PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2017GL074972
Key Points:• Heat uptake over the Southern
Ocean under global warming has
pronounced inﬂuences on global
atmospheric circulations• The delayed warming over theSouthern Ocean extends northward to
the tropics, shifting the tropicalprecipitation northward• Distinct zonal variations in tropical SST
and precipitation responses develop,
accompanying the interhemisphericasymmetry

Supporting Information:• Supporting Information S1
Correspondence to:Y.-T. Hwang,
ythwang@ntu.edu.tw

Citation:
Hwang, Y.-T., S.-P. Xie, C. Deser, andS. M. Kang (2017), Connecting tropicalclimate change with Southern Ocean
heat uptake, Geophys. Res. Lett., 44,doi:10.1002/2017GL074972.
Received 19 JUL 2017
Accepted 11 AUG 2017
Accepted article online 16 AUG 2017

Connecting tropical climate change with Southern
Ocean heat uptake
Yen-Ting Hwang1, Shang-Ping Xie2

, Clara Deser3, and Sarah M. Kang4

1

Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan, 2Scripps Institution of Oceanography,
University of California San Diego, La Jolla, California, USA, 3Climate and Global Dynamics, National Center for AtmosphericResearch, Boulder, Colorado, USA, 4School of Urban and Environmental Engineering, Ulsan National Institute of Science andTechnology, Ulsan, South Korea

Abstract Under increasing greenhouse gas forcing, climate models project tropical warming that isgreater in the Northern than the Southern Hemisphere, accompanied by a reduction in the northeast
trade winds and a strengthening of the southeast trades. While the ocean-atmosphere coupling indicates a
positive feedback, what triggers the coupled asymmetry and favors greater warming in the northern
tropics remains unclear. Far away from the tropics, the Southern Ocean (SO) has been identiﬁed as the
major region of ocean heat uptake. Beyond its local effect on the magnitude of sea surface warming, weshow by idealized modeling experiments in a coupled slab ocean conﬁguration that enhanced SO heatuptake has a profound global impact. This SO-to-tropics connection is consistent with southward
atmospheric energy transport across the equator. Enhanced SO heat uptake results in a zonally asymmetric
La-Nina-like pattern of sea surface temperature change that not only affects tropical precipitation but also
has inﬂuences on the Asian and North American monsoons.

1. Introduction
Robust patterns of sea surface temperature (SST) and rainfall response to global warming have been identiﬁed in recent studies. For example, almost all models participating in the Coupled Model Intercomparison
Project Phase 3 and Phase 5 (CMIP3 and CMIP5) show a larger increase in SST and rainfall over the northern
than southern tropics by the end of the 21st century [Xie et al., 2010; Friedman et al., 2013; Liu et al., 2005].
Further, the delayed warming and drying trend in the southern tropics exhibit zonal asymmetries, being mostpronounced over the southeastern portion of the ocean basins. These subtropical drying regions also extend
to the western coasts of continents at the edge of the Hadley Cell, over South America, southern Africa, andAustralia (see Figure SPM.7 in Intergovernmental Panel on Climate Change [2014]).
The hemispheric asymmetry of tropical SST and rainfall changes is associated with an intensiﬁcation of thesoutheast trades and a slight weakening of the northeast trades. The strengthened southeasterly trade winds
enhance sea surface evaporation, and further cool the SST. This wind-evaporation-SST (WES) feedback indicates the importance of ocean-atmosphere interaction but does not explain why the Northern Hemisphere(NH) warms faster than the Southern Hemisphere (SH) [Xie et al., 2010].
Another well-known SST response pattern is the muted warming over the SO where upwelling and deep vertical mixing occur [Manabe et al., 1991; Marshall et al., 2014]. In CMIP5 models, more than half of the excess
heat due to increased radiative forcing is stored in the SO over the past century [Frölicher et al., 2015;
Kuhlbrodt and Gregory, 2012]. Indeed recent observations suggest the SO accounts for most of the heatstorage (67 to 98%) over the past decade (2006 to 2013) [Roemmich et al., 2015]. SO heat uptake has beenrecognized as an important factor affecting many aspects of the climate response to increased greenhousegas (GHG), including westerly winds over the SO, Antarctic sea ice, and climate sensitivity [Manabe et al., 1991;
Raper et al. 2002; Armour et al., 2013]. In addition to the effects in the SH extratropics, some recent studies
have investigated its nonlocal inﬂuences on patterns of climate feedbacks [Rose et al., 2014; Rugenstein
et al., 2016; Rose and Rencurrel, 2016]. Furthermore, Cabre et al. [2017] have identiﬁed a global scale teleconnection driven by multidecadal SO convection.

