The net heat flux
and meridional temperature advection in the ocean are two factors in the
North Pacific subtropical sea surface temperature front (NPSTF) frontogenesis
occurring from October to the following February. However, the relative
importance of these two factors has been rarely explored. In this study,
frontogenesis of the NPSTF is examined quantitatively based on the
mixed-layer heat budget equation to clarify the relative importance of net
heat flux and meridional temperature advection and to further explore its
connection with the atmosphere above. Diagnosis results show that the net
heat flux dominates the frontogenesis from October to December, while the
meridional temperature advection in the ocean contributes equally as or even
more than the net heat flux in January and February. The atmosphere is
critical to the frontogenesis of the NPSTF, including the direct effect of
the net heat flux and the indirect effect through the Aleutian low. Further
analyses demonstrate that the latent heat flux (the shortwave radiation)
dominates the net heat flux in October (from November to February). The
meridional temperature advection in the ocean is mostly due to the meridional
Ekman convergence, which is related to the Aleutian low. Climatologically,
the strengthening and southward migration of the Aleutian low from October to
the following February are characterized by the acceleration and southward
shift of the westerly wind to the south, respectively, which can drive
southward ocean currents. Correspondingly, the southward ocean currents
provide for colder meridional advection to the north of the NPSTF in January
and February, favoring frontogenesis. In addition, the Aleutian low plays a
role in transforming the dominant effect of the net heat flux into the joint
effect of meridional temperature advection and net heat flux in January.
Introduction
The North Pacific Ocean is featured by two zonal sea surface temperature
(SST) fronts at the midlatitudes and subtropics, respectively. The midlatitude
front, with greater magnitude, is referred to as the North Pacific subarctic
SST front (NPSAF), and the subtropical one is the North Pacific subtropical
SST front (NPSTF). Due to the smaller magnitude, the NPSTF has been rarely
studied. However, it also exerts significant influences on the overlying
atmosphere (Xie, 2004; Kobashi et al., 2008; Wang et al., 2016; Zhang et
al., 2017a, b). On the synoptic scale, Kobashi et al. (2008) found that
the subsynoptic lows along the NPSTF are enhanced by condensational
heating and baroclinicity associated with the NPSTF during April to May. On
the interannual scale, the intensified NPSTF in spring can not only
accelerate the East Asian westerly jet (Zhang et al., 2017a), but also serve
as a precursor to the following La Niña event (Zhang et al., 2017b).
From the perspective of seasonal variation, the NPSAF can exist
throughout the year, but the NPSTF is robust in winter and spring and is
absent in summer and autumn (Fig. 1; Kobashi and Xie, 2012). Thus, several
studies have focused on the frontogenesis and frontolysis of the NPSTF
(Roden, 1975; Kazmin and Rienecker, 1996; Qiu and Kawamura, 2012). It is
pointed out that the net heat flux is responsible for the frontolysis of the
NPSTF (Qiu and Kawamura, 2012; Qiu et al., 2014). In terms of the
frontogenesis, Roden (1975) found that meridional Ekman convergence is the
primary reason for the frontogenesis of the NPSTF. However, Kazmin and
Rienecker (1996) diagnosed the mixed-layer heat budget equation using
observation data from 1982 to 1990 and pointed out that both the net heat
flux and the Ekman convergence are frontogenetic and equally important to
provide the observed frontogenesis in winter, rather than the Ekman
convergence alone. This finding is further confirmed by Dinniman and
Rienecker (1999) based on a 10-year (1985–1995) simulation of a
primitive equation model (Geophysical Fluid Dynamics Laboratory's MOM2).
However, they argued that these two factors are not equally important: the
net heat flux (the Ekman convergence) dominates the frontogenesis in the
western subtropical Pacific (the central and eastern subtropical Pacific).
Thus, the relative role of the net heat flux and the Ekman convergence in
the frontogenesis of the NPSTF remains unclear due to limited data used in
previous studies. Meanwhile, the net heat flux is associated with the
air–sea interaction, and the Ekman convergence is driven by the surface wind
stress, implying that both frontogenesis factors are closely related to the
atmospheric circulation. Kazmin (2017) demonstrated that the long-term
(quasi-decadal) variability of the subtropical SST front is determined by
the variability of the meridional shear of the zonal wind. Thus, the role of
the atmosphere in the frontogenesis of the NPSTF deserves further study.
