JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C10, 3304, doi:10.1029/2002JC001750, 2003

An active role of extratropical sea surface temperature anomalies indetermining anomalous turbulent heat flux
Youichi Tanimoto,1 Hisashi Nakamura,2 Takashi Kagimoto, and Shozo Yamane
Frontier Research System for Global Change, Yokohama, Japan
Received 18 February 2002; revised 23 April 2003; accepted 23 June 2003; published 1 October 2003.

[1] Temporal and spatial structures of turbulent latent and sensible heat flux anomalies are

examined in relation to dominant patterns of sea surface temperature anomalies (SSTA)
observed over the North Pacific. Relative importance among observed anomalies in SST,
surface air temperature, and wind speed in determining the anomalous turbulent heat fluxes
is assessed through linearizing the observed flux anomalies. Over the central basin of theNorth Pacific, changes in the atmospheric variables, including air temperature and windspeed, are primarily responsible for the generation of local SST variations by changing
turbulent heat flux, which supports a conventional view of extratropical air-sea interaction.
In the region where ocean dynamics is very important in forming SSTAs, in contrast,
SSTAs that have been formed in early winter play the primary role in determining mid- andlate-winter turbulent heat flux anomalies, indicative of the SST forcing upon the overlying
atmosphere. Specifically, both decadal scale SSTAs in the western Pacific subarctic
frontal zone and El Niño related SSTAs south of Japan are found to be engaged actively insuch forcing on the atmosphere. The atmospheric response to this forcing appears toinclude the anomalous storm track activity. The observed atmospheric anomalies, which
may be, in part, forced by the preexisting SSTAs in those two regions, act to force SSTAs inother portions of the basin, leading to the time evolution of SSTAs as observed in the course
INDEX TERMS: 4504 Oceanography: Physical: Air/sea interactions (0312); 1620
of the winter season.
Global Change: Climate dynamics (3309); 9355 Information Related to Geographic Region: Pacific Ocean;1223 Geodesy and Gravity: Ocean/Earth/atmosphere interactions (3339); KEYWORDS: sea surface temperature,
ocean-to-atmosphere effect, air-sea interaction, decadal variability, subpolar frontal zone
Citation: Tanimoto, Y., H. Nakamura, T. Kagimoto, and S. Yamane, An active role of extratropical sea surface temperature
anomalies in determining anomalous turbulent heat flux, J. Geophys. Res., 108(C10), 3304, doi:10.1029/2002JC001750, 2003.

1. Introduction[2] As a thermal boundary condition for persistent atmospheric circulation anomalies, it is of certain importance tocapture spatial and temporal characteristics of sea surface
temperature anomalies (SSTAs) on interannual and longer
timescales. On interannual timescales, a horseshoe-shapestructure, characterized by equatorially symmetric SSTAs in
the tropics and zonally elongated monopole-like anomalies
with the opposite polarity in the central North Pacific, ismost dominant in the Pacific SST variability in boreal
winter. This SST pattern is associated primarily with thewell-known Pacific/North American (PNA) and/or Western
Pacific teleconnection patterns in the atmosphere [Wallace
and Gutzler, 1981], which induce basin-wide changes in the
midlatitude surface westerlies. On decadal timescales, adifferent pattern has been identified from a careful selection
1
Also at Graduate School of Environmental Earth Science, Hokkaido
University, Sapporo, Japan.
2
Also at Department of Earth and Planetary Science, University ofTokyo, Tokyo, Japan.

Copyright 2003 by the American Geophysical Union.
0148-0227/03/2002JC001750$09.00

of an analysis domain for modal decomposition of thevariability. Nakamura et al. [1997a] and Nakamura andYamagata [1999] identified decadal SST variability along
the western Pacific subarctic frontal zone (SAFZ) in [36 
44N, 140  180E], which is simultaneously uncorrelatedwith tropical SSTAs, as confirmed in a local correlation
analysis by Tomita et al. [2001]. Since a wave-activity fluxof stationary Rossby waves applied to the associated anomaly patterns of 500-hPa geopotential height is strongly
divergent over the SAFZ and pointing equatorward to thesouth [Nakamura and Yamagata, 1999], those atmosphericanomalies are likely to be locally forced and/or maintained
rather than associated with the extratropical remote responseto the tropical SST forcing.
[3] Since temporal fluctuations of atmospheric teleconnection patterns in the extratropics strongly affect wintertime climatic conditions, atmospheric general circulation
model (AGCM) experiments have been carried out to assess
the relative importance of tropical and extratropical SSTAs
in forcing year-to-year atmospheric variations [e.g., Lau andNath, 1994]. In most of the experiments, distinct remoteresponse was simulated in the North Pacific to the tropical
Pacific SSTAs. The response was realized under both
perpetual and time-varying SSTA forcing. In contrast, most

