Geophysical Research Letters
RESEARCH LETTER
10.1029/2018GL077325
Key Points:• Ocean dynamics alters the coupledclimate response to an abrupt loss ofArctic sea ice loss within 25 years• Ocean dynamics produces anequatorially symmetric response in
the tropics; thermodynamic air-sea
coupling causes an antisymmetricresponse• Vertical advection tied to warming at
depth produces the equatorial Paciﬁcsea surface temperature warming
maximum

Supporting Information:• Supporting Information S1
Correspondence to:
C. Deser,cdeser@ucar.edu

Citation:
Wang, K., Deser, C., Sun, L., & Tomas, R. A.
(2018). Fast response of the tropics to an
abrupt loss of Arctic sea ice via ocean
dynamics. Geophysical Research
Letters, 45. https://doi.org/10.1029/
2018GL077325
Received 30 JAN 2018
Accepted 11 APR 2018
Accepted article online 24 APR 2018

Fast Response of the Tropics to an Abrupt Lossof Arctic Sea Ice via Ocean Dynamics
Kun Wang1,2,3, Clara Deser1, Lantao Sun4

, and Robert A. Tomas1

1

National Center for Atmospheric Research, Boulder, CO, USA, 2Laboratory for Climate and Ocean-Atmosphere Studies,
Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China, 3Department ofAviation Meteorology, College of Air Trafﬁc Management, Civil Aviation University of China, Tianjin, China, 4Cooperative
Institute for Research in Environmental Sciences, University of Colorado Boulder and NOAA Earth System ResearchLaboratory, Boulder, CO, USA

Abstract The role of ocean dynamics in the transient adjustment of the coupled climate system to an
abrupt loss of Arctic sea ice is investigated using experiments with Community Climate System Modelversion 4 in two conﬁgurations: a thermodynamic slab mixed layer ocean and a full-depth ocean that
includes both dynamics and thermodynamics. Ocean dynamics produce a distinct sea surface temperature
warming maximum in the eastern equatorial Paciﬁc, accompanied by an equatorward intensiﬁcation of theIntertropical Convergence Zone and Hadley Circulation. These tropical responses are established within
25 years of ice loss and contrast markedly with the quasi-steady antisymmetric coupled response in theslab-ocean conﬁguration. A heat budget analysis reveals the importance of anomalous vertical advection tied
to a monotonic temperature increase below 200 m for the equatorial sea surface temperature warming
maximum in the fully coupled model. Ocean dynamics also rapidly modify the midlatitude atmospheric
response to sea ice loss.

Plain Language Summary The effect of projected Arctic sea ice loss on the global climate system isinvestigated using a state-of-the-art coupled climate model. This study shows that the tropics respond to theice loss within two to three decades via dynamical ocean processes and air-sea interaction. This tropicalresponse in turn modiﬁes the atmospheric circulation and precipitation responses over the North Paciﬁc. This
fast response indicates that ocean dynamics needs to be represented for an accurate picture of the global
impact of Arctic sea ice loss.
1. Introduction
A seasonally ice-free Arctic Ocean is expected by midcentury under the “business-as-usual” greenhouse gas
emissions scenario (IPCC, 2013), with nearly half of the September ice cover already lost (Perovich et al., 2017).
As sea ice disappears, the newly open waters of the Arctic Ocean ﬂux heat and moisture to the overlying
atmosphere, warming the lower troposphere and increasing precipitation locally (Deser et al., 2010; Screen& Simmonds, 2010). Remote impacts are also anticipated via changes in the large-scale atmospheric circulation, but this link is more difﬁcult to observe due to confounding inﬂuences from internal variability (Barnes &Screen, 2015; Blackport & Kushner, 2016; Sun et al., 2016).

©2018. The Authors.
This is an open access article under theterms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and distribution in any medium, provided theoriginal work is properly cited, the use isnon-commercial and no modiﬁcations
or adaptations are made.

WANG ET AL.

