Re-examining tropical expansion - WxMaps.org

Review Article https://doi.org/10.1038/s41558-018-0246-2

Re-examining tropical expansion Paul W. Staten1, Jian Lu 2*, Kevin M. Grise 3, Sean M. Davis4 and Thomas Birner5 Observations reveal a poleward expansion of the tropics in recent decades, implying a potential role of human activity. However, although theory and modelling suggest increasing GHG concentrations should widen the tropics, previous observational-based studies depict disparate rates of expansion, including many that are far higher than those simulated by climate models. Here, we review the rates and possible causes of observed and projected tropical widening. By accounting for methodological differences, the tropics are found to have widened about 0.5° of latitude per decade since 1979. However, it is too early to detect robust anthropogenically induced widening imprints due to large internal variability. Future work should target the seasonal and regional signatures of forced widening, as well as the associated dynamical mechanisms.

M

eteorologists often conceptually partition the Earth’s atmosphere into the ‘tropics’ and the ‘extratropics’, with the boundary between them being defined roughly by east-towest lines of high pressure, dry climate and subsiding air at roughly 30° S and 30° N (Fig. 1). In the tropics (meaning the moist, deep tropics and the dry subtropics together), climate is dominated by a pair of global-scale circulations known as the Hadley cells, which consist of moist ascent in the deep tropics, poleward flow aloft, dry descent in the subtropics and equatorward flow near the surface. Polewards of this boundary, the stronger Coriolis force helps to produce sinuous upper-tropospheric jet streams in either hemisphere, which result in substantial day-to-day and week-to-week variability in the weather, relative to the tropics. Between 2006 and 2008, a flurry of studies indicated that the circulation patterns associated with the subtropics had been shifting polewards in the preceding decades1–5. Any such change in tropical width, and in particular the Hadley cells, has the potential to exert myriad impacts on land and at sea. On land, increasing tropical width has the potential to shift rain belts6,7, expand subtropical deserts, and exacerbate droughts and wildfires8–10. At sea, the subtropical ridge coincides with belts of high salinity and low marine bioproductivity, and an increase in tropical width may imply an expansion of these belts11–13. Furthermore, any change in temperature and wind patterns may shift tropical cyclone tracks14. Considering that about half of the world’s population either lives in or near these subtropical semi-arid climate zones15, the implications of tropical widening are potentially significant. The changing width of the tropics thus continues to be an active area of research16. Numerous theoretical and modelling studies link the widening Hadley cells to human activity, in particular to the increase in GHG concentrations and the depletion of stratospheric ozone17–22. But some studies claim that the observed tropical widening outpaces that expected from modern climate change, suggesting that some ‘hidden forcing’ may be unaccounted for. Here we strive to resolve this apparent mystery by synthesizing results from the growing body of literature on the quantification, attribution and underlying processes of tropical widening. We review metrics, causes, observations and simulations of tropical widening, and find that the widening of the global mean tropical belt may not be predominantly human-induced.

Quantifying tropical width

The width of the tropics in the context of this review should not be confused with the Tropic of Cancer or Tropic of Capricorn on a global map; it instead refers to the edge of the arid or semi-arid regions in the subtropics, set predominantly by the descending branch of the Hadley circulation. The overturning Hadley circulation is partly thermally driven (that is, ascent and descent are driven by latent heating and radiative cooling, respectively) and partly eddy-driven. The eddy-driven aspect (the ‘eddy pump’) arises from large-scale turbulence in and near the subtropical jet streams along either side of the Hadley cells, with so-called Rossby waves propagating eastwards along the jet and frequently breaking like giant ocean waves when they reach sufficient amplitude. These waves redistribute momentum and heat in the atmosphere, reinforcing the thermally direct Hadley cells in the subtropics, and resulting in the thermally indirect Ferrel cells and eddy-driven jets at mid-latitudes. The eddy pump is one of the two ways in which eddies help to define the edge of the tropics, and relates several commonly used metrics via zonal mean momentum budgets. The boundary between the tropical Hadley cells and mid-latitude Ferrel cell is among the most common metrics for the width of the tropics (that is, the edge of the Hadley cells, with subsidence shown in wide blue arrows in Fig. 1a and the descending branch of the overturning cell shown in purple dashes in Fig. 1b; see also Ψ500 in Table 1). A related metric is the surface wind zero-crossing (Fig. 1b; see Us in Table 1)23, as the friction on the surface wind is in balance with the Coriolis forcing acting on the surface meridional wind. Subsidence in the same neighbourhood relates several other metrics, namely the subtropical sea-level pressure ridge (SLP, in Table 1), dry belts (Fig. 1, yellow shading; P − E, Table 1) and subtropical maxima in top-of-theatmosphere outgoing longwave radiation (OLR in Table 1, Fig. 1a). The second way in which eddies help to define the edge of the tropics is by mixing chemical, thermodynamical and dynamical quantities such as ozone, potential temperature and potential vorticity (a dynamical tracer due to its near-conservation). Mixing these fields creates a maximum meridional gradient of potential vorticity near the 4 PVU surface24 (or 4 PV unit surface, where 1 PVU ≡ 106 K m2 kg−1 s−1), and the drop off in the height of the troposphere (Fig. 1; see also Ztrop and ΔZtrop in Table 1), all in the same neighbourhood. As most of the Earth’s ozone resides in the stratosphere, total column ozone is generally lower where the troposphere

Indiana University Bloomington, Bloomington, IN, USA. 2Pacific Northwest National Laboratory, Richland, WA, USA. 3University of Virginia, Charlottesville, VA, USA. 4NOAA ESRL Chemical Sciences Division, Boulder, CO, USA. 5Ludwig-Maximilians-University Munich, Munich, Germany. *e-mail: [email protected] 1

768

NatUre Climate Change | VOL 8 | SEPTEMBER 2018 | 768–775 | www.nature.com/natureclimatechange

Review Article

NaTure ClimaTe CHanGe a High OLR

Altitude (km)

20

Low OLR

10

0

Dry

Wet

0°

60°N

b Mixing Jet P–E=0 OLR = 240 W m–2

Ψ=0 Us = 0

Fig. 1 | Schematics of the tropical/subtropical atmosphere. a, A threedimensional depiction of the circulation, radiation and hydrological features used to define the width of the tropics. b, Zonal mean zonal wind (blue contours) is shown, with the outermost streamline for the Hadley cell overturning circulation (purple dashed line), the tropopause level (black line) on the height–latitude plane and the profile of P − E on the horizontal plane (green is net precipitation, yellow is net evaporation). The mixing (indicated by the dotted black line) between tropical and extratropical air mass across the tropopause break is the key process linking the upper- and lower-tropospheric metrics (see text for detailed descriptions).

