GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L05804, doi:10.1029/2011GL049922, 2012
Dependence of the climatological axial latitudes of the tropospheric
westerlies and storm tracks on the latitude of an extratropical
oceanic front
Fumiaki Ogawa,1 Hisashi Nakamura,1 Kazuaki Nishii,1 Takafumi Miyasaka,1
and Akira Kuwano-Yoshida2
Received 18 December 2011; revised 9 February 2012; accepted 10 February 2012; published 9 March 2012.
[1] Major “storm tracks”, where migratory cyclones and
anticyclones recurrently develop, are observed around
midlatitude oceanic frontal zones with strong meridional
gradient of sea-surface temperature (SST). A set of atmospheric general circulation model experiments is performed
with zonally uniform SST prescribed at the model lower
boundary. The latitudinal SST profile for each hemisphere
is characterized by a single front. The frontal latitude is
varied systematically from one experiment to another, while
the intensity of the frontal gradient is kept unchanged.
Though idealized, the experiments reveal a climatological
tendency for a low-level storm track to be organized
along or slightly poleward of the SST front if located in the
subtropics or midlatitudes. As a surface manifestation of
an eddy-driven polar-front jet (PFJ), surface westerly axis
tends to form on the poleward flank of the front. This
anchoring effect of the SST front is also hinted at upper
levels, but the climatological positions of the storm track
and PFJ are less sensitive to the frontal latitude. For the
SST front at subpolar latitude, the joint primary axes of
the upper-level storm track and PFJ form in midlatitudes
away from the SST front. Their positions correspond to
their counterpart simulated with a particular SST profile
from which frontal gradient has been removed, suggesting
that the anchoring effect of a subpolar SST front on the
storm track and PFJ is overshadowed by atmospheric internal
dynamics, namely, the self-maintenance mechanism of a
midlatitude storm track and PFJ through their interactions.
Citation: Ogawa, F., H. Nakamura, K. Nishii, T. Miyasaka, and
A. Kuwano-Yoshida (2012), Dependence of the climatological
axial latitudes of the tropospheric westerlies and storm tracks on
the latitude of an extratropical oceanic front, Geophys. Res. Lett.,
39, L05804, doi:10.1029/2011GL049922.
1. Introduction
[2] A midlatitude oceanic frontal zone is a confluent
region of warm and cool ocean currents, characterized by
strong meridional gradient in both sea-surface temperature
(SST) and surface air temperature (SAT). Nakamura et al.
[2004] pointed out that core regions of the major “storm
tracks”, zonally elongated regions where migratory cyclones
1
Research Center for Advanced Science and Technology, University of
Tokyo, Tokyo, Japan.
2
Earth Simulator Center, JAMSTEC, Yokohama, Japan.
Copyright 2012 by the American Geophysical Union.
0094-8276/12/2011GL049922
and anticyclones recurrently develop, are observed along or
just downstream of the major oceanic frontal zones. They
hypothesized that this collocation arises from the anchoring
of a surface baroclinic zone through effective restoration of
cross-frontal SAT gradient via differential heat supply from
the ocean (“oceanic baroclinic adjustment” or “oceanic restoration of surface baroclinicity”), which has been verified
by numerical studies by Nakamura et al. [2008], Nonaka
et al. [2009], Taguchi et al. [2009], Sampe et al. [2010],
and Hotta and Nakamura [2011].
[3] In addition to moisture supply from warm ocean currents [Hoskins and Valdes, 1990; Minobe et al., 2008], the
anchoring effect can be important for maintaining a storm
track and the associated polar-front jet (PFJ), which is driven
by westerly momentum transport from a subtropical jet
(STJ) by transient eddies [Lee and Kim, 2003; Nakamura
et al., 2004]. In the presence of poleward heat flux due to
baroclinic growth of those eddies, PFJ accompanies strong
surface westerlies. They in turn drive ocean currents
[Trenberth et al., 1990], whose heat transport acts to confine
tight SST gradients into narrow frontal zones [Nakamura
et al., 2004]. Therefore, the anchoring effect on both storm
track and PFJ can be of certain importance in understanding
the extratropical atmospheric general circulation interacting
with the underlying ocean. In fact, through their experiments
with an atmospheric general circulation model (AGCM),
Nakamura et al. [2008] and Sampe et al. [2010] showed that
the anchoring effect of SST on a storm track would be
reduced substantially without its frontal gradient.
