Asia-Pac. J. Atmos. Sci., 50(1), 31-43, 2014

DOI:10.1007/s13143-014-0024-7

REVIEW

A Theory for Polar Amplification from a General Circulation Perspective

Sukyoung Lee1,2

1The Pennsylvania State University, University Park, Pennsylvania, U. S. A.2School of Earth and Environmental Sciences, Seoul National University, Seoul, Korea

(Manuscript received 15 November 2013; accepted 19 December 2013)© The Korean Meteorological Society and Springer 2014

Abstract: Records of the past climates show a wide range of values

of the equator-to-pole temperature gradient, with an apparent univer-

sal relationship between the temperature gradient and the global-

mean temperature: relative to a reference climate, if the global-mean

temperature is higher (lower), the greatest warming (cooling) occurs

at the polar regions. This phenomenon is known as polar amplifica-

tion. Understanding this equator-to-pole temperature gradient is fun-

damental to climate and general circulation, yet there is no established

theory from a perspective of the general circulation. Here, a general-

circulation-based theory for polar amplification is presented. Rec-

ognizing the fact that most of the available potential energy (APE) in

the atmosphere is untapped, this theory invokes that La-Niña-like

tropical heating can help tap APE and warm the Arctic by exciting

poleward and upward propagating Rossby waves.

Key words: Polar amplification, general circulation, equable climate

1. Introduction

In spite of the complexity of the climate system, there is a

recurring global-scale phenomenon known as polar amplifica-

tion. Polar amplification refers to the phenomenon that

warming or cooling trends are strongest over high latitudes,

particularly over the Arctic (e.g., Manabe and Wetherald, 1975;

Huber et al., 2000; Rigor et al., 2000; Johannessen et al.,

2004). Geological data suggest that during the warm periods of

earth history, the equator-to-pole temperature gradients were

much smaller than the present-day value. Fossilized remains of

frost-averse plants and animals around the Arctic Ocean (e.g.,

McKenna, 1980; Tarduno et al., 1998) suggest that above-

freezing temperatures were not uncommon during the Creta-

ceous and early Cenozoic. Estimates of tropical temperatures

during these warm periods vary, ranging from below to above

the present-day values (Douglas and Savin, 1978; Bralower et

al., 1997; Pearson et al., 2001).

The evidence collectively suggests that the surface equator-

to-pole temperature gradients were weaker during warm periods

(Huber et al., 2000). Similarly, CLIMAP (Climate: Long range

Investigation, Mapping, and Prediction) reconstruction data

show that the meridional surface temperature gradient during

the Last Glacial Maximum (LGM) was greater than that of the

present-day climate. Based on this paleoclimate evidence,

Budyko and Izrael (1991) hypothesized that there may be a

universal relationship between changes in the surface meridio-

nal temperature gradient and global-mean temperature change.

This hypothesis was tested by Hoffert and Covey (1992),

and their key figure is reproduced here as Fig. 1. The top panel

of Fig. 1 shows the latitudinal distribution of the deviation of

surface temperatures from the present-day temperature, nor-

malized by the global mean of the temperature deviations.

Supporting the hypothesis, the latitudinal profiles are remark-

ably similar for the three different warm periods. As a way of

testing of the idea further, Hoffert and Covey (1992) con-

structed a least square fit to the data shown in Fig. 1a (shown

as a solid line), and then the resulting ‘universal’ relationship

was applied to the global mean temperatures of the middle

Cretaceous and LGM. As can be seen in the lower panel, the

latitudinal distributions of the temperature predicted by this

relationship agrees well with the data from both time periods.

This apparent universality is also evident in Fig. 2 which is

from more recent work by Miller et al. (2010). Across four

different periods (LGM, Holocene Thermal Maximum, Last

Interglaciation, and middle Pliocene), there is a nearly constant

linear relationship between the global temperature change and

the corresponding Arctic temperature change.

Polar amplification is also evident in the current-day climate.

Figure 3 shows that fluctuations in surface temperature are

small in the tropics, but they increase with latitude, with largest

values in the polar latitudes. Climate models in general show

polar amplification in response to enhanced greenhouse gas

(GHG) forcing (e.g., Manabe and Stouffer, 1980; Hansen et

al., 1984; Winton, 2006; Meehl et al., 2007). The models,

however, deviate rather substantially in their prediction of the

magnitude of the Arctic warming (Masson-Delmotte et al.,

2006; Walsh et al., 2008). This indicates that key physical

processes behind the polar amplification are not well simulated

by the models.

Why is this phenomenon of polar amplification interesting?

There are several reasons. On the practical side, there are some

obvious biological and socio-economical consequences. For

instance, the melting of sea ice will open up new, shorter

passages for ships between Asia and Europe. Meteorologically,

it can have a global scale impact. Recent studies on sea ice and

Corresponding Author: Sukyoung Lee, Department of Meteor-ology, The Pennsylvania State University, 503 Walker Building,University Park, PA 16802, U. S. A. E-mail: sxl31@psu.edu

32 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

snow cover indicate that the melting of Arctic sea ice during

the summer and snow over land areas during the fall can in-

fluence the hemispheric scale circulation during the following

winter. These changes in the circulation are associated with

fluctuations in winter precipitation, blocking frequency, cold-

air outbreaks, and snow cover over a substantial fraction of the

middle latitude Northern Hemisphere (NH) (e.g., Singarayer et

al., 2006; Honda et al., 2009; Overland and Wang, 2010;

Petoukhov and Semenov, 2010; Liu et al., 2012).

