The Evolving Role of External Forcing in North Atlantic SST Variability Over the Last Millennium

2022 ◽  
pp. 1-44
Keyword(s):  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Transitional Period ◽  
External Forcing ◽  
Sst Variability ◽  
Instrumental Observations ◽  
Large Ensembles ◽  
Sea Surface Temperature Anomalies ◽  
Active Debate

Abstract Atlantic Multidecadal Variability (AMV) impacts temperature, precipitation, and extreme events on both sides of the Atlantic basin. Previous studies with climate models have suggested that when external radiative forcing is held constant, the large-scale ocean and atmosphere circulation are associated with sea-surface temperature anomalies that have similar characteristics to the observed AMV. However, there is an active debate as to whether these internal fluctuations driven by coupled atmosphere-ocean variability remain influential to the AMV on multidecadal timescales in our modern, anthropogenically-forced climate. Here we provide evidence from multiple large ensembles of climate models, paleo reconstructions, and instrumental observations of a growing role for external forcing in the AMV. Prior to 1850, external forcing, primarily from volcanoes, explains about one third of AMV variance. Between 1850 and 1950, there is a transitional period, where external forcing explains half of AMV variance, but volcanic forcing only accounts for about 10% of that. After 1950, external forcing explains three quarters of AMV variance. That is, the role for external forcing in the AMV grows as the variations in external forcing grow, even if the forcing is from different sources. When forcing is relatively stable, as in earlier modeling studies, a higher percentage of AMV variations are internally generated.

scholarly journals Relationship between Shortwave Cloud Radiative Forcing and Local Meteorological Variables Compared in Observations and Several Global Climate Models

2006 ◽  
Vol 19 (17) ◽  
pp. 4344-4359 ◽  
Author(s):  
Markus Stowasser ◽  
Kevin Hamilton
Keyword(s):  
Relative Humidity ◽  
Vertical Velocity ◽  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Grid Point ◽  
Global Climate Models ◽  
Coupled Models ◽  
Cloud Radiative Forcing ◽  
Cloud Forcing

Abstract The relations between local monthly mean shortwave cloud radiative forcing and aspects of the resolved-scale meteorological fields are investigated in hindcast simulations performed with 12 of the global coupled models included in the model intercomparison conducted as part of the preparation for Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In particular, the connection of the cloud forcing over tropical and subtropical ocean areas with resolved midtropospheric vertical velocity and with lower-level relative humidity are investigated and compared among the models. The model results are also compared with observational determinations of the same relationships using satellite data for the cloud forcing and global reanalysis products for the vertical velocity and humidity fields. In the analysis the geographical variability in the long-term mean among all grid points and the interannual variability of the monthly mean at each grid point are considered separately. The shortwave cloud radiative feedback (SWCRF) plays a crucial role in determining the predicted response to large-scale climate forcing (such as from increased greenhouse gas concentrations), and it is thus important to test how the cloud representations in current climate models respond to unforced variability. Overall there is considerable variation among the results for the various models, and all models show some substantial differences from the comparable observed results. The most notable deficiency is a weak representation of the cloud radiative response to variations in vertical velocity in cases of strong ascending or strong descending motions. While the models generally perform better in regimes with only modest upward or downward motions, even in these regimes there is considerable variation among the models in the dependence of SWCRF on vertical velocity. The largest differences between models and observations when SWCRF values are stratified by relative humidity are found in either very moist or very dry regimes. Thus, the largest errors in the model simulations of cloud forcing are prone to be in the western Pacific warm pool area, which is characterized by very moist strong upward currents, and in the rather dry regions where the flow is dominated by descending mean motions.

scholarly journals Atmospheric Dynamics Feedback: Concept, Simulations, and Climate Implications

2018 ◽  
Vol 31 (8) ◽  
pp. 3249-3264 ◽  
Author(s):  
Michael P. Byrne ◽  
Tapio Schneider
Keyword(s):  
Climate Change ◽  
Atmospheric Circulation ◽  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Atmospheric Dynamics ◽  
Future Climate Change ◽  
Coupled Models ◽  
Near Surface ◽  
Circulation Changes

