scholarly journals Sulfate Aerosol Control of Tropical Atlantic Climate over the Twentieth Century

2011 ◽  
Vol 24 (10) ◽  
pp. 2540-2555 ◽  
Author(s):  
C.-Y. Chang ◽  
J. C. H. Chiang ◽  
M. F. Wehner ◽  
A. R. Friedman ◽  
R. Ruedy
Keyword(s):  
Twentieth Century ◽  
Atlantic Multidecadal Oscillation ◽  
Forced Response ◽  
Sulfate Aerosol ◽  
Tropical Atlantic ◽  
Atlantic Sector ◽  
The North ◽  
Aerosol Forcing ◽  
Climate Forcings ◽  
Aerosol Emissions

Abstract The tropical Atlantic interhemispheric gradient in sea surface temperature significantly influences the rainfall climate of the tropical Atlantic sector, including droughts over West Africa and Northeast Brazil. This gradient exhibits a secular trend from the beginning of the twentieth century until the 1980s, with stronger warming in the south relative to the north. This trend behavior is on top of a multidecadal variation associated with the Atlantic multidecadal oscillation. A similar long-term forced trend is found in a multimodel ensemble of forced twentieth-century climate simulations. Through examining the distribution of the trend slopes in the multimodel twentieth-century and preindustrial models, the authors conclude that the observed trend in the gradient is unlikely to arise purely from natural variations; this study suggests that at least half the observed trend is a forced response to twentieth-century climate forcings. Further analysis using twentieth-century single-forcing runs indicates that sulfate aerosol forcing is the predominant cause of the multimodel trend. The authors conclude that anthropogenic sulfate aerosol emissions, originating predominantly from the Northern Hemisphere, may have significantly altered the tropical Atlantic rainfall climate over the twentieth century.

scholarly journals Reply to “Comment on ‘Rethinking the Lower Bound on Aerosol Radiative Forcing’”

2017 ◽  
Vol 30 (16) ◽  
pp. 6585-6589 ◽  
Author(s):  
Bjorn Stevens ◽  
Stephanie Fiedler
Keyword(s):  
Twentieth Century ◽  
Lower Bound ◽  
Radiative Forcing ◽  
Climate Models ◽  
Anthropogenic Aerosols ◽  
Anthropogenic Aerosol ◽  
Aerosol Radiative Forcing ◽  
Clear Sky ◽  
The North ◽  
Aerosol Forcing

Kretzschmar et al., in a comment in 2017, use the spread in the output of aerosol–climate models to argue that the models refute the hypothesis (presented in a paper by Stevens in 2015) that for the mid-twentieth-century warming to be consistent with observations, then the present-day aerosol forcing, [Formula: see text] must be less negative than −1 W m−2. The main point of contention is the nature of the relationship between global SO2 emissions and [Formula: see text] In contrast to the concave (log-linear) relationship used by Stevens and in earlier studies, whereby [Formula: see text] becomes progressively less sensitive to SO2 emissions, some models suggest a convex relationship, which would imply a less negative lower bound. The model that best exemplifies this difference, and that is most clearly in conflict with the hypothesis of Stevens, does so because of an implausible aerosol response to the initial rise in anthropogenic aerosol precursor emissions in East and South Asia—already in 1975 this model’s clear-sky reflectance from anthropogenic aerosol over the North Pacific exceeds present-day estimates of the clear-sky reflectance by the total aerosol. The authors perform experiments using a new (observationally constrained) climatology of anthropogenic aerosols to further show that the effects of changing patterns of aerosol and aerosol precursor emissions during the late twentieth century have, for the same global emissions, relatively little effect on [Formula: see text] These findings suggest that the behavior Kretzschmar et al. identify as being in conflict with the lower bound in Stevens arises from an implausible relationship between SO2 emissions and [Formula: see text] and thus provides little basis for revising this lower bound.

