nec 535m

J Oceanogr
DOI 10.1007/s10872-014-0253-5
ORIGINAL ARTICLE
Variability of the Pacific North Equatorial Current from repeated
shipboard acoustic Doppler current profiler measurements
Shijian Hu • Dunxin Hu
Received: 28 March 2014 / Revised: 14 October 2014 / Accepted: 16 October 2014
Ó The Oceanographic Society of Japan and Springer Japan 2014
Abstract Interannual variability and 16-year trend of the
Pacific North Equatorial Current (NEC) are examined using
repeated shipboard acoustic Doppler current profiler (SADCP) measurements in the upper 200-m layer along 137°E from
1993 until 2008 and compared with previous results inferred
from hydrological and satellite data. Interannual variability of
the NEC is prominent and tied to the Niño 3.4 index but lags
the latter by 6 months. We find that the average NEC transport
under El Niño conditions is 68 Sv and greater than that under
La Niña conditions (59 Sv). Their relationship, however,
involves decadal difference: the correlation coefficient is
statistically significant between 2000 and 2008, but non-significant during 1993–1999. Composite analysis suggests that
the change of wind response to El Niño–Southern Oscillation
(ENSO) from the 1990s to 2000s might account for the difference in NEC–ENSO relationships between the past two
decades. We also find that the NEC transport has intensified in
the past 16 years and present the vertical structure of the NEC
trend. But it is shown that the NEC enhancement is slightly
canceled by the anomalous eastward Ekman current in the
Ekman layer due to the strengthened northward wind stress
above the NEC. Possible uncertainties related to the resolution
of the data are discussed.
Keywords
NEC Interannual variability Trend ENSO
S. Hu (&) D. Hu
Institute of Oceanology, Chinese Academy of Sciences,
7 Nanhai Road, Qingdao 266071, China
e-mail: [email protected]
D. Hu
e-mail: [email protected]
S. Hu D. Hu
Key Laboratory of Ocean Circulation and Wave,
Chinese Academy of Sciences, Qingdao 266071, China
1 Introduction
As a connection between the tropical and subtropical gyres
in the North Pacific Ocean, the North Equatorial Current
(NEC) is of great importance for the ocean and climate
(e.g., Nitani 1972; Hu and Cui 1991). The NEC flows
westward in between the tropical gyre and subtropical gyre,
bifurcates into two branches as it encounters the Philippine
coast, and feeds the northward-flowing Kuroshio and
southward-flowing Mindanao Current (MC). Numerous
studies examined the nature, variability, and climate effects
of the NEC through observational, theoretical, and
numerical approaches (e.g., Meyers 1975; Hu 1989; Hu
and Cui 1989; Hu and Cui 1991; Taft and Kessler 1991;
Qiu and Joyce 1992; Qiu and Lukas 1996; Qu et al. 1998;
Wang and Hu 2006; Kashino et al. 2009; Qiu and Chen
2012; Yan et al. 2014).
The interannual variability of NEC is linked to the El
Niño–Southern Oscillation (ENSO) cycle (e.g., Lukas
1988; Qiu and Joyce 1992). Toole et al. (1990) found that
the NEC strength was enhanced in the spring of 1988 (La
Niña phase) as compared with that in the fall of 1987 (El
Niño phase) through analyzing hydrographic data from the
United States–People’s Republic of China Cooperative
Studies of Air–Sea Interaction in the Tropical Western
Pacific. On the basis of onboard observations, Kashino
et al. (2009) showed that the NEC under El Niño conditions
(late 2006) was stronger than that under La Niña conditions
(early 2008), and they suggested that the dynamic height
rather than the local wind variability contributed to the
interannual variation of NEC. The transport of the southern
branch of NEC was found to be highly related to ENSO via
upwelling/downwelling Rossby waves generated by wind
stress curl anomalies in the tropical northwestern Pacific
Ocean (Zhai and Hu 2013). Besides the observation
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S. Hu, D. Hu
studies, some numerical studies also focused on the interannual variability of the NEC transport. Qiu and Lukas
(1996) suggested that the NEC bifurcated at a lower
(higher) latitude during La Niña (El Niño) years, but the
seasonal variability of the NEC transport near the Philippine coast was not significant. As Qiu and Lukas (1996)
pointed out, prior to El Niño events, the positive wind
stress curl shifted the zero wind stress curl line northward
and hence induced the northward shift of the NEC bifurcation 1 year later. Through analyzing outputs from an
ocean general circulation model, Kim et al. (2004) found
that the interannual variability of the NEC bifurcation
latitude was highly correlated to the NEC transport and was
mainly induced by ENSO-related westward propagation of
Rossby waves from the central equatorial Pacific Ocean.
