3. The Ice Age cometh

Ice Age ◽

Ice Ages ◽

Astronomical Theory ◽

Meridional Overturning ◽

Potential Impact

What turns ice ages on and off? ‘The Ice Age cometh’ considers the potential impact global warming may have on the arrival of the next expected Ice Age. Will global warming fend it off or will it accelerate the onset of the next big freeze? Conditions on Earth during the great freezes of the Cryogenian and the most recent Quaternary ice ages are described and what triggers them considered, including the Croll–Milankovitch astronomical theory. The Little Ice Age (c.ad 1450–1850) and the Medieval Warm Period (c.ad 1000–1300) are discussed along with the role ocean circulation (especially the Atlantic Meridional Overturning Circulation) has to play.

Tropical Air–Sea Interactions Accelerate the Recovery of the Atlantic Meridional Overturning Circulation after a Major Shutdown

Journal of Climate ◽

10.1175/jcli4296.1 ◽

2007 ◽

Vol 20 (19) ◽

pp. 4940-4956 ◽

Cited By ~ 81

Author(s):

Uta Krebs ◽

A. Timmermann

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Tropical Atlantic ◽

Trade Winds ◽

The North ◽

Salinity Anomaly ◽

Meridional Overturning ◽

The North Atlantic

Abstract Using a coupled ocean–sea ice–atmosphere model of intermediate complexity, the authors study the influence of air–sea interactions on the stability of the Atlantic Meridional Overturning Circulation (AMOC). Mimicking glacial Heinrich events, a complete shutdown of the AMOC is triggered by the delivery of anomalous freshwater forcing to the northern North Atlantic. Analysis of fully and partially coupled freshwater perturbation experiments under glacial conditions shows that associated changes of the heat transport in the North Atlantic lead to a cooling north of the thermal equator and an associated strengthening of the northeasterly trade winds. Because of advection of cold air and an intensification of the trade winds, the intertropical convergence zone (ITCZ) is shifted southward. Changes of the accumulated precipitation lead to the generation of a positive salinity anomaly in the northern tropical Atlantic and a negative anomaly in the southern tropical Atlantic. During the shutdown phase of the AMOC, cross-equatorial oceanic surface flow is halted, preventing dilution of the positive salinity anomaly in the North Atlantic. Advected northward by the wind-driven ocean circulation, the positive salinity anomaly increases the upper-ocean density in the deep-water formation regions, thereby accelerating the recovery of the AMOC considerably. Partially coupled experiments that neglect tropical air–sea coupling reveal that the recovery time of the AMOC is almost twice as long as in the fully coupled case. The impact of a shutdown of the AMOC on the Indian and Pacific Oceans can be decomposed into atmospheric and oceanic contributions. Temperature anomalies in the Northern Hemisphere are largely controlled by atmospheric circulation anomalies, whereas those in the Southern Hemisphere are strongly determined by ocean dynamical changes and exhibit a time lag of several decades. An intensification of the Pacific meridional overturning cell in the northern North Pacific during the AMOC shutdown can be explained in terms of wind-driven ocean circulation changes acting in concert with global ocean adjustment processes.

Two Decades of the Atlantic Meridional Overturning Circulation: Anatomy, Variations, Extremes, Prediction, and Overcoming Its Limitations

Journal of Climate ◽

10.1175/jcli-d-12-00478.1 ◽

2013 ◽

Vol 26 (18) ◽

pp. 7167-7186 ◽

Cited By ~ 45

Author(s):

Carl Wunsch ◽

Patrick Heimbach

Keyword(s):

Ocean Circulation ◽

Extreme Values ◽

Statistical Significance ◽

Western Boundary ◽

Boundary Current ◽

Gaussian Stochastic Process ◽

Overturning Circulation ◽

Abstract The zonally integrated meridional volume transport in the North Atlantic [Atlantic meridional overturning circulation (AMOC)] is described in a 19-yr-long ocean-state estimate, one consistent with a diverse global dataset. Apart from a weak increasing trend at high northern latitudes, the AMOC appears statistically stable over the last 19 yr with fluctuations indistinguishable from those of a stationary Gaussian stochastic process. This characterization makes it possible to study (using highly developed tools) extreme values, predictability, and the statistical significance of apparent trends. Gaussian behavior is consistent with the central limit theorem for a process arising from numerous independent disturbances. In this case, generators include internal instabilities, changes in wind and buoyancy forcing fields, boundary waves, the Gulf Stream and deep western boundary current transports, the interior fraction in Sverdrup balance, and all similar phenomena arriving as summation effects from long distances and times. As a zonal integral through the sum of the large variety of physical processes in the three-dimensional ocean circulation, understanding of the AMOC, if it is of central climate importance, requires breaking it down into its unintegrated components over the entire basin.