[4] In the present study, a set of AGCM experiments with
zonally uniform SST prescribed as the model boundary
condition is conducted to examine the sensitivity of the
climatological-mean latitudes of the primary storm track and
PFJ to the latitude of frontal SST gradient. Brayshaw et al.
[2008] and Chen et al. [2010] have revealed notable sensitivity of the latitudinal positions and intensities of a storm
track and attendant PFJ to prescribed SST gradient anomalies. Each of the SST profiles they used is, however, characterized by an unrealistically broad zone of moderate
gradient without realistic frontal gradient. Furthermore,
modifications they added to the SST profile are likely
to alter SST gradient not only in midlatitudes but also in
the Tropics. The latter alternation can directly modifies
the Hadley cell and associated STJ that acts as an uppertropospheric Rossby waveguide, which makes it difficult to
isolate the influence of midlatitude SST gradient. In the
present study, in contrast, the extratropical SST profile is
characterized by a single frontal zone with realistic intensity.
We perform sensitivity experiments by varying the frontal
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Figure 1. Latitudinal profiles of (a) SST and (b) its meridional gradient, prescribed as the lower-boundary condition
for our AGCM experiments. Black lines represent observed
profiles for the South Indian Ocean as an average between
60°E and 80°E, characterized by a distinct front at 45° in
each of the hemispheres. The winter profile is assigned to
the model SH and the summer profile to the NH. Lines with
other colors indicate the profiles for sensitivity experiments,
where the latitude of the SST front is displaced to other latitudes (30°, 35°, 40°, 50°, 55°) while the magnitude of SST
gradient is kept fixed.
latitude from one experiment to another with Tropical and
subtropical SST kept unchanged, to isolate the influence of
the extratropical frontal SST gradient on the troposphere.
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robust statistics. In our aqua-plant setting without any landmass, seasonality of the tropospheric circulation arises
mainly from the equatorial asymmetry in the Hadley cell and
associated STJ, which is controlled mainly by the prescribed
SST profile in the Tropics and subtropics through determining the position of the rising branch of the Hadley cell.
In this particular profile, SST peaks in the NH Tropics,
whereas subtropical SST is apparently lower in the SH. The
model SH (NH) can thus be regarded as the “winter (summer)
hemisphere”. It should be noted that our idealized experiment is not designed for reproducing the observed circulation in the SH. The mean states of storm tracks and
westerlies simulated with this SST profile are almost identical to those for the CTL experiment of Sampe et al. [2010],
which are nevertheless similar to their observational counterpart for the Indo-Australian sector.
[6] Five 60-month integrations were then repeated with
modified SST profiles in which the frontal latitude had been
shifted artificially from 45° equatorward to 30° or poleward
to 55° with 5° intervals. For the modifications, not only the
intensity of the frontal gradient but also the SST profile
equatorward of 25° was kept unchanged (Figure 1b), so as
to fix the thermal forcing on the model Hadley cells
[cf. Brayshaw et al., 2008; Chen et al., 2010]. Our experiments are thus designed to give some insight into the climatological sensitivity of the atmospheric general circulation
to the latitude of an extratropical SST front. Owing to our
aqua-planet setting, the simulated climatological-mean state
exhibits a high degree of zonal symmetry. In the following we therefore present zonally averaged statistics obtained
from our experiments.
2. Experimental Design
[5] The AGCM we used is called AFES (AGCM for
Earth Simulator) [Ohfuchi et al., 2004], with 56 vertical
levels. Its horizontal resolution (T79; equivalent to 150 km
grid interval) is sufficient for resolving the effect of an
oceanic frontal zone on large-scale atmospheric circulation.
