scholarly journals Abrupt cold events in the North Atlantic Ocean in a transient Holocene simulation

2018 ◽  
Vol 14 (8) ◽  
pp. 1165-1178 ◽  
Author(s):  
Andrea Klus ◽  
Matthias Prange ◽  
Vidya Varma ◽  
Louis Bruno Tremblay ◽  
Michael Schulz

Abstract. Abrupt cold events have been detected in numerous North Atlantic climate records from the Holocene. Several mechanisms have been discussed as possible triggers for these climate shifts persisting decades to centuries. Here, we describe two abrupt cold events that occurred during an orbitally forced transient Holocene simulation using the Community Climate System Model version 3. Both events occurred during the late Holocene (4305–4267 BP and 3046–3018 BP for event 1 and event 2, respectively). They were characterized by substantial surface cooling (−2.3 and −1.8 ∘C, respectively) and freshening (−0.6 and −0.5 PSU, respectively) as well as severe sea ice advance east of Newfoundland and south of Greenland, reaching as far as the Iceland Basin in the northeastern Atlantic at the climaxes of the cold events. Convection and deep-water formation in the northwestern Atlantic collapsed during the events, while the Atlantic Meridional Overturning Circulation was not substantially affected (weakening by only about 10 % and 5 %, respectively). The events were triggered by prolonged phases of a positive North Atlantic Oscillation that caused substantial changes in the subpolar ocean circulation and associated freshwater transports, resulting in a weakening of the subpolar gyre. Our results suggest a possible mechanism by which abrupt cold events in the North Atlantic region may be triggered by internal climate variability without the need of an external (e.g., solar or volcanic) forcing.

2017 ◽  
Author(s):  
Andrea Klus ◽  
Matthias Prange ◽  
Vidya Varma ◽  
Louis Bruno Tremblay ◽  
Michael Schulz

Abstract. Abrupt cold events have been detected in numerous North Atlantic climate records from the Holocene. Several mechanisms have been discussed as possible triggers for these climate shifts persisting decades to centuries. Here, we describe two cold events that occurred during an orbitally forced transient Holocene simulation using the Community Climate System Model version 3. Both events occurred during the late Holocene (event 1 referring to 4305–4267 BP and event 2 referring to 3046–3018 BP) and were characterized by substantial surface cooling (−2.7 and −2.2 °C, respectively) and freshening (−0.7 and −0.6 PSU, respectively) as well as severe sea ice advance east of Newfoundland and south of Greenland. Sea ice even reached the Iceland Basin in the northeastern Atlantic at the climaxes of the cold events. Convection and deep-water formation in the northwestern Atlantic collapsed during the events, while the Atlantic meridional overturning circulation was not significantly affected. The events were triggered by prolonged phases of a positive North Atlantic Oscillation which, through changes in surface winds, caused substantial changes in the sub-polar ocean circulation and associated freshwater transports, resulting in a weakening of the sub-polar gyre. Our results suggest a possible mechanism by which abrupt cold events in the North Atlantic region may be triggered by internal climate variability without the need of an external (e.g. solar or volcanic) forcing.


2015 ◽  
Vol 28 (19) ◽  
pp. 7659-7677 ◽  
Author(s):  
Claude Frankignoul ◽  
Guillaume Gastineau ◽  
Young-Oh Kwon

