GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L24607, doi:10.1029/2011GL049902, 2011

Locally enhanced wintertime air-sea interaction and deep oceanic
mixed layer formation associated with the subarctic front
in the North Pacific
Hiroyuki Tomita,1 Shinya Kouketsu,1 Eitarou Oka,2 and Masahisa Kubota3
Received 10 October 2011; revised 9 November 2011; accepted 10 November 2011; published 22 December 2011.

[1] Satellite-derived high-resolution air-sea flux data andArgo float data were analyzed to reveal the fine-scalespatial structures of air-sea turbulent heat and momentum
fluxes and their relationship with the deep oceanic mixed
layer around the subarctic front (SAF) in the North Pacific.
An important feature inferred from the satellite-derived
dataset is the presence of a pair of positive and negative
air-sea flux anomaly bands with a width of 100–200 km on
both sides of the SAF. Such a fine-scale structure in air-sea
fluxes has not been captured in previous datasets. The SAF
plays a significant role in determining the fine-scale structureof air-sea fluxes. The oceanic mixed layer is deeper to thesouth of the SAF, and the maximum mixed layer depth isobserved near the positive anomaly band in air-sea fluxes.
This study concludes that active air-sea interactions due tothe presence of the SAF enhances upward air-sea heat andmomentum fluxes to the south of SAF and selectively
produces a deep oceanic mixed layer where the formation ofTransition Region Mode Water occurs. Citation: Tomita, H.,
S. Kouketsu, E. Oka, and M. Kubota (2011), Locally enhanced wintertime air-sea interaction and deep oceanic mixed layer formation
associated with the subarctic front in the North Pacific, Geophys.
Res. Lett., 38, L24607, doi:10.1029/2011GL049902.

1. Introduction[2] In the western North Pacific, the Kuroshio and Oyashio
approach each other and then turn eastward as the Kuroshio
and Oyashio Extensions, respectively [e.g., Yasuda, 2003].
The Kuroshio and Oyashio Extensions are associated withtwo major oceanic fronts in the upper ocean, namely, theKuroshio Extension front (KEF) and the subarctic front(SAF), respectively. Such mid-latitude upper-oceanic fronts
significantly influence the atmosphere through local air-sea
interaction processes and the associated surface heat flux
(HF) [Xie, 2004; Small et al., 2008; Minobe et al., 2008;
Kwon et al., 2010].
[3] The upper-oceanic fronts are also closely related to thedevelopment of deep oceanic mixed layers (MLs) in thewinter, and thus, to the formation of mode waters [Oka andQiu, 2011]. Using historical hydrographic data [Suga et al.,2004] and Argo float observations [Ohno et al., 2009], the1
Research Institute for Global Change, Japan Agency for Marine andEarth Science Technology, Yokosuka, Japan.
2
Atmosphere and Ocean Research Institute, University of Tokyo,
Kashiwa, Japan.
3
School of Marine Science and Technology, Tokai University, Shimizu,
Japan.

Copyright 2011 by the American Geophysical Union.
0094-8276/11/2011GL049902

presence of two distinct regions of deep winter MLs in thewestern North Pacific has been determined; these regions areassociated with the KEF and SAF. One region of deep ML iscentered at 32°N, 145–150°E to the south of the KEF andcorresponds to the region of formation of the SubtropicalMode Water (STMW) [Masuzawa, 1969]. The other ML iscentered at 42°N, 155–160°E to the southeast of the SAF andis considered to be the region of formation of the recently
identified Transition Region Mode Water (TRMW) [Saito
et al., 2007].
[4] While the formation mechanism of the STMW has
been thoroughly examined, that of the deep winter ML to thesoutheast of the SAF has not been studied in detail [Oka andQiu, 2011]. Recently, the existence of a quasi-stationary jet
along the SAF has been inferred from high-resolution sea
surface temperature (SST) and current data obtained fromsatellite observations [Isoguchi et al., 2006]. This jet is
expected to transport warm and saline water from the south of
the SAF to the surface layer in the region of formation of the
TRMW. This results in an increase in the winter oceanic heatloss and surface water density, leading to the formation of the
deep ML [Saito et al., 2007]. However, it has not been
determined whether a close relationship exists between thewinter distribution of ML depth and the air-sea flux aroundthe SAF; the inability to determine the relationship is probably because of the insufficient horizontal resolution of boththe climatological ML data and the conventional air-sea flux
data derived from atmospheric reanalysis.
[5] In order to better understand the influence of the SAF on
the formation of the deep winter ML to its southeast, highresolution air-sea flux data and in situ hydrographic observation
data are indispensable. Recently, multisatellite observations
with high spatial and temporal resolutions were used toobtain a new air-sea heat and momentum flux dataset known
as the second version of the Japanese Ocean Flux Data Sets
with Use of Remote Sensing Observations (J-OFURO2)[Tomita et al., 2010; Kubota et al., 2002]. This is a potentiallypowerful tool that can be used to describe the fine-scale
structure of air-sea flux over frontal regions. In this paper, weanalyze the high-resolution (0.25°  0.25° grid) version of
the J-OFURO2 air-sea flux dataset and Argo profiling float
data to demonstrate the locally enhanced wintertime air–sea flux and deep ML formation associated with the SAF.

