This study analyzes Coupled Model Intercomparison Project phase 5 (CMIP5) model output to examine the covariability of interannual Southern Hemisphere Hadley cell (HC) edge latitude shifts and ...shortwave cloud radiative effect (SWCRE). In control climate runs, during years when the HC edge is anomalously poleward, most models substantially reduce the shortwave radiation reflected by clouds in the lower midlatitude region (LML; ∼28°S–∼48°S), although no such reduction is seen in observations. These biases in HC‐SWCRE covariability are linked to biases in the climatological HC extent. Notably, models with excessively equatorward climatological HC extents have weaker climatological LML subsidence and exhibit larger increases in LML subsidence with poleward HC edge expansion. This behavior, based on control climate interannual variability, has important implications for the CO2‐forced model response. In 4×CO2‐forced runs, models with excessively equatorward climatological HC extents produce stronger SW cloud radiative warming in the LML region and tend to have larger climate sensitivity values than models with more realistic climatological HC extents.
Key Points
CMIP5 model SW cloud radiative response and climate sensitivity is linked to the SH Hadley cell extent in each model's control climate
With 4×CO2 forcing, models with narrower Hadley cells in the control climate warm more in the SH lower midlatitudes (28–48°S)
And a larger warming in the SH lower midlatitudes strongly correlates with higher climate sensitivity
ABSTRACT
Southern South America (SSA), considered as the continental region south of 20°S, has experienced significant precipitation variability and trends in the last decades. This article uses ...monthly quality‐controlled precipitation data from rainfall stations with continuous observations during at least 100 years to quantify long‐term trends as well as interannual‐to‐centennial variability. Several statistical methods are applied to the data, primarily to detect jumps and look for changes due to relocation of the gauge stations, as well as to identify significant trends. Most of the regions have registered an increase in annual rainfall, largely attributable to changes in the warm season. On the other hand, during winter most stations in Argentina and Brazil do not have significant trends, although eastern Patagonia registered an increase in precipitation and Chile, a marked decrease in rainfall.
In order to look into the physical mechanisms behind the observed variability, the changes in mean sea level pressure and precipitable water are quantified for different sub‐periods. Also explored is the variability related to the Hadley cell width and strength over the region around SSA. Results show that the Hadley cell has shrunk and shifted towards the equator in winter over the area, which has caused an enhancement of the sinking motion over much of Argentina, Chile and Brazil, while likely increasing the baroclinicity (and associated precipitation) over Patagonia. In summer, the strength of the subsidence decreased and this was associated with an increase of the low‐level moisture advection, favouring more rainfall. The observational evidence presented here suggests that the zonal asymmetry in the change of the Hadley cell position over SSA could be linked to the presence of the Andes Cordillera.
The variability of Antarctic sea ice (ASI) has great potential to affect atmospheric circulation, with impacts that can extend from the surface to the middle and high levels of troposphere. The ...present study has evaluated the response of South Atlantic tropospheric circulation to increased coverage in area and volume of ASI. Monthly data of air temperature, zonal and meridional wind and mean sea level pressure were obtained from two ensemble simulations performed with the GDFL/CM2.1 model, covering the period from July 2020 to June 2030. In general, the response of South Atlantic tropospheric circulation to increased ASI showed that the climatic signal extended up from the surface to the high levels, propagating as a South Pole-Tropics teleconnection. The results show a general cooling of the southern troposphere, which for instance lead to the strengthening and northward shift of the polar jet and the southward shift of the subtropical jet and to an inversion from the positive to negative phase of the Southern Annular Mode. This study has great relevance for understanding the global climate changes in short term, by assessing the sensitivity of South Atlantic tropospheric circulation to extreme variations in ASI.
The Pliocene warm interval has been difficult to explain. We reconstructed the latitudinal distribution of sea surface temperature around 4 million years ago, during the early Pliocene. Our ...reconstruction shows that the meridional temperature gradient between the equator and subtropics was greatly reduced, implying a vast poleward expansion of the ocean tropical warm pool. Corroborating evidence indicates that the Pacific temperature contrast between the equator and 32°N has evolved from ∼2°C 4 million years ago to ∼8°C today. The meridional warm pool expansion evidently had enormous impacts on the Pliocene climate, including a slowdown of the atmospheric Hadley circulation and El Niño-like conditions in the equatorial region. Ultimately, sustaining a climate state with weak tropical sea surface temperature gradients may require additional mechanisms of ocean heat uptake (such as enhanced ocean vertical mixing).
Upwelling in eastern boundary current regions is crucial to bringing nutrient‐rich water to the photic zone and supporting the associated ecosystems. This upwelling is a result of the wind‐driven ...ocean circulation and is therefore susceptible to changes in the atmospheric circulation. We use the Community Earth System Model and observational data to explore the response of upwelling in the California Current and Canary Current systems to shifts in the Northern Hemisphere subtropical high‐pressure systems. We find that shifts in the North Pacific subtropical high explain a substantial fraction of both the short‐term variability and projected trend in upwelling in the California Current system during boreal summer. By contrast, the Canary Current system is less affected by shifts of the North Atlantic subtropical high, mostly because the strongest wind anomalies associated with shifts of this high‐pressure system occur too far north. We also find little impact from the Northern Hemisphere Hadley cell.