©2017. American Geophysical Union.
All Rights Reserved.

HWANG ET AL.

Here we design two sets of idealized coupled model experiments in a slab ocean setting, described in
section 2, to explore the inﬂuence of Southern Ocean heat uptake on global atmospheric circulation, witha focus on its remote inﬂuence on tropical climate. In addition to the zonal-mean changes in Hadley

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Circulation that can be understood within the framework of energetic constraints [Kang et al., 2008; Hwanget al., 2013; Schneider et al., 2014] (section 3.1), we investigate the zonally asymmetric response shaped by
the climatological wind pattern and cloud properties (sections 3.2–3.3). We compare our coupled slab ocean
model results with fully coupled models that include dynamical ocean processes in section 4 and summarize
our ﬁndings in section 5.

2. Experimental Design
We perform two sets of idealized experiments with the Community Atmospheric Model version 5 (CAM5)
coupled to a motionless slab ocean. The model features active sea ice and realistic land-sea distributions
and topography. We use the default q-ﬂux that aims to reproduce the climatological SST distribution fromthe fully coupled Community Earth System Model version 1 (CESM1) of which CAM5 is the atmospheric component [see He et al., 2017]. In both experiments, we prescribe an abrupt quadrupling of CO2 and perform a
20 year simulation. In the control experiment (hereafter termed control), we use a constant mixed-layer depth(MLD) of 50 m. In the perturbation experiment (hereafter termed deepSO), we set the MLD to 500 m over theregion 40–80°S to mimic the effects of enhanced SO heat uptake; elsewhere, the MLD remains at 50 m. The
500 m MLD value over the entire SO is not meant to be realistic [see de Boyer Montégut et al., 2004] but isdesigned to mimic the key effects of the SOs climatological meridional overturning circulation (MOC) thatmake SH middle-to-high latitudes an optimal spot for heat uptake, e.g., the effects of upwelling of cold deep
water and advection by equatorward Ekman ﬂow which spreads the absorbed heat to the north of the upwelling branch of the MOC [Marshall et al., 2014; Armour et al., 2016]. The magnitude of the change in net surface
energy ﬂux over the SO in response to a quadrupling of CO2 in the deepSO experiment is comparable to thatin the CMIP5 models (e.g., models with a dynamical ocean; Figure S1). We evaluate the effects of enhanced
SO heat uptake by comparing the responses to CO2 quadrupling in the deepSO and control experiments. The
evolution of zonal-mean SST after quadrupling CO2 in deepSO and control experiments can be seen in Figure S2.
During years 10–19, the control experiment approaches quasi-steady equilibrium with ampliﬁed zonal-mean
warming at high latitudes in both hemispheres. In contrast, there is signiﬁcantly less warming in middle-tohigh latitudes in deepSO by this time.

