Low-Latitude Freshwater Influence on Centennial Variability of the Atlantic Thermohaline Circulation
TL;DR: In this paper, the authors analyzed the variability of the thermohaline circulation (THC) in a long control simulation by the Met Office's Third Hadley Centre Coupled Ocean-Atmosphere General Circulation Model (HadCM3) and showed that internal THC variability in the coupled climate system is concentrated at interannual and centennial time scales, with the centennial mode being dominant.
Abstract: Variability of the thermohaline circulation (THC) has been analyzed in a long control simulation by the Met Office's Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). It is shown that internal THC variability in the coupled climate system is concentrated at interannual and centennial time scales, with the centennial mode being dominant. Centennial oscillations of the THC can impact surface climate via an interhemispheric SST contrast of 0.1°C in the Tropics and more than 0.5°C in mid- and high latitudes. A mechanism is proposed based on detailed process analysis involving large-scale air–sea interaction on multidecadal time scales. Anomalous northward ocean heat transport associated with a strong phase of the Atlantic THC generates a cross-equatorial SST gradient. This causes the ITCZ to move to a more northerly position with increased strength. The extra rainfall resulting from the anomalous ITCZ imposes a freshwater flux and produces a salinity anomaly in the trop...
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01 Jan 2007
TL;DR: Drafting Authors: Neil Adger, Pramod Aggarwal, Shardul Agrawala, Joseph Alcamo, Abdelkader Allali, Oleg Anisimov, Nigel Arnell, Michel Boko, Osvaldo Canziani, Timothy Carter, Gino Casassa, Ulisses Confalonieri, Rex Victor Cruz, Edmundo de Alba Alcaraz, William Easterling, Christopher Field, Andreas Fischlin, Blair Fitzharris.
Abstract: Drafting Authors: Neil Adger, Pramod Aggarwal, Shardul Agrawala, Joseph Alcamo, Abdelkader Allali, Oleg Anisimov, Nigel Arnell, Michel Boko, Osvaldo Canziani, Timothy Carter, Gino Casassa, Ulisses Confalonieri, Rex Victor Cruz, Edmundo de Alba Alcaraz, William Easterling, Christopher Field, Andreas Fischlin, Blair Fitzharris, Carlos Gay García, Clair Hanson, Hideo Harasawa, Kevin Hennessy, Saleemul Huq, Roger Jones, Lucka Kajfež Bogataj, David Karoly, Richard Klein, Zbigniew Kundzewicz, Murari Lal, Rodel Lasco, Geoff Love, Xianfu Lu, Graciela Magrín, Luis José Mata, Roger McLean, Bettina Menne, Guy Midgley, Nobuo Mimura, Monirul Qader Mirza, José Moreno, Linda Mortsch, Isabelle Niang-Diop, Robert Nicholls, Béla Nováky, Leonard Nurse, Anthony Nyong, Michael Oppenheimer, Jean Palutikof, Martin Parry, Anand Patwardhan, Patricia Romero Lankao, Cynthia Rosenzweig, Stephen Schneider, Serguei Semenov, Joel Smith, John Stone, Jean-Pascal van Ypersele, David Vaughan, Coleen Vogel, Thomas Wilbanks, Poh Poh Wong, Shaohong Wu, Gary Yohe
7,720 citations
Oeschger Centre for Climate Change Research1, Swiss Federal Institute of Aquatic Science and Technology2, University of Edinburgh3, Free University of Berlin4, ETH Zurich5, Université catholique de Louvain6, École Polytechnique7, University of Bristol8, Russian Academy of Sciences9, University of Birmingham10
TL;DR: The authors used selected proxy-based reconstructions of different climate variables, together with state-of-the-art time series of natural forcings (orbital variations, solar activity variations, large tropical volcanic eruptions, land cover and greenhouse gases), underpinned by results from GCMs and Earth System Models of Intermediate Complexity (EMICs), to establish a comprehensive explanatory framework for climate changes from the mid-Holocene (MH) to pre-industrial time.
1,539 citations
Cites background or methods from "Low-Latitude Freshwater Influence o..."
...Simulations using models including a dynamic ocean have shown that, in response to an increase in solar irradiance, the magnitude of the AMOC tends to decrease by a few percent probably because of a warming and reduced ocean surface density at high latitudes (e.g. Weber et al., 2004)....
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...While the fluctuations with a period of about 24 years are caused by variations in the AMOC, the associated changes in SST and sea ice distribution in the Arctic also imprint this periodicity on the NAO....
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...These are then advected with the AMOC into the areas of deepwater formation where they accelerate the AMOC....
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...These oscillations are driven by surface density anomalies influencing the AMOC, which itself feeds back on the advection of the surface anomalies from lower latitudes....
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...Multi-decadal variability is also identified in climate model simulations and there are multiple lines of evidence that this is due to variations in the North Atlantic meridional overturning circulation (AMOC)....