2-1

2-2

TANIMOTO ET AL.: SST-HEAT FLUX RELATIONof the AGCMs failed to generate any systematic response to
the local extratropical SSTAs even to the perpetuallyprescribed SSTAs. These AGCM experiments carried out
thus far have led to a general consensus that the tropical(extratropical) upper ocean is an active (a passive) medium
to the local atmospheric forcing [see Kushnir et al., 2002].
[4] One of the reasons for the strong model dependenceof extratropical atmospheric response to given local SSTAs
is the differences in the treatment of planetary boundary
layer (PBL) processes among those models. Several experiments in each of which an AGCM is coupled thermallywith a one-dimensional ocean mixed layer [e.g., Lau andNath, 1996, 2001; Watanabe and Kimoto, 2000] indicatedthat the extratropical ocean acts to reinforce overlying
atmospheric teleconnection patterns induced by tropical
SSTAs, modulating their spatial structure and prolonging
their intraseasonal persistence. This weak local feedback
from SSTAs to atmospheric anomalies is known as ‘‘reduced
thermal damping’’ [Kushnir et al., 2002], as elucidated in a
simple, linearized coupled model by Barsugli and Battisti[1998].
[5] Very recently, Xie et al. [2002] showed that a regional
atmospheric model (MM5) with a carefully treated PBL
exhibits a clear sensitivity of regional cyclogenesis to the
sharpness of local SST gradient in the East China Sea. In
addition, the decadal variability confined in the extratropical
Pacific as observed is simulated in a coupled generalcirculation model where air-sea coupling is allowed onlyin the extratropics [Liu et al., 2002]. These numericalexperiments suggest a two-way nature of air-sea interactionin the extratropics, although the aforementioned observational studies have clearly revealed only the atmosphericforcing with no solid evidence for possible, though maybe
subtle, feedback from the ocean. Observational detection of
the latter feedback thus requires a careful diagnosis ofobserved data, including those of turbulent heat fluxes.
[6] As well established, the turbulent latent and sensible
heat fluxes (upward positive) are determined by in situ
scalar wind speed at the sea surface, temperature andmoisture differences, respectively, between that surface
and the bottom of PBL, through the so-called aerodynamic
bulk formulae as below,
QE ¼ ra LCE Ua ðqs ðTs Þ  qa ðTa ÞÞ
ð1Þ
QH ¼ ra cp CH Ua ðTs  Ta Þ:

In these formulae, QE and QH are the latent and sensible
heat fluxes, respectively, ra is the air density, cp the specificheat of air at constant pressure, L the latent heat ofvaporization for water, Ua the scalar wind speed at 10 m
above the sea surface, and CH and CE are bulk coefficientsfor temperature (T ) and humidity (q), respectively. Subscripts s and a indicate values measured at the sea surface
and 10 m aloft, respectively. A change in any of thesevariables leads to anomalies in QE and QH. As mentioned
earlier, the most dominant mode of basin-scale SST
variability is accompanied by large-scale atmosphericcirculation anomalies. Associated changes in scalar windspeed, surface air temperature (SAT) and surface air
humidity are therefore spatially and temporally coherent
to cause large-scale turbulent heat flux variations that force

the basin-scale SSTA pattern. Those atmospheric changes
thus tend to generate temporal variations of the leading SST
mode in a month or two, as clearly shown in many previous
observational studies, [e.g., Davis, 1976; Frankignoul,
1985; Wallace and Jiang, 1987; Iwasaka et al., 1987;
Frankignoul and Kestenare, 2002].
[7] Though most dominant, however, the leading SST
mode cannot, of course, explain all the observed interannual
variance. Modal decomposition analyses, including empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses, tend to define large-scale anomaly
patterns as the leading modes of the variability that, if
combined, explain a major fraction of the total variancewithin an analysis domain. When applied to a basin-scale
SSTA field, an EOF or SVD analysis thus extracts theleading mode of SST variability that is always associatedwith planetary or basin-scale atmospheric anomalies. Since
the ocean has a deformation radius inherently much smaller
than the atmospheric counterpart, those analysis methods
are therefore not particularly suited for extracting physicallysignificant but smaller-scale SSTA patterns induced primarily by oceanic processes.
[8] In addition to the one-dimensional thermal coupling
across the sea surface as discussed in Barsugli and Battisti[1998] and Kushnir et al. [2002], three-dimensional ocean
dynamics is also essential in forming SSTAs in such a region
as near the SAFZ [Qiu and Kelly, 1993; Qiu, 2000]. Carefulinspections of ocean general circulation model (OGCM)
hindcasts, experiments, and assimilated data for several
decades have suggested that interannual to decadal SST
variations near SAFZ or Kuroshio-Oyashio extension arevery much influenced by changes in the ocean conditions inthis inter-gyre region [Xie et al., 2000; Seager et al., 2001;
Schneider and Miller, 2001; Schneider et al., 2002; Tomita etal., 2002]. A steady strong current of the Kuroshio andOyashio extension advects a large amount of heat into theregion, where a dry cold air mass continuously supplied by
the East Asian winter monsoon leads to a huge amount ofheat loss from the sea surface. Therefore changes in either ofthese independent processes or both in the ocean andatmosphere regulate the air-sea temperature difference. Specifically, SSTAs formed through ocean dynamics can act tochange the turbulent heat fluxes above the sea surface, while
anomalies in marine meteorological variables (Ua, qa, andSAT) can act to modify those SSTAs by altering the fluxes.
Most of the previous statistical studies have presented
evidence that the atmospheric variables are primarily
responsible for determining turbulent heat fluxes. In fact,
generally over the extratropical North Pacific, SSTAs arenegatively correlated with local anomalous upward turbulent
heat fluxes, indicative of the predominance of the atmospheric forcing upon the SSTAs [e.g., Hanawa et al., 1995].
A closer look of Figure 6a of Hanawa et al. [1995], however,reveals positive SST-flux correlation hinted in the westerly
boundary current region. Although the spatial resolution of
the data is too sparse to solidly identify the positive correlation as a physically meaningful signal, this positive correlation suggests a potential that SSTAs in that particular
extratropical region can force the overlying atmosphere likein the tropics.
[9] A conventional cross-correlation analysis can show alocal relationship between SSTAs and anomalous turbulent

TANIMOTO ET AL.: SST-HEAT FLUX RELATION

heat fluxes, but it does not tell us which variable is the mostdeterministic. To diagnose relative importance among
anomalies in SST, qa, SAT, and Ua in determining latentand sensible heat flux anomalies at a given instance, we
linearized the formulae (1) and thereby decomposed thetotal amount of turbulent heat fluxes into three components,
following Halliwell and Mayer [1996] as described in
section 2. In that way, we will see whether or not extratropical SSTAs on the interannual and decadal timescales
can play an active role in determining local turbulent heat
fluxes, based on the observational archives.
[10] The rest of paper is organized as follows. Section 2
describes a new data set of linearized turbulent heat fluxes
constructed for the present study. Section 3 shows the local
cross-correlation between SSTAs and other meteorologicalvariables such as the scalar wind speed and surface airtemperature anomalies (SATAs). Section 4 presents total
and linearized heat flux anomalies associated with thedominant patterns of SST variations. Section 5 discusses a
reliability of our results after a summary is given.

