast can represent the AMOC change is still unclear. δ18Oc contrast across the Florida Straits has been hypothesized as a proxy for the AMOC evolution during the last deglaciation, but the strength of Florida Current could also be influenced by wind forcing. Here we examine the ability of the zonal δ18Oc contrast to reconstruct AMOC in a deglacial model simulation. The model simulation suggests that the deglacial variation of the Florida Current strength is dominated by AMOC, with the wind effect on the variation of the Florida Current being negligible. Furthermore, the δ18Oc contrast across the western boundary along the entire Atlantic and the basin‐wide δ18Oc contrast in the North Atlantic in the upper ocean can also be used to reconstruct AMOC. However, using basin‐wide δ18Oc contrast to reconstruct AMOC in the South Atlantic is not possible at all water depths. In the subtropical South Atlantic, the basin‐wide δ18Oc contrast is decoupled from the density contrast between 400 to 600 m through the deglaciation because of the deglacial change of the Antarctic Intermediate Water. Therefore, δ18Oc is a useful proxy to reconstruct past density and in turn past AMOC, but caution has to be used when using the basin‐wide δ18Oc contrast to reconstruct the basin‐wide density contrast in the South Atlantic.

Coherent deglacial changes in western Atlantic Ocean circulation

Abrupt climate changes in the past have been attributed to variations in Atlantic Meridional Overturning Circulation (AMOC) strength. However, the exact timing and magnitude of past AMOC shifts remain elusive, which continues to limit our understanding of the driving mechanisms of such climate variability. Here we show a consistent signal of the 231Pa/230Th proxy that reveals a spatially coherent picture of western Atlantic circulation changes over the last deglaciation, during abrupt millennial-scale climate transitions. At the onset of deglaciation, we observe an early slowdown of circulation in the western Atlantic from around 19 to 16.5 thousand years ago (ka), consistent with the timing of accelerated Eurasian ice melting. The subsequent weakened AMOC state persists for over a millennium (~16.5–15 ka), during which time there is substantial ice rafting from the Laurentide ice sheet. This timing indicates a role for melting ice in driving a two-step AMOC slowdown, with a positive feedback sustaining continued iceberg calving and climate change during Heinrich Stadial 1.

Atlantic deep water circulation during the last interglacial

Understanding how the Atlantic Meridional Overturning Circulation (AMOC) evolved during crucial past geological periods is important in order to decipher the interplay between ocean dynamics and global climate change. Previous research, based on geological proxies, has provided invaluable insights into past AMOC changes. However, the causes of the changes in water mass distributions in the Atlantic during different periods remain mostly elusive. Using a state-of-the-art Earth system model, we show that the bulk of NCW in the deep South Atlantic Ocean below 4000 m migrated from the western basins at 125 ka to the eastern basins at 115 ka, though the AMOC strength is only slightly reduced. These changes are consistent with proxy records, and it is mainly due to more penetration of the AABW at depth at 115 ka, as a result of a larger density of AABW formed at 115 ka. Our results show that depth changes in regional deep water pathways can result in large local changes, while the overall AMOC structure hardly changes. Future research should thus be careful when interpreting single proxy records in terms of large-scale AMOC changes, and considering variability of water-mass distributions on sub-basin scale would give more comprehensive interpretations of sediment records.

Continental climate gradients in North America and Western Eurasia before and after the closure of the Central American Seaway

The Gulf Stream, as part of the Atlantic Meridional Overturning Circulation (AMOC), is known as a major driver of latitudinal energy transport in the North Atlantic presently causing mild winters over northwestern Eurasia. The intensity of the AMOC throughout the Neogene, prior to the final closure of the Central American Seaway (CAS) in the early Pliocene, is still poorly known, but most authors assume that the circulation was considerably weaker than present. Here we address this issue from a continental point of view. We studied the past AMOC intensity by analyzing Neogene continental climate patterns along North American and Western Eurasian transects. Based on a total of 317 palaeofloras thermal latitudinal gradients are reconstructed for three Neogene time slices, namely the middle Miocene, late Miocene, and late Pliocene using the Coexistence Approach to obtain quantitative climate data. The obtained proxy-based, continental temperature gradients are evaluated against data from a selection of published General Circulation Model (GCM) simulations for the three time slices studied.Our study suggests that shallow thermal latitudinal gradients existed in North America and Western Eurasia throughout the Miocene but became strongly steepened in the late Pliocene. In both Miocene time slices studied, the higher latitudes were by up to 30 °C warmer than present (cold month mean), also at times with presumed pre-industrial CO2 such as the late Miocene. In the late Pliocene high-latitude, the temperature difference with respect to the present had decreased by up to 10 °C (cold month mean). Both mean annual temperatures and cold month means of the lower mid and low latitudes were at the present-day level throughout all three time slices, or even slightly below. In both Miocene time slices, zonal temperature means at both continental transects were similar in the mid and higher latitudes. However, several northwest European sites reveal very mild winter condition suggesting the early existence of a probably less intense Palaeo-Gulf Stream. The distinct thermal anomaly (annual and cold month means) today existing between North America and Western Eurasia appeared for the first time in the late Pliocene, attaining about 50% of the present-day magnitude. This supports the assumption that the AMOC intensified after the final closure of the CAS during the early Pliocene. The results obtained from the palaeobotanical proxies are in line with data from coeval marine archives, particularly with North Atlantic sea surface temperatures (SSTs) inferred from oxygen isotopes. However, the proxy-based thermal gradients are not well reproduced by a selection of GCM simulations, due to a well-known systematic underestimation of high latitude warming by GCMs for the Miocene and Pliocene time slices.