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TL;DR: In this article, the authors used a 1400 year climate model calculation to simulate the observed pattern and amplitude of the Atlantic Multidecadal Oscillation (AMO) and found that the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation.
Abstract: [1] Analyses of global climate from measurements dating back to the nineteenth century show an ‘Atlantic Multidecadal Oscillation’ (AMO) as a leading large-scale pattern of multidecadal variability in surface temperature. Yet it is not possible to determine whether these fluctuations are genuinely oscillatory from the relatively short observational record alone. Using a 1400 year climate model calculation, we are able to simulate the observed pattern and amplitude of the AMO. The results imply the AMO is a genuine quasi-periodic cycle of internal climate variability persisting for many centuries, and is related to variability in the oceanic thermohaline circulation (THC). This relationship suggests we can attempt to reconstruct past THC changes, and we infer an increase in THC strength over the last 25 years. Potential predictability associated with the mode implies natural THC and AMO decreases over the next few decades independent of anthropogenic climate change.
1,137 citations
Geophysical Fluid Dynamics Laboratory1, Princeton University2, University of Reading3, Met Office4, University of Victoria5, University of Tokyo6, National Center for Atmospheric Research7, Max Planck Society8, University of Washington9, Potsdam Institute for Climate Impact Research10, Complutense University of Madrid11, University of Toronto12, Massachusetts Institute of Technology13, Royal Netherlands Meteorological Institute14
TL;DR: In this article, the authors compare the performance of different models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere-ocean general circulation models (AOGCMs) to document and improve understanding of the causes of wide variations in the modeled THC response.
Abstract: The Atlantic thermohaline circulation (THC) is an important part of the earth’s climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-S v( 1 Sv 10 6 m 3 s 1 ) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.
856 citations
TL;DR: The results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.
Abstract: The climate of the North Atlantic region exhibits fluctuations on decadal timescales that have large societal consequences. Prominent examples include hurricane activity in the Atlantic1, and surface-temperature and rainfall variations over North America2, Europe3 and northern Africa4. Although these multidecadal variations are potentially predictable if the current state of the ocean is known5, 6, 7, the lack of subsurface ocean observations8 that constrain this state has been a limiting factor for realizing the full skill potential of such predictions9. Here we apply a simple approach—that uses only sea surface temperature (SST) observations—to partly overcome this difficulty and perform retrospective decadal predictions with a climate model. Skill is improved significantly relative to predictions made with incomplete knowledge of the ocean state10, particularly in the North Atlantic and tropical Pacific oceans. Thus these results point towards the possibility of routine decadal climate predictions. Using this method, and by considering both internal natural climate variations and projected future anthropogenic forcing, we make the following forecast: over the next decade, the current Atlantic meridional overturning circulation will weaken to its long-term mean; moreover, North Atlantic SST and European and North American surface temperatures will cool slightly, whereas tropical Pacific SST will remain almost unchanged. Our results suggest that global surface temperature may not increase over the next decade, as natural climate variations in the North Atlantic and tropical Pacific temporarily offset the projected anthropogenic warming.
694 citations
References
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TL;DR: In this article, a step-by-step guide to wavelet analysis is given, with examples taken from time series of the El Nino-Southern Oscillation (ENSO).
Abstract: A practical step-by-step guide to wavelet analysis is given, with examples taken from time series of the El Nino–Southern Oscillation (ENSO). The guide includes a comparison to the windowed Fourier transform, the choice of an appropriate wavelet basis function, edge effects due to finite-length time series, and the relationship between wavelet scale and Fourier frequency. New statistical significance tests for wavelet power spectra are developed by deriving theoretical wavelet spectra for white and red noise processes and using these to establish significance levels and confidence intervals. It is shown that smoothing in time or scale can be used to increase the confidence of the wavelet spectrum. Empirical formulas are given for the effect of smoothing on significance levels and confidence intervals. Extensions to wavelet analysis such as filtering, the power Hovmoller, cross-wavelet spectra, and coherence are described. The statistical significance tests are used to give a quantitative measure of change...
12,803 citations
"Low-Latitude Freshwater Influence o..." refers background or methods in this paper
...Figure 4 shows a wavelet analysis of the annual-mean maximum overturning streamfunction at 458N (Torrence and Compo 1998)....
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...More details on wavelet analysis are given by Torrence and Compo (1998). quantify variability of the THC on an interannual time scale....
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TL;DR: An evaluation of the atmospheric moisture budget reveals coherent large-scale changes since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and parts of Scandinavia have generally experienced wetter than normal conditions.
