Article

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Contrary Responses of the Gulf Stream and the Kuroshio to
Arctic Sea Ice Loss
Kun Wang 1,*, Linyue Wu 1, Haiwen Liu 1, Bo Dan 2,*, Haijin Dai 3 and Clara Deser 4

Citation: Wang, K.; Wu, L.; Liu, H.;
Dan, B.; Dai, H.; Deser, C. Contrary
Responses of the Gulf Stream and

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Department of Aviation Meteorology, Civil Aviation University of China, Tianjin 300300, China;
170446129@cauc.edu.cn (L.W.); hwliu@cauc.edu.cn (H.L.)
2 National Marine Data and Information Service, Tianjin 300012, China
3 College of Meteorology and Oceanography, National University of Defense Technology, Changsha 410000,
Hunan, China; hj_dai@nudt.edu.cn
4 National Center for Atmospheric Research, Boulder CO 80307, USA; cdeser@ucar.edu* Correspondence: wang_k@cauc.edu.cn (K.W.); danb2012@mail.bnu.edu.cn

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Abstract: The impact on the Gulf Stream and Kuroshio from Arctic sea ice loss is investigated using
the Community Climate System Model version 4 (CCSM4) model for their important roles during
climate change. Results show that the Gulf Stream (Kuroshio) weakens (strengthens) in response to
Arctic sea ice loss via ocean (atmosphere) adjustments. More precisely, the Kuroshio acceleration ismainly due to the anomalous wind stress over the North Pacific, while the ocean gyre adjustmentsin the Atlantic are responsible for the weakened Gulf Stream. As positive buoyancy fluxes induced
by Arctic sea ice loss triggers a slowdown of the Atlantic Meridional Overturning Circulation(AMOC), the Gulf Stream decelerates evidently, which current speed decreases about 5-8 cm/s inthe upper ocean. Resulted from less advection and horizontal diffusion in the temperature budget,less poleward warm water leads to a narrow sea surface cooling sandwiched between strong warming in the subpolar and subtropical Atlantic. Furthermore, colder surface decreases the upward heat
flux (mainly latent heat flux) along the Gulf Stream Extension (GE) path, which leads to a warming
hole in the atmosphere.

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Keywords: Arctic sea ice loss; subtropical western boundary currents; AMOC; air-sea interaction;
warming hole

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the Kuroshio to Arctic Sea Ice Loss.
Atmosphere 2022, 13, x.
https://doi.org/10.3390/xxxxx
Academic Editor(s):2627Received: date
Accepted: datePublished: datePublisher’s Note: MDPI stays neutral with regard to jurisdictional
claims in published maps and institutional affiliations.

Copyright: © 2022 by the authors.
Submitted for possible open accesspublication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

1. Introduction

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The Gulf Stream in the North Atlantic and the Kuroshio in the North Pacific are important subtropical western boundary currents (WBCs) in the Northern Hemisphere (NH),influencing the weather and climate through both hydrodynamics and thermodynamics[1–3]. These two currents are characterized by fast ocean velocities, high sea surface temperature (SST), and intensive ocean heat loss [4]. The poleward ocean heat transport by
the Gulf Stream and the Kuroshio contributes to the global heat balance [5,6]. Still, thelarge amount of water mass, salinity, and nutrient exchanges can significantly impact thefisheries and environments [7]. So it is necessary to understand the evolution of the Gulf
Stream and the Kuroshio.
With global warming occurring as a result of increased greenhouse gas (GHG) concentrations in the atmosphere, there has been a growing interest in the evolution of WBCs.
Wu et al. [8] found a regionally accelerated warming (2-3 times larger than the global
surface ocean warming rate) since 1900 over the path of WBCs in all the ocean basins. The
accelerated warming is associated with a poleward shift and/or intensification of WBCs.
While Dong et al. [9] used satellite altimetry data to demonstrate that the Gulf Stream
experienced a southward shift east of 65°W, accompanied by a slowdown trend during
1993–2016. In agreement with Dong et al. [9], climate models project a weaker Gulf Stream

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Atmosphere 2022, 13, x. https://doi.org/10.3390/xxxxx

