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  • Climate variability  (12)
  • American Meteorological Society  (12)
  • Institute of Physics (IOP)
  • 2015-2019  (12)
  • 1995-1999
  • 1
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    Multidecadal Indian Ocean variability linked to the Pacific and implications for preconditioning Indian Ocean dipole events (2017)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 1739-1751, doi:10.1175/JCLI-D-16-0200.1.
    Description: The Indian Ocean has sustained robust surface warming in recent decades, but the role of multidecadal variability remains unclear. Using ocean model hindcasts, characteristics of low-frequency Indian Ocean temperature variations are explored. Simulated upper-ocean temperature changes across the Indian Ocean in the hindcast are consistent with those recorded in observational products and ocean reanalyses. Indian Ocean temperatures exhibit strong warming trends since the 1950s limited to the surface and south of 30°S, while extensive subsurface cooling occurs over much of the tropical Indian Ocean. Previous work focused on diagnosing causes of these long-term trends in the Indian Ocean over the second half of the twentieth century. Instead, the temporal evolution of Indian Ocean subsurface heat content is shown here to reveal distinct multidecadal variations associated with the Pacific decadal oscillation, and the long-term trends are thus interpreted to result from aliasing of the low-frequency variability. Transmission of the multidecadal signal occurs via an oceanic pathway through the Indonesian Throughflow and is manifest across the Indian Ocean centered along 12°S as westward-propagating Rossby waves modulating thermocline and subsurface heat content variations. Resulting low-frequency changes in the eastern Indian Ocean thermocline depth are associated with decadal variations in the frequency of Indian Ocean dipole (IOD) events, with positive IOD events unusually common in the 1960s and 1990s with a relatively shallow thermocline. In contrast, the deeper thermocline depth in the 1970s and 1980s is associated with frequent negative IOD and rare positive IOD events. Changes in Pacific wind forcing in recent decades and associated rapid increases in Indian Ocean subsurface heat content can thus affect the basin’s leading mode of variability, with implications for regional climate and vulnerable societies in surrounding countries.
    Description: This research was supported by a Research Fellowship by the Alexander von Humboldt Foundation, as well as the Ocean Climate Change Institute and the Investment in Science Fund at WHOI.
    Description: 2017-08-15
    Keywords: Indian Ocean ; Ocean dynamics ; Climate variability ; Multidecadal variability ; Pacific decadal oscillation
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Variability and trends in surface meteorology and air–sea fluxes at a site off northern Chile (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 28 (2015): 3004–3023, doi:10.1175/JCLI-D-14-00591.1.
    Description: Time series of surface meteorology and air–sea exchanges of heat, freshwater, and momentum collected from a long-term surface mooring located 1600 km west of the coast of northern Chile are analyzed. The observations, spanning 2000–10, have been withheld from assimilation into numerical weather prediction models. As such, they provide a unique in situ record of atmosphere–ocean coupling in a trade wind region characterized by persistent stratocumulus clouds. The annual cycle is described, as is the interannual variability. Annual variability in the air–sea heat flux is dominated by the annual cycle in net shortwave radiation. In austral summer, the ocean is heated; the 9-yr mean annual heating of the ocean is 38 W m−2. Ocean cooling is seen in 2006–08, coincident with La Niña events. Over the full record, significant trends were found. Increases in wind speed, wind stress, and latent heat flux over 9 yr were 0.8 m s−1, 0.022 N m−2, and 20 W m−2 or 13%, 29%, and 20% of the respective 9-yr means. The decrease in the annual mean net heat flux was 39 W m−2 or 104% of the mean. These changes were found to be largely associated with spring and fall. If this change persists, the annual mean net air–sea heat flux will change sign by 2016, when the magnitude of the wind stress will have increased by close to 60%.
    Description: This work is supported by the NOAA Climate Observation Division (NA09OAR4320129).
    Keywords: Climate variability ; Trends
    Repository Name: Woods Hole Open Access Server
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  • 3
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    Variations of the global net air–sea heat flux during the “hiatus” period (2001–10) (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 29 (2016): 3647-3660, doi:10.1175/JCLI-D-15-0626.1.