2. A Linearized Turbulent Heat Flux Data Set[11] A data set of marine meteorological variables with 2
longitude-latitude resolution has been constructed for the
North Pacific from quality-controlled ship and buoy observations compiled in the Long Marine Reports in fixed
length record (LMRF) of the so-called Comprehensive
Ocean-Atmosphere Data Set (COADS) [Woodruff et al.,1987]. Fields of SST, humidity, SAT, vector and scalarsurface wind were used in constructing fields of latent andsensible heat fluxes. The fluxes have been calculated on thebasis of Kondo’s [1975] aerodynamic bulk method. Detailsof the calculation are described by Tanimoto et al. [1997]
and Tanimoto and Xie [2002]. Although 10-day mean fields
were originally prepared, flux anomaly fields averaged over
two or three winter months are used in this analysis.
[12] As explained in Introduction, latent and sensible heat
fluxes inherently depend on SST, qa, SAT, and Ua. We
linearized each of those fluxes, to assess which is the mostdominant factor in determining the sign of a flux anomaly.
By dividing each of the variables into the climatologicalmean and the deviation from it, each of the fluxes can be
expressed as a sum of the contributions from the climatological mean and anomalies; i.e.,
QE ¼ ra LCE Ua ðqs  qa Þ    ¼ ra LCE Ua þ Ua0  qs T s þ q0s Ts0
  
 
 qa T a þ q0a Ta0

n 


o
QH ¼ ra cp CH Ua Ts  Ta þ Ua0 Ts0  Ta0 :

2-3
ð5Þ

Thus the anomalous fluxes at a particular instance at a given
location can be expressed as follows:
Q0E ¼ QE  QE

¼ ra LCE Ua q0s  Ua q0a þ Ua0 ðqs  qa Þ
h 


io
þ Ua0 q0s  q0a  Ua0 q0s  q0a

ð6Þ

Q0H ¼ QH  QH

¼ ra cp CH Ua Ts0  Ua Ta0 þ Ua0 Ts  Ta
h 


io
:
þ Ua0 Ts0  Ta0  Ua0 Ts0  Ta0

ð7Þ

We focus on the first three terms of the right-hand side ofequations (6) and (7) in the following analysis since the last
two terms have indeed been found negligible. The first threeterms of equation (7) are considered to represent therespective contributions from SSTA, SATA, and Ua0, in thisorder, to the total sensible heat flux anomaly. This
linearization does not necessary mean that these threevariables are always independent of each other. To avoid
any complication in the following section, we will use thesame terminology for the corresponding individual contributions of equation (6) to the anomalous latent heat flux
because surface humidity anomalies strongly depend on
local SATAs. This is reasonable in the light of goodcorrelation generally observed between SATAs and surface
humidity anomalies [Kleeman and Power, 1995]. Weshould note that the bulk coefficients in Kondo’s formulaealso depend on the air-sea temperature difference and scalarwind speed. For simplifying interpretation of our analysis,
however, any contribution from that particular dependency
to the linearized components is ignored in the present study.
[13] Neither spatial smoothing nor filtering for obtaining
better-looking pictures was imposed on the flux fields. As
explained in the Introduction, this study aims to identify arather subtle signal in the extratropical air-sea interaction
whose spatial scale may be much smaller than that of thebasin. In particular, at least 2 resolution is required for acareful analysis of the frontal regions. We take the advantage of avoiding data distortion, despite relatively large
sampling errors that may be included in those unsmoothed
maps with relatively high spatial resolution, even averaged
over the entire winter season.

ð2Þ

3. Local Cross-Correlation Analysis
QH ¼ ra cp CH Ua ðTs  Ta Þ

 
 

¼ ra cp CH Ua þ Ua0  Ts þ Ts0  Ta þ Ta0 :ð3Þ

The climatological-mean heat flux is given as the sum of acontribution from the climatological means of the individual
variables and a much smaller contribution from theiranomalies; i.e.,noQE ¼ ra LCE Ua ðqs  qa Þ þ Ua0 q0s  q0a

ð4Þ

[14] Figure 1c shows a map of local correlation coefficientsover the North Pacific between the January – February –March (JFM) mean SSTAs and the December – January –February (DJF) mean combined latent and sensible heat flux
anomaly. In Figures 1a and 1b, the same SSTAs are locallycorrelated with the individual contributions of Ua0 and
SATAs, respectively, to that flux anomaly. The lead-lag
relationship between the SSTAs and each of the other threevariables is reversed in Figure 2. When anomalies in Ua leadto local SSTAs, their correlation is strongly negative partic-

