Meteorol Atmos Phys 100, 275–289 (2008)
DOI 10.1007/s00703-008-0309-4
Printed in The Netherlands
1
College of Atmospheric Sciences, Nanjing University of Information Science
and Technology, Nanjing, P.R. China
2
International Pacific Research Center and Department of Meteorology, University of Hawaii, Hawaii, USA
3
LED, South China Sea Institute of Oceanology, Chinese Academy of Science, Guangzhou, P.R. China
Orographic effects on South China Sea summer climate
H. Xu1 , S.-P. Xie2 , Y. Wang2 , W. Zhuang3 , D. Wang3
With 9 Figures
Received 14 April 2007; Accepted 16 July 2007
Published online 14 August 2008 # Springer-Verlag 2008
Summary
New satellite observations reveal several distinct features of
the South China Sea (SCS) summer climate: an intense lowlevel southwesterly wind jet off the coast of South Vietnam,
a precipitation band on the western flank of the north–south
running Annam mountain range, and a rainfall shadow to
the east in the western SCS off the east coast of Vietnam. A
high-resolution full-physics regional atmospheric model is
used to investigate the mechanism for the formation of SCS
summer climate. A comparison of the control model simulation with a sensitivity experiment with the mountain
range artificially removed demonstrates that the aforementioned features form due to orographic effects of the Annam
mountains. Under the prevailing southwesterly monsoon,
the mountain range forces the ascending motion on the
windward and subsidence on the lee side, giving rise to
bands of active and suppressed convection, respectively.
On the south edge of the mountain range, the southwesterlies are accelerated to form an offshore low-level wind jet.
The mid-summer cooling in the SCS induced by this wind
jet further helps reduce precipitation over the central SCS.
A reduced-gravity ocean model is used to investigate the
ocean response to the orographically induced wind forcing,
which is found to be important for the formation of the
double-gyre circulation observed in the summer in SCS,
in particular for the northern cyclonic circulation. Thus,
Correspondence: Haiming Xu, College of Atmospheric Sciences,
Nanjing University of Information Science and Technology, 114
New Street, Pancheng, Pukou District, Nanjing 210044, P.R. China
(E-mail: haimingx@gmail.com)
orography is a key to shaping the SCS summer climate both
in the atmosphere and in the ocean.
1. Introduction
The South China Sea (SCS) is a large semienclosed marginal ocean basin with a total area
of 3.5 million km2. It is connected to the East
China Sea to the northeast, the Pacific Ocean to
the east, and the Indian Ocean to the southwest.
The SCS climate is part of the East Asian monsoon system (Lau et al. 1998). In winter the SCS
is dominated by the strong northeasterly monsoon while in summer the winds reverse the direction to southwesterly (e.g., Liu and Xie 1999).
It is well known that in summer the southwesterly winds reach a maximum east of Ho Chi
Minh City (e.g., Xie et al. 2002). Figure 1a shows
the summer surface wind climatology along with
land topography over the Indochina peninsula. A
narrow mountain range, called the Annam cordillera, runs in a north–south direction on the east
coast of Indochina peninsula on the VietnamLaos borders and ends just north of Ho Chi
Minh City. Noting that the SCS wind jet is located just offshore of the south edge of the Annam
cordillera, Xie et al. (2003) suggest that the orographic blockage of the southwesterly monsoon
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Fig. 1. Summer (JJA) climatology: (a)
QuikSCAT 10-m wind velocity (m s1 ),
and (b) TRMM-PR precipitation (mm
day1 ). Land orography is plotted in (a) in
gray shaded (at 0.5, 1.0, 1.5 km) and star
marks the location of Ho Chi Minh City
and the wind acceleration at the south corner of
the mountain range are the cause of the wind jet.
They go on to show that the strong curls of this
wind jet are the major drive of the summer SCS
circulation, giving rise to a number of important climatic features of the region such as a cold
upwelling filament (Huang et al. 1994; Kuo
et al. 2000) that disrupts the summer warming.
The wind jet varies with the El Nino=Southern
Oscillation (ENSO), driving interannual variability of the SCS in both ocean circulation and sea
surface temperature (SST). While plausible, the
orographic hypothesis of Xie et al. (2003) has
never been rigorously tested in numerical models.
The Annam cordillera also leaves a clear signature on precipitation. Figure 1b shows the
summer precipitation climatology in the region.
