OSDOcean Science DiscussionsOSDOcean Sci. Discuss.1812-0822Copernicus GmbHGöttingen, Germany10.5194/osd-12-275-2015An estimate of the Sunda Shelf and the Strait of Malacca transports: a numerical studyDaryaborF.fdaryabor@um.edu.myfarshiddaryabor@gmail.comhttps://orcid.org/0000-0001-8217-8504SamahA. A.OoiS. H.ChenoliS. N.National Antarctic Research Center, Institute of Postgraduate Studies, University of Malaya, 50603, Kuala Lumpur, MalaysiaInstitute of Ocean and Earth Sciences, Institute of Postgraduate Studies, University of Malaya, 50603 Kuala Lumpur, MalaysiaF. Daryabor (fdaryabor@um.edu.my, farshiddaryabor@gmail.com)17February201512127531318December20143February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://os.copernicus.org/articles/.htmlThe full text article is available as a PDF file from https://os.copernicus.org/articles/.pdf
Using the Regional Ocean Modeling System (ROMS), this
study aims to provide an estimate of the volume,
freshwater, heat, and salt
transports through the Sunda Shelf and the Strait of Malacca in the southern
region of the South China Sea (SSCS). The modeling
system is configured with two one-way nested domains
representing parent and child with resolutions of 1/2 and
1/12∘, respectively.
The simulated currents, sea surface
salinity, temperature and various transports
(e.g.,
volume, heat, etc) agree well with
the observed values as well as those estimated from the Simple Ocean Data
Assimilation (SODA) re-analysis
product. The ROMS estimated seasonal and mean annual
transports are in accord with those calculated from SODA and those of
limited observations. The ROMS estimates of mean annual
volume, freshwater, heat and salt
transports through the Sunda Shelf into the Java Sea are
0.32 Sv (1Sv=106m3s-1), 0.023 Sv, 0.032 PW (1PW=1015js-1), and
0.010×109kgs-1 respectively. The corresponding
ROMS estimates for mean annual transports through the Strait of Malacca into
Andaman Sea are 0.14, 0.009 Sv, 0.014
PW, and 0.0043×109kgs-1 respectively. The relative
percentages of mean annual transports computed individually from those of
volume, heat,
salinity, and freshwater between the Strait of Malacca
and the Sunda Shelf range from 39 to 43.8 %. This reflects that the
Strait of Malacca plays an equally significant role in the annual transports
from the SSCS into the Andaman Sea.
Introduction
The southern region of the South China Sea (SSCS) has complex bathymetry and
water circulation near the equator in which the monsoon plays a dominant
role (Daryabor et al.,
2010,
2014, 2015). The SSCS connects to the Java
Sea, which is a source of
low-salinity water mass (Wyrtki,
1961) through the Sunda Shelf and Karimata Strait, as
well as the Andaman Sea through the Strait of Malacca
(Fig. 1).
Therefore, the SSCS is important for water exchange
through these Straits.
The SSCS receives heat mostly from the sun and the
atmosphere, as well as freshwater from
precipitation, which can significantly affect the
climate system of the region (Gong and Wang,
1999; Zhang et al., 2003). The
Asian–Australian monsoon system largely influences the
general current circulation in the South China Sea
(SCS), particularly in the southern region
(Wyrtki, 1961; Chu et al., 1999;
Daryabor et al., 2015) and hence
regulates various transports across the Sunda Shelf/Karimata Strait and
Strait of Malacca. A better estimation of transports
across the SSCS is important for understanding
inter-ocean exchanges, particularly
between the Indian and Pacific Oceans. Numerous studies
have estimated volume, freshwater,
heat and salt transports in the SCS from surface observations and numerical
models. Wyrtki (1961), based on the
ship drift data, observed that outflow from the SCS into
Java Sea through the Karimata Strait is 4.5 Sv during
winter and noted an inflow of 3 Sv into the SCS during summer (1Sv=106m3s-1). Cai et al. (2002), using the State Key
Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical
Fluid Dynamics/Institute of Atmospheric Physics Climate Ocean Model (LICOM)
with horizontal resolution of 1/2∘,
estimated the mean annual volume transport through the Sunda Shelf into the
Java Sea is 0.93 Sv. In another study
using the same model but with different configuration,
Cai et al. (2005a) estimated 2.26 Sv
from the Sunda Shelf to the Java Sea. Song
(2006), using sea surface height from satellites and
ocean bottom pressure data, estimated a mean transport
of 7.5 Sv flowed out of the SSCS into the Java Sea
through the Karimata Strait for winter season.
Furthermore, Fang et al. (2003), using the Princeton/NOAA GFDL Modular Ocean
Model (MOM2) with a resolution of 1/6∘ for the SCS and
adjacent seas and 3∘ for the global ocean,
estimated the mean annual volume (3.1 Sv), salt (0.110×109kgs-1), and heat
(0.35 PW) transports through Sunda Shelf into the Java
Sea (1PW=1015js-1). For the Strait of
Malacca the corresponding estimates are 0.5 Sv, 0.017×109kgs-1, and 0.06 PW
respectively. Fang et al. (2009), using the same model but with different
configuration, estimated the annual
volume, heat and salt tansports from the Sunda Shelf
into the Java Sea to be 1.16 Sv,
0.113 PW, 0.039×109kgs-1 respectively, while those into
the Strait of Malacca are 0.16 Sv,
0.016 PW, 0.005×109kgs-1. Recently,
Fang et al. (2010), based on the
Acoustic Doppler current profiler observations,
estimated a mean volume transport of 3.6 Sv from the
SSCS into the Java Sea for a period of 13 January to 12 February 2008.