Increased risk of near term global warming due to a recent AMOC weakening

Nature Communications ◽

10.1038/s41467-021-26370-0 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Rémy Bonnet ◽

Didier Swingedouw ◽

Guillaume Gastineau ◽

Olivier Boucher ◽

Julie Deshayes ◽

...

Keyword(s):

Global Warming ◽

Low Frequency ◽

Internal Variability ◽

The Past ◽

Increased Risk ◽

Meridional Overturning ◽

Near Term ◽

New Generation

AbstractSome of the new generation CMIP6 models are characterised by a strong temperature increase in response to increasing greenhouse gases concentration1. At first glance, these models seem less consistent with the temperature warming observed over the last decades. Here, we investigate this issue through the prism of low-frequency internal variability by comparing with observations an ensemble of 32 historical simulations performed with the IPSL-CM6A-LR model, characterized by a rather large climate sensitivity. We show that members with the smallest rates of global warming over the past 6-7 decades are also those with a large internally-driven weakening of the Atlantic Meridional Overturning Circulation (AMOC). This subset of members also matches several AMOC observational fingerprints, which are in line with such a weakening. This suggests that internal variability from the Atlantic Ocean may have dampened the magnitude of global warming over the historical era. Taking into account this AMOC weakening over the past decades means that it will be harder to avoid crossing the 2 °C warming threshold.

Larger Role for Shallow Intermediate Waters in Ocean Circulation

Eos ◽

10.1029/2020eo141674 ◽

2020 ◽

Vol 101 ◽

Author(s):

Sara Pratt

Keyword(s):

Ocean Circulation ◽

Global Climate ◽

Water Masses ◽

Overturning Circulation ◽

Meridional Overturning ◽

Intermediate Waters

Water masses formed off southeastern Greenland may contribute more than previously thought to the variability of the Atlantic Meridional Overturning Circulation, which strongly influences global climate.

Sensitivity of the Atlantic Meridional Overturning Circulation to Model Resolution in CMIP6 HighResMIP Simulations

10.5194/egusphere-egu2020-18216 ◽

2020 ◽

Author(s):

Dorotea Iovino ◽

Malcolm J. Roberts ◽

Laura C. Jackson ◽

Christopher D. Roberts ◽

Virna Meccia ◽

...

Keyword(s):

Heat Transport ◽

Ocean Circulation ◽

The Arctic ◽

Climate Modelling ◽

Florida Current ◽

Model Resolution ◽

Overturning Circulation ◽

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the three-dimensional ocean circulation that transports warm and salty water northward, and exports cold and dense water from the Arctic southward.The simulated AMOC in Coupled Model Intercomparison Project models (both coupled and ocean-only) has been studied extensively. However, correctly simulating the AMOC with these models remains a challenge for the climate modelling community. One model aspect that can affect the AMOC representation is the model&#160;resolution (i.e. grid spacing).Here, we examine key aspects of the North Atlantic Ocean circulation using a multi-model, multi-resolution ensemble based on the CMIP6 HighResMIP coupled experiments.&#160;The AMOC and associated heat transport tend to become stronger as&#160;model resolution increases, particularly when the ocean resolution changes from non-eddying to eddy-present and eddy-rich. However, the circulation remains too&#160;shallow compared to observations for most models, and this, together with temperature biases, cause the northward&#160;heat transport to be too low for a given overturning&#160;strength.In the period 2015-2050, the overturning circulation tends to decline more rapidly in the higher&#160;resolution models by more than 20% compared to the control state, which is related to both themean state and to the subpolar gyre contribution to deep water formation. The main part of the&#160;decline comes from the Florida Current component of the circulation.

Freshening of the Labrador Sea as a trigger for Little Ice Age development

10.5194/cp-2016-80 ◽

2016 ◽

Author(s):

Montserrat Alonso-Garcia ◽

Helga F. Kleiven ◽

Jerry F. McManus ◽

Paola Moffa-Sanchez ◽

Wallace Broecker ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Little Ice Age ◽

