Previous studies using reanalysis data suggest an intensification and poleward expansion of the tropical Hadley circulation (HC) throughout the twentieth century, yet the HC climatology and trends ...remain undocumented for many of the newest reanalyses. An intercomparison of eight reanalyses is presented to better elucidate the mean state variability and trends concerning HC intensity and width. Significant variability between reanalyses was found in the mean HC intensity with less variability in HC width. Certain reanalyses (e.g., ERA40 and the Climate Forecast System Reanalysis) tend to produce stronger meridional overturning, while others (National Centers for Environmental Prediction–National Center for Atmospheric Research and Modern‐Era Retrospective‐Analysis for Research and Applications) are constantly weaker. The NOAA–Cooperative Institute for Research in Environmental Sciences Twentieth Century Reanalysis best matched the ensemble averages with the exception of a poleward shift in the subtropical terminus. Ensemble trends regarding HC intensity and width are broadly consistent with previous work, indicating a 0.40 (0.07) × 1010 kg s−1 decade−1 intensification in the northern (southern) cell and a 1.1° decade−1 widening in the past 30 years, although some uncertainty remains regarding the intensity of the southern cell. Longer‐term ensemble trends (i.e., 1958–2008) containing fewer ensemble members suggest a weaker northern cell intensification but stronger southern cell intensification and a more modest widening of the HC (i.e., 0.53° decade−1) compared to the last 30 years. Separation of the seasonally averaged stream function magnitudes by the El Niño–Southern Oscillation (ENSO) phase revealed a weak clustering and statistically significant strengthening of the mean circulation for El Niño compared to ENSO neutral and La Niña events for the winter cell with little difference in the summer cell intensity.
Key Points
Significant mean state variability exists among reanalyses for the Hadley cell
Trends in new data sets suggest intensification and widening of the Hadley cell
The winter hemispheric cell shows a strong ENSO dependency not seen in summer
Global climate warming poses a significant challenge to humanity; it is associated with, e.g., rising sea level and declining Arctic sea ice. Increasing extreme events are also considered to be a ...result of climate warming, and they may have widespread and diverse effects on health, agriculture, economics, and political conflicts. Still, the detection and quantification of climate change, both in observations and climate models, constitute a main focus of the scientific community. Here, we develop an approach based on network and percolation frameworks to study the impacts of climate changes in the past decades using historical models and reanalysis records, and we analyze the expected upcoming impacts using various future global warming scenarios. We find an abrupt transition during the evolution of the climate network, indicating a consistent poleward expansion of the largest cluster that corresponds to the tropical area, as well as the weakening of the strength of links in the tropic. This is found both in the reanalysis data and in the Coupled Model Intercomparison Project Phase 5 (CMIP5) 21st century climate change simulations. The analysis is based on high-resolution surface (2 m) air temperature field records. We discuss the underlying mechanism for the observed expansion of the tropical cluster and associate it with changes in atmospheric circulation represented by the weakening and expansion of the Hadley cell. Our framework can also be useful for forecasting the extent of the tropical cluster to detect its influence on different areas in response to global warming.
The subtropical highs have a zonal mean and a zonally asymmetric component related to the Hadley cell and land‐sea contrast, respectively. Based on 37 Coupled Model Intercomparison Project phase 5 ...models, relative roles of the Hadley cell and land‐sea contrast in future changes of the North Pacific and North Atlantic subtropical highs (NPSH and NASH) are evaluated. Both the NPSH and NASH are significantly enhanced during boreal spring (April–June) but not in summer (July–September). Although the zonally asymmetric component contributes to more than half of the enhancement during spring, the zonal mean component is responsible for the interseasonal contrast of the responses of the NPSH and NASH between spring and summer. The seasonally dependent Hadley cell changes are due to changes in tropical precipitation related to sea surface temperature (SST) warming, while enhanced land‐sea contrast has comparable effects on the NPSH and NASH during both spring and summer, with important implications to U.S. regional precipitation.