Climatological meridional SST gradients (∂SST/∂y, units: ∘C (100 km)-1) in
(a) winter, (b) spring, (c) summer and (d) autumn. Winter refers to the
time period of December in the preceding year and January–February in the
current year (DJF). The spring, summer and autumn refer to the time
periods March–Apil–May (MAM), June–July–August (JJA) and
September–October–November (SON), respectively. The black boxes in (a) and
(b) indicate the key area of the NPSTF.
Therefore, this paper aims to figure out the relative importance of the net
heat flux and the oceanic meridional temperature advection (including the
Ekman convergence) in the frontogenesis of the NPSTF, especially the role of
the atmosphere in this process. The rest of the paper is organized as
follows. We introduce the data and methods in Sect. 2. We analyze the
frontogenesis of the NPSTF using the mixed-layer heat budget equation in
Sect. 3 to explore the relative importance of the net heat flux and the
oceanic meridional temperature advection. Section 4 further investigates the
role of the atmosphere in frontogenesis. A conclusion and discussion are
given in Sect. 5.
Data and methodsData
We use monthly ocean temperature, current velocities and wind stress
from the Simple Ocean Data Assimilation (SODA; Carton and Giese, 2008)
version 2.2.4 at 0.5∘×0.5∘ grid resolution with 40 levels from the depth of 5 to 2000 m. We also use surface heat fluxes from
the Objectively Analyzed Air–sea Fluxes Project (OAFlux; Yu and Weller, 2007)
at 2.5∘×2.5∘ grid resolution to examine the mixed-layer
heat budget. All heat fluxes are defined to be positive downward. For
consistency, all variables are interpolated onto a 0.5∘×0.5∘ grid, and they cover the period from January 1984 to December 2009. The ocean temperature at 1.0∘×1.0∘
grid resolution
with 27 levels from the International Pacific Research Center (IPRC) Argo
Product, together with ocean currents (on 40 levels) and surface heat fluxes
at 0.3∘×1.0∘ grid resolution from the NCEP Global Ocean
Data Assimilation System (GODAS; Saha et al., 2006), is used to confirm our
results based on the SODA data. The Argo and GODAS data are interpolated
onto a 1.0∘×1.0∘ grid at 27 depths and only
cover the period from January 2005 to December 2013.
The atmospheric geopotential height and winds used in this study are monthly
ERA-Interim reanalysis from the European Center for Medium-Range Weather
Forecasts (ECMWF; Dee et al., 2011). They are on a 1.5∘×1.5∘ grid and cover the period from January 1984 to December 2009.
The mixed-layer heat budget equation
The temporal variation of SST is governed by mixed-layer dynamics, which can
be represented by the mixed-layer heat budget equation (Dinniman and
Rienecker, 1999; Zhang et al., 2013):
∂SST∂t=-u∂SST∂x-v∂SST∂y-wΔTH+Qnetρ0cpH+R,
where SST denotes sea surface temperature (here, we assume that SST equals
mixed-layer mean temperature), and ΔT represents the temperature
difference between the mixed layer and the interior ocean immediately below
the mixed layer. u and v are mixed-layer zonal and meridional oceanic current
velocities, respectively; w is the vertical velocity at the bottom of the
mixed layer. H is mixed-layer depth. Qnet is the net surface heat flux,
including sensible and latent heat fluxes, as well as longwave and shortwave
radiation. A positive value of Qnet means that the ocean gains heat
from the atmosphere. ρ0 and cp are the density and heat
capacity of seawater, respectively. R is the residual term, including
sub-grid-scale processes and dissipation. The zonal temperature advection
(-u∂SST/∂y), meridional temperature advection (-v∂SST/∂y) and vertical temperature advection (-wΔT/H) are
intrinsic processes in the ocean (Yu and Boer, 2004; Chen et al., 2014),
while the net heat flux term (Qnet/ρ0cpH) represents
air–sea interaction. The SST tendency (∂SST/∂t) in a
particular month is obtained through the central finite difference.
