PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2016GL067792
Key Points:• Air-sea coupling ampliﬁes the extratropical response to Arctic sea ice loss
• A slab ocean model is inadequate forinferring the effects of air-sea coupling• The atmospheric response is largest in
winter despite that the sea ice loss isgreatest in autumn

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

Citation:
Deser, C., L. Sun, R. A. Tomas, and
J. Screen (2016), Does ocean coupling
matter for the northern extratropicalresponse to projected Arctic sea ice
loss?, Geophys. Res. Lett., 43, doi:10.1002/
2016GL067792.
Received 14 JAN 2016
Accepted 9 FEB 2016
Accepted article online 12 FEB 2016©2016. 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.

DESER ET AL.

Does ocean coupling matter for the northern extratropicalresponse to projected Arctic sea ice loss?
Clara Deser1, Lantao Sun2, Robert A. Tomas1, and James Screen3
1

Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado, USA, 2Cooperative
Institute for Research in Environmental Sciences, University of Colorado Boulder and NOAA Earth System ResearchLaboratory, Boulder, Colorado, USA, 3College of Engineering, Mathematics and Physical Sciences, University of Exeter,
Exeter, UK

Abstract The question of whether ocean coupling matters for the extratropical Northern Hemisphere
atmospheric response to projected late 21st century Arctic sea ice loss is addressed using a series ofexperiments with Community Climate System Model version 4 at 1° spatial resolution under different
conﬁgurations of the ocean model component: no interactive ocean, thermodynamic slab ocean, andfull-depth (dynamic plus thermodynamic) ocean. Ocean-atmosphere coupling magniﬁes the response to
Arctic sea ice loss but does not change its overall structure; however, a slab ocean is inadequate for inferring
the role of oceanic feedbacks. The westerly winds along the poleward ﬂank of the eddy-driven jet weaken in
response to Arctic sea ice loss, accompanied by a smaller-magnitude strengthening on the equatorward
side, with largest amplitudes in winter. Dynamical and thermodynamic oceanic feedbacks amplify this
response by approximately 50%. Air temperature, precipitation, and sea level pressure responses also show
sensitivity to the degree of ocean coupling.

1. Introduction
The continued loss of Arctic sea ice is one of the most anticipated consequences of global warming, withprojections for a seasonally ice-free Arctic Ocean by 2100 if current rates of greenhouse gas (GHG) emissions
continue unabated [Intergovernmental Panel on Climate Change, 2013]. The melting of Arctic sea ice is
expected to have important consequences for atmospheric circulation and climate at middle and high latitudes due to the altered surface energy balance of the Arctic Ocean as highly reﬂective ice cover is replacedby darker open water [e.g., Serreze and Barry, 2011]. Without the insulating layer of sea ice, newly exposed(and relatively warm) surface waters ﬂux heat and water vapor into the overlying atmosphere, warming
and moistening the lower troposphere [e.g., Screen and Simmonds, 2010]. High-frequency atmosphericmotions transport this excess heat and moisture over the adjacent northern continents, leading to increases
in air temperature and precipitation [Deser et al., 2010; Screen et al., 2015b]. In addition to local thermodynamic
effects, diminished Arctic sea ice cover weakens the tropospheric westerly winds along the poleward ﬂank of
the jet stream, especially in winter. The weakened westerlies occur in association with a reduced north-southtemperature gradient due to enhanced warming of the Arctic lower troposphere [Deser et al., 2010; Peingsand Magnusdottir, 2014; Deser et al., 2015; Harvey et al., 2013, 2015; Sun et al., 2015]. Inﬂuences of Arctic
sea ice loss on the north-south meandering of the jet stream and associated synoptic activity including blocking events are less certain [Barnes, 2013; Screen and Simmonds, 2013; Cohen et al., 2014; Barnes and Screen,
2015; Hassanzadeh and Kuang, 2015; Francis and Vavrus, 2015]. In some regions, for example, central
Eurasia, Arctic sea ice loss may paradoxically lead to surface cooling as a result of an enhanced Siberian
anticyclone and associated cold-air advection [Honda et al., 2009; Mori et al., 2014; Sun et al., 2015; Kuget al., 2015], although this dynamical cooling is not expected to outweigh warming due to other atmosphericcirculation changes in the future [Mori et al., 2014].
While most of the climate impacts from Arctic sea ice loss occur at middle and high latitudes of the Northern
Hemisphere (NH), recent work demonstrates that ocean-atmosphere coupling may extend their reach
into the tropics and Southern Hemisphere [Deser et al., 2015, hereafter D15]. In that study, dynamical
ocean processes were shown to play a central role in communicating the effects of Arctic sea ice loss tolower latitudes. In addition to expanding the geographical inﬂuence of Arctic sea ice loss, air-sea interaction
also modiﬁes the local (extratropical NH) zonal mean temperature and zonal wind responses to Arctic sea
ice loss (D15).

