Current Climate Change Reports https://doi.org/10.1007/s40641-018-0114-1 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 Abstract Purpose of Review Subtropical highs are an important component of the climate system with clear implications on the local climate regimes of the subtropical regions. In a climate change perspective, understanding and predicting subtropical highs and related climate is crucial to local societies for climate mitigation and adaptation strategies. We review the current understanding of the subtropical highs in the framework of climate change. Recent Findings Projected changes of subtropical highs are not uniform. Intensification, weakening, and shifts may largely differ in 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 understanding and evaluation of the projected changes may also depend on the metrics considered and may require investigations separating thermodynamical 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 in atmospheric 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 between 20 and 40° of latitude in each hemisphere. At the surface the subtropical high-pressure belt divides the easterly Subtropical highs (or subtropical anticyclones) are regions of trade winds from the mid-latitude westerly winds. This deter- semi-permanent high atmospheric pressure typically located mines, along with the subtropical jet in the upper troposphere, the poleward boundary of the tropical circulation, which moves equatorward of its annual-mean latitude during the This article is part of the Topical Collection on Climate Change and Atmospheric Circulation winter and poleward during the summer related to the seasonal migration of the Intertropical Convergence Zone [1]. ’ * Annalisa Cherchi Subtropical highs occupy about 40% of the Earth ssurface [email protected] and contribute, through the surface wind stress curl, to the maintenance of subtropical oceanic gyres with warm pole- 1 Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici, ward western boundary currents and cool equatorward cur- Istituto Nazionale di Geofisica e Vulcanologia, Viale Berti Pichat 6/2, rents off the west coasts of the continents [2]. 40128 Bologna, Italy In the Northern Hemisphere, subtropical highs are located 2 University of São Paulo, São Paulo, SP, Brazil over the North Pacific and the North Atlantic Oceans, and in 3 Application Laboratory, Japan Agency for Marine Earth Science and the Southern Hemisphere, they are located in the South Technology, Yokohama, Japan Atlantic, southern Indian, and South Pacific Ocean (Fig. 1). 4 Federal University of Itajubá (UNIFEI), Itabira, MG, Brazil In both hemispheres, subtropical highs strongly influence the moisture transport from subtropical oceans, thus affecting re- 5 Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China gional precipitation over Caribbean [3], the USA [4, 5], East Curr Clim Change Rep ab c d e f Fig. 1 Eight hundred fifty-hectopascal eddy streamfunction (106 m2/s) as differences between the end of the twenty-first century (2075–2100) in (a) JJA and (b) DJF climatology (1980–2005) computed from ERA- the CMIP5 RCP8.5 scenario and the CMIP5 historical simulation, in JJA Interim [138]. c and d are the same as a and b but computed as CMIP5 and DJF, respectively. In the differences, dotted shading denotes statistical multi-model mean of the twentieth century simulations. Eddy significance exceeding 99% confidence level based on a t test. The list of streamfunction is computed removing the zonal mean. e and f are the models used to compute the multi-model ensemble mean is as in [139] Asia [6], southern and eastern Africa [7, 8], and southern precipitation likely driven by the intensification and poleward South America [9–11]. Subtropical highs also interact with expansion of both subtropical dry zones [23, 24]andHadley monsoon circulation [12, 13], tropical cyclone tracks cell [25, 26]. Considering their large contribution to global [14–16], marine stratus clouds with associated radiation bud- climate, this work intends to review the current knowledge on get [17–19], and Convergence Humidity Zones [10, 11]. the subtropical highs in the perspective of climate change and to Subtropical highs are important components of the atmo- highlight the main areas of future research for this topic. spheric circulation, and they play a major role in the formation of the world’s subtropical deserts and the zones of Mediterranean climate [2, 20]. In recent decades, in the context Background Framework of anthropogenic effects on climate and related global warming, changes have been observed in the position and intensity of the The subtropical highs can be distinguished into two main subtropical highs [21, 22], with decreased subtropical types: (i) zonally asymmetric highs, which are associated with Curr Clim Change Rep rotational flow and are strongest in summer, and (ii) zonally summer, a zonal wavenumber-3 component dominates the symmetric highs driven by Hadley cell descent, which are upper-level planetary wave field, showing some correspon- associated with divergent flow and are strongest in winter. dence with the three high-pressure cells at the surface [2]. This distinction is ruled by removal of the zonal mean and There, the intensity of the subtropical highs decreases from works well in describing the differences between subtropical austral winter to summer (Fig. 1). Possible reasons are related highs in the Northern and Southern Hemispheres, respective- with continents in the Southern Hemisphere being zonally ly, but also between the seasons. In fact, if the dynamic of the narrower than their counterpart in the Northern Hemisphere subtropical highs is considered similar in the two hemispheres [2], monsoons being less important generators of zonal [2], the dominant mechanisms responsible for their formation asymmetries in the mostly ocean-covered Southern vary with the season. In the summer hemisphere, the zonal Hemisphere, and topographic effects becoming stronger in asymmetry in diabatic heating associated with the land-sea winter as the flow intensifies [35]. Also, inter-hemispheric thermal contrast is dominant, whereas during winter, when teleconnections have been found to contribute to either main- convective activity over the continents is weaker, the presence taining or strengthening the southern subtropical anticyclones of the orography and the subsidence caused by the Hadley during the winter because of the heating and forcing from the circulation prevail [20, 27]. The observed mean position and summer monsoons and deep tropical convection over warm intensity of the subtropical highs in the two hemispheres and sea surface temperatures (SSTs) in the Northern Hemisphere in the different peak seasons is summarized in Fig. 1a, b in [35–37]. terms of eddy streamfunction at 850 hPa. Hadley circulation, radiation balances, and SSTs all con- Idealized numerical models with prescribed heating and tribute to the annual cycle of the subtropical highs [27, 38]. As realistic topography provided the hypothesis that the basic the transition to the early summer occurs, the wintertime zonal cause of the summer anticyclone formation is the latent heat band of high pressure underlying the subsiding branch of the released by the monsoons over the neighboring continents to Hadley cell starts to break when convection over land com- theeastorwest[28]. Remote diabatic heating in the monsoon mences. This is accomplished through vorticity balance, in- region induces a stationary Rossby wave pattern to the west cluding poleward low-level flow into the regions of deep con- that, interacting with the mid-latitude westerlies, produces adi- vection on the western flank of the subtropical high, while abatic descent with the aid of local orography. Also, local radiatively driven subsidence and equatorward flow dominate diabatic enhancement can lead to a strengthening of the de- on the eastern flank [22]. As the summer ends, the east-west scent [20]. With this monsoon-desert mechanism, the exis- asymmetry in SST tends to be damped by vertical flux of tence of the subtropical anticyclones in the Northern Pacific moist static energy. Hence, subtropical anticyclones ultimately and Atlantic sectors has been related to the Asian and North become more zonally symmetric when precipitation in the American monsoon, respectively, and that in the South subtropics weakens and eventually