Since the meridional gradient of SST overwhelmingly dominates over its zonal
counterpart in the frontal region, the gradient magnitude (GM) of the NPSTF
is defined as GM=-∂SST/∂y to measure the intensity
of the NPSTF in a particular month (Qiu and Kawamura, 2012; Qiu et al.,
2014). Accordingly, GM is always positive because the climatological mean
SST is higher in the south. Its tendency can be derived from Eq. (1) as
follows:
∂GM∂t=∂∂yu∂SST∂x+∂∂yv∂SST∂y+∂∂ywΔTH2-∂∂yQnetρ0cpH-∂R∂y.
A bigger (smaller) GM indicates a stronger (weaker) NPSTF. A positive GM
tendency (∂GM/∂t) suggests a process through which GM gradually
increases, corresponding to the frontogenesis of the NPSTF. A negative GM
tendency indicates the decreasing of GM, corresponding to the frontolysis of
the NPSTF.
Definition of the mixed-layer depth
Three definitions of mixed-layer depth H are used in this study: (a) SST-TH=0.5∘C (Qiu et al., 2014), where TH is
the temperature at the base of the mixed layer, and the depth of
0.5 ∘C lower than the SST is defined as H. (b) SST-TH=1.0∘C (Suga and Hanawa, 1990), so the depth of
1.0 ∘C lower than the SST is defined as H. (c) The mixed-layer depth
is taken from the GODAS. Figure 2a shows the latitude–time section of the
climatological mean mixed-layer depth calculated by method (a) averaged from
140∘ E to 170∘ W (longitudinal region of the NPSTF in
Fig. 1; Zhang et al., 2017a). The mixed-layer depth exhibits significant
seasonal variation, namely deep in winter and spring with a maximum of
60–80 m and shallow in summer with a minimum of 20 m. Figure 2b shows the
latitude–depth section of the climatological mean zonal current velocities
and ocean temperature gradients averaged in winter and spring when the NPSTF
exists. The maximum center of the ocean temperature gradients (NPSTF) is
mainly located between 24 and 30∘ N at the surface and
could expand downward to the depth of 60 m. The vertical scale of the
maximum center is consistent with the deeper mixed layer in winter and
spring calculated by method (a), suggesting that the variation of
the mixed-layer-averaged temperature gradient can represent the variation
of the NPSTF well. The mixed-layer depth is also computed by methods (b) and (c).
Except for the deeper depth in winter and spring (∼80 m),
their temporal evolutions of the mixed-layer depth agree well with that in
Fig. 2a, and the diagnosis results of Eqs. (1) and (2) do not change
qualitatively (not shown). Therefore, method (a) is used to define the
mixed-layer depth in this study. In addition, two subsurface subtropical
temperature fronts are located between 80 and 180 m in Fig. 2b, associated
with the two branches of the North Pacific subtropical countercurrent,
consistent with the findings of Kobashi et al. (2006). Note that the
eastward velocities are relatively weak over 25–30∘ N where the ocean temperature gradients are strongest. This may be due to
the offset of the salinity gradients, which yield westward zonal velocities
there (not shown).
(a) Latitude–time section of the climatological monthly mean
mixed-layer depth (units: meters) calculated by SST-TH=0.5∘C. (b) Latitude–depth section of the
climatological zonal current velocity (black contour; units: m s-1),
superimposed with ocean temperature gradient (shading; units: ∘C (100 km)-1); both are averaged over winter and spring. All three
fields are averaged zonally over 140∘ E–170∘ W.
Frontogenesis of the NPSTF
Figure 3 shows latitude–time sections of the climatological mean GM and its
tendency averaged over 140∘ E–170∘ W. The GM tendency
is positive and moves southward from September to the following February.
The NPSTF that forms in December is characterized by an SST gradient of 0.6 ∘C (100 km)-1, which is the threshold for the emergence and
disappearance of the NPSTF according to Qiu et al. (2014). Then, it
strengthens and slightly migrates southward until March, with a maximum of
0.9 ∘C (100 km)-1 at 27∘ N. Although the NPSTF is
still robust in spring, it exhibits an evident northward shift with a
strengthening in the northern part and a weakening in the southern and
central parts. It finally disappears in July, consistent with previous
studies (Dinniman and Rienecker, 1999; Qiu et al., 2014). In this study, we
mainly focus on the frontogenesis period of the NPSTF, which is from October
to the following February when the GM tendency is significantly positive. As
the NPSTF is located between 24 and 30∘ N during this
period, the frontogenesis region of the NPSTF is defined as 140∘ E–170∘ W, 24–30∘ N.
