Ocean–Atmosphere Dynamical Coupling Fundamental to the Atlantic Multidecadal Oscillation

Abstract The North Atlantic has shown large multidecadal temperature shifts during the twentieth century. There is ongoing debate about whether this variability arises primarily through the influence of atmospheric internal variability, through changes in ocean circulation, or as a response to anthropogenic forcing. This study isolates the mechanisms driving Atlantic sea surface temperature variability on multidecadal time scales by using low-frequency component analysis (LFCA) to separate the influences of high-frequency variability, multidecadal variability, and long-term global warming. This analysis objectively identifies the North Atlantic subpolar gyre as the dominant region of Atlantic multidecadal variability. In unforced control runs of coupled climate models, warm subpolar temperatures are associated with a strengthened Atlantic meridional overturning circulation (AMOC) and anomalous local heat fluxes from the ocean into the atmosphere. Atmospheric variability plays a role in the intensification and subsequent weakening of ocean overturning and helps to communicate warming into the tropical Atlantic. These findings suggest that dynamical coupling between atmospheric and oceanic circulations is fundamental to the Atlantic multidecadal oscillation (AMO) and motivate approaching decadal prediction with a focus on ocean circulation.

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North Atlantic Ocean Circulation Response to Stochastic Mesoscale Weather Systems

10.5194/egusphere-egu21-1310 ◽

2021 ◽

Author(s):

Shenjie Zhou ◽

Xiaoming Zhai ◽

Ian Renfrew

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Climate Models ◽

Meridional Overturning Circulation ◽

Ocean Dynamics ◽

Ensemble Prediction ◽

Subpolar Gyre ◽

Atmospheric Forcing ◽

The North ◽

The North Atlantic

The ocean is forced by the atmosphere on a range of spatial and temporal scales. In ocean and climate models the resolution of the atmospheric forcing sets a limit on the scales that are represented. For typical climate models this means mesoscale (< 400 km) atmospheric forcing is absent. Previous studies have demonstrated that mesoscale forcing significantly affects key ocean circulation systems such as the North Atlantic Subpolar gyre and the Atlantic Meridional Overturning Circulation (AMOC). However, the approach of these studies has either been ad hoc or limited in resolution. Here we present ocean model simulations with and without realistic mesoscale atmospheric forcing that represents scales down to 10 km. We use a novel stochastic parameterization &#8211; based on a cellular automaton algorithm that is common in weather forecasting ensemble prediction systems&#160;&#8211; to represent spatially coherent weather systems over a range of scales, including down to the smallest resolvable by the ocean grid. The parameterization is calibrated spatially and temporally using marine wind observations. The addition of mesoscale atmospheric forcing leads to coherent patterns of change in the sea surface temperature and mixed-layer depth. It also leads to non-negligible changes in the volume transport in the North Atlantic subtropical gyre (STG) and subpolar gyre (SPG) and in the AMOC. A non-systematic basin-scale circulation response to the mesoscale wind perturbation emerges &#8211; an in-phase oscillation in northward heat transport across the gyre boundary, partly driven by the constantly enhanced STG, correspoding to an oscillatory behaviour in SPG and AMOC indices with a typical time scale of 5-year, revealing the importance of ocean dynamics in generating non-local ocean response to the stochastic mesoscale atmospheric forcing. Atmospheric convection-permitting regional climate simulations predict changes in the intensity and frequency of mesoscale weather systems this century, so representing these systems in coupled climate models could bring higher fidelity in future climate projections.

