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Robson J, Sutton R, Menary MB, Lai MWK. Contrasting internally and externally generated Atlantic Multidecadal Variability and the role for AMOC in CMIP6 historical simulations. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2023; 381:20220194. [PMID: 37866382 PMCID: PMC10590668 DOI: 10.1098/rsta.2022.0194] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/17/2023] [Accepted: 08/01/2023] [Indexed: 10/24/2023]
Abstract
Atlantic multidecadal variability (AMV) has long been thought to be an expression of low-frequency variability in the Atlantic Meridional Overturning Circulation (AMOC). However, alternative hypotheses have been forwarded, including that AMV is primarily externally forced. Here, we review the current state of play by assessing historical simulations made for the sixth coupled model intercomparison project (CMIP6). Overall, the importance of external forcing is sensitive to the type of AMV index used, due to the importance of globally coherent externally forced signals in the models. There are also significant contrasts between the processes that drive internally and externally forced AMV, but these processes can be isolated by exploring the multivariate expression of AMV. Specifically, internal variability in CMIP6 models is consistent with an important role of ocean circulation and AMOC and the externally forced AMV is largely a surface-flux forced mechanism with little role for the ocean. Overall, the internal multivariate fingerprint of AMV is similar to the observed, but the externally forced fingerprint appears inconsistent with observations. Therefore, climate models still suggest a key role for ocean dynamics, and specifically AMOC, in observed AMV. Nevertheless, models remain deficient in a number of areas, and a stronger role for externally forced dynamical changes cannot be ruled out. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.
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Affiliation(s)
- Jon Robson
- Department of Meteorology, National Centre for Atmospheric Science, University of Reading, Reading, UK
| | - Rowan Sutton
- Department of Meteorology, National Centre for Atmospheric Science, University of Reading, Reading, UK
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Yuan T, Yu H, Chin M, Remer LA, McGee D, Evan A. Anthropogenic Decline of African Dust: Insights From the Holocene Records and Beyond. GEOPHYSICAL RESEARCH LETTERS 2020; 47:e2020GL089711. [PMID: 33281243 PMCID: PMC7685148 DOI: 10.1029/2020gl089711] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Revised: 10/14/2020] [Accepted: 10/16/2020] [Indexed: 05/22/2023]
Abstract
African dust exhibits strong variability on a range of time scales. Here we show that the interhemispheric contrast in Atlantic SST (ICAS) drives African dust variability at decadal to millennial timescales, and the strong anthropogenic increase of the ICAS in the future will decrease African dust loading to a level never seen during the Holocene. We provide a physical framework to understand the relationship between the ICAS and African dust activity: positive ICAS anomalies push the Intertropical Convergence Zone (ITCZ) northward and decrease surface wind speed over African dust source regions, which reduces dust emission and transport. It provides a unified framework for and is consistent with relationships in the literature. We find strong observational and proxy-record support for the ICAS-ITCZ-dust relationship during the past 160 and 17,000 years. Model-projected anthropogenic increase of the ICAS will reduce African dust by as much as 60%, which has broad consequences.