Key Points
North Pacific subtropical high shifts explain much of the modeled trend in California Current system upwelling during boreal summer
Shifts of the North Atlantic subtropical high have only modest impacts on the Canary Current system
Both eastern boundary current systems are relatively insensitive to changes in the Northern Hemisphere Hadley cell
The impact of cloud‐radiative interactions on the tropical circulation and its response to surface warming are studied in aquaplanet model simulations with prescribed sea‐surface temperatures from ...eight global atmosphere models. Simulations with enabled and disabled cloud‐radiative interactions are compared. In a present‐day‐like climate, the presence of cloud‐radiative interactions strengthens the Hadley cell, narrows and strengthens tropical ascent, and widens subtropical descent. These cloud impacts are robust across models and are shown to be related to the energetics and mass constraints of the tropical atmosphere. Cloud‐radiative interactions have no robust impact on the circulation response to surface warming but amplify model differences in the response of the ascent and the Hadley cell strength. The lack of robust cloud impacts is consistent with the fact that surface warming‐induced changes in atmospheric cloud‐radiative effects are small compared to the cloud‐radiative effects in the present‐day‐like climate.
Plain Language Summary
The atmospheric circulation is crucial for the global and regional climate and climate change. A main source of uncertainty in climate models is clouds. An important way of how clouds impact the circulation is by absorbing, emitting, and scattering radiation. We study the impact of cloud‐radiative interactions on the tropical circulation by considering an ensemble of climate models that simulate a water‐covered aquaplanet. In one set of simulations, clouds fully interact with radiation. In another set, clouds are made transparent to radiation, which disables cloud‐radiative interactions. We find that cloud‐radiative interactions strengthen the tropical Hadley circulation, narrow and strengthen the regions of upward motion, and widen the regions of downward motion. We relate these cloud impacts to basic energetics and mass constraints of the tropical atmosphere. In response to increasing surface temperatures, cloud‐radiative interactions have no robust impact on the circulation across the models but amplify model differences. Our results help to better understanding the role of clouds in the climate system and contribute to the World Climate Research Programme's Grand Challenge on Clouds, Circulation, and Climate Sensitivity.
Key Points
Atmospheric cloud‐radiative effects cause narrower and stronger ascent, stronger Hadley cell, and wider descent
Robust cloud impacts result from energetics and mass constraints of the tropical atmosphere
Clouds have no robust impact on circulation response to surface warming but increase model spread in response
Across a range of simulations with a coupled atmosphere–ocean climate model, shifts in the intertropical convergence zone (ITCZ) are induced by an interhemispheric heating contrast. The response to ...heating anomalies which are polar amplified are contrasted with those which are largest in the tropics. First, we find that ITCZ shifts are always damped relative to simulations in which the ocean circulation is held fixed, irrespective of the heating distribution, keeping the ITCZ “stuck” to latitudes near the equator. The damping is primarily due to the ocean’s anomalous cross-equatorial energy transport associated with the coupling of the trade winds to an oceanic cross-equatorial cell (CEC). Second, we find that the damping effect is strongest when the forcing distribution is polar-amplified, which enhances the gross stability of the CEC and maximizes the efficiency of its cross-equatorial energy transport. Third, we argue that the ocean’s energy transport can have secondary impacts on ITCZ shifts through its interaction with climate feedbacks. Finally, we discuss the implications of our study for our understanding of the role of CECs in damping ITCZ shifts and the atmosphere’s energy balance.
This study examines the cause of the spread of extratropical circulation responses to the inclusion of atmospheric cloud radiative effects (ACRE) across atmospheric general circulation models. The ...ensemble of Clouds On‐Off Klimate Intercomparison Experiment aquaplanet simulations shows that these responses include both equatorward and poleward shifts of the eddy‐driven jet of varying magnitudes. These disparate extratropical responses occur despite the relatively consistent response in the tropics: a heating in the upper troposphere, which leads to a strengthening of the Hadley cell. It is argued that the eddy‐driven jet response is a competition between two effects: the local influence of clouds driving shifts of the jet through meridional gradients in ACRE and the remote impact of a strengthened Hadley cell causing an equatorward shift of the eddy‐driven jet. Simulations in which cloud radiative effects are separately turned on in the tropics and extratropics demonstrate this explicitly.
Key Points
The response of the eddy‐driven jet latitude to the inclusion of cloud radiative effects varies widely across models
The Hadley cell and subtropical jet response to clouds is the primary control on how each model's eddy‐driven jet shifts
An important secondary control arises from the local impact of clouds onto the baroclinicity of the midlatitude atmosphere in each model
This study investigates the behaviour of subtropical high-pressure systems and the Hadley cell, which affect the weather of South Africa, using the ERA-Interim database and ensemble of 14 global ...circulation models from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). Mass stream function was used to represent the Hadley cell. To analyse the behaviour of the subtropical anticyclones, monthly sea level pressure, the 1018 hPa isobar and the maximum isobar in the study area were used. The seasonal variation of the anticyclones and Hadley circulation is consistent with rainfall over South Africa. During austral summer, a less intense, narrow mass stream function, South Atlantic Subtropical Anticyclone and Mascarene High are located more southwards, causing rainfall over the eastern parts of South Africa. During the austral winter, Hadley circulation, as well as the anticyclones, is stronger and located more northwards, causing rainfall over the southern and southwestern parts of South Africa.
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