    Description: An assessment is made of the mean and variability of the net air–sea heat flux, Qnet, from four products (ECCO, OAFlux–CERES, ERA-Interim, and NCEP1) over the global ice-free ocean from January 2001 to December 2010. For the 10-yr “hiatus” period, all products agree on an overall net heat gain over the global ice-free ocean, but the magnitude varies from 1.7 to 9.5 W m−2. The differences among products are particularly large in the Southern Ocean, where they cannot even agree on whether the region gains or loses heat on the annual mean basis. Decadal trends of Qnet differ significantly between products. ECCO and OAFlux–CERES show almost no trend, whereas ERA-Interim suggests a downward trend and NCEP1 shows an upward trend. Therefore, numerical simulations utilizing different surface flux forcing products will likely produce diverged trends of the ocean heat content during this period. The downward trend in ERA-Interim started from 2006, driven by a peculiar pattern change in the tropical regions. ECCO, which used ERA-Interim as initial surface forcings and is constrained by ocean dynamics and ocean observations, corrected the pattern. Among the four products, ECCO and OAFlux–CERES show great similarities in the examined spatial and temporal patterns. Given that the two estimates were obtained using different approaches and based on largely independent observations, these similarities are encouraging and instructive. It is more likely that the global net air–sea heat flux does not change much during the so-called hiatus period.
    Description: This paper is funded in part by the NOAA Climate Observation Division, Climate Program Office, under Grant NA09OAR4320129 and by the NOAA MAPP Climate Reanalysis Task Force Team under Grant NA13OAR4310106. The study was initiated when X. Liang was a postdoc at MIT, where he was supported in part by the NSF through Grant OCE-0961713, by NOAA through Grant NA10OAR4310135, and by the NASA Physical Oceanography Program through ECCO.
    Description: 2016-11-15
    Keywords: Physical Meteorology and Climatology ; Heat budgets/fluxes ; Surface fluxes ; Models and modeling ; Reanalysis data ; Variability ; Climate variability ; Interannual variability ; Seasonal variability
    Repository Name: Woods Hole Open Access Server
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  • 4
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    How climate change affects extremes in maize and wheat yield in two cropping regions (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 28 (2015): 4653–4687, doi:10.1175/JCLI-D-13-00326.1.
    Description: Downscaled climate model projections from phase 5 of the Coupled Model Intercomparison Project (CMIP5) were used to force a dynamic vegetation agricultural model (Agro-IBIS) and simulate yield responses to historical climate and two future emissions scenarios for maize in the U.S. Midwest and wheat in southeastern Australia. In addition to mean changes in yield, the frequency of high- and low-yield years was related to changing local hydroclimatic conditions. Particular emphasis was on the seasonal cycle of climatic variables during extreme-yield years and links to crop growth. While historically high (low) yields in Iowa tend to occur during years with anomalous wet (dry) growing season, this is exacerbated in the future. By the end of the twenty-first century, the multimodel mean (MMM) of growing season temperatures in Iowa is projected to increase by more than 5°C, and maize yield is projected to decrease by 18%. For southeastern Australia, the frequency of low-yield years rises dramatically in the twenty-first century because of significant projected drying during the growing season. By the late twenty-first century, MMM growing season precipitation in southeastern Australia is projected to decrease by 15%, temperatures are projected to increase by 2.8°–4.5°C, and wheat yields are projected to decline by 70%. Results highlight the sensitivity of yield projections to the nature of hydroclimatic changes. Where future changes are uncertain, the sign of the yield change simulated by Agro-IBIS is uncertain as well. In contrast, broad agreement in projected drying over southern Australia across models is reflected in consistent yield decreases for the twenty-first century. Climatic changes of the order projected can be expected to pose serious challenges for continued staple grain production in some current centers of production, especially in marginal areas.
    Description: This work was initiated at the Dissertations Initiative for the Advancement of Climate Change Research (DISCCRS) V Symposium, supported by the U.S. National Science Foundation through collaborative Grants SES-0932916 and SES-0931402. CCU was supported by a University of New South Wales Vice-Chancellor Fellowship and the Penzance Endowed Fund and John P. Chase Memorial Endowed Fund at WHOI. TET was supported by the U.S. Department of Energy Award DE-EE0004397. NC was funded by NSF Grant EAR-1204774. We are indebted to the FORMAS-funded Land Use Today and Tomorrow (LUsTT) project (Grant 211-2009-1682) for financial support.
    Keywords: Australia ; North America ; Climate change ; Climate models ; Climate variability ; Agriculture
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Wintertime atmospheric response to North Atlantic Ocean circulation variability in a climate model (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 28 (2015): 7659–7677, doi:10.1175/JCLI-D-15-0007.1.
    Description: 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.