The remote response to ice loss can be isolated in modeling studies provided the ensemble size is large
enough to reduce the internal variability. In atmospheric models, this response is largely conﬁned to northof 30°N, while in fully coupled ocean-atmosphere models it spans the full globe (Deser et al., 2015 [D15]).
Ocean dynamics play a crucial role in the coupled response, altering the spatial structure and amplitude especially at low latitudes (Tomas et al., 2016 [T16]). An open question is how long it takes for ocean dynamics tomodify the coupled response to Arctic sea ice loss, since previous studies focused only on the equilibrium
response. (Blackport & Kushner, 2016, compared the high-latitude response at equilibrium with that averaged over the ﬁrst 50 years but did not investigate the transient evolution in detail.)
The purpose of the present study is to investigate the transient adjustment of the global coupled climate system to a sudden loss of Arctic sea ice loss, with a particular focus on the role of ocean dynamics. To isolate therole of ocean dynamics, we perform two 100-year ensembles of coupled model experiments, each subject tothe same Arctic sea ice loss but with different ocean conﬁgurations: a thermodynamic slab mixed layer and afull-depth ocean that includes both dynamics and thermodynamics. Here we focus on the evolution of the1

Geophysical Research Letters10.1029/2018GL077325

Figure 1. Sea surface temperature (SST; °C) responses to an abrupt loss of Arctic sea ice in full ocean model (FOM) and slabocean model (SOM) averaged over years (a, b) 6–25, (c, d) 41–60, (e, f) 81–100, and (g, h) 100–360 (equilibrium). Values
not signiﬁcant at the 90% conﬁdence level are hatched.

tropical Paciﬁc coupled response and associated changes over the North Paciﬁc. Our results show that ocean
dynamics become important within two decades of a sudden loss of Arctic sea ice, considerably faster thanoften assumed (Cvijanovic et al., 2017).

2. Model and Experiments
The model and experimental design are identical to those in T16 and brieﬂy summarized here. We use theCommunity Climate System Model version 4 (CCSM4) at 1° horizontal resolution in two ocean conﬁgurations:full ocean model (FOM) and slab-ocean model (SOM). SOM has FOM’s spatially varying mixed layer depth
climatology and a spatially varying “q-ﬂux” so that its sea surface temperature (SST) mean seasonal cycle(1980–1999) matches FOM’s. Twenty (10) pairs of 100-year long simulations are conducted with FOM(SOM) under year 2000 radiative forcing; ensemble spread is generated by perturbing the initial atmospherictemperatures by order 10 14 K. Each pair consists of two different Arctic sea ice states, which approximate
those simulated by CCSM4 during 1980–1999 and 2080–2099 under historical and RCP8.5 radiative forcing,respectively. These ice conditions are achieved by adding a seasonally varying longwave radiative ﬂux to thesea ice model at each grid box and time step in the Arctic only; this ﬂux is invisible to the atmosphere and
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Figure 2. As in Figure 1 but for precipitation (mm/day).

ocean. Differencing the paired simulations isolates the coupled response to late 21st-century Arctic sea ice
loss relative to present day. Statistical signiﬁcance is assessed using a two-sided Student’s t test at the 10%
conﬁdence level.
Unlike our ensemble, T16 conducted only a single pair of simulations with FOM and SOM, but theirs were
360 years long, allowing them to analyze the equilibrium portion of the response based on years 100–360.
They also demonstrated that the SOM reproduces the FOM’s equilibrium response when both the sea ice loss
and the heat transport response to that ice loss deduced from the FOM are imposed (the latter by means of a
q-ﬂux). Thus, the role of ocean heat transport in the coupled response to sea ice loss may be inferred by comparing the SOM and FOM responses.

3. Results
The SST responses in FOM and SOM (Figure 1) show very different evolutions and geographical patterns,
depicted by averages over three 20-year periods (years 6–25, 41–60, and 81–100; the ﬁrst 5 years are omitted
while the sea ice is transitioning). The SOM response is dominated by an interhemispheric contrast in all three
periods, with signiﬁcant poleward ampliﬁed warming in the Northern Hemisphere (NH) and generally