AbstractThe regional climate response to radiative forcing is largely controlled by changes in the atmospheric circulation. It has been suggested that global climate sensitivity also depends on the circulation response, an effect called the “atmospheric dynamics feedback.” Using a technique to isolate the influence of changes in atmospheric circulation on top-of-the-atmosphere radiation, the authors calculate the atmospheric dynamics feedback in coupled climate models. Large-scale circulation changes contribute substantially to all-sky and cloud feedbacks in the tropics but are relatively less important at higher latitudes. Globally averaged, the atmospheric dynamics feedback is positive and amplifies the near-surface temperature response to climate change by an average of 8% in simulations with coupled models. A constraint related to the atmospheric mass budget results in the dynamics feedback being small on large scales relative to feedbacks associated with thermodynamic processes. Idealized-forcing simulations suggest that circulation changes at high latitudes are potentially more effective at influencing global temperature than circulation changes at low latitudes, and the implications for past and future climate change are discussed.

scholarly journals Variable External Forcing Obscures the Weak Relationship between the NAO and North Atlantic Multidecadal SST Variability

2019 ◽  
Vol 32 (13) ◽  
pp. 3847-3864 ◽  
Author(s):  
Jeremy M. Klavans ◽  
Amy C. Clement ◽  
Mark A. Cane
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
External Forcing ◽  
Atlantic Multidecadal Variability ◽  
The North ◽  
Sst Variability ◽  
Atmospheric Noise ◽  
The North Atlantic Oscillation ◽  
The Relationship ◽  
Spurious Signals

Abstract North Atlantic sea surface temperatures (SST) exhibit a lagged response to the North Atlantic Oscillation (NAO) in both models and observations, which has previously been attributed to changes in ocean heat transport. Here we examine the lagged relationship between the NAO and Atlantic multidecadal variability (AMV) in the context of the two other major components of the AMV: atmospheric noise and external forcing. In preindustrial control runs, we generally find that after accounting for spurious signals introduced by filtering, the SST response to the NAO is only statistically significant in the subpolar gyre. Further, the lagged SST response to the NAO is small in magnitude and offers a limited contribution to the AMV pattern, statistics, or predictability. When climate models include variable external forcing, the relationship between the NAO and AMV is obscured and becomes inconsistent. In these historically forced runs, knowledge of the prior NAO offers reduced predictability. The differences between the preindustrial and the historically forced ensembles suggest that we do not yet have enough observational data to surmise the true NAO–AMV relationship and add evidence that external forcing plays a substantial role in producing the AMV.

scholarly journals Investigating the Effects of Subgrid Cell Dynamic Heterogeneity on the Large-Scale Modeling of Albedo in Boreal Forests*

2016 ◽  
Vol 20 (5) ◽  
pp. 1-23 ◽  
Author(s):  
Jean-Sébastien Landry ◽  
Navin Ramankutty ◽  
Lael Parrott
Keyword(s):  
Boreal Forests ◽  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Computing Time ◽  
Grid Cell ◽  
Tree Cover ◽  
Similar Amount ◽  
Earth System ◽  
Dynamic Heterogeneity

Abstract Stand-clearing disturbances, which remove most of the tree cover but are followed by forest regrowth, affect extensive areas annually, yet each event is usually much smaller than a typical grid cell in Earth system climate models. This study argues that the approach taken to account for the resulting subgrid cell dynamic heterogeneity substantially affects the computation of land–atmosphere exchanges. The authors investigated in a simplified model the effects of three such approaches on the computation of albedo over boreal forests. It was found that the simplest approach—in which any new disturbance-created patch was immediately merged with the rest of the grid cell—underestimated the annual reflected solar radiation by ~3 W m−2 on average (a relative error of 15%) compared with the most accurate approach—in which albedo computations were performed for each individual subgrid patch. This study also investigated an intermediate approach, in which each patch was tracked individually, but albedo was estimated from a much smaller number of subgrid tiles grouping patches having a similar amount of tree cover. Results from this third approach converged quickly toward the most accurate results as the number of tiles increased and were robust to changes in the thresholds used to assign patches to specific tiles. When computing time prevents implementing the most accurate approach in Earth system climate models, the results advocate for using strategies similar to the intermediate approach in order to avoid biasing the net radiative forcing of stand-clearing disturbances toward a warming impact, at least over boreal forests.

scholarly journals Process-oriented analysis of aircraft soot-cirrus interactions constrains the climate impact of aviation