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2 of 17in the twenty-first century in response to global warming [7]. To explain the variations in
the path and strength of the Gulf Stream, many studies have considered the effect of theAtlantic meridional overturning circulation (AMOC) changes [10–12]. A slower AMOC isfound when the Gulf Stream is weaker and displaced southward [10]. On the other hand,
Zhang et al. [13] suggested that the weakening trend of the Gulf Stream from 1993 to 2016
was resulted from the decline of the North Atlantic Oscillation (NAO). With regard to the
Kuroshio, Wang et al. [14] showed a weakened Kuroshio during the period 1993-2013 despite enhanced warming along its path. The Kuroshio decelerates during the negative
phase of Pacific Decadal Oscillation (PDO) [15]. However, the projected global warming
intensifies and shifts the Kuroshio northerly in models [12,16]. Sakamoto et al. [17]thought that the acceleration of the Kuroshio was due to the changes in the large-scalewind stress over the North Pacific, but some researchers demonstrated that the sea surface
warming, not the wind changes, dominated the intensification of the upper-layer Kuroshio in a warming climate [12].
Another significant consequence of anthropogenic warming is the sea ice reduction
in Arctic [18–20]. Substantial evidence from observations and numerical simulations indicates that Arctic sea ice has declined rapidly in recent decades [21,22], with broad impacts
on surface albedo, air-sea heat/moisture fluxes as well as atmospheric and oceanic circulations [23]. For example, the AMOC is projected to slow down in response to Arctic sea
ice loss across climate models [24–26]. Previous studies also suggested that Arctic sea ice
reduction induced a negative phase of NAO during winter [27–29]. Moreover, Arctic sea
ice loss can bring about much stronger warming in Arctic compared with the global
warming, which is called Arctic amplification (AA) [30], and causes the anomalies in windfield around the world [31,32]. The AA contributes to weather and climate changes in midlatitudes, including WBCs in the Northern Hemisphere [33,34]. The Arctic Ocean may influence the North Pacific Ocean via water exchange through the Bering Strait, and it even
may affect the water properties (e.g., temperature and salinity) in the Kuroshio [35,36].
Meanwhile, the Arctic Ocean affects the North Atlantic Ocean across the Fram Strait andthe Barents Sea. The Gulf Stream’s influence would be amplified over the Barents Sea region by interacting with sea-ice anomaly, promoting cold Eurasian [37,38]. Compared to
the narrow Bering Strait, the water exchange between the Arctic Ocean and the North
Atlantic Ocean is more efficient. On the other hand, the Gulf Stream and the Kuroshio canboth be influenced indirectly by Arctic via atmosphere [5,37,39]. As a result, the responsesof the Gulf Stream and the Kuroshio to Arctic sea ice loss may be different, but the details
and physical processes of how Arctic sea ice loss affects these two important currents remain uncertain.
Although previous studies have focused on the responses of the Gulf Stream and theKuroshio to global warming [7,12,17], and the Arctic sea ice impacts on climate change[29,40], the variations in the Gulf Stream and the Kuroshio induced by Arctic sea ice loss
alone are not clear. The first question rises that whether the responses of the Gulf Stream
and the Kuroshio to Arctic sea ice loss are the same. If the answer is no, how do the Gulf
Stream and the Kuroshio in response to the Arctic sea ice loss, and what mechanisms drive
them to occur? To answer these questions, using the numerical simulations, we investigated the different responses of the Gulf Stream and the Kuroshio to Arctic sea ice loss inthis paper—that is to investigate only the sea ice loss response without CO2-induced
warming. This paper is organized as the followings: the model and design of the various
experiments are described in the section 2. The section 3 shows the different responses of
the Gulf Stream and the Kuroshio to Arctic sea ice loss, and the possible mechanisms driving the Gulf Stream and the Kuroshio anomalies, respectively. Finally, a summary andsome discussion are given in section 4.