    Description: Support from the NOAA Climate Program Office (NA10OAR4310202 and NA13OAR4310139), NSF EaSM2 (OCE 1242989) and the European Community 7th framework programme (FP7 2007-2013) under Grant Agreement 308299 (NACLIM) is gratefully acknowledged. The analysis benefited from the IPSL Prodiguer-Ciclad facility, which is supported by CNRS, UPMC, Labex L-IPSL funded by the ANR (Grant ANR-10-LABX-0018) and by the European FP7 IS-ENES2 project (Grant 312979).
    Description: 2016-04-01
    Keywords: Meridional overturning circulation ; North Atlantic Oscillation ; Climate models ; Climate variability
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  • 6
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    Can Australian multiyear droughts and wet spells be generated in the absence of oceanic variability? (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 29 (2016): 6201-6221, doi:10.1175/JCLI-D-15-0694.1.
    Description: Anomalous conditions in the tropical oceans, such as those related to El Niño–Southern Oscillation and the Indian Ocean dipole, have been previously blamed for extended droughts and wet periods in Australia. Yet the extent to which Australian wet and dry spells can be driven by internal atmospheric variability remains unclear. Natural variability experiments are examined to determine whether prolonged extreme wet and dry periods can arise from internal atmospheric and land variability alone. Results reveal that this is indeed the case; however, these dry and wet events are found to be less severe than in simulations incorporating coupled oceanic variability. Overall, ocean feedback processes increase the magnitude of Australian rainfall variability by about 30% and give rise to more spatially coherent rainfall impacts. Over mainland Australia, ocean interactions lead to more frequent extreme events, particularly during the rainy season. Over Tasmania, in contrast, ocean–atmosphere coupling increases mean rainfall throughout the year. While ocean variability makes Australian rainfall anomalies more severe, droughts and wet spells of duration longer than three years are equally likely to occur in both atmospheric- and ocean-driven simulations. Moreover, they are essentially indistinguishable from what one expects from a Gaussian white noise distribution. Internal atmosphere–land-driven megadroughts and megapluvials that last as long as ocean-driven events are also identified in the simulations. This suggests that oceanic variability may be less important than previously assumed for the long-term persistence of Australian rainfall anomalies. This poses a challenge to accurate prediction of long-term dry and wet spells for Australia.
    Description: This study was supported by the Australian Research Council (ARC) under ARC-DP1094784, ARC-DP-150101331, ARC-FL100100214, and funding for C.C.U. from the National Science Foundation under AGS-1602455 and the ARC Centre of Excellence for Climate System Science.
    Description: 2017-02-19
    Keywords: Circulation/ Dynamics ; Atmosphere-ocean interaction ; Atm/Ocean Structure/ Phenomena ; Drought ; Precipitation ; Physical Meteorology and Climatology ; Climate variability ; Forecasting ; Climate prediction ; Variability
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  • 7
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    On the movements of the North Atlantic Subpolar Front in the preinstrumental past (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 29 (2016): 1545-1571, doi:10.1175/JCLI-D-15-0509.1.
    Description: Three sediment records of sea surface temperature (SST) are analyzed that originate from distant locations in the North Atlantic, have centennial-to-multicentennial resolution, are based on the same reconstruction method and chronological assumptions, and span the past 15 000 yr. Using recursive least squares techniques, an estimate of the time-dependent North Atlantic SST field over the last 15 kyr is sought that is consistent with both the SST records and a surface ocean circulation model, given estimates of their respective error (co)variances. Under the authors’ assumptions about data and model errors, it is found that the 10°C mixed layer isotherm, which approximately traces the modern Subpolar Front, would have moved by ~15° of latitude southward (northward) in the eastern North Atlantic at the onset (termination) of the Younger Dryas cold interval (YD), a result significant at the level of two standard deviations in the isotherm position. In contrast, meridional movements of the isotherm in the Newfoundland basin are estimated to be small and not significant. Thus, the isotherm would have pivoted twice around a region southeast of the Grand Banks, with a southwest–northeast orientation during the warm intervals of the Bølling–Allerød and the Holocene and a more zonal orientation and southerly position during the cold interval of the YD. This study provides an assessment of the significance of similar previous inferences and illustrates the potential of recursive least squares in paleoceanography.
    Description: OM acknowledges support from the U.S. National Science Foundation. CW acknowledges support from the European Research Council ERC Grant ACCLIMATE 339108.
    Description: 2016-08-19
    Keywords: Geographic location/entity ; North Atlantic Ocean ; Circulation/ Dynamics ; Fronts ; Mathematical and statistical techniques ; Inverse methods ; Kalman filters ; Variability ; Climate variability ; Oceanic variability
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  • 8
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    Blocking statistics in a varying climate: Lessons from a "traffic jam" model with pseudostochastic forcing (2019)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2019. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of the Atmospheric Sciences 76(10), (2019): 3013-3027, doi:10.1175/JAS-D-19-0095.1.