The evolution of deep‐ocean flow speeds and δ13C under large changes in the Atlantic overturning circulation: Toward a more direct model‐data comparison

AbstractTo investigate the dynamics of the Atlantic meridional overturning circulation (AMOC) on timescales longer than the observational records, model‐data comparisons of past AMOC variability are imperative. However, this remains challenging because of dissimilarities between different proxy‐based AMOC tracers and the difficulty of comparing these to model output. We present an iLOVECLIM simulation with tuned AMOC evolution and focus on AMOC tracers that are directly comparable to reconstructions: flow speeds and δ13C. We deduce their driving factors and show that they yield different but complementary information about AMOC changes. Simulated flow speed changes are only linked to AMOC changes in regions bathed by North Atlantic Deep Water; however, in those regions they do provide details on vertical migration and thickness changes of the water masses. Simulated δ13C changes in the North Atlantic Deep Water region are again related to AMOC changes. Yet in regions bathed by Antarctic Bottom Water or Antarctic Intermediate Water, the δ13C evolution is driven by Southern Hemisphere source water δ13C changes, while in the Nordic Seas and the two major overflow regions it is driven by Northern Hemisphere source water δ13C changes. This shows that AMOC changes are not necessarily recorded by δ13C and stresses the need for combining both tracers in paleoclimate studies. A preliminary model‐data comparison for Last Interglacial flow speeds and δ13C changes in the Deep Western Boundary Current shows that this integrated approach is far from straightforward and currently inconclusive on the Last Interglacial AMOC evolution. Nonetheless, the approach yields potential for more direct and in‐depth model‐data comparisons of past AMOC changes.

The influence of Greenland ice sheet melting on the Atlantic meridional overturning circulation during past and future warm periods: a model study

The sensitivity of the climate system to changes in radiative forcing is crucial for our understanding of past and future climates. Especially important are feedbacks related to melting of ice sheets like the Greenland ice sheet (GIS) and its potential impact on the Atlantic meridional overturning circulation (AMOC). These effects are likely to delay and dampen predicted long-term warming trends. Estimates of climate sensitivity may be deduced from palaeoclimate-reconstructions, but this raises the question whether past climate sensitivity is applicable to the future. Therefore we have analysed the impact of GIS melt water on the AMOC strength in two past warm climates (last interglacial and early present interglacial) and three future scenarios with three different model parameter sets. These model parameter sets represent three different model sensitivities to freshwater perturbation: low, moderate and high. In both the moderate and high sensitivity versions, we find for lower GIS melt rates (below 54 mSv, Sv = 106 m3/s) a clear difference between past and future warm climates in the sensitivity of the AMOC to GIS melt. This difference is connected to the convective activity in the Labrador Sea and the amount of additional surface freshening due to sea ice melting. In contrast, for higher GIS melt rates (over 54 mSv) we find similar reductions of the AMOC strength in all cases. Considering the low sensitivity version of our model, we find that for all GIS melt rates the influence of freshwater forcing on the AMOC is independent of the background climate. Our results and implications are thus strongly determined by the parameter set considered in our model. Nonetheless, our results from two out of three model versions suggest that proxy-based reconstructions of past AMOC sensitivity to GIS melt are likely to be misleading if interpreted for future applications.

Testing the231Pa/230Th paleocirculation proxy: A data versus 2D model comparison

[1] Variations of the Atlantic Meridional Overturning Circulation (AMOC) are believed to have crucially influenced Earth's climate due to its key role in the inter-hemispheric redistribution of heat and carbon. To assess its past strength, the sedimentary 231Pa/230Th proxy has been developed and improved but also contested due to its sensitivity to other factors beyond ocean circulation. In order to provide a better basis for the understanding of the Atlantic 231Pa/230Th system, and therefore to shed light on the controversy, we compare new measurements of Holocene sediments from the north Brazilian margin to water column data and the output of a two-dimensional scavenging-circulation model, based on modern circulation patterns and reversible scavenging parameters. We show that sedimentary 231Pa/230Th data from one specific area of the Atlantic are in very good agreement with model results suggesting that sedimentary 231Pa/230Th is predominantly driven by the AMOC. Therefore, 231Pa/230Th represents an appropriate method to reconstruct past AMOC at least qualitatively along the western margin.