3. Results
3.1. Zonal Mean Responses
We focus on the difference between the average of years 10–19 after quadrupling CO2 and the 20 year average climatology under preindustrial CO2 in the two experiments. The zonal mean responses of SST, meridional mass stream function, and precipitation in the two experiments and their differences are plotted in
Figure 1. The polar ampliﬁed warming in the control run is slightly larger in the NH than the SH due presumably to the different land-sea distributions; there is also greater warming in the northern tropics compared tothe southern tropics (Figure 1a). The meridional asymmetry in the control experiment is weak compared tothat in the deepSO experiment (compare Figure 1a and Figure 1b). As expected, a deep MLD in the SO mutes
the surface warming response at high southern latitudes [Manabe et al., 1991; Marshall et al., 2014; Armour
et al., 2016]. Remarkably, however, the muted surface warming extends all the way into the tropics, creatinga strong asymmetry across the equator. The enhanced heat uptake over the SO reduces the overall warming:
the tropical-mean (20°N–20°S) SST increase is reduced from 5.4 K in the control to 3 K in deepSO. The
enhanced heat uptake also increases the interhemispheric tropical SST asymmetry (obtained by subtracting
the average over 0–20°S from the average over 0–20°N) from 0.54 K in control to 1.36 K in deepSO.
In addition to interhemispheric tropical SST asymmetry, the responses of the atmospheric overturning circulation also exhibit distinct characteristics in the two experiments (Figures 1d and 1e). Gross moist stability, theratio between energy and mass ﬂuxes, changes by about 30% at the equator in our experiments. The change
is small enough to allow us to interpret the relationships between energy, mass, and moisture transport
within the energetic framework proposed in previous studies [Chiang and Bitz, 2005; Kang et al., 2008;
Hwang et al. 2013; Schneider et al., 2014; Chiang and Friedman, 2012]. In response to SO heat uptake, an anomalous thermally direct meridional atmospheric overturning circulation develops to adjust to the hemisphericasymmetry: Anomalous northerly ﬂow aloft transports air with high gravitational potential energy to thesouthern tropics (red arrow in Figure 1f), and the southerly return ﬂow at low levels transports moisture to
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Geophysical Research Letters10.1002/2017GL074972

Figure 1. Zonal mean responses to an abrupt quadrupling of CO2 in coupled slab-ocean experiments. (a–c) Responses ofzonal mean SST (black solid line) and tropical mean (20°S–20°N) SST (dashed line) in control, deepSO, and deepSO
minus control. (d–f) Responses of meridional mass stream function (shading) in control, deepSO, and deepSO minus
10
control. Black contours show the climatological mass stream function (contour interval of 2.5 × 10 kg/s). In Figure 1f, thered arrow refers to anomalous southward energy transport, the blue arrow refers to anomalous northward moisturetransport, and the grey arrows indicate anomalous upward and downward motion. (g–i) Responses of zonal mean
precipitation (mm/yr × 100; black line) in control, deepSO and deepSO minus control. The thin grey lines show theclimatological zonal mean precipitation. The responses from control are added in Figures 1b and 1h in light red line for easycomparison with deepSO.

the NH (blue arrow in Figure 1f). The anomalous meridional circulation is accompanied by changes in verticalmotion (grey arrows in Figure 1f, upward north and downward south of the equator) that correspond toincreased precipitation and drying, respectively (Figure 1i). The precipitation center, deﬁned as theprecipitation centroid between 20°S and 20°N, shifts northward by 2.3° of latitude, and the anomalous
cross-equatorial atmospheric energy transport changes by 0.5 PW, in deepSO (see Figure 1h for
precipitation changes; energy transport is not shown). This is in contrast to the more symmetric circulation
changes with little anomalous cross-equatorial atmospheric energy transport (Figure 1d), along with amoistening trend at the equator and a drying trend on either side (Figure 1g) in control. The difference
between deepSO and control shows a 2.1° northward shift of the precipitation centroid and a southward
cross-equatorial atmospheric energy transport of 0.65 PW (see Figure 1i for precipitation changes; energy
transport is not shown); the ratio between the precipitation shift and energy transport change is similar tothat based on interannual variability in reanalysis data (1°/0.34 PW: Donohoe et al. [2014]).
3.2. Patterns of SST and Precipitation Response
Patterns of SST, surface wind, and precipitation response are plotted in Figure 2. The effects of enhanced SO
heat uptake are comparable in magnitude to, but distinct in spatial pattern from, GHG-forced climate change
as simulated by a slab ocean model of uniform MLD (compare Figures 2a with 2c and Figures 2d with 2f).
While the MLD perturbation in deepSO is zonally uniform, the climate change response exhibits marked

