A hierarchy of idealized models is used to investigate the roles of Hadley cell forcing and latent heat release from carbon dioxide condensation in determining the annular potential vorticity ...structure of the Martian winter polar vortex. The angular momentum conserving the Hadley cell model of Lindzen and Hou with summer hemisphere heating maximum of appropriate strength and latitude produces a strong westerly jet near 60°N, which is similar in strength to the winter polar night jet on Mars. Although the corresponding potential vorticity profile in the angular momentum conserving and thermal wind regions has no annular structure resembling the Martian one, there is an implied δ‐function at the discontinuity in zonal wind. This δ‐function is smoothed out by explicit diffusion in full axisymmetric model integrations forming a partial annular structure, though a local maximum in potential vorticity at the pole persists and is further enhanced when cooling representing the polar night is included. A distinct polar minimum and clear annular potential vorticity structure are obtained, however, when an additional representation of polar latent heating is also included. Full eddy‐permitting shallow‐water model integrations confirm the basic structure obtained by the axisymmetric model and suggest a nominal value of viscosity appropriate as a representation of the effects of eddy mixing. Instability of the polar annulus leads to vacillation‐type behaviour involving eddy growth and annulus disruption, followed by re‐formation under the influence of radiative relaxation. The degree of transience and mean eddy activity both show an increase with stronger latent heating and the resulting deeper polar potential vorticity minimum, showing that mixing in polar regions may be dependent on details of polar carbon dioxide condensation. Vacillation time‐scales are also shown to vary with radiative time‐scales, but through a modification of instability growth rate rather than as a result of direct radiative restoration.
Potential vorticity distribution over the North Pole due to Hadley cell forcing and additional polar latent heating, increasing left to right.
Numerous observational studies have found that the Hadley cells are expanding poleward in both the Northern and Southern Hemispheres, and model results suggest that such expansion is likely to ...continue throughout this century as a result of global warming. This has led to concerns that the subtropical dry zones may also shift poleward. However, precipitation changes associated with Hadley cell width are zonally asymmetric—especially in the Northern Hemisphere—suggesting that a more regional focus may give a clearer picture of these changes. In this study, we consider the influence of the Northern Hemisphere subtropical highs on summertime precipitation in North America and contrast this with the influence of Hadley cell expansion. Specifically, we consider the North Pacific and North Atlantic subtropical highs (NPSH and NASH, respectively) and define, for each of these, three indices representing longitude, latitude, and strength. We find that short‐term (monthly) precipitation variability over North America has comparable sensitivities to all six indices. However, for projections of 21st century climate change, North American summertime precipitation trends are driven more strongly by the longitudes of the highs than by their latitudes or strengths. Since the Hadley cells are defined with zonal‐mean quantities, much of the impact of these subtropical high shifts cannot be captured by Hadley cell metrics.
Key Points
North American summer precipitation is more sensitive to shifts of the subtropical highs than to changes in the zonal‐mean Hadley cell
Projected 21st century summer precipitation trends are driven more by the longitudes of the highs than by their latitudes or strengths
Subtropical high shifts account for a considerable fraction of the total projected precipitation trends
This study conducts an attribution analysis of long‐term changes in the southern edge of the local Hadley cell (HC) during austral summer for the past three decades (1979–2009). The southern edges of ...the local overturning circulations (local HC) are defined as the latitudes of maximum sea level pressure in the Southern Hemisphere subtropics, and the long‐term variations of local HC edges from multireanalyses are compared with those from Coupled Model Intercomparison Project Phase 5 (CMIP5) multimodel simulations by using the optimal fingerprinting technique. The observed local HC exhibits a poleward expansion in the Atlantic and Indian Ocean regions, which is successfully reproduced by the CMIP5 models including anthropogenic forcing (ANT) but with a weaker amplitude. The detection analyses further show that ANT signals are detected robustly in both Atlantic and Indian HC trends. More importantly, anthropogenic forcings other than greenhouse gas forcing are found to be clearly detected in isolation, indicating a possible attribution of the observed local HC widening over these regions to stratospheric ozone depletion.