Previous studies facilitated our understanding of the NEC,
but it seems that the interannual variability has little been
examined on the basis of sustained direct velocity measurements, and the response of NEC to the ENSO cycle
needs further research.
The climate system in the Pacific Ocean shifted after the
early 1990s (e.g., Merrifield 2011; Qiu and Chen 2012).
Although the atmospheric Walker circulation above the
tropical Pacific Ocean was weakened over the past several
decades, it has been strengthened since the early 1990s
(Merrifield 2011; Tokinaga et al. 2012a, b). As a result, the
NEC bifurcation shifts southward after the early 1990s
(Qiu and Chen 2010; Chen and Wu 2012). Qiu and Chen
(2012) examined the shift of the sea level trend using tide
gauge records and satellite altimeter data and pointed out
that the NEC transport had an increasing trend after the
early 1990s. But the trend of NEC and the vertical structure
of the NEC trend need to be further examined by direct
observations.
The objectives of this paper are twofold. Firstly, we
study the interannual variability of the NEC transport and
its relationship with ENSO. Secondly, the trend of the NEC
transport during 1993–2008 is investigated. Both the
interannual variability and 16-year trend are compared with
those inferred from hydrological and satellite data. The rest
of this paper is arranged as follows. Section 2 introduces
the data and corresponding processing methods; Sect. 3
specifies the climatology, variability, and trend of the NEC;
and Sect. 4 summarizes the major results.
2 Data and method
2.1 Data sets
Oceanic current data along the 137°E section between 8
and 18°N are provided by the Japan Oceanographic Data
Center (JODC)/the Hydrographic Department, Maritime
Safety Agency (see http://www.jodc.go.jp/NEW_JDOSS_
HP/FETI_vector_doc_e.html). All of the data were collected by shipboard acoustic Doppler current profilers
5 vertical layers
10 vertical layers
18
17
16
Latitude (°N)
15
14
13
12
11
10
9
8
1993/1
1994/9
1996/5
1998/1
1999/9
2001/5
2003/1
2004/9
2006/5
2008/1
Date (year/month)
Fig. 1 Latitudinal–temporal distribution of SADCP stations along 137°E (big/small cycles denote stations with 10/5 vertical layers in the upper
200 m)
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Variability of the Pacific North Equatorial Current
(SADCPs). The quality of the original data is initially
controlled by the JODC (the quality flags are stored within
the data sets). We then process the SADCP data as below:
firstly exclude the outliers including maximum and minimum and then remove the values whose anomalies are 2.5
times greater than their standard deviations at different
depths. As a result, 38 quality-controlled SADCP sections
across 137°E remain. Figure 1 shows the latitudinal–temporal distribution of the quality controlled data. Most of the
oceanic current profiles have five vertical layers in the
upper 200-m ocean with resolution of about 40 m. The
SADCP records are temporally uneven with general temporal interval of half a year.
Outputs of the European Centre for Medium-Range
Weather Forecasts (ECMWF) Ocean Analysis/Reanalysis
System 3 (ORA-S3) including temperature, current, and
wind stress are also used (Balmaseda et al. 2008). They
cover the global ocean spanning 53 years from January
1959 to December 2011, with a horizontal resolution of
1° 9 1°.
In addition, the monthly Niño 3.4 index (defined as the
SST anomaly averaged over 5°S–5°N, 120–170°W) estimated from optimum interpolation (OI) sea surface temperature (SST) is provided by the Climate Prediction
Center at National Oceanic and Atmospheric Administration (NOAA)/National Weather Service (http://www.cpc.
ncep.noaa.gov/data/indices/). Here Niño 3.4 index is
applied to examine the relationship between the NEC
transport and ENSO.