The numerical estimates of various transports in the SSCS as mentioned above
vary widely mainly due to the differences in model configuration and
resolution. Also, observations of
the SSCS transports remain limited compared with those in the northern
region of the SCS. Owing to the complexity of bathymetry
and flow patterns in the SSCS region,
a high-resolution regional ocean model may be required
for a better estimation of the transports.
Hence, the motivation of this work is to use the
Regional Ocean Modeling System (ROMS) with finer horizontal and vertical
resolutions to compute the volume,
freshwater, salt, and heat
transports for the Sunda Shelf and Strait of Malacca.
This study also estimates the transports in the Strait of
Malacca, which have not been in previous studies
(e.g., Cai et al., 2005a;
Qingye et al.,
2009). Section 2 discusses the
model setup and data. Section 3 describes the analysis
method. Section 4 discusses the modeled transports in
the SSCS and Sect. 5 summarizes the findings.
Model description and data sources
The IRD (the Institut de Recherche pour le Développement (http://www.romsagrif.org))
version of the ROMS with two-nested domains is employed
in this study. The coarse grid has horizontal spacing of
resolution of 1/2∘ and 30 vertical
S-levels. The coarse model domain
covers 20∘ S to 30∘ N and 90 to
140∘ E, encompassing the eastern Indian Ocean
and western Pacific Ocean. The horizontal resolution of
the fine grid is 1/12∘, with 30 vertical
S-levels also. The fine model
domain is from 2.7∘ S to 15∘ N and
97.2 to
116.7∘ E (Fig. 1). Thus, the model comprises
99 × 103 × 30 and 234 × 210 × 30 fixed grid
points for the parent and child domains,
respectively. This model solves incompressible primitive
equations in a split-explicit,
free-surface,
topography-following coordinate system with Boussinesq
and hydrostatic approximations (Shchepetkin and
McWilliams, 2003, 2005) which are
described as follows:
∂u∂t+U⋅∇u-fv=-1ρ0∂p∂x-∂∂z-KM∂u∂z-ν∂u∂z+Fu+Du∂v∂t+U⋅∇v+fu=-1ρ0∂p∂y-∂∂z-KM∂v∂z-ν∂v∂z+Fv+Dv
where Uu,v,w is the
velocity vector; f is the Coriolis parameter
calculated with the formula 2ωsinφ, in which ω is
the rotational angular velocity of the Earth and φ is
latitude; ρ0 a reference
density of the sea water (approximately 1023 kgm-3); p, the
total pressure (≅-ρ0gz);
KM, the
vertical eddy viscosity; ν, the
molecular viscosity; Fu and
Fv, the frictional terms; Du and Dv, the
diffusive terms; and t, the time
variable. The hydrostatic equation is given by:
∂ρ∂z=-ρg
Whereby ρ is the density and g=9.8ms-2 is acceleration due to
gravity. The continuity equation for an incompressible fluid is
expressed as follows:
∇⋅U=0
The primitive equations for temperature and salinity are advection diffusion
equations given as follows:
∂C∂t+U⋅∇C=-∂∂z-KC∂C∂z-νθ∂C∂z+FC+DC
The Symbol C is the tracer field (such as temperature and
salinity); KC is the vertical eddy diffusivity in m2s-1; νθ is the molecular
diffusivity coefficient in m2s-1; FC and DC are
friction and diffusive terms, respectively.
Lateral tracer and momentum advection in the model is associated with the
third-order upstream-biased scheme
(Shchepetkin and McWilliams, 1998).
The advection–diffusion split resolves spurious diapycnal mixing in
S-coordinate models caused by the implementation of
higher-order diffusive advection schemes (Marchesiello
et al., 2009).
The method used in this nested simulation maintains the low dispersion and
diffusion capabilities of the original scheme.
Moreover, vertical mixing is based on
a non-local K-Profile
Parameterization (KPP) scheme proposed by Large et al. (1994). The bottom boundary layer is generated by KPP
bottom-boundary layer parameterization and a quadratic
bottom drag (Veitch et al.,
2010).
The nested model is implemented based on the parallel runs of the parent and
child domains. This configuration is necessary to
achieve interaction when more than one domain is used (Spall and
Holland, 1991). The parent and child
bathymetry is based on the ETOPO2 (http://www.ngdc.noaa.gov)
of 1/30∘ horizontal resolution. ETOPO2 is
derived from depth soundings and satellite gravity observations (Smith and
Sandwell, 1997). The topography is
smoothed to reduce the pressure gradient error with a maximum relative
topographic gradient (r=∇hh) no larger
than 0.2 (Daryabor et al., 2015). The
vertical axis is resolved using 30 vertical
fine-resolution layers from the bottom to the surface
for both parent and child domains. The four lateral
boundaries, A,
B, C, and D, are
specified at the Karimata Strait, the east of Luzon and
Taiwan, the north of Taiwan, and
east of the Andaman Sea at the northern tip of the Strait of
Malacca, respectively (see Fig. 1). Open boundary conditions are prescribed in the
lateral boundaries where an active, implicit,
upstream-biased, radiation condition is implemented
(Marchesiello et al., 2001).The boundary conditions
are supplied by the climatological monthly mean salinity and temperature of
the World Ocean Atlas 2005 (WOA 2005) (refer to http://www.nodc.noaa.gov/OC5/WOA05/pubwoa05.html)
recommended by Antonov et al. (2006) and Locarnini et al. (2006),
respectively. Moreover, hydrostatic
and geostrophic equations (Eqs. 3 and 6) prescribed by
Marchesiello et al. (2001) are used to compute the
elevation and velocity at the boundaries.