2-4

TANIMOTO ET AL.: SST-HEAT FLUX RELATION

Figure 1. (a) Map of local cross-correlation between DJFmean scalar wind speed (Ua0 ) contribution as a componentof linearized turbulent heat flux anomalies and JFM-mean
SSTA in the North Pacific. (b) Same as in Figure 1a, but for
SATA contribution (DJF) and SSTAs (JFM). Note that theformer is negatively proportional to SATA. (c) Same as in
Figure 1a, but for the total linearized flux anomalies (DJF)
and SSTAs (JFM). In Figure 1c, a region [35  45N, 140 
165E] of the positive correlation is indicated by an inset
rectangle. Coloring convention is represented at the bottomof the figure.
ularly over the central basin (Figure 1a). The region of thestrongest negative correlation (stronger than 0.6) corresponds to the center of action of the dominant SST variability [Nitta and Yamada, 1989; Tanimoto et al., 1997; Zhang etal., 1997]. This spatially coherent structure reflects thestrengthening (weakening) of the surface westerlies thatenhances (suppresses) local evaporative cooling, in cooperation with additional contributions from enhancement (suppression) of both southward Ekman transport andmechanical vertical mixing. The apparent tendency that theircorrelation becomes weaker and spatially less coherent when
SSTA is taken as the lead variable (Figure 2a) is consistent
with the primary role of the surface westerlies in generating
SSTAs.
[15] In both of the lead and lag correlations, the local
SATA contribution to the flux anomalies, which is negatively proportional to the SATA itself, is negatively correlated with local SSTAs over most of the domain (Figures 1band 2b). The region of the strongest negative correlation ineach of their correlations almost coincides with the regionof the strongest negative correlation in Figure 1a. Thisstrong negative correlation between DJF mean SATA con-Figure 2. Same as in Figure 1, but for the reversed leadlag relationship.
tribution and JFM SSTAs indicates that the enhanced(reduced) coolness of the maritime air associated with thestrong (weak) westerlies under the climatological-mean
eastward SAT gradient also contributes to the enhanced(suppressed) evaporation, as supported by the significant
negative correlation of Ua0 and SATAs over the central basin(Figure 3). In fact, the correlation between total linearized
flux anomalies and the SSTAs is negative over the central
basin (Figure 1c), reflecting the prevailing atmosphericforcing on SSTAs through Ua0 and SATAs. However, evenwhen SSTAs are taken as the lead variable (Figure 2b),
negative correlation is still significant around the center ofaction. This tendency indicates a two-way air-sea interaction
at the sea surface, where SATAs may be adjusted toward athermally more equilibrated state with the underlying

Figure 3. Map of local cross-correlation between DJF
mean scalar wind speed and JFM mean SATA in the North
Pacific. Contour lines (interval: 4C) of the climatological
DJF-mean SAT are superimposed.

TANIMOTO ET AL.: SST-HEAT FLUX RELATION

2-5

Figure 4. (a –c) Difference maps of bi-monthly SSTAs (C) associated with the dominant mode of theNorth Pacific decadal climate variability between the two 4-year periods of 1968/1969 –1971/1972(category DC+) and 1982/1983 – 1985/1986 (category DC; i.e., DC+ minus DC) for the(a) November– December, (b) January – February, and (c) March –April periods. (d – f ) As in Figures
4a – 4c, respectively, but for the difference maps for the ENSO-related SSTAs, based on the composite for1967/1968, 1970/1971, 1973/1974, 1984/1985 winters (category IA+) and 1969/1970, 1972/1973, 1982/
1983, 1987/1988 winters (category IA; i.e., IA+ minus IA). Coloring convention is represented at thebottom of the figure.

SSTAs that have been already generated by the predominant
atmospheric forcing.
[16 ] It is noteworthy that the negative correlation
between Ua0 and SSTA is very weak or their correlation
is even weakly positive in the coastal regions of the basin
and over the western North Pacific (Figures 1a and 2a),which is consistent with a recent study by Nonaka and Xie[2003]. Enhanced vertical mixing in the atmosphericboundary layer over warm SSTAs may contribute topositive correlation coefficients between SSTA and Ua0[Xie et al., 2002, Nonaka and Xie, 2003]. This positive
correlation is indicative of an active role of SSTAs in airsea interaction over those regions. In fact, when the sum of
all the linearized components of the flux leads SSTAs,
their correlation is positive along the SAFZ in [35N 
45N, 140E  165E] indicated by the inset rectangle of
Figure 1c, in which no spatially coherent correlation
structure is observed between the atmospheric contributions to the flux and SSTAs (Figures 1a and 1b). This
positive correlation along the SAFZ suggests that a contribution of SSTA to local heat flux anomalies dominates
over a counteracting contribution from the atmosphericanomalies. In fact, when we take the SSTAs as the leadvariable, positive correlation becomes even more significant and spatially coherent along the SAFZ (Figure 2c).
Even in some other parts of the basin, the correlation turns
into weakly positive. This tendency of the correlation
between SSTA and total turbulent heat flux supports thenotion of the two-way air-sea interaction at the sea surface
in the extratropics. The tendency also suggests the dominant role of oceanic influence in determining the local
turbulent heat flux anomalies over the coastal region and
SAFZ. In the next section, we will assess the relative
importance of individual components of the linearized
turbulent heat flux anomalies observed in association withthe dominant SSTA structures over the central basin andalong SAFZ, separately. This separate treatment of these

two regions is suggested by previous modal decomposition
analyses through which the loadings of the two leading
modes have been found to be fairly well separated, one
over the central basin and the other in and along SAFZ,
respectively [Kawamura, 1994; Deser and Blackmon,
1995; Tanimoto et al., 1997; Nakamura et al., 1997a].