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Geophysical Research Letters10.1002/2017GL074972

Figure 2. Spatial patterns of response to an abrupt quadrupling of CO2 in coupled slab ocean experiments. Responses in near surface wind (vectors, Figures 2a–2c,m/s), SST (shading, Figures 2a–2c, K), and precipitation (shading, Figures 2e–2f, mm/yr) in (a and d) control, (b and e) deepSO, and (c and f) their difference. Notethat the color bars in Figures 2a–2c are centered on the tropical mean value, with red (blue) denoting SST changes larger (smaller) than the tropical mean. An
attribution analysis for the asymmetric warming in the blue box and the red box in Figure 2c is presented in section 3.3 and Figure 3.

zonal variations at low latitudes (Figures 2b and 2e), especially in the southern subtropics. The zonal
variations are mainly due to the SO heat uptake effect, obtained by differencing the deepSO and control
experiments (Figures 2c and 2f). The SO heat uptake alters the meridional temperature gradient, leading tointensiﬁed trade winds over the southeast ocean basins, expansion and strengthening of the subtropicalhighs, and drying in South America, southwest Australia, and south of Africa (Figures 2c and 2f).
In addition to delayed warming and enhanced drying in eastern oceanic basins in southern subtropics, theanomalous meridional circulation is accompanied by anomalous easterlies, increased zonal gradient, and astrengthened Walker Circulation in the deep tropics (Figure 2c). The enhanced east-west SST gradient shifts
the convective region from the Paciﬁc warm pool to the Maritime Continent, similar to that which occurs
during La Niña events. The anomalous tropical SST pattern and associated change in tropical convection
may trigger a Rossby wave train with northward energy propagation and drive the circulation changes in
the North Paciﬁc [Ting and Sardeshmukh, 1993; Trenberth et al., 1998]. In particular, the Aleutian low pressure
system weakens, bringing warm, moist air to the western North Paciﬁc and Asia, and cold, dry air to the eastern North Paciﬁc and North America.
3.3. Attribution of the Zonally Asymmetric SST Response in the Tropical South Paciﬁc Based On
Surface Energy Budget Analysis
In section 3.2, we reported an enhanced east-west SST gradient associated with the increased interhemispheric SST gradient in control and deepSO. Figure 3a demonstrates this linear relationship between
north-south and east-west asymmetries. The linear relationship exists in all ocean basins, but it is most

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Geophysical Research Letters10.1002/2017GL074972

Figure 3. The east-west SST asymmetry. (a) Anomalous zonal mean tropical SST asymmetry (anomalous SST in EQ ~ 20°N
minus EQ ~ 20°S) versus anomalous eastwest SST asymmetry in southern tropical Paciﬁc basin (anomalous SST in the bluebox (120°W ~ 90°W, 10 ~ 20°S) minus the red box (160°E ~ 170°W, 10 ~ 20°S) in Figure 2c). Blue dots are from deepSO
experiment and red dots are from control experiment. Each dot is the annual average of 1 year; and there are 19 dots foreach experiment (year 1 ~ 19 after quadrupling CO2). (b) The attribution of the east-west SST asymmetry. See equations(S1)–(S7) for details of the calculations and the deﬁnitions of each term. From left to right, the seven groups of bars refer tothe contribution to east-west SST asymmetry by (1) changes in shortwave cloud radiative forcing (T 0 sw, cld), (2) changes
in shortwave radiation in clear sky (T 0 sw, clr), (3) changes in longwave radiation (T 0 lw), (4)–(6) changes in evaporation due tochanges in wind (T 0 lh, W), relative humidity (T 0 lh, RH), stability (T 0 lh, ΔT), and (7) changes in sensible heat ﬂux (T 0 sh),respectively. The changes are calculated as in Figures 1 and 2 (the differences between the average of years 10–19 after
quadrupling CO2 concentration and climatology). Blue bars are from deepSO, red bars are from control, and green bars are
the difference of the two experiments (deepSO-control). The term with negative green bars contribute positively to thedifferences in east-west asymmetry in the two experiments.