The Value of Initialization on Decadal Timescales: State-Dependent Predictability in the CESM Decadal Prediction Large Ensemble

2020 ◽  
Vol 33 (17) ◽  
pp. 7353-7370
Author(s):  
H. M. Christensen ◽  
J. Berner ◽  
S. Yeager
Keyword(s):  
Time Scale ◽  
Initial Conditions ◽  
Heat Content ◽  
Atlantic Multidecadal Oscillation ◽  
Forced Response ◽  
Climate Change Signal ◽  
Seasonal Forecasts ◽  
The North ◽  
State Dependent ◽  
Start Dates

AbstractInformation in decadal climate prediction arises from a well-initialized ocean state and from the predicted response to an external forcing. The length of time over which the initial conditions benefit the decadal forecast depends on the start date of the forecast. We characterize this state-dependent predictability for decadal forecasts of upper ocean heat content in the Community Earth System Model. We find regionally dependent initial condition predictability, with extended predictability generally observed in the extratropics. We also detect state-dependent predictability, with the year of loss of information from the initialization varying between start dates. The decadal forecasts in the North Atlantic show substantial information from the initial conditions beyond the 10-yr forecast window, and a high degree of state-dependent predictability. We find some evidence for state-dependent predictability in the ensemble spread in this region, similar to that seen in weather and subseasonal-to-seasonal forecasts. For some start dates, an increase of information with lead time is observed, for which the initialized forecasts predict a growing phase of the Atlantic multidecadal oscillation. Finally we consider the information in the forecast from the initial conditions relative to the forced response, and quantify the crossover time scale after which the forcing provides more information. We demonstrate that the climate change signal projects onto different patterns than the signal from the initial conditions. This means that even after the crossover time scale has been reached in a basin-averaged sense, the benefits of initialization can be felt locally on longer time scales.

scholarly journals Influence of the North Atlantic SST Variability on the Atmospheric Circulation during the Twentieth Century

2015 ◽  
Vol 28 (4) ◽  
pp. 1396-1416 ◽  
Author(s):  
Guillaume Gastineau ◽  
Claude Frankignoul
Keyword(s):  
Twentieth Century ◽  
North Atlantic ◽  
Lower Troposphere ◽  
Atlantic Multidecadal Oscillation ◽  
Early Winter ◽  
Tropical Forcing ◽  
The North ◽  
Sst Variability ◽  
Sst Anomaly ◽  
The North Atlantic

Abstract The ocean–atmosphere coupling in the North Atlantic is investigated during the twentieth century using maximum covariance analysis of sea surface temperature (SST) and 500-hPa geopotential height analyses and performing regressions on dynamical diagnostics such as Eady growth rate, wave activity flux, and velocity potential. The North Atlantic Oscillation (NAO) generates the so-called SST anomaly tripole. A rather similar SST anomaly tripole, with the subpolar anomaly displaced to the east and a more contracted subtropical anomaly, which is referred to as the North Atlantic horseshoe pattern, in turn influences the atmosphere. In the fall and early winter, the response is NAO like and primarily results from subpolar forcing centered over the Labrador Sea and off Newfoundland. In summer, the largest atmospheric response to SST resembles the east Atlantic pattern and results from a combination of subpolar and tropical forcing. To emphasize the interannual to multidecadal variability, the same analysis is repeated after low-pass filtering. The SST influence is dominated by the Atlantic multidecadal oscillation (AMO), which also has a horseshoe shape, but with larger amplitude in the subpolar basin. A warm AMO phase leads to an atmospheric warming limited to the lower troposphere in summer, while it leads to a negative phase of the NAO in winter. The winter influence of the AMO is suggested to be primarily forced by the Atlantic SSTs in the northern subtropics. Such influence of the AMO is found in winter instead of early winter because the winter SST anomalies have a larger persistence, presumably because of SST reemergence.

scholarly journals Aerosol-forced multidecadal variations across all ocean basins in models and observations since 1920

2020 ◽  
Vol 6 (29) ◽  
pp. eabb0425 ◽  
Author(s):  
Minhua Qin ◽  
Aiguo Dai ◽  
Wenjian Hua
Keyword(s):  
North Atlantic ◽  
Atlantic Multidecadal Oscillation ◽  
Anthropogenic Aerosols ◽  
Surface Solar Radiation ◽  
The North ◽  
Aerosol Forcing ◽  
New Finding ◽  
The North Atlantic ◽  
Earth’S Climate ◽  
Aerosol Effects