As the southwesterly monsoon impinges on the
mountain range, the rising motion creates a
windward rain band and the subsequent subsidence produces a rain shadow on the lee. Such
orographic rain bands are also observed on the
west coasts of Myanmar, Thailand, Cambodia,
and the Philippines. With a numerical experi-
Orographic effects on SCS summer climate
ment, Xie et al. (2006) suggest that such orographic rain bands are not simply a local phenomenon but exert important remote influences
on the continental monsoon because of strong
interaction between circulation and convection
in the region during summer. The Annam cordillera is more than 500 m high on average but only
200 km or less in width, posing a serious problem
for numerical simulations. With typical horizontal resolution of 2–3 , the current global general
circulation models represent the Annam mountain range poorly and fail to simulate either
the SCS wind jet or the orographic rain band=
shadow (not shown).
The summer circulation of the central SCS is
dominated by a double-gyre circulation, with an
eastward inter-gyre jet in between that advects
the cold coastal water to form the cold filament
east of south Vietnam. This pair of anticyclonic
and cyclonic gyres south and north of roughly
12 N are observed from in-situ current measurements (Fang et al. 2002) and satellite altimetry
(Shaw et al. 1999; Ho et al. 2000). With its strong
wind curls, the orographic-induced southwest
wind jet is considered to be the major cause of
this double-gyre circulation pattern (Xie et al.
2003; Gan et al. 2006; Wang et al. 2006).
The present study test the above orographic
hypothesis for the formation of the wind jet and
the couplet of the rain band and shadow in the
summer SCS by using a high-resolution regional
atmospheric model. The 0.2 grid size of the
model is equivalent to T520 resolution for a
global spectral model. Our results show that indeed, the Annam cordillera exerts a great influence on the wind and precipitation distributions
over the Indochina peninsula and SCS. We further apply the atmospheric model results to assess the orographic effect on ocean circulation
using a reduced-gravity ocean model. We find a
strong effect on the northern cyclonic gyre north
of the wind jet.
The rest of the paper is organized as follows.
Section 2 describes the regional atmospheric
model, experimental design, and observational
data sets used for verification. Section 3 presents
the atmospheric simulation results and investigates the orographic effects of the Annam mountain range. Section 4 describes the ocean model
and presents the experiment results. Section 5
presents a summary and discussion.
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2. Regional atmospheric model
and experimental design
2.1. Atmospheric model
The regional atmospheric model (RAM) developed at the International Pacific Research
Center (IPRC), University of Hawaii, is used in
this study. It is a primitive equation model with
sigma as the vertical coordinate, solved on a longitude-latitude grid system. The model domain
is 5 S–25 N, 90–135 E, including the SCS,
Indochina peninsula, east part of the Bay of
Bengal, and part of the western Pacific (Fig. 2).
The model uses a grid spacing of 0.2 in both
longitude and latitude, and has 28 levels in the
vertical. A detailed description of the model
and its performance in simulating regional climate of East Asia can be found in Wang et al.
(2003). The model has also been used to simulate the regional climate over the eastern
Pacific, including the atmospheric response to
tropical instability ocean waves (Small et al.
2003), boundary layer clouds over the southeast
Pacific (Wang et al. 2004), the effects of the
Andean and Central American mountains (Xu
et al. 2004, 2005), and more recent the diurnal
cycle of precipitation over the Maritime continent region (Zhou and Wang 2006; Wang et al.
2007).
The model includes a detailed cloud microphysics scheme for grid-scale moist processes
(Wang 2001). The mixing ratios of cloud water,
rainwater, cloud ice, snow, and graupel are all
prognostic variables in the model. Condensation
(evaporation) of cloud water takes place instantaneously when the air is supersaturated
(subsaturated). Subgrid-scale convective processes, such as shallow convection, midlevel
convection, and penetrative deep convection,
are considered based on the mass flux cumulus
parameterization scheme originally developed by Tiedtke (1989) and later modified by
Nordeng (1995).
The subgrid-scale vertical mixing is accomplished by the so-called E–" closure scheme, in
which both the turbulence kinetic energy (TKE)
and its dissipation rate are prognostic variables
(Detering and Etling 1985). Turbulent fluxes at
the ocean surface are calculated using the TOGA
COARE algorithm (Fairall et al. 1996; Wang
2002). Over the land, the bulk aerodynamic meth-
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Fig. 2. Model domain with topography
(shaded at 0.25, 0.5, 1.0 km) and SST
(contours at 0.25 C intervals) for (a) CTL
and (b) NoTopSmSST runs
od is used in the land surface model, which uses
the Biosphere-Atmosphere Transfer Scheme
(Dickinson et al. 1993). Soil moisture is initialized using a method described by Giorgi and
Bates (1989) such that the initial soil moisture
depends on the vegetation and soil type defined
for each grid cell.