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Affiliation(s)
- Tianle Yuan
- Earth Sciences DivisionNASA Goddard Space Flight CenterGreenbeltMDUSA
- Joint Center for Earth Systems TechnologyUniversity of Maryland at Baltimore CountyBaltimoreMDUSA
| | - Hongbin Yu
- Earth Sciences DivisionNASA Goddard Space Flight CenterGreenbeltMDUSA
| | - Mian Chin
- Earth Sciences DivisionNASA Goddard Space Flight CenterGreenbeltMDUSA
| | - Lorraine A. Remer
- Joint Center for Earth Systems TechnologyUniversity of Maryland at Baltimore CountyBaltimoreMDUSA
| | - David McGee
- Department of Earth, Atmosphere, and Planetary SciencesMassachusetts Institute of TechnologyBostonMAUSA
| | - Amato Evan
- Scrips Institute of OceanographyUniversity of CaliforniaSan DiegoCAUSA
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Yu H, Yang Y, Wang H, Tan Q, Chin M, Levy RC, Remer LA, Smith SJ, Yuan T, Shi Y. Interannual variability and trends of combustion aerosol and dust in major continental outflows revealed by MODIS retrievals and CAM5 simulations during 2003-2017. ATMOSPHERIC CHEMISTRY AND PHYSICS 2020; 20:139-161. [PMID: 33204243 PMCID: PMC7668156 DOI: 10.5194/acp-20-139-2020] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Emissions and long-range transport of mineral dust and combustion-related aerosol from burning fossil fuels and biomass vary from year to year, driven by the evolution of the economy and changes in meteorological conditions and environmental regulations. This study offers both satellite and model perspectives on the interannual variability and possible trends of combustion aerosol and dust in major continental outflow regions over the past 15 years (2003-2017). The decade-long record of aerosol optical depth (AOD, denoted as τ), separately for combustion aerosol (τ c) and dust (τ d), over global oceans is derived from the Collection 6 aerosol products of the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard both Terra and Aqua. These MODIS Aqua datasets, complemented by aerosol source-tagged simulations using the Community Atmospheric Model version 5 (CAM5), are then analyzed to understand the interannual variability and potential trends of τ c and τ d in the major continental outflows. Both MODIS and CAM5 consistently yield a similar decreasing trend of -0.017 to -0.020 per decade for τ c over the North Atlantic Ocean and the Mediterranean Sea that is attributable to reduced emissions from North America and Europe, respectively. On the contrary, both MODIS and CAM5 display an increasing trend of +0.017 to +0.036 per decade for τ c over the tropical Indian Ocean, the Bay of Bengal, and the Arabian Sea, which reflects the influence of increased anthropogenic emissions from South Asia and the Middle East in the last 2 decades. Over the northwestern Pacific Ocean, which is often affected by East Asian emissions of pollution and dust, the MODIS retrievals show a decreasing trend of -0.021 per decade for τ c and -0.012 per decade for τ d, which is, however, not reproduced by the CAM5 model. In other outflow regions strongly influenced by biomass burning smoke or dust, both MODIS retrievals and CAM5 simulations show no statistically significant trends; the MODIS-observed interannual variability is usually larger than that of the CAM5 simulation.
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Affiliation(s)
- Hongbin Yu
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
| | - Yang Yang
- Pacific Northwest National Laboratory, Richland, WA, USA
| | - Hailong Wang
- Pacific Northwest National Laboratory, Richland, WA, USA
| | - Qian Tan
- Bay Area Environmental Research Institute, Petaluma, CA, USA
- NASA Ames Research Center, Moffett Field, CA, USA
| | - Mian Chin
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
| | - Robert C. Levy
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
| | - Lorraine A. Remer
- Joint Center for Earth Science & Technology, University of Maryland at Baltimore County, Baltimore, MD, USA
| | | | - Tianle Yuan
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
- Joint Center for Earth Science & Technology, University of Maryland at Baltimore County, Baltimore, MD, USA
| | - Yingxi Shi
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
- Joint Center for Earth Science & Technology, University of Maryland at Baltimore County, Baltimore, MD, USA
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Yu H, Tan Q, Chin M, Remer LA, Kahn RA, Bian H, Kim D, Zhang Z, Yuan T, Omar AH, Winker DM, Levy R, Kalashnikova O, Crepeau L, Capelle V, Chedin A. Estimates of African Dust Deposition Along the Trans-Atlantic Transit Using the Decade-long Record of Aerosol Measurements from CALIOP, MODIS, MISR, and IASI. JOURNAL OF GEOPHYSICAL RESEARCH. ATMOSPHERES : JGR 2019; 124:7975-7996. [PMID: 32637291 PMCID: PMC7340100 DOI: 10.1029/2019jd030574] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/01/2019] [Accepted: 06/29/2019] [Indexed: 05/11/2023]