2. Results[6] The location of the two major oceanic fronts, SAF and
KEF, can be inferred from the winter SST and its southward
gradient distributions (Figure 1). The SAF extends northeastward from 39°N, 146°E to 43°N, 158°E along almost thesame route as that described by Isoguchi et al. [2006]. The

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Figure 1. Spatial distribution of sea surface temperature(SST) (contour, °C) and its high-pass filtered southward
gradient (color, °C/100 km) averaged over the period fromJanuary to April from 2002 to 2007 obtained using J-OFURO2
data.
KEF extends eastward from 35°N, 140°E, although it appears
to be weak to the east of 146°E because of variation in its
position [Mizuno and White, 1983; Chen, 2008]. The larger
southward SST gradient along the SAF, compared with thatalong the KEF, reflects the quasi-stationary nature of the SAF[Isoguchi et al., 2006].
[7] Turbulent winter air-sea fluxes, namely, sensible heat
flux (SHF), latent heat flux (LHF), and momentum flux

L24607(MF), averaged over the period from January to April from2002 to 2007, demonstrate the fine-scale spatial structure of
the air-sea fluxes in the open North Pacific (Figure S1 in theauxiliary material).1 Before temporal averaging, a spatial
high-pass filter was applied to highlight variation at scales
smaller than 1000 km (Figure 2). The obtained spatial pattern has not been clearly captured in previous datasets,
including NCEP/NCAR reanalysis (Figures 2 and S1). Thus,
J-OFURO2 is suitable for investigating mesoscale air-sea
interaction in the vicinity of the SAF, which has spatial
scales of <1000 km.
[8] An important feature detected from the J-OFURO2
data is a pair of positive and negative HF anomaly bands witha width of 100–200 km on both sides of the SAF. It is inferredthat the SAF influences the air-sea HFs and enhances oceanic
heat loss to the south of the front. The magnitude of thevariation in the SHF is somewhat larger than that of thevariation in the LHF. This contrasts with the results of previous studies on the air-sea HF around the KEF, where LHF
is found to be larger than SHF [Tokinaga et al., 2009].
[9] The cross-frontal variations in the turbulent heat flux
(THF = SHF + LHF) anomaly and their contribution terms
are shown in Figure 3a. The contribution terms were estimated by decomposing the bulk formula of HFs; this method
has been applied in previous studies [e.g., Tanimoto et al.,2003]. In our formulation, the THF anomaly from thecross-frontal meridional mean of unfiltered data is decomposed into contributions from the ocean (O), atmosphere (A),and surface wind (WND).