4. Composite Analysis[17] Figure 4 presents two types of spatial structures
dominant in interannual SST variability in the North Pacific.
Nakamura et al. [1997a] and Nakamura and Yamagata[1999] presented that SST time series at several different
locations around SAFZ exhibit pronounced decadal fluctuations that are uncorrelated with the tropical SST variability.
To represent the warm and cold phases of this wintertime
decadal variability in the SAFZ, we selected a pair of 4-year
periods; 1968/1969 – 1971/1972 winters (category DC+) and1982/1983 – 1985/1986 winters (category DC), based on
Figure 3a of Nakamura et al. [1997a]. In Figure 4b, for theJanuary –February period, significant positive SSTAs over
1.0C are confined in the narrow zonal band [36  44N] in
the western half of the domain, indicating SAFZ fluctuations.
From the early winter (November – December; Figure 4a) tolate winter (March – April; Figure 4c), the SSTAs near SAFZ
extend to the east while broadening meridionally with thedevelopment of secondary maxima centered at [30N,
160W] by the March – April period. Note that negative
anomalies localized at two particular grid points [40N,
175W] and [40N, 165W] are less reliable due to erroneous
ship reports included in the original COADS LMRF archives(S. Minobe, personal communication, 2002).
[18] A timescale separation method applied to the North
Pacific SSTA field has revealed that fluctuations withinterannual and decadal timescales are found comparable
in amplitude over the North Pacific [Tanimoto et al., 1997;
Zhang et al., 1997; Nakamura and Yamagata, 1999;

2-6

TANIMOTO ET AL.: SST-HEAT FLUX RELATION

Figure 5. Difference maps of surface wind velocity (arrows) and scalar wind speed (colors; unit: m s1)
between the categories DC+ and DC (i.e., DC+ minus DC) for the bi-monthly periods of(a) December – January and (b) February – March. These bi-monthly periods have been selected in such a
way that the wind-forcing could correspond to SSTA tendencies between Figures 4a and 4b and between
Figures 4b and 4c. Scaling of arrows (unit: m s1) is given at the bottom of each panel. Coloring
convention is represented at the bottom of the figure.
Nakamura and Kazmin, 2003]. The former fluctuations in
the central basin are associated primarily with the El NiñoSouthern Oscillation (ENSO). On the basis of Table 3 ofTanimoto et al. [1997], we chose 1967/1968, 1970/1971,
1973/1974, 1984/1985 winters (category IA+) and 1969/
1970, 1972/1973, 1982/1983, 1987/1988 winters (categoryIA) as the particularly warm and cold winters, respectively, with respect to interannual SSTAs in the central
basin. Three out of four winters in the category IA+ andall the four winters in the category IA thus correspond tothe cold and warm phases of ENSO, respectively. In the
midwinter interannual composite (Figure 4e), positive
SSTAs over 1.0C spread more widely in the latitudinal
direction over the central basin than in its decadal counterpart (Figure 4b), while negative SSTAs are found westof 160E to the south of Japan.
4.1. Decadal Scales (Categories DC ++ and DC
)[19] Composite difference maps between the categories
DC+ and DC have been constructed for surface windvelocity (Figure 5) and the linearized latent heat flux
including its individual components (Figure 6). Those maps
were constructed separately for the December –January andFebruary –March periods. These bi-monthly periods havebeen selected in such a way that the anomalous heat flux
forcing corresponds to the tendency of bi-monthly SSTAs in
Figure 4. These maps correspond to the warm phase for
SAFZ associated with the decadal variability. The sign ofeach of the flux anomalies should be reversed when
discussing the cold phase. We should note here that thepositive (negative) heat flux anomalies mean enhanced(suppressed) heat release for the ocean surface, thus represented with blue and purple (orange and red) colors in thesefigures. This coloring is thus counterintuitive from theviewpoint of the atmospheric thermal forcing.
[20] During the December– January period, surface wind
anomalies (Figure 5a) are anticyclonic mostly over thebasin, accompanied by the weakening of the surface westerlies in the central basin [20  40N, 150E  150W]. A
northerly monsoonal flow also weakens south of Japan andover the East China Sea, whereas the monsoonal northwesterlies prevailing to the north undergo no substantialchanges. Off the west coast of North America, northwesterly wind anomalies weaken the warm air advection fromthe south, acting to form negative SSTAs along the coast(Figures 4b and 4c), in cooperation with the additionalcooling via induced anomalous coastal upwelling. The
aforementioned atmospheric forcing associated with theanticyclonic wind anomalies acts to expand the existing
warm SSTAs eastward and southward in the central part ofthe basin, while generating cool SSTAs along the west coastof North America, which accounts for the seasonal evolution of the observed SSTAs. Though weakened, the anticyclonic wind anomalies still remain to the south of the
Aleutian Islands during the February – March period, reinforcing SSTAs that have been generated by midwinter.
[21] Reflecting those atmospheric anomalies that areincorporated in the individual components of the turbulent
heat flux, the total latent heat flux anomalies shown in
Figure 6a display a southwest-northeast tilted band of suppressed heat release extending from [20N, 120E] all theway to [40N, 160W]. The resultant surface warming
generated in this tilted band during the December – January
period is in good correspondence to the warming SST
tendency observed in the same region (Figures 4a and 4b).
A half degree temperature increase in a mixed layer of 125
m depth is nearly equivalent to 50 W m2 of total (latentplus sensible) surface heat flux anomalies imposed over 2
months, which is consistent with the SSTA changes alongthat tilted band as represented in the composite maps. The
effect of the weakened westerlies (Figures 6g and 5a) andpositive SATAs (Figure 6e) associated with the anticyclonicanomalies are major contributors to the surface warming,
dominating over the compensating effect of the positive
SSTAs (Figure 6c). It can be thus interpreted that theobserved warming within the particular band is forced by
the atmospheric anomalies, and so is the cooling tendency
observed off the west coast of North America.
[22] The situation, however, seems the opposite in the
SAFZ (Figures 6c and 6d), where SSTAs are positive(Figures 4a – 4c) and so are correlation coefficients between
SSTAs and upward turbulent heat flux anomalies (Figures 1cand 2c). In a small region off the east coast of Japan at
40N indicated by the inset rectangle in Figure 6a,TANIMOTO ET AL.: SST-HEAT FLUX RELATION