Ice Age ◽

Labrador Sea ◽

The North ◽

External Forcings ◽

The North Atlantic ◽

Arctic Ice

Abstract. Arctic freshwater discharges to the Labrador Sea from melting glaciers and sea-ice can have a deep impact on ocean circulation dynamics in the North Atlantic modifying climate and deep water formation in this region. In this study, we present for the first time a high resolution record of ice-rafting in the Labrador Sea over the last millennium to assess the effects of freshwater discharges in this region on ocean circulation and climate. The occurrence of ice-rafted debris (IRD) in the Labrador Sea was studied using sediments from Site GS06-144-03 (57.29° N, 48.37° W, 3432 m water depth). IRD from the fraction 63–150 µm show higher concentration during the intervals: ~ 1000–1100, ~ 1150–1250, ~ 1400–1450, ~ 1650–1700 and ~ 1750–1800 yr AD. The first two intervals occurred during the Medieval Climate Anomaly (MCA), whereas the others took place within the Little Ice Age (LIA). Mineralogical identification indicates that the main IRD source during the MCA was SE Greenland. In contrast, the concentration and relative abundance of hematite-stained grains (HSG) reflects an increase in the contribution of Arctic ice during the LIA. The comparison of our Labrador Sea IRD records with other climate proxies from the subpolar North Atlantic allowed us to propose a sequence of processes that led to the cooling events during the LIA, particularly in the Northern Hemisphere. This study reveals that the warm climate of the MCA may have enhanced iceberg calving along the SE Greenland coast and, as a result, freshened the subpolar gyre (SPG). Consequently, SPG circulation switched to a weaker mode through internal feedbacks that reduced convection in the Labrador Sea decreasing its contribution to the Atlantic Meridional overturning circulation and, thus, the amount of heat transported to high latitudes. This mechanism very likely preconditioned the North Atlantic inducing a state in which external forcings (e.g. solar irradiance and volcanic input) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. The outcomes of this work indicate that a freshening of the SPG may play a crucial role in the development of cold events during the Holocene, which may be of key importance for predictions about future climate.

Nonlinear Effects of Wind on Atlantic Ocean Circulation

Eos ◽

10.1029/2022eo220013 ◽

2022 ◽

Vol 103 ◽

Author(s):

Jack Lee

Keyword(s):

Atlantic Ocean ◽

Wind Stress ◽

Ocean Circulation ◽

Nonlinear Effects ◽

Overturning Circulation ◽

Simulations reveal the influence of reduced and enhanced wind stress on the Atlantic Meridional Overturning Circulation.

Atlantic Meridional Overturning Circulation is stable under global warming

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1604187113 ◽

2016 ◽

Vol 113 (20) ◽

pp. E2760-E2761 ◽

Cited By ~ 9

Author(s):

Albert Parker

Keyword(s):

Global Warming ◽

Overturning Circulation ◽

Theoretical Models Advance Knowledge of Ocean Circulation

Eos ◽

10.1029/2019eo132047 ◽

2019 ◽

Vol 100 ◽

Author(s):

Sarah Stanley

Keyword(s):

Ocean Circulation ◽

Theoretical Models ◽

Future Research ◽

Overturning Circulation ◽

Meridional Overturning ◽

Advance Knowledge

A review of recent advancements highlights key insights into the Atlantic Meridional Overturning Circulation and what might be in store for future research.

An Indicator of the Multiple Equilibria Regime of the Atlantic Meridional Overturning Circulation

Journal of Physical Oceanography ◽

10.1175/2009jpo4215.1 ◽

2010 ◽

Vol 40 (3) ◽

pp. 551-567 ◽

Cited By ~ 74

Author(s):

Selma E. Huisman ◽

Matthijs den Toom ◽

Henk A. Dijkstra ◽

Sybren Drijfhout

Keyword(s):

Ocean Circulation ◽

Circulation Model ◽

Multiple Equilibrium ◽

Global Ocean ◽

Overturning Circulation ◽

Freshwater Transport ◽

Freshwater Budget ◽

Abstract Recent model results have suggested that there may be a scalar indicator Σ monitoring whether the Atlantic meridional overturning circulation (MOC) is in a multiple equilibrium regime. The quantity Σ is based on the net freshwater transport by the MOC into the Atlantic basin. It changes sign as soon as the steady Atlantic MOC enters the multiple equilibrium regime because of an increased freshwater input in the northern North Atlantic. This paper addresses the issue of why the sign of Σ is such a good indicator for the multiple equilibrium regime. Changes in the Atlantic freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model. The authors show that the net anomalous freshwater transport into or out of the Atlantic, resulting from the interactions of the velocity perturbations and salinity background field, is coupled to the background (steady state) state freshwater budget and hence to Σ. The sign of Σ precisely shows whether this net anomalous freshwater transport is stabilizing or destabilizing the MOC. Therefore, it can indicate whether the MOC is in a single or multiple equilibrium regime.