signiﬁcant in the southern tropical Paciﬁc. In order to understand the cause of this east-west asymmetry, we
perform an energy budget analysis of the mixed layer ocean, which can be written as
ρC p H

∂T
0
0
0
0
0
¼ Qsw þ Qlw þ Qlh þ Qsh þ O∂t(1)[Xie et al., 2010; Deser et al., 2010]. The term on the left-hand side is the storage term (or the tendency term),
which is very close to zero in the quasi-equilibrium tropics. ρ is density, Cp is the speciﬁc heat of seawater, H is∂T
mixed layer depth, and ∂t is temperature tendency. Following Xie et al. [2010], Jia and Wu [2013], and Zhang
and Li [2014], we can rewrite equation (1) as a diagnostic equation for changes in SST (equation (2) below).
Please see the supporting information for the detailed derivation. In brief, based on the linearized bulk
formula for evaporation, the latent heat ﬂux can be decomposed to changes related to (1) variations in windspeed, (2) variations in relative humidity, (3) variations in air-sea temperature difference, and (4) a term
proportional to climatological evaporation and SST anomaly (i.e., the change in evaporation in the absenceof changes in wind speed, relative humidity, and air-sea temperature difference). We can then organize
the equation, making it a diagnostic equation for SST anomaly:
T 0 ¼ T 0 sw;cld þ T 0 sw;clr þ T 0 lw þ T 0 lh;W þ T 0 lh;RH þ T 0 lh;ΔT þ T 0 sh

(2)

T 0 is changes in SST. The seven terms on the right-hand side refer to changes in SST due to different factors,
they are changes in shortwave cloud radiative forcing, changes in shortwave radiation in clear sky, changes inlongwave radiation, changes in latent heat ﬂux due to variations in wind, relative humidity, or stability, andchanges in sensible heat ﬂux, respectively.
Figure 3b examines each term in equation (2) in the southeastern Paciﬁc minus the same term in thesouthwestern Paciﬁc (the blue box minus the red box in Figure 2c). Negative values contribute to increasing
east-west SST gradient (larger warming in the west compared to the east).
The changes in latent heat ﬂux related with wind changes (T 0 lh , W term in Figure 3b) explain some of theenhanced east-west asymmetry over the southern tropical Paciﬁc in deepSO relative to control. The intensiﬁcation of the southeast trades discussed previously is most signiﬁcant in eastern oceanic basins. The prevailing southeast trade winds are favorable for this WES feedback, whereas the mean winds in the western basins

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Geophysical Research Letters10.1002/2017GL074972

Figure 4. The N-S and E-W asymmetries in CMIP5 fully coupled models. Time series of (a) anomalous tropical mean SST, (b) anomalous zonal mean tropical SST asymmetry (anomalous SST in EQ ~ 20°N minus EQ ~ 20°S), and (c) anomalous east-west SST asymmetry in southern tropical Paciﬁc basin (the blue box (120°W ~ 90°W,
10 ~ 20°S) minus the red box (160°E ~ 170°W, 10 ~ 20°S) labeled in Figure 2c) in CMIP5 fully coupled models in abrupt 4 × CO2 and 1%CO2 experiments. Black lines are
ensemble means of 1%CO2 experiment, and blue lines are ensemble mean of abrupt 4 × CO2 experiment. Shading represents one standard deviation of model
spread. A 10 year running mean is applied to the time series. (d) Anomalous zonal mean tropical SST asymmetry versus anomalous east-west SST asymmetry insouthern tropical Paciﬁc basin. Black dots are from 1%CO2 experiment and blue dots are from abrupt 4 × CO2 experiment. The dots of the ﬁrst 19 years of abrupt
4 × CO2 experiment are labeled with red open circles. Each dot is annual average of 1 year of ensemble model mean.

