Download Analysis of Equatorial Currents Observed by Eastern Indian Ocean

ATMOSPHERIC AND OCEANIC SCIENCE LETTERS, 2012, VOL. 5, NO. 4, 280−283
Analysis of Equatorial Currents Observed by Eastern Indian Ocean
Cruises in 2010 and 2011
ZENG Xue-Zhi1,2, LI Yi-Neng1, and PENG Shi-Qiu1
1
State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Received 20 March 2012; revised 24 April 2012; accepted 24 April 2012; published 16 July 2012
Abstract Hydrographic and direct current measurements were made in the Eastern Equatorial Indian Ocean
in May 2010 and April 2011 as part of the Eastern Indian
Ocean Cruises (EIOC) organized by the South China Sea
Institute of Oceanology (SCSIO). Analyses of the shipdrift Acoustic Doppler Current Profiler (ADCP) data indicate that the equatorial currents observed in May 2010
are characterized by a strongly eastward surface current
(Wyrtki Jets, WJs) with a maximum velocity of 0.9 m s−1,
while that observed in April 2011 is weak and without a
consistent direction. The strongly eastward WJ transports
the surface water eastward, resulting in a deeper upper
mixed layer, as shown in the temperature and salinity profiles. However, it was found that the Equatorial Undercurrent (EUC) in the Eastern Indian Ocean is strong in
April 2011 and weak in May 2010. The EUC was located
approximately at the position of the thermocline, and it
had higher salinity (up to approximately 35.5 psu) than
the upper and lower waters.
Keywords: equatorial currents, equatorial undercurrents,
Eastern Indian Ocean
Citation: Zeng, X.-Z., Y.-N. Li, and S.-Q. Peng, 2012:
Analysis of equatorial currents observed by Eastern Indian Ocean cruises in 2010 and 2011, Atmos. Oceanic Sci.
Lett., 5, 280–283.
1
Introduction
The surface currents in the equatorial Indian Ocean reverse their directions four times a year with the transition
of the Indian Ocean monsoons. These currents flow
westward during the winter, weakly westward in the central and western Indian Ocean during the summer, and
strongly eastward during the spring and fall (Mariano et
al., 1995; Han et al., 1999; Schott and McCreary, 2001).
The strongly eastward equatorial Indian Ocean Currents,
which have an amplitude of approximately 1 m s−1, are
known as the Wyrtki Jets (WJs) in recognition of Wyrtki’s
discovery of the currents (Wyrtki, 1973). Han et al. (1999,
Fig. 1) showed that the WJs’ zonal component, averaged
from 1°S to 1°N, can exceed 30 cm s−1 in a large meridional range (55°E–90°E) from March to July. Wyrtki
(1973) suggested that the eastward jets are forced directly
by the equatorial westerlies between the summer and
winter monsoons. His opinion was quickly supported by a
Corresponding author: PENG Shi-Qiu, [email protected]
two-layer model (O’Brien and Hurlburt, 1974), which
showed that a strong eastward jet develops as a direct
response to the switched-on westerlies. Other model results indicate the WJs’ intensity is sensitive to the sea
surface wind fields in the Indian Ocean (Anderson and
Carrington, 1993; Han et al., 1999). WJs transport surface
water eastward, leading to an increase of the upper mixed
layer and sea surface height in the Eastern Indian Ocean
(EIO) and a decrease in the west. The WJs show large
seasonal variability, with a monthly eastward transport of
35 Sv in November 1993, but only 5 Sv in May 1994
(Reppin et al., 1999).
Another important current in the equatorial Indian
Ocean is the Equatorial Undercurrent (EUC). This undercurrent is located in the thermocline and is of low magnitude (Knauss and Bruce, 1964), similar to the EUC in the
Pacific Ocean and Atlantic Ocean. Compared with the
quasi-permanent EUC in the Pacific and Atlantic (Yu and
McPhaden, 1999; Stalcup and Metcalf, 1966), however,
the EUC in the Indian Ocean is highly transient because
of the large, seasonally varying force. For its highly transient nature, Schott and McCreary (2001) suggested
viewing the EUC in the Indian Ocean as a transient equatorial wave phenomenon rather than a mean circulation.
The EUC in the Indian Ocean was first reported by
Knauss and Bruce (1964). Subsequently, other observations, most of which were located in the western and central Indian Ocean, confirmed its existence in different
months (Swallow, 1967; Knox, 1976; Leetmaa and
Stommel, 1980; Schott et al., 1997; Reppin et al., 1999,
Figure 1 Route (line) of the EIOC for the equatorial section and the
CTD sites for 2010 (cycle) and 2011 (star).
