JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, C06006, doi:10.1029/2011JC007530, 2012

Flow fields around the Emperor Seamounts detectedfrom current data
Taku Wagawa,1 Yutaka Yoshikawa,2 Yutaka Isoda,3 Eitarou Oka,4,5 Kazuyuki Uehara,6
Toshiya Nakano,7 Kenshi Kuma,3 and Shogo Takagi8
Received 19 August 2011; revised 25 March 2012; accepted 19 April 2012; published 7 June 2012.

[1] Bathymetric influences of the Emperor Seamounts (ESs) on the local and large-scale
flow fields in the subarctic North Pacific were investigated by analyzing data of directly
measured velocity obtained using Argo floats, near-surface buoys, and moored current
meters. The local influences were evident in notable deflections of two relatively strong(0.05 m s1) eastward flows over ESs. A Taylor cap induced by the eastward flow
impinging against the Nintoku Seamount (around 42.0 N, 170.5 E) was clearly identified
in the climatological flow field at 1000 m depth. This flow pattern was also identified in
the near-surface flow field (15 m depth) and dynamic height field at 200 dbar relative
to 2000 dbar, indicating the Taylor cap is stable or frequently generated and extends to thesurface. An anticyclonic flow was found over the Suiko Seamount (around 45.0 N,
170.5 E). The influences on the large-scale flow field were found in data obtained fromcurrent meters moored over the southeastern flank of the Suiko Seamount and Argo
float trajectories. These data indicate for the first time from velocity observations that
there is a seasonally varying boundary current along the eastern side of ESs partiallycompensating for the Sverdrup transport to the east of ESs, and hence, the seasonallyvarying component of the subarctic gyre in the North Pacific is divided into two subgyres
by ESs.
Citation: Wagawa, T., Y. Yoshikawa, Y. Isoda, E. Oka, K. Uehara, T. Nakano, K. Kuma, and S. Takagi (2012), Flow fields
around the Emperor Seamounts detected from current data, J. Geophys. Res., 117, C06006, doi:10.1029/2011JC007530.

1. Introduction[2] The subarctic North Pacific has been the subject ofrecent climate studies because of its major role in the longterm variability of the Earth’s climate [e.g., Mann and Park,
1996; White and Cayan, 1998]. This region has also beenstudied extensively from a fisheries viewpoint because it is
known for high biological productivity supported by nutrientrich water [e.g., Saijo and Ichimura, 1960; Taniguchi and
Kawamura, 1972; Takahashi and Ichimura, 1972; Kasai
1

Tohoku National Fisheries Research Institute, Shiogama, Japan.
Research Institute for Applied Mechanics, Kyushu University, Kasuga,
Japan.
3
Graduate School of Fisheries Science, Hokkaido University, Hakodate,Japan.
4
Atmosphere and Ocean Research Institute, University of Tokyo,
Kashiwa, Japan.
5
Research Institute for Global Change, Japan Agency for Marine-Earth
Science and Technology, Yokosuka, Japan.
6
School of Marine Science and Technology, Tokai University,
Shizuoka, Japan.
7
Global Environment and Marine Department, Japan Meteorological
Agency, Tokyo, Japan.
8
School of Fisheries Science, Hokkaido University, Hakodate, Japan.
2

Corresponding author: T. Wagawa, Tohoku National Fisheries
Research Institute, Shinhama-cho, Shiogama, Miyagi 985-0001, Japan.
(wagawa@affrc.go.jp)©2012. American Geophysical Union. All Rights Reserved.

et al., 1997; Taniguchi, 1999]. To clearly understand themechanisms of climate variation or the habitat selectivityof fisheries resources, we need detailed understanding of
the flow fields in the subarctic North Pacific, which controls horizontal distributions of temperature and salinity(T–S) and various other material properties.
[3] The flow field of the subarctic North Pacific isthought to be strongly affected by bathymetry since density
stratification and hence the baroclinic component of thecirculation in that region is relatively weak over timescalesof a year [e.g., Levine and White, 1981; Roden, 1984; Kono
and Kawasaki, 1997; Ito et al., 2004; Kuragano and
Kamachi, 2004; Wagawa et al., 2006, 2010]. The Emperor
Seamounts (ESs) are notable bathymetric features in thesubarctic North Pacific [Dietz, 1954]. They range approximately from 35 N to 52 N along about 170 E and crossalmost the whole region of the subarctic North Pacific, as
shown in Figure 1. ESs are very high and steep; the top of thehighest seamount is at about 1300 m depth. Therefore, large
bathymetric influences of ESs on the flow field of the subarctic North Pacific are expected.
[4] Bathymetric influences of ESs on a local flow field
and hydrographic structure have been debated by several
studies [Levine and White, 1981, 1983; Roden, 1984, 1987].
Using accumulated expendable bathythermograph data,
Levine and White [1983] showed that surface fronts are
sometimes deflected around the Shatsky Rise and Hess Rise

