314 research outputs found
A last millennium perspective on North Atlantic variability: exploiting synergies between models and proxy data
The North Atlantic is a key region for decadal prediction
as it has experienced significant multi-decadal variability
over the observed period. This variability, which is
thought to be intrinsic to the region, can potentially
modulate, either by amplifying or mitigating, the
global warming signal from anthropogenic greenhouse
emissions. For example, studies suggest that the North
Atlantic contributed to the recent hiatus period between
1998 and 2012, by triggering an atmospheric response
which impacted on the eastern tropical Pacific (e.g.
McGregor et al., 2014). The subpolar North Atlantic is
also a major CO2
sink, and therefore of great importance
for the global carbon cycle (Perez et al., 2013).
One of the key players in the North Atlantic region is the
Atlantic Meridional Overturning Circulation (AMOC),
which is associated with sinking due to deep water
formation in the Labrador and Nordic Seas. The AMOC is
the primary control of the poleward heat transport in the
Atlantic region. Therefore, the AMOC is associated with
important climate impacts, and plays an active role in
various feedback mechanisms with, for example, sea ice
(Mahajan et al., 2011) and the atmospheric circulation
(Gastineau and Frankignoul, 2012). The AMOC has
exhibited abrupt variations in the past (e.g. the last glacial
period, Rahmstorf, 2002) and could experience a major
slowdown in the future due to the combined effect of
surface warming and Greenland ice sheet melting on deep
water formation (Bakker et al., 2016). The possibility
of such a shutdown has stimulated considerable
international efforts to observe and reconstruct the
past AMOC changes. Only by understanding its natural
variability will we be able to detect and anticipate an
anthropogenic impact on the AMOC.
Decadal modulations are also found in other large-scale
modes of climate variability, such as the North Atlantic
Oscillation (NAO) (Stephenson et al., 2000), the Subpolar
Gyre strength (SPG) (Häkkinen and Rhines, 2004) and
the Atlantic Multidecadal Variability (AMV) (Enfield et al.,
2001), which have all been linked with widespread climate
impacts over the surrounding continents. Modelling
studies suggest that all these modes interact with the
AMOC (Gastineau and Frankignoul, 2012; Hátún et al.,
2005; Knight et al., 2005) but the exact interrelationships
are complex and remain to be disentangled. Also to be
determined are the underlying mechanisms responsible
for the decadal and centennial AMOC modulations, with
different climate models showing different key drivers
(Menary et al., 2015a). Similarly, the exact impact of the
natural external forcings (e.g. volcanic aerosols, solar
irradiance) on the variability of these different largescale
climate modes still remains unclear
Ocean temperature impact on ice shelf extent in the eastern Antarctic Peninsula
The recent thinning and retreat of Antarctic ice shelves has been attributed to both atmosphere
and ocean warming. However, the lack of continuous, multi-year direct observations
as well as limitations of climate and ice shelf models prevent a precise assessment on how
the ocean forcing affects the fluctuations of a grounded and floating ice cap. Here we show
that a +0.3–1.5 °C increase in subsurface ocean temperature (50–400 m) in the northeastern
Antarctic Peninsula has driven to major collapse and recession of the regional ice
shelf during both the instrumental period and the last 9000 years. Our projections following
the representative concentration pathway 8.5 emission scenario from the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change reveal a +0.3 °C subsurface ocean
temperature warming within the coming decades that will undoubtedly accelerate ice shelf
melting, including the southernmost sector of the eastern Antarctic Peninsula.J.E. and C.E. are financially supported by the Spanish Ministerio de Economia y Competitividad
(CTM2014–60451-C2–1-P) co-funded by the European Union through
FEDER funds. J.-H.K. was supported by the grants funded by the Korea Polar Research
Institute (KOPRI, NRF-2015M1A5A1037243 and PE19010). S.S. and J.S.S.D. are supported
by the Netherlands Earth System Science Center funded by the Dutch Ministry of
Education and Science (OCW). G.S. and D.S. were funded by the EMBRACE project
(European Union’s FP7, Grant Number: 282672). We also acknowledge funding from the
French ANR CLIMICE, ERC ICEPROXY 203441, ESF PolarClimate, HOLOCLIP 625
and FP7 Past4Future as well as the Netherlands Organisation of Scientific Research
(NWO) through a VICI grant to S.S. The HOLOCLIP Project, a joint research project of
ESF PolarCLIMATE programme, is funded by national contributions from Italy, France,
Germany, Spain, Netherlands, Belgium and the United Kingdom. The research leading to
these results has also received support from the European Union’s Seventh Framework
programme (FP7/2007–2013) under Grant Agreement No. 243908, “Past4Future, Climate
change – Learning from the past climate”
