CLIMATE CHANGE AND ATMOSPHERIC CIRCULATION (R CHADWICK, SECTION EDITOR)

The Response of Subtropical Highs to Climate Change

Annalisa Cherchi1 & Tercio Ambrizzi2 & Swadhin Behera3 & Ana Carolina Vasques Freitas4 & Yushi Morioka3 &

Tianjun Zhou5

# Springer Nature Switzerland AG 2018

AbstractPurpose of Review Subtropical highs are an important component of the climate system with clear implications on the localclimate regimes of the subtropical regions. In a climate change perspective, understanding and predicting subtropical highs andrelated climate is crucial to local societies for climate mitigation and adaptation strategies.We review the current understanding ofthe subtropical highs in the framework of climate change.Recent Findings Projected changes of subtropical highs are not uniform. Intensification, weakening, and shifts may largely differin the two hemispheres but may also change across different ocean basins. For some regions, large inter-model spread represen-tation of subtropical highs and related dynamics is largely responsible for the uncertainties in the projections. The understandingand evaluation of the projected changes may also depend on the metrics considered and may require investigations separatingthermodynamical and dynamical processes.Summary The dynamics of subtropical highs has a well-established theoretical background but the understanding of its vari-ability and change is still affected by large uncertainties. Climate model systematic errors, low-frequency chaotic variability,coupled ocean-atmosphere processes, and sensitivity to climate forcing are all sources of uncertainty that reduce the confidence inatmospheric circulation aspects of climate change, including the subtropical highs. Compensating signals, coming from a tug-of-war between components associated with direct carbon dioxide radiative forcing and indirect sea surface temperature warming,impose limits that must be considered.

Keywords Subtropical highs . Climate projections . Atmospheric circulation .Model biases

Introduction

Subtropical highs (or subtropical anticyclones) are regions ofsemi-permanent high atmospheric pressure typically located

between 20 and 40° of latitude in each hemisphere. At thesurface the subtropical high-pressure belt divides the easterlytrade winds from the mid-latitude westerly winds. This deter-mines, along with the subtropical jet in the upper troposphere,the poleward boundary of the tropical circulation, whichmoves equatorward of its annual-mean latitude during thewinter and poleward during the summer related to the seasonalmigration of the Intertropical Convergence Zone [1].Subtropical highs occupy about 40% of the Earth’s surfaceand contribute, through the surface wind stress curl, to themaintenance of subtropical oceanic gyres with warm pole-ward western boundary currents and cool equatorward cur-rents off the west coasts of the continents [2].

In the Northern Hemisphere, subtropical highs are locatedover the North Pacific and the North Atlantic Oceans, and inthe Southern Hemisphere, they are located in the SouthAtlantic, southern Indian, and South Pacific Ocean (Fig. 1).In both hemispheres, subtropical highs strongly influence themoisture transport from subtropical oceans, thus affecting re-gional precipitation over Caribbean [3], the USA [4, 5], East

This article is part of the Topical Collection on Climate Change andAtmospheric Circulation

* Annalisa Cherchiannalisa.cherchi@ingv.it

1 Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici,Istituto Nazionale di Geofisica e Vulcanologia, Viale Berti Pichat 6/2,40128 Bologna, Italy

2 University of São Paulo, São Paulo, SP, Brazil3 Application Laboratory, Japan Agency for Marine Earth Science and

Technology, Yokohama, Japan4 Federal University of Itajubá (UNIFEI), Itabira, MG, Brazil5 Institute of Atmospheric Physics, Chinese Academy of Sciences,

Beijing, China

Current Climate Change Reportshttps://doi.org/10.1007/s40641-018-0114-1

Asia [6], southern and eastern Africa [7, 8], and southernSouth America [9–11]. Subtropical highs also interact withmonsoon circulation [12, 13], tropical cyclone tracks[14–16], marine stratus clouds with associated radiation bud-get [17–19], and Convergence Humidity Zones [10, 11].

Subtropical highs are important components of the atmo-spheric circulation, and they play a major role in the formationof the world’s subtropical deserts and the zones ofMediterranean climate [2, 20]. In recent decades, in the contextof anthropogenic effects on climate and related global warming,changes have been observed in the position and intensity of thesubtropical highs [21, 22], with decreased subtropical

precipitation likely driven by the intensification and polewardexpansion of both subtropical dry zones [23, 24] and Hadleycell [25, 26]. Considering their large contribution to globalclimate, this work intends to review the current knowledge onthe subtropical highs in the perspective of climate change and tohighlight the main areas of future research for this topic.

Background Framework

The subtropical highs can be distinguished into two maintypes: (i) zonally asymmetric highs, which are associated with

a b

d

fe

c

Fig. 1 Eight hundred fifty-hectopascal eddy streamfunction (106 m2/s) as(a) JJA and (b) DJF climatology (1980–2005) computed from ERA-Interim [138]. c and d are the same as a and b but computed as CMIP5multi-model mean of the twentieth century simulations. Eddystreamfunction is computed removing the zonal mean. e and f are the

differences between the end of the twenty-first century (2075–2100) inthe CMIP5 RCP8.5 scenario and the CMIP5 historical simulation, in JJAandDJF, respectively. In the differences, dotted shading denotes statisticalsignificance exceeding 99% confidence level based on a t test. The list ofmodels used to compute the multi-model ensemble mean is as in [139]

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rotational flow and are strongest in summer, and (ii) zonallysymmetric highs driven by Hadley cell descent, which areassociated with divergent flow and are strongest in winter.This distinction is ruled by removal of the zonal mean andworks well in describing the differences between subtropicalhighs in the Northern and Southern Hemispheres, respective-ly, but also between the seasons. In fact, if the dynamic of thesubtropical highs is considered similar in the two hemispheres[2], the dominant mechanisms responsible for their formationvary with the season. In the summer hemisphere, the zonalasymmetry in diabatic heating associated with the land-seathermal contrast is dominant, whereas during winter, whenconvective activity over the continents is weaker, the presenceof the orography and the subsidence caused by the Hadleycirculation prevail [20, 27]. The observed mean position andintensity of the subtropical highs in the two hemispheres andin the different peak seasons is summarized in Fig. 1a, b interms of eddy streamfunction at 850 hPa.

Idealized numerical models with prescribed heating andrealistic topography provided the hypothesis that the basiccause of the summer anticyclone formation is the latent heatreleased by the monsoons over the neighboring continents tothe east or west [28]. Remote diabatic heating in the monsoonregion induces a stationary Rossby wave pattern to the westthat, interacting with the mid-latitude westerlies, produces adi-abatic descent with the aid of local orography. Also, localdiabatic enhancement can lead to a strengthening of the de-scent [20]. With this monsoon-desert mechanism, the exis-tence of the subtropical anticyclones in the Northern Pacificand Atlantic sectors has been related to the Asian and NorthAmerican monsoon, respectively, and that in the SouthAtlantic Ocean to the South American monsoon [20].Therefore, the monsoon-desert mechanism provides a linkbetween tropical and subtropical climates through Rossbywave propaga t ion . For South Asia and eas te rnMediterranean, this mechanism is identified also at interannu-al time scale [29] with a precise cycle and seasonally lockedanticyclones over the Middle East because of the local topog-raphy effect [30].

Results from Atmospheric General Circulation Models(AGCM) experiments confirm the conclusions based on linearmodels in terms of the influence of monsoonal heating, localdiabatic enhancement, and land-atmosphere coupling process-es for the formation of the subtropical highs [31, 32], mostlyfor the Northern Hemisphere summer. Including an oceanmixed layer, the importance of air-sea interaction to enhancesimulated subtropical anticyclones, to position them in theeastern basins, and also to regulate their seasonal cycle hasbeen reported [27].