C06006

1 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

C06006

Figure 1. Bottom topography around the Emperor Seamounts (ESs) in the subarctic North Pacific. The
red dashed line represents the area investigated for the local effects of the ESs (section 3), while the red
dotted line shows the area examined for large-scale effects of the ESs (section 4). The red cross indicatesthe observation point for moored current meters.
(located near ESs; Figure 1) and bifurcated around ESs.
Roden [1984, 1987] analyzed three years of hydrographic
data and showed that surface fronts and corresponding baroclinic flows are deflected around ESs. They suggested that
intense frontal meanders or eddies are associated with steep
bathymetries. Wagawa et al. [2006] analyzed six years ofhydrographic data and found that two surface fronts andcorresponding baroclinic flows approaching ESs merge
when passing over the narrow gap between the seamounts.
Note that these studies analyzed hydrographic data and discussed only the relative geostrophic current (i.e., baroclinic
components), although large barotropic components are
expected in this region. To better understand the flow field
affected by seamount topography, directly observed velocity
data including the barotropic components should beanalyzed.
[5] ESs are expected to affect not only a local flow field asmentioned above but also the large-scale subarctic gyre of
the North Pacific. This gyre is a cyclonic circulation driven
by positive wind stress curl [e.g., Sekine, 1999], and theeffect of ESs on the gyre was first noted by Kono andKawasaki [1997]. From moored current meters, they estimated the Oyashio (the western boundary current of thesubarctic gyre) volume transport and compared it with the
Sverdrup transport. They found that the Oyashio transport
corresponds well with the Sverdrup transport integrated
from ESs to the western boundary (the Japanese coast) rather
than from the eastern boundary of the subarctic gyre (thewest coast of North America). In addition, Ito et al. [2004]
analyzed moored current meter data and satellite altimetry
data; they showed that the Oyashio transport is better correlated with the Sverdrup transport integrated from ESs to
the western boundary for shorter period such as the seasonal
cycle. These findings imply that ESs act as a vertical wall
dividing the subarctic gyre into two subgyres, and that aboundary current like the Oyashio flows along the easternside of ESs. In fact, Kuragano and Kamachi [2004] represented such a boundary current along the eastern side of ESs
in numerical experiments in which satellite and in situ T–S
data were assimilated. Wagawa et al. [2010] examined thedependence of the boundary current on the seamount height
and width and the density stratification in a series of simplified numerical experiments and suggested the high possibility of the presence of the boundary current along theactual ESs. These studies have suggested bathymetricinfluences of ESs on the large-scale subarctic gyre of theNorth Pacific. However, some studies [Isoguchi et al., 1997;
Isoguchi and Kawamura, 2006] considered that thesebathymetric influences are negligible; they described that thewidth of ESs is so small that their effects are unimportant forthe subarctic gyre. To confirm the bathymetric influences ofESs, the presence of the boundary current along ESs should
be validated with in situ current measurements.
[6] One factor that has limited our understanding of thebathymetric influences of ESs is a paucity of direct measurements of velocity at middle and deep layers. Recently,
trajectory data obtained using Argo floats, which can beused to estimate a mid-level or deep velocity field, have been
accumulated. We also moored current meters to measure
mid-level and deep velocities. This study aims to clarify thebathymetric influences of ESs on the flow field of the subarctic North Pacific using these directly measured velocity
data. The data and methods used in the following aredescribed in section 2. Results and a discussion are presented in sections 3–4. A summary is given in section 5.

2. Data Source and Field Observations
2.1. Argo Float Trajectories[7] We used Argo float position data in the region
enclosed by red dashed lines in Figure 1 (35.0 N–55.0 N
and 160.0 E–180.0 E) from February 2000 through2 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

C06006

Figure 2. Data locations (red open circles) of (a) parking-depth velocities obtained from Argo floats
and (b) drift velocities obtained from near surface buoys. The green cross indicates the location of mooredcurrent meters. Symbols “S” and “N” denote the Suiko and Nintoku Seamounts, respectively.
February 2010. A total of 7653 float position data wereobtained by 175 Argo floats with parking depths greater than
1000 m. The data were downloaded from the Web site of the
Argo Global Data Assembly Center.
[8] Velocities at the parking depth were evaluated fromthe float ARGOS position fixes applying extrapolation [e.g.,Park et al., 2005; Menna and Poulain, 2010]; the float
diving and resurfacing locations were first estimated byextrapolating the ARGOS position fixes using the diving
and resurfacing times which are derived from the float’s
metadata and fitting a linear drift and an inertial oscillation
to the position fixes. Unfortunately, the diving and resurfacing times were often inaccurate or not reported by the float.
In such cases, the times were inferred from the times of firstand last ARGOS position fixes for a given surface trajectoryand programmed residence time at the surface [Willis andFu, 2008]. The parking-depth velocities were then obtainedby dividing the distance between the diving and resurfacing
locations by the elapsed time. The velocity data larger than
0.7 m s1 were regarded as error and removed manually[Book et al., 2002]. In our analysis, the position and time ofthe parking-depth velocities were defined as the middle ofthe diving and resurfacing positions and times. The locationof data are indicated in Figure 2a.
[9] The velocity field on a regular grid (0.5  0.5 ) was
estimated using a method of the optimal interpolation in
space for two-dimensional nondivergent vector field[Papoulis, 1972; Bretherton et al., 1976]. Spatial means of
the horizontal velocities were first calculated by taking
arithmetic averages of the parking-depth velocities over thewhole region and the mean velocities were subtracted fromthe parking-depth velocities. The optimal interpolation wasthen applied to the resulting anomaly fields and the mean

velocities were finally added back into the interpolated
anomaly fields. Space covariance functions used in theinterpolation were
1 ∂C ðrÞ
r
r
exp 
;
¼ U2 1 þ
r ∂r
Rc
Rc

ð1Þ


 

∂2 C ðrÞ
r
r2
r
2
exp

;
¼
U
1
þ

∂r2
Rc R2cRc

ð2Þ

f ðr Þ ¼ 

g ðr Þ ¼ 





r
1 r2
r
exp

;
C ðr Þ ¼ F 2 1 þ þ
Rc 3 R2cRc

ð3Þ

where f (r) and g(r) are the longitudinal and ptransverse
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
covariance functions as a function of distance (r = x2 þ y2),respectively, C(r) is the covariance function of the corresponding stream function field and U2 and F2 (=3U2R2c ) are
the variances of velocities and stream functions, respectively. The characteristic length scale (Rc) was set to 100 km.
In the above, the data within 150 km from a cell center (L0 =150 km) were used. Variances of velocities, stream functions
and measurement errors were calculated by extrapolating theobserved structure function [Mizuno, 1995]. The cells where
the ratios of the velocity estimation errors (that are also
obtained from the optimal estimation) to the estimatedvelocities larger than 0.1 were regarded as missing. These
parameters and threshold values were used unless otherwise
noted. We adopted grid spacing (0.5 ≈ 40 km) smaller than
the width of ESs (100  200 km) and the short characteristic
length scale to investigate the effects of the seamounts on adetailed velocity field.