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The purpose of this study is to provide a more comprehensive analysis of the effects of air-sea coupling on
the extratropical NH atmospheric circulation and surface climate response to projected Arctic sea ice loss
using a hierarchy of climate model simulations in which oceanic feedbacks are either suppressed entirely,
limited to thermodynamic processes, or represented in full (e.g., including dynamical mechanisms). Unlike
D15 who focused on only two models in this hierarchy, we systematically compare all three conﬁgurations.
In addition, we analyze the spatial patterns of response in atmospheric circulation, temperature, precipitation, and sea surface temperature (SST), going beyond the zonal mean view presented in D15. Changes inocean circulation and heat transport in the fully coupled experiment are presented in D15.
The remainder of this study is organized as follows. The models and experimental design are provided in
section 2. Results are presented in section 3. Key ﬁndings are discussed and summarized in section 4.

2. Models and Experimental Design
We make use of existing simulations presented in D15, along with two new experiments, to reveal the effects of
air-sea coupling in the atmospheric response to projected late 21st century Arctic sea ice loss. All experiments
use the Community Climate System Model version 4 (CCSM4) at 1° spatial resolution, conﬁgured with different
versions of the ocean component: (1) no (interactive) ocean model (NOM: SSTs and sea ice are prescribed), (2)slab (thermodynamic mixed layer) ocean model (SOM), and (3) full-depth (dynamic plus thermodynamic) ocean
model (FOM). Details of the model conﬁgurations, experimental design, and Arctic sea ice loss may be found in
D15; only a brief synopsis is given below. CCSM4 realistically simulates the climatological distribution of Arctic
sea ice concentration and thickness, as well as its interannual variability and trends over the past few decades[Kay et al., 2011; Jahn et al., 2012; Wettstein and Deser, 2014]. Other aspects of the model’s climatology and variability are presented in the CCSM4 Special Issue Collection of the Journal of Climate (2012).
To artiﬁcially control Arctic sea ice in the SOM and FOM, D15 introduced an additional seasonally varying
longwave radiative ﬂux (LRF) to the sea ice model at each Arctic grid box to achieve a desired seasonal
cycle of sea ice concentration and thickness under present-day GHG conditions (see Appendix in D15
for details of the LRF). Two such experiments were conducted: one that targets sea ice conditions at
the end of the twentieth century (the 1980–1999 average from six CCSM4 historical simulations), andone that targets conditions at the end of the 21st century (the 2080–2099 average from six CCSM4
Representative Concentration Pathway 8.5 (RCP8.5) simulations). These simulations are termed SOM_20
and SOM_21 (FOM_20 and FOM_21) based on the SOM (FOM) model conﬁgurations with late 20th and21st centuries sea ice conditions, respectively (Table S1 in the supporting information). Differencing
the 20th and 21st century SOM (FOM) simulations isolates the impact of projected Arctic sea ice loss on
the thermodynamically coupled (fully coupled) climate system, since both are run under present-day(year 2000) GHG concentrations.
We emphasize that (1) all of the prescribed LRF goes directly into the sea ice model component (e.g., the LRF
is a “ghost ﬂux” to both the atmosphere and ocean model components); (2) there is no conduction of heat
between the sea ice and ocean model components; and (3) the amount of LRF speciﬁed to the ice model
at a particular grid box at any given time is proportional to the ice fraction in the grid box at that time.