In addition to the reasons described above, polar amplifica-

tion is a fascinating phenomenon from a theoretical point of

view. Regardless of the initial cause of the warming or cooling

of the Arctic, the two temperature profiles (one for the Creta-

ceous period and the other for the LGM) in Fig. 1 raise a

fundamental question as to how the different equator-to-pole

temperature gradients were maintained. Consider the idealized

global energy balance at the top of the atmosphere (TOA), as

depicted in Fig. 4a. In the tropics and subtropics, there is

surplus of net radiation since absorbed solar radiation exceeds

outgoing long wave radiation (OLR). The opposite holds in

high latitudes, producing a deficit of net radiation. In

equilibrium, this TOA energy imbalance is compensated by

poleward energy transport. During the cold season when solar

radiation does not reach polar latitudes, the TOA deficit in

polar regions is determined almost entirely by the OLR. Thus,

in so far as the OLR responds positively to an increasing

surface temperature, the warming of the Arctic implies a

stronger poleward energy flux1 (Fig. 4b). [Of course, it is

possible that Arctic OLR may actually decrease as the surface

warms if the Arctic becomes covered by tall, convective

clouds. However, this does not occur at least in the present-day

Fig. 1. (a) Zonal-mean surface temperature deviations (∆T) from thepresent-day climate divided by <∆T>, where < > denotes a globalaverage. The solid line is a least-squares fit to the data displayed for afourth-order polynomial in the sine of latitude; NT(x) = ∆T(x)/<∆T> =0.36 + 3.21x4, where x = sin(latitude). (b) ∆T for the mid-Cretaceousmaximum warming and LGM cooling. Points are derived from paleo-data and lines indicate the best-fit NT(x) shown in (a) multiplied bythe corresponding <∆T>. [From Hoffert, M. I., and C. Covey, 1992].

Fig. 3. Time series of the seven-year running mean of the anomalouszonally averaged surface temperature (

oC). [From Bekryaev et al.,

2010)].

Fig. 2. Paleodata estimates of Arctic summer surface temperature de-viations from the present-day values (ordinate), their NH or globalmean values (abscissa), and their uncertainties. The time periods shownare: the last Glacial Maximum (LGM), Holocene Thermal Maximum(HTM), Last Interglaciation (LIG), and middle Pliocene (MP). [FromMiller et al., (2010)].

1One may argue that a weak temperature gradient is caused by an enhanced poleward energy transport which is in turn triggered by an initial state

with a strong temperature gradient. However, in a steady state (or over time scales longer than the energy transport processes), since the polewardenergy transport is not solely dependent on this presumed initial state, one is still left with the conclusion that a stronger poleward energy flux mustbe required to maintain an equable climate.

31 January 2014 Sukyoung Lee 33

climate.] This leads to the following question. How does the

poleward energy flux increase as the climate warms?

In this essay, this question is addressed from the perspective

of the general circulation. For a recent review of observational

and modeling work on the Arctic warming, readers are re-

ferred to Serreze and Barry (2011). As their review paper

recounts, most studies on the Arctic climate have focused on

local processes such as ice-albedo feedback (Budyko, 1969).

While these authors also discuss heat transport by the large-

scale atmospheric circulation, a theoretical treatment of this

topic in the context of polar amplification is lacking in the

literature. This essay is an attempt to advance such a theory.

2. Theories of equable climates

a. The flux-gradient relationship and baroclinic adjustment

Until recently, proposed mechanisms on equable climates

that have considered poleward heat transports include a Hadley

cell which occupies the entire hemisphere (Farrell, 1990), an

enhanced poleward oceanic heat transport (e.g., Barron et al.,

1993; Sloan et al., 1995), and an intensification of the thermo-

haline circulation due to driving by tropical cyclones (Sriver

and Huber, 2007; Korty et al., 2008). There are other proposed

mechanisms that do not involve poleward heat transports.

These include a convection-cloud radiative forcing feedback

(Sewall and Sloan, 2004; Abbot and Tziperman, 2008a, b), a

vegetation-climate feedback (Otto-Bliesner and Upchurch,

1997; DeConto et al., 2000), and decreased cloud reflectivity

due to a reduction in the number of cloud condensation nuclei

(Kump and Pollard, 2008). But again, as was discussed above,

an equilibrium climate requires an increased poleward energy

flux. Therefore, a mechanism that can account for the increased

poleward energy flux is still needed.

Given the fact that the present-day extratropical (i.e.,

poleward of 30o) poleward heat transport is carried out mostly

by atmospheric baroclinic eddies (here eddies are defined as

motion that deviates from a zonal mean), it is interesting to

note that none of the above theories involve baroclinic eddies.

There is a good reason for this. To a very good approximation,

the baroclinic eddy heat flux is negatively proportional to the

gradient of the mean temperature:

[v’T’] = −D ∂[T]/∂y,

where v is meridional wind, T the temperature, the square

bracket an appropriately defined mean, and prime the deviation

form the mean. According to this flux-gradient relationship, if

the meridional temperature gradient were to decrease, a new

equilibrium state would be achieved through a weaker

poleward eddy heat flux. Therefore, a downgradient (poleward)

baroclinic eddy heat flux cannot account for the poleward heat

transport which must increase in order to maintain the weak

temperature gradient of a warm climate.