3 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS[10] Since the number of floats was not high enough toillustrate the flow field at each parking depth (111 floats at
1000 m depth, 30 floats at 1500 m depth, 2 floats at 1650 m
depth and 32 floats at 2000 m depth), we calculated thevelocities at a depth of 1000 m from the velocities at theother depths with climatological geostrophic shears [e.g.,Lavender et al., 2005; Willis and Fu, 2008]. The geostrophic
shears were computed from T–S data of the World Ocean
Atlas 2001 climatology [Boyer et al., 2005].
[11] There are two sources of error in the estimates of theparking-depth velocity. One is associated with estimating
either diving or resurfacing locations by extrapolating
ARGOS position fixes. To assess this uncertainty, we artificially predicted the last (n-th; n is the number of ARGOS
position fixes in the surface cycle) point fix by extrapolating
the previous (<n-th) point fixes and compared it with theactual last fix (which is known) in the same way as describedby Park et al. [2005], Chen et al. [2007] and Menna andPoulain [2010]. In the present data set with n ≥ 6, the distances between the predicted and the actual position fixes
had a median of about 740 m.
[12] The second error source is related to the velocity
shear above the parking depth. The velocity shear isassumed to follow the shape of the first baroclinic mode andis constructed from the difference in climatological geostrophic velocities between the surface and 2000 m depth[e.g., Park et al., 2005; Willis and Fu, 2008; Menna andPoulain, 2010] estimated using the T–S data of the World
Ocean Atlas 2001 climatology [Boyer et al., 2005]. The
ascent time (the time taken for ascent from the parking depth
to the surface) was not reported for about 31% of floats. In
such cases, a median ascent time of the remaining 69%
floats was used as an approximation [e.g., Willis and Fu,
2008]. In the present data set, the climatological velocity
difference was 0.011 m s1 and the median ascent time was
4.6 hours. Assuming an ascent rate is constant, the computed extra float drifts during ascent had a median of about
184 m. As argued by Park et al. [2005] and Chen et al.
[2007], we approximated the descending float drift withthe same distance (184 m).
are uncorrelated, the total[13] Since the estimation errors
qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ
uncertainty can be evaluated as 2  ð740 mÞ2 þ 2  ð184 mÞ2 ≃
1078 m. Dividing by 182.0 hours, the average residence timeof the floats below the surface, the total drift uncertainty led
to an error estimate of about 0.002 m s1 for the parkingdepth velocities. This error is small enough to allow detailed
analysis of the relationship between the mid/deep-layer flow
patterns and bathymetry.

2.2. Argo Float CTD Profiles[14] We also used 4215 T–S profiles obtained by 145 Argo
floats from June 2001 to March 2010 in the same region andperiod as those of the trajectory data (the region enclosed by
red dashed lines in Figure 1). Only profiles with a maximum
profiling depth greater than 2000 dbar were used in thepresent analysis to investigate the mid/deep hydrographic
feature. In addition, we used only the profiles evaluated as“good” in the delayed quality controlled data of the Argo
Global Data Assembly Center to avoid pressure bias errors.

C06006

The T–S profiles were vertically averaged in 100 layers from
20 dbar to 2000 dbar at 20 dbar intervals. They were thenhorizontally interpolated for each cell as was done for theparking-depth velocities (section 2.1) except using the spacecovariance function for dynamic height field whose functional form is given by equation (3).
2.3. Surface Drift Buoys[15] To examine the bathymetric influences on the nearsurface flows, drift velocities of near-surface buoys (at 15 m
depth) were used. We used 152,240 drift velocity (304,480
float position) data obtained by 286 buoys in the same region
as that of the Argo float data (the region enclosed by reddashed lines in Figure 1) from November 1990 to September
2009. The data were downloaded from the Web site of theAtlantic Oceanographic and Meteorological Laboratory of
the National Oceanic and Atmospheric Administration(NOAA/AOML). Preliminary quality control of the velocity
data was carried out by NOAA/AOML. Near-surface drift
velocities of each buoy are calculated from the buoy positions transmitted to satellites every 1.2–1.5 hours, and they
are interpolated via kriging to regular six-hour intervals[Hansen and Poulain, 1996]. We computed the wind-driven(Ekman) velocity at 6-hourly intervals along the drifter trackusing the empirical formula of Niiler et al. [2003]
uE ¼ a f 1=2 W expðib Þ;

ð4Þ

where uE = uE + ivE is the Ekman current at 15 m depth,
a = 7  107 s1/2 relates to the speed factor of the winddriven flow with respect to the wind speed, i is the imaginary
unit, f is the Coriolis parameter and W = Wx + iWy is 10 mheight wind of the National Centers for Environmental Prediction (NCEP) reanalysis interpolated to each drifter track
and b = 54 represents the deflection angle of the winddriven flow relative to the wind. The Ekman velocity (uE)was subtracted from the drift velocities. These velocity data
were then averaged every 10 day (over 40 samples) in orderfor its timescale to be matched with that of the parking-depth
velocities of Argo floats. The velocity data were then gridded
using the same optimal interpolation as used for the parkingdepth velocities (section 2.1). The location of data are indicated in Figure 2b.
2.4. Moored Current Meters[16] To investigate the vertical structure and the temporalvariation of flows near ESs, the velocities obtained frommoored current meters were investigated. The observationsof the current velocity with moored current meters (RCM8;
Aanderaa Data Instruments) were conducted for about one
year from July 26 in 2006 to July 7 in 2007 on the southeastern slope of the Suiko Seamount (44.0 N, 170.5 E andwater depth of 3552 m; red cross in Figure 1). The mooring
was deployed on the 169-th cruise of the T/S Oshoro-Maru(Hokkaido University) and recovered on the 180-th cruise. At
the mooring, three current meters at about 1000 m, 2000 m
and 3000 m depths were set and hourly velocity data wereobtained. Tidal harmonic analysis was then performed toremove 49 tidal components: 48 tidal components that canbe decomposed from the M2 tidal component in our

4 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

Figure 3. Climatological velocity vector at 1000 m depth
estimated from Argo float trajectories. In the cells where
the ratios of the velocity estimation errors to the estimatedvelocities are larger than 0.1, the vectors are not depicted
regarding as missing. Both color and length of the arrow represent the velocity magnitude. Black cross indicates the position of the moored current meters. Symbols “S” and “N”
denote the Suiko and Nintoku Seamounts, respectively.