1
Auxiliary materials are available in the HTML. doi:10.1029/
2011GL049902.

Figure 2. High-pass filtered spatial distributions (color and black contours) of the sensible heat flux (SHF), latent heat flux
(LHF) (W/m2; positive upward), and momentum flux (MF) (N/m2) anomalies averaged over the period from January to April
from 2002 to 2007 obtained using (a–c) J-OFURO2 and (d–f) NCEP/NCAR reanalysis. The contour intervals of the heat flux
(HF) and MF anomalies are 10 W/m2 and 0.01 N/m2, respectively. High-pass filtered southward gradient of sea surface
temperature (SST) obtained from J-OFURO2 was superimposed for the value of 1.5°C/100 km (white contour).
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Figure 3. Composite cross-frontal sections of anomalous (a) turbulent heat flux and its contribution terms and (b) surface
variables. (c and d) Anomalous turbulent heat and momentum fluxes with their regression and residuals. The fluxes are positive upward anomalies from the meridional mean. All values were averaged over the period from January to April from2002 to 2007. The horizontal axis shows the distance (positive northward) relative to the subarctic front (SAF), whose location was defined as the maximum of the southward gradient of the high-pass filtered SST.
[10] The THF anomaly is positive (negative) to the south(north) of the SAF and changes sharply across the SAF(Figure 3a). This change in the THF anomaly is largely
explained by the air-sea differences in temperature andhumidity, shown as O + A in Figure 3a. Although the contribution of the surface wind speed to the THF anomalies is
considerably smaller (<10 W/m2) than that of the otherterms, it tends to have the same sign as that of the THF
anomalies. The influence of wind speed as a mechanicalforcing mechanism on the upper ocean will be discussed
later.
[11] To understand the physical mechanisms resulting in
the cross-frontal variations and the local maximum of air-sea
flux to the south of SAF, it is helpful to examine the natureof near-surface conditions. While both SST (Ts) and airtemperature (Ta) increase from north to south as expected,
across the SAF, the gradient of Ts is stronger than that of Ta;
this causes the large air-sea temperature difference (Ts - Ta)
to the south of the SAF (Figure 3b). This implies that acrossthe SAF, Ta does not rapidly adjust to SST, and the northwesterly wind transports dry and cold air to the southeast of
the SAF. Thus, the air-sea temperature difference has a local
maximum to the south of SAF.
[12] To estimate the front-induced THF difference acrossthe SAF, the THF anomaly was divided into two, namely,
the anomaly arising from the influence of the large-scale
meridional difference and that arising from the local influence of the SAF (Figure 3c); this was done under theassumption that by studying the linear regression field
information regarding the large-scale field (without local
oceanic front effect) can be obtained. The difference
between THFs on both sides of the SAF at a distance of
75 km from the front (approximately 80 W/m2) is muchlarger than that (approximately 40 W/m2) calculated from thelinear regression field of the THF across the front (Figure 3c).
This suggests that the SAF could cause a meridional difference twice as large as one without the oceanic front.
[13] Surface wind speed (W) also changes significantly
across the front, being 1 m/s larger to the south of SAF than
to the north (Figure 3b). In addition, it exhibits a local
maximum to the south of SAF corresponding to local maxima of THF and Ts - Ta. This is consistent with the verticalmixing mechanism proposed by Wallace et al. [1989] andsupported by observations [Nonaka and Xie, 2003; Tokinaga
et al., 2006]. That is, enhanced THF activates vertical mixing in the atmospheric boundary layer to the south of the
SAF, and, as a consequence, momentum is transported to thesurface from the higher atmosphere. Although the crossfrontal changes in the wind speed do not have a remarkableinfluence on THF (<10 W/m2), the influence of the SAF on
MF is clear (Figure 3d). As in the case of the THF, the SAF
is estimated to enhance meridional difference of momentum
flux twice as large as one without the front.
[14] To investigate the influence of fine-scale air-sea fluxes
on the upper ocean, the distribution of the winter oceanic ML
around the SAF was examined; this distribution is based on

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L24607100–200 W/m2 occurs at approximately 100–400 km distance from the SAF and little heat gain is observed near thefront. On the other hand, to the north of the SAF, the HSR
indicates small heat losses (<50 W/m2). This suggests that theocean requires heat loss of approximately 100–200 W/m2 to
produce the distribution of ML south of the SAF.
[16] To the south of the SAF, the heat loss by air-sea HF isapproximately 150–200 W/m2, which explains the HSR.
Near the SAF heat loss by the air-sea HF greatly exceeds the
HSR. The difference is probably caused by transport of warm
water from the upstream region by the quasi-stationary jet[Isoguchi et al., 2006].
[17] Locally deep MLs approximately 100 km south of the
SAF mostly have salinities lower than 33.9 (Figure 4a),which are characteristic of the TRMW rather than the centralmode water (CMW) [Oka et al., 2011]. On the other hand,MLs approximately 200–300 km south of the SAF arecharacteristic of the CMW, which is characterized by moresaline (>33.9) and warmer waters. This provides strong evidence that a deep ML constituting the source of the TRMW islocally formed south of the SAF.