In a zonal mean perspective, the Hadley circulation existseven in an idealized framework without the monsoon [33, 34],and the formation of the subtropical highs in winter is associ-ated with its descending branch. In the Southern Hemisphere

summer, a zonal wavenumber-3 component dominates theupper-level planetary wave field, showing some correspon-dence with the three high-pressure cells at the surface [2].There, the intensity of the subtropical highs decreases fromaustral winter to summer (Fig. 1). Possible reasons are relatedwith continents in the Southern Hemisphere being zonallynarrower than their counterpart in the Northern Hemisphere[2], monsoons being less important generators of zonalasymmetries in the mostly ocean-covered SouthernHemisphere, and topographic effects becoming stronger inwinter as the flow intensifies [35]. Also, inter-hemisphericteleconnections have been found to contribute to either main-taining or strengthening the southern subtropical anticyclonesduring the winter because of the heating and forcing from thesummer monsoons and deep tropical convection over warmsea surface temperatures (SSTs) in the Northern Hemisphere[35–37].

Hadley circulation, radiation balances, and SSTs all con-tribute to the annual cycle of the subtropical highs [27, 38]. Asthe transition to the early summer occurs, the wintertime zonalband of high pressure underlying the subsiding branch of theHadley cell starts to break when convection over land com-mences. This is accomplished through vorticity balance, in-cluding poleward low-level flow into the regions of deep con-vection on the western flank of the subtropical high, whileradiatively driven subsidence and equatorward flow dominateon the eastern flank [22]. As the summer ends, the east-westasymmetry in SST tends to be damped by vertical flux ofmoist static energy. Hence, subtropical anticyclones ultimatelybecome more zonally symmetric when precipitation in thesubtropics weakens and eventually shifts to the other hemi-sphere in winter [22].

Subtropical Highs: Observations, Modeling,and Related Dynamics

Subtropical Highs of the Northern Hemisphere

The western North Pacific subtropical high (WNPSH) is animportant component of East Asian summer monsoon(EASM) system [39]. The WNPSH and the EASM have asignificant effect on droughts, heat waves, and tropical cy-clone tracks over East Asia and the northwest Pacific[40–43] and can modulate summer rainfall extending fromthe eastern China to Japan [44, 45]. Seasonal and intra-seasonal variations of the WNPSH are related with the onsetand withdrawal of the EASM [39], but also to remote ENSO(El Niño Southern Oscillation)-SST forcing [46, 47].

The interannual variability of the WNPSH is dominated bytwo leading modes with the positive phases featuring ananomalous anticyclone over the western North Pacific(WNP). The first mode is closely connected with the SST

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anomalies over the tropical Indian Ocean, the MaritimeContinent, and the equatorial central Pacific, while the secondmode is closely connected with the SST anomalies over theWNP [48]. Changes in the sensible heating over the TibetanPlateau also modulate the interannual variability of theWNPSH: when the heating is stronger in spring, it is usuallyfollowed by an enhanced and westward-extended WNPSH insummer [49]. At both interannual and decadal time scales, thedistributions of summer monsoon rainfall over East Asia aredominated by the variations ofWNPSH in intensity, structure,and location [6, 39, 44].

Since the early 1990s, the interannual variability of theWNPSH is more strongly regulated by the SST anomaliesover the equatorial central Pacific and the MaritimeContinent. The early decays of El Niños in strong WNPSHyears and the early development of El Niños in weakWNPSHyears contributed to this change [50]. Both decaying El Niñoand developing La Niña events accompanied by a warmIndian Ocean and cold central Pacific, respectively, are favor-able to hotter summers in the central eastern China becausethese patterns strengthen and extend the WNPSH toward west[51–53]. The Pacific Decadal Oscillation (PDO) accounts forthe low-frequency variability of the WNPSH through the me-ridional shift of the subtropical jet [54]. In winter, tropicalAtlantic warming associated with the positive phase of theAtlantic Meridional Oscillation (AMO) provides favorableconditions for the intensification and northwestward extensionof the WNPSH [55].

Since the late 1970s, the WNPSH has extended westward,which has resulted in a monsoon rain band shift over China,with excessive rainfall along the middle and lower reaches ofthe Yangtze River valley along ~ 30° N over eastern China,and deficient rainfall in north China [56, 57]. Numerical ex-periments suggest that negative heating in central and easternPacific and increased convective heating in the equatorialIndian Ocean and Maritime Continent sector, associated withlocal warming, favor the westward extension of the WNPSH[58].

The subtropical high located over the North Atlantic(NASH) is referred to as BBermuda high^ (because of theplace where it is centered) but sometimes also BAzores high^considering its eastern extension. The NASH has a majorinfluence on weather and climate over the eastern USA [21,59], Western Europe, and northwestern Africa [60].

In boreal summer, the Azores anticyclone has high pressuredominating the Atlantic basin, and in boreal winter, it extendsover eastern North America and western Africa [61]. Themovement of the NASH western ridge toward the Americancontinent regulates moisture transport and vertical motionsover the southeastern USA. In fact, when the NASH westernridge is located southwest of its mean climate position, exces-sive summer precipitation is observed over the southeasternUSA due to an enhanced moisture transport. On the contrary,

when the western ridge is located in the northwest, a precipi-tation deficit prevails with downward motion dominating theregion [21]. The NASH variability, in terms of both intensityand position, is dominated by a quasi-biennial oscillation inwinter and by a lower frequency (~ 7 years) in summer, likelyforced by the North Atlantic Ocean variability as the correla-tions with ENSO is nonsignificant [62].

In the last 30 years, the variability of the precipitation overthe eastern USA increased because of changes in the intensityand position of the western ridge of the Bermuda high [4, 63].Strong anomalous shallow cross-equatorial circulation en-ables extreme cold surges from extra-tropical South Americato influence surface temperature and circulation over the trop-ical and subtropical Atlantic Ocean and the southern edge ofthe NASH, increasing its pressure and leading to equatorwardexpansion of its southern boundary, providing a potentialsource of predictability for lower tropospheric circulationanomalies during boreal summer [64].

The Azores high is the southern branch of the NorthAtlantic Oscillation (NAO) [65] which is associated withchanges in temperature and rainfall in Europe and NorthAmerica with day-to-day variability linked with weather sys-tem and seasonal and longer term variability with some usefulpredictive skill [66]. Multi-decadal variations of the NAO caninduce multi-decadal variations in the Atlantic meridionaloverturning circulation and poleward ocean heat transport inthe Atlantic up to the Arctic. This is likely to contribute to lossof Arctic sea-ice, warming of the Northern Hemisphere andchanges in the Atlantic tropical storm activities [67]. Thesemulti-decadal variations are superimposed on long-term an-thropogenic forcing trends.

Subtropical Highs of the Southern Hemisphere

The South Atlantic subtropical high (SASH) is very importantfor the regional climate, influencing the rainfall variability inSouth America and southern Africa, whose orography has animportant role on its localization over the ocean [36]. TheSASH is strongest and largest during the solstitial months,when its center is either closest to the equator and on thewestern side of the South Atlantic basin as in austral winter[68] or farthest poleward in the center of the South Atlanticbasin as in late austral summer [22]. In austral summer, thespatial variations of the SASH on the interannual time scaleare primarily in the meridional direction and are dominated byENSO [22]. During the austral winter, multiple physicalmechanisms, like Southern Annular Mode (SAM), ENSO,variations in the Asian-African monsoon, anomalous forcingfrom convection over the Arabian Sea and over the Coral Seato the northeast of Australia, as well as the variations ofSouthern Hemisphere storm tracks, appear to impact the po-sition of the SASH but none of them clearly dominate over theanticyclone variability, both in terms of position and intensity

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[22]. Intensification and poleward shift of the SASH in therecent decades have been explained in terms of intensificationof the westward mixed layer ocean currents and Benguelaupwelling region along the Namibian coast [69].