C06006

northern part of the S-shaped meandering of the eastward
flow-passing over the Nintoku Seamount. The S-shaped
flow pattern can also be identified in the climatologicaldynamic height field at 200 dbar relative to 2000 dbar estimated from Argo float T–S profiles (Figure 5). These results
show that the bathymetric influences of the Nintoku Seamount extend to the surface.
[19] A similar flow pattern is found in the summerhydrographic data of Roden [1984] (see his Figure 14) andRoden and Taft [1985]. They suggested that the flow is apart of the Taylor cap formed by the impinging flow on
the Nintoku Seamount. The S-shaped flow pattern foundin the present results is similar to the theoretical pattern of
the Taylor cap [e.g., Verron and LeProvost, 1985]. Note that
the Taylor cap can be identified even in the climatologicalsurface flow field; it is stable or frequently generated andextends to the surface. This result is consistent with results ofChapman and Haidvogel [1992], who performed a series ofidealized numerical experiments for Taylor caps. For stratified flows over a tall seamount (Burger number NH/fL ≥ 1
and Hm/H ≃ 0.8, which are typical around the Nintoku Seamount, where N, H, f, L and Hm are the Brunt-Väisälä frequency (≈0.002 s1), water depth (≈6000 m), the Coriolis
parameter (≈104 s1), seamount width (≈105 m) and seamount height (≈5000 m)), steady Taylor caps can be generated if the Rossby number (Ro = U/fL, where U is theimpinging flow velocity) is below the critical Rossby
number (Roc = 0.15) [Chapman and Haidvogel, 1992].
Around the Nintoku Seamount, the characteristic flow
velocity (U) was smaller than 0.3 m s1 (Figures 3 and 4),

observational period of 345 days (from M2 to q1) and theMf tidal component (which had high energy).

3. Bathymetric Influences of Emperor Seamounts
on the Local Flow Field[17] Figure 3 shows the horizontal distributions of theclimatological velocity vector at 1000 m depth estimatedfrom the Argo float trajectories. There were two relatively
strong (0.05 m s1) eastward flows at 42 N and 45 N to
the west of the ESs (170 E), and these flows were deflected
over the ESs and other major bathymetric features. This
result clearly indicates the bathymetric influences of ESs on
the flow field in the mid/deep layer, which were not obvious
in previous large-scale circulation studies such as Reid
[1997]. Similar features are found in summer hydrographic
data sets of Roden [1984, 1987]. The present analysis (of thedata including all seasons) clarifies that these features are
deep and stable or frequently generated over timescales of ayear.
[18] A more detailed examination of Figure 3 reveals an
anticyclonic flow pattern over the Nintoku Seamount. A
similar flow pattern was found in the climatological surface
flow field derived from surface drift buoys (Figure 4), in
which the anticyclonic flow pattern was embedded in a

Figure 4. Same as Figure 3 except for data of the drift
velocities obtained from near-surface (15 m depth) buoys.

5 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

Figure 5. Climatological dynamic height at 200 dbar relative to 2000 dbar estimated from Argo float T–S profiles.
White thick and thin contours indicate 3000 m depth and5000 m depth, respectively. Black cross indicates the position of the moored current meters. Symbols “S” and “N”
denote the Suiko and Nintoku Seamounts, respectively.
and hence, the Rossby number was well below the criticalvalue (Ro < 0.03 < Roc).
[20] The other eastward flow at 45 N appeared to besteered southward near 167 E–168 E along the northeastern
slope of the Shatsky Rise. The flow then flowed to the eastover the gap between Suiko and Nintoku Seamounts andthen flowed to the north over the eastern flank of the Suiko
Seamount. The climatological surface velocity field(Figure 4) and dynamic height field (Figure 5) also show thisflow pattern. Although the eastward flow at 45 N does notseem to impinge on the Suiko Seamount in the climatological flow fields, close examination of the flow field over the
Suiko Seamount reveals anticyclonic flow over the Suiko
Seamount. This was clear in the high-resolution horizontal
distributions (0.25  0.25 grid) of the velocity vectors(Figure 6). The anticyclonic flow was also evident in the
trajectory paths (Figure 7), which show trapping of several
floats over the seamount.
[21] Roden [1984] and Roden and Taft [1985] found theflow patterns of Taylor caps over the Suiko Seamount anddescribed the caps as being more predominant than the cap
over the Nintoku Seamount. However, such features were
not clearly found in the climatological flow fields; the flow
patterns of Taylor caps was more predominant over theNintoku Seamount than over the Suiko Seamount(Figures 3–5). One possible explanation for the discrepancy
is that the flow does not always impinge against the Suiko
Seamount and the Taylor cap is thus transient; therefore, theC06006Taylor cap over the Suiko Seamount is less clear than thatover the Nintoku Seamount in the climatological field(Figures 3–5). However, the anticyclonic flow was identified
over the Suiko Seamount in the climatological flow field(Figures 3 and 4). The trajectory paths show that some of thefloats trapped over the Suiko Seamount are not traced backto the eastward impinging flow (Figure 7). Different scenario is required for the generation of the anticyclonic eddy.
[22] Current variability over the Suiko Seamount can beinvestigated in detail using the velocity data obtained frommoored current meters. The daily means of the processedhourly velocity data were calculated by taking the 25-hourrunning mean and subsampling at intervals of 1 day.
Figure 8 shows the daily current velocities during ourobservation. Noticeable temporal variations are evident, andthey were larger at greater depths; root-mean square velocity
fluctuations were 0.014 m s1 at 1000 m depth, 0.019 m s1
at 2000 m depth and 0.021 m s1 at 3000 m depth. Figure 9
represents the rotary spectrum of the current velocity. Significant peak in the spectrum was found at 10–12-day (240–288-hour) period at 3000 m depth, and power spectra werelarger at greater depths; total rotary spectra at 10-day (240hour) period were 6.1  107 (m s1)2 cph1 at 1000 m
depth, 6.1  106 (m s1)2 cph1 at 2000 m depth and 1.6 
105 (m s1)2 cph1 at 3000 m depth. Sources of thesebottom-intensified disturbances are unknown; they might be
excited by barotropic flows induced by the meandering of
the oceanic frontal jet or atmospheric disturbances andpropagate as bottom-trapped waves. Energetic eddies shownin Figure 7 seem related to those variations. In fact, Isoda
and Fukai [2004] reported that there were often eddy-likemotions over the Suiko Seamount from analysis of satellite
altimetry data.
[23] These data allow us to speculate that the anticycloniceddy over the Suiko Seamount is generated by eddy-