The lower boundary of the AGCM is set as the fully global
ocean with six different latitudinal profiles of zonally uniform SST (Figure 1a). With this idealized “aqua-planet”
setting without any landmass, we can eliminate planetaryscale atmospheric stationary waves forced by land-sea thermal contrasts and topography as observed in the Northern
Hemisphere (NH). As in the work by Sampe et al. [2010],
one of these SST profiles was taken from the OISST data
for the South Indian Ocean [6080°E], where the warm
Agulhas Return Current is confluent with the cool Antarctic
Circumpolar Current to maintain frontal SST gradient. The
NOAA Optimum Interpolation (OI) SST V2 is available
at http://www.cdc.noaa.gov/cdc/data.noaa.oisst.v2.html. For
any of its profiles, SST poleward of 70° is set to 1.79°C
and linearly interpolated equatorward to the poleward flank
of the SST front, to realize ice-free condition and eliminate
strong thermal contrast across the sea-ice margin. Examining
its climatic impact is, however, beyond the scope of this
study. The profile for austral winter (Jun.–Aug.) was
assigned to the model Southern Hemisphere (SH) and the
corresponding summertime profile (Dec.–Feb.) to the model
NH. With this SST profile characterized by the frontal gradient at 45° latitude in both hemispheres, the AGCM was
integrated for 60 months under insolation fixed to its solstice
condition after six-month spin-up in order for obtaining
3. Latitudes of Surface Baroclinic Zone and
Midlatitude Low-Level Storm Track
[7] The mean-flow baroclinicity can be measured by the
Eady growth rate s [Eady, 1949], which is proportional to
meridional temperature gradient and inversely to static stability [Hoskins and Valdes, 1990]. Near-surface s, which is
of critical importance for baroclinic development of synoptic-scale eddies, has been evaluated from the climatological
zonal-mean temperature at the 850 and 1000 hPa levels.
Figure 2a shows meridional profiles of the near-surface s in
the “winter” hemisphere for the six experiments. Each of the
profiles exhibits a distinct peak (dots) collocated with the
SST front, reflecting the anchoring of a surface baroclinic
zone through effective oceanic restoration of SAT gradient
across the SST front [Nakamura et al., 2008].
[8] Activity of baroclinically developing transient eddies
can be measured by their poleward heat transport [v′T′],
where the bracket denotes zonal averaging and the primes
denote fluctuations that have been extracted through highpass filtering with a half-power cutoff period of eight days.
It is evident in the climatological-mean profile of 850 hPa
[v′T′] for each of the experiments (dots in Figure 2b), a welldefined single low-level storm track forms in the “winter
hemisphere”. (The storm track latitude exhibits no significant trend during the analysis period.) As summarized in
Figure 2c, the climatological storm track in the “winter
hemisphere” shows a strong tendency to form in the
immediate vicinity of the surface baroclinic zone anchored
by the SST front whose latitude is 40° or higher. Meanwhile, the mean storm track forms slightly poleward of the
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Figure 2. Meridional profiles of the “wintertime” climatologies of (a) Eady growth rate [s] estimated for the 1000–
850 hPa layer and (b) 850-hPa poleward eddy heat flux
([v′T′], K m/s) associated with sub-weekly disturbances.
Based on 60-month AGCM integrations with the SST profiles shown in Figure 1, each of which is characterized by
a front whose position is indicated with a colored triangle.
(c) Diagram showing the climatological-mean latitudes
(ordinate) of maximum [s] (red) and [v′T′] (green) for the
“winter” hemisphere as function of the frontal latitude
(abscissa; also dotted line). (d) As in Figure 2c, but for the
“summer” hemisphere.
SST front whose latitude is 35° or lower. In fact, the surface
storm track in these experiments, defined as the latitudinal
maximum of 1000-hPa [v′T′] is situated at SST front (not
shown). At higher levels, however, the storm track axis
shifts toward a mid-tropospheric baroclinic zone associated
with the midlatitude PFJ, which may be a factor that deviates the eddy activity from its linear relationship with Eady
growth rate.
[9] In the model “summer hemisphere” (Figure 2d), the
low-level storm track forms systematically poleward of
the surface baroclinic zone if the SST front is located at 45°
or equatorward. In contrast, the mean storm track forms
equatorward of the front if located at 55°. The simulated
“summertime” storm track exhibits a clear tendency to stay
in midlatitudes, although it nevertheless shows a noticeable
tendency to follow the latitudinal shift of the SST front.
4. Axial Latitudes of the Surface Westerlies
and Their Eddy Forcing
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transport of westerly momentum via poleward eddy heat
transport, while the upper-tropospheric PFJ is maintained by
the meridional eddy momentum transport mainly from STJ
[Lee and Kim, 2003; Nakamura et al., 2004]. In the transformed Eulerian mean (TEM) framework, eddy forcing of
the surface westerlies can be approximated as the nearsurface divergence of the Eliassen-Palm (E-P) flux [Andrews
and McIntyre, 1976]. The meridional and vertical components of the E-P flux are proportional to the meridional eddy
transport of westerly momentum [u′v′] and heat [v′T′],
respectively. Figures 3a and 3b show the peak latitude of
the climatological-mean eddy forcing for the “winter” and
“summer” hemispheres, respectively, as approximated by
925-hPa E-P flux divergence associated with sub-weekly
disturbances [Edmon et al., 1980].