Thus, the prescribed LRF does not directly affect the climate system: it impacts the ocean and atmosphere
only via the changes it induces in Arctic sea ice.
A climatological spatially varying monthly “Qﬂux” derived from FOM_20 is speciﬁed to both SOM_20 and
SOM_21 to represent the mean effects of present-day ocean heat transport on SST; in addition, climatologicalspatially varying annual mean mixed layer depths (MLDs) from FOM_20 are speciﬁed to both SOM_20
and SOM_21.
Two new simulations are conducted here using the NOM version of the model in which sea ice concentration and thickness as well as SSTs are speciﬁed to the atmospheric model. For the NOM_20 experiment, theglobal sea ice and SST conditions from FOM_20 are speciﬁed; for the NOM_21 experiment, the Arctic sea ice
and SST conditions from FOM_21 are speciﬁed, with sea ice and SST conditions over the rest of the globe
taken from FOM_20 (Table S1). The difference between the two NOM experiments isolates the effect of
21st century Arctic sea ice loss on the atmosphere-land system in the absence of ocean feedbacks (GHG
levels were ﬁxed at the year 2000 in both NOM simulations). (Note that the equivalent NOM experiments
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presented in D15 were based on sea ice and SST conditions from the CCSM4 historical and RCP8.5 simulations, which are slightly different from those in the FOM_20 and FOM_21 experiments used here.)
The SOM and FOM experiments were run for 300 and 360 years, respectively, and the last 260 years are used
for analysis following D15. The NOM experiments were run for 260 years, and all years are used for analysis. In
the following, we refer to the difference between the 260 year averages from NOM_21 and NOM_20 as
ΔICE_NOM, and similarly for the SOM and FOM experiments. The statistical signiﬁcance of all responses isassessed using a two-sided Student’s t test applied to the difference between the two 260 year averages
and their corresponding standard deviations.
The seasonal cycles of the change in Arctic sea ice area between the late 20th and late 21st centuries arenearly identical across the three experiments (ΔICE_NOM, ΔICE_SOM, and ΔICE_FOM), with the greatest
losses in late summer and autumn (maximum amplitude ~ 7 × 106 km2 in October) and the smallest reductions in late winter (minimum amplitude ~ 2 × 106 km2 in March; Figure S1a). This means that we can attributeany differences in their atmospheric responses mainly to the effects of ocean coupling. Spatially, sea ice loss
in autumn (September–November) is largely conﬁned to the central Arctic basin, whereas in winter, it occurs
mainly in the peripheral seas (Sea of Okhotsk, Bering and Beaufort Seas in the Paciﬁc, and Labrador,
Greenland and Barents Seas and Hudson Bay in the Atlantic: Figure S1b).
The reduction in sea ice cover results in an anomalous net upward surface energy ﬂux (Qnet) from the Arctic
Ocean to the atmosphere, with the maximum Qnet response (70 W m 2 in November–January) lagging thepeak sea ice loss by 2–3 months in all three experiments (Figure S1a). This delay is due to the effect of theseasonal cycle of the climatological air-sea temperature difference on the turbulent energy ﬂux response
as discussed in Deser et al. [2010]. There are some small (5–10 W m 2) differences in the magnitude of thewintertime Qnet response among the experiments, due to slight differences in the amount of sea ice loss
and surface temperature of the ice (not shown). However, the overall similarity of the sea ice loss and Qnet
responses in the three experiments allows us to deduce the effects of dynamical and thermodynamic oceanic
feedbacks on the atmospheric response to projected Arctic sea ice loss.