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2. Model and Experiments

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A fully coupled global climate model, the Community Climate System Model version
4 (CCSM4) is used in this study. The atmospheric component has 26 vertical levels, with

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the finite volume nominal 0.9°×1.25° in the horizontal direction. The oceanic resolution isabout 1° in the horizontal and 60 levels in the vertical direction. The ice model has thesame horizontal grid as the ocean model.
A suite of experiments (Table 1) are designed to explore the differences in the mechanism of the Gulf Stream and the Kuroshio responses to Arctic sea ice loss. The model isconfigured in two ocean configurations: full ocean model (FOM) and slab-ocean model(SOM). SOM has FOM’s spatially varying mixed layer depth climatology. FOM and SOM
both have two kinds of simulations with different Arctic sea ice states. One ice state(ICE_20) is the average Arctic sea ice conditions representative of the late 20th-century(1980–1999) under historical radiative forcing (the black line in Figure 1), and the otherone (ICE_21) is representative of the late 21st-century (2080–2099) under representative
concentration pathway 8.5 (RCP8.5) radiative forcing (the blue line in Figure 1). As Figure
1 shown, each Arctic sea ice state has a seasonal cycle of Arctic sea ice extent (defined as
the area with at least 15% fractional ice cover). The greatest sea ice extent is in March, andthe least ice extent appears in September. A year-round reduction in Arctic sea ice extent
happens in response to RCP8.5 radiative forcing. The Arctic becomes nearly ice-free from
August to October during 2080–2099. To achieve the sea ice conditions for the late twentieth and twenty-first centuries, a seasonally varying longwave radiative flux is artificiallyadded to the sea ice model at each grid box and time step in the Arctic only. The spatial
distribution of the additional longwave radiative flux is related with the sea ice reduction(Figure 1b, S1). For example, Figure 1b shows the added longwave radiative flux to theice model in September in ICE_21 compared to ICE_20. The additional longwave radiativeflux is the largest in the central Arctic close to the Greenland where the sea ice loss is thestrongest in September. The value is southward decreasing and becomes zero where there
is no ice. It should be noted that apart from the different sea ice state, radiative forcing
conditions are kept fixed at the year 2000 in FOM and SOM to isolate the response to sea
ice loss.

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Table 1. Design of the model experiments. The results use the ensemble mean of the runs. Each run
is integrated for 100 years. ∆FOM = ICE_21_FOM - ICE_20_FOM, ∆SOM = ICE_21_SOM ICE_20_SOM.

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Name

Radiative forcing

ICE_20_FOM
ICE_21_FOM
ICE_20_SOM
ICE_21_SOM

Year 2000
Year 2000
Year 2000
Year 2000

Arctic sea ice
state
1989-1999
2080-2099
1980-1999
2080-2099

Ocean
configurations
Full ocean
Full ocean
Slab ocean
Slab ocean

Ensemble
20 runs
20 runs
10 runs
10 runs

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Figure 1. (a) Monthly Arctic sea ice extent (10 km ) in the late 20th-century (1980–1999) under historical radiative forcing (the black line) and the late 21st-century (2080–2099) under RCP8.5 radiative
forcing (the blue line). (b) The longwave radiative flux (W/m2) artificially added to the sea ice modelin September in ICE_21 compared to ICE_20. The direction of longwave radiative flux is downward.

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In order to reduce the noise from the inherent variability of the ocean currents, 20 (10)pairs of runs with FOM (SOM) are performed to ensure the robustness of results. Eachrun is integrated for 100 years. The difference between the ensemble mean of 20 (10) simulations with different Arctic sea ice states, referred to as ∆FOM (∆SOM), represents theclimate response to late 21st-century Arctic sea ice loss relative to present day. Comparing∆FOM with ∆SOM, the role of ocean adjustments is isolated in the response to sea ice loss.
In this study, we focus on the quasi-equilibrium responses over the last 20 years of eachexperiment [26]. Statistical significance of the responses is assessed at the 95% confidence
level by a two-tailed Student’s t test. Details of the method and model components coupling can be found in [26,41].