Based on a scaling argument relevant for an idealized two-

layer atmosphere of differing densities, Held and Larichev

(1996) suggested D ~ Uλξ2 where U is the velocity wind shear

which is proportional to the meridional temperature gradient, λ

the internal Rossby radius of deformation, and ξ the super-

criticality which can be thought of as the meridional tempera-

ture gradient of a radiative equilibrium state. Under equable

climate conditions, both U and ξ decrease. According to Fig. 1,

these values during the Cretaceous are roughly 50% of the

present-day values. Assuming that λ does not change, the

diffusivity would actually be smaller under equable climate

conditions.

There is also the view that the statistical equilibrium state of

the atmosphere corresponds to the marginal state for baroclinic

instability. This equilibration process, known as baroclinic

adjustment (Stone, 1978; Lindzen et al., 1980; Cehelsky and

Tung, 1991), is based on Charney-Stern-Pedlosky stability

criterion (Charney and Stern, 1962; Pedlosky, 1964). That is, if

Fig. 4. Schematic diagram of top of the atmosphere (TOA) net radiation and poleward energy flux. According to Fig. 1, underequable climates, warming is much greater in high latitudes. This implies that the increase in outgoing long wave radiation(OLR) is greater in the high latitudes, hence strengthening the equator-to-pole gradient in TOA net radiation. This implies thatthe poleward energy flux must be stronger under equable climates. Note that this balance between the TOA radiation gradientand the poleward energy flux, on its own, cannot be used to infer a causal relationship.

34 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

baroclinic adjustment is appliacable, the atmospheric state

exhibits neutrality with respect to a spontaneous growth of

eddies from arbitrarily small perturbations in a stratified

rotating fluid. This implies that an increase in the eddy heat

flux cannot take place beyond that of the neutral state.

Therefore, in this theory, which excludes further poleward heat

flux by finite-amplitude disturbances, the reduced meridional

temperature gradient of the equable climate cannot be realized.

None of the above two equilibration processes is caused by

an exhaustion of zonal-mean available potential energy (ZAPE;

Lorenz, 1955). In the present-day atmosphere, eddy available

potential energy is only about 10% of the zonal-mean available

potential energy (Peixoto and Oort, 1994; Li et al., 2007).

According to the above equilibration theories, the baroclinic

eddy heat flux is either saturated with respect to linear inst-

ability or constrained in the context of homogeneous baroclinic

turbulence (Salmon, 1980; Vallis, 1988; Held and Larichev,

1996). In reality, however, the atmosphere is neither devoid of

finite amplitude disturbances nor does it resemble homo-

geneous turbulence: for example, the atmosphere is almost

always subject to finite-amplitude disturbances from the tropics

(think of El Niño-Southern Oscillation or the Madden-Julian

Oscillation (MJO)), and there is inhomogeneity introduced by

orography and the land-sea distribution. Thus, if, somehow, the

mostly untapped ZAPE can be released through these forcings,

in principle, enhanced poleward heat transport, required to

maintain the equable climates, can be realized.

Of course, this is not to say that the contributions from

oceanic heat transport can be ignored. The point is that short-

comings in the above equilibration theories should not prevent

us from searching for other atmospheric processes that can

contribute to the maintenance of an equable climate. Such a

theory is needed because oceanic heat transport alone cannot

explain the high-latitude continental winters of above-freezing

temperatures of the Cretaceous and early Cenozoic. In fact, the

hemisphere-wide Hadley cell (Farrell, 1990), whose sinking

branch occurs in the Arctic, can warm high-latitude continental

interior. However, this theory requires a tropopause height of

~30 km, 2-3 times that of the present-day value. In this essay,

we present another plausible theory for equable climates which

involves a circulation that is more similar to that of the

present-day climate.

b. Criteria for an alternative mechanism

Is there an alternative mechanism that (1) can help explain

equable climates, but (2) does not require a radically different

atmospheric state, (3) does not involve either the flux-gradient

relationship or baroclinic adjustment, and possibly (4) accounts

for the apparent universal relationship? As was mentioned

earlier, given the fact that only a small fraction of the ZAPE is

being tapped, processes that can tap the remaining vast storage

of ZAPE may potentially satisfy all of the four conditions. Is

there such a process?

A hint can be seen from present-day stationary waves.

During the boreal winter, poleward of ~40oN, the poleward

transport of moist static energy is dominated by its stationary

wave contribution (Oort and Peixoto, 1992). This can be

accounted for by linear wave theory: vertically propagating

Rossby waves, as signified by upward and westward tilt in the

pressure (or geopotential) field, transport heat poleward. Ac-

cording to the linear theory of Charney and Drazin (1961),

vertical wave propagation is possible for waves with large

horizontal scales embedded in a weak westerly wind. The NH

winter circulation is in fact most favorable to this vertical

propagation.

Stationary waves are forced by both orography and heating,

although these two forcings are not independent of each other.

For example, orographic forcing is dependent on pressure

difference across the orography, and the pressure field may be

influenced by a heat-induced circulation. Likewise an oro-

graphically-forced circulation can influence heating by affecting

air temperatures over the ocean, hence the ensuing convection

and convective heating. Since these ‘forced tappings’ of ZAPE

are independent of baroclinity and instability, this mechanism

satisfies the condition (3).

Stationary waves are not the only agent of a forced tapping.

