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1. Vertically Varying Cloud Responses to Southern Hemisphere Jet Shifts and Their Role in Southern Annular Mode Persistence in Observations and CMIP6 Models

   https://doi.org/10.1029/2024GL114550  

   Liu, Xinhuiyu; Grise, Kevin M (July 2025, Geophysical Research Letters)

   This study investigates how clouds and their atmospheric radiative effects respond to meridional shifts in the Southern Hemisphere (SH) mid‐latitude jet, represented by the Southern Annular Mode (SAM), using reanalysis data, CloudSat/CALIPSO observations, and CMIP6 models. Consistent with previous studies, poleward jet shifts displace storm‐track clouds, creating lower tropospheric radiative heating anomalies poleward of the mean jet latitude and cooling anomalies on the equatorward side of the mean jet latitude where large‐scale subsidence increases low cloud fraction. Whether these radiative heating anomalies can affect SAM persistence is also investigated in CMIP6 models. If observed sea surface temperatures are prescribed, models that simulate low cloud responses more realistically show less SAM persistence, aligning more closely with observations. Our results based on CMIP6 models agree with a recent idealized modeling study and suggest that atmospheric cloud radiative heating anomalies, induced by the poleward jet shift, contribute to a reduction in SAM persistence. 

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2. Coupled Stratospheric Ozone and Atlantic Meridional Overturning Circulation Feedbacks on the Northern Hemisphere Midlatitude Jet Response to 4xCO2

   https://doi.org/10.1175/JCLI-D-23-0119.1  

   Orbe, Clara; Rind, David; Waugh, Darryn W; Jonas, Jeffrey; Zhang, Xiyue; Chiodo, Gabriel; Nazarenko, Larissa; Schmidt, Gavin A (May 2024, Journal of Climate)

   Abstract Stratospheric ozone, and its response to anthropogenic forcings, provides an important pathway for the coupling between atmospheric composition and climate. In addition to stratospheric ozone’s radiative impacts, recent studies have shown that changes in the ozone layer due to 4xCO₂have a considerable impact on the Northern Hemisphere (NH) tropospheric circulation, inducing an equatorward shift of the North Atlantic jet during boreal winter. Using simulations produced with the NASA Goddard Institute for Space Studies (GISS) high-top climate model (E2.2), we show that this equatorward shift of the Atlantic jet can induce a more rapid weakening of the Atlantic meridional overturning circulation (AMOC). The weaker AMOC, in turn, results in an eastward acceleration and poleward shift of the Atlantic and Pacific jets, respectively, on longer time scales. As such, coupled feedbacks from both stratospheric ozone and the AMOC result in a two-time-scale response of the NH midlatitude jet to abrupt 4xCO₂forcing: a “fast” response (5–20 years) during which it shifts equatorward and a “total” response (∼100–150 years) during which the jet accelerates and shifts poleward. The latter is driven by a weakening of the AMOC that develops in response to weaker surface zonal winds that result in reduced heat fluxes out of the subpolar gyre and reduced North Atlantic Deep Water formation. Our results suggest that stratospheric ozone changes in the lower stratosphere can have a surprisingly powerful effect on the AMOC, independent of other aspects of climate change. 

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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)

   null (Ed.)

   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 jet 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. 

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4. Response of Northern Hemisphere Midlatitude Circulation to Arctic Amplification in a Simple Atmospheric General Circulation Model

   https://doi.org/10.1175/JCLI-D-15-0602.1  

   Wu, Yutian; Smith, Karen L. (March 2016, Journal of Climate)

   This study examines the Northern Hemisphere midlatitude circulation response to Arctic amplification (AA) in a simple atmospheric general circulation model. It is found that, in response to AA, the tropospheric jet shifts equatorward and the stratospheric polar vortex weakens, robustly for various AA forcing strengths. Despite this, no statistically significant change in the frequency of sudden stratospheric warming events is identified. In addition, in order to quantitatively assess the role of stratosphere–troposphere coupling, the tropospheric pathway is isolated by nudging the stratospheric zonal mean state toward the reference state. When the nudging is applied, rendering the stratosphere inactive, the tropospheric jet still shifts equatorward but by approximately half the magnitude compared to that of an active stratosphere. The difference represents the stratospheric pathway and the downward influence of the stratosphere on the troposphere. This suggests that stratosphere–troposphere coupling plays a nonnegligible role in establishing the midlatitude circulation response to AA. 

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5. 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 increases 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. 

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