2.2 Estimation of NEC transport
As a result of the limited vertical range of SADCP data, the
NEC transport referred to in this paper is defined as the
westward transport across 137°E above 200-m depth
between 8 and 18°N:
tranSADCP ðtÞ ¼ 18 N 200
Z m
Z
8 N
uNEC ðy; z; tÞdy dz;
ð1Þ
processes might influence the results. To investigate these
possible influences, we estimate both the westward transport defined in the above equation and net transport of
NEC. It shows that the difference between the westward
transport and net transport is relatively small and that the
interannual variabilities of the westward and net transports
are consistent with each other (not shown).
The bottom of the NEC is obviously deeper than 200 m.
To illustrate the complete NEC structure at 137°E, we
employ the ECMWF ORA-S3 data and plot Fig. 2a
showing the climatology of zonal current and temperature.
We find that the isoline of -0.05 m s-1 (westward) is
much deeper than 400 m. Though the variability of NEC
below 200 m is suggested to be of much importance, there
are two reasons that we focus on the upper 200-m NEC
besides the data limitation.
First, the transport in the upper 200-m ocean contributes
the largest part of the NEC transport and its interannual
variability. We integrate the westward velocity from surface to different depths, to calculate the westward transport
in different layers, and then estimate the rate of different
layer transport over the whole NEC transport (upper
535 m). It is revealed that the transport in the upper 200 m
exceeds 70 % of the whole NEC transport (Fig. 2b). The
standard deviation of 13-month running mean velocity
across 137°E over 1993–2008 is also estimated on the basis
of the monthly ECMWF ORA-S3 data. It shows that the
interannual variability in the upper 200 m is much stronger
than that in the ocean deeper than 200-m depth (Fig. 2c).
Second, the upper 200-m NEC plays an essential role in
the climate system. As shown in Fig. 2, the bottom of the
western Pacific warm pool (WPWP) and mixed layer are
completely enclosed in the upper 200-m ocean. The thermocline is also located at around 200-m depth (Fig. 2a). As
an interface between the ocean and atmosphere, the mixed
layer is of critical importance, and zonal current in the
mixed layer plays an essential role in the variability of
WPWP and ENSO cycle (e.g., Picaut et al. 1996; Guan
et al. 2013).
0m
where uNEC(y,z,t) is the westward velocity (negative value)
across 137°E. To examine the interannual variability of the
NEC, tranSADCP is then linearly interpolated to form a
monthly NEC transport tranmon. Since the NEC is defined
in a fixed region, variabilities induced by the potential
northward/southward shift of the current axis are included.
But it seems that the variation due to the meridional
movement of the NEC is small relative to the interannual
variability of NEC strength (figure not shown). In addition,
because these SADCP measurements provide only snapshots of the NEC, eddy activities and/or small scale
3 Results
3.1 Climatology
The zonal velocity across the 137°E section is averaged
over all the 38 cruises during 1993–2008 to demonstrate
the vertical structure of climatological NEC. As shown in
Fig. 3, the mean zonal current in the upper 200-m layer is
completely westward. It is interesting that the NEC transport in the direct observation is relatively stronger than
those derived from hydrographic data. Qu et al. (1998)
123
S. Hu, D. Hu
Fig. 2 a Climatological temperature (contours with interval of 1 °C)
superimposed on climatological zonal current (color, m s-1) across
137.1°E in ECMWF ORA-S3. The bottom of the western Pacific
warm pool (28 °C isotherm) is highlighted by the thick black line.
b Rates of westward current transports integrated from surface to
different depths over that integrated from surface to about 535 m.
c Standard deviation of 13-month running mean zonal velocities
across 137°E in ECMWF ORA-S3
claimed that the mean transport of the NEC is 41 Sv.
Through analyzing 40-year hydrographic observations
from the Japan Meteorological Agency (JMA), Zhai and
Hu (2013) recently suggested that the mean transport of the
NEC is 51 Sv. But in the present study, the climatological
speed averaged over 8–18°N and 0–200 m is 27 cm s-1.