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3. Results

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3.1. SST changes

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The SST’s response to the Arctic sea ice loss exhibits significantly different patterns
in SOM (Figure 2a) and FOM experiments (Figure 2b). In SOM experiments (Figure 2a),there is pronounced warming in the NH for lack of ocean dynamics. The warming amplitude is poleward amplified significantly in the North Atlantic and North Pacific, suggesting that the meridional gradient of the SST response is much stronger than the zonal gradient response. The SST warming maximum appears in the subpolar to polar regions
where the sea-ice reduction is the largest. However, the SST anomaly distributes in a more
complex pattern in FOM (Figure 2b). The different patterns between SOM and FOM indicate the important role of ocean dynamics in the SST response.

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Figure 2. The quasi-equilibrium response of sea surface temperature (SST, ℃) to Arctic sea ice lossin (a) SOM and (b) FOM. Values not significant at the 95% confidence level are hatched.

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In contrast to whole warming NH in SOM, there is enhanced warming at high latitudes and along the equator in the Eastern Pacific in FOM. In addition, the FOM responseshows that a cooling occurs along the path of the Gulf Stream Extension (GE) among thewarming in the North Atlantic (black box A in Figure 2b). The SST decreasing in the Labrador Sea is hatched indicating that it is not significant at the 95% confidence level, as wellas the cooling along the Oyashio Extension in the North Pacific. The warming is relatively
weak in the Kuroshio and its extension’s region compared to other regions in the North
Pacific (black box B in Figure 2b). The warming in the eastern equatorial Pacific has been
discussed in [26]. However, the mechanism of surface cooling along the GE path has notbeen explained, which is another key difference between FOM and SOM. Moreover, thesign of SST response is opposite over the WBCs in the North Atlantic and North Pacific(black box A and black box B in Figure 2b), associated with an interesting phenomenon
that the two important currents show contrary responses to Arctic sea ice loss—that is, theGulf Stream weakens (Figure 3b) while the Kuroshio intensifies (Figure 4b).

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Figure 3. (a) Climatological ocean currents averaged in the upper 100-m depth of the North Atlantic
in FOM. (b) The quasi-equilibrium responses of ocean currents averaged in the upper 100-m depth(vectors, cm/s) and SST (color shading, ℃) to Arctic sea ice loss in FOM.

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Figure 4. As in Figure 3 but for the North Pacific.

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3.2. Mechanisms for the Gulf Stream response

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The Climatological Gulf Stream and GE with fast ocean velocities are well captured
by CCSM4 in ICE_20_FOM with full ocean configuration (Figure 3a). When the prescribedlongwave radiative flux is added into the sea ice model component in FOM case, the Arctic
and subpolar Atlantic get warmer (Figure 2b, S2a), where the upper ocean stratification
becomes stable, hindering deep-water formation and triggering the weakening of the
AMOC (Figure 5). Thereafter, the sea-ice melting starts to take effect, freshening the upper
ocean, which causes the AMOC diminishes further (Figure S3). The AMOC index is defined as the maximum value of the streamfunction between 20°-70°N and 300-2000 m in
the Atlantic. In climatology, the AMOC strength is about 22 Sv (Figure not shown) andthe maximum value is at about 1000 m between 30°N and 40°N. The AMOC reduces bynearly 6 Sv within the first 30 years of ice loss and 4.5 Sv by year 100 (Figure 5a). The
weakened strength is approximately 20.5% of the AMOC before the Arctic sea ice loss and33.3% of the total AMOC decline under RCP8.5 in CCSM4 [26]. The maximum change for
AMOC happens in the depth 1000-2000 m between 30°N and 50°N (Figure 5b). As part of
the upper branch of the AMOC, the Gulf Stream and GE slow down evidently. The meridional streamfunction reduces significantly above 300 m in the subtropical North Atlantic (the black box in Figure 5b) and the upper ocean current velocity decreases about 5-8
cm/s (vectors in Figure 3b) with a narrow cooling along the Gulf Stream and GE path. The
vertically integrated volume transport in the upper 1000 m of the Gulf Stream and GE
averaged over 32°-42°N where the deceleration of the current is large decreases by about

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24.2%. The weakening of the Gulf Stream caused by the slowdown of the AMOC in our
experiments is in agreement with some previous studies [7,12].