is taller. The tropopause height gradient thus coincides with a stratospheric ozone gradient (ΔO3, Table 1), as well as a meridional temperature gradient, which is in thermal wind balance with the subtropical jet (Fig. 1a, white arrow; ΔT and STJ in Table 1). Part of the perceived discrepancy between observed and modelled tropical width changes arises from the choice of which of the above metrics to use. Many commonly used metrics, such as ΔOLR, ΔO3, Ztrop, ΔZtrop, ΔT and STJ rely on ‘upper tropospheric’ observables. Use of these metrics suffers from a number of drawbacks; they yield a large range of widening rates (from negligible change to 2° of latitude per decade)2, co-vary weakly from year to year23,25 and show inconsistent responses to forcings23,25. The cause for the disagreement between some metrics is unclear; some of the perceived disagreement results from the use of arbitrary thresholds, which can be particularly problematic in a changing climate26,27. But even when care is taken to avoid arbitrary thresholds, many metrics disagree. Furthermore, as most of these metrics are only readily calculable from model data and model-based reanalyses, it is possible that some of the disagreements arise from biases in the underlying models. Identifying the cause of these disagreements represents an avenue for future research28. Other metrics (Ψ500, Us, SLP and P − E) rely loosely on ‘lower tropospheric’ observables. These metrics co-vary more closely from year to year, and show a more consistent response to external forcings21,23,27,29. They also have more direct linkages to the aforementioned surface impacts. For these reasons, we focus on tropical width measured by the lower tropospheric metrics in the remainder of this Review. We do not include SLP, because we find that although it is well correlated with the other three metrics over the Southern Hemisphere, over the Northern Hemisphere the metric performs less well, particularly over land25. Specifically, we focus on Ψ500, Us, and P − E. The first metric (Ψ500) is considered canonical for studying the width of the tropics, the second (Us) is dynamically related to the first (but also more readily observable from the surface) and the third (P − E) directly informs the impact on the hydrological cycle at the surface. But even for these metrics interannual variability is large, and trends differ between metrics and datasets (see Fig. 2).

Table 1 | Table of common metrics of tropical width Name

References Definition

Ψ500

5

The zero-crossing, polewards of the ITCZ, of the meridional overturning stream function at 500 hPa.

Us

23

Utrop

34,35,42

SLP

63,76,77

OLR*

5,29

The near surface wind zero crossing. This often occurs near Ψ500, and divides tropical easterlies from mid-latitude westerlies. The latitude of the maximum of the vertically integrated zonal momentum. This is a blended metric, capturing some of the behaviour of the subtropical jet and eddy-driven jets. The subtropical high-pressure ridge. This is generally correlated with the Ψ500 metric over the Southern Hemisphere, and the latitudinal maxima are often used for regional widening studies — particularly over the ocean. The isoline of outgoing longwave radiation, either remotely sensed or simulated, demarcating the edge of the dry zone. Values of 240 W m−2 and 250 W m−2 are commonly used. A slope-based (as opposed to threshold-based) OLR width metric. The tropopause break. The tropical tropopause typically occurs well above 15 km altitude, and the extratropical well below. This metric is generally chosen to be the latitude at which the relative frequency of tropopause occurrence above ~15 km surpasses a given percentage (for example, 55%, or 200 days out of the year). The latitude at which the greatest gradient in tropopause height occurs. The column ozone threshold. Column ozone should be lower in the tropics (where the troposphere is taller) than at high latitudes, polewards of the tropopause break. No standard threshold has been set; total column ozone is instead algorithmically separated into regimes. The subtropical jet stream inferred from the latitudinal tropospheric temperature gradient and thermal wind balance, or from lower stratospheric temperature change with latitude. The subtropical jet stream calculated from the location maximum winds aloft. Alternatively, the (near) surface wind may be subtracted from the winds aloft to isolate the STJ from the eddy-driven jet.

ΔOLR Ztrop*

ΔZtrop ΔO3*

27,43 2,18,23,26,50

27,50 4,78

ΔT

1,30

STJ

27,34,79

P−E

17,80

The latitude of the zero-crossing of precipitation minus evaporation, often associated with the climatological mean subsidence in the vicinity of (although generally polewards of) Ψ500.

Metrics used in this study are highlighted in bold. *Note that some of the metrics have been found to be problematic (see corresponding references), and have been included here for completeness.


769

Review Article

NaTure ClimaTe CHanGe

a

20

4 2 0 –2 –4 1960

Tropical widening (°)

b

1980

2000

2020

2040

2060

2080

2100

GHG-induced heating

6

Wave reflection

4 2

Rising tropopause height

Enhanced temperature gradient

Year Wave breaking

Enhanced static stability

Modified barolinicity Pollution

0 Decadal SST variability

0 90° S

–2 –4 1960

1980

2000

2020

2040

2060

2080

2100

2040

2060

2080

2100

Year

c 6 Tropical widening (°)

Stratospheric ozone-induced cooling

Altitude (km)

Tropical widening (°)

6

4 2

0°

90° N

Fig. 3 | Factors and mechanisms for tropical expansion. The key factors hypothesized to be responsible for the recent tropical expansion are tropical upper-tropospheric warming related to GHG forcing, enhanced extratropical stratospheric cooling related to O3 depletion in the Southern Hemisphere, emissions of black carbon and tropospheric ozone (pollution, primarily over the Northern Hemisphere) and decadal SST variability and trends. A green dot is used to denote the source of Rossby waves.

0 –2 –4 1960

1980

2000

2020 Year

Fig. 2 | 1960–2100 modelled versus observed widening for three metrics. a, P − E. b, Us. c, Ψ500. Individual model time series (black curves) are from CMIP5; historical simulation output is used to 2005, after which RCP8.5 simulation output is used. The multimodel ensemble mean is shown by the thick black curve. The envelope of observed estimates (red shading) is calculated from four modern retrospective reanalyses: ERA-Interim, MERRA2, CFSR and JRA55, each shown as a red curve. The 2σrange of the annual mean widths from pre-industrial control runs is shown by the dashed blue lines. See Table 1 for a description of each metric. All time series are plotted with respect to the mean during the 1981–2010 period.

