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GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L18612, doi:10.1029/2008GL034858, 2008
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North-south asymmetry of warm water volume transport related with
El Niño variability
Akio Ishida,1 Yuji Kashino,1 Shigeki Hosoda,1 and Kentaro Ando1
Received 2 June 2008; revised 24 July 2008; accepted 19 August 2008; published 27 September 2008.
[1] A better understanding of variability in the volume of
warm water in the equatorial Pacific pycnocline (warm
water volume, or WWV) is critical to understanding El
Niño-Southern Oscillation (ENSO) variability. We have
investigated the meridional WWV transport in the Northern
(NH) and Southern Hemispheres (SH) using a highresolution general circulation model. The transport in the
western boundary region compensates the interior transport
in the SH, where Sverdrup balance holds approximately. In
contrast, such compensation does not hold in the NH,
because the boundary transport lags interior transport by
about 7 months. Hence, the WWV exchange in the NH has
more impact on the recharge-discharge of the equatorial
WWV. The north-south asymmetry of the WWV transport,
which is interpreted based on the linear Rossby theory, is
related with the southward migration of westerly wind and
the negative wind curl over the northwestern off-equatorial
region after the mature stage of ENSO. Citation: Ishida, A.,
Y. Kashino, S. Hosoda, and K. Ando (2008), North-south
asymmetry of warm water volume transport related with El
Niño variability, Geophys. Res. Lett., 35, L18612, doi:10.1029/
2008GL034858.
1. Introduction
[2] El Niño-Southern Oscillation (ENSO) variability is
related with interannual variation of warm water volume
(WWV), i.e., oceanic heat content (OHC) build-up and
discharge, in the tropical Pacific [e.g., Wyrtki, 1985; Zebiak,
1989]. Jin [1997] constructed a new conceptual model for
ENSO based upon the recharge-discharge of the equatorial
heat content as the phase-transition mechanism. Meinen and
McPhaden [2000, 2001] (hereafter MM00, MM01) used in
situ measurements of subsurface temperature to confirm that
variations in WWV were consistent with the rechargedischarge oscillator, but they were unable to address variations in the transport in the western boundary region due to
insufficient amount of observed data. They speculated that
the majority of the eastward warm water transport across
156°E must have been supplied by meridional convergence
west of 156°E. Studies on the OHC in the tropical Pacific
[e.g., Kessler, 1990; Hasegawa and Hanawa, 2003] have
revealed the westward propagation of the OHC in the offequatorial North Pacific centered around 16°N and the OHC
exchange between the equatorial and off-equatorial region
support the recharge-discharge oscillator.
1
Institute of Observational Research for Global Change, Japan Agency
for Marine-Earth Science and Technology, Yokosuka, Japan.
Copyright 2008 by the American Geophysical Union.
0094-8276/08/2008GL034858$05.00
[3] The WWV transports in the western boundary region
play an important role on the equatorial WWV variability
because they could compensate the recharge-discharge in
the interior regions. In fact, some observational studies
indicate that the interannual variations in the transport in
the western boundary regions are related to ENSO variability.
In the Southern Hemisphere (SH), Ueki et al. [2003]
estimated the western boundary current transport during
the period 1995 –2000 and found that the boundary current
transport was anti-correlated with Sverdrup transport and
tended to compensate the interior transport. In the northern
hemisphere (NH) Lukas [1988] analyzed the sea level data
during the period 1974 – 1985 and reported that the Mindanao
Current (MC) was stronger than average during the ENSO
year; however, there was no apparent relationship between
the fluctuations of the MC and the ENSO signal because the
strongest fluctuations were recognized in the non-ENSO
period. Kashino et al. [2005] showed that the MC velocity
was high during onset of the 2002 – 03 El Niño during the
period 1999 –2002; however, the data was limited only for
the onset of El Niño. In contrast to the SH, a simple
relationship between the boundary current variations and
ENSO variability has not been reported.
[4] Some numerical studies discussed the relation of the
transport variability between the western boundary and
interior regions. Lee and Fukumori [2003] focused on the
variability of pycnocline transports across 10°N and 10°S
from a numerical hindcast experiment and found that the
larger compensation between the boundary and interior
pycnocline at 10°S rather than 10°N. Kug et al. [2003]
investigated water exchanges in the upper 200-m depth
between the equatorial and off-equatorial Pacific Ocean.
The results showed that ENSO-related meridional transport
in the NH is larger than in the SH. Although the observational and numerical studies infer asymmetric variability of
the transport in the western boundary region, the factors that
induce the north-south difference are not fully understood.