Figure 6. Same as Figure 3 except for a high-resolution
grid (0.25  0.25 ) cells with Rc = 50 km and L0 = 75 km
in order to show the anticyclonic circulation over the Suiko
Seamount.

6 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

C06006

Figure 7. Trajectory paths of Argo floats around the Nintoku and Suiko Seamounts illustrated byconnecting the locations of the parking-depth velocities.
topography interaction. It is well known that the mean current can be generated if these perturbations mix the potential
vorticity of water across the slope [e.g., Holloway, 1987;
Adcock and Marshall, 2000]. Such current is anticyclonicover a seamount. Thus, eddies may be cause of the formationof anticyclonic flow over the Suiko Seamount found in the
climatological flow fields.

4. Bathymetric Influences of Emperor Seamounts
on the Subarctic Gyre of the North Pacific[24] Kuragano and Kamachi [2004] simulated theboundary current along the eastern side of ESs in theirnumerical assimilation. Wagawa et al. [2010] suggested thehigh possibility of the presence of the boundary current
along the ESs. If the boundary current actually exists, it
should covary with the interior Sverdrup flows to the east of
ESs on shorter time-scales. This is because in the subarctic
North Pacific where the density stratification is weak, thebarotropic Rossby waves excited at the eastern boundary(the west coast of North America) propagate to ESs within aweek whereas the baroclinic Rossby waves take more time(>10 years) to reach ESs and hence are damped to a larger
degree before arriving at the ESs. The short baroclinic
Rossby waves excited at the ESs due to incident long barotropic waves from the eastern boundary and the local wind
forcing [e.g., Wang and Koblinsky, 1994] are expected to bedamped quickly because of its large wave number and small
group velocity [Wagawa et al., 2010]. Thus, the phase of
shorter fluctuations such as a seasonal variation in theboundary current is expected to be less affected by the baroclinic waves as found by Qiu [2002] and Wagawa et al.
[2010]. This section focuses on the seasonal variations in
the boundary current along ESs and the interior Sverdruptransport to the east.
[25] Figure 10 shows the horizontal distribution of theclimatological monthly wind stress curl in winter (February)and summer (August) estimated from the daily wind stressdata set of the NCEP reanalysis data set [Kalnay et al.,1996]. Prominent seasonal variation in the wind stress curl
field can be seen in the subarctic North Pacific; the windstress curl was largest (smallest) in winter (summer).

7 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

C06006

Figure 8. Time series of daily current velocities during our observation of current meters moored on thesouthern slope of the Suiko Seamount.
Figure 11 shows the climatological seasonally varying
component (which is defined as anomaly from the annual
mean component) of the Sverdrup transport estimated byintegrating the wind stress curl over 171.6 E–229.7 E andaveraging meridionally over 41.9 N–51.4 N. It is found thatthe seasonally varying component of the Sverdrup transport
was northward (southward) from December to April (fromMay to November). If there is a boundary current along theeastern side of ESs, its direction should be opposite to that of
the Sverdrup transport.
[26] The current meters moored on the southeastern slopeof the Suiko Seamount were used again to investigate seasonal variations in the current along the eastern side of ESs.
Figure 12 shows low-pass filtered daily current velocity
anomalies relative to the annual mean obtained by taking the
40-day running mean to remove shorter fluctuations. Notethat the amplitude (≃0.010 m s1) and direction (71.8
clockwise from the north) of the annual mean velocity at1000 m depth (denoted by blue vector on the left side ofeach panel in Figure 12) almost corresponded to the amplitude (0.017 m s1) and direction (81.8 clockwise from thenorth) of the Argo float parking depth velocity at the cell of
44.0 N, 170.5 E on the mooring point (Figure 3). It is foundthat the seasonally varying component of the velocitychanged from northward to southward in December at all
three depths; the flow was southward (northward) from
December to June (from August to November). The timingof this directional change corresponded well to the timing of
the directional change in the interior Sverdrup transport fromsouthward to northward (Figure 11). The period of thenorthward velocity component (August to November;
Figure 12) was not inconsistent with that of the southward
Sverdrup transport (Figure 11) although the opposite directional change from southward to northward in the daily
moored velocity time series could not be confirmed owing to
the lack of sampling from late June to early August(Figure 12). The weakening of the southward flow in Marchand April seemed to be due to eddies with periods less than
90 days. The coefficients of correlation between the meridional components of the daily moored velocities (Figure 12)
and the interior Sverdrup transport obtained from the NCEP
reanalysis data set in the same period were 0.60 at 1000 m
depth, 0.54 at 2000 m depth and 0.70 at 3000 m depth.
Therefore, the phase of the seasonally varying component of
the velocities obtained from the current meters moored for345 days (Figure 12) corresponded well to that of the climatological interior Sverdrup transport (Figure 11).