3. Results
3.1. Global Zonal Mean Response to Arctic Sea Ice Loss
We begin with a global view of the annual zonal mean atmospheric temperature and zonal wind responses to
Arctic sea ice loss in the three ocean model conﬁgurations (Figure 1). Signiﬁcant responses in ΔICE_NOM are
conﬁned to the northern extratropics, with surface-intensiﬁed warming north of 50°N (maximum values > 8°C
at 1000 hPa in the Arctic) and weakened (strengthened) westerlies on the poleward (equatorward) ﬂank of
the eddy-driven jet (maximum amplitudes approximately 0.5 m s 1 at 60°N and 0.2 m s 1 at 35°N in the upper
troposphere near 250–300 hPa). Compared to the uncoupled experiment, the coupled experiments show stronger high-latitude warming in the free troposphere, with values of approximately 0.15°C in ΔICE_NOM, 1.40°C in
ΔICE_SOM, 0.60°C in ΔICE_FOM at 500 hPa. The ampliﬁed polar warming in the free troposphere in the coupledexperiments leads to a larger-amplitude zonal wind response, with maximum values at 60°N and 300 hPa of
1.0 m s 1 in ΔICE_FOM and 1.2 m s 1 in ΔICE_SOM, compared to 0.5 m s 1 in ΔICE_NOM. A notable difference
between the westerly wind responses in ΔICE_SOM and ΔICE_FOM is their latitudinal extent: ΔICE_SOM shows abroader band (40°–70°N) of reduced westerlies compared to ΔICE_FOM (50°–70°N). In addition, ΔICE_SOM
shows no signiﬁcant zonal wind change on the equatorward side of the eddy-driven jet (although the equatorward side of the thermally driven subtropical jet weakens in the latitude band 10°–25°N), unlike ΔICE_FOM whichshows a strengthening of the westerlies in the region 30°–40°N. In summary, ΔICE_FOM shows a similar patternof response in the northern extratropics as ΔICE_NOM but with approximately twice the amplitude; and both
exhibit structural differences with the response in ΔICE_SOM.
In addition to local effects, ocean coupling extends the atmospheric response to Arctic sea ice loss into thetropics and Southern Hemisphere (SH). However, the latitudinal structures of the global-scale responses differ
markedly between ΔICE_FOM and ΔICE_SOM. In particular, ΔICE_SOM shows a strong interhemisphericgradient in tropospheric warming magnitude, whereas ΔICE_FOM shows a more equatorially symmetricstructure. These distinctive thermal patterns are associated with different zonal mean zonal wind changes.
In particular, the cross-equatorial temperature gradient in ΔICE_SOM results in opposite-signed zonal wind
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Figure 1. Annual zonal mean (a, c, e) temperature (°C) and (b, d, f) zonal wind (m s ) responses to Arctic sea ice loss in the in ΔICE_NOM (Figures 1a and 1b),
ΔICE_SOM (Figures 1c and 1d), and ΔICE_FOM (Figures 1e and 1f) model conﬁgurations (color shading: color bars at the bottom of each column; note thenonlinear color scales). Stippling indicates where the response is statistically signiﬁcant at the 95% conﬁdence level. Contours indicate the twentieth century
1climatology (contour interval of 10°C for temperature and 5 m s for zonal wind with the zero contour thickened).

anomalies on the equatorward ﬂanks of the thermally driven subtropical jet of the two hemispheres, whereas
the equatorially symmetric tropical warming in ΔICE_FOM is associated with like-signed subtropical zonal
wind changes. In addition, ΔICE_SOM shows a small but statistically signiﬁcant weakening of the westerlies
on the poleward ﬂank of the eddy-driven jet in the SH, whereas ΔICE_FOM shows a weak positive zonal windresponse near Antarctica. Both coupled experiments feature enhanced warming at upper levels in the tropics,
as well as some ampliﬁcation of the lower tropospheric warming at high southern latitudes.
The response of the NH westerlies to Arctic sea ice loss exhibits a pronounced seasonal cycle, with largest
amplitudes during boreal winter (Figure S2) when the net upward surface energy ﬂux response over theArctic Ocean is greatest (recall Figure S1a). The zonal mean temperature and zonal wind responses in thetropics and NH in December-January-February (DJF) are similar to those in the annual mean, but theirmagnitudes are approximately twice as large (Figure S3). The SH responses in DJF are generally insigniﬁcant
except in ΔICE_FOM, which shows a small but statistically signiﬁcant weakening of the westerlies on thepoleward ﬂank of the eddy-driven jet near 50°–60°S, analogous to that in the NH (Figure S3).
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Figure 2. DJF 700 hPa zonal wind (U700; m s ), 500 hPa geopotential height (Z500; m), and SLP (hPa) responses to Arctic
sea ice loss in the (a–c) ΔICE_NOM, (d–f) ΔICE_SOM, and (g–i) ΔICE_FOM model conﬁgurations. Stippling indicates where
the response is statistically signiﬁcant at the 95% conﬁdence level. Contours indicate the twentieth century climatology
1(contour interval of 5 m s for U700 with the zero contour thickened, 500 m for Z500 and 10 hPa for SLP).

