The projected weakening of the Northern Hemisphere Hadley cell will have large climatic impacts at low latitudes. Several mechanisms have been proposed to explain this weakening. In order to isolate ...and assess their relative importance, we here use the abrupt 4 × CO2 experiment of the Coupled Model Intercomparison Project phase 5, as this forcing separates the different mechanisms which respond on different time scales. We find that the Hadley circulation responds relatively quickly to quadrupling CO2 concentrations, reaching its steady‐state value after less than a decade. This fast response demonstrates that the weakening could not be solely due to the much slower increase in surface temperature. In addition, we show that the Hadley cell's weakening results from a combination of an increase in tropical static stability, partially offset by an increase in the latitudinal gradient of latent heating.
Plain Language Summary
The Hadley circulation has large effects on the variability of temperature and precipitation at low latitudes. By the end of the 21st century, climate models project a weakening of the circulation, which will have great societal impacts. It is thus critical to elucidate the underlying mechanisms of the projected weakening. Here, we assess different mechanisms for the projected weakening and show that it stems from an increase in the vertical temperature gradient in the tropics, which is partially offset by an increase in latent heating.
Key Points
Surface warming alone is unable to explain Hadley cell weakening under 4 × CO2 forcing, as the weakening exhibits a much faster response
Each of the previously proposed mechanisms analyzed in this study cannot fully explain the weakening of the Hadley cell
The weakening stems from a combination of the opposing effects of increases in stratification and in meridional gradient of latent heating
The projected widening and weakening of the Hadley circulation hold large societal impacts at low and subtropical latitudes. Previous studies suggested that ocean heat transport (OHT) might play a ...central role in future circulation's changes. Here, using ensembles of model integrations, we quantify the role of OHT in the evolution of the Hadley cell over the 20th and 21st centuries. We find that by the end of the current century OHT reduces the widening of the circulation by ∼35% (0.42°) and its weakening by ∼60% (1.3 × 1010 kg s−1). As a result, OHT delays the emergence from internal variability of the widening by 30 years and of the weakening by 20 years. Lastly, while oceanic heat uptake accounts for most of the reduced widening, and thus merely delays it by reducing surface warming, horizontal heat transport and net heat uptake have comparable impacts to reduce the weakening of the circulation.
Key Points
By the end of the 21st century ocean heat transport is projected to reduce the widening of the Hadley cell by ∼35% and its weakening by ∼60%
The reduced widening is mostly due to the increase in oceanic heat uptake, which thus delays the expansion of the tropics
Horizontal heat distribution and net heat uptake are both responsible for weakening the circulation by redistributing latent heating
The cosmogenic radionuclide 7Be is one of the best tracers for aerosol transport since its half-life of 53 days is in the time scale of many atmospheric circulation phenomena. In this work, we ...analyze a 12-years-long daily time-series for the airborne 7Be concentration for nine air filtering stations in the Southern Hemisphere or close to it. The observed latitudinal distribution of 7Be concentration, with its maximum at the southern subtropical high-pressure belt, is similar to the one in the Northern Hemisphere. A good time correlation was found between the 7°-shift of the 7Be concentration latitudinal distribution and the seasonal displacement of the extent of the Hadley cell. This is consistent with tropopause folding events, mostly occurring in spring, being the main contribution for the injection of stratospheric 7Be into the descending branch of the Hadley cell.
•We analyzed 12-year long 7Be concentrations time series from the Southern Hemisphere.•The 7Be latitudinal distribution shows a maximum at the subtropical high-pressure belt.•Time correlation found between the shift of this distribution and the seasonal displacement of the Hadley cell.
The South Atlantic Subtropical Anticyclone (SASA) is the main feature of the atmospheric circulation over the South Atlantic Ocean, and its study is of great importance to explain many ...characteristics of the Brazilian weather and climate. Therefore, this study aims to present (1) a review of the literature on SASA including the drivers of the semi-permanent anticyclones and (2) the main features of the SASA in the future climate obtained through the projections of three global climate models (HadGEM2-ES, GFDL-ESM2M, and MPI-ESM-MR), from the Coupled Model Intercomparison Project (CMIP5), using the Representative Concentration Pathway 8.5 (RCP8.5) scenario. SASA is zonally wider in winter and retracted to the east in summer, when it presents a more circular format. These features of the SASA in the present climate (1979–2005) are well represented by the three global climate models, which also project this same SASA seasonal pattern for the future climate (2065–2095). Considering the projections, they indicate a slightly poleward expansion of the SASA, which is associated with the widening of the Hadley cell. At the SASA core, the pressure can be similar or slightly more intense than the present climate.
The interannual sea level variations in the Northwestern Pacific are significantly tied to the El Niño-Southern Oscillation (ENSO), with opposite responses and asymmetric strength to the south and ...north of 20°N. Using a 1.5-layer reduced gravity model, we show that the spatial pattern of sea level interannual variations is not sensitive to the existence of the Luzon Strait located at around 20°N, but is dominated by the basin-scale wind stress curl associated with ENSO. Dynamically, sea level variations tied to ENSO are attributed to long baroclinic Rossby wave adjustment driven by wind stress curl in the western/central Pacific, as previous studies have indicated. However, the responses of wind stress curl to ENSO are dynamically different to the south and north of 20°N. In the tropics, the wind stress curl anomaly results from the ENSO event directly, through the Matsuno-Gill response with the transition of zonal wind anomaly at approximately 20°N, while in the subtropics, the wind stress curl anomaly is mainly set up by atmospheric vertical motions through the Hadley Cell.