3. Conclusion

Figure 4. (a) Composite cross-frontal sections of oceanic
mixed layer depth (small colored dots) in the period from January to April from 2002 to 2007. Large black dots and lines
show the average and standard deviation for each bin of
50-km width. Colored bars indicate the proportion of themixed layers deeper than 150 m to all mixed layers. (b) Composite cross-frontal sections of heat storage rate and its budget,
averaged over January to April from 2002 to 2007.
the hydrographic data obtained by Argo profiling floats(Figure 4a). Here, ML depth was defined as the depth atwhich the potential density increases by 0.03 kg/m3 from thevalue at the sea surface, following de Boyer Montegut et al.
[2004]. The winter ML is much deeper on the southern sideof the SAF than on the northern side, with a maximum depth
reached approximately 50–100 km away from the SAF,
where air-sea fluxes also reach their local maxima. This
covariation suggests that mesoscale distribution of air-sea
fluxes in association with the SAF has a significant influenceon the layers in the surrounding upper ocean.
[15] To allow us to discuss the influence of air-sea fluxes
quantitatively, the contribution of the HF to the upper ocean
was estimated. For completeness, the contribution of the MF
through the forcing of entrainment and other factors also
needs to be quantified, but such a detailed analysis will be
performed in a future study. Firstly, the thermal influence of
surface HF on upper ocean temperature was evaluated. The
temporal changes in heat storage between early winter(December to February) and late winter (March to May) were
calculated from Argo temperature profiles of up to 300-m
depth (hereafter, we refer to this as the heat storage rate,
HSR; Figure 4b). A large amount of heat loss occurs in mostpart of the region to the south of SAF; the largest heat loss of[18] A satellite-derived high-resolution air-sea flux dataset, J-OFURO2, and Argo profiling float data during theperiod from January to April from 2002 to 2007 have been
analyzed to reveal the fine-scale structure of air-sea turbulent
heat and momentum fluxes in winter and its relation to theformation of the deep oceanic ML around the SAF. An
important feature detected by J-OFURO2 is a pair of positive
and negative air-sea flux anomaly bands with a width of 100–
200 km on both sides of the SAF. Such a fine-scale structure
in air-sea fluxes has not been captured in previous datasets
including the NCEP/NCAR reanalysis. The SAF plays asignificant role in determining the fine-scale structure of airsea fluxes. We concluded that the active air-sea interaction
attributed to the existence of the SAF enhances the air–sea
heat and momentum fluxes to the south of the SAF andselectively produces the deep MLs where the formation of
TRMW occurs, as inferred by Isoguchi et al. [2006] andSaito et al. [2007].
[19] The mesoscale air-sea interaction and the productionof deep oceanic ML revealed in this study and in precedingstudies [Uehara et al., 2003; Itoh et al., 2011; Kouketsu et al.,2011] have the potential to influence broader-scale ocean
circulations and global climate through the variation in mode
waters. Furthermore, the meridional migration of the SAF on
decadal and interannual timescales [Nonaka et al., 2006] canaffect the processes. To further improve understanding, collaborative studies using both high-resolution satellite-derived
air-sea HF data and Argo float data are important, andintensive and sustainable deploying of Argo floats in theworld oceans, especially in the frontal regions, is essential.
[20] Acknowledgments. The authors thank Shinsuke Iwasaki for help
in preparing the J-OFURO2. This research was supported in part by JAXA
(Japan Aerospace eXploration Agency). The Argo float data used in this study
were collected and made freely available by the International Argo Project andthe national programs that contribute to it (http://www.argo.ucsd.edu, http://
argo.jcommops.org). The author EO was supported in part by the Japan Society
for the Promotion of Science [KAKENHI, Grant-in-Aid for Scientific Research(B) 21340133].
[21] The Editor thanks an anonymous reviewer for his or her assistancein evaluating this paper.

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S. Kouketsu and H. Tomita, Research Institute for Global Change, Japan
Agency for Marine and Earth Science Technology, Yokosuka, Kanagawa
237-0061, Japan. (tomitah@jamstec.go.jp)
M. Kubota, School of Marine Science and Technology, Tokai University,
Shimizu, Shizuoka 424-8610, Japan.
E. Oka, Atmosphere and Ocean Research Institute, University of Tokyo,
Kashiwa, Chiba 277-8564, Japan.

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