3.2. Spatial Patterns of Response to Arctic Sea Ice Loss
For the remainder of this study, we focus on the wintertime (DJF) extratropical NH atmospheric response toprojected Arctic sea ice loss. Spatial patterns of the DJF 700 hPa zonal wind (U700), 500 hPa geopotential
height (Z500), and sea level presssure (SLP) responses are shown in Figure 2 (annual means are shown in
Figure S4). The weakening of the lower tropospheric zonal winds at high latitudes is apparent over all longitudes in all three model conﬁgurations; however, the midlatitude strengthening of the westerlies occurs preferentially in the Paciﬁc and east Atlantic sectors. The positive U700 anomalies in the east Atlantic/west
African sector are indicative of a strengthening of the equatorward ﬂank of the eddy-driven jet, but in the
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Paciﬁc sector they represent a downstream strengthening of the jet core. Thus, caution is needed when relatingclimatology and response based on zonal averages. Overall, the U700 response patterns are very similar
between ΔICE_NOM and ΔICE_FOM, with differences primarily in magnitude, consistent with the zonal mean
results. ΔICE_SOM, however, shows some notable differences: in particular, the high-latitude band of easterly
wind anomalies in the Atlantic extends farther south into the main core of the jet, and the midlatitude bandof westerly wind anomalies in the Paciﬁc is virtually absent.
The Z500 response in ΔICE_NOM projects onto the negative polarity of the Northern Annular Mode (NAM), withpositive values in the polar region and negative values farther south in the eastern Atlantic and Paciﬁc (Figure 2).
The elevated Z500 values over the polar cap are a direct consequence of the warming of the lower tropospherein response to Arctic sea ice loss, as found in many previous studies [e.g., Deser et al., 2010; Screen et al., 2015a].
The polar Z500 response is ampliﬁed in both coupled experiments, consistent with the greater warming of thefree troposphere. The negative NAM response pattern is still discernible in ΔICE_FOM, although it is embedded
within a positive background Z500 response. In contrast, ΔICE_SOM is dominated by a general increase in Z500.
The SLP response in each of the experiments exhibits a continental wave-one pattern, with positive values overnorthern Eurasia and negative values over Canada (Figure 2). The magnitude of the SLP response over Eurasia
is largest in ΔICE_FOM and smallest in ΔICE_NOM (maximum values ~ 4.0 hPa and 2.7 hPa, respectively),while the response over Canada is greatest in ΔICE_SOM (maximum values ~ 4 hPa compared to ~ 3 hPa in
the other experiments). In addition to this continental dipole pattern, negative SLP anomalies occur overthe eastern Atlantic and Paciﬁc, with larger magnitudes in ΔICE_FOM compared to ΔICE_NOM. ΔICE_SOM
lacks the deepened Aleutian Low and overestimates the negative SLP response over the eastern Atlanticand Mediterranean compared to ΔICE_FOM and ΔICE_NOM. These differences can be traced to differencesin their tropical precipitation responses (not shown). The dynamical mechanisms that give rise to the distinctive
continental wave-one SLP pattern remain to be elucidated. Previous atmosphere-only studies indicate that thehigh-pressure response over Eurasia is related to sea ice loss in the Barents-Kara Sea region [e.g., Mori et al.,2014; Kug et al., 2015]. Further work is needed to assess the impact of sea ice loss in other areas.