123
Variability of the Pacific North Equatorial Current
0
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8
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12
13
14
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Latitude (°N)
Fig. 3 Zonal component of SADCP velocities (m s-1) across 137°E averaged over 1993–2008. Negative denotes westward direction
The NEC transport of the mean flow (the average of all the
SADCP sections shown in Fig. 3) is about 58 Sv and the
mean transport of all the individual flows (i.e., the mean
value of the observed NEC transports) is approximately
63 Sv with a standard deviation of about 19 Sv. So the
NEC transport observed by the SADCP seems to be relatively larger than previous studies. But this difference
between documented NEC transports and our results is
probably induced by the inconformity of our definitions
(they applied different depth/longitudinal domain and net
transport).
Qu et al. (1998) reported that the NEC across 130°E in
the upper 200 m had several velocity cores, including two
cores in the upper 100 m at 9.5°N/12.5°N and one core in
between 100 and 200 m with maximum NEC velocity less
than 35 cm s-1. Kashino et al. (2009) also mentioned the
multi-core structure of the snapshots of NEC (December
2006 and January 2008) in the upper 200 m between 8 and
13°N. The maximum westward velocity of the snapshot
NEC during December 2006 exceeded 60 cm s-1 (Kashino
et al. 2009). Here, the maximal velocity in the direct
observed climatological NEC is about 47 cm s-1 (Fig. 3).
Two surface velocity cores at 9 and 10.5°N and another
core at 14°N, 140 m are found. These velocity cores extend
northward with increasing depth, which is consistent with
previous studies (Qu and Lukas 2003).
3.2 Interannual variability related to ENSO
NEC transport is estimated cruise by cruise through
Eq. (1). Kashino et al. (2009) estimated the NEC transport
(Ekman transport plus geostrophic transport relative to
1000 db) and found that the NEC is 61.1 (44.0) Sv in the
December 2006 (January 2008). We have no observation in
the exact same dates, but over the time nearby, as shown in
Fig. 4a, the NEC transports are 64.0 Sv in the January of
2007 and 50.0 Sv in the August of 2008, which are close to
the results reported by Kashino et al. (2009). During the 38
cruises, the maximum and minimum monthly NEC transports are 101 and 26 Sv, respectively (Fig. 4a). While the
temporal distribution is uneven, the annual mean NEC
transport features significant and periodic interannual variability, which changes between 40.7 and 85.9 Sv with
standard deviation of 11.7 Sv (Fig. 4b). This is in good
agreement with previous results derived from hydrographic
data. The maximum geostrophic NEC transport is less than
70 Sv and the minimum more than 30 Sv with standard
deviation of 6–16 Sv (Qu et al. 1998; Zhai and Hu 2013).
Figure 4c shows the standard deviations of zonal velocities
among the 38 cruises. It suggests that the zonal velocity
standard deviations are much stronger in the NEC bands
south of 10°N and north of 15°N, i.e., it is relatively weaker
inside the NEC bifurcation band around 12°N (Qiu and
Chen 2012). In the latitude band 11–14°N, the minimums
of velocity standard deviations extend northward with
increasing depth. It should be noted that possible bias
induced by the seasonality or sample frequency exists, but
it is effectively excluded after low-pass filtering (annual
mean or 13-month running mean in the present study).
Many foregone studies have linked the interannual
variability of NEC to ENSO (e.g., Lukas 1988; Toole et al.
1990; Qiu and Joyce 1992; Qiu and Lukas 1996; Qu et al.
1998; Kim et al. 2004; Kashino et al. 2009; Zhai and Hu
2013). The power spectra of tranmon and Niño 3.4 index are
shown in Fig. 5. The peak period of the tranmon is around
4–5 years and the Niño 3.4 index has an interannual period
123
S. Hu, D. Hu
Fig. 4 SADCP measured upper 200-m NEC transports (Sv, a), annual means with error bars (red) defined as the standard deviation of the
samples in a calendar year (b), and standard deviations of zonal velocities (m s-1) of the 38 cruises (c) (color figure online)
123
Variability of the Pacific North Equatorial Current
0.25
NEC PSD
0.2
95% confidence−NEC
Nino−3.4 PSD
95% confidence−Nino−3.4
PSD
0.15
0.1
0.05
0
1
2
3
4
5
6 7 8 9 10
Period (Years)
Fig. 5 Power spectrum density (PSD) of NEC transport anomaly and
Niño 3.4 index with 95 % confidence interval
Table 1 NEC transports during El Niño/La Niña events
ENSO phase
El Niño
La Niña
Period
Transport (Sv)
Mean (Sv)
68
Oct 1994–Feb 1995
88
Oct 1997–Feb 1998
Oct 2002–Feb 2003
68
56
Oct 2006–Feb 2007
59
Oct 1995–Feb 1996
51
Oct 1998–Feb 1999
55
Oct 1999–Feb 2000
63
Oct 2007–Feb 2008
67
59
of 4–8 years, implying a clear overlap of their frequencies
on an interannual time scale.
