Paulo Ceppi

The response of the Southern Hemispheric eddy-driven jet to future changes in shortwave radiation in CMIP5

A strong relationship is found between changes in the meridional gradient of absorbed shortwave radiation (ASR) and Southern Hemispheric jet shifts in 21st century climate simulations of CMIP5 (Coupled Model Intercomparison Project phase 5) coupled models. The relationship is such that models with increases in the meridional ASR gradient around the southern midlatitudes, and therefore increases in midlatitude baroclinicity, tend to produce a larger poleward jet shift. The ASR changes are shown to be dominated by changes in cloud properties, with sea ice declines playing a secondary role. We demonstrate that the ASR changes are the cause, and not the result, of the intermodel differences in jet response by comparing coupled simulations with experiments in which sea surface temperature increases are prescribed. Our results highlight the importance of reducing the uncertainty in cloud feedbacks in order to constrain future circulation changes.

Mixed‐phase cloud physics and Southern Ocean cloud feedback in climate models

Increasing optical depth poleward of 45° is a robust response to warming in global climate models. Much of this cloud optical depth increase has been hypothesized to be due to transitions from ice-dominated to liquid-dominated mixed-phase cloud. In this study, the importance of liquid-ice partitioning for the optical depth feedback is quantified for 19 Coupled Model Intercomparison Project Phase 5 models. All models show a monotonic partitioning of ice and liquid as a function of temperature, but the temperature at which ice and liquid are equally mixed (the glaciation temperature) varies by as much as 40 K across models. Models that have a higher glaciation temperature are found to have a smaller climatological liquid water path (LWP) and condensed water path and experience a larger increase in LWP as the climate warms. The ice-liquid partitioning curve of each model may be used to calculate the response of LWP to warming. It is found that the repartitioning between ice and liquid in a warming climate contributes at least 20% to 80% of the increase in LWP as the climate warms, depending on model. Intermodel differences in the climatological partitioning between ice and liquid are estimated to contribute at least 20% to the intermodel spread in the high-latitude LWP response in the mixed-phase region poleward of 45°S. It is hypothesized that a more thorough evaluation and constraint of global climate model mixed-phase cloud parameterizations and validation of the total condensate and ice-liquid apportionment against observations will yield a substantial reduction in model uncertainty in the high-latitude cloud response to warming.

Mechanisms of the Negative Shortwave Cloud Feedback in Middle to High Latitudes

AbstractIncreases in cloud optical depth and liquid water path (LWP) are robust features of global warming model simulations in high latitudes, yielding a negative shortwave cloud feedback, but the mechanisms are still uncertain. Here the importance of microphysical processes for the negative optical depth feedback is assessed by perturbing temperature in the microphysics schemes of two aquaplanet models, both of which have separate prognostic equations for liquid water and ice. It is found that most of the LWP increase with warming is caused by a suppression of ice microphysical processes in mixed-phase clouds, resulting in reduced conversion efficiencies of liquid water to ice and precipitation. Perturbing the temperature-dependent phase partitioning of convective condensate also yields a small LWP increase. Together, the perturbations in large-scale microphysics and convective condensate partitioning explain more than two-thirds of the LWP response relative to a reference case with increased SSTs, and ...

On the Speed of the Eddy-Driven Jet and the Width of the Hadley Cell in the Southern Hemisphere