[11] In the “winter” hemisphere (Figure 3a), the peak
latitude of the eddy acceleration follows the latitudinal shift
of the SST front and low-level storm track (Figure 2c), while
being displaced systematically poleward from the front.
The displacement is consistent with a slight poleward tilt of
the maximum [v′T′] with height (not shown). In a manner
consistent with the eddy acceleration, the axis of the climatological-mean 925-hPa westerlies [U] is systematically
poleward of the front (Figure 3a). The near-surface [U] axis
tends to be farther poleward of the maximum eddy acceleration if the SST front is located at 40° or a lower latitude.
For the SST front at subpolar latitude (50° or 55°), in contrast, the climatological-mean near-surface westerlies exhibit
double jet structure with another axis around 40°. Driven by
eddies away from the SST front, the presence of the secondary branch of the near-surface [U] implies the importance of atmospheric internal dynamics that is not directly
related to the thermal influence of the frontal SST gradient.
[12] In the “summer” hemisphere (Figure 3b), the surface
westerly axis is situated systematically poleward of the SST
front as long as it is at 45° or lower latitude. This poleward
displacement is consistent with the positions of the storm
track and associated eddy forcing. Unlike in the “winter”
hemisphere, no double jet structure emerges for the SST
situated at subpolar latitude (50° or 55°). In this case, the
surface westerly axis again forms in midlatitudes away from
the SST front. This result suggests a large contribution from
internal dynamics via eddy-mean flow interaction to the
maintenance of the surface westerlies. The particular contribution tends to be comparable to or even dominant over
the anchoring effect on the surface westerly jet to the SST
front when located at subpolar latitude, in agreement with
Brayshaw et al. [2008]. Interestingly, the midlatitude westerly axis almost coincides with the single westerly axis
simulated in the NF experiment by Nakamura et al. [2008]
and Sampe et al. [2010], where the frontal gradient is artificially removed from their SST profile (their “NF” profile).
This implies that the thermal forcing of a subpolar SST front
on the lower-tropospheric circulation tends to be less effective than that of a midlatitude or subtropical SST front,
leading to the dominance of the internal dynamics postulated
by Robinson [2006].
5. Latitudes of Upper-Level Storm Track and PFJ
[10] As a surface manifestation of a PFJ, midlatitude surface westerlies are maintained mainly through the downward
[13] Figure 3c shows the climatological-mean axis of an
upper-level storm track in the “winter hemisphere” simulated in each of the experiments, defined as the peak latitude
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Shimpo, 2004], a secondary storm-track forms along the
STJ without any notable baroclinic eddy growth (Figure 3c),
featuring the role of the STJ as a waveguide.
[14] In the model “summer” hemisphere (Figure 3d), the
upper-level sensitivity of the latitudes of the midlatitude
storm track and PFJ to the latitudinal position of the SST
front is qualitatively the same as its wintertime counterpart,
but this particular sensitivity is diminished if the SST front is
located at subpolar latitude (50° or 55°). Located just a few
degrees equatorward of the latitudes of storm track and
maximum eddy acceleration, the summertime midlatitude
jet is essentially a PFJ.
6. Summary and Discussion
Figure 3. (a) As in Figure 2c, but for the latitudes of maximum [U] (blue) and eddy westerly acceleration (orange)
derived from the E-P flux divergence, both at the 925 hPa
level. Gray dashed line indicates the maximum of 925-hPa
[U] simulated in the NF experiment by Sampe et al. [2010]
with SST profile with no frontal gradient. (b) As in Figure 3a,
but for the “summer” hemisphere. (c) As in Figure 3a, but for
the axial latitudes of the 300-hPa storm-track activity ([v′v′],
green) and westerly jet ([U], blue), in addition to the latitudes of maximum eddy westerly acceleration (solid orange)
and deceleration (dashed orange) based on 300-hPa [u′v′]
convergence and divergence, respectively. Gray dashed line
indicates the maximum of 300-hPa [v′v′] simulated in the
NF experiment by Sampe et al. [2010]. (d) As in Figure 3c,
but for the “summer” hemisphere.