u=-1ρf∂∂y∫z0ρgdz-gf∂ζ∂yv=1ρf∂∂x∫z0ρgdz+gf∂ζ∂x
The term (u, v) on the left side of Eq. () is
baroclinic velocity components in ms-1, and the symbol
ζ in meters is sea surface height. Radiation
boundary conditions of Flather's (1976) and Chapman's (1985) are used for
the 2-D momentum and elevation fields respectively.
However, the Orlanski (1976) radiative boundary
condition is used for the 3-D fields. The time steps for
the parent and child domains are 18 and 3 min
respectively. Climatological monthly mean surface forces
(wind stress, freshwater, and net
heat) from the Comprehensive Ocean–Atmosphere Data Set
(COADS from http://iridl.ldeo.columbia.edu/SOURCES/.DASILVA/.SMD94/.climatology/)
recommended by Da Silva et al. (1994) are used to force
the parent and child domains. The model also includes
a relaxation to COADS climatological temperature and
salinity. The model is initialized by using
climatological values of temperature and salinity fields from the WOA
2005. The model is run for 10 years in total with three
years spin-up time estimated from the surface and
volume averaged kinetic energy (Daryabor et al., 2015). The
analysis is then based on the data from Year 4 to Year 10 of the model
run. Seasonal and monthly climatology is computed based
on this period. The modeled currents and estimation of
mass transports are compared with the Simple Ocean Data Assimilation (SODA)
Version 2.2.6 (Carton and
Giese, 2008). The SODA is a global
ocean reanalysis data set with 1/2∘ horizontal resolution spanning
from 1865 to 2008. There are 40 levels in the vertical
direction from 5 to 5375 m with a finer resolution in the upper
ocean. Fang et al. (2012) indicated
that SODA is a reasonable product for model validation in the
SCS. Apart from the SODA, the
monthly mean of Ocean Surface Current Analyses-Real
(OSCAR) time with a horizontal resolution of 1/3 ∘ derived from
satellite altimeter and scatterometer data (available from http://www.oscar.noaa.gov/)
for the period 2000 to 2006 is used as a secondary dataset for
validation. In addition, the model
simulated climatological Sea Surface Temperature (SST) is compared with that
from the Group for High Resolution Sea Surface Temperature (GHRSST) with
a horizontal resolution of 0.05∘ for the
period 2000 to 2006 (refer to http://podaac.jpl.nasa.gov/dataset/NCDC-L4LRblend-GLOB-AVHRR_OI). This product uses optimal interpolation (OI) using
data from the 4 km Advanced Very High Resolution Radiometer (AVHRR)
Pathfinder Version 5 (Reynolds et al., 2007), and
in situ ship and buoy observations. Hydrographic
climatological data “HydroBase” version 2 with 1∘ horizontal
resolution from http://www.whoi.edu/science/PO/hydrobase/php/index.php
for the period 2000 to 2006 is also used to validate the model simulated
climatological Sea Surface Salinity (SSS).
Methods of analysis
This section presents the calculations on the
inter-ocean transport between the Sunda Shelf in the
SSCS and the Strait of Malacca. The present study uses
prognostic-mode momentum components and tracer field to
calculate and analyze the transport. The
cross-sections of the Sunda Shelf (transect T1) and
Strait of Malacca (transect T2) are specified by heavy red solid lines from
the southern end of the Peninsular Malaysia to western Borneo
(1.5∘ N from 104 to
109∘ E) and from the east coast of Sumatra to the southern end of
Peninsular Malaysia (103.7∘ E from
0.2 to 1.5∘ N), respectively (Fig. 1). The SCS is a marginal sea with a depth of about
4000–5000 m in the central region and becoming
shallower towards the southern region with depths of
50–70 m in the Sunda Shelf.
The Strait of Malacca is located between the east coast of Sumatra and the
west coast of Peninsular Malaysia, connecting the SSCS
and the Andaman Sea. The minimum depth of the Strait of
Malacca (≤20m) is in the southern part of the Strait connected to the
Sunda Shelf whereas the maximum depth is in the northern part
(Fig. 1). The transports in these
sections are calculated from the surface to the bottom in the
z coordinate. For the volume
transport the following equation is used:
FV=∫Av⋅n^dA
Where A represents the transect from the surface to the desired
depth, dA denotes the area
element, v is the velocity
component, and n^ indicates the normal vector perpendicular to the
transect. Thus, the direction of the
normal velocity component relative to the normal vector is considered as the
direction of transports. Hereafter,
positive and negative values are referred to as “outflow” form and
“inflow” to the SSCS,
respectively. The heat transport FH is calculated
using:
FH=ρcp∫AT-T0v⋅n^dA
The symbol ρ is the water density with a mean temperature of 28 ∘C and a mean salinity of 33 psu, taken as
1023 kgm-3. The specific heat cp for the above temperature and salinity is determined based on the
calculation by Millero et al. (1973). T denotes the water
temperature, and T0 represents the reference
temperature of 3.72 ∘C (Schiller et al., 1998;
Fang et al.,
2009, 2010). The salinity and
freshwater transports are evaluated according to Eqs. () and
(), respectively, where S is the
salinity (unit of psu), and S0 is the reference
salinity set to 34.544 (Fang et al., 2009,
2010).
