scholarly journals Anthropogenic aerosols modulated twentieth‐century Sahel rainfall variability via their impacts on North Atlantic sea surface temperature

2021 ◽  
Author(s):  
Shipeng Zhang ◽  
Philip Stier ◽  
Guy Dagan ◽  
Minghuai Wang
Keyword(s):  
Twentieth Century ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Rainfall Variability ◽  
Sea Surface ◽  
Anthropogenic Aerosols ◽  
Sahel Rainfall

Anthropogenic aerosols modulated twentieth-century Sahel rainfall variability via impacts on North Atlantic sea surface temperature

2021 ◽  
Author(s):  
Shipeng Zhang ◽  
Philip Stier ◽  
Guy Dagan ◽  
Minghuai Wang
Keyword(s):  
Twentieth Century ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Rainfall Variability ◽  
Hadley Cell ◽  
Sea Surface ◽  
Anthropogenic Aerosols ◽  
Multidecadal Variability ◽  
Sahel Rainfall

<p>Sahel rainfall experienced significant multidecadal variability over the twentieth century. Previous work have proposed several drivers to explain the severe drought and the subsequent recovery of Sahel rainfall in the past century, including anthropogenic aerosols, GHGs, and internal variabilities. However, the attribution remained ambiguous. Sahel summertime monsoon has close teleconnections with North Atlantic sea surface temperature (NASST) variability, which has been proven to be affected by aerosols. Therefore, changes in regional aerosols emission can potentially drive multidecadal Sahel rainfall variability.</p><p>Here we use ensembles of state-of-the-art global climate models (the CESM-large ensemble and CMIP6 models) and observational datasets to demonstrate that anthropogenic aerosols have significant impacts on twentieth-century Sahel rainfall multidecadal variability through modifying NASST. Aerosol-induced multidecadal variations of downward solar fluxes over the North Atlantic Ocean cause NASST variability during the 20<sup>th</sup> century, altering the strength of the Hadley cell and the ITCZ position, therefore, dynamically linking aerosol effects to Sahel rainfall variability. While the observed linear trend of NASST might still be affected by a mix of various external and internal drivers, our results suggest that NASST variability is most likely caused by aerosol-induced changes in radiative fluxes rather than changes in ocean circulations, and that anthropogenic aerosols can explain most of the detrended Sahel rainfall variability. CMIP6 models further suggest that aerosol-cloud interactions contributed more to the variability than aerosol-radiation interactions. These findings highlight the importance of accurate representation of regional aerosol radiative effects for the simulation of Sahel rainfall variability.</p>

scholarly journals The Role of Forcings in the Twentieth-Century North Atlantic Multidecadal Variability: The 1940–75 North Atlantic Cooling Case Study

2017 ◽  
Vol 30 (18) ◽  
pp. 7317-7337 ◽  
Author(s):  
A. Bellucci ◽  
A. Mariotti ◽  
S. Gualdi
Keyword(s):  
Twentieth Century ◽  
Greenhouse Gases ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
Sea Surface ◽  
Atlantic Multidecadal Variability ◽  
Anthropogenic Aerosols ◽  
Multimodel Ensemble ◽  
Multidecadal Variability

Abstract Results from a study inspecting the origins of multidecadal variability in the North Atlantic sea surface temperature (NASST) are presented. The authors target in particular the 1940–75 “warm-to-cold” transition, an event that is generally framed in the context of the longer-term Atlantic multidecadal variability (AMV) cycle, in turn associated with the Atlantic meridional overturning circulation (AMOC) internal variability. Here the authors examine the ability of uninitialized, historical integrations from the phase 5 of the Coupled Model Intercomparison Project (CMIP5) archive to retrospectively reproduce this specific episode of twentieth-century climatic history, under a hierarchy of forcing conditions. For this purpose, both standard and so-called historical Misc CMIP5 simulations of the historical climate (combining selected natural and anthropogenic forcings) are exploited. Based on this multimodel analysis, evidence is found for a significant influence of anthropogenic agents on multidecadal sea surface temperature (SST) fluctuations across the Atlantic sector, suggesting that anthropogenic aerosols and greenhouse gases might have played a key role in the 1940–75 North Atlantic cooling. However, the diagnosed forced response in CMIP5 models appears to be affected by a large uncertainty, with only a limited subset of models displaying significant skill in reproducing the mid-twentieth-century NASST cooling. Such uncertainty originates from the existence of well-defined behavioral clusters within the analyzed CMIP5 ensembles, with the bulk of the models splitting into two main clusters. Such a strong polarization calls for some caution when using a multimodel ensemble mean in climate model analyses, as averaging across fairly distinct model populations may result, through mutual cancellation, in a rather artificial description of the actual multimodel ensemble behavior. A potentially important role for both anthropogenic aerosols and greenhouse gases with regard to the observed North Atlantic multidecadal variability has clear implications for decadal predictability and predictions. The uncertainty associated with alternative aerosol and greenhouse gas emission scenarios should be duly accounted for in designing a common protocol for coordinated decadal forecast experiments.