The SASH exhibits distinct decadal and multi-decadal var-iability. Once the SST anomalies are generated by the atmo-spheric variations through changes in meridional and verticalheat transports, they initiate to migrate eastward as quasi-stationary oceanic Rossby waves along the eastwardAntarctic Circumpolar Current [70]. This eastward migrationfurther contributes to the decadal SST variability in the south-ern Indian Ocean [71, 72]. Since both the SST and sea levelpressure (SLP) anomalies exhibit distinct eastward propaga-tion, highs over the South Atlantic and southern Indian sub-tropical oceans may have remote links on decadal time scale.

The subtropical high in the southern Indian Ocean is alsocalled BMascarene High.^ It is one major SouthernHemisphere circulation system that can also affect theEASM through changes in the Somali jet [39, 73]. Since thedecadal variability in the Mascarene High is accompaniedwith basin-wide SST anomalies in the southern IndianOcean [74–76], local air-sea interaction may be involved insustaining the subtropical variations, both in intensity and po-sition, on decadal time scale. Also, the decadal variability ofMascarene High is likely responsible for low-frequency rain-fall variability over Southern Africa [77], through changes inmoisture transport [74, 75, 78].

As a potential source of the decadal SST variability, thedecadal modulation of the Indonesian Throughflow (ITF)may induce decadal changes in ocean heat transport fromthe tropical Pacific to the Indian Ocean [79]. Since the ITFtransport receives strong influences from the tropical Pacificcondition, the decadal variability in the tropical Pacific Oceanmay play indirect roles in inducing decadal variability of theMascarene High through modulation of ocean heat transportand hence air-sea interaction in the southern Indian Ocean. Onthe other hand, the decadal modulation of the SAM involvingozone variability in the high latitudes is suggested to be theother remote factor for the decadal variability of theMascarene High [8].

Over the South Atlantic and southern Indian Oceans, thesubtropical anticyclones show a common tendency towardsouthward displacement [23, 80–82], consistently with thepoleward expansion of the Hadley cell [80, 81, 83–85]. Apoleward shift in the subtropical anticyclones in both hemi-spheres has been identified, consistently with the observedrecent trend of positive polarity of the SAM and the observedpoleward expansion of the Hadley circulation and widening ofthe tropical belt [81, 86]. Coupled Model IntercomparisonProject Phase 5 (CMIP5, [87]) twentieth century simulationsreproduce the observed Hadley cell expansion mostly in theSouthern Atlantic and South Indian Oceans, but greenhousegases (GHGs) forcing alone cannot explain the local Hadley

cell widening [83]. Other anthropogenic forcing, like strato-spheric ozone depletion, may likely play an important role[88, 89].

The South Pacific subtropical high (SPSH) is the mainatmospheric system that influences precipitation in southwest-ern South America. The maintenance of the equatorward por-tion of the SPSH and the equatorward low-level jet along theSouth American coast, during the austral winter, is due to thecontribution of the adiabatic subsidence over the southeasterntropical Pacific, produced by the warm pool convection [37].This interhemispheric response to heating in the NorthernHemisphere depends critically on the configuration of themean zonal winds in the Southern Hemisphere, and it is moredramatic in the South Pacific, as numerical model simulationsindicate that the SPSH nearly disappears in the austral winterwithout the influence from the Northern Hemisphere [35].

Subtropical Ocean Dipoles

The interannual variations of the subtropical highs in theSouthern Hemisphere generate meridional dipole patterns ofwarm and cold SST anomalies, named Bsubtropical oceandipoles.^ Three subtropical dipole SST modes have been, infact, identified in each of the three subtropical ocean basins:Indian [7], Atlantic [90, 91], and Pacific [92, 93]. Subtropicaldipoles develop during austral summer when the continentalregions in the subtropical Southern Hemisphere receive mostof annual rainfalls and when the local atmospheric subtropicalhighs are less intense. Generation mechanisms of subtropicaldipoles include air-sea interaction processes [91, 94, 95] andremote teleconnection with the ENSO or the SAM [91,96–98]. These SST dipole events modulate the atmosphericcirculation and convection with a clear impact on the rainfallover the subtropical continents [92, 99].

The subtropical South Atlantic experienced a cooling trendin the last two decades [86] consistent with the response asso-ciated with the positive phase of the South Atlantic subtropicaldipole [100, 101]. This suggests that the subtropical dipolepattern has likely been intensifying and/or become moreprominent over the past three decades. In the southernIndian Ocean, the positive phase of the subtropical dipole ischaracterized by the occurrence of a cold (warm) SST anom-aly in the southeastern (southwestern) sector during the australsummer [7]. During the mature phase, the subtropical highstrengthens and shifts slightly southward. In the SouthPacific, the subtropical dipole was identified for the first timeas part of a global wavenumber-3 dipole SST mode [92],linearly independent from both ENSO and the SAM. Lateron, it has been described as a mode associated with anortheast-southwest-oriented dipole of positive and negativeSST anomalies in the central basin with the SST poles devel-oping during austral spring, peaking in austral summer, and,then, gradually decaying afterward [93]. The SLP anomalies

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that generate the South Pacific subtropical dipole are linkedwith the geopotential height anomalies in the upper tropo-sphere and are associated with a stationary Rossby wave pat-tern along the westerly jet in the mid-latitudes, suggestingremotely induced signals in the generation of this dipole mode[93].

Subtropical Highs in Future WarmingScenarios and Related Climatic Consequences

The performance of CMIP5 models in representing mean po-sition and intensity of the subtropical highs in the differentseasons, as described in the section above, is summarized inFig. 1c, d in terms of eddy streamfunction at 850 hPa, to becompared with the observations.

Alterations in the location or strength of the subtropicalanticyclones have major implications for climate change atboth regional and global scales. In the following, a reviewon the investigations of these changes is given based on stud-ies using results from twenty-first century scenario experi-ments performed in the framework of the 3rd and 5th phasesof the Coupled Model Intercomparison Project (CMIP3 [102]and CMIP5 [87], respectively), as well as on sensitivityexperiments.

Unambiguous intensification of the subtropical highs inboth Northern and Southern Hemispheres is reported basedon CMIP3- and CMIP5-coupled model projections [103,104]. In the Northern Hemisphere, CMIP3 future warmingscenarios agree on the strengthening of the subtropical highsas primarily caused by enhanced diabatic heating over conti-nents and cooling over the ocean favoring a stronger near-surface anticyclonic circulation [103]. Same mechanismshave been found responsible for the intensification of the sub-tropical highs in the Southern Hemisphere summer comparingCMIP3- and CMIP5-coupled model future warming scenari-os, plus a positive feedback with the marine boundary layerclouds mostly over the Atlantic and the Pacific subtropicaloceans [104]. In all CMIP5 scenarios, the WNPSH isprojected to enlarge, strengthen, and extend westward withclear implications for the attribution and prediction of climatechanges in East Asia. However, the ridge line of the high doesnot evidence any long-term trend [105]. The WNPSH and theEast Asian Jet (EAJ) will remain the dominant systemsinfluencing East Asian summer precipitation in the twenty-first century but the role of theWNPSH is projected to weakencompared to that of the EAJ [106].

A more recent comparison based on CMIP5 scenariosshows slightly different results, stating that subtropical highsover the North Pacific, South Atlantic, and southern IndianOcean are projected to become weaker, whereas the NASHwill intensify as well as the SPSH though there is uncertaintyin its projections [82]. Differently from previous studies, this

more recent comparison is based on changes in subsidence,low-level divergence, and rotational wind. In addition, eddygeopotential height is used instead of the traditionalgeopotential height to measure the intensity of the subtropicalhighs. This is because the latter is supposed to systematicallyincrease with increased temperature, thus appearing less ap-propriate to be used in climate change studies [107, 108].Diagnostic analyses and idealized simulations with linearbaroclinic model suggest that the projected changes in thesubtropical anticyclones are well-explained by the combinedeffect of increased tropospheric static stability and changes indiabatic heating. The pattern of change in diabatic heating isdominated by latent heating associated with changes in pre-cipitation, which is enhanced over theWNP under the Brichestget richer^ mechanism but is reduced over subtropical NorthAtlantic and South Pacific due to a local minimum in theamplitude of SSTwarming. The change in the diabatic heatingpattern substantially enhances the subtropical highs over theNorth Atlantic and South Pacific but weakens the NorthPacific one [82].