Abstract
Deposition of mineral dust into ocean fertilizes ecosystems and influences biogeochemical cycles and climate. In-situ observations of dust deposition are scarce, and model simulations depend on the highly parameterized representations of dust processes with few constraints. By taking advantage of satellites' routine sampling on global and decadal scales, we estimate African dust deposition flux and loss frequency (LF, a ratio of deposition flux to mass loading) along the trans-Atlantic transit using the three-dimensional distributions of aerosol retrieved by spaceborne lidar (CALIOP) and radiometers (MODIS, MISR, and IASI). On the basis of a ten-year (2007-2016) and basin scale average, the amount of dust deposition into the tropical Atlantic Ocean is estimated at 136 - 222 Tg yr-1. The 65-83% of satellite-based estimates agree with the in-situ climatology within a factor of 2. The magnitudes of dust deposition are highest in boreal summer and lowest in fall, whereas the interannual variability as measured by the normalized standard deviation with mean is largest in spring (28-41%) and smallest (7-15%) in summer. The dust deposition displays high spatial heterogeneity, revealing that the meridional shifts of major dust deposition belts are modulated by the seasonal migration of the intertropical convergence zone (ITCZ). On the basis of the annual and basin mean, the dust LF derived from the satellite observations ranges from 0.078 to 0.100 d-1, which is lower than model simulations by up to factors of 2 to 5. The most efficient loss of dust occurs in winter, consistent with the higher possibility of low-altitude transported dust in southern trajectories being intercepted by rainfall associated with the ITCZ. The satellite-based estimates of dust deposition can be used to fill the geographical gaps and extend time span of in-situ measurements, study the dust-ocean interactions, and evaluate model simulations of dust processes.
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Affiliation(s)
- Hongbin Yu
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
| | - Qian Tan
- Bay Area Environmental Research Institute, Petaluma, California, USA
- Earth Science Division, NASA Ames Research Center, Moffett Field, California, USA
| | - Mian Chin
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
| | - Lorraine A Remer
- JCET, University of Maryland at Baltimore County, Baltimore, Maryland, USA
| | - Ralph A Kahn
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
| | - Huisheng Bian
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
- JCET, University of Maryland at Baltimore County, Baltimore, Maryland, USA
| | - Dongchul Kim
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
- GESTAR, Universities Space Research Association, Columbia, Maryland, USA
| | - Zhibo Zhang
- JCET, University of Maryland at Baltimore County, Baltimore, Maryland, USA
- Physics Department, University of Maryland at Baltimore County, Baltimore, Maryland, USA
| | - Tianle Yuan
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
- JCET, University of Maryland at Baltimore County, Baltimore, Maryland, USA
| | - Ali H Omar
- Earth Science Division, NASA Langley Research Center, Hampton, Virginia, USA
| | - David M Winker
- Earth Science Division, NASA Langley Research Center, Hampton, Virginia, USA
| | - Robert Levy
- Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
| | - Olga Kalashnikova
- Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
| | | | | | - Alain Chedin
- Laboratoire deMeteorologie Dynamique, Palaiseau, France
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Yuan T, Oreopoulos L, Platnick SE, Meyer K. Observations of Local Positive Low Cloud Feedback Patterns and Their Role in Internal Variability and Climate Sensitivity. GEOPHYSICAL RESEARCH LETTERS 2018; 45:4438-4445. [PMID: 30034051 PMCID: PMC6049961 DOI: 10.1029/2018gl077904] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/12/2018] [Revised: 04/11/2018] [Accepted: 04/15/2018] [Indexed: 06/08/2023]
Abstract