[5] In this study, we investigated the interannual variability
of the WWV transport associated with El Niño by diagnosing the result from a high-resolution ocean general circulation model (OGCM) experiment. This diagnosis allowed us
to (1) quantify the transport in the western boundary and
interior regions and (2) discuss the mechanism of the
transport variability, and is based on the WWV across 8°N
and 8°S above 20°C isotherm. Our diagnosis is the same as
MM01, except that we extended the analysis into the
western region west of 156°E. Since the model accurately
simulated mean and variability of the equatorial currents
and hydrography including the western boundary regions as
reported in the previous works [e.g., Kim et al., 2004; Qu et
al., 2004; Ishida et al., 2005], the diagnosis of the model is
valuable to discuss the transport precisely and to also better
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Figure 1. Comparison of the 20°C isotherm depth time-series in the (a) western and (b) eastern sites between the TAO/
TRITON data (black lines) and model (red line for 8°N, green line for the equator, and blue line for 8°S). The longitudes are
156°E for 8°N and equator, and 165°E for 8°S in Figure 1a and 110°W in Figure 1b.
understand the processes in the western boundary regions
where observed data is still limited.
2. Model and Experiment
[6] The model is based on the Modular Ocean Model
(MOM2) [Pacanowski, 1995]. The domain is zonally global
between 75°S and 75°N, with a horizontal grid spacing of
0.25 degrees and 55 levels in the vertical [Ishida et al.,
2005]. In the spin-up stage, the model is forced with annual
mean wind stress for the first 2 years and with climatological
monthly wind stress for the following 18 years. Next, the
model is forced by 3-day averaged ECMWF wind from
January 1982 to December 2004. The modeled sea surface
temperature is relaxed to the monthly sea surface temperature
[Smith and Reynolds, 2003] and sea surface salinity to
Levitus’ [1982] monthly climatology.
[7] The depth of the 20°C isotherm (Z20) representing
the main thermocline is computed by linear interpolation of
the gridded model output. The WWV transport per unit
width is computed by integrating the modeled velocity
vertically between the surface and Z20. Similar to MM01,
we compute the zonal integration of the meridional WWV
transport at 8°N and 8°S from 156°E to the eastern
boundary (west coast of America), which is referred to as
the WWV transport in the interior region (WWVTINT) in
this study. We also define the WWV transport in the western
boundary region (WWVTWB) as the zonal integration from
the western boundary (the east coast of Mindanao Island for
8°N and New Guinea for 8°S) to 156°E.
3. Results
[8] Comparison of Z20 between the model and TAO/
TRITON (Tropical Atmosphere Ocean/TRIangle Trans
Ocean buoy Network) data indicated high consistency
between the data and model in interannual variability
related to the 1997 – 98 and 2002 –03 El Niño events as
well as in high frequency variability (Figure 1). The mean
correlation coefficient at the stations in Figure 1 is about
0.74 and its standard deviation among the stations is 0.16.
The comparison with satellite sea surface height anomaly
indicates slightly higher correlation about 0.77. It is noted
that Z20 during El Niño is the shallowest in the northwestern site earlier than in the southwestern site (i.e., early
January 1998 at 8°N, 156°E and June 1998 at 8°S, 165°E)
as seen in both observation and model (Figure 1a). This
infers that the discharge of WWV in the off-equatorial NH
occurs earlier than in the SH in the western region.
[9] The WWVTINT in the model (black lines in Figures 2a
and 2d) shows poleward discharge during the 1997 – 98
El Niño event with an intermittent decrease (or break of
discharge) at 8°S between late 1997 and early 1998, which is
consistent with MM01 (their Figure 9). Although the peak
magnitude of the model southward transport across 8°S in
September 1997 is larger than the observation by factor of
two, the value and time of northward discharge at 8°N early
1998 and southward discharge at 8°S in June 1998 are
consistent with the observed estimates. The model also
simulated poleward WWV discharge during the other
El Niño events in 1982– 83, 1986 – 88, 1991 – 92, 1997 –98,
and 2002 – 03.
[10] The WWVTWB exhibited different behaviors in the
NH and SH (blue lines in Figures 2a and 2d). In the NH, the
WWVTWB lagged behind the WWVTINT, which was recognized especially for the 3 El Niño events in 1982 –83, 1991–
92 and 1997 – 98 (red double-headed arrows in Figure 2a).