Abstract Maximum covariance analysis of a preindustrial control simulation of the NCAR Community Climate System Model, version 4 (CCSM4), shows that a barotropic signal in winter broadly resembling a negative phase of the North Atlantic Oscillation (NAO) follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 7 yr. The delay is due to the cyclonic propagation along the North Atlantic Current (NAC) and the subpolar gyre of a SST warming linked to a northward shift and intensification of the NAC, together with an increasing SST cooling linked to increasing southward advection of subpolar water along the western boundary and a southward shift of the Gulf Stream (GS). These changes result in a meridional SST dipole, which follows the AMOC intensification after 6 or 7 yr. The SST changes were initiated by the strengthening of the western subpolar gyre and by bottom torque at the crossover of the deep branches of the AMOC with the NAC on the western flank of the Mid-Atlantic Ridge and the GS near the Tail of the Grand Banks, respectively. The heat flux damping of the SST dipole shifts the region of maximum atmospheric transient eddy growth southward, leading to a negative NAO-like response. No significant atmospheric response is found to the Atlantic multidecadal oscillation (AMO), which is broadly realistic but shifted south and associated with a much weaker meridional SST gradient than the AMOC fingerprint. Nonetheless, the wintertime atmospheric response to the AMOC shows some similarity with the observed response to the AMO, suggesting that the ocean–atmosphere interactions are broadly realistic in CCSM4.


2021 ◽  
Author(s):  
Leonard F. Borchert ◽  
Alexander J. Winkler

<p>Vegetation in the northern high latitudes shows a characteristic pattern of persistent changes as documented by multi-decadal satellite observations. The prevailing explanation that these mainly increasing trends (greening) are a consequence of external CO<sub>2</sub> forcing, i.e., due to the ubiquitous effect of CO2-induced fertilization and/or warming of temperature-limited ecosystems, however does not explain why some areas also show decreasing trends of vegetation cover (browning). We propose here to consider the dominant mode of multi-decadal internal climate variability in the north Atlantic region, the Atlantic Multidecadal Variability (AMV), as the missing link in the explanation of greening and browning trend patterns in the northern high latitudes. Such a link would also imply potential for decadal predictions of ecosystem changes in the northern high latitudes.</p><p>An analysis of observational and reanalysis data sets for the period 1979-2019 shows that locations characterized by greening trends largely coincide with warming summer temperature and increasing precipitation. Wherever either cooling or decreasing precipitation occurs, browning trends are observed over this period. These precipitation and temperature patterns are significantly correlated with a North Atlantic sea surface temperature index that represents the AMV signal, indicating its role in modulating greening/browning trend patterns in the northern high latitudes.</p><p>Using two large ensembles of coupled Earth system model simulations (100 members of MPI-ESM-LR Grand Ensemble and 32 members of the IPSL-CM6A-LR Large Ensemble), we separate the greening/browning pattern caused by external CO<sub>2</sub> forcing from that caused by internal climate variability associated with the AMV. These sets of model simulations enable a clean separation of the externally forced signal from internal variability. While the greening and browning patterns in the simulations do not agree with observations in terms of magnitude and location, we find consistent internally generated greening/browning patterns in both models caused by changes in temperature and precipitation linked to the AMV signal. These greening/browning trend patterns are of the same magnitude as those caused by the external forcing alone. Our work therefore shows that internally-generated changes of vegetation in the northern lands, driven by AMV, are potentially as large as those caused by external CO<sub>2</sub> forcing. We thus argue that the observed pattern of greening/browning in the northern high latitudes could originate from the combined effect of rising CO<sub>2</sub> as well as the AMV.</p>


2014 ◽  
Vol 27 (1) ◽  
pp. 101-110 ◽  
Author(s):  
Wilbert Weijer ◽  
Erik van Sebille

Abstract The impact of Agulhas leakage variability on the strength of the Atlantic meridional overturning circulation (AMOC) in the Community Climate System Model, version 4 (CCSM4) is investigated. In this model an advective connection exists that transports salinity anomalies from the Agulhas region into the North Atlantic on decadal (30–40 yr) time scales. However, there is no identifiable impact of Agulhas leakage on the strength of the AMOC, suggesting that the salinity variations are too weak to significantly modify the stratification in the North Atlantic. It is argued that this study is inconclusive with respect to an impact of Agulhas leakage on the AMOC. Salinity biases leave the South Atlantic and Indian Oceans too homogeneous, in particular erasing the observed salinity front in the Agulhas retroflection region. Consequently, salinity variability in the southeastern South Atlantic is found to be much weaker than observed.