2-7

Figure 6. Same as in Figure 5, but for the (a, b) total upward latent heat flux anomalies, (c, d) SSTA
contribution, (e, f ) SATA contribution, and (g, h) scalar wind speed contribution to Figures 6a and 6b,
respectively (unit: W m2). An inset rectangle in Figures 6a – 6d indicates the domain [35  45N, 140 
165E], as in Figures 1c and 2c.
turbulent latent heat flux anomalies are upward, acting toyield the surface cooling in the warm SAFZ during theDecember – January period. In this region where SSTAs
and anomalous turbulent heat fluxes are positively correlated
in the rectangles in Figures 1c and 2c, the effect of SATAs istoward the SST warming (Figure 6e), whereas UA0 is notstrong enough to make any significant contribution to theanomalous heat flux (Figure 6g). It is the warm SSTAs that
dominantly contribute to the SST cooling (Figure 6c). In
other words, the observed SSTAs act to force the lower
troposphere, while damping themselves through theenhanced heat release into the atmosphere. It is noteworthy
that the contributions from SSTAs (Figure 6c) and SATAs(Figure 6e), which normally tend to compensate one another,
exhibit different meridional scales in the western Pacific,
perhaps reflecting different deformation radii between atmospheric and oceanic flows. Since SSTAs are significantly
positive only along the narrow SAFZ, this is the only region
where the SSTA contribution to the total latent heat flux
anomalies dominates over the counteracting contributions
from the atmospheric anomalies. As the atmospheric anticy-

clonic anomalies gradually weaken by late winter and early
spring (Figure 5b), the atmospheric contribution to theanomalous heat flux reduces (Figures 6f and 6h), leading to
the increasing role of the SSTA contribution (Figure 6d). As aconsequence, surface cooling along the warm SAFZ
becomes even stronger in late winter, extending its zonal
extent (Figure 6b). The weakening of the atmospheric anticyclonic anomalies, in turn, implies that the SSTA forcing on
the atmosphere, if exists, would be rather modest. Thoughconsiderably weaker, the anomalous sensible heat flux is
distributed spatially in a manner very similar to that of theanomalous latent heat flux (not shown).
4.2. Interannual Timescales (Categories IA++ and IA)[23] Figure 7 shows composite difference maps of thesurface wind velocity and its speed between the categories
IA+ and IA. The most striking feature for the December–
January period is a northwest-southeast tilted band of theweakened westerlies, extending central and eastern portionsof the basin (Figure 7a). To the south, surface wind speed
tends to be stronger by up to 1 m s1 than its climatological2-8

TANIMOTO ET AL.: SST-HEAT FLUX RELATION

Figure 7. Same as in Figure 5, but for difference maps between the categories IA+ and IA (i.e., IA+minus IA).
mean, although the anomalous wind velocity exhibits nocoherent pattern in the same area. Wind speed also tends tobe weaker off the west coast of North America, despite thefact that the alongshore wind component is in the same
sense as in the case shown in Figure 5. Since the alongshore
component is perpendicular to the climatological southwesterlies, the anomalous wind speed depends sensitively on theamplitude of the anomalous surface wind velocity. Thesesurface wind anomalies shown in Figure 7a are associatedwith distinct anticyclonic anomalies over the basin, as aremote response to the tropical SSTAs as in La Niñawinters. Sign of each of the anomalies should be reversed
in discussing the situation for El Niño winters.
[24] In a difference composite map of the total latent heat
flux for the same early winter season (Figure 8a), suppressed heat release prevails over that tilted region of the
reduced wind speed, and the enhanced heat release to its
southwest. The anomalous wind speed contributes only alittle, though positively, to the observed surface flux forcing(Figure 8g). Rather, SATAs are the major contributor to theanomalous heat flux (Figure 8e). The spatial correspondence between the SATA contribution and the anomalous
heat flux is good over the entire basin, except off Baja
California, where the SSTA determines the heat flux
anomaly (Figures 8a and 8c). The reasonably good correspondence between the SATA contribution (Figure 8e) andobserved SST evolution into midwinter (Figures 4d and 4e)suggests the prevailing atmospheric forcing via SATA on
the seasonal evolution of the observed SSTA, which overcomes the counteracting effect from SSTAs, as indicated in
the previous studies.
[25] A difference composite map in Figure 7b indicatesthat the wind speed anomaly pattern undergoes a marked
change by the February –March period. The midlatitude
region of the weakened surface westerlies becomes more
zonally oriented, extending entirely across the basin. A
striking feature found in the latent heat flux anomalies is
the suppressed heat release that occurs off the east coastof Japan and farther to the east (Figure 8b). To the eastof 150E, both the positive SATAs and reduced windspeed (Figures 8f and 8h) contribute to this suppression(Figure 8b). Presumably, the weakened westerlies act toreduce the supply of a cold and dry air mass fromthe Eurasian continent, which apparently contributes tothe maintenance of the local positive SSTAs. Just off thesouthern coast of Japan, in contrast, SATAs have been

negative since early winter (Figures 8e and 8f), despite
the slight weakening of the East Asian winter monsoon(Figure 7). The suppressed latent heat release observed
just south of Japan is found due primarily to the negative
SSTAs in that region (Figure 8d), indicative of oceanic
forcing on the overlying atmosphere. Essentially the samecharacteristics as mentioned above are found in theanomalous sensible heat flux field (not shown).