of 300 hPa meridional wind variance [v′v′]. Though somewhat less obvious than near the surface, the storm-track axis
still exhibits a tendency to follow the latitudinal displacement of the SST front as long as it is located in the subtropics or midlatitudes. For the SST front at subpolar latitude
(50° or 55°), in contrast, the storm track forms near 45°,
away from the front. Figure 3c also shows a tendency for the
upper-level secondary jet to coincide with the midlatitude
storm track, reflecting its characteristics of an eddy-driven
PFJ. In fact, the convergence of eddy momentum flux [u′v′]
tends to maximize just a few degrees poleward of the secondary jet. Meanwhile, the strongest divergence of [u′v′]
occurs on the poleward flank of a STJ, indicating systematic
poleward transport of westerly momentum by eddies from
the STJ toward the midlatitude PFJ located poleward of
the SST front. As observed in the SH [Nakamura and
[15] Though performed under the idealized setting, our
AGCM experiments have revealed certain sensitivity of the
climatological-mean positions of a midlatitude storm track
and an eddy-driven PFJ to the latitudinal position of an SST
front. The sensitivity, which is unambiguous for the SST
front situated in the subtropics or midlatitudes, is manifested
as the tendency for the positions of the storm track and
PFJ to follow the latitudinal displacement of the SST front.
This tendency arises probably from the anchoring effect of
the SST front on the storm track and associated PFJ by
maintaining the surface baroclinic zone along the SST front,
via effective “oceanic restoration of surface baroclinicity”
[Nakamura et al., 2008] and by supplying moisture to individual storms and thereby energizing them. The particular
sensitivity tends to be stronger in the lower troposphere
than in the upper troposphere, while it is more obvious in
the “winter” hemisphere with an intensified STJ than in the
“summer” hemisphere. If the SST front is situated at subpolar latitude (50° or 55°), however, both the PFJ and upperlevel storm track are situated in midlatitudes away from the
front. Rather, their positions correspond to those realized
without frontal SST gradient [Nakamura et al., 2008; Sampe
et al., 2010]. In other words, the anchoring effect of the
SST front, if located at subpolar latitude, tends to be overshadowed by atmospheric internal dynamics, especially in
the upper troposphere. We argue that the internal dynamics
involve self-maintenance mechanisms of a midlatitude storm
track and eddy-driven jet via eddy-mean-flow interactions
[Robinson, 2006], can be operative regardless of the thermal
forcing from the surface.
[16] It should be pointed out that our results must be
regarded as an upper bound of potential impacts of an
extratropical SST front on the atmosphere, as it is prescribed
for the model in a zonally symmetric manner. Nevertheless,
our preliminary results provide some insight into the fundamental nature of the extratropical atmospheric general
circulation, especially the mean state of a midlatitude
storm track and PFJ, while invoking some fundamental
questions. Specifically, what mechanisms cause the “wintersummer” difference in the sensitivity of their axial positions
to the location of the SST front, and why are their upperlevel positions less sensitive to the frontal location than
their low-level positions? We hypothesize that these hemispherically and vertically contrasting sensitivities may arise
from the corresponding contrasts in the relative importance
between the thermal forcing with an extratropical SST front
and the internal dynamics that acts to situate the storm
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track and PFJ around 40° latitude regardless of the surface
influence. Verification of our hypothesis and investigation
of variability around the mean state both remain for our
future work.
[17] Acknowledgments. We thank the two anonymous referees for
their sound criticism and constructive comments and suggestions on the
earlier version of this paper, which have led to its substantial improvement.
We used the Earth Simulator in support of JAMSTEC. We thank the AFES/
CFES working team of JAMTEC, Y. Kosaka, T. Sampe and A. Goto for
their advices for our experiments. This study is supported in part by the
Japanese Ministry of Education, Culture, Sports, Science and Technology
(MEXT) through the Grant-in-Aid for Scientific Research 22340135 and
that on Innovative Areas 2205 and by the Japanese Ministry of Environment
through the Global Environment Research Fund (S-5).
[18] The Editor thanks the two anonymous reviewers for their
assistance in evaluating this paper.
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A. Kuwano-Yoshida, Earth Simulator Center, JAMSTEC, 3173–25
Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236–0001, Japan.
([email protected])
T. Miyasaka, H. Nakamura, K. Nishii, and F. Ogawa, Research Center
for Advanced Science and Technology, University of Tokyo, 4-6-1
Komaba, Meguro-ku, Tokyo 153–8904, Japan. ([email protected]; [email protected]; [email protected].
ac.jp; [email protected])
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