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Figure 5. (a) The temporal evolution of the AMOC index (unit: Sv, 1 Sv = 106 m3/s) in FOM. The
AMOC index is defined as the maximum value of the streamfunction in the region of 20–70°N, 300–
2000 m in the Atlantic. (b) The quasi-equilibrium AMOC responses to Arctic sea ice loss in FOM.

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3.3. SST cooling induced by the Gulf Stream variations

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Strong warming happens in North Atlantic except profound negative SST anomalies
over the Gulf Stream and GE region (Figure 2b). Since the net surface heat flux (Qnet)
response has a warming effect on the ocean along the Gulf Stream and GE path (Figure9a), the sea surface cooling is mainly due to the slowdown of the Gulf Stream and GE. As
the largest warm current in the world, the Gulf Stream and GE transports lots of heat tohigh-latitude regions. In order to understand the processes attribute to the SST cooling,
the heat budget terms are calculated online in FOM. The heat budget equation can be
simply written as,

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0

0

0

1
𝜕𝑇
1
𝜕𝑇
𝜕𝑇
𝜕𝑇
1
∫
𝑑𝑧 = ∫ −(𝑢+𝑣+ 𝑤 )𝑑𝑧 + ∫ (𝐴𝐻 ∇2 𝑇) 𝑑𝑧
𝐻 −𝐻 𝜕𝑡
𝐻 −𝐻
𝜕𝑥
𝜕𝑦
𝜕𝑧
𝐻 −𝐻
1 0 𝜕 𝜕𝑇1+ ∫ ( 𝜅 ) 𝑑𝑧 +𝑄
𝐻 −𝐻 𝜕𝑧 𝜕𝑧
𝐻𝜌0 𝐶𝑝 𝑛𝑒𝑡

(1)

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where T is temperature. The term on the left-hand side of Equation (1) is the temperaturetendency (TEND). The right-hand side terms are temperature advection (ADV), horizontal diffusion (HDIFF), vertical diffusion (VDIFF), and the heat flux term (Q), respectively.
u, v, and w are zonal, meridional, and vertical velocities. AH and 𝜅 are the horizontal andvertical diffusivity coefficient respectively, which are used to calculate the diffusive temperature flux resulting from diapycnal diffusion and parameterized isopycnal diffusion.
𝜌0 is the reference density of seawater. cp is the specific heat of seawater. The net surface
heat flux (Qnet) is a sum of solar radiation flux (SW), long wave radiation flux (LW), latent
heat flux (LH), and sensible heat flux (SH). A positive value of Qnet indicates that theocean gets energy. We perform a heat budget analysis along the Gulf Stream and GE path,focusing on the upper 100 m (H = 100 m) [12]. The location is 42°-47°N, 20°-55°W. Figure
6a shows the responses of the terms to Arctic sea ice loss (ICE_21_FOM-ICE_20_FOM).
Apart from the temperature tendency (TEND), the positive value means a warming effect,
while the negative value represents a cooling effect.
As Figure 6a shown, the temperature tendency (the grey curve) keeps negative in thefirst 45 years, indicating that the temperature decreasing with time. Thereafter, the temperature tendency fluctuates around zero. The temperature response stays nearly stable.
To analyze the heat budget terms on the right-hand side of Equation (1), the temperatureadvection (the green curve) and horizontal diffusion (the purple curve) contribute to thecooling, while the heat flux term (the red curve) and the vertical diffusion (the orange
curve) act as warming terms. The Gulf Stream and its extension region has the highest
level of eddy variability in the North Atlantic. Eddies play an important role in thetransport of heat and nutrients [42]. As the ocean model resolution is about 1°, the horizontal mixing is parameterized as part of horizontal diffusion for the mesoscale turbulence and submesoscale processes in the ocean model, which makes the value of horizontal diffusion large. As a result, the role of horizontal diffusion is important for the temperatures changes. When the Gulf Stream current velocities decrease, the temperature advection and horizontal diffusion weaken, whose negative pattern is well matched with the
sandwiched cooling region in the North Atlantic (Figure 6b). The net surface heat flux is
positive along the Gulf Stream and GE path (Figure 9a). The space pattern also reveals
that the sea surface cooling is resulted from the ocean adjustments, not the heat exchange
between ocean and atmosphere.