    Description: Recently Nakamura and Huang proposed a semiempirical, one-dimensional model of atmospheric blocking based on the observed budget of local wave activity in the boreal winter. The model dynamics is akin to that of traffic flow, wherein blocking manifests as traffic jams when the streamwise flux of local wave activity reaches capacity. Stationary waves modulate the jet stream’s capacity to transmit transient waves and thereby localize block formation. Since the model is inexpensive to run numerically, it is suited for computing blocking statistics as a function of climate variables from large-ensemble, parameter sweep experiments. We explore sensitivity of blocking statistics to (i) stationary wave amplitude, (ii) background jet speed, and (iii) transient eddy forcing, using frequency, persistence, and prevalence as metrics. For each combination of parameters we perform 240 runs of 180-day simulations with aperiodic transient eddy forcing, each time randomizing the phase relations in forcing. The model climate shifts rapidly from a block-free state to a block-dominant state as the stationary wave amplitude is increased and/or the jet speed is decreased. When eddy forcing is increased, prevalence increases similarly but frequency decreases as blocks merge and become more persistent. It is argued that the present-day climate lies close to the boundary of the two states and hence its blocking statistics are sensitive to climate perturbations. The result underscores the low confidence in GCM-based assessment of the future trend of blocking under a changing climate, while it also provides a theoretical basis for evaluating model biases and understanding trends in reanalysis data.
    Description: The main results of this paper emerged from a group project during Rossbypalooza, a student-led summer school at the University of Chicago in June 2018, with the theme of “Understanding climate through simple models.” The authors thank the participants of the summer school for their valuable feedback. Constructive criticisms of the two anonymous reviewers greatly improved the quality of the manuscript. The work is supported by NSF Grants AGS1563307 and AGS1810964
    Keywords: Blocking ; Nonlinear dynamics ; Planetary waves ; Potential vorticity ; Wave breaking ; Climate variability
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  • 9
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    Climate variability, volcanic forcing, and last millennium hydroclimate extremes (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 4309-4327, doi:10.1175/JCLI-D-17-0407.1.
    Description: Multidecadal hydroclimate variability has been expressed as “megadroughts” (dry periods more severe and prolonged than observed over the twentieth century) and corresponding “megapluvial” wet periods in many regions around the world. The risk of such events is strongly affected by modes of coupled atmosphere–ocean variability and by external impacts on climate. Accurately assessing the mechanisms for these interactions is difficult, since it requires large ensembles of millennial simulations as well as long proxy time series. Here, the Community Earth System Model (CESM) Last Millennium Ensemble is used to examine statistical associations among megaevents, coupled climate modes, and forcing from major volcanic eruptions. El Niño–Southern Oscillation (ENSO) strongly affects hydroclimate extremes: larger ENSO amplitude reduces megadrought risk and persistence in the southwestern United States, the Sahel, monsoon Asia, and Australia, with corresponding increases in Mexico and the Amazon. The Atlantic multidecadal oscillation (AMO) also alters megadrought risk, primarily in the Caribbean and the Amazon. Volcanic influences are felt primarily through enhancing AMO amplitude, as well as alterations in the structure of both ENSO and AMO teleconnections, which lead to differing manifestations of megadrought. These results indicate that characterizing hydroclimate variability requires an improved understanding of both volcanic climate impacts and variations in ENSO/AMO teleconnections.
    Description: This work is supported by NSF EaSM Grants AGS-1243125 and NCAR-1243107 to The University of Arizona.
    Description: 2018-11-03
    Keywords: Drought ; Climate variability ; ENSO ; Paleoclimate ; Climate models ; Multidecadal variability
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  • 10
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    The continuum of drought in southwestern North America (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 8627-8643, doi:10.1175/JCLI-D-18-0010.1.