For instance, the 30-60 day tropical convective system, known

as Madden-Julian Oscillation (MJO; Madden and Julian, 1971,

1972), can excite poleward propagating Rossby waves

(Matthews et al., 2004). Sardeshmukh and Hoskins (1986)

showed that waves excited by tropical convective heating can

escape into the extratropics through a subtropical Rossby wave

source. In the NH, this subtropical Rossby wave source is

located over southeast Asia and the adjacent western sub-

tropical Pacific Ocean where the convective divergent flow

from the Indo-western Pacific warm pool (simply ‘warm pool’

hereafter) interacts with the tight absolute vorticity gradient

associated with the subtropical jet. The wave that emanates

from the Rossby wave source can propagate poleward until it

encounters a turning latitude. The larger the zonal scale of the

wave, the farther it can propagate into high latitudes (Hoskins

and Karoly, 1981). Therefore, when MJO convection is over

the warm pool region, the Rossby wave excited by this MJO

heating constructively interferes with the stationary wave

forced by climatological warm pool convection (Garfinkel et

al., 2010; 2012). This constructive interference can strengthen

the forced tapping, hence enhance poleward heat transport.

Thus, stationary or non-stationary, large horizontal scale (zonal

wave numbers 1 or 2) tropical convective heating can tap

ZAPE. This possibility is depicted in Fig. 5a. (Figure 5a also

shows the transient baroclinic eddy heat flux. This will be

discussed in Section 3.)

The forced tapping of ZAPE can also arise from an eddy

momentum flux which is strongest in the upper troposphere.

Since the potential vorticity gradient is positive in the upper

troposphere, in the latitude of the wave source, or wave

stirring, there is eastward acceleration by an eddy momentum

flux convergence. Likewise, in the latitude of a wave sink or

wave dissipation, there is a westward acceleration by an eddy

31 January 2014 Sukyoung Lee 35

momentum flux divergence. Thus, in the stirring (dissipation)

region, the vertical shear of zonal wind increases (decreases).

To maintain thermal wind balance, the meridional temperature

gradient must increase (decrease) across the stirring (dissipa-

tion) region. Thus, poleward (equatorward) of the dissipation

region, there must be a sinking (rising) motion so as to bring

adiabatic warming (cooling). In the atmosphere, the Hadley

cell is driven in part by wave dissipation in the subtropics

(Pfeffer, 1982; Kim and Lee, 2002; Walker and Schneider,

2006); by the same token, in midlatitudes where most of the

stirring occurs (due to baroclinic eddies), there is the thermally

indirect Ferrel cell.

In the same way, if a forced wave, say, excited by tropical

convective heating, manages to propagate into the extratropics

and dissipates in mid-to-high latitudes, there should be adia-

batic warming poleward of these latitudes. Figure 5b illustrates

this possibility: A tropical convective heat source excites a

Rossby wave, and as this Rossby wave propagates poleward,

westerly momentum is transferred into the wave source region

(indicated by the symbol, , in Fig. 5b). If this momentum

flux convergence is sufficiently strong, the equatorial wind

becomes westerly, i.e., the circulation evolves into a super-

rotating state. If the angular momentum of the atmosphere is to

be conserved, the eastward acceleration in the tropics must be

balanced by a westward acceleration elsewhere. From the

arguments presented above, this westward acceleration will

take place in the region of wave dissipation. This region is

indicated by the symbol, ⊗, in Fig. 5b. As mentioned above,

thermal wind adjustment requires that this momentum sink

produce downward motion on its poleward side. The down-

ward motion compresses the air and warms it adiabatically.

Therefore, if this downward motion occurs in polar regions, a

localized tropical heat source can warm high latitudes. In the

atmosphere, such a localized heat source can be found over the

warm pool. If this process were to intensify as the climate

warms, then the Rossby wave response could be an important

contributor toward polar amplification. Thus, if this tropical

stirring mechanism is responsible for polar amplification, one

can expect that the resulting equable climate must be accom-

panied by an equatorial upper tropospheric wind which is less

easterly or even superrotating.

Indeed, one of the most spectacular examples of such a

forced wave effect can be found in a modeling study of

equatorial superrotation. With an idealized two-layer GCM,

Saravanan (1993) examined the transition from a ‘normal’ state

to a superrotating state by subjecting the model atmosphere to

zonal-wavenumber-two heating on the equator. The transition

takes place as the heat source is gradually strengthened.

Because the zonal mean of the heat source is zero, the zonal

mean ‘radiative equilibrium’ temperature field is identical bet-

ween the normal and superrotating states. In spite of their

identical radiative equilibrium temperature field, Fig. 1e of

Saravanan (1993) shows that the midlatitude eddy heat flux of

the superrotating state is only about half that of the normal

state. Although not discussed in that study, according to the

flux-gradient relationship, this implies that the meridional tem-

perature gradient of the superrotating state should be weaker

than that of the normal state. This, in turn, implies that there

must be a process, other than the eddy heat flux, that warms

high latitudes and/or cools the tropics in that model. Because

there is no ocean in the model, the high-latitude warming must

take place through an atmospheric process, most likely through

adiabatic warming. Although this result is from a highly

idealized model, it nevertheless alludes to the possibility that

Fig. 5. Schematic diagram of the TEAM mechanism. The storm trackeddy response in step (4) and the cloud cover increase in step (8) arefound to accompany the forced tapping (of ZAPE) mechanism (Lee et

al., 2011a). Step (7) is shown by Yoo et al. (2012a, b), and (8) by Yooet al. (2012b). It is worth noting that the equatorial superrotation canbe a byproduct of the TEAM mechanism.

36 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

an increase in strength and a localization of tropical convection

may contribute toward polar amplification.

Now turning our attention to condition (4), if these forced

tapping processes were to explain the universal relationship

between the global mean temperature and the meridional tem-

perature gradient, there must be a process where localized

warming (cooling) promotes (hinders) the forced tapping.