8 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

Figure 9. Rotary spectra (m2 s2 (cycle per hour)1) of the current velocity from current meters mooredon the Suiko Seamount at (a) 1000 m, (b) 2000 m and (c) 3000 m depths. The total rotary spectra (solid
thick lines) are shown in each diagram, as well as the clockwise (dotted lines) and anticlockwise (dashedlines) rotary components. Vertical bar is 95% confidence interval. The upper horizontal axis shows periods(unit: hour) whereas the lower horizontal axis represents frequency (unit: cycles per hour).
9 of 14

C06006

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

C06006

Figure 10. Horizontal distribution of the climatological monthly wind stress curl estimated from the
NCEP reanalysis data set: (a) February and (b) August. Contour interval is 107 N m3.

[27] The vertical structure of the low-pass filtered velocities (Figure 12) was highly barotropic; root-mean squarevelocity fluctuations were about 0.010 m s1, 0.013 m s1
and 0.012 m s1 at 1000 m, 2000 m and 3000 m depth,
respectively. These velocity amplitudes were of the sameorder (O (102 m s1)) as those in Kuragano and Kamachi[2004] and Wagawa et al. [2010]. All the results strongly
suggest the existence of boundary current along ESs, indicating that ESs divide the subarctic gyre of the North Pacificfor shorter period fluctuations.
[28] To determine the horizontal structure of the boundary
current, we attempted to extract the seasonal variation in theparking-depth velocities from the Argo float trajectories. We
here adopted larger parameter and threshold (Rc = 2000 km
and L0 = 2500 km) and used the data in the extended region
enclosed by red dotted lines in Figure 1 (35.0 N–55.0 N and
142.0 E–210.0 E) to extract the flow field of large-scale
subarctic gyre of the North Pacific. Figures 13a and 13b
show the velocity field at 1000 m depth in the 6-monthmean (December to May (uI) and June to November (uII)).
The mean periods were determined on the basis of themagnitude of the climatological interior Sverdrup transport
to the east of ESs (Figure 11). Although eastward flows
dominate as suggested by Reid [1997] in both periods, there
are also differences in these two velocity fields. In order todecompose the flow field into annual mean component andseasonally varying component, we also show (uI + uII)/2(which corresponds to the annual mean component) and(uI  uII)/2 (which corresponds to the seasonally varying
component) in Figures 13c and 13d, respectively. The
annual mean field shows eastward flow as a whole, whichcorresponds to climatological flow field by Reid [1997].
Note that annual mean field is nontopographic because of

the baroclinic Rossby wave adjustment. The seasonallyvarying component shows the southward current of a few
cm s1 along the eastern side of the ESs when the windstress curl was positive (December to May; Figure 13d).
Although the current was less clear in 45.5 N–47.5 N, we
consider that it is caused by eddies which contaminate seasonally varying flow field. The seasonal change in current
direction was consistent with that expected from the
Sverdrup transport and its velocity amplitude was of thesame order (O (102 m s1)) as those in the moored current
meters (Figure 12), Kuragano and Kamachi [2004] andWagawa et al. [2010]. These results indicate that ESs divide

Figure 11. Seasonally varying component (anomaly fromthe annual mean component) of the interior Sverdrup transport to the east of the Emperor Seamounts (ESs) calculated
from the climatological wind stress curl integrated from theeastern boundary to the ESs (171.6 E–229.7 E) and averaged meridionally over the region of 41.9 N–51.4 N.

10 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

C06006

Figure 12. Same as Figure 8 except that shorter fluctuations are subtracted by taking the 40-day runningmean and that the annual mean is removed. Blue vector on the left side of each panel indicates the annualcomponent of the velocity at each depth.
the seasonally varying component of the subarctic gyre into
two subgyres.

5. Summary and Discussion[29] Bathymetric influences of Emperor Seamounts (ESs)on the flow fields in the subarctic North Pacific were
investigated by analyzing data of the directly measured
velocity. The influences were examined from two viewpoints: influences on the local flow field in the vicinity of
ESs and influences on the large-scale subarctic gyre of theNorth Pacific. Climatological mean velocity fields were
investigated for the former influences while seasonal variation in the current along the eastern side of ESs was examined for the latter influences. Our findings are:[30] 1. Two relatively strong (0.05 m s1) eastward
flows were found at 1000 m depth. Notable deflections ofthese flows over ESs or the Shatsky Rise were identified.
Eddies were also evident at 1000 m depth.
[31] 2. A Taylor cap that causes anticyclonic deflection
induced by the eastward flow impinging against the Nintoku
Seamount (42.0 N) was clearly identified in the climatological flow field. This feature was also identified in thenear-surface flow field (15 m depth) and dynamic height
field at 200 dbar relative to 2000 dbar. These results elucidate that the Taylor cap over the Nintoku seamount is stableor frequently generated and extends to the surface layer.
[32] 3. An anticyclonic flow was found over the Suiko
Seamount (45.0 N). It was difficult to explain the generationof this flow using only the Taylor cap theory.
[33] 4. Data obtained from the current meter moored overthe southeastern flank of the Suiko Seamount suggest thatthe boundary current partially compensates for the Sverdruptransport to the east of ESs. This boundary current was also
detected in the Argo float trajectories.
[34] Although anticyclonic flows were found over both theNintoku and Suiko Seamounts, their generation mechanisms
seem different. The different mechanisms result in different
distributions of water properties and nutrients; the Taylor
cap induces continuous upwelling to the left of the impinging flow (looking downstream) [Chapman and Haidvogel,1992], while eddy-induced flows are not accompanied bysuch organized vertical motions since they are not generated
by the impinging flow. Thus, continuous upwelling to thenear-surface layer, which can contribute to local biologicalactivity, is expected over the northwestern flank of the

11 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

Figure 13. Velocity field of the 6-month mean ((a) December to May (uI) and (b) June to November (uII)),(c) the annual mean component ((uI + uII)/2) and (d) the seasonally varying component ((uI  uII)/2)(the ratios of the velocity estimation errors to the estimated velocities are less than 0.1). Here, larger
parameter and threshold (Rc = 2000 km and L0 = 2500 km) and the data in the large region enclosed by
red dotted lines in Figure 1 (35.0 N–55.0 N and 142.0 E–210.0 E) are used. Both color and length of
the arrow represent the velocity magnitude.
12 of 14

