Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer

Gertler, Charles G.; O’Gorman, Paul A.

doi:10.1073/pnas.1812312116

Cited by 31 publications

(36 citation statements)

References 47 publications

(60 reference statements)

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“…In addition to the stability changes discussed above, Gertler and O'Gorman [16] showed that the weakening of the surface meridional temperature gradient in mid-latitudes around NH land also contributes to the decrease of nonconvective MAPE (ΔMAPE ≈…”

Section: Decreased Surface Meridional Temperature Gradientmentioning

confidence: 95%

Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation

Shaw

2019

Curr Clim Change Rep

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State-of-the-art climate models predict the zonal mean mid-latitude circulation will undergo a poleward shift and seasonally and hemispherically dependent intensity changes in the future. Here I review the mechanisms put forward to explain the zonal mean mid-latitude circulation response to increased carbon dioxide (CO 2) concentration. The mechanisms are grouped according to their thermodynamic starting point, which are thought to arise from processes independent of the zonal mean mid-latitude circulation response. There are 24 mechanisms and 8 thermodynamic starting points: (i) increased latent heat release aloft in the tropics, (ii) increased dry static stability and tropopause height outside the tropics, (iii) radiative cooling of the stratosphere, (iv) Hadley cell expansion, (v) increased specific humidity following the Clausius-Clapeyron relation, (vi) cloud radiative effect changes, (vii) turbulent surface heat flux changes, and (viii) decreased surface meridional temperature gradient. I argue progress can be made by testing the thermodynamic starting points. I review recent tests of the increased latent heat release aloft in the tropics starting point, i.e., prescribing diabatic perturbations, quantifying the transient response to an abrupt CO 2 increase and imposing latitudinally dependent CO 2 concentration. Finally, I provide a future outlook for improving our understanding of predicted changes in the zonal mean mid-latitude circulation. Keywords Circulation • Climate change This article is part of the Topical Collection on Mid-latitude Processes and Climate Change

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Section: Decreased Surface Meridional Temperature Gradientmentioning

confidence: 95%

Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation

Shaw

2019

Curr Clim Change Rep

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“…Storm track intensity defined using EKE is linearly related to MAPE in idealized simulations and reanalysis data (Gertler & O'Gorman, 2019;O'Gorman, 2010;O'Gorman & Schneider, 2008). In the simulations discussed in section 2.3, EKE is linearly related to MAPE, which we define following equation 3in O'Gorman and Schneider 2008:…”

Section: Mape Frameworkmentioning

confidence: 99%

Hydrological Cycle Changes Explain Weak Snowball Earth Storm Track Despite Increased Surface Baroclinicity

Shaw

Graham

2020

Geophysical Research Letters

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Simulations show that storm tracks were weaker during past cold, icy climates relative to the modern climate despite increased surface baroclinicity. Previous work explained the weak North Atlantic storm track during the Last Glacial Maximum using dry zonally asymmetric mechanisms associated with orographic forcing. Here we show that zonally symmetric mechanisms associated with the hydrological cycle explain the weak Snowball Earth storm track. The weak storm track is consistent with the decreased meridional gradient of evaporation and atmospheric shortwave absorption and can be predicted following global mean cooling and the Clausius-Clapeyron relation. The weak storm track is also consistent with decreased latent heat release aloft in the tropics, which decreases upper tropospheric baroclinicity and mean available potential energy. Overall, both hydrological cycle mechanisms are reflected in the significant correlation between storm track intensity and the meridional surface moist static energy gradient across a range of simulated climates between modern and Snowball Earth. Plain Language Summary Storm tracks (low-and high-pressure weather systems) dominate Earth's climate in the middle latitudes. Several modern theories connect storm track intensity to the equator-to-pole near-surface temperature gradient. However, it has been known for some time that this connection fails when applied to simulations of past cold, icy climates such as the Last Glacial Maximum and Snowball Earth. Previous work explained the weak North Atlantic storm track during the Last Glacial Maximum using dry longitudinally dependent dynamical mechanisms associated with orographic forcing. Here we show that the weak Snowball Earth storm track can be explained by hydrological cycle changes that are independent of longitude. In particular, the weak storm track is consistent with the decreased equator-to-pole gradient of evaporation, absorption of sunlight, and surface moist static energy, which follow global mean cooling and the Clausius-Clapeyron relation. The decreased equator-to-pole surface moist static energy gradient is correlated with decreased latent heat release aloft in the tropics, which weakens the potential energy that is available to be converted into kinetic energy.

