Coupled High-Latitude Climate Feedbacks and Their Impact on Atmospheric Heat Transport

The response of atmospheric heat transport to anthropogenic warming is determined by the anomalous meridional energy gradient. Feedback analysis offers a characterization of that gradient and hence reveals how uncertainty in physical processes may translate into uncertainty in the circulation response. However, individual feedbacks do not act in isolation. Anomalies associated with one feedback may be compensated by another, as is the case for the positive water vapor and negative lapse rate feedbacks in the tropics. Here a set of idealized experiments are performed in an aquaplanet model to evaluate the coupling between the surface albedo feedback and other feedbacks, including the impact on atmospheric heat transport. In the tropics, the dynamical response manifests as changes in the intensity and structure of the overturning Hadley circulation. Only half of the range of Hadley cell weakening exhibited in these experiments is found to be attributable to imposed, systematic variations in the surface albedo feedback. Changes in extratropical clouds that accompany the albedo changes explain the remaining spread. The feedback-driven circulation changes are compensated by eddy energy flux changes, which reduce the overall spread among experiments. These findings have implications for the efficiency with which the climate system, including tropical circulation and the hydrological cycle, adjusts to high-latitude feedbacks over climate states that range from perennial or seasonal ice to ice-free conditions in the Arctic

Coupled High-Latitude Climate Feedbacks and Their Impact on Atmospheric Heat Transport

The response of atmospheric heat transport to anthropogenic warming is determined by the anomalous meridional energy gradient. Feedback analysis offers a characterization of that gradient and hence reveals how uncertainty in physical processes may translate into uncertainty in the circulation response. However, individual feedbacks do not act in isolation. Anomalies associated with one feedback may be compensated by another, as is the case for the positive water vapor and negative lapse rate feedbacks in the tropics. Here a set of idealized experiments are performed in an aquaplanet model to evaluate the coupling between the surface albedo feedback and other feedbacks, including the impact on atmospheric heat transport. In the tropics, the dynamical response manifests as changes in the intensity and structure of the overturning Hadley circulation. Only half of the range of Hadley cell weakening exhibited in these experiments is found to be attributable to imposed, systematic variations in the surface albedo feedback. Changes in extratropical clouds that accompany the albedo changes explain the remaining spread. The feedback-driven circulation changes are compensated by eddy energy flux changes, which reduce the overall spread among experiments. These findings have implications for the efficiency with which the climate system, including tropical circulation and the hydrological cycle, adjusts to high-latitude feedbacks over climate states that range from perennial or seasonal ice to ice-free conditions in the Arctic

The dependence of transient climate sensitivity and radiative feedbacks on the spatial pattern of ocean heat uptake

The effect of ocean heat uptake (OHU) on transient global warming is studied in a multimodel framework. Simple heat sinks are prescribed in shallow aquaplanet ocean mixed layers underlying atmospheric general circulation models independently and combined with CO_2 forcing. Sinks are localized to either tropical or high latitudes, representing distinct modes of OHU found in coupled simulations. Tropical OHU produces modest cooling at all latitudes, offsetting only a fraction of CO_2 warming. High-latitude OHU produces three times more global mean cooling in a strongly polar-amplified pattern. Global sensitivities in each scenario are set primarily by large differences in local shortwave cloud feedbacks, robust across models. Differences in atmospheric energy transport set the pattern of temperature change. Results imply that global and regional warming rates depend sensitively on regional ocean processes setting the OHU pattern, and that equilibrium climate sensitivity cannot be reliably estimated from transient observations

The remote impacts of climate feedbacks on regional climate predictability

Uncertainty in the spatial pattern of climate change is dominated by divergent predictions among climate models. Model differences are closely linked to their representation of climate feedbacks, that is, the additional radiative fluxes that are caused by changes in clouds, water vapour, surface albedo, and other factors, in response to an external climate forcing. Progress in constraining this uncertainty is therefore predicated on understanding how patterns of individual climate feedbacks aggregate into a regional and global climate response. Here we present a simple, moist energy balance model that combines regional feedbacks and the diffusion of both latent and sensible heat. Our model emulates the relationship between regional feedbacks and temperature response in more comprehensive climate models; the model can therefore be used to understand how uncertainty in feedback patterns drives uncertainty in the patterns of temperature response. We find that whereas uncertainty in tropical feedbacks induces a global response, the impact of uncertainty in polar feedbacks remains predominantly regionally confined

