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1. Energetic Constraints on the Pattern of Changes to the Hydrological Cycle under Global Warming

   https://doi.org/10.1175/JCLI-D-22-0337.1  

   Bonan, David B. ; Siler, Nicholas ; Roe, Gerard H. ; Armour, Kyle C. ( May 2023 , Journal of Climate)

   Abstract The response of zonal-mean precipitation minus evaporation ( P − E ) to global warming is investigated using a moist energy balance model (MEBM) with a simple Hadley cell parameterization. The MEBM accurately emulates zonal-mean P − E change simulated by a suite of global climate models (GCMs) under greenhouse gas forcing. The MEBM also accounts for most of the intermodel differences in GCM P − E change and better emulates GCM P − E change when compared to the “wet-gets-wetter, dry-gets-drier” thermodynamic mechanism. The intermodel spread in P − E change is attributed to intermodel differences in radiative feedbacks, which account for 60%–70% of the intermodel variance, with smaller contributions from radiative forcing and ocean heat uptake. Isolating the intermodel spread of feedbacks to specific regions shows that tropical feedbacks are the primary source of intermodel spread in zonal-mean P − E change. The ability of the MEBM to emulate GCM P − E change is further investigated using idealized feedback patterns. A less negative and narrowly peaked feedback pattern near the equator results in more atmospheric heating, which strengthens the Hadley cell circulation in the deep tropics through an enhanced poleward heat flux. This pattern also increasesmore »gross moist stability, which weakens the subtropical Hadley cell circulation. These two processes in unison increase P − E in the deep tropics, decrease P − E in the subtropics, and narrow the intertropical convergence zone. Additionally, a feedback pattern that produces polar-amplified warming partially reduces the poleward moisture flux by weakening the meridional temperature gradient. It is shown that changes to the Hadley cell circulation and the poleward moisture flux are crucial for understanding the pattern of GCM P − E change under warming. Significance Statement Changes to the hydrological cycle over the twenty-first century are predicted to impact ecosystems and socioeconomic activities throughout the world. While it is broadly expected that dry regions will get drier and wet regions will get wetter, the magnitude and spatial structure of these changes remains uncertain. In this study, we use an idealized climate model, which assumes how energy is transported in the atmosphere, to understand the processes setting the pattern of precipitation and evaporation under global warming. We first use the idealized climate model to explain why comprehensive climate models predict different changes to precipitation and evaporation across a range of latitudes. We show this arises primarily from climate feedbacks, which act to amplify or dampen the amount of warming. Ocean heat uptake and radiative forcing play secondary roles but can account for a significant amount of the uncertainty in regions where ocean circulation influences the rate of warming. We further show that uncertainty in tropical feedbacks (mainly from clouds) affects changes to the hydrological cycle across a range of latitudes. We then show how the pattern of climate feedbacks affects how the patterns of precipitation and evaporation respond to climate change through a set of idealized experiments. These results show how the pattern of climate feedbacks impacts tropical hydrological changes by affecting the strength of the Hadley circulation and polar hydrological changes by affecting the transport of moisture to the high latitudes.« less
2. The Circulation Response to Volcanic Eruptions: The Key Roles of Stratospheric Warming and Eddy Interactions

   https://doi.org/10.1175/JCLI-D-18-0099.1  

   DallaSanta, Kevin ; Gerber, Edwin P. ; Toohey, Matthew ( February 2019 , Journal of Climate)

   Proxy data and observations suggest that large tropical volcanic eruptions induce a poleward shift of the North Atlantic jet stream in boreal winter. However, there is far from universal agreement in models on this effect and its mechanism, and the possibilities of a corresponding jet shift in the Southern Hemisphere or the summer season have received little attention. Using a hierarchy of simplified atmospheric models, this study examines the impact of stratospheric aerosol on the extratropical circulation over the annual cycle. In particular, the models allow the separation of the dominant shortwave (surface cooling) and longwave (stratospheric warming) impacts of volcanic aerosol. It is found that stratospheric warming shifts the jet poleward in both the summer and winter hemispheres. The experiments cannot definitively rule out the role of surface cooling, but they provide no evidence that it shifts the jet poleward. Further study with simplified models demonstrates that the response to stratospheric warming is remarkably generic and does not depend critically on the boundary conditions (e.g., the planetary wave forcing) or the atmospheric physics (e.g., the treatment of radiative transfer and moist processes). It does, however, fundamentally involve both zonal-mean and eddy circulation feedbacks. The time scales, seasonality, and structuremore »of the response provide further insight into the mechanism, as well as its connection to modes of intrinsic natural variability. These findings have implications for the interpretation of comprehensive model studies and for postvolcanic prediction.