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ZENG ET AL.: EQUATORIAL CURRENTS OBSERVED IN 2010 AND 2011
Iskandar et al., 2009). The latest month in a year when the
EUC is observed is August (Reppin et al., 1999). Surprisingly, this undercurrent is absent during some years, such
as 1974 (Knox, 1976) and 2003 (Iskandar et al., 2009). It
is generally believed that the EUC is driven by the eastward pressure gradient force caused primarily by upwelling equatorial Kelvin waves excited by prevailing easterlies (Iskandar et al., 2009).
The currents in the equatorial Indian Ocean have been
depicted by some moorings and ship-drift Acoustic Doppler Current Profilers (ADCPs). However, most of these
moorings and ship-drift ADCPs are located in the western
and central Indian Ocean, and observations in the EIO are
still limited. This paper gives a general description of the
equatorial currents in the EIO in May 2010 and April
2011, based on observations along the EIO in these
months. The paper is organized as follows: the data
sources are given in the next section; section 3 describes
the currents, temperature, and salinity in the equatorial
EIO; finally, a conclusion is given in section 4.
2
Data
The Eastern Indian Ocean Cruise (EIOC) organized by
the South China Sea Institute of Oceanology (SCSIO)
was successfully performed in 2010 and 2011. The EIOC
collected a number of hydrological and meteorological
data to understand the oceanic and atmospheric state of
the EIO. The equatorial section is a routine observation
section in this cruise. Because of the weather conditions,
the route and the observation sites (Fig. 1) along the
equatorial section in these two years are not exactly the
same, but are still highly valuable for understanding the
currents in the EIO.
The data that we use in this paper include currents
from a ship-drift ADCP; temperature and salinity from
Conductivity, Temperature, and Depth (CTD) and wind
vectors from an Advanced Scatterometer (ASCAT). The
sampling frequencies of ADCP and CTD are set to
12/hour and 24/second, respectively. ADCP provides current data with a vertical resolution of 8 m from 22 m to
574 m at an extremely high horizontal resolution. CTD (5
in 2010 and 15 in 2011, Fig. 1) provides temperature and
salinity data with an extremely high vertical resolution
(approximately 0.2 m, processed by the CTD processing
software SBEDataProc) from the surface to 1500 m. The
ASCAT data we used are the daily winds, with a spatial
resolution of 0.25°×0.25°.
281
(Reppin et al., 1999). The jet also tends to occupy a
deeper layer in the east because of an increased upper
mixed layer (Fig. 3) induced by strong eastward surface
transports (Schott and McCreary, 2001). The jet reached a
depth of 80 m in the east but a depth of only 50 m in the
west. Another weaker eastward current with a maximum
speed of 0.5 m s–1 is seen below the WJ (at a depth of 90
m to 150 m) from 82°E to 92°E. To discuss the position of
the undercurrent, the 28°C and 20°C isotherms are shown
in Fig. 2 as a proxy for the top and bottom of thermoclines, respectively (Iskandar and McPhaden, 2011). Noting that this current coincides approximately with the
thermocline as indicated by the 28°C and 20°C isotherms
(Iskandar and McPhaden, 2011) as well as a slow westward current separating this eastward current and the WJ
(Fig. 2), we infer that this weak eastward current is a
weak equatorial undercurrent, not an extension of the WJ
in the subsurface. One can see that this current has the
typical characteristics of equatorial undercurrents in the
Indian Ocean, including the strong eastward velocity, and
is located in the thermocline with May as the time of occurrence.
In contrast to the phenomenon in May 2010, the shipdrift ADCP data indicate a strong equatorial eastward
undercurrent from 80°E to 95°E on 15–22 April 2011 (Fig.
2). This undercurrent is located at a depth of 100 m to 150
m but not exactly in the thermocline. The core of the EUC
was below the 20°C isotherms in the east, which is inconsistent with the description given by Knauss and Bruce
(1964). This undercurrent reached the same strength as
3 Equatorial currents in the Eastern Indian
Ocean
A typical WJ with a strong eastward velocity was observed from 80°E to 92.5°E in the equatorial Indian
Ocean on 10–14 May 2010 (Fig. 2). This WJ developed
better in the east and with a greater velocity than in the
west. The maximum eastward velocity reached 0.9 m s–1
at the surface at approximately 91°E. The maximum
speed of WJs can exceed 1.5 m s–1, as observed at an
equatorial mooring from September 1993 to January 1994
Figure 2 Observed zonal currents in the equatorial section from 22 m
to 575 m on 10–14 May 2010 (top) and 15–22 April 2011 (bottom)
(units: m s−1). The black and blue curves show the 28°C and 20°C isotherms as a proxy for the upper and lower boundaries of the thermocline,
respectively. ADCP has a much higher horizontal resolution than CTD
(see Data section), so this current figure appears noisier than the temperature and salinity figures (Figs. 3 and 4).