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
Bernd Kärcher ◽  
Fabian Mahrt ◽  
Claudia Marcolli
Keyword(s):  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Climate Model ◽  
Global Climate ◽  
Climate Impact ◽  
Global Climate Models ◽  
Soot Particles ◽  
Cloud Ice ◽  
The Impact

AbstractFully accounting for the climate impact of aviation requires a process-level understanding of the impact of aircraft soot particle emissions on the formation of ice clouds. Assessing this impact with the help of global climate models remains elusive and direct observations are lacking. Here we use a high-resolution cirrus column model to investigate how aircraft-emitted soot particles, released after ice crystals sublimate at the end of the lifetime of contrails and contrail cirrus, perturb the formation of cirrus. By allying cloud simulations with a measurement-based description of soot-induced ice formation, we find that only a small fraction (<1%) of the soot particles succeeds in forming cloud ice alongside homogeneous freezing of liquid aerosol droplets. Thus, soot-perturbed and homogeneously-formed cirrus fundamentally do not differ in optical depth. Our results imply that climate model estimates of global radiative forcing from interactions between aircraft soot and large-scale cirrus may be overestimates. The improved scientific understanding reported here provides a process-based underpinning for improved climate model parametrizations and targeted field observations.

scholarly journals Integrated approaches to climate–crop modelling: needs and challenges

2005 ◽  
Vol 360 (1463) ◽  
pp. 2049-2065 ◽  
Author(s):  
Richard A. Betts
Keyword(s):  
Climate Change ◽  
Atmospheric Chemistry ◽  
Land Surface ◽  
Radiative Forcing ◽  
Crop Production ◽  
Large Scale ◽  
Climate Models ◽  
Atmospheric Composition ◽  
Short Time Scale ◽  
Crop Models

This paper discusses the need for a more integrated approach to modelling changes in climate and crops, and some of the challenges posed by this. While changes in atmospheric composition are expected to exert an increasing radiative forcing of climate change leading to further warming of global mean temperatures and shifts in precipitation patterns, these are not the only climatic processes which may influence crop production. Changes in the physical characteristics of the land cover may also affect climate; these may arise directly from land use activities and may also result from the large-scale responses of crops to seasonal, interannual and decadal changes in the atmospheric state. Climate models used to drive crop models may, therefore, need to consider changes in the land surface, either as imposed boundary conditions or as feedbacks from an interactive climate–vegetation model. Crops may also respond directly to changes in atmospheric composition, such as the concentrations of carbon dioxide (CO 2 ), ozone (O 3 ) and compounds of sulphur and nitrogen, so crop models should consider these processes as well as climate change. Changes in these, and the responses of the crops, may be intimately linked with meteorological processes so crop and climate models should consider synergies between climate and atmospheric chemistry. Some crop responses may occur at scales too small to significantly influence meteorology, so may not need to be included as feedbacks within climate models. However, the volume of data required to drive the appropriate crop models may be very large, especially if short-time-scale variability is important. Implementation of crop models within climate models would minimize the need to transfer large quantities of data between separate modelling systems. It should also be noted that crop responses to climate change may interact with other impacts of climate change, such as hydrological changes. For example, the availability of water for irrigation may be affected by changes in runoff as a direct consequence of climate change, and may also be affected by climate-related changes in demand for water for other uses. It is, therefore, necessary to consider the interactions between the responses of several impacts sectors to climate change. Overall, there is a strong case for a much closer coupling between models of climate, crops and hydrology, but this in itself poses challenges arising from issues of scale and errors in the models. A strategy is proposed whereby the pursuit of a fully coupled climate–chemistry–crop–hydrology model is paralleled by continued use of separate climate and land surface models but with a focus on consistency between the models.

scholarly journals The Dynamical Influence of the Atlantic Multidecadal Oscillation on Continental Climate

2017 ◽  
Vol 30 (18) ◽  
pp. 7213-7230 ◽  
Author(s):  
Christopher H. O’Reilly ◽  
Tim Woollings ◽  
Laure Zanna
Keyword(s):  
Atmospheric Circulation ◽  
Large Scale ◽  
Climate Models ◽  
Surface Air Temperature ◽  
Atlantic Multidecadal Oscillation ◽  
The North ◽  
Sst Variability ◽  
Western Sahel ◽  
Prediction Systems ◽  
Sahel Region