•Inter-annual sea level variations in the Northwestern Pacific are opposite and asymmetric to the south and north of 20°N.•The ENSO-related wind forcing dominates the spatial pattern of regional sea level change.•The tropical response is due to Matsuno-Gill response and the subtropical response is tied to Hadley Cell.
Significance Biogeophysical effects such as albedo and evapotranspiration changes due to deforestation were shown by several studies in the past to exert strong influence on local surface ...temperatures. In this study, we assess the remote versus local effects of large-scale deforestation on precipitation in the monsoon regions of the world. In contrast to the dominant role of local effects on temperature changes, we find that the remote effects have a larger influence than local effects on shifting the location of the Intertropical Convergence Zone and hence precipitation in all the monsoon regions. This result has important implications for assessing the net benefits of climate change mitigation strategies such as afforestation/reforestation and for understanding changes in monsoon rainfall in past climates.
In this paper, using idealized climate model simulations, we investigate the biogeophysical effects of large-scale deforestation on monsoon regions. We find that the remote forcing from large-scale deforestation in the northern middle and high latitudes shifts the Intertropical Convergence Zone southward. This results in a significant decrease in precipitation in the Northern Hemisphere monsoon regions (East Asia, North America, North Africa, and South Asia) and moderate precipitation increases in the Southern Hemisphere monsoon regions (South Africa, South America, and Australia). The magnitude of the monsoonal precipitation changes depends on the location of deforestation, with remote effects showing a larger influence than local effects. The South Asian Monsoon region is affected the most, with 18% decline in precipitation over India. Our results indicate that any comprehensive assessment of afforestation/reforestation as climate change mitigation strategies should carefully evaluate the remote effects on monsoonal precipitation alongside the large local impacts on temperatures.
Projected precipitation changes in a warming climate vary considerably, spatially, and between intensities. The changes can be greater or less than the ~7% K-1 Clausius-Clapeyron (CC) prediction, ...owing to dynamic effects. Using two global-climate-model large ensembles, we quantify the dynamically induced changes to precipitation extremes from the present (1996–2005) to late-21st-century (2071–2080) climates, as a function of recurrence interval, focusing on the subtropics. We separate non-CC changes into a term proportional to the present-day vertical-velocity spatial pattern (i.e., an amplification or damping thereof by a constant factor) and a residual. The amplitude term varies with recurrence interval, approximately canceling (doubling) CC for moderate (large) extremes, increasing precipitation variability. Contrastingly, the residual is quasi-uniform across recurrence intervals but spatially heterogeneous, weakening extremes over dry zones. This residual may be related to Hadley cell expansion, although this explanation is insufficient to explain many features, and other possible mechanisms are discussed.
A consistent weakening and poleward expansion of the Hadley circulation is diagnosed in the climate change simulations of the IPCC AR4 project. Associated with this widening is a poleward expansion ...of the subtropical dry zone. Simple scaling analysis supports the notion that the poleward extent of the Hadley cell is set by the location where the thermally driven jet first becomes baroclinically unstable. The expansion of the Hadley cell is caused by an increase in the subtropical static stability, which pushes poleward the baroclinic instability zone and hence the outer boundary of the Hadley cell.
Several studies have reported that global climate models underestimate the observed trend in tropical expansion, with the implication that such models are missing key processes of the climate system. ...We show here that integrations of a chemistry‐climate model forced with observed sea surface temperatures (SSTs), greenhouse gases, and ozone‐depleting substances can produce 1980 to 2009 expansion trends comparable to those found in most reanalyses data products. Correct representation of the SSTs changes is important for the Northern Hemisphere, while correct representation of stratospheric ozone changes is important for the Southern Hemisphere. The ensemble mean trend (which captures only the forced response) is nearly always much weaker than trends in reanalyses. This suggests that a large fraction of the recently observed changes may, in fact, be a consequence of internal atmospheric variability and not a response of the climate system to anthropogenic forcings.
Key Points
Climate models can capture recent HC expansion if internal variability is considered
SSTs important for Northern Hemisphere, ozone depletion for Southern Hemisphere
Models are not necessarily missing key processes
IntroductionThe results of the study showed that the correlation headley cell and subtropical jet on the atmosphere Iran at the level 200 hPa has a positive correlation with a value of 0.4-0.7 to 35 ...° latitude and also regression analysis showed that in latitudes between 15 35 degrees north of the subtropical jet 1(m/s) is higher than normal, although in 2017 up to latitudes 30 degrees north showed an increase of 2(m/s), which had a negative effect on rainfall.Data and MethodThe relationship between Hadley cell and olr in the southern, southwestern and southeastern regions of Iran with a value of 0.4 and the Zagros and northwestern heights of Iran with a value of 0.7 and regression with a value of (w/m2) 0.01 more than normal.Results and DiscussionIt acts as a tangible source of heat in the middle Wordspehr and the heat is added directly to the middle Wordspehr and causes heating of the upper half of the Wordspehr.ConclusionRegression 2 to 1 is shown. Low relative humidity along with the dried air mass is located below the descending branches of the headley cell, which has ruled the drought conditions (-0/7) showed that it creates conditions for lack of rainfall and drought.
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