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ATMOSPHERIC AND OCEANIC SCIENCE LETTERS
the surface current in May 2010. In contrast, the surface
current, which is westward in the east and eastward in the
west, is weak in April 2011. This current pattern is similar
to the results observed from March to May 1994 (Reppin
et al., 1999). An obvious decreasing of the upper mixed
layer depth in the east of the EIO is shown in Fig. 4. Iskandar et al. (2009) pointed out that the onset of the EUC
coincides with the shoaling of the thermocline, and they
supposed that the eastward surface currents depress the
development of the EUC so that a strong eastward equatorial undercurrent is always accompanied by a westward
surface current, the same as a typical equatorial undercurrent in the Pacific Ocean and Atlantic Ocean (Yu and
McPhaden, 1999; Stalcup and Metcalf, 1966) or a weak
eastward surface current (as observed by Reppin et al.,
1999) in the Indian Ocean.
The EUC has a higher salinity (up to 35.5 psu) than the
upper or lower waters (Figs. 3 and 4) whether it is strong
(in 2011) or not (in 2010). Previous studies have shown
that high salinity waters originating in the Arabian Sea are
advected along the equator to the Eastern Indian Ocean
(Han and McCreary, 2001; Hase et al., 2008) and have
noted that the zonal transport of the EUC is considerable.
Reppin et al. (1999) reported that the EUC had a large
transport of 17 Sv in March and April 1994. Iskandar et al.
(2009) calculated the zonal transport per unit width within
the depth of the EUC, which reached to 35 m2 s−1 in April.
Such a transport is important because it balances the salt
budget between the western and Eastern Indian Ocean.
Other studies (Hase et al., 2008) have suggested that the
high salinity water in the thermocline after the onset of
the EUC may be responsible for the formation of a barrier
layer in the eastern Indian Ocean.
It is generally believed that the WJs are directly forced
by the equatorial westerlies (Wyrtki, 1973; O’Brien and
Hurlburt, 1974; Anderson and Carrington, 1993; Han et
al., 1999). Figure 5 shows the difference of the wind field
during 10–14 May 2010 and 15–22 April 2011. Obviously,
the wind during 10–14 May 2010, which is strong and
eastward, is beneficial to the development of equatorial
surface currents, while the wind during 15–22 April 2011,
which is weak and without a consistent direction, is not.
4
Figure 3 Observed temperature (top, units: °C) and salinity (bottom,
units: psu) profiles in the equatorial Indian Ocean from 5 m to 500 m on
10–14 May 2010. The black curve shows the mixed layer depth (MLD).
The MLD is defined as the depth of the density, which is calculated by
the surface salinity and a temperature 0.5°C lower than the surface temperature (Sprintall and Tomczak, 1992).
Figure 4 Same as Fig. 3, but on 15–22 April 2011.
VOL. 5
Conclusion
The upper layer equatorial current system in the EIO is
investigated by analyzing the ADCP and CTD data collected by the EIOC. A typical strong eastward equatorial
surface current (WJ) and a typical strong eastward equatorial undercurrent in the EIO are clearly observed in May
2010 and April 2011, respectively. Another weak eastward
equatorial undercurrent below the strong eastward surface
current was observed in May 2010. The EUCs are located
in and below the thermocline in May 2010 and April 2011,
respectively. The strong eastward-WJ transported the surface water eastward and deepened the upper mixed layer.
The salinity profiles in these two years both show a high
salinity core in the EUC. This high salinity water is important because it balances the salinity budget across the
basin and may be responsible for the formation of barrier
lays in the EIO.
These observations, along with those collected by the
EIOC in the future, will be further investigated to obtain a
full understanding of the current system in the EIO.
Figure 5 Averaged wind fields during the cruises (top: 10–14 May
2010, bottom: 15–22 April 2011).
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ZENG ET AL.: EQUATORIAL CURRENTS OBSERVED IN 2010 AND 2011
Acknowledgements. The authors express their gratitude to the
captain, scientists, and crew of the R/V SHIYAN #1 of the EIOCs in
2010 and 2011. This study is jointly supported by the Ministry of
Science and Technology (MOST) of China (Grant No.
2011CB403504), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant Nos. KZCX2-EW-208 and
KZCX2-YW-Q11-02), and the National Natural Science Foundation
of China (Grant No. 41076009).
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