In climate projections, the weakening and the eastwardretreat of the WNPSH are robust in the middle troposphere,while at low-levels, the projected changes in the WNPSHintensity are approximately zero in the multi-model ensemblemean though with large inter-model spread [53]. The uncer-tainty ofWNPSH projection adds uncertainty to the projectionof future monsoon changes over East Asia. Under bothRCP4.5 and RCP8.5 scenarios, CMIP5 models with a signif-icantly increased (decreased) WNPSH intensity have a signif-icant increase in the precipitation over the northern (southern)part of eastern Asia and an enhanced (weakened) southerlywind [109].

Direct effect of the changing natural and anthropogenicforcing factors on atmospheric and land-surface temperature,indirect effect on climate of these factors through SSTand sea-ice changes, and internal atmospheric and oceanic variabilityall contribute to the climate change and variability observed sofar [110]. Combining CMIP5 results and AGCM sensitivityexperiments, the response of the Northern Hemisphere sum-mertime subtropical highs can be linearly decomposed into aresponse to the carbon dioxide (CO2) direct effect and one toSST warming [111]. In this framework, the response of thesubtropical high in the North Pacific is weak because it repre-sents the outcome of a tug-of-war between CO2 direct effect,which strengthens the high, consistently with enhanced land-ocean moist entropy contrast, and the SST warming which inturn weakens the high because of a weakened land-oceancontrast. The opposing influences on the North Pacific sub-tropical high also affect the response of the Pacific jet streams[111]. Consistently, the dynamics component of the moisturetransport over South and East Asia dominates near-termchanges, while the thermodynamic component dominates inthe long-term projections [112].

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On the other hand, the response of the NASH and its west-ward shift is dominated by CO2 direct effect [111], specificallyfor increased CO2 effect over land in the Eastern Hemisphere[113]. Similar arguments, i.e., increased radiative forcing overNorth Africa and land-sea heating contrast in response to theforcing, have been used to explain the westward shift ofNASH documented in the Mid-Holocene driven by changesin the orbital parameters [114].

TheMediterranean climate with dry summers and wet win-ters, typical of the densely populated warm-temperate regionsof the world, is projected to shift northward and eastward inCMIP5 climate scenarios with the equatorward margins re-placed by arid climate type [115]. These changes appear lessrobust over California in winter. There, in fact, CMIP5models largely disagree in the projections, reflecting aprecarious balance between the subtropical highsexpanding from the south and the Aleutian low extend-ing southeastward [116, 117].

Impacts from changes in the intensity and characteristics ofthe subtropical highs extend to Australia over rangelands[118] due to the expansion of subtropical dry zone and tothe western coasts zone management [119] because of thevariability of the Southern Ocean storm belt related with thesubtropical high-pressure ridge. Also, the health of coralfauna over southwestern Atlantic [120] and in the sub-tropics [121] as well as the urban climate in subtropicalregions [122], including heat waves and related mortality[123], may largely depend on subtropical high variabilityand change.

In a warmer climate, changes in the strength and width ofthe Hadley cell may significantly alter stationary Rossby wavesources and characteristics of propagation [124], with impor-tant consequences for winter conditions in South America andwestern Africa [125]. The recent expansion of the Hadley cellin the Southern Hemisphere has been associated with the in-tensification and poleward shift of the subtropical highs [126],likely followed by an increased aridity on the eastern flanks ofsubtropical anticyclones under enhanced greenhouse gas forc-ing [23, 127]. The poleward expansion has been found to belargest in autumn, raising questions as to whether it is a de-layed effect from ozone depletion that climate models fail tocapture or entirely due to global warming, whose impact isunderestimated in CMIP3 models unless forced with observedSSTs [23]. In austral summer, rainfall over the SouthernHemisphere subtropics is projected to increase because of arobust positive trend projected for the SAM [128].

The changes in mean intensity and position of the subtrop-ical highs as described in the cited literature are summarized inFig. 1e, f in terms of 850 hPa eddy streamfunction differencesof the climatology at the end of the twenty-first century fol-lowing the RCP8.5 scenario and of the climatology at the endof the twentieth century in historical simulations using CMIP5model results.

Challenges and Future Prospects

Subtropical highs undergo significant seasonal as well as in-terannual variations; hence, potential predictability skills oftheir variations are essential for local societies to implementmeasures to mitigate climate risks and for climate adaptationpolicies. The WNPSH may be highly predictable as it is pri-marily controlled by the central Pacific cooling/warming and apositive feedback with the Indo-Pacific warm pool [42], en-abling improved prediction skills for the EASM rainfall andtropical storm activity in the western North Pacific. Skill forthe predictability of the WNPSH also becomes important tothe prediction of heatwaves over East Asia [52].

Most CMIP5 models are able to capture the spatial distri-bution and variability of the 500-hPa geopotential height andzonal wind fields in the subtropical western North Pacific, butthey tend to underestimate the mean intensity of the WNPSH[105]. This underestimation may be associated with the coldsystematic bias of sea surface temperature in the tropicalIndian and western Pacific oceans in the models. Nearly allCMIP3 and CMIP5 AGCMs exhibit northward shift in themean state of the WNPSH ridge line [129]. Atmosphere-ocean coupling in the North Pacific remains a major sourceof uncertainty for the relationship between the northward shiftof WNPSH, the widening of Hadley cell, and related implica-tions for winter precipitation in California, for example [117].

The eastern South Atlantic is a region where most climatemodels typically exhibit serious errors in the form of a severewarm bias in simulated SSTs, which tends to be accompaniedby an erroneous westward shift of the SASH [27, 130]. In thecase of uncoupled atmospheric simulations with prescribedSSTs, serious deficiencies in the simulation of the SASH arealso documented [36]. These issues highlight the need to im-prove the representation of the SASH in the models, whichdemands the nontrivial task of capturing the intricate loop offeedbacks involving the subtropical zonal SST gradient, land-sea heating contrasts, and ocean dynamics [131]. The interac-tion between the SASH and the continental thermal low needsfurther investigation because of implications in present-dayclimate variability and future climate change [86]. The inabil-ity of the CMIP5 models to capture the co-variability betweenSAM and ENSO substantially limits the confidence in theirfuture Southern Hemisphere rainfall projections, especiallyregarding projections of extreme seasonal rainfall, thatmay depend on the concurrence of SAM and ENSO events[128, 132]. The fact that most models underestimate the in-tensification of subtropical ridge relates to the underestimationof the Hadley cell expansion in present climate and in futurewarming scenarios [124] and opens up the possibility thatdeclines of projected rainfall in the subtropical regions maybe underestimated [133].

It is also important to distinguish weak projected circula-tion responses to global warming arising because of genuine

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uncertainty and lack of model agreement, from weak re-sponses arising because of competing effects that are robustand physically understood [111]. These situations are not un-common; for example, the future Southern Hemisphere sum-mertime circulation is expected to involve a tug-of-war be-tween ozone recovery and greenhouse gas increases [111].

Conclusions

Semi-permanent high-pressure systems of the global atmo-spheric circulation and their influence on regional weatherand climate are studied since the end of the nineteenth century,when, for the first time, their centers of actions have beenidentified from maps of monthly mean sea-level pressure[134]. It is only in the mid-twentieth century that the charac-teristics of these features and the relative variability have beenmathematically described [135]. The lack of an adequate net-work of stations in middle and high latitudes of the SouthernHemisphere limited the knowledge of the circulation patterns.As new stations were set up in these relatively unobservedareas, attempts were conducted to describe synoptic patterns,although only for limited regions [136]. In late twentieth cen-tury, the theoretical grounds setting the existence and charac-teristics of the subtropical highs as peculiar aspects of theclimate system have been defined and recognized [27, 28,137]. Based on that background, efforts have been dedicatedto study specific aspects of the subtropical highs addressingdifferent studies and communities for each sector of relevancein the world, such as the Southern Hemisphere or the westernNorth Pacific region.

