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

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.

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

Climate of the Past ◽

10.5194/cp-13-317-2017 ◽

2017 ◽

Vol 13 (4) ◽

pp. 317-331 ◽

Cited By ~ 10

Author(s):

Montserrat Alonso-Garcia ◽

Helga (Kikki) F. Kleiven ◽

Jerry F. McManus ◽

Paola Moffa-Sanchez ◽

Wallace S. Broecker ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Deep Water ◽

Little Ice Age ◽

Ice Age ◽

Labrador Sea ◽

Deep Water Formation ◽

The North ◽

Water Formation ◽

The North Atlantic

Abstract. Arctic freshwater discharges to the Labrador Sea from melting glaciers and sea ice can have a large 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 shows particularly high concentrations during the intervals ∼  AD 1000–1100, ∼  1150–1250, ∼  1400–1450, ∼  1650–1700 and ∼  1750–1800. 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 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 that occurred 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 and reduced convection in the Labrador Sea, decreasing its contribution to the North Atlantic deep water formation and, thus, reducing the amount of heat transported to high latitudes. This situation of weak SPG circulation may have made the North Atlantic climate more unstable, inducing a state in which external forcings (e.g. reduced solar irradiance and volcanic eruptions) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. This analysis indicates 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.

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Bunker Cave stalagmites: an archive for central European Holocene climate variability

Climate of the Past Discussions ◽

10.5194/cpd-8-1687-2012 ◽

2012 ◽

Vol 8 (3) ◽

pp. 1687-1720 ◽

Cited By ~ 9

Author(s):

J. Fohlmeister ◽

A. Schröder-Ritzrau ◽

D. Scholz ◽

C. Spötl ◽

D. F. C. Riechelmann ◽

...

Keyword(s):

Climate Variability ◽

North Atlantic ◽

Little Ice Age ◽

Ice Age ◽

Holocene Climate ◽

Central European ◽

The North ◽

The North Atlantic ◽

Meteoric Precipitation ◽

Precipitation And Temperature

Abstract. Holocene climate was characterised by variability on multi-centennial to multi-decadal time scales. In central Europe, these fluctuations were most pronounced during winter. Here we present a new record of past winter climate variability for the last 10.8 ka based on four speleothems from Bunker Cave, Western Germany. Due to its central European location, the cave site is particularly well suited to record changes in precipitation and temperature in response to changes in the North Atlantic realm. We present high resolution records of δ18O, δ13C values and Mg/Ca ratios. We attribute changes in the Mg/Ca ratio to variations in the meteoric precipitation. The stable C isotope composition of the speleothems most likely reflects changes in vegetation and precipitation and variations in the δ18O signal are interpreted as variations in meteoric precipitation and temperature. We found cold and dry periods between 9 and 7 ka, 6.5 and 5.5 ka, 4 and 3 ka as well as between 0.7 to 0.2 ka. The proxy signals in our stalagmites compare well with other isotope records and, thus, seem representative for central European Holocene climate variability. The prominent 8.2 ka event and the Little Ice Age cold events are both recorded in the Bunker cave record. However, these events show a contrasting relationship between climate and δ18O, which is explained by different causes underlying the two climate anomalies. Whereas the Little Ice Age is attributed to a pronounced negative phase of the North Atlantic Oscillation, the 8.2 ka event was triggered by cooler conditions in the North Atlantic due to a slowdown of the Thermohaline Circulation.

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Co-variability of salinity and temperature changes in the North Atlantic

10.5194/egusphere-egu21-5324 ◽

2021 ◽

Author(s):

Levke Caesar ◽

Gerard McCarthy

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Climate Model ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Geophysical Research ◽