The radiation package originally developed by
Edwards and Slingo (1996) and later modified
by Sun and Rikus (1999) is used, which includes
seven=four bands for longwave=shortwave radiation. Seasonal-varying climatological ozone and
a constant mixing ratio of carbon dioxide for the
present climate are used.
2.2 Experimental design
The initial and lateral boundary conditions are
obtained from the National Centers for the
Environmental Prediction=National Center for
Atmospheric Research (NCEP=NCAR) global
reanalysis (Kalnay et al. 1996), available on a
2.5 2.5 grid with 17 vertical pressure levels.
They are interpolated onto the model grid by
Orographic effects on SCS summer climate
cubic spline interpolation in the horizontal and
linear interpolation in both the vertical and time
based on the four times daily reanalysis. Over the
ocean, the NOAA optimal interpolation weekly
SST dataset on a 1 1 grid is used as the lower
boundary condition (Reynolds et al. 2002).
The following three sets of experiments
are carried out to examine the effects of the
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Annam cordillera on the summer climate of
SCS. Each consists of an ensemble of three
simulations that are initialized at 0000UTC on
26, 27, and 28 June 2001, respectively, and
integrated to the end of July 2001. July 2001
is selected during which the SCS wind jet is
well developed (Fig. 3a). The rest of the paper discusses the July means, averaged for
Fig. 3. Wind velocity (contours with
1 m s1 intervals) at 10 m in (a)
QuikSCAT observations, and (b) the
CTL run, averaged for July 2001. Land
orography (shaded at 0.25, 0.5, 1.0 km)
is also plotted
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three ensemble simulations based on their hourly
output.
Control (CTL) run. The model topography is
based on the U. S. Geophysical Survey 1=12
topographic dataset and smoothed with an
envelope topographic algorithm (Wang et al.
2003). At our model resolution, the main
Annam cordillera is represented reasonably
well (Fig. 2a), at about 0.5 km high and 200 km
in width.
No-topography (NoTop) run. The Annam range
south of 17.5 N is flattened by setting land
elevation at 0.5 m (Fig. 2b). Strictly speaking,
the design of this no-topography run may
be physically inconsistent with the imposed
lateral boundary conditions that are influenced
by the presence of these Annam mountains in
the first place. Nevertheless, a comparison of
the CTL and NoTop runs can help identify the
regional effects of the mountain range within
the context of this model.
SST
No-topography
and
smoothed
(NoTopSmSST) run. While the comparison of
the CTL and NoTop runs can identify the
direct effect of the Annam mountain range,
this mountain range leaves marked signatures
in the SST field in the form of a cold filament
through the action of the wind jet (Fig. 2a; Xie
et al. 2003). This cold filament, in turn, affects
the atmospheric circulation and convection,
and may be considered as an indirect effect
of Annam cordillera. In the NoTopSmSST
run, we remove this orographically induced
anomaly in SST as well as the mountain range.
The smoothed SST field is obtained by first
setting SST to 29.0 C if it is less than this
value off the southeast coast of Vietnam, then
applying a 5-point smoother 5 times in the
coastal region. Figure 2b shows the resultant
SST field, which is nearly uniform near the
southeast coast of Vietnam.
2.3 Observational data
To evaluate the model simulations, we use 3hourly Tropical Rainfall Measuring Mission
(TRMM) Real-Time (RT) multi-satellite precipitation analysis (3B42RT) available since January
2002 on a 0.25 grid covering 50 N–50 S
(Huffman et al. 2007). The 3B42RT product combines measurements by the TRMM satellite’s
Microwave Imager (TMI) and TMI-calibrated
infrared estimates of precipitation from geosynchronous satellites. We also use TRMM
Precipitation Radar (PR) surface rainfall product
3A25G2 (Kummerow 2000) from December 1997
to August 2005 on a 0.5 grid. The PR makes the
best estimation of precipitation but its narrow
swath introduces larger sampling errors. We limit
the use of the PR-only 3A25G2 product to construct a multi-year climatology in Fig. 1b while
using the 3B42RT product for the July 2001
analysis.
The microwave scatterometer on the National
Aeronautics and Space Administration’s (NASA)
QuikSCAT satellite measures daily surface wind
velocity over the world ocean (Liu et al. 2000).
QuikSCAT observations have revealed rich wind
structures on short spatial scales around the
world (Chelton et al. 2004). Here we use a
monthly product for wind velocity available from
August 1999 to August 2005 on a 0.25 grid.