Abstract: Greenland ice-core data have revealed large decadal climate variations over the North Atlantic that can be related to a major source of low-frequency variability, the North Atlantic Oscillation. Over the past decade, the Oscillation has remained in one extreme phase during the winters, contributing significantly to the recent wintertime warmth across Europe and to cold conditions in the northwest Atlantic. An evaluation of the atmospheric moisture budget reveals coherent large-scale changes since 1980 that are linked to recent dry conditions over southern Europe and the Mediterranean, whereas northern Europe and parts of Scandinavia have generally experienced wetter than normal conditions.
7,593 citations
"Low-Latitude Freshwater Influence o..." refers background in this paper
...In (NAO; Hurrell 1995) years of a positive NAO index, there is anomalous downwelling in the Atlantic near 408N, and upwelling to the north and south....
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TL;DR: In this article, a simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating, showing that there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent.
Abstract: A simple analytic model is constructed to elucidate some basic features of the response of the tropical atmosphere to diabatic heating. In particular, there is considerable east-west asymmetry which can be illustrated by solutions for heating concentrated in an area of finite extent. This is of more than academic interest because heating in practice tends to be concentrated in specific areas. For instance, a model with heating symmetric about the equator at Indonesian longitudes produces low-level easterly flow over the Pacific through propagation of Kelvin waves into the region. It also produces low-level westerly inflow over the Indian Ocean (but in a smaller region) because planetary waves propagate there. In the heating region itself the low-level flow is away from the equator as required by the vorticity equation. The return flow toward the equator is farther west because of planetary wave propagation, and so cyclonic flow is obtained around lows which form on the western margins of the heating zone. Another model solution with the heating displaced north of the equator provides a flow similar to the monsoon circulation of July and a simple model solution can also be found for heating concentrated along an inter-tropical convergence line.
3,799 citations
"Low-Latitude Freshwater Influence o..." refers background in this paper
...…section 5e, the freshwater flux resulted from a such a shift of the ITCZ plays a crucial role in the centennial oscillation of the THC. Anomalous rainfall in the Tropics is not limited to the Atlantic sector: anomalies can be readily communicated elsewhere via the equatorial waveguide (Gill 1980)....
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...Anomalous rainfall in the Tropics is not limited to the Atlantic sector: anomalies can be readily communicated elsewhere via the equatorial waveguide (Gill 1980)....
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TL;DR: A new version of the Hadley Centre coupled model (HadCM3) that does not require flux adjustments to prevent large climate drifts in the simulation is presented in this article.
Abstract: Results are presented from a new version of the Hadley Centre coupled model (HadCM3) that does not require flux adjustments to prevent large climate drifts in the simulation The model has both an improved atmosphere and ocean component In particular, the ocean has a 125° × 125° degree horizontal resolution and leads to a considerably improved simulation of ocean heat transports compared to earlier versions with a coarser resolution ocean component The model does not have any spin up procedure prior to coupling and the simulation has been run for over 400 years starting from observed initial conditions The sea surface temperature (SST) and sea ice simulation are shown to be stable and realistic The trend in global mean SST is less than 0009 °C per century In part, the improved simulation is a consequence of a greater compatibility of the atmosphere and ocean model heat budgets The atmospheric model surface heat and momentum budget are evaluated by comparing with climatological ship-based estimates Similarly the ocean model simulation of poleward heat transports is compared with direct ship-based observations for a number of sections across the globe Despite the limitations of the observed datasets, it is shown that the coupled model is able to reproduce many aspects of the observed heat budget
2,674 citations
"Low-Latitude Freshwater Influence o..." refers background or methods in this paper
...The model is described in detail by Gordon et al. (2000), Pope et al. (2000), and Cox et al. (1999)....
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...This is the most recent version of D ow nloaded from http://journals.am etsoc.org/jcli/article-pdf/17/23/4498/3789109/3219_1.pdf by guest on 23 Septem ber 2020 the Met Office’s Hadley Centre climate model (Gordon et al. 2000)....
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...the Met Office’s Hadley Centre climate model (Gordon et al. 2000)....
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...It takes about 400 yr of integration before heat and freshwater budgets in the ocean attain a balance (Gordon et al. 2000; Pardaens et al. 2003)....
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...A validation of the model simulation of some important climate quantities is given by Gordon et al. (2000)....
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TL;DR: In this article, the authors attempt hemispheric temperature reconstructions with proxy data networks for the past millennium, focusing not just on the reconstructions, but the uncertainties therein, and important caveats.
Abstract: Building on recent studies, we attempt hemispheric temperature reconstructions with proxy data networks for the past millennium. We focus not just on the reconstructions, but the uncertainties therein, and important caveats. Though expanded uncertainties prevent decisive conclusions for the period prior to AD 1400, our results suggest that the latter 20th century is anomalous in the context of at least the past millennium. The 1990s was the warmest decade, and 1998 the warmest year, at moderately high levels of confidence. The 20th century warming counters a millennial-scale cooling trend which is consistent with long-term astronomical forcing.
1,742 citations