As mentioned above, Kashino et al. (2009) found that
the NEC under El Niño conditions was stronger than that
under La Niña conditions on the basis of two cases in
December 2006 and January 2008. Here we focus on the
differences of the upper 200-m NEC between El Niño and
La Niña conditions in composited observations. Four welldefined El Niño events (1994/1995, 1997/1998, 2002/2003,
and 2006/2007) and four La Niña events (1995/1996,
1998/1999, 1999/2000, and 2007/2008) occurred during
the 1993–2008 period. Since the El Niño and La Niña
events are phase locked to boreal winters, here we composite NEC transports during October to the following
February (Table 1). It is concluded that NEC transport
under El Niño conditions is generally greater than that
under La Niña conditions except for the La Niña event in
1999/2000. This is consistent with the result presented by
Kashino et al. (2009), but the mean NEC transports in both
the El Niño phase and La Niña phase are relatively stronger
than their results: here it is 68 Sv under El Niño conditions
and 59 Sv under La Niña conditions.
It may be interesting to examine the difference in the
NEC’s vertical structure between the warm and cold phases
of ENSO. Here we composite the velocity across the 137°E
section over El Niño/La Niña events during 1993–2008. As
shown in Fig. 6, velocities between 11 and 18°N are predominantly intensified under El Niño conditions relative to
that under normal or La Niña conditions. But it indicates
that the westward currents between 8 and 11°N, especially
around 9.6–10.6°N, are significantly weakened during El
Niño events. NEC under La Niña conditions between 9.6
and 11.4°N is much stronger than that under El Niño
conditions. Thus, the response of NEC has clear latitudinal
differences.
To further demonstrate the NEC–ENSO relationship, we
plot in Fig. 7 the 11-month running mean tranmon and
Niño 3.4 index. The interannual variability of NEC transport is by and large tied to the Niño 3.4 index with a
simultaneous correlation coefficient of about 0.46 at the
99 % confidence level. Qiu and Chen (2012) suggested that
the transport of the north branch of the NEC lags the
Niño 3.4 index by 9 months with a maximum linear correlation of 0.61, but Zhai and Hu (2013) claim that the
NEC transport leads the Niño 3.4 index by 1–2 months.
Here we find that the maximum linear correlation coefficient between tranmon and Niño 3.4 index is 0.66 when the
NEC transport lags the Niño 3.4 index by 6 months.
The relationship between NEC transport and ENSO also
involves decadal difference. As shown in Fig. 7, the relationship is close after 2000 (2000s) with a high simultaneous correlation coefficient of 0.75. But the condition
during 1993–1999 (1990s) is different: their correlation
coefficient in the 1990s is 0.25 below the 99 % confidence
level.
Why is the simultaneous NEC–ENSO relationship relatively poorer in the 1990s? As suggested in previous
studies, the ENSO cycle exerts impacts on the interannual
variability of the NEC by wind stress curl anomaly-induced
upwelling/downwelling Rossby waves in the tropical
Pacific Ocean (e.g., Zhai and Hu 2013). We thus hypothesize that the poor correlation between ENSO and NEC in
the 1990s might be due to the change of the wind stress
field in response to ENSO cycles from the 1990s to 2000s.