Glacial climate sensitivity to different states of the Atlantic Meridional Overturning Circulation: results from the IPSL model
Paleorecords from distant locations on the globe show rapid and large amplitude climate variations during the last glacial period. Here we study the global climatic response to different states of the Atlantic Meridional Overturning Circulation (AMOC) as a potential explanation for these climate variations and their possible connections. We analyse three glacial simulations obtained with an atmosphere-ocean coupled general circulation model and characterised by different AMOC strengths (18, 15 and 2 Sv) resulting from successive ~0.1 Sv freshwater perturbations in the North Atlantic. These AMOC states suggest the existence of a freshwater threshold for which the AMOC collapses. A weak (18 to 15 Sv) AMOC decrease results in a North Atlantic and European cooling. This cooling is not homogeneous, with even a slight warming over the Norwegian Sea. Convection in this area is active in both experiments, but surprisingly stronger in the 15 Sv simulation, which appears to be related to interactions with the atmospheric circulation and sea-ice cover. Far from the North Atlantic, the climatic response is not significant. The climate differences for an AMOC collapse (15 to 2 Sv) are much larger and of global extent. The timing of the climate response to this AMOC collapse suggests teleconnection mechanisms. Our analyses focus on the North Atlantic and surrounding regions, the tropical Atlantic and the Indian monsoon region. The North Atlantic cooling associated with the AMOC collapse induces a cyclonic atmospheric circulation anomaly centred over this region, which modulates the eastward advection of cold air over the Eurasian continent. This can explain why the cooling is not as strong over western Europe as over the North Atlantic. In the Tropics, the southward shift of the Inter-Tropical Convergence Zone appears to be strongest over the Atlantic and Eastern Pacific and results from an adjustment of the atmospheric and oceanic heat transports. Finally, the Indian monsoon weakening appears to be connected to the North Atlantic cooling via that of the troposphere over Eurasia. Such an understanding of these teleconnections and their timing could be useful for paleodata interpretation
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Tentative reconstruction of the 1998–2012 hiatus in global temperature warming using the IPSL–CM5A–LR climate model
The period running from 1998 to 2012 has experienced a slower increase in global temperature at the surface of the Earth than the decades before. Several explanations have been proposed, ranging from internal variability of the climate system to a contribution of the natural external forcing. In this study, we use the IPSL-CM5A-LR climate model to test these different hypotheses. We consider historical simulations, including observed external forcing, in which nudging towards observed sea surface temperature has been applied to different regions of the ocean to phase the decadal variability of large-scale modes in the Atlantic and the Pacific to observations. We find that phasing the tropical Pacific is reducing the warming trend detected in historical simulations by a factor of two, but the remaining trend is still twice as large as the observed one. Combining the tropical Pacific phasing and the potential effect of recent eruptions allows us to fully reproduce the observed hiatus. Conversely, nudging the Atlantic does not drive any hiatus in this model
Role of the Atlantic Multidecadal Variability in modulating the climate response to a Pinatubo-like volcanic eruption
The modulation by the Atlantic multidecadal variability (AMV) of the dynamical climate response to a Pinatubo-like eruption is investigated for the boreal winter season based on a suite of large ensemble experiments using the CNRM-CM5 Coupled Global Circulation Model. The volcanic eruption induces a strong reduction and retraction of the Hadley cell during 2 years following the eruption and independently of the phase of the AMV. The mean extratropical westerly circulation simultaneously weakens throughout the entire atmospheric column, except at polar Northern latitudes where the zonal circulation is slightly strengthened. Yet, there are no significant changes in the modes of variability of the surface atmospheric circulation, such as the North Atlantic Oscillation (NAO), in the first and the second winters after the eruption. Significant modifications over the North Atlantic sector are only found during the third winter. Using clustering techniques, we decompose the atmospheric circulation into weather regimes and provide evidence for inhibition of the occurrence of negative NAO-type circulation in response to volcanic forcing. This forced signal is amplified in cold AMV conditions and is related to sea ice/atmosphere feedbacks in the Arctic and to tropical-extratropical teleconnections. Finally, we demonstrate that large ensembles of simulations are required to make volcanic fingerprints emerge from climate noise at mid-latitudes. Using small size ensemble could easily lead to misleading conclusions especially those related to the extratropical dynamics, and specifically the NAO.This research was carried out within the pro-