Overturning Circulation ◽

The North ◽

Cold Anomaly ◽

The North Atlantic

While there is increasing paleoclimatic evidence that the Atlantic Meridional Overturning Circulation (AMOC) has weakened over the last one to two hundred years (Caesar et al., 2018; Thornalley et al., 2018), this is not confirmed by climate model simulations. Instead, the new simulations from the 6th Coupled Model Intercomparison Project (CMIP6) show a slight strengthening of the multimodel mean AMOC from 1850 until about 1985 (Menary et al., 2020), attributed to anthropogenic aerosol forcing. Arguing for a recent weakening of the AMOC, some studies attribute the emergence of the North Atlantic warming hole as a sign of the reduced meridional heat transport associated with a weaker AMOC (e.g. Caesar et al., 2018), yet this cold anomaly has also been interpreted as being aerosol-forced (Booth et al., 2012) and therefore not necessarily a sign of a weakening AMOC but rather a possible driver of a strengthening of the AMOC.Looking beyond temperature, a fresh anomaly has recently emerged in the subpolar North Atlantic (Holliday et al., 2020). While a strengthening AMOC has been linked with an increase in salinity in the subpolar gyre region (Menary et al., 2013), an AMOC weakening would, due to the salt-advection feedback, likely lead to a reduction in salinity in the North Atlantic region. To shed some light on the question of whether the cold anomaly is internally (AMOC) or externally (aerosol-forced) driven we consider the co-variability of salinity and temperature in the North Atlantic in respect of changes in surface fluxes or alternate drivers.&#160;ReferencesBooth, B.B.B., Dunstone, N.J., Halloran, P.R., Andrews, T. and Bellouin, N., 2012. Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature, 484(7393): 228&#8211;232.Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G. and Saba, V., 2018. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700): 191-196.Holliday, N.P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-L&#243;pez, C., H&#225;t&#250;n, H., Johns, W., Josey, S.A., Larsen, K.M.H., Mulet, S., Oltmanns, M., Reverdin, G., Rossby, T., Thierry, V., Valdimarsson, H. and Yashayaev, I., 2020. Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1): 585.Menary, M.B., Roberts, C.D., Palmer, M.D., Halloran, P.R., Jackson, L., Wood, R.A., M&#252;ller, W.A., Matei, D. and Lee, S.-K., 2013. Mechanisms of aerosol-forced AMOC variability in a state of the art climate model. Journal of Geophysical Research: Oceans, 118(4): 2087-2096.Menary, M.B., Robson, J., Allan, R.P., Booth, B.B.B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J., Ringer, M., Sutton, R., Wilcox, L. and Zhang, R., 2020. Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations. Geophysical Research Letters, 47(14): e2020GL088166.Thornalley, D.J.R., Oppo, D.W., Ortega, P., Robson, J.I., Brierley, C.M., Davis, R., Hall, I.R., Moffa-Sanchez, P., Rose, N.L., Spooner, P.T., Yashayaev, I. and Keigwin, L.D., 2018. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature, 556(7700): 227-230.

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Identification and quantification of the North Atlantic Deep Water pathways

10.5194/egusphere-egu21-10358 ◽

2021 ◽

Author(s):

Philippe Miron ◽

Maria J. Olascoaga ◽

Francisco J. Beron-Vera ◽

Kimberly L. Drouin ◽

M. Susan Lozier

Keyword(s):

North Atlantic ◽

Deep Water ◽

Sea Water ◽

Lower Layer ◽

North Atlantic Deep Water ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Western Boundary ◽

The North ◽

The North Atlantic

The North Atlantic Deep Water (NADW) flows equatorward along the Deep Western Boundary Current (DWBC) as well as interior pathways and is a critical part of the Atlantic Meridional Overturning Circulation. Its upper layer, the Labrador Sea Water (LSW), is formed by open-ocean deep convection in the Labrador and Irminger Seas while its lower layers, the Iceland&#8211;Scotland Overflow Water (ISOW) and the Denmark Strait Overflow Water (DSOW), are formed north of the Greenland&#8211;Iceland&#8211;Scotland Ridge.In recent years, more than two hundred acoustically-tracked subsurface floats have been deployed in the deep waters of the North Atlantic. &#160;Studies to date have highlighted water mass pathways from launch locations, but due to limited float trajectory lengths, these studies have been unable to identify pathways connecting &#160;remote regions.This work presents a framework to explore deep water pathways from their respective sources in the North Atlantic using Markov Chain (MC) modeling and Transition Path Theory (TPT). Using observational trajectories released as part of OSNAP and the Argo projects, we constructed two MCs that approximate the lower and upper layers of the NADW Lagrangian dynamics. The reactive NADW pathways&#8212;directly connecting NADW sources with a target at 53&#176;N&#8212;are obtained from these MCs using TPT.Preliminary results show that twenty percent more pathways of the upper layer(LSW) reach the ocean interior compared to &#160;the lower layer (ISOW, DSOW), which mostly flows along the DWBC in the subpolar North Atlantic. Also identified are the Labrador Sea recirculation pathways to the Irminger Sea and the direct connections from the Reykjanes Ridge to the eastern flank of the Mid&#8211;Atlantic Ridge, both previously observed. Furthermore, we quantified the eastern spread of the LSW to the area surrounding the Charlie&#8211;Gibbs Fracture Zone and compared it with previous analysis. Finally, the residence time of the upper and lower layers are assessed and compared to previous observations.