Zhai and Hu (2013) suggested that the interannual variability of the NEC is controlled by the first mode baroclinic
Rossby wave dynamics and can be explained by the sea
surface height (SSH) variation. They further solved the
vorticity equation in the framework of a 1.5-layer reduced
gravity model and obtained the solution (Zhai and Hu
2013). On the basis of their solution, it is suggested that the
interannual variability of the NEC at latitude y0 is
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S. Hu, D. Hu
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10
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18
Latitude(°N)
Fig. 6 Vertical structure of upper 200-m NEC velocity across 137.1°E composited over El Niño events and La Nina events (m s-1)
Fig. 7 Time series of 11-month
low-pass filtered Niño 3.4 and
the NEC transport. Both the
variables are normalized by
their standard deviations. R90
and R00 denote the correlation
coefficients between them
during the 1990s (left) and
2000s (right). Student’s t test
shows that in both the 1990s and
2000s, a correlation coefficient
above 0.30 is of 99 %
confidence level
2
NEC transport anomaly
Nino−3.4
1.5
1
0.5
0
−0.5
−1
R90=0.25
−1.5
1993/1
1995/1
R00=0.75
1997/1
determined mainly by the wind stress curl variation (integration of the wind stress curl) along the same latitude.
To examine the response of the wind field to the El Niño
events in the 1990s and 2000s, the wind stress curl
anomalies are composited over the El Niño periods in the
1990s and 2000s. As shown in Fig. 8, significant positive
wind stress curl anomaly occurs in the NEC region centered at about 15°N west of the dateline during the El Niño
events in the 2000s (2002/2003, 2004/2005, and
2006/2007). However, this wind stress curl anomaly is
relatively weaker during the El Niño periods in the 1990s
(1994/1995 and 1997/1998) compared with that in the
2000s. One can expect that the response of the NEC to the
123
1999/1
2001/1
2003/1
2005/1
2007/1
El Niño events in the 2000s should be much more significant than that to the El Niño events in the 1990s. Thus, we
suggest that the decadal difference in the interannual
relationship between ENSO and NEC might result from the
response difference in wind stress curl to ENSO between
the past two decades.
3.3 Linear trend
A previous study reported that the geostrophic NEC had
been strengthened after the early 1990s (Qiu and Chen
2012). Here we examine the linear trend of the upper
200-m NEC in the repeated SADCP measurements. As
Variability of the Pacific North Equatorial Current
Fig. 8 Maps of wind stress curl
anomaly (10-7 N m-3)
composited over El Niño events
in the 1990s (1994/1995 and
1997/1998) and 2000s (2002/
2003, 2004/2005, and
2006/2007). The climatology is
defined as the average of wind
stress curl during 1993–2008
mentioned above, the discrete and temporal asymmetrical
NEC transport is linearly interpolated into monthly time
series of traninterp. The 13-month running mean series of
traninterp is then estimated as shown in Fig. 9a. The linear
trend of the NEC transport is calculated as well with a
least-squares method. It is revealed that the NEC transport
has been intensified at a rate of about 0.47 Sv year-1. In
other words, the NEC transport has increased by about
7.46 Sv over the 16 years. This is similar to the result
derived from altimeter data: Qiu and Chen (2012) suggested that the NEC transport over 1993–2009 showed an
increase of 8.3 Sv, i.e., 0.49 Sv year-1. We also calculate
the seasonal mean transports (summer, May–July; winter,
October–December) and group them into summer and
winter series of NEC transports. To exclude the influence
of seasonality on the estimation of linear trend, we calculate the linear trends of summer and winter transports
separately. As shown in Fig. 9, both the summer and winter
transports have an increasing trend: the summer and winter
transports increased by 0.24 and 0.39 Sv year-1 during
1993–2008.
To examine the vertical patterns in the linear trend of
NEC, we display the velocity as a function of depth and
time, and the linear trend of velocity as a function of latitude and depth in Fig. 10. It is suggested that the NEC in
the latitude bands including 8.0–12.0°N, 14.5–15.2°N, and
15.8–18.0°N is clearly enhanced. But the NEC in the latitude band 12.5°N–14.5°N is significantly weakened. The
most intensification occurs in the 100–200-m layer around
8, 11, and 16.5°N. Qiu and Chen (2012) claimed that the
increase of NEC transport is mostly due to the
strengthening of the NEC south of 12.5°N. However, here
we find that the NEC in the latitude band north of 15°N is
also intensified significantly. The difference might be
induced by the poor representation of the subsurface (here
is 100–200 m) current trend in the surface geostrophic
current.