A strong correlation between the speed of the eddy-driven jet and the width of the Hadley cell is found to exist in the Southern Hemisphere, both in reanalysis data and in twenty-first-century integrations from the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report multimodel archive. Analysis of the space‐time spectra of eddy momentum flux reveals that variations in eddy-driven jet speed are related to changes in the mean phase speed of midlatitude eddies. An increase in eddy phase speeds induces a poleward shift of the critical latitudes and a poleward expansion of the region of subtropical wave breaking. The associated changes in eddy momentum flux convergence are balanced by anomalous meridional winds consistent with a wider Hadley cell. At the same time, faster eddies are also associated with a strengthened polewardeddymomentumflux,sustainingastronger westerlyjet in midlatitudes. The proposedmechanismis consistent with the seasonal dependence of the interannual variability of the Hadley cell width and appears to explain at least part of the projected twenty-first-century trends. One of the most salient features of Earth’s general circulation is the presence of Hadley cells (HCs) in the tropical belts of both hemispheres. Associated with their large-scale motions are some of the main characteristics of tropical and subtropical climates: the intertropical convergence zone in the rising branch and the subtropical dry zones in the descending branches (e.g., Hartmann 1994). Studying the factors determining the meridional extent of these overturning circulations is therefore essential to understand the spatial distribution of tropical and subtropical climates and its changes. The role of the HCs is particularly crucial in the context of climate change and the observed widening of the atmospheric circulation. Observations show a poleward shift of the eddy-driven jets and an expansion of the HCs in recent decades, particularly in the Southern Hemisphere (Hu and Fu 2007; Johanson and Fu 2009; Lu et al. 2009), and the role of radiative forcings induced by greenhouse gases and stratospheric ozone has been shown to be crucial in this context (Polvani et al. 2011b;

Observational evidence for a negative shortwave cloud feedback in middle to high latitudes

Exploiting the observed robust relationships between temperature and optical depth in extratropical clouds, we calculate the shortwave cloud feedback from historical data, by regressing observed and modeled cloud property histograms onto local temperature in middle to high southern latitudes. In this region, all CMIP5 models and observational data sets predict a negative cloud feedback, mainly driven by optical thickening. Between 45° and 60°S, the mean observed shortwave feedback (−0.91 ± 0.82 W m−2 K−1, relative to local rather than global mean warming) is very close to the multimodel mean feedback in RCP8.5 (−0.98 W m−2 K−1), despite differences in the meridional structure. In models, historical temperature-cloud property relationships reliably predict the forced RCP8.5 response. Because simple theory predicts this optical thickening with warming, and cloud amount changes are relatively small, we conclude that the shortwave cloud feedback is very likely negative in the real world at middle to high latitudes.

Clouds and the Atmospheric Circulation Response to Warming

AbstractThe authors study the effect of clouds on the atmospheric circulation response to CO2 quadrupling in an aquaplanet model with a slab ocean lower boundary. The cloud effect is isolated by locking the clouds to either the control or 4xCO2 state in the shortwave (SW) or longwave (LW) radiation schemes. In the model, cloud radiative changes explain more than half of the total poleward expansion of the Hadley cells, midlatitude jets, and storm tracks under CO2 quadrupling, even though they cause only one-fourth of the total global-mean surface warming. The effect of clouds on circulation results mainly from the SW cloud radiative changes, which strongly enhance the equator-to-pole temperature gradient at all levels in the troposphere, favoring stronger and poleward-shifted midlatitude eddies. By contrast, quadrupling CO2 while holding the clouds fixed causes strong polar amplification and weakened midlatitude baroclinicity at lower levels, yielding only a small poleward expansion of the circulation. Th...

Connections between clouds, radiation, and midlatitude dynamics: a review

We review the effects of dynamical variability on clouds and radiation in observations and models and discuss their implications for cloud feedbacks. Jet shifts produce robust meridional dipoles in upper-level clouds and longwave cloud-radiative effect (CRE), but low-level clouds, which do not simply shift with the jet, dominate the shortwave CRE. Because the effect of jet variability on CRE is relatively small, future poleward jet shifts with global warming are only a second-order contribution to the total CRE changes around the midlatitudes, suggesting a dominant role for thermodynamic effects. This implies that constraining the dynamical response is unlikely to reduce the uncertainty in extratropical cloud feedback. However, we argue that uncertainty in the cloud-radiative response does affect the atmospheric circulation response to global warming, by modulating patterns of diabatic forcing. How cloud feedbacks can affect the dynamical response to global warming is an important topic of future research.