FS=ρ∫ASv⋅n^dAFW=∫AS0-SS0v⋅n^dA
Results and discussion
Generally, the modeled SST, SSS and
circulation pattern in the SSCS are in good agreement with observations and
previous studies
(e.g., Chu et al., 1999;
Xie et al.,
2003; Daryabor et al.,
2014, 2015). During winter
(December-January-February) and
summer
(June-July-August),
the model simulated the circulation patterns and major features including
the western boundary currents along the Peninsular Malaysia's eastern
continental shelf (PMECS) reasonably.
Sea surface temperature and salinity
The patterns of model simulated SST (Fig. 2) and SSS
(Fig. 3) compare well with the observations of GHRSST
and those of HydroBase climatological data,
respectively. The seasonal variations in the modeled SST
show that the SSTs are relatively lower during winter with an average of
26.5 ∘C north of
1.5∘ N. Because of the
advection of cold waters from the north to the SSCS and relatively warmer
through the Sunda Shelf from the south of
1.5∘ N towards the Java Sea with an
average temperature of approximately 29 ∘C these well agree
with the GHRSST (Fig. 2) and previous studies (Hu et al., 2000;
Morimoto et al.,
2000; Cai et al., 2005b, 2007; Daryabor et al., 2014,
2015). The modeld SSTs in the basin are relatively
warmer with the average temperature 30 ∘C during summer
because of increased solar radiation and the advection of warm waters from
the southern region, especially from the Java Sea
(Yanagi et al.,
2001; Cai et al., 2005b;
Daryabor et al., 2014, 2015). The major feature
during summer is the “cold tongue” along the PMECS with the temperature
range of 28.5 ∘C due to the existence of
upwelling filament (Daryabor et al., 2014, 2015), which is also
simulated by the model. Model output can be different
from the observation due to several possible sources,
particularly during summer monsoon because of the interaction of strong
monsoon wind flow direction with bathymetry and topography at different
entry points. In addition,
characteristic turbulent diffusion times due to vertical turbulent diffusion
and the vertical advection/convection in the upper layer during the strong
southwesterly winds explain why temperature is somewhat underestimated
(Lonin et al.,
2010). Another reason could arise from the changes in
the vertical resolution at different locations due to the vertical
configuration of the model (Lonin et al., 2010).
Changes in surface salinity reflect changes in freshwater flux (evaporation
minus precipitation). Hence sea surface salinity pattern
resembles that of surface freshwater flux (Fig. 3). Figure 3 shows the SSS is generally 32 to 34 psu in
the region of study. However, the
minimum occurs at the northern and southern coast of the PMECS ranging from
31.5 to 32 psu. In
comparsion, relatively low values of SSS averaging
approximately 30.5 and 31 psu are observed and simulated
in the Strait of Malacca during the winter and summer
respectively. This is due to the net freshwater input in
the corresponding regions where net precipitation is high
(Fig. 3c and f). In
addition, mixing between waters of different salinities
may be the other reason that causes the original salinity to change
gradually, particularly in the summer season along the
southern coast of the PMECS.
However, a deeper understanding of the issue will be
studied in future.
Figure 4 shows the comparison of the seasonal cycle of modeled SST and SSS
with those of GHRSST and hydrographic climatological data (HydroBase2)
averaged in the study area (101–109.5∘ E, 2∘ S–8∘ N).
The modeled seasonal cycle of sea surface temperature and salinity appears
to be similar to GHRSST and hydrographic climatological
SSS, respectively. Figure 4a shows
that during the monsoons (winter and summer), SST
attains a maximum and minimum value of 27 and 30 ∘C
respectively in January and June. The maximum seasonal
cycle peak of SSS occurs prior to April with the value of approximately 33 psu. This peak gradually decreases to the lowest value
of 32.5 from the months of June to September
(Fig. 4b). Figure 4 shows that
during both the winter and summer seasons the minimum and maximum SST is in
consonance with the maximum and minimum SSS. This is
attributed to the dominant monsoonal influence on the SST and SSS variations
in the SSCS.
Model current circulation
Figure 5 represents the surface currents, derived from
model, satellite altimeter and scatterometer
(OSCAR), and the SODA in the winter and summer seasons
respectively. The simulated surface circulation patterns
generally resemble those of OSCAR, SODA and the earlier
studies (Chu et al.,
1999; Morimoto et al., 2000;
Cai et al.,
2007; Tangang et al., 2011;
Daryabor et al., 2014, 2015).
The model simulated the main features of the SSCS at the sea surface and
depth of 30 m reasonably as compared with those shown in the OSCAR and
SODA during winter and summer, including the strong
boundary currents along the PMECS (Figs. 5–7). One notable feature is the
cyclonic eddies (E1w and E2w) for the
ROMS, but at slightly different locations for OSCAR and
SODA during winter. These simulated features are located
north of Natuna and north of Anambas Islands (for the island
locations, see Fig. 1), with their center situated at about 6
and 4∘ N respectively.