3. Effect on wind and precipitation
3.1 Control run
Figure 3 compares the simulated 10-m surface wind velocity averaged for July 2001 with
QuikSCAT observations over the ocean for the
same period. The model simulates well the surface wind field over the SCS, with southwesterly
winds north of about 5 N and southerly winds
to the south. In particular, the model captures the
strong wind jet off the southeast coast of Vietnam
with a maximum wind speed above 9 m s1 , in
good agreement with QuikSCAT observations.
The model also reproduces the reduced wind
speed lee of the Annam range. In the lee weakwind zone, the model exaggerates the alongshore
variations in wind speed due to the smoothed
orography with two artificial saddles. For reasons
unclear at this time, the winds tend to be weaker
than in observations south of the wind jet.
Figure 4 compares the monthly mean precipitation in the model with TRMM 3B42RT observations for July 2001. The model captures the
orographic effects of Indochina mountains, including the rain bands on the Cambodia coast at
the foothills of the Cardamom Hills and on the
west slope of the Annam cordillera as well as the
broad rain shadow lee of the Annams. We note
Orographic effects on SCS summer climate
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Fig. 4. Monthly mean precipitation (mm
day1 ) for July 2001: (a) TRMM 3B42RT
observations, and (b) the CTL run
that the rain band windward of the Annams is
not reproduced in a lower-resolution (0.5o) version of the IPRC RAM (Xie et al. 2006), suggesting the importance of sufficient resolution. The
model underestimates precipitation west of the
Philippines, a problem also noted in Xie et al.
(2006), illustrating the difficulty in simulating
the convection-circulation interaction. The southwesterly winds blowing west of Luzon Island are
weak by 2 m s1 compared to QuikSCAT observations (Fig. 3), presumably reducing orographic
effects of the island. Despite all these deficiencies, the model captures the salient features induced by Indochina mountains, encouraging us
to study the orographic effect of the Annams
with an experimental approach.
3.2 Topographic effects
With the Annam mountain range removed in the
NoTop run, the surface wind field is markedly
changed. Without the mountain barrier, air flows
freely across the Indochina Peninsula, and the
southwesterly winds become much smoother in
space over the northern SCS (Fig. 5a), especially
off the east coast of Vietnam. In particular, wind
speed markedly increases off the east coast of
Vietnam, and the wind jet core disappears. The
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Fig. 5. (a) 10-m wind velocity (m s1 ) and
land orography (shaded at 0.25, 0.5, and
1.0 km) in the NoTop run. (b) CTL-NoTop
difference in 10-m wind velocity (0.5 m s1
interval), with shade indicating values
passing a two-sided t test at the 99% significant level
effect of the Annam cordillera on the SCS wind
field can be better seen in the surface wind difference field between the CTL and NoTop runs
(Fig. 5b). The presence of the Annam range
reduces the southwesterlies in the lee by as much
as 5 m s1 while increases the wind speed at its
southern corner by up to 2 m s1 . Thus the SCS
wind jet is indeed a result of the orographic
blockage of the Annams, which forces the air
to rush through the southern flank of the mountain range.
Without the Annam mountain range, the precipitation distribution is also markedly changed
(Fig. 6a). The rain band on the windward side of
Orographic effects on SCS summer climate
283
Fig. 6. (a) July 2001 precipitation (mm
day1 ) in the NoTop run. (b) CTL-NoTop
difference in precipitation (mm day1 ),
with shade indicating values passing a twosided t test at the 99% significant level
the Annam cordillera disappears and the associated rainfall shadow weakens in the NoTop run,
resulting in a much smoother precipitation distribution over the Indochina Peninsula and western
SCS. The rainfall minimum on the coast of North
Vietnam appears due to the low-level wind divergence as wind accelerates offshore due to reduced surface roughness over the ocean. The
orographic effect on precipitation can be better
seen in the difference field between the CTL and
NoTop runs (Fig. 6b). A nearly zonally oriented
dipole of rainfall anomalies, positive and negative roughly west and east of coastal line of
Vietnam, respectively, indicates that the Annam
mountain range acts to force convection on the
windward side while suppressing it on the leeside.