Observed widening

A large majority of the tropical width metrics in Table 1 indicates expansion from around 1979 onward, or since the beginning of the satellite era3. Initial estimates for tropical widening during this time period have ranged widely, from 0.25 to 3° of latitude per decade. Much of the discrepancy can be attributed to the different metrics used for the tropical width. Upper-tropospheric observables tend to give larger widening rates and ranges (although there are exceptions1,30). In addition, many of these metrics have little direct relationship to the surface impacts associated with tropical widening. For example, widening rates of 3° per decade were inferred based on stratospheric ozone4, whereas metrics of upper tropospheric flow suggest a possible contraction of the tropics31. But neither of these metrics is closely correlated with metrics nearer the surface25. These lower-tropospheric measures, including precipitation changes, show much better agreement with each other and fewer extreme trends in most modern reanalyses23,27 (Fig. 2). The reasons for the discrepancies between the lower- and upper-tropospheric metrics are only beginning to be understood32. The timing of the satellite era and of early widening studies is also related; compared with neighbouring years, the early 1980s saw an anomalously equatorward Southern Hemisphere edge of the Hadley cell, and 2010 saw an anomalously poleward Southern 770

Hemisphere Hadley cell edge33. Observational studies performed around 2010, then, were primed by chance to show large trends in tropical width. Another source of the large trends in earlier studies of tropical width is the inhomogenous datasets available at the time. The dataset dependency is typified by the large tropical width trends (of around 2–4.5° between 1979 and 2005, based on Ψ5005) calculated from NCEP/NCAR and NCEP/DOE reanalyses; modern reanalyses generally exhibit much smaller trends (closer to ~0.5° per decade)23. Other recently derived rates of expansion of the Hadley cells in each hemisphere are roughly 0.2° per decade during 1979–199934 and 1979–200935, in good agreement with the results in Fig. 2. The observed tropical width expansion also exhibits intriguing seasonality, with the summer and autumn season witnessing larger expansion than winter and spring seasons5,18,19,36–38. This seasonality of the expansion may reflect not only the nature of the forcing agents driving tropical expansion (for example, the effect of stratospheric O3 only manifests in the Southern Hemisphere during the summer; see the section below on attribution), but also the different dynamical regimes in which the Hadley circulation resides during warm and cold seasons39.

Causes of tropical widening

Observed rates of widening tend to fall along the upper end of the range of model simulations, including both atmosphere–ocean coupled simulations with prescribed forcings and atmosphere-only simulations with prescribed forcings and observed sea surface temperature (SST) changes35,37,40–43. One challenge facing the climate community, then, is attributing changes in tropical width. One method for the attribution of trends involves simulating widening due to specific drivers individually. Ideally, each hypothesized cause produces a unique signature that can then be detected in observed (or reanalysis) data. Results from such simulations have identified an abundance of possible causes of tropical width expansion, including GHG concentration increases, stratospheric ozone depletion, volcanic forcing, increasing pollutants (such as black carbon aerosols, sulfates, and tropospheric ozone) and natural variations in SST, particularly over the Pacific Ocean11,18–22,33,44 (Fig. 3). However, the respective contribution of each forcing to the observed expansion since 1979 remains an open avenue of investigation. Here we


Review Article

NaTure ClimaTe CHanGe synthesize the current state of understanding regarding the likely contributions of each of the forcings listed above. GHG concentration increases. During the past three decades, CO2 concentrations have risen by ~50 ppm, implying a weak radiative forcing of 0.8 W m−2 based on Beer–Lambert law: ΔRF = 5.35 × ln(C/C0), where C and C0 denote the perturbed and reference concentrations of CO2, respectively. Although this direct radiative effect of increasing CO2 can cause the tropics to expand, it is the indirect effect — the warming of the ocean surface, and the subsequent resulting tropospheric warming and water vapour increase — that is more potent for the expansion of the tropics in simulations19. Historical forcing simulations suggest that GHG concentration increases produce a widening of the tropics of about 0.1° per decade, with most of that occurring over the Southern Hemisphere. Although sizable, this explains only part of the recent forced expansion of ~0.2–0.3° per decade33,35,41(black lines in Fig. 2). The increase in subtropical tropospheric stability (through mechanisms 1 and 2 in Box 1) and the Equator-to-pole temperature contrast in the upper troposphere–lower stratosphere (through mechanisms 4 and 5; Fig. 3) have been cited as a possible mechanism for widening under the influence of GHG increases, but the dynamical underpinnings of GHG-induced Hadley cell widening remain an topic of debate17,45,46. Stratospheric ozone depletion. Stratospheric ozone depletion over the Antarctic has been implicated in the shifts of the edge of the Southern Hemisphere Hadley cell during December–February on par with (or stronger than) those due to GHG concentrations in recent decades20,47–49. As the Antarctic ozone hole has deepened each spring since the early 1980s, the spring and summer polar stratosphere has cooled, steepening the Equator-to-pole temperature contrast in the upper troposphere–lower stratosphere (Fig. 3), and probably contributing to a poleward shift of the eddy-driven jet and Hadley cell edge during the following summer months. Mechanisms 4 and 5 in Box 1 might be plausible candidates for explaining ozone depletion-forced widening. Volcanic aerosols. The role of volcanic aerosols in tropical expansion is unclear. Some studies note a contraction of Ztrop-based tropical width following volcanic eruptions18,50, but separating the impacts of volcanic aerosols from interference by the El Niño/ Southern Oscillation (ENSO) is an outstanding problem, and volcanic eruptions seem to produce a weak signal at best in the troposphere23,51,52. More work seems warranted, as the same mechanisms underlying ozone depletion may operate after a volcanic eruptions, only operating in reverse, as volcanic eruptions cool the tropical upper troposphere and warm the lower stratosphere. Pollution. The impacts of pollution on tropical width are unclear. One major source of the uncertainty in the impacts of pollution is the competition between the heating effects of black carbon and tropospheric ozone and the cooling effects of sulfate aerosols. Emissions of all three have changed in recent decades, with a particular ramping up of black carbon and tropospheric aerosols over Asia in recent decades. Using slab ocean models, this has been implicated in a recent northward shift of the tropical boundary of the Northern Hemisphere by as much as 0.2° per decade53. A comparison of groups of coupled models from the Coupled Model Intercomparison Project Phase 3 (CMIP3) with and without black carbon and tropospheric ozone changes suggests a shift of about 0.3° per decade34 (see Fig. 4, bottom row). But this rate may be an overestimate, because it includes model differences as well as pollution changes34. And again, these rates of widening depend on the composition of the emissions, and whether the heating or cooling effects of aerosols dominate54.