Process Drivers, Inter-Model Spread, and the Path Forward: A Review of Amplified Arctic Warming

Arctic amplification (AA) is a coupled atmosphere-sea ice-ocean process. This understanding has evolved from the early concept of AA, as a consequence of snow-ice line progressions, through more than a century of research that has clarified the relevant processes and driving mechanisms of AA. The predictions made by early modeling studies, namely the fall/winter maximum, bottom-heavy structure, the prominence of surface albedo feedback, and the importance of stable stratification have withstood the scrutiny of multi-decadal observations and more complex models. Yet, the uncertainty in Arctic climate projections is larger than in any other region of the planet, making the assessment of high-impact, near-term regional changes difficult or impossible. Reducing this large spread in Arctic climate projections requires a quantitative process understanding. This manuscript aims to build such an understanding by synthesizing current knowledge of AA and to produce a set of recommendations to guide future research. It briefly reviews the history of AA science, summarizes observed Arctic changes, discusses modeling approaches and feedback diagnostics, and assesses the current understanding of the most relevant feedbacks to AA. These sections culminate in a conceptual model of the fundamental physical mechanisms causing AA and a collection of recommendations to accelerate progress towards reduced uncertainty in Arctic climate projections. Our conceptual model highlights the need to account for local feedback and remote process interactions within the context of the annual cycle to constrain projected AA. We recommend raising the priority of Arctic climate sensitivity research, improving the accuracy of Arctic surface energy budget observations, rethinking climate feedback definitions, coordinating new model experiments and intercomparisons, and further investigating the role of episodic variability in AA

Differences in Water Vapor Radiative Transfer among 1D Models Can Significantly Affect the Inner Edge of the Habitable Zone

An accurate estimate of the inner edge of the habitable zone is critical for determining which exoplanets are potentially habitable and for designing future telescopes to observe them. Here, we explore differences in estimating the inner edge among seven one-dimensional radiative transfer models: two line-by-line codes (SMART and LBLRTM) as well as five band codes (CAM3, CAM4_Wolf, LMDG, SBDART, and AM2) that are currently being used in global climate models. We compare radiative fluxes and spectra in clear-sky conditions around G and M stars, with fixed moist adiabatic profiles for surface temperatures from 250 to 360 K. We find that divergences among the models arise mainly from large uncertainties in water vapor absorption in the window region (10 μm) and in the region between 0.2 and 1.5 μm. Differences in outgoing longwave radiation increase with surface temperature and reach 10–20 W m^(−2); differences in shortwave reach up to 60 W m^(−2), especially at the surface and in the troposphere, and are larger for an M-dwarf spectrum than a solar spectrum. Differences between the two line-by-line models are significant, although smaller than among the band models. Our results imply that the uncertainty in estimating the insolation threshold of the inner edge (the runaway greenhouse limit) due only to clear-sky radiative transfer is ≈10% of modern Earth's solar constant (i.e., ≈34 W m^(−2) in global mean) among band models and ≈3% between the two line-by-line models. These comparisons show that future work is needed that focuses on improving water vapor absorption coefficients in both shortwave and longwave, as well as on increasing the resolution of stellar spectra in broadband models