The highest correlation between the WWVT INT and
WWVTWB occurred when the WWVTINT was shifted by
7 months. As the result, there was a significant net WWV
exchange across 8°N (red-green shade in Figure 2a). Contrasting to the NH, the WWVTWB counterbalanced to the
WWVTINT in the SH so that the net WWV exchange across
8°S was small (Figure 2d). The magnitudes of the variations
(standard deviation) of the transport in the interior, western
boundary and whole equatorial regions were 4.7 Sv, 4.2 Sv
and 5.6 Sv at 8°N, while they were 8.8 Sv, 7.8 Sv and 3.8 Sv
at 8°S, respectively.
[11] To investigate the dynamics determining the WWV
transport variability, we investigated the transport variability
and wind forcing based on the linear Rossby wave theory
[Meyers, 1979; Kessler, 1990; Qiu and Lukas, 1996]. The
oceanic response in the reduced gravity model is given by
@h
@h
~
t
;
cR
¼ r @t
@x
f
ð1Þ
where h is the pycnocline depth anomaly, and ~
t is the
surface wind stress vector. The long Rossby wave speed is
cR = bc2/f2, where f is the Coriolis parameter, b is the
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meridional derivative of f, and c is the internal gravity wave
speed. Since meridional transport (V) is given by the sum of
the geostrophic component (c2/f @h/@x), and Ekman
transport (t x/f), equation (1) is written as
V ¼
Figure 2. (a) Northward transport anomaly upper 20°C
isotherm in the model across 8°N: black line for the interior
(156°E– eastern boundary), blue line for western boundary
(western boundary – 156°E), red-green shading for total
transport across whole equatorial longitude. The red arrows
indicate time lag of the interior and boundary WWV
transport in three El Niño events. (b and c) The model
northward transport anomaly (black line) and the sum of the
Sverdrup transport and vortex stretching terms (red line),
and the Sverdrup transport (blue line) and stretching term
(green). (d – f) Same as Figures 2a – 2c except for 8°S.
r ~
t f @h
þ
b
b @t
ð2Þ
[12] On the right hand side, the first term represents the
Sverdrup transport and the second term represents additional transport induced by vortex stretching or horizontal
convergence. These two terms were determined by using
the ECMWF wind stress data for ~
t and the model Z20
anomaly for h.
[13] In the NH, both the Sverdrup transport and stretching
terms had much higher variability compared with the
WWV transport so that the Sverdrup balance did not hold
(Figure 2c). Large northward (southward) Sverdrup transport in the developing (decaying) period of El Niño was
induced by large wind stress curl variation significantly in
the 1986 –88 and 1997– 98 El Niño events. The stretching
term also had high variability with large positive values at
the peaks of the El Niño events. Although both of the
Sverdrup transport and the stretching terms had large
amplitudes they could on the most part cancel each other
out so that the sum of these terms approximated the WWV
transport well (Figure 2b). In fact, the correlation of the
WWV transport with the Sverdrup transport and the stretching terms was low compared with the sum of them together
(Table 1).
[14] Compared to the result in the NH, the Sverdrup
transport itself in the SH approximated well to the WWV
transport (Figure 2f). Relatively small amplitude of the
stretching term resulted in high correlation between the
WWV transport and Sverdrup transport (Table 1). Although
the contribution by the stretching term was relatively small,
the sum of the Sverdrup transport and stretching terms
showed high correlation (0.89) with the WWV transport
as well (Figure 2e). Therefore, inclusion of the stretching
term improves the simulation of the WWV transport including the intermittent decrease of southward transport (or
break of discharge) in the period between late 1997 and
early 1998.
[15] Clarke et al. [2007] argued that vortex stretching as
well as Sverdrup transport should be taken into account at
ENSO time scales based on their theoretical order estimates.
The results from the realistic simulation model in this study
indicates that the inclusion of the vortex stretching term is
more important in the NH than in the SH. In the SH, the
WWVTWB is anti-correlated with the WWVTINT and therefore anti-correlated with the Sverdrup transport, which is
Table 1. Correlation Coefficient Between the WWV Transport
Anomaly in the Interior and Sverdrup Transport Term, Stretching
Term, and Sum of the Sverdrup Transport and Stretching Terms
8°N
8°S
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Sverdrup
Stretching
Sum
0.35
0.66
0.42
0.31
0.72
0.89
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ISHIDA ET AL.: WARM WATER VOLUME TRANSPORT
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Figure 3. Composite maps of wind stress anomaly vectors and its curls (a– c) and the model Z20 anomaly (d – f) of the El
Niño events in 1982– 83, 1991 – 92, and 1997 – 98. The time-series of the composite northward transport anomaly at 8°N
(g) and 8°S (h): black for interior region, blue for western boundary region, and red for total transport.
consistent with the observational study by Ueki et al.