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3. Midlatitude Cloud Radiative Effect Sensitivity to Cloud Controlling Factors in Observations and Models: Relationship with Southern Hemisphere Jet Shifts and Climate Sensitivity

   https://doi.org/10.1175/JCLI-D-20-0986.1  

   Grise, Kevin M. ; Kelleher, Mitchell K. ( April 2021 , Journal of Climate)

   Abstract An effective method to understand cloud processes and to assess the fidelity with which they are represented in climate models is the cloud controlling factor framework, in which cloud properties are linked with variations in large-scale dynamical and thermodynamical variables. This study examines how midlatitude cloud radiative effects (CRE) over oceans co-vary with four cloud controlling factors: mid-tropospheric vertical velocity, estimated inversion strength (EIS), near-surface temperature advection, and sea surface temperature (SST), and assesses their representation in CMIP6 models with respect to observations and CMIP5 models. CMIP5 and CMIP6 models overestimate the sensitivity of midlatitude CRE to perturbations in vertical velocity, and underestimate the sensitivity of midlatitude shortwave CRE to perturbations in EIS and temperature advection. The largest improvement in CMIP6 models is a reduced sensitivity of CRE to vertical velocity perturbations. As in CMIP5 models, many CMIP6 models simulate a shortwave cloud radiative warming effect associated with a poleward shift in the Southern Hemisphere (SH) midlatitude jet stream, an effect not present in observations. This bias arises because most models’ shortwave CRE are too sensitive to vertical velocity perturbations and not sensitive enough to EIS perturbations, and because most models overestimate the SST anomalies associated with SH jetmore »shifts. The presence of this bias directly impacts the transient surface temperature response to increasing greenhouse gases over the Southern Ocean, but not the global-mean surface temperature. Instead, the models’ climate sensitivity is correlated with their shortwave CRE sensitivity to surface temperature advection perturbations near 40°S, with models with more realistic values of temperature advection sensitivity generally having higher climate sensitivity.« less
4. ITCZ Width Controls on Hadley Cell Extent and Eddy-Driven Jet Position and Their Response to Warming

   https://doi.org/10.1175/JCLI-D-18-0434.1  

   Watt-Meyer, Oliver ; Frierson, Dargan M. W. ( February 2019 , Journal of Climate)

   The impact of global warming–induced intertropical convergence zone (ITCZ) narrowing onto the higher-latitude circulation is examined in the GFDL Atmospheric Model, version 2.1 (AM2.1), run over zonally symmetric aquaplanet boundary conditions. A striking reconfiguration of the deep tropical precipitation from double-peaked, off-equatorial ascent to a single peak at the equator occurs under a globally uniform +4 K sea surface temperature (SST) perturbation. This response is found to be highly sensitive to the SST profile used to force the model. By making small (≤1 K) perturbations to the surface temperature in the deep tropics, varying control simulation precipitation patterns with both single and double ITCZs are generated. Across the climatologies, narrower regions of ascent correspond to more equatorward Hadley cell edges and eddy-driven jets. Under the global warming perturbation, the experiments in which there is narrowing of the ITCZ show significantly less expansion of the Hadley cell and somewhat less poleward shift of the eddy-driven jet than those without ITCZ narrowing. With a narrower ITCZ, the ascending air has larger zonal momentum, causing more westerly upper-tropospheric subtropical wind. In turn, this implies 1) the subtropical jet will become baroclinically unstable at a lower latitude and 2) the critical (zero wind) linemore »will shift equatorward, allowing midlatitude eddies to propagate farther equatorward. Both of these mechanisms modify the Hadley cell edge position, and the latter affects the jet position.

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5. Model Uncertainty in Cloud–Circulation Coupling, and Cloud-Radiative Response to Increasing CO ₂ , Linked to Biases in Climatological Circulation

   https://doi.org/10.1175/JCLI-D-17-0665.1  

   Lipat, Bernard R. ; Voigt, Aiko ; Tselioudis, George ; Polvani, Lorenzo M. ( December 2018 , Journal of Climate)

   Recent analyses of global climate models suggest that uncertainty in the coupling between midlatitude clouds and the atmospheric circulation contributes to uncertainty in climate sensitivity. However, the reasons behind model differences in the cloud–circulation coupling have remained unclear. Here, we use a global climate model in an idealized aquaplanet setup to show that the Southern Hemisphere climatological circulation, which in many models is biased equatorward, contributes to the model differences in the cloud–circulation coupling. For the same poleward shift of the Hadley cell (HC) edge, models with narrower climatological HCs exhibit stronger midlatitude cloud-induced shortwave warming than models with wider climatological HCs. This cloud-induced radiative warming results predominantly from a subsidence warming that decreases cloud fraction and is stronger for narrower HCs because of a larger meridional gradient in the vertical velocity. A comparison of our aquaplanet results with comprehensive climate models suggests that about half of the model uncertainty in the midlatitude cloud–circulation coupling stems from this impact of the circulation on the large-scale temperature structure of the atmosphere, and thus could be removed by improving the climatological circulation in models. This illustrates how understanding of large-scale dynamics can help reduce uncertainty in clouds and their response to climatemore »change.

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