Key Points
Human contribution to the recent widening of local Hadley cells is analyzed in the austral summer by using the subtropical ridge location
Anthropogenic signals are detected robustly in the local Hadley cell expansion over the Atlantic and Indian Oceans
The observed broadening of local Hadley cells is likely attributable to stratospheric ozone depletion
The steady and transient response of “dynamically” dry and moist atmospheres to uniform sea‐surface temperature (SST) is studied using an aquaplanet general circulation model (GCM). Specifically, the ...latent heat (Lv) of water vapour is varied, so that for small Lv, water substance is essentially a passive tracer from a dynamical point of view. Despite the lack of SST gradients, Hadley and Ferrel cells in the same direction as that on present‐day Earth are observed for relatively stronger moist coupling. Organized convergence by equatorial waves drives tropical overturning, and along with the equatorial deformation radius, the Hadley cell width increases with coupling strength. An abrupt switch to a much shallower tropical cell is noted when the system becomes completely passive. Moist static energy is transported equatorward in the tropics and a larger amount is directed poleward in the midlatitudes. As a whole, there is an almost invariant poleward transport of moist static energy for relatively strong coupling of water substance. Transient extratropical activity is seen in the form of intense warm‐core vortices for strong coupling, and these systems become weaker and smaller as Lv decreases. The drift of these moist vortices results in the observed poleward energy transport in the midlatitudes. In the tropics, intraseasonal variability is dominant and systematically shifts to longer time periods with stronger coupling. In fact, large‐scale, low‐frequency Kelvin waves and MJO‐like modes disappear as water vapour becomes passive in nature. Further, the direction of tropical overturning reverses as Lv decreases due to the off‐equatorial convergence imposed by tropical Rossby waves. Finally, extreme rainfall events associated with cyclonic storms vanish as water vapour becomes dynamically inactive, however, moderate precipitation events increase leading to higher total precipitation for weaker coupling of water substance. Tropospheric heating due to a saturation of the outgoing longwave radiation results in an increase in the stability of the atmosphere for strong coupling, and provides a plausible physical mechanism for interpreting the behaviour of precipitation.
Mean meridional mass stream function (black contours; solid clockwise and dashed counterclockwise) and angular momentum (grey contours) averaged over the last two years of the runs. The divergence of eddy momentum flux is shown in colour. The streamfunction contours are logarithmic in nature, with its magnitude doubling between successive contours. The lowest contour has a value of 2.5×109kg·s−1 and zero contour is not shown. Angular momentum contours are at intervals of 0.1Ωa2, with decreasing values away from the Equator
Sea surface temperature (SST) could significantly affect the dynamic and thermodynamic conditions of the atmospheric circulation and consequently the cloud variations. Here we use several different ...satellite records to extract the spatial‐time modes of total cloud cover (TCC) by employing a pairwise rotation of Empirical Orthogonal Function analysis. The results show that the first two principal oscillation modes of TCC are closely associated with the Central Pacific El Niño Southern Oscillation (CP ENSO) and Eastern Pacific (EP) ENSO during the 1980s–2000s, while the ENSO‐like mode of TCC can provide an evident contribution to the TCC change during the 2000s–2010s. In CP El Niño, the cloud vertical structure decomposed from CloudSat observations shows an increase of cloud occurrence frequency near the equator and around 40°, and a decrease around 10° in both hemispheres, suggesting a symmetric tightening of Hadley cell (HC). In addition, cloud occurrence frequency increases around 180°, which is accompanied by an eastward shift of Walker circulation (WC). In EP El Niño, TCC increases (decreases) over the Equatorial Eastern Pacific (Western Pacific warm pool), and decreases asymmetrically over the subtropical Pacific Ocean, indicating a weakening of WC and an asymmetric tightening of HC, respectively. The different responses of circulation and clouds to CP and EP El Niño highlight the nonlinearity of El Niño SST forcing. We also construct a trend mode of TCC to investigate cloud long‐term responses to SST warming by transferring the linear trends of the rest modes to a specific mode. The principal components (PCs) of TCC trend modes are strongly correlated with global‐mean SST (GSST) with correlation a coefficient of about 0.60 during the 1980s–2000s and 0.45 during the 2000s–2010s, suggesting a continued influence of global SST warming on TCC. The global TCC change is mainly influenced by the combined effects of Atlantic Multi‐decadal Oscillation (AMO), Pacific Decadal Oscillation (PDO) and Indian Ocean Dipole (IOD). The variation about the trend mode of TCC is closely associated with PDO and IOD.