are less organized, with a small northerly component due to the subtropical high. Thus, the climatologicalcirculation links the meridional asymmetry with the zonal asymmetry. Note that the changes in windspeed are more signiﬁcant in the monthly mean wind ﬁeld compared with the annual mean wind ﬁeld
plotted in Figure 2c. The WES feedback has been recognized as a key factor leading to the delayed
warming over southeast ocean basins [Xie et al., 2010]. There are northwest-southeast oriented cooling
patches in the southern tropics in all three ocean basins.
In addition to WES feedback, shortwave cloud radiative forcing (T 0 sw , cld term in Figure 3b) also contributes
signiﬁcantly to the enhanced east-west asymmetry over the southern tropical Paciﬁc in deepSO relative tocontrol. Low stratus clouds over the cold eastern basins amplify SST perturbations, while high convective
clouds over warm western basins damp them. With reduced warming, stratus clouds increase in deepSO(relative to control), leading to decreasing shortwave cloud radiative forcing in eastern basins. There is alsoless warming in the western ocean basins in deepSO (relative to control) where the cloud radiative effectis opposite: convective clouds decrease, leading to increased shortwave radiation at the surface.
The link between meridional and zonal SST gradients was previously recognized by Chiang et al. [2008] andFedorov et al. [2015]. While these studies concern equatorial upwelling and the equatorial zonal SST gradient,the mechanisms of the east-west asymmetries we report here are related to atmospheric processes andthermodynamic ocean-atmosphere interactions in the subtropics. Controlled by climatological wind ﬁeld
and cloud properties, the zonal gradient associated with the hemispheric asymmetry is most signiﬁcant insouthern subtropical ocean basins in our experiments.

4. Comparison With CMIP5 Fully Coupled Models
The simplicity of the experimental design allows us to demonstrate causal relationships and highlight the
global response to enhanced ocean heat uptake through atmospheric teleconnections and thermodynamic
air-sea coupling. Since the only difference between deepSO and control is the MLD over 40°S–80°S, all of the
response patterns in Figures 2c and 2f can be attributed to enhanced SO heat uptake. However, caution
should be taken when quantifying the uncertainties of these responses.
To evaluate the role of dynamical ocean processes, Figure 4 shows the time evolution of relevant quantities in
the CMIP5 models. In abrupt 4xCO2 experiments, the tropical mean temperature increases rapidly during theﬁrst 20 years (Figure 4a). This rapid warming is not spatially uniform, with northern tropics warming fasterthan the southern tropics (Figure 4b) and southeastern Paciﬁc warming less than southwestern Paciﬁc(Figure 4c). The linear relationship between hemispheric asymmetry and zonal SST gradient in southern subtropical Paciﬁc also holds for CMIP5 models (Figure 4d). The ratio between the meridional and zonal

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asymmetry are remarkably similar in the idealized slab ocean and the CMIP5 dynamical ocean models, withthe linear regression slopes being 1 K tropical hemispheric asymmetry corresponding to 2.6 K zonal SST
gradient in southern subtropical Paciﬁc in both Figures 3b and 4d.
Although the mean tropical SST increase is similar to our idealized deepSO experiment (3 K), the hemisphericasymmetry in warming is signiﬁcantly reduced in the fully coupled models, which is likely due to the compensation from the shallow oceanic overturning cells connecting the subtropics and tropics [Held, 2001;
Schneider et al., 2014]. As the easterly trades strengthen in the southern tropics and weaken in the northern
tropics, the anomalous winds induce an anomalously southward near-surface Ekman ﬂow. The anomalous
southward Ekman ﬂow in both hemispheres transports warm water across the equator, decreases the hemispheric asymmetry in warming, and reduces the cross-equatorial atmospheric energy transport [Green andMarshall, 2017].
Another complication arises due to the slowdown of the Atlantic Meridional Overturning Circulation (AMOC)
in response to CO2 increase [Held et al. 2010; Cheng et al., 2013]. As the northward transport of AMOC
decrease, warming in the NH and N-S asymmetry also reduces. About 15 years after CO2 quadrupling, somemodels show a gradual reduction in the N-S and E-W asymmetries (blue lines in Figures 4b and 4c) while thetropical mean SST continues to increase (blue line in Figure 4a). The reversal of hemispheric asymmetry is notapparent in the more realistic 1%CO2 experiments; the N-S and E-W asymmetries both increase with time,with N-S asymmetry becoming more positive and E-W asymmetry becoming more negative throughout
throughout the 140 years of the experiments (black lines in Figures 4b and 4c).