The current availability of global observations, 3-D atmo-spheric re-analyses, and results from a large number of numer-ical simulations, performed with sophisticated global-coupledmodels from international frameworks like CMIP, largelyhelped the progresses in the understanding of the dynamicsof the subtropical highs and their representation. However,there are still many dark or gray zones which are yet to beexplored. In particular, multi-model comparisons highlighteddeficiencies in the representation of the subtropical highs,mostly related to coupled ocean-atmosphere processes, lowfrequency variability, sensitivity to climate forcing, and inter-relationships among different climate phenomena and theirvariability. These reported biases and uncertainties open fur-ther questions on the reliability of models used to performfuture climate projections, thereby reducing the confidencein atmospheric circulation aspects of climate change and thecorresponding regional patterns of precipitation change.Compensating signals, coming from a tug-of-war betweencomponents associated with direct CO2 radiative forcing andindirect SST warming, impose limits that must be consideredin terms of the confidence in the attribution, detection, and

model projections of the regional circulation responses toglobal warming.

Acknowledgements We are grateful to the two anonymous reviewerswhose comments helped in improving the shape and content of the man-uscript. A special thank is due to Dr. X Chen for the help in redrawingFig. 1 using CMIP5 model results.

Compliance with Ethical Standards

Conflict of Interest On behalf of all authors, the corresponding authorstates that there is no conflict of interest.

References

1. Saha K. Monsoon over Australia (region – IV). In: Tropical cir-culation systems and monsoons. Berlin Heidelberg: Springer-Verlag; 2010. https://doi.org/10.1007/978-3-642-03373-5_7.

2. Miyasaka T, Nakamura H. Structure andmechanisms of the south-ern hemisphere summertime subtropical anticyclones. J Clim.2010;23:2115–30.

3. Gamble DW, Parnell DB, Curtis S. Spatial variability of theCaribbean mid-summer drought and relation to North Atlantichigh circulation. Int J Climatol. 2008;28:343–50.

4. Li W, Li L, Fu R, Deng Y, Wang H. Changes to the North Atlanticsubtropical high and its role in the intensification of summer rain-fall variability in the southeastern United States. J Clim. 2011;24:1499–506.

5. Luchetti NT, Nieto Ferreira R, Rickenbach TM, NissenbaumMR,McAuliffe JD. Influence of the North Atlantic subtropical high onwet and dry sea-breeze events in North Carolina, United States.Investigaciones Geograficas. 2017;68:9–25. https://doi.org/10.14198/INGEO2017.68.01.

6. Zhou T, Gong D, Li J, Li B. Detecting and understanding themulti-decadal variability of the East Asian Summer Monsoon -recent progress and state of affairs. Meteorol Z. 2009a;18(4):455–67.

7. Behera SK, Yamagata T. Subtropical SST dipole events in thesouthern Indian Ocean. Geophys Res Lett. 2001;28:327–30.

8. Manatsa D, Morioka Y, Behera SK, Matarira CH, Yamagata T.Impact of Mascarene high variability on the east African Bshortrains^. Clim Dyn. 2014;42(5–6):1259–74.

9. Doyle ME, Barros VR. Midsummer low-level circulation and pre-cipitation in subtropical South America and related sea surfacetemperature anomalies in the South Atlantic. J Clim.2002;15(23):3394–410.

10. Reboita MS, Gan MA, Da Rocha RP, Ambrizzi T. Precipitationregimes in South America: a bibliographic review. J Meteor.2010;25:185–204. Available in Portuguese

11. Gilliland JM, Keim BD. Position of South Atlantic anticycloneand its impact on surface conditions across Brazil. J ApplMeteor Clim doi. 2018;57:535–53. https://doi.org/10.1175/JAMC-D-17-0178.1.

12. Sun S, Ying M. Subtropical high anomalies over the western pa-cific and its relations to the Asianmonsoon and SSTanomaly. AdvAtmos Sci. 1999;16:559–68.

13. Ding YH, Chan JCL. The East Asian summer monsoon: an over-view. Meteor Atmos Phys. 2005;89:117–42.

14. Bannister AJ, Boothe MA, Carr LE, Elsberry RL (1997) Southernhemisphere application of the systematic approach to tropical cy-clone track forecasting: part I: environmental structure

Curr Clim Change Rep

characteristics. Tech Rep NPS-MR-98-001 96 pp NavalPostgraduate School, Monterey, CA.

15. Wu L,Wang B, Geng S. Growing typhoon influence on East Asia.Geophys Res Lett. 2005;32:L18703.

16. Stowasser M, Wang Y, Hamilton K. Tropical cyclone changes inthe western North Pacific in a global warming scenario. J Clim.2007;20:2378–96.

17. Klein SA, Hartmann DL. The seasonal cycle of low stratiformclouds. J Clim. 1993;6:1587–606.

18. Rahn DA, Garreaud R. Marine boundary layer over the subtropi-cal southeast Pacific during VOCALS-REx—part 1: mean struc-ture and diurnal cycle. Atmos Chem Phys. 2010;10:4491–506.

19. Wei W, Wenhong Li, Yi Deng, Song Yang, Jonathan H. Jiang, LeiHuang, W. Timothy Liu (2017) Dynamical and thermodynamicalcoupling between the North Atlantic subtropical high and the ma-rine boundary layer clouds in boreal summer. Clim Dyn DOIhttps://doi.org/10.1007/s00382-017-3750-6, 50, 2457, 2469.

20. Rodwell MJ, Hoskins BJ. Subtropical anticyclones and summermonsoons. J Clim. 2001;14:3192–211.

21. Li L, Li W, Kushnir Y. Variation of the North Atlantic subtropicalhigh western ridge and its implication to southeastern US summerprecipitation. Clim Dyn. 2012a;39:1401–12. https://doi.org/10.1007/s00382-011-1214-y.

22. Sun X, Cook KH, Vizy EK. The South Atlantic subtropical high:climatology and interannual variability. J Clim. 2017;30:3279–96.

23. Cai W, Cowan T, Thatcher M. Rainfall reductions over southernhemisphere semi-arid regions: the role of subtropical dry zoneexpansion. Sci Rep. 2012;2:702. https://doi.org/10.1038/srep00702.

24. Scheff J, Frierson D. Twenty-first-century multimodel subtropicalprecipitation declines are mostly midlatitude shifts. J Clim.2012;25:4330–47. https://doi.org/10.1175/JCLI-D-11-00393.1.

25. Hu Y, Zhou C, Liu J. Observational evidence for poleward expan-sion of the Hadley circulation. Adv Atm Sci. 2011;28:33–44.

26. Nguyen H, Hendon HH, Lim EP, Boschat G, Maloney E, TimbalB. Variability of the extent of the Hadley circulation in the south-ern hemisphere: a regional perspective. Clim Dyn. 2018;50:129–42. https://doi.org/10.1007/s00832-017-3592-2.

27. Seager R, Murthugudde R, Naik N, Clement A, Gordon N, MillerJ. Air-sea interaction and the seasonal cycle of the subtropicalanticyclones. J Clim. 2003;16:1948–66.

28. Rodwell MJ, Hoskins BJ. Monsoons and the dynamics of deserts.Quart J Roy Meteor Soc. 1996;122:1385–404.

29. Cherchi A, Annamalai H, Masina S, Navarra A. South Asiansummer monsoon and the eastern Mediterranean climate: themonsoon-desert mechanism in CMIP5 simulations. J Clim.2014;27:6877–903. https://doi.org/10.1175/JCLI-D-13-00530.1.