Sea surface temperature (SST) patterns could significantly affect dynamic and thermodynamic conditions of atmosphere circulation and cloud variations as shown in the image. The principal oscillation modes of total cloud cover (TCC) are closely associated with the Central Pacific El Niño Southern Oscillation and Eastern Pacific El Niño Southern Oscillation. The trend mode of TCC is strongly correlated with the global sea surface temperature (GSST), indicating an important influence of global SST warming on the TCC change during the 1980s–2000s.
Southwestern South America (SWSA) has undergone frequent and persistent droughts in recent decades with severe impacts on water resources, and consequently, on socio-economic activities at a ...sub-continental scale. The local drying trend in this region has been associated with the expansion of the subtropical drylands over the last decades. It has been shown that SWSA precipitation is linked to large-scale dynamics modulated by internal climate variability and external forcing. This work aims at unravelling the causes of this long-term trend toward dryness in the context of the emerging climate change relying on a large set simulations of the state-of-the-art IPSL-CM6A-LR climate model from the 6th phase of the Coupled Model Intercomparison Project. Our results identify the leading role of dynamical changes induced by external forcings, over the local thermodynamical effects and teleconnections with internal global modes of sea surface temperature. Our findings show that the simulated long-term changes of SWSA precipitation are dominated by externally forced anomalous expansion of the Southern Hemisphere Hadley Cell (HC) and a persistent positive Southern Annular Mode (SAM) trend since the late 1970s. Long-term changes in the HC extent and the SAM show strong co-linearity. They are attributable to stratospheric ozone depletion in austral spring-summer and increased atmospheric greenhouse gases all year round. Future ssp585 and ssp126 scenarios project a dominant role of anthropogenic forcings on the HC expansion and the subsequent SWSA drying, exceeding the threshold of extreme drought due to internal variability as soon as the 2040s, and suggest that these effects will persist until the end of the twenty-first century.
ABSTRACT
From a local point of view, in May/June, there is an important and positive 2 m temperature trend at Palma (Mallorca), which is simultaneous and highly correlated with a strong increase in ...the 500 hPa geopotential height. This study analyses this fact as well as the observed tendencies in a wider seasonal and geographical context.
We confirm the particularly high correlation between 2 m temperature and 500 hPa geopotential during the warm months as opposed to the much weaker correlation in winter. This suggests that mechanisms for thermal changes act differently throughout the year in this region. Besides the direct radiative effect, warm season near‐surface temperatures are linked to the presence of deep anticyclones, which effectively determine the northern edge of the Hadley cell. Accordingly, the strong warming trend in the area of Palma during the warm months of the year is purportedly related to the poleward extension of the Hadley cell.
The fact that May/June shows the highest low‐level temperature trend among all bi‐monthly series is a common regional feature over a relatively wide area over the Western Mediterranean. Different geographical patterns emerge in other periods of the year. In July/August, the strongest low‐level warming area drifts to the east‐northeast, towards Ukraine and Russia. Coincidentally, the 500 hPa geopotential tendencies show a coherent pattern, with an intense positive trend ridge over the Western Mediterranean area in May/June, and a displacement of this ridge to the north‐east in July/August.
We show the connection between 500 hPa geopotential height and near‐surface temperature by means of a multiple lineal regression that attributes half of the local temperature tendency in Palma to the intensification of a 500 hPa ridge centred over the Western Mediterranean and surroundings.