The dynamics that are responsible for the SADCP
observed strengthening of the NEC and the vertical structure of NEC’s linear trend might be the long-term change
in the wind field. Figure 11 delineates the spatial pattern of
wind stress difference between the 2000s and 1990s and
implies that the easterly trade winds in the tropical Pacific
Ocean, i.e., the sea surface branch of the atmospheric
Walker circulation, have been sharply intensified. This is in
concert with the linear trend of wind stress in the tropical
Pacific Ocean (e.g., Merrifield 2011). As Qiu and Chen
(2012) suggested, the increasing of NEC transport might be
a result of the strengthened atmospheric Walker
circulation.
However, this intensification is slightly canceled by the
Ekman current anomalies in the Ekman layer. According to
the Ekman theory (1905), the zonal component of the
Ekman current velocity uE is determined by the meridional
wind stress sy :
Z0
sy
;
ð2Þ
uE ðzÞdz qE f
DE
where DE is the Ekman layer depth (74 m at 15°N under
5 m s-1 wind speed), qE the density of seawater (typically
1025 kg m-3),
and
f
the
Coriolis
parameter
123
S. Hu, D. Hu
Fig. 9 a Monthly, b summer mean (May–July), and c winter mean (October–December) of interpolated SADCP observed upper 200-m NEC
transports and linear trends over 1993–2008
(3:6 105 s1 at 15°N). As shown in Fig. 11, the northward wind stress anomaly in the 2000s is stronger than that
in the 1990s in the NEC region. At the 137°E section, this
northward wind stress anomaly is about 0:01 N m2 . One
can expect that the intensification of northward wind stress
shall give rise to an enhanced eastward Ekman transport.
According to Eq. (2), an anomalous sy of about
0:01 N m2 drives a change of the Ekman current velocity
u E:
123
Z0
uE ðzÞdz 0:26 m2 s1 :
ð3Þ
DE
In other words, supposing the Ekman layer depth is
74 m, the strengthening of northward wind stress causes a
vertically averaged eastward current anomaly of about
0.35 cm s-1 every 8 years (from the 1990s to 2000s). This
partly explains the vertical difference in linear velocity
Variability of the Pacific North Equatorial Current
Fig. 10 a Meridional averaged zonal velocity across 137°E and b vertical structure of the linear trend of zonal velocities in the upper 200-m
ocean
trends between the 0–100-m layer and 100–200-m layer
(Fig. 10b).
4 Summary and discussion
Variability of the Pacific NEC is intensively studied using
satellite records, hydrographic data, and simulations. But
the NEC variabilities in direct observations remain unclear.
Based on 16-year repeated SADCP measurements, the
present paper aims to characterize and understand the
interannual variability and long-term trend of the NEC
across 137°E during 1993–2008.
The vertical structure of the NEC was described in some
previous studies with hydrographic measurements (e.g.,
Qiu and Joyce 1992; Qu et al. 1998; Zhai and Hu 2013) and
instantaneous SADCP observations (Kashino et al. 2009).
In this study, we examined the mean state of the repeated
SADCP observations and found that the climatological
NEC has a maximal velocity of about 47 cm s-1 and
shows a multi-core structure: two surface velocity cores at
9 and 10.5°N and another core at 14°N, 140 m depth, in
123
S. Hu, D. Hu
Fig. 11 ECMWF ORA-S3
wind stress vector (arrows) and
its meridional components
(color) in the 2000s minus that
in the 1990s. The blue line
marks the 137°E section (color
figure online)
30oN
20oN
10oN
0o
10oS
20oS
−2
0.01 N m
125oE
−0.02
150oE
−0.015
concert with previous studies (e.g., Qu et al. 1998; Kashino
et al. 2009). The average NEC transport in the upper 200-m
layer is 63 Sv and relatively larger than the documented
results derived from hydrographic data.
NEC transport features significant and periodic interannual variability with a significant period of 4–5 years
and varies between 26 and 101 Sv with standard deviation
of 11.7 Sv. We further show the features of the latitudinal
distribution of the zonal velocity variations. By performing
composite analysis, we find that the average of NEC
transport under the El Niño condition is 68 Sv and greater
than that under the La Niña conditions which is 59 Sv. This
is similar to the documented case study by Kashino et al.
(2009).