Given the theoretical expectation that an enhanced large-scale,

localized tropical convection (acting as wave stirring) can cause

winter Arctic warming, if GHG warming can intensify the

existing zonally localized convection (currently the strongest

convection occurs over the warm pool), then condition (4) can

be satisfied. In other words, if the GHG warming intensifies

the existing east-west gradient in tropical convection, according

to the theory presented here, poleward heat transport will

intensify, hence the Arctic will warm more than the tropics.

Here, the zonal localization is emphasized because the zonally

symmetric part of the tropical heating does not excite the

forced waves.

c. Does the west-east gradient in the tropical convection in-

crease with warming?

In the current climate, it is uncertain whether or not the zonal

asymmetry in convective heating over the tropical Pacific

would intensify in a warming world. In fact, this has been a

subject of debate in the climate science community in the con-

text of the Walker Circulation which is characterized by rising

motion over the warm pool region and sinking motion over the

eastern tropical Pacific. Most climate model simulations of the

twentieth century show a weakened Walker circulation (Vecchi

et al., 2006; Vecchi and Soden, 2007), and this model response

has been attributed to GHG warming (Held and Soden, 2006).

However, the opposite trend has been found in satellite data

analysis (Sohn and Park, 2010), and also in reanalysis, recon-

struction, and in-situ measurements (L’Heureaux et al., 2013).

Since a trend toward rising (sinking) motion coincides with

increased (decreased) SST and active (inactive) convection,

the Walker Circulation trend has often been equated with

trends in SST trend and/or convective precipitation. However,

this is not necessarily the case. The experiment by Clement et

al. (1997) shows that when their model’s SST is uniformly

raised, while keeping the trade wind (the surface branch of the

Walker circulation) speed constant, the model responds by in-

creasing the east-west SST contrast, producing a La-Niña-like

SST field. Similarly, based on the thermodynamic energy

balance in the tropics, one can expect that the Walker circula-

tion may weaken while the east-west contrast in convective

heating increases; this is possible if the fractional increase in

static stability is greater than that of the convective heating.

In paleoclimate studies, there is emerging evidence that

during times when the climate was warm (cold), the tropical

SST took on a La-Niña-like (El-Niño-like) spatial structure.

Stott et al. (2002) analyzed seawater temperature and salinity

reconstruction data for the late Pleistocene, and found that

during interstadials (periods of warm high latitudes) the tropical

Pacific warm pool was fresher, implying enhanced convection,

while the stadials (periods of cold high latitudes) coincide with

a saltier warm pool, suggestive of suppressed convection. This

finding is at odds with the conclusions of an earlier study by

Lea et al. (2000), but Stott et al. (2002) pointed out that the site

used by Lea et al. (2000) may be too far to the east to be

considered as representative of the western warm pool. The

association of a warm (cold) climate with La-Niña-like (El-

Niño-like) conditions is reported in other studies such as

Koutavas et al. (2002) who showed that the LGM coincided

with El-Niño-like conditions in the tropical Pacific, and Visser

et al. (2003) who found that warm pool SST increased by 3.5-

4.0oC during the last two glacial to interglacial transitions.

3. Testing of the forced tapping mechanism

The forced tapping mechanism was tested with a GCM

experiment (Lee et al., 2011a) in the context of the equable

climate of the Mesozoic and Cenozoic2 when high-latitude

continental winters are believed to have undergone above-

freezing temperatures (Huber, 2008; Spicer et al., 2008). In a

series of model runs, a localized heat source was prescribed in

the western corner of the tropical Pacific ocean (see the red

box in Fig. 6a) where warm pool convection was postulated to

have existed by the authors; elsewhere in the tropics, the same

amount of heating is subtracted (see the blue box in Fig. 6a) so

that the zonal mean net heating in the tropics is zero. Figure 6a

shows the difference in January surface temperature between a

run with 150 W m−2 contrast (between the red and blue regions)

and a run with no heating contrast. Although the zonal mean

diabatic heating is identical between the two runs, the run with

the tropical heat perturbation produces Arctic temperatures

that are as large as 16oC warmer. Figure 6b shows evidence of

poleward propagating Rossby wave trains emanating from the

heating region in the tropics, supporting the hypothesis that

large-scale, localized tropical heating can warm the winter

Arctic by exciting poleward propagating Rossby waves.

As the tropical heat perturbation intensifies, the Arctic sur-

face air temperature exhibits a marked increase (Fig. 7b), yet

the temperature in the equatorial region (Fig. 7a) remains close

to being constant. This behavior is reminiscent of the ‘univer-

sal’ relationship depicted in Fig. 1. It is unknown whether

there was a warm pool during the Mesozoic and Cenozoic, nor

how strong was the convective heating. However, this proof-

of-concept calculation shows that stirring of the tropical

atmosphere (by way of a heat perturbation) can produce cold

season polar amplification with a muted equatorial temperature

change.

Does this polar amplification occur as was theorized in

Section 2b? Lee et al. (2011b) found that between 40oN and

2The Mesozoic and Cenozoic range from about 145 to 50 million years ago.

31 January 2014 Sukyoung Lee 37

60oN, the warming is due to the stationary eddy heat flux (the

prescribed tropical heating is stationary); between 60oN and

80oN the warming is by the transient eddy heat flux (see Fig.