Box 1 | | Mechanisms for eddy-driven tropical expansion

Classic theory treats the tropospheric circulation as occurring in a two-dimensional latitude–height plane81,82, and predicts a reasonable climatological Hadley circulation, but cannot account for changes in tropical width associated with zonally asymmetric phenomena such as ENSO, and fails to predict intermodel differences in the forced response to warming17,83. Analysis of Earth-like climates has shown that the descending motion near the edge of the Hadley cell is largely driven by eddies through the ‘eddy-pump’ effect84,85. Thus, any relevant mechanism for tropical expansion must include the effect of eddies86. Plausible (and not mutually exclusive) explanations for Hadley circulation widening in recent literature include the following: (1) The edge of the Hadley cell is set by the latitude at which the thermally forced, angular momentum-conserving, upper-tropospheric poleward flow becomes baroclinically unstable45,87, or favourable for the growth of baroclinic waves. This scaling theory predicts a Hadley cell expansion provided the following conditions are met: (1) increased static stability17,88,89, (2) decreased radius of the Earth, (3) decreased rotation rate of the Earth and (4) increased tropopause height17. (2) The edge of the Hadley cell is set by the latitude at which mid-latitude eddies propagating upwards from the surface first approach the tropopause. The penetration depth of these eddies is determined by the poleward temperature gradient, divided by the vertical stability. This formulation scales in a manner that is qualitatively similar to the first46,88. (3) An increase in the tropospheric baroclinicity, often through a change in the poleward temperature gradient, fosters increased wave growth polewards of the Hadley cell90–93. Sensitivity studies suggest that heating immediately equatorwards of the eddy-driven jet, that is, around 30°, is particularly effective in this regard34,94. (4) Changes in wave reflection and wave absorption can result in increased wave dissipation or breaking on the flanks of the Hadley cells and hence their expansion67,95–99. The effects of this wave breaking and the resulting mixing can be quantified in terms of eddy diffusivity100. (5) A shift in the mode of wave breaking from cyclonic wave breaking (LC2; from a satellite over the Northern Hemisphere, this resembles an ocean wave breaking to the left) to anticyclonic wave breaking (LC1; from a satellite over the Northern Hemisphere, this resembles an ocean wave breaking to the right) increases mixing on the equatorward side of the jet, in the neighbourhood of the Hadley cells67,98. Each of the causes for Hadley cell widening discussed here probably operates through one or more of the mechanisms above, although realistic widening also includes the complicating effects of terrain and season. The efficacy of aerosols in driving changes in tropical width may arise partly from their location. The largest increases in pollution have taken place near the subtropics, where they have the greatest impact on the extent of the Northern Hemisphere Hadley cell via mechanism 3 in Box 153. Furthermore, the rapid increase in southeast Asia raises the possibility that pollution may force major modes of Pacific SST variability35,55–58. Natural variability. Tropical width can vary on its own as a manifestation of internal climate variability over a range of timescales,


771

Review Article


P–E

Us/Utrop

4 D

J 10 A

P–E

10 J

G

H

Ψ

I

P–E

13 M

Us/Utrop

2 B

A 6 F AB D 4

6 F

H

F 6

A

F A 6 A

6 F

Ψ

Us/Utrop

A

F

J 10 A

10 J

AA

G

10 J 10 J A A

C E L 12

10 J

A. Staten et al., 201219 (1870–2000)*a B. Allen et al., 201234 (1979–2009)b C. Amaya et al., 201733 (1980–2013) 12 L

E

13 M

J

C J

D. Hu et al., 201337 (1979–2005) E. Quan et al., 201369 (1979–2009)*

J F. Allen et al., 201234 (1979-2009)c

E 12 L C 14 N

Ψ

K 11 13 M J

B

P–E

C 13 M 12 L N J 14

E

G. Son et al., 200947 (1960–1999)

K 11

H. Son et al., 201049 (1960–1999) I. Min and Son, 201320 (1979–2009)

B

B 2

J. Allen et al., 201435 (1979–2009)c Us/Utrop

B

K. Hu et al., 201136 (1979–2009)*

B 2

L. Mantsis et al., 201743 (1993–2012)*

Ψ

B

B

B 2

M. Allen et al., 201742 (1979–2008)c N. Garfinkel et al., 201565 (1980–2009)*

–0.5

–0.25

0

0.25

0.5

Expansion rate (° decade–1) Southern Hemisphere expansion

Northern Hemisphere expansion

Fig. 4 | Tropical expansion rates in previous studies. Circles and squares represent estimates from coupled and AMIP-style (prescribed SST) experiments, respectively, for the Northern Hemisphere (red) and Southern Hemisphere (blue) and the time period labelled for each study (see list on the right). The squares with whiskers show the range of four reanalysis trends. Asterisks indicate that the values come from a single model. Open symbols indicate that only the DJF mean trend is analysed. Results are grouped (from top to bottom) by cause: GHGs, stratospheric ozone depletion, observed SST and black carbon plus tropospheric ozone. Superscripts denote deviations from other studies. aTrends are estimated over 1870–2000 for GHGs and 1980–2000 for stratospheric ozone. bThe black carbon and tropospheric ozone trend is the difference between nine models with time varying black carbon and ozone forcings and six without. cUtrop is shown in place of Us.

without the application of an external forcing59, as evidenced by the year-to-year differences in the individual curves in Fig. 2. At interannual timescales, it has been well established that the warm phase of ENSO and the associated enhanced tropical heating can drive a contraction of the Hadley circulation and an equatorward shift of the jet stream60,61 — and vice versa for the cold phase of ENSO, or La Niña. Specifically, warm SST anomalies centred around 30° N in the northwestern Pacific (such as those that contribute to a negative ENSO/Pacific decadal oscillation (PDO) pattern) have been identified as part of the optimal SST configuration for generating zonally elongated ridge in the subtropics, which led to prolonged dry conditions in mid-latitudes, especially in North America, during the period 1998–200262. Given the strong resemblance between the patterns 772

of the PDO and ENSO, it is perhaps unsurprising that a shift towards the negative phase of the PDO during the past few decades may have played a part in the recent expansion of the Hadley cell35. Forced or naturally varying, this low-frequency SST effect is not captured by the average of CMIP-type climate simulations, as the phases of various modes of oceanic variability (such as the PDO) differ from one simulation to the next, and any natural decadal signal will be averaged out by the multimodel ensemble mean. Ensemble means of fully coupled CMIP5 simulations can only capture about a quarter of the observed tropical expansion35,63. By comparing coupled and Atmospheric Model Intercomparison Project (AMIP)-style (uncoupled, with prescribed SSTs chosen to match those observed) historical simulations, the importance of