[2003].
4. Discussion
[16] Our results indicated asymmetry of the WWVTINT in
the SH and NH, i.e., the Sverdrup balance holds in the SH
but does not hold in the NH. In the NH, significant
stretching caused the imbalance between planetary vorticity
advection and wind stress curl. The results also indicated
asymmetry of the WWVTWB, which compensated for the
WWVTINT in the SH but not in the NH. The noncompensation in the NH was caused by the phase-lag of
the WWVTINT and WWVTWB.
[17] To explain the WWVTWB lags about 7 months
behind the WWVTINT in the NH, we focused on the
composite maps of wind forcing and Z20 for 3 El Niño
events in 1982 – 83, 1991 – 92, and 1997 – 98 and a timeseries of the WWV transport at 8°N and 8°S (Figure 3).
These 3 events were selected because of the clearly recognizable lags as seen and indicated with red arrows in
Figure 2a. Year (0) corresponds to the developing period
of El Niño events (1982, 1991, 1997) and Year (+1)
corresponds to the decaying periods (1983, 1992, 1998).
The plots are presented in 3 stages (early, mature, and later
stages); the maps for the Z20 anomaly are presented
3 months later than that for wind forcing in the early and
later stages to take into account of the time lag of Z20
response to wind forcing.
[18] At the beginning of El Niño, a westerly wind
anomaly existed north of the equator as indicated with a
red arrow in Figure 3a and it induced anomalous water
divergence (i.e., shoaling of Z20) east of the Philippine
Islands (Figure 3d). As El Niño progressed, the westerly
shifted southward and crossed the equator in the mature
stage (Figure 3b). There were symmetric structures in the
Z20 anomaly: two shallow regions north and south of the
equator in the western equatorial Pacific, and one deep
region in the eastern equatorial Pacific (Figure 3e). After the
mature stage, the westerly moved further to the south and
reached at around 5°S, and a negative wind stress curl in the
northwestern equatorial Pacific was generated as indicated
with a red square in Figure 3c. This wind forcing deepened
Z20 east of the Philippine Islands (red square in Figure 3f)
through convergence of Ekman transport. The deepening of
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Z20 offshore of the Philippine Islands implies that the
dynamic height is higher there than that on the western
boundary, therefore, the northward WWVTWB anomaly is
generated based on the geostrophic relation. This scenario
was consistent with the time-series of the WWV transport
composite at 8°N (Figure 3g), which showed that the
northward WWVTWB is generated after the northward
WWVTINT by about half a year. In contrast, the transport
variability at 8°S showed low variability in Year (0) and the
large WWVTINT was compensated with the WWVTWB in
Year (+1).
5. Conclusions
[19] To summarize, our study on north-south asymmetry
in the WWV transport variations shows that the amplitude
of the WWVTINT in the SH is larger than that in the NH;
however, the net WWV transport across the whole equatorial region is more significant in the NH because most of the
WWVTINT is compensated with the WWVTWB in the SH.
Therefore the WWV variability in the whole equatorial
region, a key variable in El Niño cycle, is determined by
the net transport variability in the NH. The WWVTINT is
explained by Sverdrup transport and vortex stretching terms
predicted based on the linear long Rossby wave theory in
both hemispheres; however, Sverdrup transport is a better
approximation in the SH because of relatively smaller
contribution by the vortex stretching term. The forcing
mechanism in this study is different from the studies based
equilibrated Ekman transport and geostrophic transport. The
net exchange in the NH is associated with the lags of the
WWVTINT and WWVTWB. Negative wind stress curl in the
north equatorial Pacific after the mature state of El Niño is
key to generating the lagged northward WWVTWB, which
causes the north-south asymmetry of the discharge of the
equatorial WWV. In this study, we focused on the WWV
exchange between equatorial and off-equatorial regions.
The zonal redistribution of the WWV in the equatorial band
and the relation with the meridional exchange is left as a
future study.
[20] Acknowledgments. This research was supported partly by the
Japan Society for Promotion of Science through a Grant-in-Aid for
Scientific Research (C) 18540440.
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Hasegawa, T., and K. Hanawa (2003), Heat content variability related to
ENSO events in the Pacific, J. Phys. Oceanogr., 33, 407 – 421.