Earth’s climate fluctuates considerably on decadal-multidecadal time scales, often causing large damages to our society and environment. These fluctuations usually result from internal dynamics, and many studies have linked them to internal climate modes in the North Atlantic and Pacific oceans. Here, we show that variations in volcanic and anthropogenic aerosols have caused in-phase, multidecadal SST variations since 1920 across all ocean basins. These forced variations resemble the Atlantic Multidecadal Oscillation (AMO) in time. Unlike the North Atlantic, where indirect and direct aerosol effects on surface solar radiation drive the multidecadal SST variations, over the tropical central and western Pacific atmospheric circulation response to aerosol forcing plays an important role, whereas aerosol-induced radiation change is small. Our new finding implies that AMO-like climate variations in Eurasia, North America, and other regions may be partly caused by the aerosol forcing, rather than being originated from the North Atlantic SST variations as previously thought.

scholarly journals The Atlantic Multidecadal Oscillation Inferred from the Forced Climate Response in Coupled General Circulation Models

2009 ◽  
Vol 22 (7) ◽  
pp. 1610-1625 ◽  
Author(s):  
Jeff R. Knight
Keyword(s):  
North Atlantic ◽  
General Circulation ◽  
Internal Variability ◽  
Atlantic Multidecadal Oscillation ◽  
Forced Response ◽  
Climate System ◽  
Climate Response ◽  
The North ◽  
Ensemble Mean ◽  
The North Atlantic

Abstract Instrumental sea surface temperature records in the North Atlantic Ocean are characterized by large multidecadal variability known as the Atlantic multidecadal oscillation (AMO). The lack of strong oscillatory forcing of the climate system at multidecadal time scales and the results of long unforced climate simulations have led to the widespread, although not ubiquitous, view that the AMO is an internal mode of climate variability. Here, a more objective examination of this hypothesis is performed using simulations with natural and anthropogenic forcings from the Coupled Model Intercomparison Project phase 3 (CMIP3) database. Ensemble means derived from these data allow an estimate of the response of models to forcings, as averaging leads to cancellation of the internal variability between ensemble members. In general, the means of individual model ensembles appear to be inconsistent with observed temperatures, although small ensemble sizes result in uncertainty in this conclusion. Combining the ensembles from different models creates a multimodel ensemble of sufficient size to allow for a good estimate of the forced response. This shows that the variability in observed North Atlantic temperatures possess a clearly distinct signature to the climate response expected from forcings. The reliability of this finding is confirmed by sampling those models with low decadal internal variability and by comparing simulated and observed trends. In contrast to the inconsistency with the ensemble mean, the observations are consistent with the spread of responses in the ensemble members, suggesting they can be accounted for by the combined effects of forcings and internal variability. In the most recent period, the results suggest that the North Atlantic is warming faster than expected, and that the AMO entered a positive phase in the 1990s. The differences found between observed and ensemble mean temperatures could arise through errors in the observational data, errors in the models’ response to forcings or in the forcings themselves, or as a result of genuine internal variability. Each of these possibilities is discussed, and it is concluded that internal variability within the natural climate system is the most likely origin of the differences. Finally, the estimate of internal variability obtained using the model-derived ensemble mean is proposed as a new way of defining the AMO, which has important advantages over previous definitions.

scholarly journals A Limited Role for Unforced Internal Variability in Twentieth-Century Warming

2019 ◽  
Vol 32 (16) ◽  
pp. 4893-4917 ◽  
Author(s):  
Karsten Haustein ◽  
Friederike E. L. Otto ◽  
Victor Venema ◽  
Peter Jacobs ◽  
Kevin Cowtan ◽  
...  
Keyword(s):  
Twentieth Century ◽  
Surface Temperature ◽  
Radiative Forcing ◽  
Low Frequency ◽  
Internal Variability ◽  
External Forcing ◽  
Anthropogenic Aerosol ◽  
Transient Climate Response ◽  
The North ◽  
Aerosol Forcing