Review Article

NaTure ClimaTe CHanGe the SSTs in driving the expansion is beginning to be recognized, with an expansion of 0.2° per decade in the Northern Hemisphere and a smaller expansion in the Southern Hemisphere (Fig. 4, third row) resulting from the observed SSTs. Much of that may be attributed to the recent negative PDO-like SST pattern33,42. To the extent that the PDO is an intrinsic coupled mode of climate variability, this suggests that the natural low-frequency variability due to ocean dynamics has probably caused at least as much of the recent trend as human activity33,42. The Hadley cell expansion under a negative phase of the PDO ought to share common dynamical processes with that under the cold phase of ENSO, which has been examined from the perspectives of baroclinicity (mechanism 3), wave refraction64 and wave breaking (mechanism 5)60. It should also be noted that the observed climate is just one realization of possible Earth-like climates. Referring to Fig. 2, the reanalyses time series (red curves) behave much more like the individual model time series (grey curves) than the multimodel mean (thick black curve). And although reanalyses may exhibit stronger trends than the multimodel mean, their changes are well within the envelope of the individual simulations. Observed trends from reanalyses, then, are within the range of trends in individual model simulations from CMIP523,38,65. Furthermore, observed trends are within the realm of natural variability, so no hidden forcing is needed to explain the discrepancy between forced and observed rate of widening; natural variability, including (but not limited to) the PDO, may be enough42.

Future widening

Although discerning a forced signal in the observational record is challenging, the detection of such a signal in the future is not so much a question of if, but when, if humanity continues on the business-as-usual path of GHG emissions. In the near term, natural variability muddles the widening signal of increasing GHG concentrations. But models project that the forced tropical widening will break out of the envelope of natural year-to-year variability some time in the middle of this century33(see Fig. 2). The timescale for this emergence will depend largely on the hemisphere and season, as well as on emissions. Thus far, the widening signal over the Southern Hemisphere has been stronger, owing to the comparative longitudinal uniformity of the Southern Hemisphere and to a strong summertime (December–February, DJF) contribution to widening by stratospheric ozone depletion. The recovery of the ozone hole will therefore probably counteract much of the Southern Hemisphere DJF widening due to GHG concentrations over the coming half-century21,44,48,66,67. Zonal mean widening over the Northern Hemisphere is most clear during SON, when the Pacific and Atlantic jets are aligned38,39,68. This may thus be the season to inspect for early evidence of a forced expansion signal over the Northern Hemisphere. Over all hemispheres and seasons, SST variability and future changes in anthropogenic emissions of GHGs and pollutants may each advance or delay the time to detection. Neither of these drivers can be predicted with confidence at present.

Discussion

A flurry of studies during the past decade or so has shown compelling evidence for the widening of the tropics in both observations and model simulations. But recently, more careful analyses indicated that the widening trend since late 1970s may not be as rapid as some metrics have indicated, and may not be happening in all metrics for tropical width. By surveying studies based on more reliable tropospheric metrics and contemporary reanalysis datasets, we find that the expansion rate may be dialled down to approximately 0.2° per decade in each hemisphere. Individual numerical experiments can reproduce this reduced rate of tropical width expansion, implying that no hidden forcing is needed to explain observed expansion.

Accounting for the observed multidecadal evolution of the SST can fill in some of the gap between the modelled and observed trends. The earlier notion that anthropogenic forcings are the predominant cause for the observed expansion seems to arise from a few early studies limited to the use of the problematic tropopause-based and OLR-based tropical width metrics (marked by asterisks in Table 1) and older generation reanalyses, and therefore should be revised. Including recent evidence, it is fair to assert that the natural swings in decadal atmospheric and oceanic variability may have driven at least as much of the observed expansion as human activity. Natural variability thus looms large over tropical width attribution studies. We may be on the cusp of robust detectability69, however, and our synthesis reveals several promising paths forward. Much remains to be done to understand the processes that determine tropical width. The diverse mechanisms summarized in Box 1 for the eddy-driven Hadley cell expansion are incomplete — other factors may cause the tropics to expand independently from the eddy pump effect. A mechanistic explanation for seasonal and longitudinal differences is also in order, given the considerable seasonality and interhemispheric asymmetry in tropical expansion under increasing GHG concentrations23,39,70,71. More surgically designed numerical experiments are needed to discern which of the hypothesized mechanisms listed above (or other novel mechanisms) are most relevant to each of the forcings discussed here. Regarding impacts, the climatic changes that matter are regional. In addition to informing us about potential impacts, the study of regional changes in tropical width is useful for attribution studies. For instance, stratospheric ozone depletion, GHG concentration increases, aerosols and natural SST variability may all produce annual and zonal mean changes in tropical width, but with very different seasonal and regional manifestations. Several recent studies show promise in regressing regional changes (in the sea-level pressure field or P − E, for example) to zonal mean tropical width to identify the spatial fingerprint of the variability in tropical width33,72–74. This approach should be encouraged, and may be extended to identifying the seasonal fingerprint of the forced change and variability in tropical width. Part of the challenge with measuring the tropical width is that the most easily observed metrics globally (for example, tropopause height or OLR) are the least correlated with other measures or with surface impacts, and the most highly correlated (such as the overturning stream function) are not always directly observable, but require global, gridded data. To produce gridded observational data, reanalyses use forecast models to perform what may be thought of as a physically based interpolation, but they are no ‘silver bullet’ — reanalyses inherit some of the faults of the models on which they are based, as well as discontinuities in the observations they ingest75. Earlier reanalyses produce spurious trends compared with modern reanalyses and with datasets based on raw observations23. Ideally, one would hope for a single metric that is calculable from models, reanalyses and observational datasets alike, and can in turn be robustly related to the Hadley cell edge and other metrics. In the absence of such a metric, an improved understanding of the poor agreement between the observable tropopause-based metrics and other reanalysis-derived metrics is worth pursuing. Our synthesis shows strong evidence that the tropics have widened by about 0.5° per decade since the beginning of the satellite era. The robust detectability of this widening trend is to some extent an artefact of a fortuitous swing in the PDO. Since no one has a crystal ball to tell when the PDO will switch phases (as decadal prediction remains a major challenge to the climate community), it is impossible to predict whether or not the Earth’s tropical belt is going to continue bulging in the coming decade33. But the ozone hole is beginning to recover, and judging from the timescale of the PDO, it is likely to switch sign in the coming decade or two; indeed, the last few years have seen El Niño-like conditions that may be the


773

Review Article


harbinger of such a shift. Each of these trends functions to counteract tropical widening. Thus, for the coming few decades, the tropics may contract. At centennial timescales, however, the effect of GHG emissions will begin to dominate, and eventually further widen the tropics, unless action is taken to mitigate them. Received: 14 September 2017; Accepted: 13 July 2018; Published online: 30 August 2018