Abstract The Atlantic multidecadal oscillation (AMO) in sea surface temperature (SST) has been shown to influence the climate of the surrounding continents. However, it is unclear to what extent the observed impact of the AMO is related to the thermodynamical influence of the SST variability or the changes in large-scale atmospheric circulation. Here, an analog method is used to decompose the observed impact of the AMO into dynamical and residual components of surface air temperature (SAT) and precipitation over the adjacent continents. Over Europe the influence of the AMO is clearest during the summer, when the warm SAT anomalies are interpreted to be primarily thermodynamically driven by warm upstream SST anomalies but also amplified by the anomalous atmospheric circulation. The overall precipitation response to the AMO in summer is generally less significant than the SAT but is mostly dynamically driven. The decomposition is also applied to the North American summer and the Sahel rainy season. Both dynamical and residual influences on the anomalous precipitation over the Sahel are substantial, with the former dominating over the western Sahel region and the latter being largest over the eastern Sahel region. The results have potential implications for understanding the spread in AMO variability in coupled climate models and decadal prediction systems.

scholarly journals Tree-ring oxygen isotope based inferences on winter and summer moisture dynamics over the glacier valleys of Central Himalaya

2021 ◽  
Author(s):  
Nilendu Singh ◽  
Mayank Shekhar ◽  
Bikash Ranjan Parida ◽  
Anil K. Gupta ◽  
Kalachand Sain ◽  
...  
Keyword(s):  
Phase Shift ◽  
Tree Ring ◽  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Summer Precipitation ◽  
Summer Season ◽  
Glacier Mass Balance ◽  
Central Himalaya ◽  
The 1960S

Abstract. Accelerated glacier mass loss is primarily attributed to greenhouse-induced warming, but land–climate interaction has increasingly been recognized as an important forcing at the regional-local scale. However, the related effects on the Himalayan glaciers are less explored but believed to be an important factor regulating spatial heterogeneity. This study aims to present a multi-decadal approximation on hydroclimate and glacier interaction over the western central Himalaya (WCH). Three highly coherent, multi-species, tree-ring δ18O site-chronologies from WCH were used to derive regional changes in atmospheric humidity (atmospheric moisture content: AMC) since the last four centuries. Coherency analyses between AMC and glacier mass balance (GMB: tree-ring δ13C-derived) indicate an abrupt phase-shift since the 1960s within a common record of 273 years. To ascertain the cause of phase-shift, annual AMC was disintegrated into seasonal-scale, utilizing δ18O record of deciduous species. Seasonal (winter: October–March; &amp; summer-accumulation season: April–September) decomposition results reveal that winter-westerlies rather than summer precipitation from Indian summer monsoon (ISM) govern the ice-mass variability in WCH. Decadal coherency between summer-season AMC and GMB remained relatively stable since the mid-20th century, despite a decline in central Himalayan summer precipitation (tree-ring δ18O records). We hypothesize that excess water vapor brought to the atmosphere through increase in pre-monsoon precipitation and greening-mediated increase in evapotranspiration might have been recycled through the summer season to compensate for the ISM part of precipitation. However, isotope-enabled ecophysiological models and measurements would be able to strengthen this hypothesis. In addition, high-resolution radiative forcing and glacier valley-scale vegetation trend analyses point towards a probable influence of greening on GMB. Results indicate that attribution of ice-mass to large-scale dynamics is likely to be modulated by local vegetation changes. We contend that glacier-climate models fed with these feedback processes could reliably improve the projections.

Observation-constrained Radiative Forcing from historical land-cover changes in CMIP5 models

2020 ◽  
Author(s):  
Quentin Lejeune ◽  
Edouard Davin ◽  
Grégory Duveiller ◽  
Bas Crezee ◽  
Ronny Meier ◽  
...  
Keyword(s):  
Land Cover ◽  
Radiative Forcing ◽  
Large Scale ◽  
Climate Models ◽  
Mean Value ◽  
Cmip5 Models ◽  
Land Cover Changes ◽  
Conversion Rates ◽  
The One ◽  
Derived Data