30. Tyrlis E, Lelieveld J, Steil B. The summer circulation over theeastern Mediterranean and the Middle East: influence of theSouth Asian monsoon. Clim Dyn. 2013;40:1103–23. https://doi.org/10.1007/s00382-012-1258-4.

31. Shaffrey LC, Hoskins BJ, Lu R. The relationship between theNorth American summer monsoon, the Rocky Mountains andthe North Pacific subtropical anticyclone in HadAM3. Quart JRoy Meteor Soc. 2002;128:2607–22.

32. Liu Y, Wu G, Ren R. Relationship between the subtropical anti-cyclone and diabatic heating. J Clim. 2004;17:682–98.

33. Held IM, Hou AY. Nonlinear axially symmetric circulations in anearly inviscid atmosphere. J Atm Sci. 1980;37:515–33.

34. Held IM (2000) The general circulation of the atmosphere. WoodsHole Oceanographic Institute Geophysical Fluid DyamicsProgram, Woods Hole, Mass (available at https://www.gfdl.noaa.gov/wp-content/uploads/files/user_files/ih/lectures/woods_hole.pdf).

35. Lee SK, Mechoso CR, Wang C, Neelin JD. Interhemispheric in-fluence of the northern summer monsoons on southern subtropical

anticyclones. J Clim. 2013;26:10193–204. https://doi.org/10.1175/JCLI-D-13-00106.1.

36. Richter I, Mechoso CR, Robertson AW. What determines the po-sition and intensity of the South Atlantic anticyclone in australwinter? - an AGCM study. J Clim. 2008;21:214–29. https://doi.org/10.1175/2007JCLI1802.1.

37. Wang C, Lee S-K,Mechoso CR. Interhemispheric influence of theAtlantic warm pool on the southeastern Pacific. J Clim. 2010;23:404–18.

38. Hastenrath S. Climate dynamics of the tropics: Kluwer; 1991. p.488.

39. Tao SY, Chen LX. A review of recent research on the East Asiansummer monsoon in China. In: Chang CP, Krishnamurti TN, ed-itors. Review of monsoon meteorology. London: Oxford UnivPress; 1987. p. 353.

40. Du Y, Yang I, Xie SP. Tropical Indian Ocean influence onNorthwest Pacific tropical cyclones in summer following strongEl Nino. J Clim. 2011;24:315–22. https://doi.org/10.1175/2010JCLI3890.1.

41. Kosaka Y, Chowdary JS, Xie SP, Min YM, Lee JY. Limitations ofseasonal predictability for summer climate over East Asia and thenorthwestern Pacific. J Clim. 2012;25:7574–89.

42. Wang B, Xiang BQ, Lee JY. Subtropical high predictability estab-lishes a promising way for monsoon and tropical storm predic-tions. Proc Nat Academy Sci. 2013;110:2718–22.

43. Zhang I, Zhou T. Drought over East Asia: a review. J Clim.2015;28:3375–99. https://doi.org/10.1175/JCLI-D-14-00259.1.

44. Li T, Wang B, Wu B, Zhou T, Chang CP, Zhang R. Theories onformation of an anomalous anticyclone in western North Pacificduring El Nino: a review. J Meteor Res. 2017;31:987–1006.https://doi.org/10.1007/s13351-017-7147-6.

45. Nagata R,Mikami T. Changes in the relationship between summerrainfall over Japan and the North Pacific subtropical high, 1901-2000. Int J Climatol. 2017;37:3291–6.

46. DongX, Li R, Fan F. Comparison of the twomodes of theWesternPacific subtropical high between early and late summer. Atm SciLett. 2017;18:153–60. https://doi.org/10.1002/asl.737.

47. Qian W, Shi J. Tripole precipitation pattern and SST variationslinkedwith extreme zonal activities of the western Pacific subtrop-ical high. Int J Climatol. 2017;37:3018–35. https://doi.org/10.1002/joc.4897.

48. He C, Zhou T. The two interannual variability modes of the west-ern North Pacific subtropical high simulated by 28 CMIP5-AMIPmodels. Clim Dyn. 2014;43:2455–69. https://doi.org/10.1007/s00382-014-2068-x.

49. Duan A, Sun R, He J. Impact of surface sensible heating over theTibetan Plateau on the western Pacific subtropical high: a land-air-sea interaction perspective. Adv Atm Sci. 2017;34:157–68.

50. He C, Zhou T. Decadal change of the connection between summerwestern North Pacific subtropical high and tropical SST in theearly 1990s. Atm Sci Lett. 2015a;16:253–9.

51. Paek H, Yu JY, Zheng F, Lu MM. Impacts of ENSO diversity onthe western Pacific and North Pacific subtropical highs duringboreal summer. Clim Dyn. 2016; https://doi.org/10.1007/s00382-016-3288-z.

52. Chen X, Zhou T. Relative contributions of external SST forcingand internal atmospheric variability to July-August heat wave overthe Yangtze River valley during 1979-2014. Clim Dyn. 2017;https://doi.org/10.1007/s00382-017-3871-y.

53. He C, Zhou T, Lin A, Wu B, Gu D, Li C, et al. Enhanced orweakened western North Pacific subtropical high under globalwarming? Sci Rep. 2015b;5:16771. https://doi.org/10.1038/srep16771.

54. Matsumura S, Horinouchi T. Pacific Ocean decadal forcing oflong-term changes in the western Pacific subtropical high. SciRep. 2016;6:37765. https://doi.org/10.1038/srep37765.

Curr Clim Change Rep

55. Lyu K, Yu JY, Paek H. The influences of the Atlantic multidecadaloscillation on the mean strength of the North Pacific subtropicalhigh during boreal winter. J Cli. 2017;30:411–24. https://doi.org/10.1175/JCLI-D-16-0525.1.

56. Hu Z, Yang S, Wu R. Long-term climate variations in China andglobal warming signals. J Geophys Res. 2003;108:4614. https://doi.org/10.1029/2003JD003651.

57. Yu R, Zhou T. Seasonality and three-dimensional structure of theinterdecadal change in East Asian monsoon. J Clim. 2007;20:5344–55.

58. Zhou T, Yu R, Zhang J, Drange H, Cassou C, Deser C, et al. Whythe western Pacific subtropical high has extended westward sincethe late 1970s. J Clim. 2009b;22:2199–215.

59. Katz RW, Parlange MB, Tebaldi C. Stochastic modeling of theeffects of large-scale circulation on daily weather in the southeast-ern US. Clim Ch. 2003;60:189–21.

60. Nicholson SE. The West African Sahel: a review of recent studieson the rainfall regime and its interannual variability. ISRNMeteordoi. 2013;2013:1–32. https://doi.org/10.1155/2013/453521.

61. Davis RE, Hayden BP, Gay DA, Phillips WL, Jones GV. TheNorth Atlantic subtropical anticyclone. J Clim. 1997;10:728–44.

62. Hasanean HM. Variability of the North Atlantic subtropical highand associations with tropical sea surface temperature. Int JClimatol. 2004;24:945–57. https://doi.org/10.1002/joc.1042.

63. Diem JE. Influences of the Bermuda high and atmospheric moist-ening on changes in summer rainfall in the Atlanta, Georgia re-gion, USA. Int J Climatol. 2013;33:160–72. https://doi.org/10.1002/joc.3421.

64. Bowerman AR and co-authors (2017) An influence of extremesouthern hemisphere cold surges on the North Atlantic subtropicalhigh through a shallow atmospheric circulation. J Geophys ResAtm 122: 10,135 - 10,148.

65. Walker GT, Bliss EW. World weather. V Mem Roy Meteor Soc.1932;4:53–83.

66. Scaife AA, et al. Skillful long-range prediction of European andNorth American winters. Geophys Res Lett. 2014;41:2514–9.https://doi.org/10.1002/2014GL059637.