Global Monsoon Responses to Decadal Sea Surface Temperature Variations during the Twentieth Century: Evidence from AGCM Simulations

2019 ◽  
Vol 32 (22) ◽  
pp. 7675-7695 ◽  
Author(s):  
Jie Jiang ◽  
Tianjun Zhou
Keyword(s):  
Twentieth Century ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Atmospheric Circulation ◽  
North Atlantic ◽  
Sea Surface ◽  
Monsoon Precipitation ◽  
The North ◽  
The North Atlantic ◽  
Sst Anomalies

Abstract Multidecadal variations in the global land monsoon were observed during the twentieth century, with an overall increasing trend from 1901 to 1955 that was followed by a decreasing trend up to 1990, but the mechanisms governing the above changes remain inconclusive. Based on the outputs of two atmospheric general circulation models (AGCMs) forced by historical sea surface temperature (SST) covering the twentieth century, supplemented with AGCM simulations forced by idealized SST anomalies representing different conditions of the North Atlantic and tropical Pacific, evidence shows that the observed changes can be partly reproduced, particularly over the Northern Hemisphere summer monsoon (NHSM) domain, demonstrating the modulation of decadal SST changes on the long-term variations in monsoon precipitation. Moisture budget analysis is performed to understand the interdecadal changes in monsoon precipitation, and the dynamic term associated with atmospheric circulation changes is found to be prominent, while the contribution of the thermodynamic term associated with humidity changes can lead to coincident wetting over the NHSM domain. The increase (decrease) in NHSM land precipitation during 1901–55 (1956–90) is associated with the strengthening (weakening) of NHSM circulation and Walker circulation. The multidecadal scale changes in atmospheric circulation are driven by SST anomalies over the North Atlantic and the Pacific. A warmer North Atlantic together with a colder eastern tropical Pacific and a warmer western subtropical Pacific can lead to a strengthened meridional gradient in mid-to-upper-tropospheric thickness and strengthened trade winds, which transport more water vapor into monsoon regions, leading to an increase in monsoon precipitation.

scholarly journals A spatiotemporal reconstruction of sea-surface temperatures in the North Atlantic during Dansgaard–Oeschger events 5–8

2018 ◽  
Vol 14 (6) ◽  
pp. 901-922 ◽  
Author(s):  
Mari F. Jensen ◽  
Aleksi Nummelin ◽  
Søren B. Nielsen ◽  
Henrik Sadatzki ◽  
Evangeline Sessford ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
North Atlantic ◽  
General Circulation ◽  
Sea Surface ◽  
Reconstruction Method ◽  
Sea Surface Temperatures ◽  
Proxy Data ◽  
Surface Temperatures ◽  
Temperature Reconstructions

Abstract. Here, we establish a spatiotemporal evolution of the sea-surface temperatures in the North Atlantic over Dansgaard–Oeschger (DO) events 5–8 (approximately 30–40 kyr) using the proxy surrogate reconstruction method. Proxy data suggest a large variability in North Atlantic sea-surface temperatures during the DO events of the last glacial period. However, proxy data availability is limited and cannot provide a full spatial picture of the oceanic changes. Therefore, we combine fully coupled, general circulation model simulations with planktic foraminifera based sea-surface temperature reconstructions to obtain a broader spatial picture of the ocean state during DO events 5–8. The resulting spatial sea-surface temperature patterns agree over a number of different general circulation models and simulations. We find that sea-surface temperature variability over the DO events is characterized by colder conditions in the subpolar North Atlantic during stadials than during interstadials, and the variability is linked to changes in the Atlantic Meridional Overturning circulation and in the sea-ice cover. Forced simulations are needed to capture the strength of the temperature variability and to reconstruct the variability in other climatic records not directly linked to the sea-surface temperature reconstructions. This is the first time the proxy surrogate reconstruction method has been applied to oceanic variability during MIS3. Our results remain robust, even when age uncertainties of proxy data, the number of available temperature reconstructions, and different climate models are considered. However, we also highlight shortcomings of the methodology that should be addressed in future implementations.

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