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Variability of the Meridional Overturning in the North Atlantic from the 50-Year GECCO State Estimation

Journal of Physical Oceanography ◽

10.1175/2008jpo3775.1 ◽

2008 ◽

Vol 38 (9) ◽

pp. 1913-1930 ◽

Cited By ~ 121

Author(s):

Armin Köhl ◽

Detlef Stammer

Keyword(s):

Time Scales ◽

Ocean Circulation ◽

North Atlantic ◽

Meridional Overturning Circulation ◽

Special Focus ◽

Western Boundary ◽

Consistent Estimate ◽

The North ◽

Meridional Overturning ◽

The North Atlantic

Abstract The German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) provided a dynamically consistent estimate of the time-varying ocean circulation over the 50-yr period 1952–2001. The GECCO synthesis combines most of the data available during the entire estimation period with the ECCO–Massachusetts Institute of Technology (MIT) ocean circulation model using its adjoint. This GECCO estimate is analyzed here for the period 1962–2001 with respect to decadal and longer-term changes of the meridional overturning circulation (MOC) of the North Atlantic. A special focus is on the maximum MOC values at 25°N. Over this period, the dynamically self-consistent synthesis stays within the error bars of H. L. Bryden et al., but reveals a general increase of the MOC strength. The variability on decadal and longer time scales is decomposed into contributions from different processes. Changes in the model’s MOC strength are strongly influenced by the southward communication of density anomalies along the western boundary originating from the subpolar North Atlantic, which are related to changes in the Denmark Strait overflow but are only marginally influenced by water mass formation in the Labrador Sea. The influence of density anomalies propagating along the southern edge of the subtropical gyre associated with baroclinically unstable Rossby waves is found to be equally important. Wind-driven processes such as local Ekman transport explain a smaller fraction of the variability on those long time scales.

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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 ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

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.

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Water mass distributions and transports for the 2014 GEOVIDE cruise in the North Atlantic

Biogeosciences ◽

10.5194/bg-15-2075-2018 ◽

2018 ◽

Vol 15 (7) ◽

pp. 2075-2090 ◽

Cited By ~ 21

Author(s):

Maribel I. García-Ibáñez ◽

Fiz F. Pérez ◽

Pascale Lherminier ◽

Patricia Zunino ◽

Herlé Mercier ◽

...

Keyword(s):

North Atlantic ◽

Water Mass ◽

Water Levels ◽

Water Masses ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Intermediate Water ◽

North East ◽

The North ◽

The North Atlantic

Abstract. We present the distribution of water masses along the GEOTRACES-GA01 section during the GEOVIDE cruise, which crossed the subpolar North Atlantic Ocean and the Labrador Sea in the summer of 2014. The water mass structure resulting from an extended optimum multiparameter (eOMP) analysis provides the framework for interpreting the observed distributions of trace elements and their isotopes. Central Waters and Subpolar Mode Waters (SPMW) dominated the upper part of the GEOTRACES-GA01 section. At intermediate depths, the dominant water mass was Labrador Sea Water, while the deep parts of the section were filled by Iceland–Scotland Overflow Water (ISOW) and North-East Atlantic Deep Water. We also evaluate the water mass volume transports across the 2014 OVIDE line (Portugal to Greenland section) by combining the water mass fractions resulting from the eOMP analysis with the absolute geostrophic velocity field estimated through a box inverse model. This allowed us to assess the relative contribution of each water mass to the transport across the section. Finally, we discuss the changes in the distribution and transport of water masses between the 2014 OVIDE line and the 2002–2010 mean state. At the upper and intermediate water levels, colder end-members of the water masses replaced the warmer ones in 2014 with respect to 2002–2010, in agreement with the long-term cooling of the North Atlantic Subpolar Gyre that started in the mid-2000s. Below 2000 dbar, ISOW increased its contribution in 2014 with respect to 2002–2010, with the increase being consistent with other estimates of ISOW transports along 58–59° N. We also observed an increase in SPMW in the East Greenland Irminger Current in 2014 with respect to 2002–2010, which supports the recent deep convection events in the Irminger Sea. From the assessment of the relative water mass contribution to the Atlantic Meridional Overturning Circulation (AMOC) across the OVIDE line, we conclude that the larger AMOC intensity in 2014 compared to the 2002–2010 mean was related to both the increase in the northward transport of Central Waters in the AMOC upper limb and to the increase in the southward flow of Irminger Basin SPMW and ISOW in the AMOC lower limb.