2021 ◽  
Author(s):  
Levke Caesar ◽  
Gerard McCarthy

<p>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.</p><p>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.</p><p> </p><p>References</p><p>Booth, 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): 228–232.</p><p>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.</p><p>Holliday, N.P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-López, C., Hátún, 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.</p><p>Menary, M.B., Roberts, C.D., Palmer, M.D., Halloran, P.R., Jackson, L., Wood, R.A., Müller, 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.</p><p>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.</p><p>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.</p>


2008 ◽  
Vol 38 (9) ◽  
pp. 1913-1930 ◽  
Author(s):  
Armin Köhl ◽  
Detlef Stammer

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.


2007 ◽  
Vol 20 (19) ◽  
pp. 4940-4956 ◽  
Author(s):  
Uta Krebs ◽  
A. Timmermann

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.


2017 ◽  
Vol 30 (2) ◽  
pp. 477-498 ◽  
Author(s):  
Florian Sévellec ◽  
Alexey V. Fedorov

This study investigates the excitation of decadal variability and predictability of the ocean climate state in the North Atlantic. Specifically, initial linear optimal perturbations (LOPs) in temperature and salinity that vary with depth, longitude, and latitude are computed, and the maximum impact on the ocean of these perturbations is evaluated in a realistic ocean general circulation model. The computations of the LOPs involve a maximization procedure based on Lagrange multipliers in a nonautonomous context. To assess the impact of these perturbations four different measures of the North Atlantic Ocean state are used: meridional volume and heat transports (MVT and MHT) and spatially averaged sea surface temperature (SST) and ocean heat content (OHC). It is shown that these metrics are dramatically different with regard to predictability. Whereas OHC and SST can be efficiently modified only by basin-scale anomalies, MVT and MHT are also strongly affected by smaller-scale perturbations. This suggests that instantaneous or even annual-mean values of MVT and MHT are less predictable than SST and OHC. Only when averaged over several decades do the former two metrics have predictability comparable to the latter two, which highlights the need for long-term observations of the Atlantic meridional overturning circulation in order to accumulate climatically relevant data. This study also suggests that initial errors in ocean temperature of a few millikelvins, encompassing both the upper and deep ocean, can lead to ~0.1-K errors in the predictions of North Atlantic sea surface temperature on interannual time scales. This transient error growth peaks for SST and OHC after about 6 and 10 years, respectively, implying a potential predictability barrier.


2011 ◽  
Vol 7 (1) ◽  
pp. 133-150 ◽  
Author(s):  
D. Hofer ◽  
C. C. Raible ◽  
T. F. Stocker

Abstract. The variability of the Atlantic meridional overturing circulation (AMOC) strength is investigated in control experiments and in transient simulations of up to the last millennium using the low-resolution Community Climate System Model version 3. In the transient simulations the AMOC exhibits enhanced low-frequency variability that is mainly caused by infrequent transitions between two semi-stable circulation states which amount to a 10 percent change of the maximum overturning. One transition is also found in a control experiment, but the time-varying external forcing significantly increases the probability of the occurrence of such events though not having a direct, linear impact on the AMOC. The transition from a high to a low AMOC state starts with a reduction of the convection in the Labrador and Irminger Seas and goes along with a changed barotropic circulation of both gyres in the North Atlantic and a gradual strengthening of the convection in the Greenland-Iceland-Norwegian (GIN) Seas. In contrast, the transition from a weak to a strong overturning is induced by decreased mixing in the GIN Seas. As a consequence of the transition, regional sea surface temperature (SST) anomalies are found in the midlatitude North Atlantic and in the convection regions with an amplitude of up to 3 K. The atmospheric response to the SST forcing associated with the transition indicates a significant impact on the Scandinavian surface air temperature (SAT) in the order of 1 K. Thus, the changes of the ocean circulation make a major contribution to the Scandinavian SAT variability in the last millennium.


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