Vertical velocity structures are composited over the
warm and cold ENSO phases, and we find that the response
of NEC features clear latitudinal differences. In terms of
the lead–lag relationship between NEC and ENSO, controversy exists in previous studies. Qiu and Chen (2012)
pointed out that the NEC transport lags the Niño 3.4 index
by 9 months, but Zhai and Hu (2013) suggested that the
NEC transport leads the Niño 3.4 index by 1–2 months. In
the present paper, we find that the NEC transport lags the
Niño 3.4 index by 6 months with a correlation coefficient
of 0.66.
Although the interannual variability of NEC is closely
related to the Niño 3.4 index, the NEC–ENSO relationship
shows decadal difference between the 1990s and 2000s.
The correlation coefficient is 0.75 (above the 99 % confidence level) in the 2000s but 0.25 (below the 99 % confidence level) in the 1990s. We examined the wind stress
curl field under the El Niño conditions in both the 1990s
and 2000s. It turns out that the response of wind stress curl
to the El Niño in the 2000s is quite different from that in
the 1990s: positive wind stress curl anomaly in the El Niño
events in the 2000s is much stronger than that in the 1990s.
123
−0.01
175oE
−0.005
0
160oW
0.005
0.01
135oW
0.015
This implies that the low-frequency variability in the wind
field is important in regulating the NEC–ENSO
relationship.
We then investigated the trend of the NEC velocity and
its vertical pattern during 1993–2008 following Qiu and
Chen’s work (2012). It is suggested that the NEC transport
has been intensified with a linear trend of 0.24 Sv year-1 in
the summer season and 0.39 Sv year-1 in winter during
1993–2008.
The linear trend of the NEC in this period shows significant meridional and vertical diversity. It is interesting
that the strengthening of NEC in the 100–200-m layer is
more intense than in the 0–100-m layer. In the latitude
bands including 8–12°N, 14.5–15.2°N, and 15.8–18°N, the
NEC has been clearly enhanced, but in the latitude band
12.5–14.5°N, it has been significantly weakened. Qiu and
Chen (2012) pointed out that the increase of NEC transport
is mostly due to the strengthening of the NEC south of
12.5°N, but we found that the NEC intensification is also
significant north of 15°N.
To gain an insight into the dynamics responsible for the
NEC’s linear trend, we finally checked the wind stress
difference between the 2000s and 1990s and found that the
strengthening of the NEC results from the strengthened
atmospheric Walker circulation since the early 1990s. This
is in agreement with the result reported by Qiu and Chen
(2012). However, because the northward wind stress
anomaly has increased clearly in the tropical Pacific Ocean,
an eastward Ekman current anomaly occurs in the NEC
region and slightly canceled the intensification of the NEC
in the Ekman layer. This partly explains the vertical difference in NEC intensifications between the upper 0–100m layer and bottom 100–200-m layer.
However, problems remain. First, the sparse SADCP
observations presented here cannot capture the detailed
evolution of the NEC following the ENSO cycle. The NEC
Variability of the Pacific North Equatorial Current
and its variability of course cannot be fully captured here
by the upper 200-m observations. Because of the limitation
of the data (sample frequency and size), there is possible
bias that we are unable exclude completely. Second, the
linear trend during 1993–2008 is possibly influenced by the
decadal or inter-decadal variabilities. But this issue is not
further pursued here because of the limited temporal length
of the available observation record. Finally, western
boundary currents in the Philippine Sea show significant
intraseasonal variability associated with mesoscale eddies
(Hu et al. 2013) and the NEC intraseasonal variability
might also be very important (Kashino et al. 2009). But the
role of mesoscale eddies in the intraseasonal variability of
NEC is unclear and will be considered in future work.
Acknowledgments This study was supported by the National Natural Science Foundation of China (Grant No. 41406016), the Strategic Priority Research Program of the Chinese Academy of Sciences
(Grant Nos. XDA11010101 and XDA11010100) and the NSFCShandong Joint Fund for Marine Science Research Centers (Grant No.
U1406401). We are obliged to the Japan Oceanographic Data Center,
the European Centre for Medium-Range Weather Forecasts, and the
National Oceanic and Atmospheric Administration for providing data
sets, and to those unknown people who had joined in producing and
maintaining these data. Constructive comments from two anonymous
reviewers and Dr. Tomoki Tozuka are particularly valued and much
appreciated.
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