Similarly, during summer monsoon but with an
anticyclonic rotation, the eddies (E1S and
E2S) around the north of Natuna Islands and the north of
Anambas Islands is simulated also. During
summer, the western boundary current leaves the PMECS
and bifurcates approximately at the latitude 8∘ N
(Figs. 5 and 6). Recent study by
Daryabor et al. (2015) pointed out that it may due to
the formation of an adverse pressure gradient force and an adverse vorticity
downstream in the near-shore waters around the coastal
region. The currents at depths of 30 and 50 m
exhibit patterns similar to those near the surface (Figs. 5–7). The cyclone and anticyclone
eddies in the winter and summer seasons north of Natuna Islands and north of
the Anambas Islands still persist. This may be related
to the dominant effect of monsoons
(December-January-February and
June-July-August) that leads to
the development of baroclinic instability in the region of study (Li et al., 2011;
Chen et al.,
2012; Daryabor et al., 2014, 2015).
Seasonal and mean annual transports
This section discusses the seasonal and annual exchange of transports of
volume, freshwater,
heat, and salt between the SSCS and the Java Sea as well
as their effects on distribution of temperature and salinity in the region
of study. The same is also dealt with for the Strait of
Malacca. Further transports and the effects of these
exchanges through the main passages in the SSCS, their
roles in the mechanism and the formation of thermohaline circulation in the
upper SSCS as well as the changes of the ocean's ventilation rates and
pathways are elaborated.
Seasonal transports(a) Sunda Shelf
The seasonal cycles of various transports calculated from the sea surface to
the bottom in the pathway of the Sunda Shelf indicated by transect T1 are
depicted in Fig. 8. The ROMS estimated seasonal cycle
of volume transport agrees well with those of SODA. In
addition, both estimates are in good agreement with the
observed values of Wyrtki (1961). In early
January, the SODA and ROMS volume transports from the
SSCS into the Java Sea are 4.9 and 5.7 Sv, respectively (Fig. 8a). These
estimates of outflows into the Java Sea are consistent with the mostly
southward flow in the Sunda Shelf during winter
(Figs. 5–7).
Meanwhile, the volume transport estimate of Fang et al. (2003) during January appears to be higher as
compared to the ROMS estimated values by about 2 Sv. This higher estimate may be due to the coarse
resolution of the Fang's model which is configured with the horizontal
resolution of the coarse grid 3∘ and the fine grid of
approximately 1/6∘ with 15 vertical
levels.
The ROMS estimated outflow in February is 4.5 Sv, which is comparable with the
observed value of Wyrtki (1961). The volume transport
gradually reduces to zero around April–May and becomes
negative (inflow into the SSCS) starting from June to October when the flow
becomes dominantly northwards. By early
June, transport switches to inflow and continues
throughout the summer with a maximum of about 3.0 Sv in
August. This estimate is in good agreement with the
observed volume transport of 3 Sv by Wyrtki (1961). The
inflow then decreases in September and switches back to outflow in early
winter with estimated volume transports of 3.3 and 2.5 Sv in December for ROMS and
SODA, respectively.
Hence, the volume transport in the Sunda Shelf seems to
depend largely on the monsoon cycle. The good agreement
between ROMS, SODA and observed values of Wyrtki (1961)
seems to suggest the advantage of using
a high-resolution regional ocean model for better
estimation of transports in complex region such as the
SSCS. In fact, Fang et al. (2003) using a global ocean circulation model with
coarse resolution of 1/6∘ overestimated the volume transport
in winter but underestimated it in summer.
Furthermore, the switching of transport from outflow to
inflow occurs in the middle of August, much earlier than
those of ROMS, SODA and observed values of Wyrtki (1961)
(Fig. 8a).
Table 1 provides another comparison of volume transport of this study with
two previous modeling studies based on the coarser global ocean model during
the months of December, January, and
June. Cai et al. (2005a), based on LICOM global model of
1/2∘ horizontal resolution and 12 uniform vertical
levels, estimated 3.4 and 0.2 Sv of volume transport during December and
June, respectively. The positive
values of volume transports during both December and June indicate no
reversal of the volume transports from winter to summer.
Qingye et al. (2009), using HYCOM
global model of 1/2∘ horizontal resolution and 20 uniform
levels with bathymetry from ETOPO5, estimated volume
transport of 2.1 and -1.0 Sv during January and June
respectively. The estimates from these two modeling
studies are not consistent with the modeled and observed values from
ROMS, SODA and Wyrtki (1961).
Table 2 shows a comparison of transports from the SSCS into Java Sea through
the Sunda Shelf in the months of January and February between estimates of
a more recent observational study of Fang et al. (2010)
and those estimated from ROMS and SODA. Fang et al.'s estimates are based on the Acoustic Doppler
current profiler deployed for a period of 13 January to 12 February 2008. The corresponding volume
transports of ROMS and SODA are consistent with the observed value of Fang
et al. (2010).
The seasonal cycle of freshwater,
heat, and salt transport derived from ROMS and SODA
generally follows that of the volume transport (Fig. 8). During the peak of the winter monsoon in
January, freshwater transport enters the Java Sea at
0.23 Sv while in August the transport is into the Sunda
Shelf at 0.099 Sv. These estimates
are in accord with the values of 0.19 and
0.066 Sv for the respective months of January and August
obtained from SODA (Fig. 8b).