Latitude–time section of the climatological monthly mean gradient
magnitude (GM) of the NPSTF (black contour; units: ∘C (100 km)-1) and its
tendency (shading; units: ∘C (100 km)-1 month-1), averaged zonally over 140∘ E–170∘ W.
Latitude–time section of each term (shading; units: ∘C month-1) in Eq. (1) from October to the following February, averaged
zonally over (140∘ E–170∘ W). (a) The total SST
tendency (∂SST/∂t) and (b–f) the components
on the right-hand side of Eq. (1), namely zonal temperature advection
(Uadv), meridional temperature advection (Vadv), vertical temperature
advection (Wadv), the net heat flux (Qnet) and the residual term (R). The
black contours in each panel are the same, indicating the climatological
monthly mean GM (units: ∘C (100 km)-1) averaged zonally
over 140∘ E–170∘ W.
SST variation
Since the NPSTF is characterized by the meridional gradient of SST in the
subtropics, the SST variation during the frontogenesis of the NPSTF is the
first thing we are interested in. Figure 4 portrays the temporal evolution
of each term in Eq. (1) over the NPSTF from October to the following
February. As shown in Fig. 4a, the SST tendency is coherently negative
during frontogenesis, indicating that the SST across the NPSTF gradually
decreases. Note that the SST decreases more quickly in the north than in the
south, corresponding to the strengthening of the NPSTF. This indicates that
the largely decreasing SST in the north should be the key for the
frontogenesis of the NPSTF. A diagnosis of each contributor on the
right-hand side of Eq. (1) is given in Fig. 4b–f. The SST tendency due to
the net heat flux term (Fig. 4e) bears similarities to the SST tendency in
Fig. 4a in terms of spatial pattern and magnitude, while the residual term
(R) is mainly positive and facilitates an increasing SST. As for the oceanic
intrinsic processes, the meridional temperature advection serves as a much
more important factor in determining the SST tendency compared to the zonal
and vertical temperature advections, especially in January and February. In
addition, the meridional temperature advection experiences a significant
southward displacement, which slightly increases the SST across the NPSTF in
October and November and strongly decreases the SST in January and February.
This is similar to the southward migration of the GM tendency during
frontogenesis (Fig. 3). Overall, the SST across the NPSTF gradually
decreases during the frontogenesis, which is mainly attributed to the net
heat flux term with some contributions from the cold meridional advection in
January and February. The residual term acts to suppress this decreasing
tendency.
Same as Fig. 4, except for the terms (units: ∘C (100 km)-1 month-1) in Eq. (2).
GM variation
Figure 5a shows the temporal evolution of the climatological mean GM
tendency across the NPSTF from October to the following February. It is
positive and moves southward during the frontogenesis period, corresponding
to the gradual enhancement of the NPSTF. Similar to the SST tendency from
October to December (Fig. 4), the GM tendency is mainly caused by the net
heat flux term (Fig. 5e), while the residual term acts to suppress the
frontogenesis process (Fig. 5f). In January and February, the net heat flux
term, together with the meridional temperature advection, favors the
frontogenesis of the southern and central NPSTF and suppresses the
frontogenesis of the northern NPSTF. The effect of R is nearly the opposite.
Note that the magnitude of the meridional temperature advection is
quantitatively comparable to that of the net heat flux term in January and
February. The zonal and vertical temperature advections are
negligible due to their smaller magnitudes (Fig. 5b and d). Figure 6a
further shows the regionally averaged GM tendency across the NPSTF during
frontogenesis. The net heat flux term dominates the GM tendency from
October to December and decreases after January. The meridional temperature
advection increases gradually from October to December and plays an
important role in January and February. The residual term (R) mainly exerts
an opposing influence on frontogenesis except in January. These findings
can be quantitatively illustrated in Fig. 6b. The net heat flux term
controls the NPSTF frontogenesis from October to December, while the
meridional advection increases gradually and contributes equally as the net
heat flux in January and February. The results in January and February are
consistent with those in Kazmin and Rienecker (1996); namely, the net heat
flux and the meridional Ekman convergence are equally important for
frontogenesis in winter. In addition, the net heat flux also contributes to
the disappearance of the NPSTF in summer (not shown), which is consistent
with the finding of Qiu et al. (2014).