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Figure 6. (a) Temporal evolutions of the heat budget terms in Equation (1). The grey curve is for thetemperature tendency (TEND); green is for three-dimensional advection (ADV); purple is for thehorizontal diffusion (HDIFF); orange is for the vertical diffusion (VDIFF); and red is for heat flux
term (Q). The right y-axis is for TEND, and the left, for the other variables. The direction of Q is
downward. All variables are averaged over the box of 42°–47°N, 20°–55°W and 0–100 m. Unit: 10-8℃/s. (b) The temperature advection and horizontal diffusion anomalies (ADV+HDIFF) in the upper
ocean in FOM. Unit: 10-8 ℃/s.

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Induced by the cooling along the GE path and a band of weak warming in the western subtropical gyre (Figure 2b), the crosswind SST gradient decreases near the Gulf
Stream or SST fronts, but that increases at 30°-40°N of the ocean interior. As a result, there
is a negative (positive) wind stress curl anomaly over the Newfoundland Basin in thenorthwest Atlantic (central subtropical Atlantic) [43] (ICE_21_FOM-ICE_20_FOM, contour in Figure 7a). The Sverdrup transport is the net meridional transport diagnosed inboth the subtropical and subpolar gyres, resulting from planetary vorticity changes thatbalance Ekman pumping or Ekman suction [44]. The cyclonic wind stress anomaly in theocean interior (30°-40°N) causes the anomalous Ekman upwelling, and then the southward Sverdrup transport deceases (color shading in Figure 7a) via Sverdrup balance. The
Gulf Stream and GE weakens as the compensating current. Therefore, the wind stresschanges due to the SST anomaly can in turn partly influence the strength of the Gulf
Stream and GE. There exists a positive feedback between the wind changes over the oceaninterior and the Gulf Stream variations.

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Figure 7. Anomalies of Sverdrup transport streamfunction (color shading, Sv) and the wind stresscurl (contour, 10-8 N/m3) in the North Atlantic (a) and North Pacific (b). The anomalies are the differences between the ICE_21_FOM and ICE_20_FOM (ICE_21_FOM-ICE_20_FOM). For the
Sverdrup transport streamfunction, the positive value indicates the strengthening northwardtransport, while the negative value indicates the increasing southward transport.

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3.4. Mechanisms for the Kuroshio response274

In contrast to the weakening of Gulf Stream, the Kuroshio and its extension shows aslightly strengthening in response to the Arctic sea ice loss (Figure 4b). The increase of thecurrent velocity is up to 1.4 cm/s at southeast of Japan in comparison with the climatological Kuroshio and KE in Figure 4a. The vertically integrated volume transport of the Kuroshio and KE averaged over 26°-36°N where the acceleration of the current is large increases by about 7.1%. Not like the Gulf Stream influenced largely by the AMOC, the acceleration of the Kuroshio and KE is mainly due to the responses of wind field to Arctic
sea ice loss. Though there is a bit of cooling in the northeast of Japan, the SST response
displays a warming pattern in the whole North Pacific. The warming is fairly strong in
the high latitudes compared to the lower latitudes (recall Figure 2b). A great zonal temperature gradient response appears in the low-mid latitudes, which means it is much
warmer in the East Pacific than the West Pacific. Along the meandering Kuroshio path,
the warming is quite weak. As a result, a positive change of wind stress curl occurs in thenorthern North Pacific, but that of the opposite sign in the southwestern and southern
regions of the North Pacific (contour in Figure 7b). The cyclonic anomaly in wind stresscurl indicates the developing of Aleutian Low and the strengthening of subpolar gyre.
The anticyclonic wind stress curl anomaly in lower latitudes intensifies the subtropical

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gyre. Furtherly, the norward (southward) Sverdrup transport anomaly (shading in Figure7b) increases up to 3.5 Sv (1.8 Sv) in the subpolar (subtropical) North Pacific calculated
from wind stress via Sverdrup relation. As the frictional western boundary current, theKuroshio and KE intensifies, which balances the negative vorticity by the Ekman pumping in the subtropical ocean interior. The position of Kuroshio is almost unchanged.
Chen et al. [12] found that the acceleration of the upper-layer Kuroshio is dominantly
forced by the surface warming through increasing the isopycnal slope in representative
concentration pathway 4.5 (RCP4.5) simulations. Though the surface warming also exists
in our experiments, the amplitudes of subsurface warming do not show marked difference between the east and west of the Kuroshio to influence the isopycnal slope for lackof the local warming effect from CO2. The SST warming response in the North Pacificoriginates from Arctic sea ice loss alone, and that is why the mechanism is different fromthat in the CO2 forcing simulations. So we conclude that the Kuroshio response to Arctic
sea ice loss is mainly due to the wind changes.