C06006

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

Nintoku Seamount, while such continuous upwelling may
be transient or weak in terms of the climatological meanaround the Suiko Seamount.
[35] The boundary current along the eastern side of ESs, asa result of the bathymetric influences of ESs on the largescale subarctic gyre of the North Pacific, was found for the
first time from velocity observations. This result is contrary
to some previous studies [Isoguchi et al., 1997; Isoguchi and
Kawamura, 2006] regarding the bathymetric influences of
ESs on the subarctic gyre as negligible. Note that bathymetric influences on the subarctic gyre will be reduced on alonger timescale [e.g., Wagawa et al. 2010]. Bathymetricinfluences on the subarctic gyres in the North Pacific should,
thus, be separately discussed in terms of the timescale ofcirculation variations.
[36] To directly support the generation mechanisms of the
anticyclonic flows found over the Nintoku and Suiko Seamounts, extensive observations for velocity and density fieldwith detailed levels of precision and resolution are required.
Numerical experiments will also be useful to investigate
such dynamics. The observation period of the moored current meters was less than one year (345 days) in the present
study. In addition, there were few moorings. To obtain thehorizontal structure of the boundary current along ESs,
several velocity observations along the eastern side of ESs
with a high level of precision are needed; these observations
would need to be sufficiently precise to estimate the volumetransport and examine the spatial patterns of the transport,
and sufficiently long to analyze their seasonal variation
through harmonic or spectral analyses. In addition, a greaternumber of Argo float data would help clarify the bathymetric influences of ESs. We will carry out such work infuture studies.
[37] Acknowledgments. We are grateful to the crew of the 169-th and180-th cruises of the T/S Oshoro-Maru of Hokkaido University. The Argo
float data used in this study were collected and made freely available by
the International Argo Project and the national programs that contribute toit (http://www.argo.ucsd.edu, http://wo.jcommops.org/cgi-bin/WebObjects/
Argo). Comments made by the other reviewers and the Associate Editor
have significantly improved this study. We wish to thank Akira Masuda of
Kyushu University, Jong-Hwan Yoon of Kyushu University, Atsuhiko
Isobe of Ehime University and Kaoru Ichikawa of Kyushu University for
their instructive comments. Hiroji Onishi of Hokkaido University helps toaccomplish our mooring observation. Daisuke Ambe of National ResearchInstitute of Fisheries Science and Masatoshi Sato of Tohoku National Fisheries Research Institute provided technical advice relating to surface buoys
and the optimal interpolation, respectively.

References
Adcock, S., and D. P. Marshall (2000), Interactions between geostrophic
eddies and the mean circulation over large-scale bottom topography,
J. Phys. Oceanogr., 30(12), 3232–3238.
Book, J. W., M. Wimbush, S. Imawaki, H. Ichikawa, H. Uchida, andH. Kinoshita (2002), Kuroshio temporal and spatial variations south ofJapan determined from inverted echo sounder measurements, J. Geophys.
Res., 107(C9), 3121, doi:10.1029/2001JC000795.
Boyer, T., S. Levitus, H. Garcia, R. A. Locarnini, C. Stephens, and J.
Antonov (2005), Objective analyses of annual, seasonal, and monthly
temperature and salinity for the world ocean on a 0.25 grid, Int. J. Clim.,
25(7), 931–945.
Bretherton, F. P., R. E. Davis, and C. B. Fandry (1976), A technique forobjective analysis and design of oceanographic experiments applied to
MODE-73, Deep Sea Res. Oceanogr. Abstr., 23, 559–582.
Chapman, D., and D. Haidvogel (1992), Formation of Taylor caps over atall, isolated seamount in a stratified ocean, Geophys. Astrophys. Fluid
Dyn., 64, 31–65.