Figure 6c shows the area mean GM tendency across the NPSTF calculated using
Argo data from 2005 to 2013. Similar to Fig. 6a, the net heat flux term
dominates from October to December and the meridional temperature advection
works in January and February. However, the effect of the meridional
temperature advection is overwhelmingly large in January and February, with
a much smaller net heat flux term and R. This further confirms the dominant
effect of the net heat flux term from October to December and the important
role of the meridional temperature advection in January and February for the
frontogenesis of the NPSTF. Therefore, similar to previous studies
(Kazmin and Rienecker, 1996; Dinniman and Rienecker, 1999), both the net
heat flux and oceanic meridional temperature advection contribute to the
frontogenesis of the NPSTF. As for the relative importance, the net heat
flux dominates frontogenesis from October to December and then the
meridional temperature advection contributes equally as or even more than
the net heat flux in January and February. In addition, although the
magnitude of the net heat flux dominates the GM tendency from October to
December, the variation of GM tendency is not all consistent with that of the
net heat flux term, for example at 26.5∘ N (Fig. 5e). However, the
increasing of the GM tendency corresponds to that of meridional temperature
advection, highlighting the important role of the meridional temperature
advection in frontogenesis.
(a) The area mean GM tendency (units: ∘C (100 km)-1 month-1) over the NPSTF from October to the following February.
(b) The contribution percentages of the right-hand-side terms
in Eq. (2) to the left-hand-side term. (c) Same as Fig. 6a, except using the
Argo data from 2005 to 2013. The black dashed line in (a) and (c) is the GM
tendency of the NPSTF. Green, red, purple, blue and brown indicate zonal
temperature advection (Uadv), meridional temperature advection (Vadv),
vertical temperature advection (Wadv), the net heat flux (Qnet) and the
residual term (R), respectively.
Roles of the atmosphereDecomposition of the net heat flux
The net heat flux term is critical for the frontogenesis of the NPSTF from
October to December, which can be decomposed as follows:
Qnetρ0cpH=QSρ0cpH+QLρ0cpH+QLRρ0cpH+QSRρ0cpH,
where QS, QL, QLR and QSR represent sensible heat
flux, latent heat flux, longwave radiation and shortwave radiation,
respectively. Figure 7 shows the temporal evolution of the GM tendency
induced by individual heat flux terms in Eq. (3). The positive latent heat
flux term primarily contributes to the positive GM tendency in October,
together with the sensible heat flux and the longwave radiation terms. The
shortwave radiation term evidently strengthens in November and December and
appears to be the dominant factor in January and February. Meanwhile, the
other three terms act to suppress frontogenesis, especially the latent
heat flux term. Therefore, the four components of the net heat flux jointly
contribute to the frontogenesis of the NPSTF, with a leading effect of the
latent heat flux term in October and the shortwave radiation term from November
to February. Note that the temporal variation of the net heat flux term is
consistent with that of the latent heat flux term. Moreover, the quick
decrease in the net heat flux term in January is mainly attributed to the
reduction of the latent heat flux term.
The contribution (units: ∘C (100 km)-1 month-1) of the net heat flux term and its individual component to the
GM tendency over the NPSTF from October to the following February: the net
heat flux term (Qnet, blue), sensible heat flux term (QS, green),
latent heat flux term (QL, red), longwave radiation term (QLR,
purple) and shortwave radiation term (QSR, brown).
Cold meridional advection
As discussed above, meridional temperature advection plays an important
role in the frontogenesis of the NPSTF in January and February (Fig. 6),
which transports cold water from the north to decrease the SST across the
NPSTF. Figure 8 gives the meridional Ekman convergence of ∂(VE∂SST/∂y)/∂y calculated by the meridional Ekman
velocity VE=-τx/ρ0fH, where τx is the zonal
component of wind stress and f is the Coriolis parameter. The meridional
Ekman convergence moves southward from October and strengthens in January
and February, similar to the meridional temperature advection (Fig. 5c). In
terms of magnitude, the Ekman convergence also largely contributes to the
meridional temperature advection in frontogenesis. Thus, the meridional
temperature advection in January and February is mostly due to the
meridional Ekman convergence. Note that τx=cDρaU2,
where cD is the drag coefficient, ρa is air
density and U is the surface zonal wind speed. Accordingly, the meridional
Ekman convergence must be associated with zonal wind speed. In the
following, we focus on the possible atmospheric influence on the meridional
temperature advection.