AbstractThe early twentieth-century warming (EW; 1910–45) and the mid-twentieth-century cooling (MC; 1950–80) have been linked to both internal variability of the climate system and changes in external radiative forcing. The degree to which either of the two factors contributed to EW and MC, or both, is still debated. Using a two-box impulse response model, we demonstrate that multidecadal ocean variability was unlikely to be the driver of observed changes in global mean surface temperature (GMST) after AD 1850. Instead, virtually all (97%–98%) of the global low-frequency variability (>30 years) can be explained by external forcing. We find similarly high percentages of explained variance for interhemispheric and land–ocean temperature evolution. Three key aspects are identified that underpin the conclusion of this new study: inhomogeneous anthropogenic aerosol forcing (AER), biases in the instrumental sea surface temperature (SST) datasets, and inadequate representation of the response to varying forcing factors. Once the spatially heterogeneous nature of AER is accounted for, the MC period is reconcilable with external drivers. SST biases and imprecise forcing responses explain the putative disagreement between models and observations during the EW period. As a consequence, Atlantic multidecadal variability (AMV) is found to be primarily controlled by external forcing too. Future attribution studies should account for these important factors when discriminating between externally forced and internally generated influences on climate. We argue that AMV must not be used as a regressor and suggest a revised AMV index instead [the North Atlantic Variability Index (NAVI)]. Our associated best estimate for the transient climate response (TCR) is 1.57 K (±0.70 at the 5%–95% confidence level).

scholarly journals Impacts of the Aleutian–Icelandic Low Seesaw on Surface Climate during the Twentieth Century

2005 ◽  
Vol 18 (14) ◽  
pp. 2793-2802 ◽  
Author(s):  
Meiji Honda ◽  
Shozo Yamane ◽  
Hisashi Nakamura
Keyword(s):  
Twentieth Century ◽  
North Atlantic ◽  
North Pacific ◽  
Early Period ◽  
Recent Period ◽  
Atlantic Sector ◽  
The North ◽  
Surface Climate ◽  
The North Atlantic ◽  
The North Pacific

Abstract An interannual seesaw between the intensities of the Icelandic and Aleutian lows and its impact on surface climate observed during the twentieth century are investigated. In a recent period from the late 1960s to the early 1990s, their seesaw relationship was particularly apparent in late winter. The associated anomalies in surface air temperature were significant in many regions over the extratropical Northern Hemisphere except in central portions of the continents. The seesaw also modified the ocean–atmosphere exchange of heat and moisture extensively over the North Atlantic and North Pacific by changing evaporation and precipitation. Since the seesaw formation was triggered by eastward propagation of stationary Rossby wave trains from the North Pacific into the North Atlantic, anomalous circulation over the North Pacific in January was identified as a good precursor for February surface air temperatures in the Euro–Atlantic sector during that period. The seesaw relationship between the two lows underwent multidecadal modulations during the twentieth century. It was weak in the mid-1950s through the mid-1960s, while it was particularly strong during the preceding period from the 1920s to the 1940s with its impact on surface temperatures as extensive as in the recent period. Although it reached maturity in January, the precursory signal of the seesaw in that early period was also found in the North Pacific one month earlier, which suggests that the formation was through essentially the same mechanisms as in the recent period.

scholarly journals Importance of Late Fall ENSO Teleconnection in the Euro-Atlantic Sector

2018 ◽  
Vol 99 (7) ◽  
pp. 1337-1343 ◽  
Author(s):  
Martin P. King ◽  
Ivana Herceg-Bulić ◽  
Ileana Bladé ◽  
Javier García-Serrano ◽  
Noel Keenlyside ◽  
...  
Keyword(s):  
Southern Oscillation ◽  
Wave Train ◽  
Climate Impact ◽  
Atlantic Sector ◽  
The North ◽  
Temperature And Precipitation ◽  
Late Fall ◽  
Rossby Wave Train ◽  
Climate Forcings ◽  
European Sector