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insigniﬁcant changes in the Southern Hemisphere (SH), the high-latitude SH warming in the middle period
notwithstanding. The FOM response is more complex, with distinctive patterns that evolve over time. In
the North Atlantic, a region of cooling occurs along the path of the Gulf Stream Extension, sandwiched
between strong warming in the subpolar gyre and a band of weaker warming in the western subtropical gyrethat strengthens over time, as well as a warming center emanating from West Africa. In the North Paciﬁc,large SST increases occur in the subpolar gyre, ﬂanked to the south by a narrow center of cooling along
the Kuroshio-Oyashio Extension, as well as a tongue of weaker warming extending from the United States
into the western subtropics that strengthens over time. In the tropical Paciﬁc, a well-deﬁned center of warming occurs to the west of Central America in the ﬁrst time period, but by the middle and later periods, thedominant feature is the eastern equatorial warming maximum that strengthens with time. In the SH, positive
SST anomalies develop in the extratropics by the middle period and amplify over time. Further ampliﬁcationof the SST responses occurs beyond year 100 in both FOM and SOM, as seen by comparing years 81–100 withyears 100–360 obtained from the longer (single) pair of simulations conducted by T16, but the patterns
remain similar (Figure 1).
Like SST, the precipitation responses in FOM and SOM show very different spatial patterns and temporal evolutions, particularly in the tropics (Figure 2). The tropical rainfall response in SOM is dominated by negativevalues south of the equator and positive values north of the equator, indicative of a northward displacementof the Intertropical Convergence Zone (ITCZ; Figure 2) and accompanying Hadley Circulation (supporting
information Figure S1), with little change in strength over time (although it strengthens beyond 100 years
in conjunction with the southward ampliﬁcation of the SST anomalies to the north of the equator).
Embedded within this broad-scale pattern are regional features, including a narrow band of negative values
centered a few degrees north of the equator in the Paciﬁc. The pattern of tropical rainfall response in FOM
bears little resemblance to that in SOM, especially after year 25. In particular, FOM generally lacks negative
precipitation anomalies in the southern tropics, and the near-equatorial Paciﬁc response consists of two
narrow bands of positive anomalies centered a few degrees off the equator in the middle and late periods
as well as at equilibrium. In addition, negative anomalies are found over the subtropical North Paciﬁc, a feature largely absent in SOM. The amplitude of the tropical precipitation response is generally weaker in FOM
than SOM throughout the simulations. Consistent with the rainfall response, FOM shows an equatorward
intensiﬁcation of the Hadley Circulation, especially in the middle and late periods, a pattern that is nearly
orthogonal to the northward shift evident in SOM (Figure S1). FOM and SOM also show opposite Walker
Circulation responses in the middle and late periods (Figure S2).
The time evolution of zonal mean SST, surface wind, and precipitation responses in FOM and SOM within thetropics is depicted in Figure 3. A 10-year running mean has been applied to reduce the noise from residualinterannual El Niño–Southern Oscillation variability. The SST response is shown as the departure from the tropical (30°N to 30°S) mean to highlight the relationship between SST gradients and surface winds. The SOM
response shows a simple quasi-stationary pattern of southeasterly wind anomalies south of the equator,
southerly anomalies at the equator, and southwesterly anomalies north of the equator, consistent with thepressure-gradient force associated with the anomalous northward SST gradient and the Coriolis force. This
leads to anomalous surface wind divergence south of approximately 5°S and convergence to the north(not shown), consistent with reduced precipitation in the latitude band 5°S–30°S and enhanced precipitationin the band 5°S to 30°N. This dipole pattern represents a broad northward shift of the climatological double
ITCZ (Figure 3) and Hadley Circulation (Figure S1). The FOM response shows an evolving structure. In the ﬁrst
20 years, the SST and wind anomalies are not unlike those in SOM, although the northward directed SST gradient and associated surface wind anomalies are weaker (note the different wind vector scales). Around year
20, however, the FOM response departs markedly from the SOM, with the appearance of a narrow band of
warming along the equator, which causes northerly wind anomalies to the north (as far as 10°N), resulting
in surface wind convergence over the warm equatorial SST maximum and divergence between approximately 10°N–25°N. This pattern persists through the simulation, with a slight broadening of the southernedge of the equatorial SST warming to approximately 10°S after year 75. The FOM precipitation response follows that of the implied surface wind divergence, with increased rainfall in the band 8°S to 8°N and decreased
rainfall to the north and south extending to approximately 25°. Thus, the FOM precipitation response differs
strikingly from the SOM, even as early as the second decade after an abrupt loss of Arctic sea ice. Analogous
results are found for zonal averages restricted to the Paciﬁc sector (Figure S3).

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Figure 3. Evolution of zonally averaged (a, b) sea surface temperature (SST; °C) and surface wind (m/s) and (c, d) precipitation (mm/day) responses to an abrupt loss of Arctic sea ice in full ocean model (FOM) and slab-ocean model (SOM).
(e) Climatological zonal-mean precipitation (mm/day).