Same as Fig. 5c, except for the meridional temperature advection
term calculated by the Ekman velocity.
Figure 9a shows latitude–time sections of the climatological monthly mean
geopotential height and zonal wind speed at 1000 hPa. The Aleutian low
strengthens and heads southward from ∼48∘ N in
October to ∼35∘ N in February, with the associated
westerly wind enhanced and shifted southward. In theory, the westerly wind
stress covaries with the westerly wind, which can force southward Ekman
ocean currents in the Northern Hemisphere according to VE=-τx/ρ0fH. Thus, the southward meridional ocean currents are obviously
increased and move southward from October to the following February with
the Aleutian low (Fig. 9b). Correspondingly, the cold meridional advection
is enhanced and moves southward, cooling the SST across the NPSTF in January
and February, which is consistent with the southward migration of the
meridional temperature advection in Fig. 4c. Li (2010) found that an Aleutian-low-like anomalous wind stress can decrease the SST in the midlatitude
North Pacific (north of 25∘ N) in numerical models. Further
analysis revealed that cold meridional advection, induced by the
Aleutian-low-like anomalous wind stress, acts to decrease the SST north of
25∘ N. This previous study suggested that the strengthening and
southward migration of the Aleutian low can decrease the SST across the
NPSTF via the cold meridional advection. In addition, both the westerlies
and the southward currents reach the southern latitude of 28∘ N,
resulting in colder SST in the northern NPSTF than in the southern NPSTF,
corresponding to the frontogenesis of the NPSTF. The cooler SST in the
north is also associated with the fact that northern SST cooling
contributes greatly during frontogenesis (Fig. 4a). Thus, the meridional
Ekman convergence dominates the cold meridional advection, which may be
related to the strengthening and southward migration of the Aleutian low
from October to the following February. The associated westerly wind,
together with the wind-driven southward currents, is strengthened and shifts
southward to induce cooler SST in the northern NPSTF, favoring its
frontogenesis.
Latitude–time sections of (a) the climatological monthly mean
geopotential height (shading; units: m2 s-2) and zonal wind speed
at 1000 hPa (black contour; units: m s-1); (b) the climatological
monthly mean meridional ocean currents (units: m s-1). All variables
are averaged zonally over 140∘ E–170∘ W.
Note that the rapid decrease in the net heat flux term in January is mainly
due to the reduction of the latent heat flux term. The latent heat flux term
can be calculated by QL=ρaLCU10m(qs-qa), where
L is the latent heat of vaporization, C is the bulk coefficient, and
U10m represents the 10 m wind speed (Qiu et al., 2014). According to
Eq. (2), the GM tendency is proportional to the meridional gradient of the
10 m wind speed (-∂U10m/∂y). Figure 10 shows the
temporal evolutions of -∂U10m/∂y across the NPSTF and
GM tendency associated with the latent heat flux term. The meridional
gradient of wind speed gradually decreases from October to the following
February, consistent with the GM tendency calculated by the latent heat flux
term, especially from December to February. Interestingly, the
decreasing -∂U10m/∂y is also consistent with the
southward migration of the Aleutian low (blue line in Fig. 10). This
southward shift leads to a gradual increase in the wind speed to the south
of the Aleutian low (to the north of the NPSTF), corresponding to the
decrease in -∂U10m/∂y between the NPSTF and its
northern region, further resulting in the decease in the latent heat flux
term during the frontogenesis. Therefore, the Aleutian low acts to decrease
the effect of the net heat flux and to increase the effect of the meridional
temperature advection during the frontogenesis, which may also play an
important role in transforming the dominant effect of the net heat flux into
the joint effect of meridional temperature advection and net heat flux
in January.
Meridional gradient of 10 m wind speed (-∂U10m/∂y, black, units: 10-5 s-1) and GM tendency calculated
by the latent heat flux (QL, red, units: ∘C (100 km)-1 month-1) over the NPSTF. The
blue curve (AL) is the latitude of
climatological geopotential height at 900 m2 s-2 averaged zonally
over 140∘ E–170∘ W, representing the southward
migration of the Aleutian low.