Modeling studies have shown that cloud feedbacks are sensitive to the spatial pattern of sea surface temperature (SST) anomalies, while cloud feedbacks themselves strongly influence the magnitude of SST anomalies. Observational counterparts to such patterned interactions are still needed. Here we show that distinct large-scale patterns of SST and low-cloud cover (LCC) emerge naturally from objective analyses of observations and demonstrate their close coupling in a positive local SST-LCC feedback loop that may be important for both internal variability and climate change. The two patterns that explain the maximum amount of covariance between SST and LCC correspond to the Interdecadal Pacific Oscillation and the Atlantic Multidecadal Oscillation, leading modes of multidecadal internal variability. Spatial patterns and time series of SST and LCC anomalies associated with both modes point to a strong positive local SST-LCC feedback. In many current climate models, our analyses suggest that SST-LCC feedback strength is too weak compared to observations. Modeled local SST-LCC feedback strength affects simulated internal variability so that stronger feedback produces more intense and more realistic patterns of internal variability. To the extent that the physics of the local positive SST-LCC feedback inferred from observed climate variability applies to future greenhouse warming, we anticipate significant amount of delayed warming because of SST-LCC feedback when anthropogenic SST warming eventually overwhelm the effects of internal variability that may mute anthropogenic warming over parts of the ocean. We postulate that many climate models may be underestimating both future warming and the magnitude of modeled internal variability because of their weak SST-LCC feedback.
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Affiliation(s)
- Tianle Yuan
- Joint Center for Earth Systems TechnologyUniversity of Maryland, Baltimore CountyBaltimoreMDUSA
- Earth Sciences DivisionNASA Goddard Space Flight CenterGreenbeltMDUSA
| | | | | | - Kerry Meyer
- Earth Sciences DivisionNASA Goddard Space Flight CenterGreenbeltMDUSA
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Williams RH, McGee D, Kinsley CW, Ridley DA, Hu S, Fedorov A, Tal I, Murray RW, deMenocal PB. Glacial to Holocene changes in trans-Atlantic Saharan dust transport and dust-climate feedbacks. SCIENCE ADVANCES 2016; 2:e1600445. [PMID: 28138515 PMCID: PMC5262466 DOI: 10.1126/sciadv.1600445] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/29/2016] [Accepted: 10/20/2016] [Indexed: 05/30/2023]
Abstract
Saharan mineral dust exported over the tropical North Atlantic is thought to have significant impacts on regional climate and ecosystems, but limited data exist documenting past changes in long-range dust transport. This data gap limits investigations of the role of Saharan dust in past climate change, in particular during the mid-Holocene, when climate models consistently underestimate the intensification of the West African monsoon documented by paleorecords. We present reconstructions of African dust deposition in sediments from the Bahamas and the tropical North Atlantic spanning the last 23,000 years. Both sites show early and mid-Holocene dust fluxes 40 to 50% lower than recent values and maximum dust fluxes during the deglaciation, demonstrating agreement with records from the northwest African margin. These quantitative estimates of trans-Atlantic dust transport offer important constraints on past changes in dust-related radiative and biogeochemical impacts. Using idealized climate model experiments to investigate the response to reductions in Saharan dust's radiative forcing over the tropical North Atlantic, we find that small (0.15°C) dust-related increases in regional sea surface temperatures are sufficient to cause significant northward shifts in the Atlantic Intertropical Convergence Zone, increased precipitation in the western Sahel and Sahara, and reductions in easterly and northeasterly winds over dust source regions. Our results suggest that the amplifying feedback of dust on sea surface temperatures and regional climate may be significant and that accurate simulation of dust's radiative effects is likely essential to improving model representations of past and future precipitation variations in North Africa.
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Affiliation(s)
- Ross H. Williams
- Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
| | - David McGee
- Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
| | - Christopher W. Kinsley
- Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
- Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
| | - David A. Ridley
- Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
| | - Shineng Hu
- Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA
| | - Alexey Fedorov
- Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA
| | - Irit Tal
- Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
| | - Richard W. Murray
- Department of Earth and Environment, Boston University, Boston, MA 02215, USA
| | - Peter B. deMenocal
- Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA
- Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
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