The DJF 2 m air temperature, precipitation, and SST responses to Arctic sea ice loss are shown in Figure 3(recall that ΔICE_NOM has prescribed SST anomalies in regions of sea ice loss, taken from ΔICE_FOM). All
three experiments show warming over the high-latitude continents, with maximum values exceeding 5°C near
the Arctic border. This terrestrial warming is primarily conﬁned to the highest latitudes in ΔICE_NOM, whereas it
extends over the entire North American continent in both coupled experiments, and over all of Eurasia in
ΔICE_SOM. A swath of cooling extends across Eurasia from the Ural Mountains to the Paciﬁc Ocean in
ΔICE_NOM. This decrease in air temperature is dynamically induced by the positive SLP response over
Siberia, which brings anomalously cold northeasterly ﬂow to the region. Note that the magnitude of the coolingis modest (generally < 0.5°C). There is no trace of negative temperature anomalies in ΔICE_SOM, and only avestige of cooling in ΔICE_FOM, despite that the positive SLP response over Siberia is stronger in both coupledexperiments compared to ΔICE_NOM. It is likely that the continent-wide warming in ΔICE_SOM is due tothermodynamic effects associated with increased SSTs, which outweigh the dynamically induced coolingassociated with the Siberian high-pressure response. Similarly, the weak residual cooling in ΔICE_FOM likely
reﬂects a combination of stronger dynamically induced cooling and smaller thermodynamically induced
warming due to smaller SST increases compared to ΔICE_SOM.
The patterns of precipitation response are largely similar in all experiments, with regional differences inmagnitude. Within the Arctic, the largest precipitation increases occur directly over regions of sea ice loss,
with magnitudes in excess of 0.5 mm d 1 in ΔICE_NOM and more than 0.6 mm d 1 in ΔICE_SOM and
ΔICE_FOM. Both ΔICE_NOM and ΔICE_FOM exhibit decreased precipitation over the far northern Atlantic
and Paciﬁc, likely in association with the negative net surface heat ﬂux response in these regions (not shown,
but see Deser et al. [2010]). ΔICE_FOM shows a distinctive north-south dipole pattern of precipitation
response over the North Atlantic that coincides with underlying changes in SSTs (increased precipitation
associated with warmer SSTs and vice versa). The distinctive pattern of SST response in ΔICE_FOM is associated with a southward shift of the Gulf Stream and its extension. This dynamically induced SST response
is necessarily lacking in ΔICE_SOM, which shows instead a relatively bland pattern of oceanic warming that
increases monotonically northward in both basins. A signiﬁcant increase in precipitation occurs over theeastern North Paciﬁc and western United States in ΔICE_FOM, likely driven by the deepened Aleutian Low;
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Figure 3. DJF surface air temperature (°C; land only), precipitation (mm d ), and SST (°C) responses to Arctic sea ice loss inthe (a–c) ΔICE_NOM, (d–f) ΔICE_SOM, and (g–i) ΔICE_FOM model conﬁgurations. Stippling indicates where the response isstatistically signiﬁcant at the 95% conﬁdence level. White shading in Figures 3c, 3f, and 3i denotes absolute values < 0.01°C.