Plain Language Summary
The North Pacific and North Atlantic subtropical highs (NPSH and NASH) are the most evident low‐level atmospheric features in the Northern Hemisphere during the warm season (April–September). Both the Hadley cell and land‐sea distribution contribute to its formation and seasonal variation. Previous studies only focused on the future changes of the NPSH and NASH during their peak season (June–August). Here we found that both the NPSH and NASH will be intensified more during spring (April–June) than summer (July–September) under climate warming. The enhanced land‐sea thermal contrast under global warming strengthens the NPSH and NASH, with similar magnitude in spring and summer. However, the Hadley cell enhances the NPSH and NASH during spring while weakens them during summer. Hence, the seasonally dependent response of the NPSH and NASH is mainly driven by the Hadley cell. The seasonality changes of the NPSH and NASH have important implications for the U.S. regional precipitation.
Key Points
Both North Pacific and North Atlantic subtropical highs are more significantly enhanced during spring than summer in response to warming
Land‐sea contrast contributes more to the enhancement in spring, but the Hadley cell dominates the seasonally dependent response
The spring enhancement and seasonal dependence of subtropical high response have important implications for U.S. regional precipitation
The separate effects of ozone depleting substances (ODSs) and greenhouse gases (GHGs) on forcing circulation changes in the Southern Hemisphere extratropical troposphere are investigated using a ...version of the Canadian Middle Atmosphere Model (CMAM) that is coupled to an ocean. Circulation-related diagnostics include zonal wind, tropopause pressure, Hadley cell width, jet location, annular mode index, precipitation, wave drag, and eddy fluxes of momentum and heat. As expected, the tropospheric response to the ODS forcing occurs primarily in austral summer, with past (1960–99) and future (2000–99) trends of opposite sign, while the GHG forcing produces more seasonally uniform trends with the same sign in the past and future. In summer the ODS forcing dominates past trends in all diagnostics, while the two forcings contribute nearly equally but oppositely to future trends. The ODS forcing produces a past surface temperature response consisting of cooling over eastern Antarctica, and is the dominant driver of past summertime surface temperature changes when the model is constrained by observed sea surface temperatures. For all diagnostics, the response to the ODS and GHG forcings is additive; that is, the linear trend computed from the simulations using the combined forcings equals (within statistical uncertainty) the sum of the linear trends from the simulations using the two separate forcings. Space–time spectra of eddy fluxes and the spatial distribution of transient wave drag are examined to assess the viability of several recently proposed mechanisms for the observed poleward shift in the tropospheric jet.
Widespread public and scientific interest in the recent global warming hiatus and related multidecadal climate variability stimulated a surge in our understanding of key metrics of global climate ...change. While seeking explanations for the nearly steady global mean temperature from late 1990s through the early 2010s, the scientific community also grappled with concomitant and seemingly inconsistent changes in other metrics. For example, over that period, Arctic sea ice experienced a rapid decline but Antarctic sea ice expanded slightly, summertime warm extremes continued to rise without evidence of a pause, and the expanding Hadley cell trend maintained its course. In this article, we review recent advances in understanding the multidecadal variability of these metrics of global climate change, focusing on how internal multidecadal variability may reconcile differences between projected and recently observed trends and apparent inconsistencies between recent trends in some metrics. We emphasize that the impacts of global scale, naturally occurring patterns on multidecadal timescales, most notably the Pacific and Atlantic Multidecadal Variability, tend to be more regionally heterogeneous than those of radiatively forced change, which weakens the relationship between local climate impacts and global mean temperature on multidecadal timescales. We conclude this review with a summary of current challenges and opportunities for progress.
•Global climate change metrics vary naturally over multidecadal periods.•These metrics may track global mean surface temperature in counterintuitive ways.•The global warming hiatus provided a focus for rapid scientific progress.•Pacific and Atlantic decadal variability played a key role in recent modulations.