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Hydrological variations in central China over the past millennium and their links to the tropical Pacific and North Atlantic oceans

Climate of the Past ◽

10.5194/cp-16-475-2020 ◽

2020 ◽

Vol 16 (2) ◽

pp. 475-485

Author(s):

Fucai Duan ◽

Zhenqiu Zhang ◽

Yi Wang ◽

Jianshun Chen ◽

Zebo Liao ◽

...

Keyword(s):

North Atlantic ◽

Atmospheric Precipitation ◽

Tropical Pacific ◽

Meridional Overturning Circulation ◽

Central China ◽

Ice Age ◽

The North ◽

Future Global Warming ◽

The North Atlantic ◽

The Tropical Pacific

Abstract. Variations of precipitation, also called the Meiyu rain, in the East Asian summer monsoon (EASM) domain during the last millennium could help enlighten the hydrological response to future global warming. Here we present a precisely dated and highly resolved stalagmite δ18O record from the Yongxing Cave, central China. Our new record, combined with a previously published one from the same cave, indicates that the Meiyu rain has changed dramatically in association with the global temperature change. In particular, our record shows that the Meiyu rain was weakened during the Medieval Climate Anomaly (MCA) but intensified during the Little Ice Age (LIA). During the Current Warm Period (CWP), our record indicates a similar weakening of the Meiyu rain. Furthermore, during the MCA and CWP, our records show that the atmospheric precipitation is similarly wet in northern China and similarly dry in central China, but relatively wet during the CWP in southern China. This spatial discrepancy indicates a complicated localized response of the regional precipitation to the anthropogenic forcing. The weakened (intensified) Meiyu rain during the MCA (LIA) matches well with the warm (cold) phases of Northern Hemisphere surface air temperature. This Meiyu rain pattern also corresponds well to the climatic conditions over the tropical Indo-Pacific warm pool. On the other hand, our record shows a strong association with the North Atlantic climate as well. The reduced (increased) Meiyu rain correlates well with positive (negative) phases of the North Atlantic Oscillation. In addition, our record links well to the strong (weak) Atlantic meridional overturning circulation during the MCA (LIA) period. All abovementioned localized correspondences and remote teleconnections on decadal to centennial timescales indicate that the Meiyu rain was coupled closely with oceanic processes in the tropical Pacific and North Atlantic oceans during the MCA and LIA.

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Impact of Labrador Sea Convection on the North Atlantic Meridional Overturning Circulation

Journal of Physical Oceanography ◽

10.1175/jpo3178.1 ◽

2007 ◽

Vol 37 (9) ◽

pp. 2207-2227 ◽

Cited By ~ 99

Author(s):

Robert S. Pickart ◽

Michael A. Spall

Keyword(s):

North Atlantic ◽

World Ocean ◽

Atlantic Meridional Overturning Circulation ◽

Meridional Overturning Circulation ◽

Labrador Sea ◽

Overturning Circulation ◽

The North ◽

Meridional Overturning ◽

The North Atlantic ◽

Density Space

Abstract The overturning and horizontal circulations of the Labrador Sea are deduced from a composite vertical section across the basin. The data come from the late-spring/early-summer occupations of the World Ocean Circulation Experiment (WOCE) AR7W line, during the years 1990–97. This time period was chosen because it corresponded to intense wintertime convection—the deepest and densest in the historical record—suggesting that the North Atlantic meridional overturning circulation (MOC) would be maximally impacted. The composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass. The analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux (i.e., the transformation of light water to Labrador Sea Water). The mean overturning cell is calculated to be 1 Sv (1 Sv ≡ 106 m3 s−1), as compared with a horizontal gyre of 18 Sv. The total water mass transformation is 2 Sv. These values are consistent with recent modeling results. The diagnosed heat flux of 37.6 TW is found to result predominantly from the horizontal circulation, both in depth space and density space. These results suggest that the North Atlantic MOC is not largely impacted by deep convection in the Labrador Sea.

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