    Description: Drought has severe consequences for humans and their environment, yet we have a limited understanding of the drivers of drought across the full range of time scales on which it occurs. Here, the atmosphere and ocean conditions that drive this continuum of drought variability in southwestern North America (SWNA) are studied using the latest observationally based products, paleoclimate reconstructions, and state-of-the-art Earth system model simulations of the last millennium. A novel application of the self-organizing maps (SOM) methodology allows for a visualization of the continuum of climate states coinciding with thousands of droughts of varying lengths in last millennium simulations from the Community Earth System Model (CESM), the Goddard Institute for Space Studies Model E2-R (GISS E2-R), and eight other members from phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is found that most droughts are associated with a cool Pacific decadal oscillation (PDO) pattern, but persistent droughts can coincide with a variety of ocean–atmosphere states, including time periods showing a warm PDO or weak ocean–atmosphere anomalies. Many CMIP5 models simulate similar SWNA teleconnection patterns, but the SOM analysis demonstrates that models simulate different continuums of ocean–atmosphere states coinciding with droughts of different lengths, suggesting fundamental differences in their drought dynamics. These findings have important implications for our understanding and simulation of the drivers of persistent drought, and for their potential predictability.
    Description: The National Science Foundation EaSM2 Grant (AGS1243125) supported this work.
    Keywords: Atmosphere-ocean interaction ; Drought ; Paleoclimate ; Ensembles ; General circulation models ; Climate variability
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  • 11
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    On the predominant nonlinear response of the extratropical atmosphere to meridional shifts of the Gulf Stream (2017)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 9679-9702, doi:10.1175/JCLI-D-16-0707.1.
    Description: The North Atlantic atmospheric circulation response to the meridional shifts of the Gulf Stream (GS) path is examined using a large ensemble of high-resolution hemispheric-scale Weather Research and Forecasting Model simulations. The model is forced with a broad range of wintertime sea surface temperature (SST) anomalies derived from a lag regression on a GS index. The primary result of the model experiments, supported in part by an independent analysis of a reanalysis dataset, is that the large-scale quasi-steady North Atlantic circulation response is remarkably nonlinear about the sign and amplitude of the SST anomaly chosen over a wide range of GS shift scenarios. The nonlinear response prevails over the weak linear response and resembles the negative North Atlantic Oscillation (NAO), the leading intrinsic mode of variability in the model and the observations. Further analysis of the associated dynamics reveals that the nonlinear responses are accompanied by the shift of the North Atlantic eddy-driven jet, which is reinforced, with nearly equal importance, by the high-frequency transient eddy feedback and the low-frequency wave-breaking events. Additional sensitivity simulations confirm that the nonlinearity of the circulation response is a robust feature found over the broad parameter space encompassing not only the varied SST but also the absence/presence of tropical influence, the varying lateral boundary conditions, and the initialization scheme. The result highlights the fundamental importance of the intrinsically nonlinear transient eddy dynamics and the eddy–mean flow interactions in generating the nonlinear downstream response to the meridional shifts in the Gulf Stream.
    Description: The authors are grateful for the support from NASA (NNX13AM59G) and the NSF (AGS-1355339, OCE-1419235).
    Description: 2018-05-07
    Keywords: North Atlantic Ocean ; Blocking ; North Atlantic Oscillation ; Atmosphere-ocean interaction ; Regional models ; Climate variability
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  • 12
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    Mechanisms of multidecadal Atlantic meridional overturning circulation variability diagnosed in depth versus density space (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 27 (2014): 9359–9376, doi:10.1175/JCLI-D-14-00228.1.
    Description: Multidecadal variability of the Atlantic meridional overturning circulation (AMOC) is examined based on a comparison of the AMOC streamfunctions in depth and in density space, in a 700-yr present-day control integration of the fully coupled Community Climate System Model, version 3. The commonly used depth-coordinate AMOC primarily exhibits the variability associated with the deep equatorward transport that follows the changes in the Labrador Sea deep water formation. On the other hand, the density-based AMOC emphasizes the variability associated with the subpolar gyre circulation in the upper ocean leading to the changes in the Labrador Sea convection. Combining the two representations indicates that the ~20-yr periodicity of the AMOC variability in the first half of the simulation is primarily due to an ocean-only mode resulting from the coupling of the deep equatorward flow and the upper ocean gyre circulation near the Gulf Stream and North Atlantic Current. In addition, the density-based AMOC reveals a gradual change in the deep ocean associated with cooling and increased density, which is likely responsible for the transition of AMOC variability from strong ~20-yr oscillations to a weaker red noise–like multidecadal variability.
    Description: Support from the NOAA Climate Program Office (Grant NA10OAR4310202 and NA13OAR4310139) and NSF EaSM2 (OCE1242989) is gratefully acknowledged.
    Description: 2015-06-15
    Keywords: North Atlantic Ocean ; Meridional overturning circulation ; Ocean circulation ; Thermocline circulation ; Climate variability ; Multidecadal variability
    Repository Name: Woods Hole Open Access Server
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