References 1. 2. 3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

774

Fu, Q., Johanson, C. M., Wallace, J. M. & Reichler, T. Enhanced midlatitude tropospheric warming in satellite measurements. Science 312, 1179 (2006). Seidel, D. J. & Randel, W. J. Recent widening of the tropical belt: evidence from tropopause observations. J. Geophys. Res. Atmos. 112, D20113 (2007). Seidel, D. J., Fu, Q., Randel, W. J. & Reichler, T. J. Widening of the tropical belt in a changing climate. Nat. Geosci. 1, 21–24 (2008). Hudson, R. D., Andrade, M. F., Follette, M. B. & Frolov, A. D. The total ozone field separated into meteorological regimes – Part II: Northern Hemisphere mid-latitude total ozone trends. Atmos. Chem. Phys. 6, 5183–5191 (2006). Hu, Y. & Fu, Q. Observed poleward expansion of the Hadley circulation since 1979. Atmos. Chem. Phys. 7, 5229–5236 (2007). Si, D., Ding, Y. & Liu, Y. Decadal northward shift of the Meiyu belt and the possible cause. Chinese Sci. Bull. 54, 4742–4748 (2009). Brönnimann, S. et al. Southward shift of the northern tropical belt from 1945 to 1980. Nat. Geosci. 8, 969–974 (2015). Post, D. A. et al. Decrease in southeastern Australian water availability linked to ongoing Hadley cell expansion. Earth’s Future 2, 231–238 (2014). Scheff, J. & Frierson, D. Twenty-first-century multimodel subtropical precipitation declines are mostly midlatitude shifts. J. Clim. 25, 4330–4347 (2012). Feng, S. & Fu, Q. Expansion of global drylands under a warming climate. Atmos. Chem. Phys. 13, 10081–10094 (2013). Polovina, J. J., Howell, E. A. & Abecassis, M. Ocean’s least productive waters are expanding. Geophys. Res. Lett. 35, L03618 (2008). Irwin, A. J. & Oliver, M. J. Are ocean deserts getting larger? Geophys. Res. Lett. 36, L18609 (2009). Moore, J. K. et al. Sustained climate warming drives declining marine biological productivity. Science 359, 1139–1143 (2018). Studholme, J. & Gulev, S. Concurrent changes to Hadley crculation and the meridional distribution of tropical cyclones. J. Clim. 31, 4367–4389 (2018). Rankin, W. Population Histograms (Radical Cartography, 2008); http://www. radicalcartography.net/index.html?histpop Birner, T., Davis, S. M. & Seidel, D. J. The changing width of Earth’s tropical belt. Phys. Today 67, 38–44 (2014). Lu, J., Vecchi, G. A. & Reichler, T. Expansion of the Hadley cell under global warming. Geophys. Res. Lett. 34, L06805 (2007). Lu, J., Deser, C. & Reichler, T. Cause of the widening of the tropical belt since 1958. Geophys. Res. Lett. 36, L03803 (2009). Staten, P. W., Rutz, J. J., Reichler, T. & Lu, J. Breaking down the tropospheric circulation response by forcing. Clim. Dynam. 39, 2361–2375 (2012). Min, S.-K. & Son, S.-W. Multimodel attribution of the Southern Hemisphere Hadley cell widening: major role of ozone depletion. J. Geophys. Res. Atmos. 118, 3007–3015 (2013). Waugh, D. W., Garfinkel, C. I. & Polvani, L. M. Drivers of the recent tropical expansion in the Southern Hemisphere: changing SSTs or ozone depletion? J. Clim. 28, 6581–6586 (2015). Kim, Y.-H., Min, S.-K., Son, S.-W. & Choi, J. Attribution of the local Hadley cell widening in the Southern Hemisphere. Geophys. Res. Lett. 1015–1024 (2017). Davis, N. & Birner, T. On the discrepancies in tropical belt expansion between reanalyses and climate models and among tropical belt width metrics. J. Clim. 30, 1211–1231 (2016). Homeyer, C. R. & Bowman, K. P. Rossby wave breaking and transport between the tropics and extratropics above the subtropical jet. J. Atmos. Sci. 70, 607–626 (2012). Waugh, D. W. et al. Revisiting the relationship among metrics of tropical expansion. J. Clim. 31, 7565–7581 (2018). Birner, T. Recent widening of the tropical belt from global tropopause statistics: sensitivities. J. Geophys. Res. Atmos. 115, D23109 (2010). Davis, S. M. & Rosenlof, K. H. A multidiagnostic intercomparison of tropical-width time series using reanalyses and satellite observations. J. Clim. 25, 1061–1078 (2011). Davis, N. A., Davis, S. M. & Waugh, D. W. New insights into tropical belt metrics. Variations 16, 1–7 (2018).