However, there has been no direct observation of the
seasonal cycle of freshwater transport that can be compared with these
values. Nevertheless, Fang et al. (2010), based on observation for
a period of 13 January to 12 February 2008, provided an estimate of freshwater transport of
0.14 Sv into the Java Sea through the Sunda Shelf (see
Table 2). This value is comparable with both SODA and
ROMS estimates of 0.15 and 0.18 Sv, even though observed value represents a transport
only for a very specific period of measurement. The ROMS
and SODA estimated seasonal cycles of heat transport through the Sunda Shelf
are similar to each other (Fig. 8c).
However, the estimates of Fang et al. (2003), which are based on the
global ocean model, are much higher during winter and
lower during summer. Nevertheless,
the ROMS and SODA estimated heat transport in one month during January and
February are consistent with observed values of Fang et al. (2010) (see Table 2).
Similarly, the seasonal cycle of the salt transport
(Fig. 8d) through the Sunda Shelf follows the pattern of
volume transport. The ROMS estimated amounts of salt
transport out of and into the Sunda Shelf in January and August are 0.2×109 and 0.11×109kgs-1 respectively. The corresponding
values for SODA are 0.16×109 and 0.06×109kgs-1 respectively. The ROMS estimated
value in summer is slightly higher compared with that from
SODA. However, the ROMS estimated
salt transport for the months of January and February is in accordance with
the observed value (Fang et al.,
2010) as shown in Table 2.
(b) Strait of Malacca
Figure 9 compares the seasonal cycle of various transports estimated based
on ROMS, SODA for the pathway of the Strait of Malacca
indicated by transect T2 and those of Fang et al. (2003). As in the Sunda Shelf, ROMS
and SODA estimates of transports in the Strait of Malacca are in good
agreement with each other. However,
in contrast to Sunda Shelf, Fang et al.'s estimates of transports during winter are lower
than the ROMS estimates. The ROMS estimated volume
transport through the Strait of Malacca in early January is approximately
1.8 Sv towards the Andaman Sea,
which is slightly higher than the 1.4 Sv from
SODA. As in the Sunda Shelf, the
volume transport decreases and becomes zero in April.
The volume transport switches to an inflow in late May and continues
throughout the summer up to October with maximum values in Auguest of
approximately 0.9 and 0.6 Sv for ROMS
and SODA respectively.
The inflow decreases further in autumn and switches back to outflow in
November with a maximum rate of approximately 1.6 and
1.2 Sv for ROMS and SODA in December respectively
(Fig. 9). In
general, the volume transport estimates of the ROMS and
SODA are in good agreement throughout the years. In
contrast, Fang's study underestimated the volume
transports in summer (Fig. 9a).
The seasonal cycle of freshwater transport (Fig. 9b)
generally follows that of the volume transport. During
the peak of the winter monsoon in January, freshwater
transport enters the Strait of Malacca at a maximum rate of
0.073 and 0.065 Sv for the ROMS
and SODA respectively. However,
during summer, the inflow of the freshwater transports
into the Strait of Malacca is estimated to be approximately 0.28 and 0.23 Sv for ROMS and SODA
respectively. This inflow reverses its direction at the
end of autumn. The seasonal cycle of heat
(Fig. 9c) is also very similar to the seasonal cycle of
volume transport. Estimates from ROMS and SODA are in
accordance with each other. On the other
hand, Fang's study underestimated the heat transport
during the summer. During January the heat is
transported into the Andaman Sea through the Strait of Malacca with an
estimate of 0.18 PW for ROMS and 0.16
PW for SODA. In the summer season,
the heat is transported into the corresponding area with an estimate of 0.096 PW for ROMS and 0.079 PW for
SODA. For salt transport (Fig. 9d), ROMS and SODA estimate are also in good
agreement. During winter, salt is
transported into the Strait of Malacca with estimates of 0.068×109 and 0.067×109kgs-1 for ROMS and SODA
respectively. During summer the salt transport is into
the Strait of Malacca with estimates of 0.034×109 and 0.041×109kgs-1 for ROMS and SODA
respectively. In contrast, Fang's
study underestimated the salt transport during both summer and
winter.
Mean annual transports(a) Sunda Shelf
The ROMS mean of various annual transports through the Sunda Shelf and those
of SODA are listed in Table 3. The model estimated mean
annual volume transport shows the water flows out of the SSCS by
0.32 Sv through the Sunda Shelf and into the Java
Sea. This is comparable with the corresponding value of
0.42 Sv of SODA.
Table 3 also presents the mean annual heat,
salt, and freshwater transports through the Sunda
Shelf. The ROMS and SODA estimates of heat transport
from SSCS into the Java Sea through the Sunda Shelf are comparable with
values of 0.032 and 0.042 PW
respectively. Meanwhile, the modeled
annual salt and freshwater transports flows into the Java Sea are
0.010×109kgs-1 and 0.023 Sv, respectively. These estimates
are comparable to those of SODA
(i.e., 0.016×109kgs-1 and 0.026 Sv). However, estimates of heat and
salt transports from several numerical studies vary considerably due to
different model configurations and resolutions. Cai et al. (2005a), using LICOM global
model of horizontal of 1/2∘ and uniform 12 levels in the upper
300 m estimated 0.17 PW and
0.066×109kgs-1 of heat and salt transports through the
Sunda shelf respectively. Fang et al. (2003), based on the MOM global
ocean model with a horizontal resolution of 1/6∘ and 15
uniform vertical levels, estimated the heat and salt
transport of 0.35 PW and 0.11×109kgs-1,
respectively.