A key difference between the two coupled model conﬁgurations is the rapid development of a distinct
SST warming maximum in the eastern equatorial Paciﬁc approximately two decades after ice loss in
FOM, a feature that is absent in SOM. To understand the processes responsible for this equatorial warming
maximum, we perform a heat budget analysis following Gaspar et al. (1990) and Redi (1982) for the region
10°S to 10°N, 90°W to 150°W, focusing on the upper 100 m to avoid strong contributions from vertical diffusion and entrainment (Buckley et al., 2014; DiNezio & Deser, 2014). Text S1 provides details of the heatbudget calculation. As shown in Figure 4a, the steady increase in temperature averaged over the upper
100 m (T) is due primarily to advection, with a minor contribution from diffusion in the last 20 years, while
the net surface heat ﬂux acts as a cooling term. Horizontal advection is the primary warming term in the in
the ﬁrst 25 years (and brieﬂy around year 80), while vertical advection dominates during the remainderof the simulation (Figure 4b).
The vertical advection term can be subdivided into a term related to the anomalous vertical velocity acting
on the mean stratiﬁcation and a term related to the mean vertical velocity acting on the anomalous vertical temperature gradient (the term related to the anomalous vertical velocity acting on the anomalous
vertical temperature gradient is negligible; Figure 4c). The ﬁrst term contributes to warming during years
25–70 and cooling thereafter (Figure 4c), the latter likely a result of anomalous wind convergence anddownwelling (Figure S4c). The warming effect of the second term steadily increases in importance after
year 25 in association with the pronounced warming at depth, with maximum values between 200 and
400 m (Figure 4c); this subsurface warming is more than twice as large as that in the upper 100 m by

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Figure 4. Evolution of the eastern equatorial Paciﬁc response to an abrupt loss of Arctic sea ice in full ocean model (FOM).
(a–c) Upper 100 m T (°C) and heat budget terms (°C/s; deﬁned in Text S1); (d) T(z). Region outlined in Figure 1e.

year 100 (Figure 4d). Mean upwelling brings this subsurface thermal anomaly into the mixed layer starting
around year 30, as evidenced by the vertical connection of the anomalies. Before this time, the warming in
the upper 50–70 m is largely disconnected from the subsurface, consistent with the dominance ofhorizontal advection in the heat budget (Figure 4b).
A heat budget analysis for the 200- to 400-m layer reveals that the subsurface warming is mainly caused byanomalous downwelling and northward ﬂow (Figure S5). The equatorial subsurface warming is part of alarger-scale warming that maximizes between 30°S and 10°S (Figure S6) and which results from anomalous
downwelling linked to anomalous Ekman pumping (Figure S7). We note that the associated depression of
the 26.5-kg/m3 isopycnal surface (Figure S6) is consistent with the response to a weakened Atlantic
Meridional Overturning Circulation (AMOC; Timmermann et al., 2005). In our experiments, AMOC decreasesby 6 Sv within the ﬁrst 20 years of ice loss, followed by a partial recovery to 4.5 Sv by year 100 (Figure S8),
representing approximately one-third of the total AMOC decline under RCP8.5 in CCSM4.
During the ﬁrst 30 years, the increase in the regional temperature index reﬂects warming to north of theequator rather than along the equator per se (recall Figure 1). This initial warming west of Central America
is likely the result of a weakening of the local trade winds (i.e., southwesterly wind anomalies) in responseto ampliﬁed SST warming in the Caribbean and Gulf of Mexico (Figure S4a). A similar mechanism has been
invoked to explain observed tropical SST changes over the past three decades (Li et al., 2016). However, inthe case of observations, tropical Atlantic warming also induces easterly wind anomalies over the Indian
Ocean and western Paciﬁc, which in turn cause SSTs in the eastern tropical Paciﬁc to cool as a result of the
Bjerknes ocean dynamic feedback mechanism. Thus, the warming at 200- to 400-m depth in response to
Arctic sea ice loss plays a crucial role in determining the sign of the SST response in the eastern tropicalPaciﬁc. In this regard, our results are more in line with those of Zhang and Delworth (2005) who showed thata weakening of AMOC induced by an imposed freshwater forcing causes a warming of the eastern tropicalPaciﬁc through air-sea interactions. However, in the case of a weakened AMOC, both the North Atlantic
and North Paciﬁc cool, opposite to the warming seen in our sea ice loss experiments which contain boththe inﬂuence of a reduced AMOC and the larger inﬂuence of heat released to the atmosphere from the newly
open waters of the Arctic Ocean that is transported southward to warm the oceans (see D15 and T16 forfurther discussion). Thus, neither recent observed tropical trends nor AMOC changes in isolation provide agood analog for the coupled response to Arctic sea ice loss.

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Figure 5. As in Figure 1 but for December–February (a, b) U700 (m/s) and (c, d) precipitation (mm/day).