5a; since the imposed heat perturbation is stationary, the tran-

sient eddy heat flux should be regarded as a response to the

stationary eddy heat flux), and between 70oN and 80oN, it is

caused by adiabatic warming driven by stationary wave mo-

mentum flux. It turned out that poleward of 80oN, the warming

is caused by downward infrared radiation (IR) associated with

increased cloud cover. Because the ultimate driver of these

responses is the stationary tropical heat perturbation, the logical

conclusion is that dynamical processes - meridional heat flux

and adiabatic warming - first warms the Arctic Ocean, and the

transport of water vapor and cloud liquid water (by the trop-

ically forced Rossby wave, and perhaps by the transient eddies

that respond to the heat flux convergence by the forced wave)

into the Arctic leads to further warming through enhanced

downward IR; once it encounters the cold Arctic air, because

of the low saturation vapor pressure, the water vapor con-

denses (latent heat release) to form clouds, and as cloud cover

increases, radiative forcing by the clouds can take over the

warming. This final step in the process is sketched in Fig. 5c.

This physical process is found to contribute to the present-

day warming trend in the Arctic winter, and this is consistent

with the La-Niña-like trend of intensified tropical convection

over the warm pool region. Lee et al. (2011b) found with

global European Center for Medium-Range Weather Forecasts

reanalysis data (ERA-40) that winter Arctic warming between

1958-2001 is contributed by increases in the frequency of oc-

currence of a few teleconnection patterns; these patterns grow

and decay at intra-seasonal time scales where the growth of

aggregation of these patterns is preceded by enhanced warm

pool convection and the decay is followed by downward IR

over the Arctic Ocean. Poleward eddy heat flux and adiabatic

warming also contribute to the Arctic warming trend. Because

it takes 7-10 days for a Rossby wave to propagate from the

tropics to high latitudes (Hoskins and Karoly, 1981), the

mechanism described here, if it is relevant, should be operating

on intraseasonal time scales. This is found to be the case in Lee

et al. (2011b).

The connection between localized tropical convection and

Arctic warming at intraseasonal time scales is highlighted by

the findings of Yoo et al. (2011). Using ERA-Interim data,

they showed that when the MJO convection enhances (sup-

presses) the warm pool convection, Arctic warming (cooling)

follows. Here, it was again found that the positive warm pool

convection anomaly is followed by dynamic warming, then by

moisture transport into the Arctic, and an increase in down-

ward IR (Yoo et al., 2012a). The implied causal relationship

Fig. 6. January difference in (a) surface temperature and (b) 250-hPageopotential height and wind fields between a run with 150 W m−2

contrast (between the red and blue regions) and a run with no heatingcontrast. The model is a coupled atmosphere-mixed-layer ocean GCM.The CO

2 level in this model is 4 × PAL (Preindustrial Atmospheric

Level, 1 PAL = 280 ppmv). [Adopted from Lee et al., 2011a.].

Fig. 7. January zonal mean surface air temperature simulated by acoupled atmosphere-mixed-layer ocean GCM. In the legend, thevalues indicate the imposed heating difference between the red andblue boxes (W m

−2) shown in Figure 6a. [Adopted from Lee et al.,

2011a.].

38 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

between the convection and the Arctic warming was supported

by model calculations (Yoo et al., 2012b). This warm pool

convection-Arctic warming linkage is also present at inter-

annual time scales, since La-Niña (El-Niño) years are accom-

panied by enhanced (suppressed) poleward moist energy flux

in the extratropics and warmer (cooler) Arctic (Lee, 2012). To

emphasize the linkage between the tropical convection and

Arctic warming, Lee (2012) named this forced tapping process

as the Tropically Excited Arctic warming (TEAM) mechanism3.

4. More on poleward heat flux and downward IR

Without invoking the TEAM mechanism, some recent

studies have questioned the importance of the ice-albedo feed-

back, and have instead stated that a poleward atmospheric heat

(or moist static energy) transport may be the key process

behind the Arctic amplification (Alexeev, 2005; Graversen,

2007; Graversen and Wang, 2009; Lu and Cai, 2010). Held

and Soden (2006) showed that the fourth IPCC assessment

report (AR4) models, under Special Report on Emissions

Scenarios (SRES) A1B forcing scenario, produce an increase

in the poleward atmospheric heat flux, while the changes in the

oceanic heat transport are negligible. Consistent with this

increase in poleward heat flux, Wu et al. (2010) and Zelinka

and Hartmann (2012) showed that the equator-to-pole gradient

in the TOA net radiation strengthens in response to GHG-

driven warming. Analyzing the time mean states of the models,

both studies found that the increase in the TOA radiation

gradient is caused by water vapor and cloud feedbacks.

Wu et al. (2010) interpreted that these feedback processes

increase the baroclinicity, and that the transient eddies respond

to this increase in baroclinicity. However, their Fig. 5 shows

that the match between baroclinicity and the poleward eddy

flux is poor: the increase in baroclinicity occurs mostly in the

upper troposphere, yet the increase in eddy heat flux is largest

in the lower troposphere. Recognizing this discrepancy, they

suggested that eddies are influenced by the upper tropospheric

baroclinicity. Even though the mechanism that drives this

process remains unclear, it appears that this view, that an

increase in the poleward eddy heat transport is a response to a

strengthening of the equator-to-pole gradient in the TOA net

radiation, or upper tropospheric baroclinity, is shared by others

(Lu and Cai, 2010; Zelinka and Hartmann, 2012).

The prevalence of this view is understandable, given the

importance of baroclinic instability and the flux-gradient rela-

tionship in our understanding of the atmospheric circulation.