Ishida, A., H. Mitsudera, Y. Kashino, and T. Kadokura (2005), Equatorial
Pacific subsurface countercurrents in a high-resolution global ocean circulation model, J. Geophys. Res., 110, C07014, doi:10.1029/
2003JC002210.
Jin, F.-F. (1997), An equatorial ocean recharge paradigm for ENSO. Part I:
Conceptual model, J. Atmos. Sci., 54, 811 – 829.
Kashino, Y., A. Ishida, and Y. Kuroda (2005), Variability of the Mindanao
Current: Mooring observation results, Geophys. Res. Lett., 32, L18611,
doi:10.1029/2005GL023880.
Kessler, W. S. (1990), Observations of long Rossby waves in the northern
tropical Pacific, J. Geophys. Res., 95, 5183 – 5217.
Kim, Y. Y., T. Qu, T. Jensen, T. Miyama, H. Mitsudera, H.-W. Kang,
and A. Ishida (2004), Seasonal and interannual variations of the North
Equatorial Current bifurcation in a high-resolution OGCM, J. Geophys.
Res., 109, C03040, doi:10.1029/2003JC002013.
Kug, J., I. Kang, and S. An (2003), Symmetric and antisymmetric mass
exchanges between the equatorial and off-equatorial Pacific associated
with ENSO, J. Geophys. Res., 108(C8), 3284, doi:10.1029/
2002JC001671.
Lee, T., and I. Fukumori (2003), Interannual-to-decadal variations of
tropical-subtropical exchange in the Pacific Ocean: Boundary versus
interior pycnocline transports, J. Clim., 16, 4022 – 4042.
Levitus, S. (1982), Climatological Atlas of the World Ocean, NOAA Prof.
Pap., vol. 13, 173 pp., U. S. Gov. Print. Off., Washington, D. C.
Lukas, R. (1988), Interannual fluctuations of the Mindanao Current inferrered from sea level, J. Geophys. Res., 93, 6744 – 6748.
Meinen, C. S., and M. J. McPhaden (2000), Observations of warm water
volume changes in the equatorial Pacific and their relationship to El Niño
and La Niña, J. Clim., 13, 3551 – 3559.
Meinen, C. S., and M. J. McPhaden (2001), Interannual variability in warm
water volume transports in the equatorial Pacific during 1993 – 99,
J. Phys. Oceanogr., 31, 1324 – 1345.
Meyers, G. (1979), Annual variation in the slope of the 14°C isotherm
along the equator in the Pacific Ocean, J. Phys. Oceanogr., 9, 885 – 891.
Pacanowski, R. C. (1995), MOM2 documentation: User’s guide and reference manual, GFDL Ocean Group Tech. Rep. 3, 246 pp., Geophys. Fluid
Dyn. Lab., NOAA, Princeton, N. J.
Qiu, B., and R. Lukas (1996), Seasonal and interannual variability of the
North Equatorial Current, the Mindanao Current, and the Kuroshio along
the Pacific western boundary, J. Geophys. Res., 101, 12,315 – 12,330.
Qu, T., Y. Y. Kim, M. Yaremchuk, T. Tozuka, A. Ishida, and T. Yamagata
(2004), Can Luzon Strait transport play a role in conveying the impact of
ENSO to the South China Sea?, J. Clim., 17, 3644 – 3657.
Smith, T. M., and R. W. Reynolds (2003), Extended reconstruction of
global sea surface temperatures based on COADS data (1854 – 1997),
J. Clim., 16, 1495 – 1510.
Ueki, I., Y. Kashino, and Y. Kuroda (2003), Observation of current variations off the New Guinea coast including the 1997 – 1998 El Niño period
and their relationship with Sverdrup transport, J. Geophys. Res., 108(C7),
3243, doi:10.1029/2002JC001611.
Wyrtki, K. (1985), Water displacements in the Pacific and the genesis of
El Niño cycles, J. Geophys. Res., 90, 7129 – 7132.
Zebiak, S. E. (1989), Oceanic heat content variability and El Niño cycles,
J. Phys. Oceanogr., 19, 475 – 486.
References
Clarke, A. J., S. V. Gorder, and G. Colantuono (2007), Wind stress curl and
ENSO discharge/recharge in the equatorial Pacific, J. Phys. Oceanogr.,
37, 1077 – 1091, doi:10.1175/JPO3035.1.
K. Ando, S. Hosoda, A. Ishida, and Y. Kashino, Institute of Observational
Research for Global Change, Japan Agency for Marine-Earth Science and
Technology, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. (ishidaa@
jamstec.go.jp)
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