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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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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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A multi-decadal meridional displacement of the Subpolar Front in the Newfoundland Basin

Ocean Science ◽

10.5194/os-8-91-2012 ◽

2012 ◽

Vol 8 (1) ◽

pp. 91-102 ◽

Cited By ~ 7

Author(s):

I. Núñez-Riboni ◽

M. Bersch ◽

H. Haak ◽

J. H. Jungclaus ◽

K. Lohmann

Keyword(s):

North Atlantic ◽

General Circulation ◽

Sea Water ◽

Circulation Model ◽

Heat Fluxes ◽

Meridional Overturning Circulation ◽

The North ◽

Meridional Displacement ◽

The North Atlantic ◽

Newfoundland Basin

Abstract. Observations since the 1950s show a multi-decadal cycle of a meridional displacement of the Subpolar Front (SPF) in the Newfoundland Basin (NFB) in the North Atlantic. The SPF displacement is associated with corresponding variations in the path of the North Atlantic Current. We use the ocean general circulation model MPIOM with enhanced horizontal and vertical resolutions and forced with NCEP/NCAR reanalysis data to study the relation of the SPF displacement to atmospheric forcing, intensities of the subpolar gyre (SPG) and Meridional Overturning Circulation (MOC), and Labrador Sea Water (LSW) volume. The simulations indicate that the SPF displacement is associated with a circulation anomaly between the SPG and the subtropical gyre (STG), an inter-gyre gyre with a multi-decadal time scale. A sensitivity experiment indicates that both wind stress curl (WSC) and heat fluxes (which match LSW changes) contribute to the circulation anomalies in the frontal region and to the SPF displacement. An anticyclonic inter-gyre gyre is related to negative WSC and LSW anomalies and to a SPF north of its climatological position, indicating an expanding STG. A cyclonic inter-gyre gyre is related to positive WSC and LSW anomalies and a SPF south of its climatological position, indicating an expanding SPG. Therefore, the mean latitudinal position of the SPF in the NFB (a "SPF index") could be an indicator of the amount of LSW in the inter-gyre region. Spreading of LSW anomalies intensifies the MOC, suggesting our SPF index as predictor of the MOC intensity at multi-decadal time scales. The meridional displacement of the SPF has a pronounced influence on the meridional heat transport, both on its gyre and overturning components.

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Atlantic Multidecadal Variability and the U.K. ACSIS Program

Bulletin of the American Meteorological Society ◽

10.1175/bams-d-16-0266.1 ◽

2018 ◽

Vol 99 (2) ◽

pp. 415-425 ◽

Cited By ~ 36

Author(s):

R. T. Sutton ◽

G. D. McCarthy ◽

J. Robson ◽

B. Sinha ◽

A. T. Archibald ◽

...

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Heat Content ◽

Climate Impacts ◽

Environmental Research ◽

Atlantic Multidecadal Variability ◽

Multidecadal Variability ◽

The North ◽

Holistic Understanding ◽

The North Atlantic

Abstract Atlantic multidecadal variability (AMV) is the term used to describe the pattern of variability in North Atlantic sea surface temperatures (SSTs) that is characterized by decades of basinwide warm or cool anomalies, relative to the global mean. AMV has been associated with numerous climate impacts in many regions of the world including decadal variations in temperature and rainfall patterns, hurricane activity, and sea level changes. Given its importance, understanding the physical processes that drive AMV and the extent to which its evolution is predictable is a key challenge in climate science. A leading hypothesis is that natural variations in ocean circulation control changes in ocean heat content and consequently AMV phases. However, this view has been challenged recently by claims that changing natural and anthropogenic radiative forcings are critical drivers of AMV. Others have argued that changes in ocean circulation are not required. Here, we review the leading hypotheses and mechanisms for AMV and discuss the key debates. In particular, we highlight the need for a holistic understanding of AMV. This perspective is a key motivation for a major new U.K. research program: the North Atlantic Climate System Integrated Study (ACSIS), which brings together seven of the United Kingdom’s leading environmental research institutes to enable a broad spectrum approach to the challenges of AMV. ACSIS will deliver the first fully integrated assessment of recent decadal changes in the North Atlantic, will investigate the attribution of these changes to their proximal and ultimate causes, and will assess the potential to predict future changes.