However, it is not feasible to tease apart causal relationships

from time mean states. Instead of the causality proposed by the

above studies, it is also possible that a strengthening of the

poleward energy flux, for instance, driven by tropical stirring

(through localized convection), i.e., the TEAM mechanism,

causes the TOA net radiation gradient to increase. If one were

to diagnose the two model runs in Lee et al. (2011a), without

knowledge of how the model was perturbed, one may con-

clude that the clouds, by influencing the TOA net radiation

gradient, caused the energy transport to strengthen.

The AR4 model studies also found that downward IR,

caused by increased moisture and cloud cover, plays an impor-

tant role for warming the Arctic (e.g., Winton, 2006). Two

different processes can contribute to this increase in downward

IR: a local moisture feedback process (Abbot et al., 2009) and

remote process which invokes moisture transport from outside

of the Arctic. The increase in poleward latent heat flux in the

AR4 models (Held and Soden, 2006; Wu et al., 2010) suggests

that the remote process plays an important role for enhancing

the downward IR.

5. Summary, caveats and further questions

From the perspective of the atmospheric general circulation,

this article advances a conjecture that an equable climate,

which is characterized by a weak meridional temperature

gradient, can be realized if the vast storage of zonal available

potential energy (ZAPE) can be tapped with a forcing which

relies on neither baroclinic instability nor the flux-gradient

relationship. Under usual circumstances, localized tropical con-

vection is perhaps the most viable candidate for such a forcing.

From this perspective, a theory, referred to as the TEAM

(Tropically Excited Arctic warMing) mechanism, has been put

forward (Lee, 2011a, b; Yoo et al., 2011, 2012a, b; Lee, 2012)

where the main thesis is that an enhancement in the warm pool

tropical convection strengthens and/or more frequently excites

poleward propagating Rossby waves, and that the wave dynam-

ics and subsequent increase in downward infrared radiation

lead to warming over the Arctic.

This general-circulation based theory can help explain: (1)

an enhanced poleward heat transport, conforming to the TOA

energy balance; (2) that Arctic warming occurs during the

winter when solar radiation does not reach the Arctic; (3) the

paleo-equable climate warmth in the high-latitude continental

interior; (4) the apparent universal relationship between global-

mean temperature and the meridional temperature gradient; (5)

the apparent invariance of equatorial temperature in the widely

varying climatic conditions (Fig. 1); (6) the tendency of models

to produce equatorial superrotation when the model climate is

warmed (Lee, 1999; Huang et al., 2001; Cabarello and Huber,

2010).

Regarding (2), the leading theory has been that the ice-

albedo feedback process (Budyko, 1969) warms the Arctic

Ocean during the warm season and then release the heat during

3The Pacific Decadal Oscillation (PDO) may be regarded as another phenomenon which can be used to test the TEAM mechanism since the associ-ated SST pattern shares a resemblance with that of El Niño. However, care needs to be taken in doing so because it is the tropical convection patternthat matters in the TEAM mechanism; during the positive phase of the PDO (El-Niño-like SST) which started in late 1970s, the hydrographic salin-ity record indicates a continued freshening in the western warm pool region (Durack and Wijeffels, 2010), suggesting that there has been a La-Niña-like trend in convective precipitation.

31 January 2014 Sukyoung Lee 39

the following winter (Serreze and Barry, 2011). Figure 8a is

adopted from Stroeve et al. (2012), and shows a schematic

description of the ice-albedo feedback process in more detail.

In this framework, the GHG-driven warmer air thins the spring

sea ice and melts the summer sea ice, with latter process being

amplified by ice-albedo feedback. The warmer ocean then

releases the stored heat in the following fall and winter, a pro-

cess which hinders sea ice formation and thickening. However,

recent studies have raised questions on this picture: Over re-

gions covered by sea ice, the surface during the cold season is

warmed by heat flux from the atmosphere (Graversen and

Wang, 2009; Lee et al., 2011a). Park et al. (2014) and Flourney

et al. (2014) found that during winter and early spring, positive

downward IR anomalies over the Arctic are preceded by

anomalously strong warm pool convection, and followed by a

statistically significant reduction in Arctic sea ice; Persson

(2012) analyzed measurements from the Surface Heat Flux of

the Arctic Ocean (SHEBA) experiment and found that melt

onset is preceded by springtime free-tropospheric warming,

and is punctuated by atmospheric synoptic events.

The autocorrelation of Arctic sea ice further suggests that

winter processes such as the TEAM mechanism may play

more active role than was previously thought. Figure 9 shows

that the April sea-ice area (its lag correlations are highlighted

by the first vertical line in Fig. 9) is uncorrelated with the

previous melting season (May-September), but is correlated

with the previous cold season (November - March). The April

sea-ice area is also correlated with the sea-ice area for much of

the following year, until the month of March. In contrast, the

September sea-ice area (highlighted by the second vertical line

in Fig. 9) is significantly correlated with all of the previous

months’ sea-ice area, but for those months with positive lags,

statistically significant correlations are found only until the

following March. One possible explanation for these lag

correlations is that winter climate conditions may be more

important than summer conditions. In fact, Persson (2012)

reports for a synoptic weather event that during the SHEBA

experiment the ice-albedo feedback effect was much weaker

than warming by downward IR. Graversen and Wang (2009)

also found in their model experiment that the ice-albedo effect

is weaker than the downward IR effect. They pointed out that

this is consistent with the fact that the Arctic is covered by

clouds most of the time (Wang and Key, 2005). Therefore,

contrary to the view that summer season warming is the main

driver, it is possible that the winter warming, perhaps driven by

the TEAM mechanism, may act as the main player by its

hindering of the formation of sea ice, and thus by promoting

summer sea ice melting. This possibility is presented sche-

Fig. 8. (a) Adopted from Stroeve et al. (2012), and shown here to becompared with (b). (b) Supported by the sea-ice-area autocorrelationshown in Fig. 9, the physical processes described by the TEAMmechanism suggest that winter Arctic conditions may influencesummer Arctic sea ice. The question marks indicate that the causalrelationship may not be as solid as previously suggested (e.g., byStroeve et al., 2012) for the present warming. These processes maybecome more important as sea ice continues to melt.