this feature is also present to some extent in both ΔICE_NOM and ΔICE_SOM but with weaker amplitude andfewer areas of statistical signiﬁcance.

4. Discussion and Conclusions
We have examined the NH extratropical atmospheric response to projected Arctic sea ice loss using a hierarchyof model conﬁgurations in which oceanic feedbacks are either suppressed entirely, limited to thermodynamic
processes, or represented in full. We ﬁnd that ocean-atmosphere coupling ampliﬁes the atmospheric circulation
response to Arctic sea ice loss by approximately 50% but does not change its overall structure; however, both

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dynamical and thermodynamic processes must be included for a proper representation of oceanic feedbacks.
As in many other studies, we ﬁnd that Arctic sea ice loss weakens the tropospheric westerlies along thepoleward ﬂank (50°–70°N) of the eddy-driven jet, accompanied by a smaller-magnitude increase to the south(30°–40°N). This response is strongest in winter, despite that the sea ice loss is largest in autumn. The weakeningof the westerly winds on the poleward side of the jet occurs at all longitudes, whereas the increase to the south
occurs preferentially over the eastern ocean basins and shows a different relationship to the climatologicalwinds in the Atlantic and Paciﬁc. The reduction in high-latitude westerlies is consistent with thermal windbalance in response to warming of the Arctic lower troposphere. The SLP response shows a distinctive continental wave-one pattern, with positive anomalies over northern Eurasia and negative anomalies over northern
North America. The dynamical mechanisms that give rise to this pattern remain to be elucidated, in particular,
the contributions from sea ice loss in different regions.
Air-sea coupling also enhances the magnitude of the near-surface air temperature and precipitation responses
to projected Arctic sea ice loss, in particular, warming over the northern continents and precipitation increases
over the central Arctic and the eastern North Paciﬁc extending into western North America. These climate
impacts are driven by a combination of thermodynamic and dynamical (circulation) effects. Although the
dynamically induced cooling over central Eurasia is stronger with ocean feedbacks owing to the larger SLP
response, it is outweighed by an even greater increase in thermodynamically induced warming resulting fromelevated SSTs: the net effect of ocean coupling is thus to reduce the magnitude of near-surface cooling incentral Eurasia. The southward shift of the Gulf Stream in response to Arctic sea ice loss produces a distinctive
dipole pattern of SST anomalies in the North Atlantic that in turn drives a local precipitation dipole response:
these features are unique to the fully coupled model.
The general amplifying effect of oceanic feedbacks on the atmospheric response to Arctic sea ice loss can betraced to an overall warming of the surface global oceans, which in turn warms and moistens the global
troposphere through a combination of local and remote processes as discussed in D15. In particular, theenhanced warming of the northern extratropical free troposphere in ΔICE_FOM compared to ΔICE_NOM isdue to increased poleward energy transport associated with a warmer and moister tropical upper troposphere as shown in D15. As a result, the atmospheric circulation response in the NH extratropics intensiﬁes,
with subsequent effects on the dynamically induced portion of the near-surface air temperature and precipitation responses. The thermodynamic (radiative) contribution to these responses is also enhanced by virtueof a warmer, moister atmosphere.
One might expect that thermodynamic air-sea coupling (e.g., as represented by a “slab” ocean mixed layer)
would adequately represent the effects of ocean feedbacks on the atmospheric and surface climate response
to Arctic sea ice loss. However, our experiments indicate that the slab ocean overestimates the local thermodynamic impact of air-sea coupling and modiﬁes the spatial structure of the response compared to thefull-depth ocean. Both shortcomings can be traced to the slab ocean’s SST response, which is larger than thatin the full-depth ocean model and which has a stronger gradient between the equator and the North Pole.
The enhanced and southward expanded meridional gradient in the SST response leads to an overestimateof the magnitude and latitudinal width of the zonal wind response in middle latitudes in the slab ocean
compared to the full-ocean coupled model conﬁguration. A caveat is that the use of annual mean MLDs in
the SOM experiments (the default conﬁguration in CCSM4) may overestimate the magnitude of the SST
response; however, the large-scale meridional (equator-to-pole) structure of the SST response is likely to berobust. Additional experiments are needed to verify these speculations.
Our experiments address the coupled climate system’s response to an abrupt loss of Arctic sea ice (e.g., thesea ice in FOM_21 was forced to melt within a year, not shown). The extratropical NH atmospheric circulation
adjusts quickly (within a few years) to the sudden loss of sea ice, even in the FOM experiment (Figure S6).
Thus, the responses shown here can be viewed as being in quasi-equilibrium with the more gradual paceof sea ice loss that occurs in the RCP8.5 simulation over the course of the 21st century (not shown, but see
Stroeve et al. [2012]). A full exploration of the mechanisms, patterns, and time scales of transient adjustmentof the global coupled climate system to an abrupt loss of Arctic sea ice is beyond the scope of this study
but will be pursued in future work. Finally, it should be noted that our study examines only the sea ice loss
induced portion of climate change associated with an increase in GHG, not the full response to GHG forcing(see D15 for additional discussion).

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Acknowledgments
R. Tomas and L. Sun gratefully
acknowledge support from the Ofﬁce of
Polar Programs at the National Science
Foundation. J. Screen is supported by
the Natural Environment ResearchCouncil. NCAR is sponsored by NSF.
We appreciate the comments from thetwo anonymous reviewers.

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COUPLED RESPONSE TO ARCTIC SEA ICE LOSS

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