AbstractRecent studies have indicated the importance of fall climate forcings and teleconnections in influencing the climate of the northern mid- to high latitudes. Here, we present some exploratory analyses using observational data and seasonal hindcasts, with the aim of highlighting the potential of the El Niño–Southern Oscillation (ENSO) as a driver of climate variability during boreal late fall and early winter (November and December) in the North Atlantic–European sector, and motivating further research on this relatively unexplored topic. The atmospheric ENSO teleconnection in November and December is reminiscent of the east Atlantic pattern and distinct from the well-known arching extratropical Rossby wave train found from January to March. Temperature and precipitation over Europe in November are positively correlated with the Niño-3.4 index, which suggests a potentially important ENSO climate impact during late fall. In particular, the ENSO-related temperature anomaly extends over a much larger area than during the subsequent winter months. We discuss the implications of these results and pose some research questions.

scholarly journals Long-Term Behavior of the Atlantic Interhemispheric SST Gradient in the CMIP5 Historical Simulations

2013 ◽  
Vol 26 (21) ◽  
pp. 8628-8640 ◽  
Author(s):  
John C. H. Chiang ◽  
C.-Y. Chang ◽  
M. F. Wehner
Keyword(s):  
Twentieth Century ◽  
Coupled Model ◽  
Atlantic Multidecadal Oscillation ◽  
Forced Response ◽  
Cmip5 Models ◽  
Anthropogenic Aerosols ◽  
Anthropogenic Aerosol ◽  
Global Environmental ◽  
Late Twentieth ◽  
Hadley Centre

Abstract Multidecadal and longer changes to the Atlantic interhemispheric sea surface temperature gradient (AITG) in phase 5 of the Coupled Model Intercomparison Project (CMIP5) historical simulations are investigated. Observations show a secular trend to this gradient over most of the twentieth century, with the southern lobe warming faster relative to its northern counterpart. A previous study of phase 3 of the CMIP (CMIP3) suggests that this trend is partially forced by anthropogenic sulfate aerosols. This analysis collectively confirms the partially forced trend for the CMIP5 and by anthropogenic aerosols. Like the CMIP3, the CMIP5 also simulates a reversal in the AITG trend in the late 1970s, which was attributed to a leveling off of the anthropogenic aerosol influence and increased influence of greenhouse gases in the late twentieth century. Two (of 25) CMIP5 models, however, systematically simulate a twentieth-century trend opposite to observed, leading to some uncertainty regarding the forced nature of the AITG trend. The observed AITG also exhibits a pronounced multidecadal modulation on top of the trend, associated with the Atlantic multidecadal oscillation (AMO). Motivated by a recent suggestion that the AMO is a forced response to aerosols, the causes of this multidecadal behavior were also examined. A few of the CMIP5 models analyzed do produce multidecadal AITG variations that are correlated to the observed AMO-like variation, but only one, the Hadley Centre Global Environmental Model, version 2 (HadGEM2), systematically simulates AMO-like behavior with both the requisite amplitude and phase. The CMIP5 simulations thus point to a robust aerosol influence on the historical AITG trend but not to the AMO-like multidecadal behavior.

scholarly journals Observed Low-Frequency Covariabilities between the Tropical Oceans and the North Atlantic Oscillation in the Twentieth Century

2006 ◽  
Vol 19 (6) ◽  
pp. 1032-1041 ◽  
Author(s):  
Martin P. King ◽  
Fred Kucharski
Keyword(s):  
Twentieth Century ◽  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Low Frequency ◽  
Reanalysis Data ◽  
Tropical Atlantic ◽  
Maximum Covariance Analysis ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract The low-frequency covariabilities of tropical sea surface temperature (SST) and the North Atlantic Oscillation (NAO) during twentieth-century winters are investigated by maximum covariance analysis (MCA) using reanalysis data. It was found that the positive NAO phase is positively correlated to an increase in tropical SST, especially during the recent decades. The western tropical Pacific SST displays high correlation with the NAO throughout the whole of the twentieth century. For this ocean region, the MCA homogeneous map has a SST spatial pattern with meridional gradients. It was also found that a cooling of tropical Atlantic SST is correlated with positive NAO. The influence of the tropical Atlantic SST on the NAO is strongest during the pre-1960s period.

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