67. Delworth TL, Zheng F, Vecchi GA, Yang X, Zhang L, Zhang R.The North Atlantic oscillation as a driver of rapid climate changein the northern hemisphere. Nat Geosci. 2016;9:509–12. https://doi.org/10.1038/ngeo2738.

68. Machel H, Kapala A, Flohn EH. Behaviour of the centres of actionabove the Atlantic since 1881. Part I: characteristics of seasonaland interannual variability. Int J Climatol. 1998;18:1–22.

69. Vizy EK, Cook KH, Sun X. Decadal change of the South AtlanticOcean Angola-Benguela frontal zone since 1980. Clim Dyn.2018; https://doi.org/10.1007/s00382-018-4077-7.

70. Morioka, Y., Taguchi, B., & Behera, S. K. (2017). Eastward-propagating decadal temperature variability in the South Atlanticand Indian Oceans. Journal of Geophysical Research: Oceans.

71. Morioka Y, Engelbrecht F, Behera SK. Potential sources of decad-al climate variability over southern Africa. J Clim. 2015b;28(22):8695–709.

72. Le Bars D, Viebahn JP, Dijkstra HA. A Southern Ocean mode ofmultidecadal variability. Geophys Res Lett. 2016;43(5):2102–10.

73. Xue F and co-authors (2015) Recent advances in monsoon studiesin China. Adv Atm Sci 32: 206–229 doi: https://doi.org/10.1007/s00376-014-0015-8.

74. Allan RJ, Lindesay JA, Reason CJ. Multidecadal variability in theclimate system over the Indian Ocean region during the australsummer. J Clim. 1995;8(7):1853–73.

75. Reason CJC. Warm and cold events in the southeast Atlantic/southwest Indian Ocean region and potential impacts on circula-tion and rainfall over southern Africa. Met Atm Phys. 1998;69(1–2):49–65.

76. Yamagami Y, Tozuka T. Interdecadal changes of the Indian Oceansubtropical dipole mode. Clim Dyn. 2015;44(11–12):3057–66.

77. Tyson PD, Dyer TG, Mametse MN. Secular changes in southAfrican rainfall: 1880 to 1972. Q J R Meteorol Soc.1975;101(430):817–33.

78. Malherbe J, Landman WA, Engelbrecht FA. The bi-decadal rain-fall cycle, Southern Annular Mode and tropical cyclones over theLimpopo River Basin, southern Africa. Clim Dyn. 2014;42(11–12):3121–38.

79. Reason CJC, Allan RJ, Lindesay JA. Evidence for the influence ofremote forcing on interdecadal variability in the southern IndianOcean. J Geophys Res Oceans. 1996;101(C5):11867–82.

80. Choi J, Son SW, Lu J, Min SK. Further observational evidence ofHadley cell widening in the southern hemisphere. Geophys ResLett. 2014;41:2590–7. https://doi.org/10.1002/2014GL059426.

81. Lucas C, NguyenH. Regional characteristics of tropical expansionand the role of climate variability. J Geophys Res Atm. 2015;120:6809–24. https://doi.org/10.1002/2015JD023130.

82. He C, Wu B, Zou L, Zhou T. Responses of the summertime sub-tropical anticyclones to global warming. J Clim. 2017;30:6465–79.

83. Kim Y-H, Min S-K, Son S-K, Choi J. Attribution of local Hadleycell widening in the southern hemisphere. Geophys Res Lett.2017;44:1015–24. https://doi.org/10.1002/2016GL072353.

84. Lu J, Vecchi GA, Reichler T. Expansion of the Hadley cell underglobal warming. Geophys Res Lett. 2007;34:L06805. https://doi.org/10.1029/2006GL028443.

85. Tao L, Hu Y, Liu J. Anthropogenic forcing on the Hadley circu-lation in CMIP5 simulations. Climate Dyn. 2016;46:3337–50.https://doi.org/10.1007/s00382-015-2772-1.

86. Vizy EK, CookKH. Understanding long-term (1982–2013) multi-decadal change in the equatorial and subtropical South Atlanticclimate. Clim Dyn. 2016;46:2087–113.

87. Taylor KE, Stouffer RJ, Meehl GA. An overview of CMIP5 andthe experiment design. Bull Am Meteor Soc. 2012;93:485–98.https://doi.org/10.1175/BAMS-D-11-00094.I.

88. Min SK, Won SW. Multimodel attribution of the southern hemi-sphere Hadley cell widening: major role of ozone depletion. JGeophys Res Atm. 2013;118:3007–15.

89. Waugh DW, Garfinkel CI, Polvani LM. Drivers of the recent trop-ical expansion in the southern hemisphere: changing SSTs orozone depletion? J Clim. 2015;28:6581–6.

90. Venegas SA, Mysak LA, Straub DN. Atmosphere–ocean coupledvariability in the South Atlantic. J Clim. 1997;10(11):2904–20.

91. Fauchereau N, Trzaska S, Richard Y, Roucou P, Camberlin P. Sea-surface temperature co-variability in the Southern Atlantic andIndian Oceans and its connections with the atmospheric circula-tion in the southern hemisphere. Int J Climatol. 2003;23(6):663–77.

92. Wang F. Subtropical dipole mode in the southern hemisphere: aglobal view. Geophys Res Lett. 2010;37:L10702. https://doi.org/10.1029/2010GL042750.

93. Morioka Y, Ratnam JV, SasakiW,MasumotoY. Generationmech-anism of the South Pacific subtropical dipole. J Clim.2013;26(16):6033–45.

94. Suzuki R, Behera SK, Iizuka S, Yamagata T. Indian Ocean sub-tropical dipole simulated using a coupled general circulation mod-el. J Geophys Res: Oceans. 2004;109(C9)

95. Morioka Y, Tozuka T, Yamagata T. Climate variability in thesouthern Indian Ocean as revealed by self-organizing maps.Clim Dyn. 2010;35(6):1059–72.

96. Colberg F, Reason CJC, Rodgers K. South Atlantic response to ElNiño–Southern Oscillation induced climate variability in an oceangeneral circulat ion model. J Geophys Res: Oceans.2004;109(C12)

Curr Clim Change Rep

97. Morioka Y, Masson S, Terray P, Prodhomme C, Behera SK,Masumoto Y. Role of tropical SST variability on the formationof subtropical dipoles. J Clim. 2014;27(12):4486–507.

98. Rodrigues RR, Campos EJ, Haarsma R. The impact of ENSO onthe South Atlantic subtropical dipole mode. J Clim. 2015;28(7):2691–705.

99. Reason CJC. Subtropical Indian Ocean SST dipole events andsouthern African rainfall. Geophys Res Lett. 2001;28(11):2225–7.

100. MoriokaY, Tozuka T, Yamagata T. On the growth and decay of thesubtropical dipole mode in the South Atlantic. J Clim.2011;24(21):5538–54.

101. Yuan C, Tozuka T, Luo JJ, Yamagata T. Predictability of the sub-tropical dipole modes in a coupled ocean atmosphere model.Climate Dyn. 2014;42:1291–308. https://doi.org/10.1007/s00382-013-1704-1.

102. Meehl GA and co-authors (2007) TheWCRP CMIP3 multimodeldataset: a new era in climate change research. Bull AmMeteor Soc88: 1383–1394 doi: https://doi.org/10.1175/BAMS-88-9-1383.

103. Li W, Li L, Ting M, Liu Y. Intensification of northern hemispheresubtropical highs in a warming climate. Nat Geosci. 2012b;5:830–4. https://doi.org/10.1038/NGEO1590.

104. LiW, Li L, TingM, DengY, Kushnir Y, LiuY, et al. Intensificationof the southern hemisphere summertime subtropical anticyclonesin a warming climate. Geophys Res Lett. 2013;40:5959–64.https://doi.org/10.1002/2013GL058124.