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“…Weakening of summertime westerlies and storm tracks is a robust feature supported by observations (Chang et al, 2016;Coumou et al, 2015), paleo-data (Routson et al, 2019), model simulations (Gertler and O'Gorman, 2019;Lehmann et al, 2014), physical understanding (Coumou et al, 2018;Hoskins and Woollings, 2015), and has been documented using multiple metrics (Chang et al, 2016;Coumou et al, 2015Coumou et al, , 2018Gertler and O'Gorman, 2019;Lehmann et al, 2014;O'Gorman, 2010;Petrie et al, 2015;Routson et al, 2019;Sussman et al, 2020). Since 2000, the Arctic has warmed at a rate at least twice as fast as the global average (Cohen et al, 2020;Coumou et al, 2015Coumou et al, , 2018.…”

Section: Introductionmentioning

confidence: 77%

“…Since 2000, the Arctic has warmed at a rate at least twice as fast as the global average (Cohen et al, 2020;Coumou et al, 2015Coumou et al, , 2018. This decreases the temperature gradient between the Arctic and (sub) tropics and weakens the mid-latitude westerlies and storm tracks (Chang et al, 2016;Coumou et al, 2015Coumou et al, , 2018Gertler and O'Gorman, 2019;Lehmann et al, 2014;O'Gorman, 2010;Petrie et al, 2015;Routson et al, 2019). These dynamical changes in boreal summer are also seen in climate models under greenhouse gas forcing (Chang et al, 2016;Coumou et al, 2018).…”

Section: Introductionmentioning

confidence: 91%

“…While in general there is substantial uncertainty associated with circulation changes within the climate system (Shepherd, 2014), there is now strong evidence that boreal summer circulation weakens with global warming (Chang et al, 2016;Coumou et al, 2015Coumou et al, , 2018Gertler and O'Gorman, 2019;Lehmann et al, 2014;O'Gorman, 2010;Routson et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

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Global warming makes weather in boreal summer more persistent

Coumou

Luca

2020

Preprint

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Abstract. Extreme summer weather often has devastating impacts on society when it lasts for many days. Stalling cyclones can lead to flooding and persistent hot-dry conditions can lead to health impacts and harvest losses. Global warming weakens the hemispheric-wide circulation in boreal summer, which has been shown in both observations and models using multiple circulation metrics. Until now, it is still largely unclear what this weakening implies for regional weather conditions, including their persistence. Using an advanced persistence metric, we show that summer weather has become more-persistent over 1979–2019. State-of-the-art climate models reproduce this upward trend in persistence indicating that it can be attributed to greenhouse gas forcing. Our persistence metric accounts for the full state of the atmosphere at any given moment and is strongly rooted in dynamical systems theory. Thereby it is able to detect dynamical changes previously unseen in more widely used clustering analyses that sharply reduce the amount of information used. We show that under future high-emission scenarios, summer weather will become increasingly more-persistent due to a weakening of the circulation. Most of this increase in persistence, and the associated societal risks, is avoided under an emission scenario compatible with the Paris agreement.

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Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer

Cited by 31 publications

References 47 publications

Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation

Mechanisms of Future Predicted Changes in the Zonal Mean Mid-Latitude Circulation

Hydrological Cycle Changes Explain Weak Snowball Earth Storm Track Despite Increased Surface Baroclinicity

Global warming makes weather in boreal summer more persistent

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