C06006

Chen, S., B. Qiu, and P. Hacker (2007), Profiling float measurements of therecirculation gyre south of the Kuroshio Extension in May to November
2004, J. Geophys. Res., 112, C05023, doi:10.1029/2006JC004005.
Dietz, R. S. (1954), Marine geology of northwestern Pacific: Description ofJapanese bathymetric chart 6901, Geol. Soc. Am. Bull., 65, 1199–1224.
Hansen, D. W., and P.-M. Poulain (1996), Quality control and interpolations of WOCE–TOGA drifter data, J. Atmos. Oceanic Technol., 13(4),900–909.
Holloway, G. (1987), Systematic forcing of large-scale geophysical flowsby eddy-topography interaction, J. Fluid Mech., 184, 463–476.
Isoda, Y., and D. Fukai (2004), Water mass characteristics around thetransitional domain in the northern North Pacific, SOLA, 79, 65–75.
Isoguchi, O., and H. Kawamura (2006), Seasonal to interannual variationsof the western boundary current of the subarctic North Pacific by a combination of the altimeter and tide gauge sea levels, J. Geophys. Res., 111,
C04013, doi:10.1029/2005JC003080.
Isoguchi, O., H. Kawamura, and T. Kono (1997), A study on wind-driven
circulation in the subarctic North Pacific using TOPEX/POSEIDON
altimeter data, J. Geophys. Res., 102(C6), 12,457–12,468.
Ito, S., K. Uehara, T. Miyao, H. Miyake, I. Yasuda, T. Watanabe, andY. Shimizu (2004), Characteristics of SSH anomaly based on TOPPEX/POSEIDON altimetry and in situ measured velocity and transport of
Oyashio on OICE, J. Oceanogr., 60(2), 425–437.
Kalnay, E., et al. (1996), The NCEP/NCAR 40-year reanalysis project, Bull.
Am. Meteorol. Soc., 77(3), 437–471.
Kasai, H., H. Saito, A. Yoshimori, and S. Taguchi (1997), Variability intiming and magnitude of spring bloom in the Oyashio region, the western
subarctic Pacific off Hokkaido, Japan, Fish. Oceanogr., 6(2), 118–129.
Kono, T., and Y. Kawasaki (1997), Results of CTD and mooring observations southeast of Hokkaido: Part 1. Annual velocity and transport variations in the Oyashio, Bull. Hokkaido Natl. Fish. Inst., 61, 65–81.
Kuragano, T., and M. Kamachi (2004), Balance of volume transports
between horizontal circulation and meridional overturn in the North
Pacific subarctic region, J. Oceanogr., 60(2), 439–451.
Lavender, K. L., W. B. Owens, and R. E. Davis (2005), The mid-depth
circulation of the subpolar North Atlantic Oceans as measured by subsurface floats, Deep Sea Res., Part 1, 52(5), 767–785.
Levine, E. R., and W. B. White (1981), Large-scale synoptic thermal fronts
in the mid-latitude North Pacific from 1976–1978, J. Geophys. Res.,
86(C7), 6567–6579.
Levine, E. R., and W. B. White (1983), Bathymetric influences upon thecharacter of North Pacific fronts, 1976–1980, J. Geophys. Res., 88(C14),
9617–9625.
Mann, M. E., and J. Park (1996), Joint spatiotemporal modes of surface
temperature and sea level pressure variability in the Northern Hemisphere
during the last century, J. Clim., 9(9), 2137–2162.
Menna, M., and P. M. Poulain (2010), Mediterranean intermediate circulation estimated from Argo data in 2003–2010, Ocean Sci., 6(1), 331–343,
doi:10.5194/os-6-331-2010.
Mizuno, K. (1995), Basin-scale hydrographic analysis and optimal interpolation method (in Japanese with English abstract), Oceanogr. Jpn., 4(3),
187–208.
Niiler, P. P., N. A. Maximenko, G. G. Panteleev, T. Yamagata, andD. B. Olson (2003), Near-surface dynamical structure of the Kuroshio
Extension, J. Geophys. Res., 108(C6), 3193, doi:10.1029/2002JC001461.
Papoulis, A. (1972), Probability, Random Variables, and Stochastic Processes, 583 pp., McGraw-Hill, New York.
Park, J. J., K. Kim, B. A. King, and S. C. Riser (2005), An advanced
method to estimate deep currents from profiling floats, J. Atmos. Oceanic
Technol., 22(8), 1294–1304, doi:10.1175/JTECH1748.1.
Qiu, B. (2002), Large-scale variability in the midlatitude subtropical andsubpolar North Pacific Ocean: Observations and causes, J. Phys. Oceanogr., 32(1), 353–375.
Reid, J. L. (1997), On the total geostrophic circulation of the Pacific Ocean:
Flow patterns, tracers, and transports, Prog. Oceanogr., 39(4), 263–352.
Roden, G. I. (1984), Mesoscale oceanic fronts of the North Pacific, Ann.
Geophys., 2(4), 399–410.
Roden, G. I. (1987), Effect of seamounts and seamounts chains on ocean
circulation and thermohaline structure, in Seamount, Islands, and Atolls,
Geophys. Monogr. Ser., vol. 43, edited by B. H. Keating et al.,pp. 335–354, AGU, Washington, D. C.
Roden, G. I., and B. A. Taft (1985), Effect of the Emperor Seamounts on
the mesoscale thermohaline structure during the summer of 1982, J. Geophys. Res., 90(C1), 839–855.
Saijo, Y., and S. Ichimura (1960), Primary production in the northwestern
Pacific Ocean, J. Oceanogr. Soc. Jpn., 16(3), 139–145.
Sekine, Y. (1999), Anomalous southward intrusion of the Oyashio east ofJapan: 2. Two-layer numerical model, J. Geophys. Res., 104(C2),3049–3058.

13 of 14

C06006

WAGAWA ET AL.: FLOW FIELDS AROUND THE EMPEROR SEAMOUNTS

Takahashi, M., and S. Ichimura (1972), Some aspects of primary productionin the northwestern Pacific Ocean, in Biological Oceanography of theNorthern Pacific Ocean, edited by A. Y. Takenouti et al., pp. 218–229,
Idemitsu Shoten, Tokyo.
Taniguchi, A. (1999), Differences in the structure of the lower trophic levelsof pelagic ecosystems in the eastern and western subarctic, Prog. Oceanogr., 43(2), 289–315.
Taniguchi, A., and T. Kawamura (1972), Primary production in the Oyashio
region with special reference to the subsurface chlorophyll maximum
layer and phytoplankton-zooplankton relationships, in Biological Oceanography of the Northern Pacific Ocean, edited by A. Y. Takenouti et al.,pp. 231–243, Idemitsu Shoten, Tokyo.
Verron, J., and C. LeProvost (1985), A numerical study of quasigeostrophic flow over isolated topography, J. Fluid Mech., 154, 231–252.
Wagawa, T., Y. Isoda, N. Saitoh, T. Azumaya, and K. Sakaoka (2006),
Hydrographic structural change in the transition domain around theC06006Emperor Seamounts (in Japanese with English abstract), Oceanogr.
Jpn., 15(3), 267–281.
Wagawa, T., Y. Yoshikawa, and A. Masuda (2010), Bathymetric influencesof the Emperor Seamounts upon the subarctic gyre of the North Pacific:
Examining boundary current dynamics along the eastern side of themountain ridge with an idealized numerical model, J. Oceanogr., 66(2),259–271.
Wang, L., and C. J. Koblinsky (1994), Influence of mid-ocean ridges on
Rossby waves, J. Geophys. Res., 99(C12), 25,143–25,153.
White, W. B., and D. R. Cayan (1998), Quasi-periodicity and global symmetries in interdecadal upper ocean temperature variability, J. Geophys.
Res., 103(C10), 21,335–21,354.
Willis, J. K., and L.-L. Fu (2008), Combining altimeter and subsurface float
data to estimate the time-averaged circulation in the upper ocean, J. Geophys. Res., 113, C12017, doi:10.1029/2007JC004690.

14 of 14