105. Liu Y, Li W, Zuo J, Hu ZZ. Simulation and projection of thewestern Pacific subtropical high in CMIP5 models. J MeteorRes. 2014;28:327–40.

106. Ren Y, Zhou B, Song L, Xiao Y. Interannual variability of westernNorth Pacific subtropical high, East Asian jet and East Asian sum-mer precipitation: CMIP5 simulation and projection. Quart Int.2017;440:64–70.

107. He C, Zhou T, Wu B. The key oceanic regions responsible for theinterannual variability of the western North Pacific subtropicalhigh and associated mechanisms. J Meteor Res. 2015a;29(4):562–75.

108. He C, Lin A, Gu D, Li C, Zheng B, Wu B, et al. Using eddygeopotential height to measure the western North Pacific subtrop-ical high in a warming climate. Theor Appl Climatol. 2018;131:681–91. https://doi.org/10.1007/s00704-016-2001-9.

109. He C, Zhou T. Responses of the western North Pacific subtropicalhigh to global warming under RCP4.5 and RCP8.5 scenariosprojected by 33 CMIP5 models: the dominance of tropicalIndian Ocean - Tropical Western Pacific SST Gradient. J Clim.2015b;28:365–80.

110. Folland CK, Sexton DMH, Karoly DJ, Johnson CE, Rowell DP,Parker DE. Influence of anthropogenic and oceanic forcing onrecent climate change. Geophys Res Lett. 1998;25:353–6.

111. Shaw TA, Voigt A. Tug of war on summertime circulation be-tween radiative forcing and sea surface warming. Nat Geosci.2015;8:560–6. https://doi.org/10.1038/NGEO2449.

112. Li X, Ting M, Li C, Henderson N. Mechanisms of Asian summermonsoon changes in response to anthropogenic forcing in CMIP5models. J Clim. 2015;28:4107–25.

113. Shaw TA, Voigt A. Land dominates the regional response to CO2

direct radiative forcing. Geophys Res Lett. 2016;43:11,383–91.114. Kelly P, Kravitz B, Lu J, Leung LR. Remote drying in the North

Atlantic as a common response to precessional changes and CO2

increase over land. Geophys Res Lett. 2018;45:3615–24. https://doi.org/10.1002/2017GL076669.

115. Alessandri A and co-authors (2014) Robust assessment of theexpansion and retreat ofMediterranean climate in the 21st century.Sci Rep 4: 7211 doi: https://doi.org/10.1038/srep07211.

116. Polade SD and co-authors (2017) Precipitation in a warmingworld: assessing projected hydro-climate changes in California

and other Mediterranean climate regions. Sci Rep 7: 10783 doi:https://doi.org/10.1038/s41598-017-11285-y.

117. Choi J, et al. Uncertainty in future projections of the North Pacificsubtropical high and its implication for California winter precipi-tation change. J Geophys Res Atm. 2016;121:795–806. https://doi.org/10.1002/2015JD023858.

118. Eldridge DJ, Beecham G (2018) The impact of climate variabilityon land use and livelihoods in Australia’s rangelands. In: GaurMK, Squires VR (eds) BClimate variability impacts on land useand livelihoods in drylands^ Springer.

119. Wandres M, Pattiaratchi C, Wijeratne EMS, Hetzel Y. The influ-ence of the subtropical high-pressure ridge on the westernAustralian wave climate. J Coast Res. 2016;75:567–71. https://doi.org/10.2112/SI75-114.1.

120. Cassola GE, et al. Decline in abundance and health state of anAtlantic subtropical gorgonian population. Mar Poll Bull.2016;104:329–34.

121. Xie JY and co-authors (2017) The 2014 summer coral bleachingevent in subtropical HongKong.Mar Poll Bull doi: https://doi.org/10.1016/j.marpolbull.2017.03.061, 124, 653, 659.

122. Tan Z, Lau KKL, Ng E. Planning strategies for roadside treeplanting and outdoor comfort enhancement in subtropical high-density urban areas. Build Environ. 2017;120:93–109. https://doi.org/10.1016/j.buildenv.2017.05.017.

123. Son JY, et al. The impact of temperature on mortality in a subtrop-ical city: effects of cold, heat and heatwaves in Sao Paulo, Brazil.Int J Biometeorol. 2016;60:113–21.

124. Freitas ACV, Frederiksen JS, O’Kane TJ, Ambrizzi T. Simulatedaustral winter response of the Hadley circulation and stationaryRossby wave propagation to a warming climate. Clim Dyn.2017;49:521–45. https://doi.org/10.1007/s00382-016-3356-4.

125. Freitas ACV, Ambrizzi T. Changes in the austral winter Hadleycirculation and the impact on stationary Rossby waves propaga-tion. Adv Meteorol. 2012;2012:1–15. https://doi.org/10.1155/2012/980816.

126. Nguyen H, Evans A, Lucas C, Smith I, Timbal B. The Hadleycirculation in re-analyses: climatology, variability, and change. JClim. 2013;26:3357–76. https://doi.org/10.1175/JCLI-D-12-00224.1.

127. Seager R, Naik N, Vecchi GA. Thermodynamic and dynamicmechanisms for large-scale changes in the hydrological cycle inresponse to global warming. J Clim. 2010;23:4651–68. https://doi.org/10.1175/2010JCLI3655.1.

128. Lim EP, et al. The impact of the southern annular mode on futurechanges in southern hemisphere rainfall. Geophys Res Lett.2016;43:7160–7. https://doi.org/10.1002/2016GL069453.

129. Song F, Zhou T. Interannual variability of East Asian summermonsoon simulated by CMIP3 and CMIP5 AGCMs: skill depen-dence on Indian Ocean-western Pacif ic anticycloneteleconnections. J Clim. 2014a;27:1679–97.

130. Richter I, Xie S-P, Wittenberg AT, Masumoto Y. Tropical Atlanticbiases and their relation to surface wind stress and terrestrial pre-cipitation. Clim Dyn. 2012;38:985–1001. https://doi.org/10.1007/s00382-011-1038-9.

131. CabosW, Sein DV, Pinto JG, et al. The South Atlantic anticycloneas a key player for the representation of the tropical Atlantic cli-mate in coupled climate models. Clim Dyn. 2017;48:4051–69.https://doi.org/10.1007/s00382-016-3319-9.

132. Hendon HH, Lim EP, Arblaster J, Anderson DTL. Causes andpredictability of the record wet spring over Australia in 2010.Clim Dyn. 2014;42:1155–74. https://doi.org/10.1007/s00382-013-1700-5.

133. Nguyen H, et al. Expansion of the southern hemisphere Hadleycell in response to greenhouse forcing. J Clim. 2015;28:8067–77.

134. Teissereng de Bort L (1883) Etude sur l’hiver de 1879–80 etrecherches sur la position des centres d’action de l’atmosphere

Curr Clim Change Rep

dans les hivers anormaux. Bureau Central Meteor. de la France,Annales, 1881, 4, 17–62.

135. Stewart HJ. Periodic properties of semi-permanent atmosphericpressure systems. Q Appl Math. 1943;1:276–7.

136. Rubin MJ, van Loon H. Aspects of the circulation of the southernhemisphere. J Meteor. 1954;11:68–76.

137. Chen PC, Hoerling MP, Dole RM. The origin of the subtropicalanticyclones. J Atmos Sci. 2001;58:1827–35.

138. Dee DP, Uppala SM, Simmons AJ, Berrisford P, Poli P, KobayashiS, et al. The ERA-interim re-analysis: configuration and perfor-mance of the data assimilation system. Quart J Roy Meteor Soc.2011;137:553–97.

139. Chen X, Zhou T. Distinct effects of global mean warming andregional sea surface warming pattern on projected uncertainty inthe South Asian summer monsoon. Geophys Res Lett. 2015;42:9433–9.

Curr Clim Change Rep