5. Concluding Remarks[26] We have examined the spatial structures of anomalous turbulent heat fluxes at the sea surface associated withthe dominant SSTA patterns in the North Pacific, by utilizing
a carefully constructed data set with relatively higher resolution (2 longitude-latitude) than currently available data
sets on the basis of in situ ship measurements. Our correlation analysis has revealed a dual role of the turbulent heat
flux anomalies in extratropical air-sea interaction. In thecentral North Pacific, the local SSTA and SATA contributions to the turbulent heat flux anomalies are negatively
correlated under very weak influence of continental air
masses (Figures 1b and 2b). These negative lead-lag correlation coefficients indicate that these two contributions are
not completely independent. The thermal adjustment occurs
at the sea surface in this portion of the basin, leading to themutual compensation between SSTA and SATA contributions (Figures 6 and 8).
[27] As argued by Xie and Seki [1997], however, air-sea
differences in temperature and specific humidity are notalways constant. The SSTA and SATA contributions to the
local turbulent heat flux anomalies are not well correlated
near the SAFZ and Kuroshio extension regions. These
contributions to the flux anomalies have different spatial
scales to one another in the composite maps (Figures 6
and 8). These findings indicate that these contributions are
determined by different processes. Hence it is meaningful toseparate individual contributions, especially in the SAFZ
and coastal regions. In the present study, linearization of theanomalous turbulent heat flux applied to a given phase of
the dominant SST variability has further elucidated thecounteracting contributions from the atmospheric and SST
anomalies to the local heat flux anomalies. In the central andeastern portions of the basin, where the wind anomalies are
relatively strong, the atmospheric contribution to the total
turbulent heat flux anomalies dominates over the SSTATANIMOTO ET AL.: SST-HEAT FLUX RELATION

2-9

Figure 8. Same as in Figure 6, but for difference maps between the categories IA+ and IA (i.e., IA+minus IA).
contribution on both decadal and interannual timescales. In
these regions, atmospheric anomalies thus locally force
SSTA by changing the heat flux, in good agreement witha number of previous studies.
[28] However, in such regions as the narrow SAFZ andthe region south of Japan, SSTAs on decadal and interannual timescales determine the sign of local heat flux
anomalies and thereby act to force the overlying atmospherethermally. In those regions where anomalies in the surface
wind speed tend to be relatively weak, the advective effectof the Kuroshio and/or Oyashio currents estimated withsatellite altimeters probably dominates in determining local
SSTAs over the direct thermal forcing from the atmosphere[Qiu and Kelly, 1993; Qiu, 2002]. The primary importanceof oceanic processes in forming SSTAs near SAFZ has been
already suggested by OGCM simulations [Xie et al., 2000;
Seager et al., 2001; Schneider and Miller, 2001; Schneider
et al., 2002] and by model-assimilated data [Tomita et al.,2002]. Our composite analysis has shown that SSTAs
confined in the SAFZ, which characterize the decadal-scalevariability in the North Pacific [Nakamura et al., 1997a], act
to force the atmosphere thermally, as suggested in a coupled
GCM study by Schneider et al. [2002]. There, as SST

increases, more heat is released into the atmosphere via
turbulent fluxes from the ocean surface, just like in thetropics. This active role of those decadal SSTAs we haverevealed in the air-sea heat exchange within the SAFZ isapparently beyond the effect of reduced thermal damping[Barsugli and Battisti, 1998].
[29] Whether or not the early-winter pattern of atmospheric circulation anomalies shown in Figure 5a is indeed
generated in response to the pre-existing SSTAs in the
SAFZ (Figure 4a), it has been shown in our analysis thatthose atmospheric anomalies force SSTAs with the opposite sign from middle to late winter over the Gulf of
Alaska, so as to form a seesaw-like relationship observed
between the two regions as a characteristic feature of thedecadal SSTA pattern inherent in the extratropical North
Pacific [Nakamura et al., 1997a]. The atmospheric anomalies also act to extend the pre-existing SSTAs in the SAFZ
into the eastern portion of the basin by late winter. It istherefore possible that two-way atmosphere-ocean interaction via anomalous turbulent heat flux over the extratropicalNorth Pacific can account for the observed seasonal evolution of SSTAs and atmospheric anomalies associated withthe decadal climate variability. Essentially the same kind of

2 - 10TANIMOTO ET AL.: SST-HEAT FLUX RELATION

Figure 9. Difference maps of (a) a wave-activity flux
(arrows; unit: m2s2), (b) storm track activity (colored; unit:
K m s1), and (c) geopotential height tendency as a measureof feedback forcing from the storm track (colored; unit: mday1) between the categories DC+ (1968/1969 – 1971/1972) and DC (1982/1983– 1985/1986) for December –January period. In Figures 9a and 9c, a difference map of 250hPa geopotential height (unit: m) is superimposed by contour
lines with 20 (m) intervals. The wave-activity flux in Figure
9a has been evaluated from 250-hPa anomalous geostrophic
stream function and corresponding wind anomalies using aformula of Takaya and Nakamura [2001]. The storm track
activity in Figure 9b is measured as the 850-hPa meridional
heat flux associated with subweekly disturbances [Nakamura
et al., 2002]. The 250-hPa geopotential height tendency in
Figure 9c has been calculated solely from the vorticity flux
convergence associated with subweekly disturbances at thesame level [Nakamura et al., 1997b]. Scaling of arrows is
given at the top. Coloring convention is represented at thebottom of each panel.