Compared to the zonal-mean Hadley cell (HC), our knowledge of the characteristics, influence factors and associated climate anomalies of the regional HC remains quite limited. Here, we examine ...interannual variability of the northern poleward HC edge over western Pacific (WPHCE) during boreal winter. Results suggest that interannual variability of the WPHCE is impacted by the El Niño-Southern Oscillation (ENSO) Modoki, North Pacific Oscillation (NPO) and North Atlantic Oscillation (NAO). The WPHCE tends to shift poleward during negative phase of the ENSO Modoki, and positive phases of the NPO and NAO, which highlights not merely the tropical forcing but also the extratropical signals that modulate the WPHCE. ENSO modoki, NPO and NAO modulate the WPHCE via inducing atmospheric anomalies over the western North Pacific. We further investigate the climatic impacts of the WPHCE on East Asia. The poleward shift of the northern descending branch of the WPHC results in anomalous upward (downward) motions and upper-level divergence (convergence) anomalies over south-central China (northern East-Asia), leading to increased (decreased) rainfall there. Moreover, pronounced cold surface air temperature anomalies appear over south-central China when the sinking branch of the WPHC moves poleward. Based on the temperature diagnostic analysis, negative surface temperature tendency anomalies over central China are mostly attributable to the cold zonal temperature advection and ascent-induced adiabatic cooling, while the negative anomalies over South China are largely due to the cold meridional temperature advection. These findings could improve our knowledge of the WPHCE variability and enrich the knowledge of forcing factors for East Asian winter climate.
Global climate models fail to represent the annual cycle of tropical‐extratropical cloud bands and produce too much summer rainfall over subtropical southern Africa. This study demonstrates that ...running a regional convective‐permitting climate simulation alleviates these biases, counteracting biases that are present in the parent model. The improvement emerges from stronger vertical mass flux in the tropics, which forces a stronger local Hadley overturning into the summer hemisphere. This enhanced overturning increases upper‐level subsidence in the subtropics and amplifies the forcing of the local subtropical jet. Together, these improvements halve the wet subtropical rainfall bias and are associated with a 50% increase to an 80% match between the simulated and observed annual cycle of crucial tropical‐extratropical cloud band rainfall systems. The results advocate for the increased use of convective‐permitting climate models with domains that include regional tropical convection hot spots, in order to fully benefit from the explicit representation of deep convection.
Plain Language Summary
Climate models, both global and regional, approximate the development and effect of thunderstorms on the simulated circulation. The inadequacy of these approximations is widely thought to produce inaccuracies in simulated regional climates. These errors are particularly severe over southern Africa where model simulations produce too much rain and poorly represent the annual cycle of the dominant summer rainfall systems. In this study, a pan‐African regional climate simulation is run for 10 years at a grid spacing that allows direct simulation of widespread outbreaks of thunderstorms, breaking the model dependence on inadequate approximations. This convection‐permitting model produces a substantially more realistic regional climate over subtropical southern Africa with the main rainfall systems occurring at the right time of year and only a moderate overestimate of monthly summer rainfall. This result is particularly striking because this regional model overcomes errors that are present in the global model which provides the boundary conditions to its pan‐African domain.
Key Points
Large‐domain convective‐permitting regional climate simulations can overcome bias in parent model
Such a model run for 10 years over Africa shows stronger local Hadley overturning into the subtropics
This improves the annual cycle of rainfall systems and reduces wet bias over subtropical southern Africa
Abstract
Hadley Cells are thermally driven cell in the tropics. On its occurrence, these cells are strongly influenced by the sea surface temperature (SST) distribution across the tropical ocean or ...the Pacific Ocean as the investigated location in this study. The SST shifting in the Pacific Ocean is mainly due to the ENSO. An opposite SST polarity between the western and eastern Pacific Ocean are captured during ENSO events. This means that ENSO could trigger an anomalous regional Hadley Cells that behave oppositely between Indonesia or the western Pacific and the eastern Pacific. This study examines the strength of the regional Hadley Cells related to the ENSO event across the Indonesian region and the Pacific Ocean. A significant correlation between the Hadley Cells and ENSO as the tropical climate variability in the Pacific Oceans are found. The strength of the Hadley Cells associated with ENSO event is examined by using the zonally average vertical velocity across the Pacific Ocean. During La Nina, the regional Hadley Cells over Indonesia or the western Pacific strengthened, whereas the regional cells over the eastern Pacific weakened. In contrast, during El Nino where the warm pool shifted to the eastern Pacific, the regional cell in the eastern Pacific strengthened, while the cell over the western Pacific weakened. These anomalous conditions clearly show that the meridional temperature gradient is strongly affecting the regional Hadley Cells strength. The stronger the meridional temperature gradient, the stronger the regional Hadley Cells.
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