Fig. 9. Lagged autocorrelation of monthly mean Arctic sea-ice areafrom 1979-2010. The data is from National Snow and Ice Data Center.The abscissa is the reference month and the ordinate is the corres-ponding lag month. For instance, (4,-7) is for correlation betweenApril and the previous September sea-ice area. Except for the dottedarea, all values are statistically significant (p < 0.05). To aid visuali-zation, the abscissa shows double of the twelve-month period.

40 ASIA-PACIFIC JOURNAL OF ATMOSPHERIC SCIENCES

matically in Fig. 8b.

Point (4) above also needs further discussion. As indicated

by Miller et al. (2010), the apparent universal relationship

(Figs. 1 and 2) may be misleading since the forcing mech-

anisms vary among the different time periods. On the other

hand, there is emerging evidence that La-Niña-like conditions

prevailed during warm periods such as Pliocene (Rickaby and

Halloran, 2005), interglacial periods (Visser et al., 2003), and

interstadials during late Pleistocene (Stott et al., 2002); whereas

El-Niño-like conditions were present during cold periods such

as stadials (Stott et al., 2002), glacial periods (Visser et al.,

2003), and LGM (Koutavas et al., 2002). Therefore, it is pos-

sible that the Arctic amplification, whether it is ultimately

caused by GHG or by orbital forcing (e.g., HTM and LGM

were forced by variations in orbital parameters), may be

realized through the influence of these forcings on tropical

convection.

There are also caveats to the TEAM mechanism. Although

the premise of the theory hinges on the forced tapping of

ZAPE by dynamical warming, it turns out that the tropical

warm pool convection also causes downward IR to increase

over the Arctic, and this process appears to have an even

greater impact on the Arctic surface temperature than does the

dynamic warming (Lee et al., 2011a,b; Yoo et al., 2012b).

Hence, for the TEAM mechanism to be effective, moisture

transport into the Arctic (and subsequent condensation into

cloud liquid water droplets) is critical. Since the moisture

content is higher in warm air than in cold air, it is to be

expected that poleward sensible heat flux caused by the forced

tapping would be also accompanied by an enhanced moisture

flux. Nevertheless, this downward IR is not part of the untapped

ZAPE in the sense of Lorenz (1955), although it is in the sense

of available energy formulated by Bannon (2012).

Secondly, it is unclear whether dynamic warming can occur

over the Arctic under different climatic conditions. Adiabatic

warming is an element of the TEAM mechanism, but the

location of the downward motion (thus the adiabatic warming)

depends on where the westward acceleration occurs (the

location indicated by ⊗ in Fig. 5b), and also on the horizontal

scale of the overturning circulation which depends on Rossby

radius of deformation. Therefore, there is no guarantee that the

downward motion would occur over polar regions under a

different basic state or during a different time period. Similarly,

even though poleward propagating waves can also transport

heat poleward, since the poleward heat flux can occur only if

waves can propagate upward and poleward, and since this in

turn depends on the background state (meridional and vertical

refractive indices for Rossby waves are dependent on the back-

ground state), it is uncertain whether eddy heat flux conver-

gence, driven by tropical convection, would occur under a

different basic state and time period.

Thirdly, the TOA energy balance of equable climates warrants

further discussion. One premise of the TEAM mechanism is

that an equable climate arises in part from a greater Rossby-

wave-driven poleward water vapor and cloud liquid water

transport which leads to an increase in cloudiness and thus a

larger outgoing IR in polar regions. The resulting increase in

the meridional gradient of the TOA net radiative flux by these

waves requires that there be a stronger poleward energy trans-

port. However, this may not be the case if the Arctic is covered

by optically thick, tall clouds, similar to the convective clouds

in the tropics. In this case, the cloud tops may be sufficiently

cold that the TOA radiative deficit over the Arctic may be

small enough for the meridional gradient in the TOA radiative

flux to be greater than that of a colder climate. It could be that

in the initial stage of warming, Arctic amplification occurs

through an enhanced poleward energy flux, such as the TEAM

mechanism, and that as the warming continues and sea ice

melts away, convection may ensue as shown by Abbot et al.

(2009).

In closing, we reiterate that the TEAM theory is incomplete,

but that progress can be made in the topic of polar amplifica-

tion and equable climates by going back to basics and con-

sidering the general circulation. Theoretical pursuit of the polar

amplification problem also points to the need for a more

accurate understanding and better simulation of tropical con-

vection in climate models. The current level of understanding

is apparently inadequate (Stevens and Bony, 2013). The follow-

ing quote by Visser et al. (2003) from a paleoclimate perspec-

tive underscores the need to further develop such a general-

circulation based theory: “A new view of the importance of the

tropics in controlling global climate change is beginning to

emerge, although the nature of the linkage between tropical

and extra-tropical regions still needs to be resolved.”

Acknowledgments: This research was supported by Seoul

National University, the Republic of Korea, and by NSF grant

AGS-1139970, USA.

Edited by: Song-You Hong, Kim and Yeh

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