Abstract
This review provides a comprehensive coverage of changes of the Hadley Cell extent and their impacts on the weather, climate, and society. The theories predicting the Hadley Cell width are introduced as a background for the understanding of the circulation changes and the metrics used for detection. A variety of metrics derived from various data sources have been used to quantify the Hadley Cell width. These metrics can be classified as dynamical, hydrological, thermal, and chemical metrics, based on the properties of the variables used. The dynamical metrics have faster trends than those based on thermal or hydrological metrics, with the values exceeding 1 degree per decade. The hydrological metric edge poleward trends were found a slightly faster expansion in the Northern Hemisphere than its southern counterpart. The chemical metrics show a poleward trend of more than 1 degree per decade in both hemispheres. We also suggest a few reasons for the discrepancy among trends in Hadley Cell expansion found in previous studies. Multiple forcings have been found responsible for the expansion, which seems to be more attributed to the natural variability than anthropogenic forcing. Validation of the scaling theories by the trends in Hadley Cell width suggests that theories considering the extratropical factor would be better models for predicting the Hadley Cell width changes. The Hadley Cell has an impact on different atmospheric processes on varying spatio-temporal scales, ranging from weather to climate, and finally on society. The remaining questions regarding Hadley Cell climate are briefly summarized at the end.
Highlights
Introduction to the SPARC Reanalysis IntercomparisonProject (S-RIP) and overview of the reanalysis systems
Over 30 years after the Montreal Protocol came into effect, the ozone hole over the Antarctica is showing signs of healing, general circulation model (GCM) studies found that ozone recovery may approximately cancel the effects of greenhouse gas (GHG) on climate change in the first half of the 21st century, resulting in a reduced or near-zero trends in the Hadley Cell width, tropopause, and jet location [77,111]
This article reviews the theories for explaining the Hadley Cell extent, and the metrics and datasets for describing the Hadley Cell edge, summarizes various long-term trends of Hadley Cell expansion from previous studies, and suggests several reasons for the discrepancy among them
Summary
The Hadley Cell is a salient feature of Earth’s atmospheric circulation spanning half the globe. The third theory based on the supercriticality of the slope of isentropes, proposed by Korty and Schneider [15], posits that the Hadley Cell edge can be characterized by a critical value of supercriticality over which vertical wave activity fluxes extend in the dry atmosphere [16] This theory does not require angular momentum conservation of the tropical upper-tropospheric flow or use an expression for baroclinic instability. In the extratropical regions with high occurrence frequencies of subtropical jet, Eady growth rates increase according to the thermal wind balance, Equation (4) becomes f 0.31 N. where f = 2Ωsinφ is Coriolis parameter with φ as latitude, and ∂U/∂z is the vertical wind shear, ∆U is the zonal wind difference between the upper and lower troposphere. The validation of each mechanism is discussed in Section 4.2: How important each cause is in driving the Hadley Cell? How sensitive the latitudinal extent of the Hadley Cell is to each cause?
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