jects: (i) MORDICUS funded by the French Agence Nationale de la Recherche (ANR-13-SENV-0002-02); (ii) SPECS funded by the European Commission’s Seventh Framework Research Programme under the grant agreement 308378; (iii) VOLCADEC funded by the Spanish program MINECO/FEDER (ref. CGL2015-70177-R). We thank Javier Garcia-Serrano for its comments about the NAO precursors, Omar Bellprat for its suggestions concerning the statistical analysis and François Massonnet for its recommendations in terms of graphical presentation. CC is grateful to Marie-Pierre Moine, Laure Coquart and Isabelle Dast for technical help to run the model. Computer resources have been provided by Cerfacs. We thank the two anonymous referees for their useful comments and suggestions to improve this manuscript.Peer ReviewedPostprint (author's final draft
Bidecadal North Atlantic ocean circulation variability controlled by timing of volcanic eruptions
International audienceWhile bidecadal climate variability has been evidenced in several North Atlantic paleoclimaterecords, its drivers remain poorly understood. Here we show that the subset of CMIP5historical climate simulations that produce such bidecadal variability exhibits a robustsynchronization, with a maximum in Atlantic Meridional Overturning Circulation (AMOC) 15years after the 1963 Agung eruption. The mechanisms at play involve salinity advection fromthe Arctic and explain the timing of Great Salinity Anomalies observed in the 1970s and the1990s. Simulations, as well as Greenland and Iceland paleoclimate records, indicate thatcoherent bidecadal cycles were excited following five Agung-like volcanic eruptions of the lastmillennium. Climate simulations and a conceptual model reveal that destructive interferencecaused by the Pinatubo 1991 eruption may have damped the observed decreasing trend of theAMOC in the 2000s. Our results imply a long-lasting climatic impact and predictabilityfollowing the next Agung-like eruption
Changes in global ocean bottom properties and volume transports in CMIP5 models under climate change scenarios
Changes in bottom temperature, salinity and density in the global ocean by 2100 for CMIP5 climate models are investigated for the climate change scenarios RCP4.5 and RCP8.5. The mean of 24 models shows a decrease in density in all deep basins except the North Atlantic which becomes denser. The individual model responses to climate change forcing are more complex: regarding temperature, the 24 models predict a warming of the bottom layer of the global ocean; in salinity, there is less agreement regarding the sign of the change, especially in the Southern Ocean. The magnitude and equatorward extent of these changes also vary strongly among models. The changes in properties can be linked with changes in the mean transport of key water masses. The Atlantic Meridional Overturning Circulation weakens in most models and is directly linked to changes in bottom density in the North Atlantic. These changes are due to the intrusion of modified Antarctic Bottom Water, made possible by the decrease in North Atlantic Deep Water formation. In the Indian, Pacific and South Atlantic, changes in bottom density are congruent with the weakening in Antarctic Bottom Water transport through these basins. We argue that the greater the 1986-2005 meridional transports, the more changes have propagated equatorwards by 2100. However, strong decreases in density over 100 years of climate change cause a weakening of the transports. The speed at which these property changes reach the deep basins is critical for a correct assessment of the heat storage capacity of the oceans as well as for predictions of future sea level rise
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The impact of salinity perturbations on the future uptake of heat by the Atlantic Ocean
Anthropogenic ocean heat uptake is a key factor in determining climate change and sea-level rise. There is considerable uncertainty in projections of freshwater forcing of the ocean, with the potential to influence ocean heat uptake. We investigatethis by adding either -0.1 Sv or +0.1 Sv freshwater to the Atlantic in global climate model simulations, simultaneously imposing an atmospheric CO2 increase. The resulting changes in the Atlantic meridional overturning circulation are roughly equal and opposite (±2Sv). The impact of the perturbation on ocean heat content is more complex, although it is relatively small (~5%) compared to the total anthropogenic heat uptake. Several competing processes either accelerate or retard warming at different depths. Whilst positive freshwater perturbations cause an overall heating of the Atlantic, negative perturbations produce insignificant net changes in heat content. The processes active in our model appear robust, although their net result is likely model- and experiment-dependent
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