&lt;p&gt;The albedo of trees is lower than the one of crops and grasses, especially in the presence of snow. It is therefore understood that the replacement of forests by croplands and grasslands used for agricultural purposes that has occurred since pre-industrial times led to large-scale albedo increases. This is reflected by the estimate of the Radiative Forcing (RF) from historical Land-Cover Changes (LCC) of the Fifth Assessment Report (AR5) of the IPCC, which amounts to -0.15 +/- 0.10 W/m&lt;sup&gt;2&lt;/sup&gt;. However, this expert judgment was intended to both account for a few studies using single climate models which put forward values close to 0.2W/m&lt;sup&gt;2&lt;/sup&gt;, and the finding that climate models usually overestimate the albedo difference between natural vegetation and croplands in comparison to satellite-derived observational evidence. Further uncertainties around this number have also been suggested by studies revealing a substantial model spread in the albedo response to historical LCC. This points at the need to revisit the IPCC AR5 conclusions in light of recent model intercomparison efforts and observational data.&lt;/p&gt;&lt;p&gt;In this study, we reconstructed the local albedo changes induced by conversions between trees and crops/grasses since 1860 for 15 CMIP5 models. We evaluated the employed methodology using factorial experiments isolating the historical LCC forcing in four models for which the required simulations are available, and obtained very similar results. Using an empirical parameterisation of the radiative kernel, we then derived estimates of the associated RF ranging between 0 and -0.22 W/m&lt;sup&gt;2&lt;/sup&gt;, with a multi-model mean value of -0.07 W/m&lt;sup&gt;2&lt;/sup&gt;.&lt;/p&gt;&lt;p&gt;Furthermore, we constrained the RF estimates with observations by replacing the albedo response to the transition between trees and crops/grasses from the models by that provided by satellite-derived data. This led to an unexpected increase in the range between the models, due to two models having unrealistic conversion rates from trees to crops/grasses. Excluding these two models, we obtain a revised multi-model mean estimate of -0.11 W/m&lt;sup&gt;2&lt;/sup&gt; (with individual model results between -0.04 and -0.16 W/m&lt;sup&gt;2&lt;/sup&gt;). We were also able to link the differences between the unconstrained and constrained RF estimates to some of the model biases in the albedo sensitivity to deforestation.&lt;/p&gt;&lt;p&gt;Since the conversions between trees and crops/grasses are responsible for almost the totality of historical albedo changes in CMIP5 models, our findings are comparable to previous estimates of the RF from all LCC. They point at values that are at the lower end of the range provided by the IPCC AR5. The approach described in this study can be applied on other model simulations, such as those from CMIP6.&lt;/p&gt;

scholarly journals The Role of Natural Climate Variability in Recent Tropical Expansion

2017 ◽  
Vol 30 (16) ◽  
pp. 6329-6350 ◽  
Author(s):  
Robert J. Allen ◽  
Mahesh Kovilakam
Keyword(s):  
Climate Variability ◽  
Large Scale ◽  
Climate Models ◽  
Southern Oscillation ◽  
Tropical Pacific ◽  
Forced Response ◽  
Natural Climate Variability ◽  
Sst Variability ◽  
Natural Climate

Observations show the tropical belt has widened over the past few decades, a phenomenon associated with poleward migration of subtropical dry zones and large-scale atmospheric circulation. Coupled climate models also simulate tropical belt widening, but less so than observed. Reasons for this discrepancy, and the mechanisms driving the expansion remain uncertain. Here, the role of unforced, natural climate variability—particularly natural sea surface temperature (SST) variability—in recent tropical widening is shown. Compared to coupled ocean–atmosphere models, atmosphere-only simulations driven by observed SSTs consistently lead to larger rates of tropical widening, especially in the Northern Hemisphere (NH), highlighting the importance of recent SST evolution. Assuming the ensemble mean SSTs from historical simulations accurately represent the externally forced response, the observed SSTs can be decomposed into a forced and an unforced component. Targeted simulations with the Community Atmosphere Model, version 5 (CAM5), show that natural SST variability accounts for nearly all of the widening associated with recent SST evolution. This is consistent with the similarity of the unforced SSTs to the observed SSTs, both of which resemble a cold El Niño–Southern Oscillation/Pacific decadal oscillation (ENSO/PDO)-like SST pattern, which is associated with a wider tropical belt. Moreover, CAM5 coupled simulations with observed central to eastern tropical Pacific SSTs yield more than double the rate of widening compared to analogous simulations without prescribed tropical Pacific SSTs and reproduce the magnitude of tropical widening in atmosphere-only simulations. The results suggest that the bulk of recent tropical widening, particularly in the NH, is due to unforced, natural SST variability, primarily related to recent ENSO/PDO variability.

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