29. Solomon, A., Polvani, L. M., Waugh, D. W. & Davis, S. M. Contrasting upper and lower atmospheric metrics of tropical expansion in the Southern Hemisphere. Geophys. Res. Lett. 43, 10496–10503 (2016). 30. Fu, Q. & Lin, P. Poleward shift of subtropical jets inferred from satelliteobserved lower-stratospheric temperatures. J. Clim. 24, 5597–5603 (2011). 31. Manney, G. L. & Hegglin, M. I. Seasonal and regional variations of long-term changes in upper-tropospheric jets from reanalyses. J. Clim. 31, 423–448 (2017). 32. Zurita-Gotor, P. & Álvarez-Zapatero, P. Coupled interannual variability of the Hadley and Ferrel cells. J. Clim. 31, 4757–4773 (2018). 33. Amaya, D. J., Siler, N., Xie, S.-P., & Miller, A. J. The interplay of internal and forced modes of Hadley Cell expansion: lessons from the global warming hiatus. Clim. Dynam. 305–319 (2017).. 34. Allen, R. J., Sherwood, S. C., Norris, J. R. & Zender, C. S. Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone. Nature 485, 350–354 (2012). 35. Allen, R. J., Norris, J. R. & Kovilakam, M. Influence of anthropogenic aerosols and the Pacific decadal oscillation on tropical belt width. Nat. Geosci. 7, 270–274 (2014). 36. Hu, Y., Zhou, C. & Liu, J. Observational evidence for poleward expansion of the Hadley circulation. Adv. Atmos. Sci. 28, 33–44 (2011). 37. Hu, Y., Tao, L. & Liu, J. Poleward expansion of the Hadley circulation in CMIP5 simulations. Adv. Atmos. Sci. 30, 790–795 (2013). 38. Grise, K. M., Davis, S. M. & Staten, P. W. Regional and seasonal characteristics of the recent expansion of the tropics. J. Clim. https://doi. org/10.1175/JCLI-D-18-0060.1 (2018). 39. McGraw, M. C. & Barnes, E. A. Seasonal sensitivity of the eddy-driven jet to tropospheric heating in an idealized AGCM. J. Clim. 29, 5223–5240 (2016). 40. Johanson, C. M. & Fu, Q. Hadley cell widening: model simulations versus observations. J. Clim. 22, 2713–2725 (2009). 41. Tao, L., Hu, Y. & Liu, J. Anthropogenic forcing on the Hadley circulation in CMIP5 simulations. Clim. Dynam. 46, 3337–3350 (2016). 42. Allen, R. J. & Kovilakam, M. The role of natural climate variability in recent tropical expansion. J. Clim. 30, 6329–6350 (2017). 43. Mantsis, D. F., Sherwood, S., Allen, R. & Shi, L. Natural variations of tropical width and recent trends. Geophys. Res. Lett. 44, 3825–3832 (2017). 44. Perlwitz, J. Tug of war on the jet stream. Nat. Clim. Change 1, 29–31 (2011). 45. Held, I. M., Salmon, R., Fields, J. & Thiffeault, J.-L. in The General Circulation of the Atmosphere: 2000 Program in Geophysical Fluid Dynamics Technical Report No. WHOI-2001-03 1–54 (Woods Hole Oceanographic Institute, 2000); http://hdl.handle.net/1912/15 46. Korty, R. L. & Schneider, T. Extent of Hadley circulations in dry atmospheres. Geophys. Res. Lett. 35, L23803 (2008). 47. Son, S.-W., Tandon, N. F., Polvani, L. M. & Waugh, D. W. Ozone hole and Southern Hemisphere climate change. Geophys. Res. Lett. 36, L15705 (2009). 48. Polvani, L. M., Waugh, D. W., Correa, G. J. P. & Son, S.-W. Stratospheric ozone depletion: the main driver of twentieth-century atmospheric circulation changes in the Southern Hemisphere. J. Clim. 24, 795–812 (2010). 49. Son, S.-W. et al. Impact of stratospheric ozone on Southern Hemisphere circulation change: A multimodel assessment. J. Geophys. Res. Atmos. 115, D00M07 (2010). 50. Lucas, C., Nguyen, H. & Timbal, B. An observational analysis of Southern Hemisphere tropical expansion. J. Geophys. Res. Atmos. 117, D17112 (2012). 51. Robock, A., Adams, T., Moore, M., Oman, L. & Stenchikov, G. Southern Hemisphere atmospheric circulation effects of the 1991 Mount Pinatubo eruption. Geophys. Res. Lett. 34, L23710 (2007). 52. Barnes, E. A., Solomon, S. & Polvani, L. M. Robust wind and precipitation responses to the Mount Pinatubo eruption, as simulated in the CMIP5 models. J. Clim. 29, 4763–4778 (2016). 53. Kovilakam, M. & Mahajan, S. Confronting the “Indian summer monsoon response to black carbon aerosol” with the uncertainty in its radiative forcing and beyond. J. Geophys. Res. Atmos. 121, 7833–7852 (2016). 54. Allen, R. J. & Ajoku, O. Future aerosol reductions and widening of the northern tropical belt. J. Geophys. Res. Atmos. 121, 6765–6786 (2016). 55. Bonfils, C. & Santer, B. D. Investigating the possibility of a human component in various Pacific decadal oscillation indices. Clim. Dynam. 37, 1457–1468 (2011). 56. Newman, M. et al. The Pacific decadal oscillation, revisited. J. Clim. 29, 4399–4427 (2016). 57. Xu, Y. & Hu, A. How would the twenty-first-century warming influence Pacific decadal variability and its connection to North American rainfall: assessment based on a revised procedure for the IPO/PDO. J. Clim. 31, 1547–1563 (2017). 58. Wills, R. C., Schneider, T., Wallace, J. M., Battisti, D. S. & Hartmann, D. L. Disentangling global warming, multidecadal variability, and El Niño in Pacific temperatures. Geophys. Res. Lett. 45, 2487–2496 (2018).