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Tropical and Extratropical Responses of the North Atlantic Atmospheric Circulation to a Sustained Weakening of the MOC

Journal of Climate ◽

10.1175/2008jcli2594.1 ◽

2009 ◽

Vol 22 (11) ◽

pp. 3146-3155 ◽

Cited By ~ 36

Author(s):

David J. Brayshaw ◽

Tim Woollings ◽

Michael Vellinga

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Western Europe ◽

Storm Track ◽

Meridional Overturning Circulation ◽

Boreal Winter ◽

Mean Flow ◽

Near Surface ◽

The North ◽

The North Atlantic

Abstract The tropospheric response to a forced shutdown of the North Atlantic Ocean’s meridional overturning circulation (MOC) is investigated in a coupled ocean–atmosphere GCM [the third climate configuration of the Met Office Unified Model (HadCM3)]. The strength of the boreal winter North Atlantic storm track is significantly increased and penetrates much farther into western Europe. The changes in the storm track are shown to be consistent with the changes in near-surface baroclinicity, which can be linked to changes in surface temperature gradients near regions of sea ice formation and in the open ocean. Changes in the SST of the tropical Atlantic are linked to a strengthening of the subtropical jet to the north, which, combined with the enhanced storm track, leads to a pronounced split in the jet structure over Europe. EOF analysis and stationary box indices methods are used to analyze changes to the North Atlantic Oscillation (NAO). There is no consistent signal of a change in the variability of the NAO, and while the changes in the mean flow project onto the positive NAO phase, they are significantly different from it. However, there is a clear eastward shift of the NAO pattern in the shutdown run, and this potentially has implications for ocean circulation and for the interpretation of proxy paleoclimate records.

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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 ◽

Meridional Overturning Circulation ◽

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.

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Abrupt cold events in the North Atlantic in a transient Holocene simulation

10.5194/cp-2017-106 ◽

2017 ◽

Cited By ~ 2

Author(s):

Andrea Klus ◽

Matthias Prange ◽

Vidya Varma ◽

Louis Bruno Tremblay ◽

Michael Schulz

Keyword(s):

Sea Ice ◽

Ocean Circulation ◽

North Atlantic ◽

Meridional Overturning Circulation ◽

Surface Cooling ◽

Climate System Model ◽

The North ◽

Cold Events ◽

The North Atlantic ◽

Polar Ocean

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.

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North Atlantic Multidecadal Climate Variability: An Investigation of Dominant Time Scales and Processes

Journal of Climate ◽

10.1175/2010jcli3471.1 ◽

2010 ◽

Vol 23 (13) ◽

pp. 3626-3638 ◽

Cited By ~ 90

Author(s):

Leela M. Frankcombe ◽

Anna von der Heydt ◽

Henk A. Dijkstra

Keyword(s):

Time Scales ◽

North Atlantic ◽

Climate Model ◽

Internal Variability ◽

Meridional Overturning Circulation ◽

Theoretical Point ◽

Intergovernmental Panel ◽

Multidecadal Variability ◽

The North ◽

The North Atlantic

Abstract The issue of multidecadal variability in the North Atlantic has been an important topic of late. It is clear that there are multidecadal variations in several climate variables in the North Atlantic, such as sea surface temperature and sea level height. The details of this variability, in particular the dominant patterns and time scales, are confusing from both an observational as well as a theoretical point of view. After analyzing results from observational datasets and a 500-yr simulation of an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate model, two dominant time scales (20–30 and 50–70 yr) of multidecadal variability in the North Atlantic are proposed. The 20–30-yr variability is characterized by the westward propagation of subsurface temperature anomalies. The hypothesis is that the 20–30-yr variability is caused by internal variability of the Atlantic Meridional Overturning Circulation (MOC) while the 50–70-yr variability is related to atmospheric forcing over the Atlantic Ocean and exchange processes between the Atlantic and Arctic Oceans.

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