Figure 10 shows that estimate of the annual volume transport (FV) from
the different model studies vary considerably and decrease
exponentially. The previous estimates of mean volume
annual transport differ between a maximum value of 3.15 Sv of Fang et al. (2003) and minimum of
0.5 Sv of Qingye et al. (2009). Table 4 shows the ocean model configurations and
specifications used in the previous modeling studies.
These previous studies are mostly based on a global ocean model of various
resolutions. As in Fig. 1 the length
of the Sunda Shelf is noted to be approximately 500 km and maximum water
depth of about 50 m across the passage. Hence the
above differences in the estimate of FV from different models may be due
to the coarse horizontal and vertical resolutions of the
models. Such a coarse model could produce relative
differences in transport estimates through the Sunda
Shelf. Fang et al. (2005) pointed
that large transport values are due to the coarse horizontal and vertical
resolutions, particularly in the shallow water region
and the entrance of the Straits where are connected to the different
regions.
(b) Strait of Malacca
Table 5 presents a comparison of the mean annual transports through the
Strait of Malacca from ROMS, SODA,
and those from the study by Fang et al. (2003). In consistent with the seasonal cycle of
transport in Fig. 9, the ROMS and SODA estimate of
various estimates are very much in good agreement with each
other. All estimates of mean annual transports are
positive indicating outflow into the Strait of Malacca.
For volume transports, ROMS and SODA estimates are
0.14 and 0.13 Sv
respectively. However, Fang et al. (2003) overestimated the mean volume annual
transport of 0.5 Sv, about four
times higher than that of ROMS estimates. ROMS and SODA
estimates of mean heat annual transports are 0.014 and
0.013 PW respectively. For mean
annual salt transport, ROMS and SODA estimates are
0.0043×109 and 0.0041×109kgs-1.
Similarly, Fang's study also overestimated the mean
annual heat and salt transports. These discrepancies may
be due to the relatively coarse horizontal resolution of the global ocean
model used in Fang's study. The width of the
cross-section in Strait of Malacca (see Fig. 1) is
about 140 km with depths of ≤20m. With such a narrow and shallow
Strait, using a global ocean model for transports
estimation may be inaccurate.
The cross-section in the Strait of Malacca is narrower
than the Sunda Shelf passage and hence lesser transports flow through
it. The maximum outflow volume transport through the
Sunda Shelf during January is 5.7 Sv, which is much higher than the corresponding outflow
volume transport of 1.8 Sv for the Strait of
Malacca. Owing to this shallow
depth, the Strait of Malacca is often not captured in
the simulation using a coarse global ocean model (Fang et al., 2005;
Cai et al.,
2005a; Qingye et al., 2009).
However, in spite of the high value of outflow and
inflow transports throughout the year, the outflow mean
annual transports through the Sunda Shelf are relatively
small. Interestingly, the outflow
mean annual transports through the Strait of Malacca are equally significant
compared to those of the Sunda Shelf. For mean volume
and heat annual transports, the estimate for Strait of
Malacca is about 43.8 % of the Sunda
Shelf. For salt and freshwater, the
percentages are about 43 and 39 %,
respectively. This reflects that the Strait of Malacca
plays equally important role in the annual transports from the Strait of
Malacca into the Andaman Sea.
Summary and conclusion
Based on the results of ROMS simulation, transports of
volume, freshwater,
heat, and salt, through the
inter-ocean passages are estimated in the SSCS for the
Sunda Shelf and the Strait of Malacca. The simulated
patterns of SST and current circulation are in good agreement with
observations, re-analysis product
of SODA, near-real time global
ocean surface currents derived from satellite altimeter and scatterometer
data (OSCAR). The estimated ROMS and SODA seasonal
transports across both the Sunda Shelf and the Strait of Malacca are almost
comparable to each other (Figs. 8 and 9). The ROMS estimated transports
are also consistent with the observed values of Fang et al. (2010) for one-month duration
of 13 January and 12 February 2008.
However, estimates of transports from coarser global
ocean model (Fang et al., 2003) are
often higher during winter and lower during summer. This
could be due to the inability of the coarser global ocean model in resolving
the complexity in the SSCS region due to rapid changes by interaction with
bathymetry and topography. This study shows that the
mean annual transports in the Sunda Shelf are outflow transports into the
Java Sea. Similarly, the mean annual
transports in the Strait of Malacca are outflow transports from the Straits
of Malacca towards the Andaman Sea. Owing to the
relatively large area of the cross-section in the Sunda
Shelf as compared to that in the Strait of Malacca, the
seasonal transports in the Sunda Shelf are higher and vary
considerably. However, in terms of
mean annual transports, ROMS estimated transports
indicate the importance of both passages in the Sunda Shelf and the Strait
of Malacca. The percentages of estimated mean annual
transports through the Strait of Malacca to those of the Sunda Shelf range
from 39 to 43.8 %. These
percentages reiterate the equally importance of the Strait of Malacca as an
inter-ocean transport passage from the SSCS into the
Andaman Sea. Overall, this study
provides estimates of various transports through
inter-ocean passages in the SSCS.