The local, remote, and global consequences of climate feedbacks

Thesis (Ph.D.)--University of Washington, 2013Climate feedbacks offer a powerful framework for revealing the energetic pathways by which the system adjusts to an imposed forcing, such as an increase in atmospheric CO2. We investigate how local atmospheric feedbacks, such as those associated with Arctic sea ice and the Walker circulation, affect both global climate sensitivity and spatial patterns of warming. Emphasis is placed on a general circulation model with idealized boundary conditions, for the clarity it provides. For this aquaplanet simulation, we account for rapid tropospheric adjustments to CO2 and explicitly diagnose feedbacks (using radiative kernels) and forcing for this precise model set-up. In particular, a detailed closure of the energy budget within a clean experimental set-up allows us to consider nonlinear interactions between feedbacks. The inclusion of a tropical Walker circulation is found to prime the Hadley Circulation for a larger deceleration under CO2 doubling, by altering subtropical stratus decks and the meridional feedback gradient. We perform targeted experiments to isolate the atmospheric processes responsible for the variability in climate sensitivity, with implications for high-sensitivity paleoclimates. The local climate response is characterized in terms of the meridional structure of feedbacks, atmospheric heat transport, nonlinearities, and forcing. Our results display a combination of positive subtropical feedbacks and polar amplified warming. These two factors imply a critical role for transport and nonlinear effects, with the latter acting to substantially reduce global climate sensitivity. At the hemispheric scale, a rich picture emerges: anomalous divergence of heat flux away from positive feedbacks in the subtropics; clear-sky nonlinearities that reinforce the pattern of tropical cooling and high-latitude warming tendencies; and strong ice-line feedbacks that drive further amplification of polar warming. These results have implications for regional climate predictability, by providing an indication of how spatial patterns in feedbacks combine to affect both the local and nonlocal climate response, and how constraining uncertainty in those feedbacks may constrain the climate response. We also consider how competing definitions of feedbacks influence interpretation of climate sensitivity. While climate feedbacks represent a convenient breakdown of the energy balance, their widespread appeal has led to a profusion of definitions, and to variations upon the traditional decomposition. We demonstrate that a locally defined feedback framework does provide several advantages from the perspective of regional climate predictability. Namely, it enables a partial temperature change analysis which quantifies contributions to spatial patterns of warming; it also ensures feedbacks are not biased at high latitudes due to polar amplification. Alternative approaches to characterizing feedbacks can also isolate and illuminate different atmospheric processes. In particular, comparison of two versions of the water vapor feedback, one focused on specific humidity and the other on relative humidity, allows for an elegant dissection of the relative importance of thermodynamical and dynamical changes in a warmer world

Sea Ice Loss, Water Vapor Increases, and Their Interactions with Atmospheric Energy Transport in Driving Seasonal Polar Amplification

Abstract: The ice–albedo feedback associated with sea ice loss contributes to polar amplification, while the water vapor feedback contributes to tropical amplification of surface warming. However, these feedbacks are not independent of atmospheric energy transport, raising the possibility of complex interactions that may obscure the drivers of polar amplification, in particular its manifestation across the seasonal cycle. Here, we apply a radiative transfer hierarchy to an idealized aquaplanet global climate model coupled to a thermodynamic sea ice model. The climate responses and radiative feedbacks are decomposed into the contributions from sea ice loss, including both retreat and thinning, and the radiative effect of water vapor changes. We find that summer sea ice retreat causes winter polar amplification through ocean heat uptake and release, and the resulting decrease in dry energy transport weakens the magnitude of warming. Moreover, sea ice thinning is found to suppress summer warming and enhance winter warming, additionally contributing to winter amplification. The water vapor radiative effect produces seasonally symmetric polar warming via offsetting effects: enhanced moisture in the summer hemisphere induces the summer water vapor feedback and simultaneously strengthens the winter latent energy transport in the winter hemisphere by increasing the meridional moisture gradient. These results reveal the importance of changes in atmospheric energy transport induced by sea ice retreat and increased water vapor to seasonal polar amplification, elucidating the interactions among these physical processes

Current-climate sea ice amount and seasonality as constraints for future Arctic amplification

The recent Arctic sea ice loss is a key driver of the amplified surface warming in the northern high latitudes, and simultaneously a major source of uncertainty in model projections of Arctic climate change. Previous work has shown that the spread in model predictions of future Arctic amplification (AA) can be traced back to the inter-model spread in simulated long-term sea ice loss. We demonstrate that the strength of future AA is further linked to the current climate’s, observable sea ice state across the multi-model ensemble of the 6th Coupled Model Intercomparison Project (CMIP6). The implication is that the sea-ice climatology sets the stage for long-term changes through the 21st century, which mediate the degree by which Arctic warming is amplified with respect to global warming. We determine that a lower base-climate sea ice extent and sea ice concentration (SIC) in CMIP6 models enable stronger ice melt in both future climate and during the seasonal cycle. In particular, models with lower Arctic-mean SIC project stronger future ice loss and a more intense seasonal cycle in ice melt and growth. Both processes systemically link to a larger future AA across climate models. These results are manifested by the role of climate feedbacks that have been widely identified as major drivers of AA. We show in particular that models with low base-climate SIC predict a systematically stronger warming contribution through both sea-ice albedo feedback and temperature feedbacks in the future, as compared to models with high SIC. From our derived linear regressions in conjunction with observations, we estimate a 21st-century AA over sea ice of 2.47–3.34 with respect to global warming. Lastly, from the tight relationship between base-climate SIC and the projected timing of an ice-free September, we predict a seasonally ice-free Arctic by mid-century under a high-emission scenario