Conclusion and discussion
Previous studies have demonstrated that both net heat flux and meridional
temperature advection in the ocean contribute to NPSTF frontogenesis
(Kazmin and Rienecker, 1996; Dinniman and Rienecker, 1999). However,
the relative importance of these two factors in frontogenesis is not stated
clearly. In this study, we investigated the frontogenesis of the NPSTF
occurring from October to the following February based on the mixed-layer
heat budget equation and further find that the net heat flux and meridional
temperature advection play different roles in the different periods of
frontogenesis. The net heat flux dominates the frontogenesis of the NPSTF
from October to December, while the meridional temperature advection
contributes equally as or even more than the net heat flux in January and
February. The zonal and vertical temperature advections can be neglected due
to their smaller magnitudes, while R acts to suppress frontogenesis
except in January.
Moreover, the role of the atmosphere in frontogenesis is also explored,
including the direct effect of the net heat flux and the indirect effect
through the Aleutian low. A decomposition of the net heat flux term reveals
that its four components jointly contribute to frontogenesis, with a
leading role by the latent heat flux in October and by shortwave radiation
from November to the following February. Further analyses of atmospheric
effects on the oceanic process show that the meridional Ekman convergence
dominates the meridional temperature advection and is associated with
Aleutian low variation. The strengthening and southward migration of the
Aleutian low are characterized by the acceleration and southward shift of
the westerly wind to the south, which benefits southward ocean currents.
Accordingly, the cold meridional advection due to the southward currents
induces cooler SST in the northern NPSTF than in the southern NPSTF and
favors the frontogenesis of the NPSTF in January and February. In addition,
the reduction of the latent heat flux term (dominating the net heat flux
term variation) during frontogenesis also results from the southward
shift of the Aleutian low, suggesting that the Aleutian low also plays a
role in transforming the dominant effect of the net heat flux into the joint
contributions of meridional temperature advection and net heat flux in
January.
Note that the residual term, including the sub-grid-scale process, is
relatively large in our results, which may be due to the eddy-induced heat
fluxes. Wunsch (1999) noted that eddy-induced heat fluxes are important
relative to the total meridional heat fluxes in western boundary current
regions of the North Atlantic and Pacific Ocean. Moreover, Qiu and Chen (2005) showed that the meridional eddy-induced heat fluxes over the
subtropical North Pacific are both poleward for warm-core and cold-core
eddies. Accordingly, poleward eddy-induced heat fluxes tend to transport
warm water from lower latitudes to the subtropics and benefit the warmer
water there. These findings are consistent with our result that the residual
term leads to increasing SST over the NPSTF. Thus, the eddy-induced heat
flux may play an important role in the residual term to increase the SST and
to further halt frontogenesis (Figs. 4f and 5f). However, it is still
hard to confirm this process at this stage because the spatial and temporal
resolutions of the observations and reanalysis data used in study are relatively
coarse. Thus, further exploration is needed when finer data become
available to us.
Data availability
The SODA
data are from
http://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=4865 (last access: 17 April 2019, Carton and Giese, 2008). The Argo
data are from http://apdrc.soest.hawaii.edu/las/v6/dataset?catitem=194 (last access: 17 April 2019, Li et al., 2017).
The GODAS data are from
https://www.esrl.noaa.gov/psd/data/gridded/data.godas.html (last access: 17 April 2019, Saha et al., 2006). The OAFlux
data are from
ftp://ftp.whoi.edu/pub/science/oaflux/data_v3/monthly/radiation_1983-2009/ (last access: 17 April 2019, Yu and Weller, 2007),
and the ERA-Interim data are from
http://apps.ecmwf.int/datasets/data/interim-full-moda/levtype=sfc/ (last access: 17 April 2019, Dee et al., 2011).
Author contributions
LZ designed the study, analyzed the data and wrote the manuscript. HX
initiated the idea and designed the study. JM, NS and JD contributed to the
study design and writing of the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This work was jointly supported by the National Science
Foundation of China (grant nos. 41575077, 41490643, 41575057, 41705054 and
41805051) and the National Key Research and Development Program of China
(2017YFA0604102). Leying Zhang was supported by the scientific research start-up
funds of Nanjing Forestry University (grant no. 163108056). Jiechun Deng was
supported by the General Program of Natural Science Research of Jiangsu
Province University (grant no. 17KJB170012) and the China Scholarship Council
(grant no. 201808320137).
Review statement
This paper was edited by Anders Levermann and reviewed by
two anonymous referees.
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