Trends in the CERES Dataset, 2000-13: The Effects of Sea Ice and Jet Shifts and Comparison to Climate Models

The Clouds and the Earth’s Radiant Energy System (CERES) observations of global top-of-atmosphere radiative energy fluxes for the period March 2000‐February 2013 are examined for robust trends and variability. The trend in Arctic ice is clearly evident in the time series of reflected shortwave radiation, which closelyfollowstherecordoficeextent.Thedataindicatethat,forevery10 6 km 2 decreaseinSeptemberseaice extent, annual-mean absorbed solar radiation averaged over 758‐908N increases by 2.5Wm 22 , or about 6Wm 22 between 2000 and 2012. CMIP5 models generally show a much smaller change in sea ice extent over the 1970‐2012 period, but the relationship of sea ice extent to reflected shortwave is in good agreement with recent observations. Another robust trend during this period is an increase in reflected shortwave radiation in the zonal belt from 458 to 658S. This trend is mostly related to increases in sea ice concentrations in the Southern Ocean and less directly related to cloudiness trends associated with the annular variability of the Southern Hemisphere. Models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) produce a scaling of cloud reflection to zonal wind increase that is similar to trend observations in regions separated from the direct effects of sea ice. Atmospheric Model Intercomparison Project (AMIP) model responses over the Southern Ocean are not consistent with each other or with the observed shortwave trends in regions removed from the direct effect of sea ice.

Relationship of tropospheric stability to climate sensitivity and Earth’s observed radiation budget

Significance In current climate models, the anticipated amount of warming under greenhouse gas forcing, quantified by the “effective climate sensitivity,” increases as time passes. Consequently, effective climate sensitivity values inferred from the historical record may underestimate the future warming. However, the mechanisms of this increase in effective climate sensitivity are not understood, limiting our confidence in climate model projections of future climate change. Here, we present observational and modeling evidence that the magnitude of effective climate sensitivity partly depends on the evolution of the vertical profile of atmospheric warming. In climate models, as the Earth warms overall, the warming becomes increasingly muted aloft, and this alters the strength of feedbacks controlling the radiative response to greenhouse gas forcing. Climate feedbacks generally become smaller in magnitude over time under CO2 forcing in coupled climate models, leading to an increase in the effective climate sensitivity, the estimated global-mean surface warming in steady state for doubled CO2. Here, we show that the evolution of climate feedbacks in models is consistent with the effect of a change in tropospheric stability, as has recently been hypothesized, and the latter is itself driven by the evolution of the pattern of sea-surface temperature response. The change in climate feedback is mainly associated with a decrease in marine tropical low cloud (a more positive shortwave cloud feedback) and with a less negative lapse-rate feedback, as expected from a decrease in stability. Smaller changes in surface albedo and humidity feedbacks also contribute to the overall change in feedback, but are unexplained by stability. The spatial pattern of feedback changes closely matches the pattern of stability changes, with the largest increase in feedback occurring in the tropical East Pacific. Relationships qualitatively similar to those in the models among sea-surface temperature pattern, stability, and radiative budget are also found in observations on interannual time scales. Our results suggest that constraining the future evolution of sea-surface temperature patterns and tropospheric stability will be necessary for constraining climate sensitivity.

Contributions of Climate Feedbacks to Changes in Atmospheric Circulation

AbstractThe projected response of the atmospheric circulation to the radiative changes induced by CO2 forcing and climate feedbacks is currently uncertain. In this modeling study, the impact of CO2-induced climate feedbacks on changes in jet latitude and speed is assessed by imposing surface albedo, cloud, and water vapor feedbacks as if they were forcings in two climate models, CAM4 and ECHAM6. The jet response to radiative feedbacks can be broadly interpreted through changes in midlatitude baroclinicity. Clouds enhance baroclinicity, favoring a strengthened, poleward-shifted jet; this is mitigated by surface albedo changes, which have the opposite effect on baroclinicity and the jet, while water vapor has opposing effects on upper- and lower-level baroclinicity with little net impact on the jet. Large differences between the CAM4 and ECHAM6 responses illustrate how model uncertainty in radiative feedbacks causes a large spread in the baroclinicity response to CO2 forcing. Across the CMIP5 models, differ...