SSTA forcing as above on the atmosphere has been foundin the Kuroshio region south of Japan as a part of the
ENSO-associated anomalies, although the spatial structureof those SSTAs is very different from that in the decadal
timescales.
[30] Accompanied by the decadal-scale SSTAs in the
SAFZ (Figures 4a – 4c), stationary anticyclonic anomalies
are observed over the central part of the basin from thesurface (Figure 5) up to the 250-hPa level (Figure 9). A
wave-activity flux developed by Takaya and Nakamura[2001] strongly diverges out of this upper-level PNA-likeanticyclonic anomalies (Figure 9a), which is consistent witha similar diagnosis by Nakamura and Yamagata [1999]
applied to the 500-hPa level. The results suggest that the
PNA-like stationary anomalies are locally forced in association with the decadal-scale SSTAs in the SAFZ. Although
the specifics of the forcing are unknown at this stage, one
possible mechanism may be the involvement of a nearby
storm track, as already suggested by recent AGCM experiments [Peng and Whitaker, 1999; Watanabe and Kimoto,2000; Kushnir et al., 2002]. As shown in Figures 6c and 6d,the SSTA contribution dominates over the counteracting
atmospheric contributions only around the SAFZ locatedslightly north of the Kuroshio axis and its downstream
extension. In the presence of warm SSTAs in the SAFZ
the associated local enhancement of heat release into theatmosphere acts to augment the meridional thermal contrast
near the surface along the northern flank of the SAFZ whilerelaxing the contrast along the Kuroshio axis. The combined
effect would lead to the slight northward shift of a nearsurface baroclinic zone over the western Pacific. Likewise,
the baroclinic zone would be slightly shifted southward in
the presence of decadal-scale cool SSTAs in the SAFZ. In
fact, Nakamura and Kazmin [2003] observed such decadalscale changes in the axial position of the SAFZ. During early
winter when westerlies are modestly strong, the storm track
activity over the North Pacific is positively correlated withthe low-level baroclinicity [Nakamura, 1992]. One would
therefore imagine that the Pacific storm track (i.e., the zonal
band of high activity of atmospheric baroclinic disturbances)would shift northward, in the presence of warm SSTAs in
SAFZ as in Figure 4a that act to shift the baroclinic zone
northward. Figure 9b indicates that is really the case. In thisfigure, the storm track activity is measured by the anomalous
poleward heat flux at the 850-hPa level associated with
subweekly disturbances [Nakamura et al., 2002], on thebasis of the NCEP/NCAR reanalysis data set. The northward
shift of the storm track activity results in the reduction in thepoleward vorticity transport by upper-tropospheric eddies at
35N. The 250-hPa geopotential height tendency due
solely to the anomalous eddy vorticity flux convergence[Nakamura et al., 1997b] exhibits the anticyclonic forcing
over the central part of the basin, reinforcing the existingstationary anticyclonic anomalies (Figure 9c). Thus theoceanic thermal forcing might indirectly reinforce atmospheric circulation anomalies throughout the winter.
[31] Our successful identification of the oceanic thermal
forcing upon the atmosphere in a midlatitude frontal zone
is owing to the characteristics of our data set and analysis
methods. Unlike in most of the previous studies, we
utilized a data set whose spatial resolution exceeds theminimal for resolving meridionally narrow frontal zones.
We avoided applying any basin-wide modal decomposition(EOF or SVD) analysis or any spatial smoothing. Our
usage of carefully analyzed in situ ship observation data
was also critical for our successful analysis. To ensure thefidelity of our observational results, latent and sensible
heat flux data in the NCEP/NCAR reanalyses are also
analyzed. In latent heat flux anomalies in the reanalysis
data (Figure 10), signature of such oceanic forcing on the

TANIMOTO ET AL.: SST-HEAT FLUX RELATION

2 - 11Figure 10. (a, b) Same as in Figures 6a and 6b, respectively, but for the latent heat flux based on the
NCEP/NCAR reanalyses.
atmosphere as identified in our COADS data set is actually
hinted, but the signature is substantially weaker (compare
Figures 6a and 6b) despite the comparable spatial resolutions between the two data sets. Recent re-evaluation
studies of the turbulent heat fluxes for the Atlantic [Sun etal., 2003] and for the western boundary current regions[Moore and Renfrew, 2002] have revealed that excessively
large values of the roughness length for the bulk formulae
were used in the reanalyses in evaluating turbulent heat
fluxes under high-wind situations. Overestimation in thelatent heat flux of reanalyses is also reported in comparisons with in situ observations in Labrador Sea [Renfrew etal., 2002] and those made during WOCE [Smith et al.,2001]. It is therefore likely that the atmospheric contribution is overestimated in the surface heat fluxes of the
NCEP/NCAR and ECMWF reanalyses, which probablymasks the oceanic contribution. Rederiving the fluxes
using surface marine meteorological variables from thereanalyses and TOGA-COARE flux algorithm was successful to reduce the overestimation [Moore and Renfrew,
2002; Sun et al., 2003]. An overall reevaluation of the
turbulent fluxes in reanalysis data sets is desired for theworld ocean.
[32] Acknowledgments. The authors are grateful to S.-P. Xie and
S. Minobe for stimulating discussions. They are also grateful to N. Iwasaka,
R. Zhang, and M. Honda for their preparing the data sets used in the present
study. A number of valuable comments given by two referees and a usefulcomment by the Editor (J. Toole) are appreciated. This work is under the
support of the Frontier Research System for Global Change sponsored by
Japan Marine Science and Technology Center.

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

T. Kagimoto and S. Yamane, Frontier Research System for Global
Change, Yokohama 236-0001, Japan. (kagimoto@jamstec.go.jp; yamane@
jamstec.go.jp)
H. Nakamura, Department of Earth and Planetary Science, University ofTokyo, Tokyo 113-0033, Japan. (hisashi@eps.s.u-tokyo.ac.jp)
Y. Tanimoto, Graduate School of Environmental Earth Science,
Hokkaido University, Sapporo 060-0810, Japan. (tanimoto@ees.hokudai.
ac.jp)