Review Article

NaTure ClimaTe CHanGe 59. Simpson, I. R. Natural variability in the width of the tropics. Variations 16, 14–20 (2018). 60. Lu, J., Chen, G. & Frierson, D. M. W. Response of the zonal mean atmospheric circulation to El Niño versus global warming. J. Clim. 21, 5835–5851 (2008). 61. Chen, G., Lu, J. & Frierson, D. M. W. Phase speed spectra and the latitude of surface westerlies: interannual variability and global warming trend. J. Clim. 21, 5942–5959 (2008). 62. Lau, N.-C. & Nath, M. J. A model study of heat waves over North America: meteorological aspects and projections for the twenty-first century. J. Clim. 25, 4761–4784 (2012). 63. Nguyen, H., Lucas, C., Evans, A., Timbal, B. & Hanson, L. Expansion of the Southern Hemisphere Hadley cell in response to greenhouse gas forcing. J. Clim. 28, 8067–8077 (2015). 64. Seager, R., Harnik, N., Kushnir, Y., Robinson, W. & Miller, J. Mechanisms of hemispherically symmetric climate variability. J. Clim. 16, 2960–2978 (2016). 65. Garfinkel, C. I., Waugh, D. W. & Polvani, L. M. Recent Hadley cell expansion: the role of internal atmospheric variability in reconciling modeled and observed trends. Geophys. Res. Lett. 42, 10824–10831 (2015). 66. Eyring, V. et al. Multimodel projections of stratospheric ozone in the 21st century. J. Geophys. Res. Atmos. 112, D16303 (2007). 67. McLandress, C. et al. Separating the dynamical effects of climate change and ozone depletion. Part II: Southern Hemisphere troposphere. J. Clim. 24, 1850–1868 (2010). 68. Simpson, I. R., Shaw, T. A. & Seager, R. A diagnosis of the seasonally and longitudinally varying midlatitude circulation response to global warming. J. Atmos. Sci. 71, 2489–2515 (2014). 69. Quan, X.-W., Hoerling, M. P., Perlwitz, J., Diaz, H. F. & Xu, T. How fast are the tropics expanding? J. Clim. 27, 1999–2013 (2013). 70. Barnes, E. A. Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes. Geophys. Res. Lett. 40, 4734–4739 (2013). 71. Grise, K. M. & Polvani, L. M. Is climate sensitivity related to dynamical sensitivity? J. Geophys. Res. Atmos. 121, 5159–5176 (2016). 72. Schmidt, D. F. & Grise, K. M. The response of local precipitation and sea level pressure to Hadley cell expansion. Geophys. Res. Lett. 44, 10,510– 573,582 (2017). 73. Huang, R., Chen, S., Chen, W. & Hu, P. Interannual variability of regional Hadley circulation intensity over Western Pacific during boreal winter and its climatic impact over Asia‐Australia region. J. Geophys. Res. Atmos. 123, 344–366 (2017). 74. Zhang, H. & Delworth, T. L. Detectability of decadal anthropogenic hydroclimate changes over North America. J. Clim. 31, 2579–2597 (2018). 75. Hartmann, D.L. et al. in Climate Change 2013: The Physical Science Basis (eds Stocker, T. F. et al.) Ch. 2 (IPCC, Cambridge Univ. Press, 2013). 76. Choi, J., Son, S.-W., Lu, J. & Min, S.-K. Further observational evidence of Hadley cell widening in the Southern Hemisphere. Geophys. Res. Lett. 41, 2590–2597 (2014). 77. Lucas, C. & Nguyen, H. Regional characteristics of tropical expansion and the role of climate variability. J. Geophys. Res. Atmos. 120, 6809–6824 (2015). 78. Lamarque, J.-F. & Solomon, S. Impact of changes in climate and halocarbons on recent lower stratosphere ozone and temperature trends. J. Clim. 23, 2599–2611 (2010). 79. Archer, C. L. & Caldeira, K. Historical trends in the jet streams. Geophys. Res. Lett. 35, L08803 (2008). 80. Zhou, Y. P., Xu, K.-M., Sud, Y. C. & Betts, A. K. Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data. J. Geophys. Res. Atmos. 116, D09101 (2011). 81. Schneider, E. K. & Lindzen, R. S. Axially symmetric steady-state models of the basic state for instability and climate studies. Part I. Linearized calculations. J. Atmos. Sci. 34, 263–279 (1977). 82. Held, I. M. & Hou, A. Y. Nonlinear axially symmetric circulations in a nearly inviscid atmosphere. J. Atmos. Sci. 37, 515–533 (1980). 83. Lindzen, R. S. & Hou, A. V. Hadley circulations for zonally averaged heating centered off the Equator. J. Atmos. Sci. 45, 2416–2427 (1988). 84. Robinson, W. A. On the midlatitude thermal response to tropical warmth. Geophys. Res. Lett. 29, 31–34 (2002). 85. Walker, C. C. & Schneider, T. Eddy influences on Hadley circulations: simulations with an idealized GCM. J. Atmos. Sci. 63, 3333–3350 (2006).

86. Rodrigo, C. Role of eddies in the interannual variability of Hadley cell strength. Geophys. Res. Lett. 34, L22705 (2007). 87. Palmén, E. & Newton, C. W. Atmospheric circulation systems: their structural and physical interpretation. Science 167, 972 (1970). 88. Schneider, T., O’Gorman, P. A. & Levine, X. J. Water vapor and the dynamics of climate changes. Rev. Geophys. 48, RG3001 (2010). 89. Lu, J., Chen, G. & Frierson, D. M. W. The position of the midlatitude storm track and eddy-driven westerlies in aquaplanet AGCMs. J. Atmos. Sci. 67, 3984–4000 (2010). 90. Yin, J. H. A consistent poleward shift of the storm tracks in simulations of 21st century climate. Geophys. Res. Lett. 32, L18701 (2005). 91. Brayshaw, D. J., Hoskins, B. & Blackburn, M. The storm-track response to idealized SST perturbations in an aquaplanet GCM. J. Atmos. Sci. 65, 2842–2860 (2008). 92. Sampe, T., Nakamura, H., Goto, A. & Ohfuchi, W. Significance of a midlatitude SST frontal zone in the formation of a storm track and an eddy-driven westerly jet. J. Clim. 23, 1793–1814 (2009). 93. Rivière, G. A dynamical interpretation of the poleward shift of the jet streams in global warming scenarios. J. Atmos. Sci. 68, 1253–1272 (2011). 94. Tandon, N. F., Gerber, E. P., Sobel, A. H. & Polvani, L. M. Understanding Hadley cell expansion versus contraction: insights from simplified models and implications for recent observations. J. Clim. 26, 4304–4321 (2012). 95. Chen, G. & Held, I. M. Phase speed spectra and the recent poleward shift of Southern Hemisphere surface westerlies. Geophys. Res. Lett. 34, L21805 (2007). 96. Williams, G. P. Circulation sensitivity to tropopause height. J. Atmos. Sci. 63, 1954–1961 (2006). 97. Lorenz, D. J. Understanding midlatitude jet variability and change using Rossby wave chromatography: poleward-shifted jets in response to external forcing. J. Atmos. Sci. 71, 2370–2389 (2014). 98. Wittman, M. A. H., Charlton, A. J. & Polvani, L. M. the effect of lower stratospheric shear on baroclinic instability. J. Atmos. Sci. 64, 479–496 (2007). 99. Kidston, J. & Vallis, G. K. The relationship between the speed and the latitude of an eddy-driven jet in a stirred barotropic model. J. Atmos. Sci. 69, 3251–3263 (2012). 100. Lu, J., Sun, L., Wu, Y. & Chen, G. The role of subtropical irreversible PV mixing in the zonal mean circulation response to global warming-like thermal forcing. J. Clim. 27, 2297–2316 (2013).

Acknowledgements

P.W.S., K.M.G., T.B. and S.M.D. are members of working groups related to the topic of this review—the International Space Science Institute (ISSI) Tropical Width Diagnostics Intercomparison Project and the US Climate Variability and Predictability Program (US CLIVAR) Changing Width of the Tropical Belt Working Group. We thank the the ISSI and US CLIVAR offices, and sponsoring agencies (the ESA, Swiss Confederation, Swiss Academy of Sciences, University of Bern, NASA, NOAA, NSF and DOE) for supporting these groups and activities. J.L. is supported by the US Department of Energy Office of Science Biological and Environmental Research (BER) as part of the Regional and Global Climate Modeling Program. We acknowledge F. Liu for his assistance in drawing Fig. 4.

Author contributions

P.W.S. wrote and revised the majority of the paper. J.L. contributed to the writing, produced the summary figure and produced the schematic figures with help from a graphic designer at PNNL. K.M.G. contributed to the writing, and created the time series plot. S.M.D. and T.B. both contributed to the writing and revision of the paper.

Competing interests

The authors declare no competing interests.

Additional information

Reprints and permissions information is available at www.nature.com/reprints. Correspondence should be addressed to J.L. Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


775