There have been several observational and modeling analyses that have indicated that the stratospheric quasi‐biennial oscillation (QBO) significantly affects the tropical troposphere. This article ...aims to identify the QBO signal in tropical deep convection. Some difficulties in previous studies were ambiguities in the identification of tropical deep convection in observations, and also in separating the El Niño/Southern Oscillation (ENSO) and other tropospheric signals from QBO influences. We use a recent cluster analysis of 21.5 years of International Satellite Cloud Climatology Project tropical weather states to identify tropical deep convection and cirrus clouds, as well as 32.25 years of precipitation data as proxies for deep convection. Correlations between the QBO, ENSO, and other tropospheric patterns such as the tropospheric biennial oscillation and Pacific decadal oscillation are taken into account to isolate the influence of the QBO. Whereas tropical deep convection is mostly related to ENSO and the annual cycle, the QBO westerly phase, independent of the annual cycle as well as linear and nonlinear impacts of ENSO, leads to an eastward shift in the strength of meridional overturning contributions to the Hadley circulation over the Pacific and thus also affects the subtropical circulation. For deep convective clouds, relative differences in convective cloud cover between the QBO easterly and QBO westerly phases can be as large as 51% ± 7% of the annual average over isolated regions in the tropical west Pacific and 103% ± 35% over the east Pacific, where the absolute values are lower and where notable deviations occur during the QBO westerly phase.
Key Points
QBO influences tropical deep convection over the central Pacific
QBO westerly phase leads to eastward shift in strength of Hadley cell
ENSO signal is removed from tropical convection
Various observational and modeling studies have shown that the Hadley cell (HC) has widened during the past few decades. Here we present further observational evidence of the widening of the HC in ...the Southern Hemisphere by tracking the location of the subtropical ridge. A robust and significant poleward shift of the southern edge of the HC has been observed during the austral summer over the past three decades with a shift of 0.22° per decade between 1980 and 2012, primarily from the South Atlantic Ocean eastward to Australia. In other seasons, significant changes in the southern edge of the HC have not been observed, with a discernable regional trend having only occurred in limited regions. The comparison of these results with those derived from reanalysis data and possible causes for the summer HC expansion in the Southern Hemisphere are briefly discussed.
Key Points
Observational evidence of Hadley cell widening and its regional structure
The seasonal variability of precipitation has a profound impact on water demand for India's agricultural and socio‐economic sectors which are associated with continental and surrounding oceanic ...moisture sources controlled by dynamic and thermodynamic processes. In this study, moisture sources have been tracked by the Lagrangian trajectory approach over four homogeneous regions: Northwest India (NWI), Central India (CEI), Northeast India (NEI) and South Peninsular India (SPI). ERA‐Interim reanalysis data are used throughout the study for three decades from 1989 to 2018. The results indicate that the multidecadal variability of precipitation has strengthened towards the mid‐decade (1999–2008), mainly attributable to moisture source transport from the Arabian Sea. These enhancement patterns of moisture sources have been widespread over Central India for all three decades. Moisture availability in the boundary layer highly affects the precipitation and it is found that 80% of the moisture above the boundary layer contributes >10% of monsoonal precipitation occurrences. Further, the NEI drying trend in monsoonal rainfall is found to be related to the consequent weakening of the land–ocean temperature gradient (LOTG) during all three decades in this region and the absence of low cloud cover over the region throughout the study period. In addition, the moisture sources are regulated by prevailing large‐scale features such as frequency of La Niña, northward shifting of low‐level jet and strengthening of Hadley cell.
Multidecadal variability of precipitation over Indian region has been regulated by moisture transported from the Arabian Sea. There is an enhancement in moisture transport over central India region in last 30 years. The moisture sources are strongly influenced by La Niña, northward shifting of low‐level monsoon jet and strengthening of Hadley cell.
The Coupled Model Intercomparison Project Phase 5 (CMIP5) 21st century climate change simulations exhibit a robust (slight) weakening of the Hadley cell (HC) during the boreal winter (summer, ...respectively) season in the future climate. Using 30 different coupled model simulations, we investigate the main mechanisms for both the multimodel ensemble mean changes in the HC strength and its intermodel changes in response to global warming during these seasons. A simple scaling analysis relates the strength of the HC to three factors: the meridional potential temperature gradient, gross static stability, and tropopause height. We found that changes in the meridional potential temperature gradients across the subtropics in a warming climate play a crucial role in the ensemble mean changes and model‐to‐model variations in the HC strength for both seasons. A larger reduction in the meridional temperature gradient in the Northern Hemisphere in boreal winter leads to the larger reduction of the HC strength in that season.
Key Points
Meridional temperature gradient change is the main cause of HC strength change
Meridional temperature gradient change explains intermodel spread of HC change
The scaling relations predict the changes in HC strength fairly well
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