We conclude our analysis of the transient response of the coupled climate system to an abrupt loss of Arctic
sea ice by showing the evolution of 700-hPa zonal winds (U700) and precipitation over the extra-tropical NH
in December–February (Figure 5; annual means are similar but with reduced amplitude). Both FOM and SOM
show a quasi-steady ring of negative U700 at high latitudes (50°N–70°N) on the poleward ﬂank of theeddy-driven jet, which is directly associated with the reduced meridional temperature gradient resulting
from warming of the Arctic lower troposphere. However, the midlatitude U700 responses differ, especiallyin the Paciﬁc where FOM shows large positive values on the equatorward side of the jet (25°N–45°N) thatdevelop in concert with the tropical Paciﬁc SST/rainfall anomalies (recall Figures 1 and 2). In SOM, this feature
is considerably weaker, generally insigniﬁcant and shifted northward (indeed, SOM shows negative U700 in
the latitude band 20°N–35°N). These differences between FOM and SOM appear within the ﬁrst 25 years andcan be traced to their distinctive tropical SST/rainfall responses via Rossby wave dynamics (see T16 forexperimental evidence) and to their opposing subtropical meridional SST anomaly gradients via thermal
wind balance (Deser et al., 2016).
The extratropical precipitation responses evolve in concert with the U700 responses (Figure 5). In particular,
the stronger and southward extended band of westerly wind anomalies over the Paciﬁc in FOM compared to

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SOM is generally associated with a larger and broader swath of increased precipitation that reaches into thewest coast of North America, with weaker precipitation deﬁcits farther south. Like U700, these differences
between FOM and SOM are already evident in the ﬁrst 25 years, although there is more noise in the evolutionof the precipitation response than in the U700 response. The contrasting amplitudes and latitudinal structures of the FOM and SOM precipitation responses are especially clear at equilibrium, not only in the
Paciﬁc sector but also over the Atlantic where a local signature of the different SST responses in the vicinityof the Gulf Stream is evident.

4. Summary and Discussion
Whereas previous modeling studies have examined the equilibrium coupled climate response to projected
Arctic sea ice loss, we have investigated aspects of the transient adjustment to an abrupt loss of Arctic sea
ice, with a particular focus on the tropics and the role of ocean dynamics. To study the relative roles of dynamical versus thermodynamic air-sea interaction, we conducted identical sets of experiments with CCSM4 in
the full-depth ocean model (FOM) and slab ocean model (SOM) conﬁgurations. The SOM response is dominated by a quasi-steady interhemispheric SST contrast (warming in the NH and little change in the SH),
accompanied by a northward shift of the ITCZ and Hadley Circulation. The FOM response is more complex,
with distinctive patterns that evolve over time. The tropical SST response is characterized by a distinct equatorial Paciﬁc maximum, which develops within approximately 20 years, accompanied by an equatorward
intensiﬁcation of the ITCZ and Hadley Circulation. These structures amplify with time and are in marked contrast to the SOM response. A heat budget analysis for the upper 100 m of the eastern equatorial Paciﬁc indicates the importance of anomalous vertical advection, which is tied to a monotonic warming at depth (below
200 m). Although further diagnostics and experiments are needed to understand the origins of this subsurface warming, it appears to be qualitatively consistent with the adjustment of the global thermohaline circulation to a density perturbation in the North Atlantic, in this case induced by a freshening and warming of the
subpolar gyre due to sea ice melt.
In addition to distinctive tropical responses, FOM and SOM also exhibit some differences in their NH midlatitude atmospheric circulation responses. In particular, FOM shows an increase in lower tropospheric westerlies over the North Paciﬁc, a response that strengthens over time. This aspect is very weak and shifted
poleward in SOM compared to FOM. In addition, SOM shows reduced westerlies farther south, a feature thatis lacking in FOM. These distinctions in the midlatitude circulation responses between FOM and SOM, apparent even within the ﬁrst 25 years, can be traced to differences in their tropical Paciﬁc SST responses and affectthe precipitation response along the west coast of North America among other regions.
In summary, the coupled ocean-atmosphere response to an abrupt loss of Arctic sea ice is rapidly (within
20–30 years) and markedly modiﬁed by dynamical ocean processes. To what extent our results depend on
the particular model used and the experimental design remains to be ascertained.

Acknowledgments
The ﬁrst author was supported by the
Chinese National Science Foundation(NSF) (grants 41376007 and 91337106).
NCAR is sponsored by the U.S. NSF.
Model data are available on the Earth
System Grid (www.earthsystemgrid.
org). We thank the two anonymous
reviewers for constructive comments.

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