Nevertheless, a better estimate of transport is
necessary to understand the changes in the ocean circulation as well as to
enhance our knowledge on the role of transport distribution such as heat and
freshwater which in turn affect the changes of the ocean's ventilation rates
and pathways. This provides a better picture to assess
the changes in the net uptake of gases such as O2, CO2
which influence the distribution of the nutrient balance in regulation
changes in the marine ecosystem.
Acknowledgements
This research study is funded by the Higher Institution Center of Excellence
(HICOE), IOES-2014A and Fundamental
Research Grant Scheme (FRGS), FP
049-2013A. It is also strongly
supported by the Vice Chancellor of the University of
Malaya.
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Seasonal volume transport (1Sv=106m3s-1) out of and into the SSCS through
the Sunda Shelf obtained from previous modeling studies.
Winter SummerDecJanJunCai et al. (2005a)3.4NA0.2Qingye et al. (2009)NA2.1-1.0Present Study3.35.7-2.0
NA: Not available.
Estimates of mean volume
(FV), heat (FH), salt
(FS), and freshwater (FW) transports during
winter (January and February) from observation (Fang et al., 2010), the
SODA and ROMS.
ReferenceFV (1Sv=106m3s-1)FH (1PW1015js-1)FS(=109kgs-1)FW(1Sv=106m3s-1)Fang et al. (2010)3.6±0.80.36±0.080.12±0.030.14±0.04SODA3.90.390.130.15ROMS4.80.480.160.18
Mean annual volume
(FV), heat (FH), salinity
(FS), and freshwater (FW) transports through the
Sunda Shelf. Positive values indicate outflow
transports.
The ocean model configurations and
specifications used in the previous modeling studies.
ReferencesModelRange of DomainTopographyHorizontal ResolutionVertical LevelsSurface ForcesFang et al. (2009)MOM2.0Outer domain: the global ocean. Inner domain: the SCS, ECS and JES.NAOuter domain: 3∘.Inner domain: 1/6∘15 levelsForced by Hellerman and Rosenstein's (1983) wind stress climatology.Cai et al. (2005a)LICOM75∘ S and 65∘ NNA1/2∘12 levelsThe net heat flux and the sea surface wind stresses from ECMWF reanalysis.Fang et al. (2005)MOM2.0Outer domain: the global ocean. Inner domain: 20∘ S–50∘ N and 99–150∘ ENAOuter domain: 2∘ Inner domain: 1/6∘18 levelsForced by Hellerman and Rosenstein's (1983) wind stress climatology.Qingye et al. (2009)HYCOM76∘ S and 70∘ NETOPO51/6∘20 levelsNA
NA: Not available.
Mean annual volume
(FV), heat (FH), salt
(FS), and freshwater (FW) transports through the
Strait of Malacca.
ReferenceFV(=106m3s-1)FH(=1015js-1)FS(=109kgs-1)FW(=106m3s-1)Fang et al. (2003)0.500.0600.0173NASODA0.130.0130.00410.009ROMS0.140.0140.00430.009
NA: Not available.
(a) Bathymetry (in meters) is in the coarse
resolution domain and the top left corner of the map is for the fine
resolution model domain. The red
dot-dashed line marked with the letters A to D in (a)
indicates the four lateral boundaries applied to
simulation. The red dashed line in the top corner map
indicates the region of study. Lower panel (b) shows
bathymetry (in meters) for the southern part of the South China Sea with
major passages of water mass transport through the Sunda Shelf and the
Strait of Malacca, marked with the red
solid-line cross sections
(i.e., transect
T1 and T2 respectively) used for the transport budget
analysis.
Seasonal variations in sea surface
temperatures in ∘C for (a–b) winter monsoon
(December-January-February) and
(c–d) summer monsoon
(June-July-August).
Seasonal variations in sea surface salinity in
psu for (a–b) winter monsoon
(December-January-February) and
(d–e) summer monsoon
(June-July-August).
(c–f) demonstrates distribution of surface freshwater flux (cm day-1) for the
winter and summer seasons computed based on amounts of evaporation (E) and
precipitation (P) from monthly climatological data (Da Silva et al., 1994).
The seasonal cycle of SST (∘C)
and SSS (psu) represented for the GHRSST and HydroBase SSS (the solid line)
and with the dashed-lines for the
model.
Seasonal pattern of the sea surface currents
(ms-1): for the ROMS (a, d), for OSCAR (b, e) and for the SODA
(c, f).
Seasonal patterns of the modeled sea currents
(ms-1): at the depth of 30 m(a, b), and in 50 m(c, d).
As in Fig. 6, but for
the SODA.
Seasonal variations in (a)
volume, (b) freshwater, (c)
heat, and (d) salt transport through the Sunda Shelf
based ROMS (solid line), SODA (dashed
line), and the dash-dot line
estimated by Fang et al. (2003).
Positive and negative values indicate outflow and
inflow, respectively. The circles in
(a) indicate observed volume transport by Wyrtki (1961).
Seasonal variations in (a)
volume, (b) freshwater, (c)
heat, and (d) salinity transports in the Strait of
Malacca based on ROMS (solid line), the SODA (dashed
line) and Fang et al. (2003) (the
dash-dot line). Positive (negative)
means flow into (out) of the Strait of Malacca.
Results of different models for the mean
volume annual transport (in 1Sv=106m3s-1)
for the SSCS through the Sunda Shelf into the Java Sea.