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3.5. Impact of the WBCs variations on atmosphere306

In consistent with the SST changes (recall Figure 2a), the air temperature in the lower
troposphere (850hpa level) increases significantly in almost the whole NH in SOM (Figure8a). While the largely atmospheric warming is approximately confined to the north of
40°N in the NH in FOM for the global energy balance constraints, suggesting that ocean
dynamics can in turn impact the mid-latitude atmospheric response to sea ice loss, especially in the subtropical western boundary currents regions. As is known, the atmosphereand the ocean transport heat from the equator to the poles, maintaining the heat balanceof the Earth [45] in climatology. Owing to the reduced meridional temperature gradientfrom the Arctic warming, the oceanic northward energy transport is reduced, cooling the
NH. The strongest cooling effect is in the mid-latitudes and near the WBCs and their extensions [46], which compensates for the excess heat from the Arctic into the northern
mid-latitudes. Therefore, the SST response in the Kuroshio region is quite weak. Although
the Kuroshio and KE has a slightly strengthening due to the wind stress anomaly, theatmospheric warming on the 850hpa and 925hpa level is relatively weak or even not significant at the 95% confidence level (the stippling regions in Figure 8b, c) compared toother regions over the North Pacific.

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Figure 8. The quasi-equilibrium response of air temperature (℃) to Arctic sea ice loss at (a) 850 hpa
level in SOM, (b) 850 hpa level in FOM, and (c) 925 hpa level in FOM. The values are statisticallysignificant at the 95% confidence level except in the stippling regions.

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Meanwhile, the whole North Atlantic warms except the GE region on the 925hpa
level in the atmosphere. The GE region is cooling, like a “hole” embedded in the warming
pattern (Figure 8c), a phenomenon known as the warming hole. The North Atlantic warming hole is characterized in the observed record as a region south of Greenland with negative trends in surface air temperature (SAT) despite global warming [47]. Some studies
associated the North Atlantic warming hole with a decline of the AMOC [48,49], but we
indicate that it involves an adjustment of the gyre circulation and the air–sea interaction
is important here. In our sea ice loss experiments, the sea surface cooling causes the upward heat flux decreasing, including the long wave radiation flux (LW), the sensible heat
flux (SH) and the latent heat flux (LH) (Figure 9b). The LH anomaly is the largest, whichmeans the atmosphere gets less energy from the ocean. As a result, a warming hole appears over the North Atlantic. The impact of the loss of upward heat flux decreases withheight, which results in the warm hole being more clear on the 925hpa level than on the
850hpa level (Figure 8b, c). It should be noted that the solar radiation also decreases, whichmakes the surface cooling amplified, especially in the GE region, where is a minima SST
center (Figure 2b, 3b). Though the solar radiation anomaly has a cooling effect on SST, the

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triggering mechanism of cooling pattern in the Atlantic is the slowdown of Gulf Stream
and GE, and then it causes the warming hole over the Atlantic.

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Figure 9. (a) Net surface heat flux (Qnet) response to Arctic sea ice loss over the North Atlantic in
FOM. A positive value indicates that the ocean gets energy. (b) The average short wave radiation
flux (SW), long wave radiation flux (LW), latent heat flux (LH), and sensible heat flux (SH) over thearea 42°-50°N, 15°-35°W. The direction is downward for SW while it is upward for LW, LH and SH.
Qnet = SW-LW-LH-SH.