5. Summary

Acknowledgments
We thank Brian Rose and an anonymous
reviewer for their constructive comments and Yu-Chia Peng and Jia-Ying
Tsai for their assistance with ﬁgures.
Y.-T. Hwang was supported by Ministryof Science and Technology of Taiwan(103-2111-M-002-013, 104-2111-M-002005, and 105-2628-M-002 -009 -MY4);
S.-P. Xie by the U.S. National Science
Foundation (637450); and S. M. Kang by
National Reseach Foundation of Korea(2016R1A1A3A04005520). The NSFsponsored Climate Simulation
Laboratory at NCAR’s Computationaland Information Systems Laboratory(CISL) provided computing resources on
Yellowstone. Request for the idealized
experiment outputs can be made to
Y.-T. Hwang. The CMIP outputs areavailable via http://cmip-pcmdi.llnl.gov/
cmip5/data_portal.html. We acknowledge the climate modeling groups for
producing and making available theirmodel output. The National Center for
Atmospheric Research is sponsored by
the National Science Foundation.

HWANG ET AL.

Our coupled slab ocean model experiments reveal that enhanced SO heat uptake has global impacts, especially with regard to the hemispheric asymmetry in GHG-induced climate response. Speciﬁcally, enhanced
heat uptake by the SO reduces the surface ocean warming over the entire SH and drives a cross-equatorial
Hadley circulation that displaces the tropical rain belt north of the equator. In addition to the cross-equatorial
asymmetry that is consistent with the zonal mean energetic theories [Kang et al., 2008; Frierson and Hwang
2012; Schneider et al., 2014], marked zonal variations develop in response to the MLD deepening in the SO.
In southern subtropical oceans, we report a linear relationship between hemispheric asymmetry and zonal
SST gradient, resulting from zonal variations in climatological wind and clouds. In the equatorial Paciﬁc, a
La Niña-like pattern of sea surface temperature change develops and causes a westward shift of deep convection, a drying trend in the southeast subtropical ocean, and a weakened Aleutian Low. These zonal asymmetries in the southern subtropics and North Paciﬁc are similar to those resulting from aerosol forcing orNorth Atlantic freshening, albeit with opposite sign and less pronounced thermal changes in the NH extratropics [Hill et al., 2015; Xie et al., 2013; Zhang and Delworth, 2005]. Similar SST or precipitation patterns emerge
associated with the CMIP5 intermodel spread in the strength of Atlantic Meridional Overturning circulation or
shortwave cloud forcing over SO [Wang et al., 2014, Figure 1; Hwang and Frierson, 2013, Figure S1]. Identifying
SO heat uptake as an important cause for hemispheric asymmetry and zonally varying SST changes is a step
toward understanding and predicting patterns of precipitation and atmospheric circulation changes underglobal warming.
The characteristics associated with enhanced SO heat uptake are signiﬁcantly weaker in the CMIP5 models
than in a slab mixed layer ocean. The wind-driven subtropical cells in the ocean act as negative feedbacks
to the hemispheric asymmetry. The degree of compensation from ocean dynamics appears to vary signiﬁcantly across models, however. Understanding how the dynamical ocean modulates the climate response
to different forcings is currently an active research area [Deser et al., 2015, Tomas et al. 2016, Kay et al.,2016, Hawcroft et al., 2016, Green and Marshall, 2017. The idealized slab ocean experiments we present here
can serve as a reference for future evaluation of ocean dynamical effects compensating the hemisphericasymmetries due to SO heat uptake.

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