Regional Widening of Tropical Overturning: Forced Change, Natural Variability, and Recent Trends

Staten, Paul W.; Grise, Kevin M.; Davis, Sean; Karnauskas, Kristopher B.; Davis, Nancy D.

doi:10.1029/2018jd030100

Cited by 43 publications

(56 citation statements)

References 95 publications

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“…Previous studies indicate that tropical expansion has strong regional dependence (Chen et al., 2014; Grise et al., 2018; Kim et al., 2017; Lucas & Nguyen, 2015; Staten et al., 2019). We find that the regional characteristics of tropical expansion are primarily determined by the regional variations of the MMTG (Figure 3).…”

Section: Discussionmentioning

confidence: 97%

Tropical Expansion Driven by Poleward Advancing Midlatitude Meridional Temperature Gradients

Yang

Lohmann

et al. 2020

JGR Atmospheres

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An abundance of evidence indicates that the tropics are expanding. Despite many attempts to decipher the cause, the underlying dynamical mechanism driving tropical expansion is still not entirely clear. Here, based on observations, multimodel simulations from the Coupled Model Intercomparison Project phase 5 (CMIP5) and purposefully designed numerical experiments, the variations and trends of the tropical width are explored from a regional perspective. We find that the width of the tropics closely follows the displacement of oceanic midlatitude meridional temperature gradients (MMTG). Under global warming, as a first‐order response, the subtropical ocean experiences more surface warming because of the mean Ekman convergence of anomalously warm water. The enhanced subtropical warming, which is partially independent of natural climate oscillations, such as the Pacific Decadal Oscillation, leads to poleward advance of the MMTG and drives the tropical expansion. Our results, supported by both observations and model simulations, imply that global warming may have already significantly contributed to the ongoing tropical expansion, especially over the ocean‐dominant Southern Hemisphere.

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Section: Discussionmentioning

confidence: 97%

Tropical Expansion Driven by Poleward Advancing Midlatitude Meridional Temperature Gradients

Yang

Lohmann

et al. 2020

JGR Atmospheres

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“…The value of the ToE is computed as the year when S / N crosses the 0.5 or 1 threshold. In aforementioned studies (Grise et al., 2019; Staten et al., 2019), the noise has been estimated using the internal climate variability in the reference period. In our case, we considered its possible variation with time.…”

Section: Methodsmentioning

confidence: 99%

Poleward Shift of Northern Subtropics in Winter: Time of Emergence of Zonal Versus Regional Signals

D’Agostino

Scambiati

Jungclaus

et al. 2020

Geophysical Research Letters

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The Northern Hemisphere mid-latitudes will be exposed to hydroclimatic risk in next coming decades because the subtropical expansion. However, it is not clear when the anthropogenic signal will emerge from the internal climate variability. For this purpose, we investigate the time of emergence (ToE) of the hemispheric and regional shift of northern subtropical margins in the Max Planck Institute Grand Ensemble. For several indicators, the ToE of the poleward shift of Northern subtropical margin will not occur by the end of the 21st century, neither at regional nor at hemispheric scale. The exceptions are the Mediterranean/Middle East and, to a lesser degree, Western Pacific, where the ToE would occur earlier. According to our results, given the fundamental role played by internal variability, trends of Northern Hemisphere subtropical poleward shift that have been identified over last decades in reanalyses cannot be considered as robust signals of anthropogenic climate change. Plain Language Summary The projected poleward shift of subtropical margins will affect densely populated areas of the Northern Hemisphere that have limited or scarce water resources. Therefore, it is fundamental for societies in these areas to know when the effects of climate change on the regional environment will become evident, because this information can make the difference for effective adaptation and mitigation strategies. The position of the subtropical margins is intimately related to the zonal-mean Hadley Circulation, but is also affected by regional processes related to the distribution of continents, oceans and to their characteristics. In our study we show that the poleward shift of Northern subtropics has strong regional connotations and only in few areas, for example, the Mediterranean/Middle East and, to a lesser extent the Western Pacific, the anthropogenic climate change signal will emerge from the natural climate variability before the end of the 21st century. Recent literature emphasized how magnitude and significance of the HC poleward shift and tropical width expansion from 1979 onward disagree among studies: Rates are ranging from few tenths to several degrees latitude per decade, and they are not always significant (

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“…This response is seen in models of varying complexity, ranging from idealized aquaplanet simulations (Frierson et al, 2007;Levine and Schneider, 2011;Tandon et al, 2013) to comprehensive general circulation model experiments (Hu et al, 2013;Lu et al, 2007;Tao et al, 2016), such as those from phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP). The poleward expansion of the Hadley circulation is anticipated to have a number of regional climate impacts in the subtropics, potentially shifting dry regions (Feng and Fu, 2013;Scheff and Frierson, 2012;Schmidt and Grise, 2017), altering zones of ocean upwelling (Cook and Vizy, 2018;Rykaczewski et al, 2015), and modifying hurricane tracks (Kossin et al, 2014;Sharmila and Walsh, 2018;Studholme and Gulev, 2018).…”

Section: Introductionmentioning

confidence: 99%

Hadley cell expansion in CMIP6 models

Grise

Davis

2020

Atmos. Chem. Phys.

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Abstract. In response to increasing greenhouse gases, the subtropical edges of Earth's Hadley circulation shift poleward in global climate models. Recent studies have found that reanalysis trends in the Hadley cell edge over the past 30–40 years are within the range of trends simulated by Coupled Model Intercomparison Project Phase 5 (CMIP5) models and have documented seasonal and hemispheric asymmetries in these trends. In this study, we evaluate whether these conclusions hold for the newest generation of models (CMIP6). Overall, we find similar characteristics of Hadley cell expansion in CMIP5 and CMIP6 models. In both CMIP5 and CMIP6 models, the poleward shift of the Hadley cell edge in response to increasing greenhouse gases is 2–3 times larger in the Southern Hemisphere (SH), except during September–November. The trends from CMIP5 and CMIP6 models agree well with reanalyses, although prescribing observed coupled atmosphere–ocean variability allows the models to better capture reanalysis trends in the Northern Hemisphere (NH). We find two notable differences between CMIP5 and CMIP6 models. First, while both CMIP5 and CMIP6 models contract the NH summertime Hadley circulation equatorward (particularly over the Pacific sector), this contraction is larger in CMIP6 models due to their higher average climate sensitivity. Second, in recent decades, the poleward shift of the NH annual-mean Hadley cell edge is slightly larger in CMIP6 models. Increasing greenhouse gases drive similar trends in CMIP5 and CMIP6 models, so the larger recent NH trends in CMIP6 models point to the role of other forcings, such as aerosols.

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Regional Widening of Tropical Overturning: Forced Change, Natural Variability, and Recent Trends

Cited by 43 publications

References 95 publications

Tropical Expansion Driven by Poleward Advancing Midlatitude Meridional Temperature Gradients

Tropical Expansion Driven by Poleward Advancing Midlatitude Meridional Temperature Gradients

Poleward Shift of Northern Subtropics in Winter: Time of Emergence of Zonal Versus Regional Signals

Hadley cell expansion in CMIP6 models

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