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4. Conclusions and Discussion

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To exclude the anthropogenic warming effects and isolate the direct effect of sea ice
loss, we used a fully coupled climate model to explore the responses of the Gulf Stream
and the Kuroshio. The artificially added longwave radiation flux provides the energy forthe sea ice melting. As the sea ice cover reduces, the Arctic Ocean warms through surface
albedo changes. The SST changes in the northern mid-latitudes seen in our sea ice loss
experiments contain two parts. One part is the influence of reduced northward ocean heattransport associated with a weakening of the AMOC, and the other part is the influenceof heat released to the atmosphere from the newly open waters of the Arctic Ocean, reducing the need for poleward heat transport within the atmosphere. This excess heat isvented from the Arctic into the northern mid-latitudes to warm the ocean [41,46]. The signof the SST response is determined by which influence is larger. For the Gulf Stream and
GE regions, the ocean circulations adjustments due to the weakened AMOC play a crucialrole in causing cooing.

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The Arctic sea ice loss causes positive buoyancy fluxes in the subpolar Atlantic, triggering the slowdown of AMOC. As the northward flowing upper limb of the AMOC, theGulf Stream and GE velocity decreases in the upper ocean. As the current velocity decreases, the diminished temperature advection (ADV) and horizontal diffusion (HDIFF)
dominates the sandwiched cooling region in the North Atlantic. The wind stress change
due to the SST gradient anomaly partly influences the strength of the Gulf Stream and GE.
The role of thermodynamic air-sea interaction is more important in the Kuroshio regionin contrast to the Gulf Stream region. Along the meandering Kuroshio and KE path, there
is a weak warming response to Arctic sea ice loss on the ocean surface. The anticyclonicanomaly in wind stress curl strengthens the subtropical gyre. The increasing southward
Sverdrup transport in the ocean interior indicates the acceleration of the Kuroshio and KE.
The variations of Gulf Stream, Kuroshio and their extensions in turn modify the midlatitude atmospheric response to sea ice loss. The warming hole, a phenomenon that isobserved cooling trend over the North Atlantic, has been simply linked to a slowdown of
the AMOC in previous studies [49,50]. Here we suggest that the warming hole is a chain
reaction caused by the responses of Gulf Stream and GE to Arctic Sea ice loss in our experiments. The sea surface cooling along the Gulf Stream and GE path causes the upward
LW, SH and LH (largest) decreasing, which leads to the warming hole in the atmosphere.
Apart from the heat exchange from ocean, the reduced downward solar radiation has acooling effect on SST, which also contributes to the warming hole formation. In the North
Pacific, the Kuroshio and KE has a slightly strengthening due to the wind stress anomaly,
releasing more heat into the atmosphere, but the reduced northward ocean heat transport
has a cooling effect on the Kuroshio and KE region. A tongue of warming stretching fromthe United States into the western subtropics in the lower atmosphere, showing a zonal
temperature gradient in the North Pacific. All the processes discussed above are summarized in Figure 10. The conclusions drawn in this study may be subject to model limitations. To what extent our results depend on the particular model and the experimental
design calls for further investigation. Studies using observations and different models
would be helpful to determine the robustness of this work.

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Figure 10. Schematic diagram showing the mechanisms for the Gulf Stream and Kuroshio responses
to Arctic sea ice loss, a series of ocean adjustments and the atmospheric teleconnections.

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Author Contributions: Conceptualization, Methodology and Writing—original draft preparation,
K.W.; Visualization, L.W.; Writing—review and editing, H.L. and H.D.; Investigation and Software,
B.D.; Resources, C.D. All authors have read and agreed to the published version of the manuscript.

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Funding: This research was funded by the National Natural Science Foundation of China (41906009),
the National Key R&D Program of China (2020YFB1600103), the joint fund of the State Key Programof National Natural Science of China and the Civil Aviation Administration of China (Grant No.

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U2033207), the fund of the China civil aviation safety capability project (Grant No. 2146903) and the
National Natural Science Foundation of China (42106016).

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Institutional Review Board Statement: Not applicable.

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Informed Consent Statement: Not applicable.

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Acknowledgments: We greatly appreciate the invaluable suggestions from anonymous reviewers
and the editor.

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Conflicts of Interest: The authors declare no conflict of interest.

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