Now we examine further the mechanism by
which the Annam mountain range affects the precipitation pattern. Figure 7 presents the cross sections of specific humidity and cloud liquid water
content in both the CTL and NoTop runs, along
with vertical velocity in the CTL run. The mountains force an updraft on the windward side
(Fig. 7c) with elevated cloud liquid water content
(Fig. 7a) and precipitation (Fig. 4b). On the lee
side, on the other hand, the mountains cause a
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Fig. 7. Vertical cross-sections for July 2001: specific humidity (contours in g kg1 ) and cloud liquid water content
(shaded in 102 g kg1 ) along 15 N in (a) the CTL and (b) NoTop runs; (c) vertical velocity (in Pa s1 , with the zero
contour omitted ) in the CTL run; (d) CTL – NoTop difference in temperature (contours at 0.5 C intervals) and specific
humidity (shaded in g kg1 ) between the CTL and NoTop runs. Topography is shaded black
strong downdraft (Fig. 7c), which along with the
windward precipitation depresses specific humidity by advecting dry air downward. About
100 km away from the coast, boundary layer
moisture recovers to values high enough for convection to become active east of 112 E.
In the NoTop run, the specific humidity
(Fig. 7b) and vertical velocity (not shown) do
not change much across the Indochina peninsula.
Without mountains, moisture depletion by orographic rainfall is greatly reduced (Fig. 7b),
and so is the subsidence off the east coast of
Vietnam (not shown). Both effects act to increase
moisture in the lower atmosphere over the western SCS near the Indochina Peninsula in the
NoTop run compared to the CTL run. The warming and drying induced by orographic subsidence
on the SCS reach as far as 100–200 km offshore,
amounting to 2 C and 1.5 g kg1 in magnitude,
respectively (Fig. 7d).
3.3 SST effects
Surface winds become uniform off the southeast
coast of Vietnam in the NoTop run (Fig. 5a),
indicating that it might be reasonable to assume
that the SST field would be uniform offshore
without the Annam cordillera. Thus, we use a
smoothed SST field (Fig. 2b) to remove the indirect orographic effect of the Annams on summer
SCS climate.
The simulated precipitation pattern over the
SCS in the NoTopSmSST run is similar to that
in the NoTop run, i.e., with a precipitation distribution quite uniform over the Indochina peninsula and western SCS (not shown). The removal
Orographic effects on SCS summer climate
285
Fig. 8. Precipitation difference of (contours
in mm day1 with the zero contour omitted)
between the NoTopSmSST and NoTop
runs, averaged for July 2001. Shading
denotes regions where the difference
passes a two-sided t test at the 95%
significant level
of the cold filament in the western SCS enhances precipitation there by about 2 mm day1,
as seen clearly in the precipitation difference
field between the NoTopSmSST and NoTop runs
(Fig. 8). Noticeable negative rainfall anomalies
also exist on the south edge of the Indochina
Peninsula, possibly a remote response to increased convection over the mid-SCS, which
forces subsidence Rossby waves to the west.
The indirect effect of the cold filament on rainfall is about 30–50% of the direct orographic
effect over the mid-SCS and is smaller on the
Indochina Peninsula.
4. Effect on ocean circulation
This section assesses the orographic effect of the
Annam cordillera on the SCS summer circulation
by using a 1.5-layer reduced-gravity ocean model. The model consists of two layers: the upper
active layer represents the upper ocean while the
lower layer represents the deep motionless ocean.
At the interface, the entrainment or detrainment
is parameterized following McCreary and Yu
(1992).
The model domain covers 0–24 N, 100–
124.5 E, and the coastline is set by the 50 m
isobath. The model grid size is 50 in both longitude and latitude. The water exchange between
the SCS and the adjacent seas is mainly through
the Luzon Strait, with all other straits closed. The
inflow velocity at the southern open boundary
(Luzon Strait) is calculated from the National
Centers for Environmental Predication Ocean
Data Assimilation System (NCEP-ODAS) tropical Pacific Ocean analysis (Ji et al. 1995). The
outflow velocity is determined by balancing volume transport. Zhuang et al. (2006) gives a detailed description of this ocean model and its
performance in the SCS.
The ocean model is forced by QuikSCAT
monthly climatological winds, spun up from the
resting state and integrated for 5 years. The annual-mean thermocline depth (102 m) and SST
(22.6 C) averaged in the SCS basin (deeper than
200 m) are used as the initial thickness and temperature of the upper layer, respectively. Sea surface heat flux is parameterized as a Haney (1971)
type relaxation toward observed SST plus the
Southampton constrained version of net heat flux
(Grist and Josey 2003). The annual cycle shows
no appreciable drift in the last three years, indicating that the model has reached a quasi-steady
state. Therefore, results of the fifth year are saved
for analyses. Besides this ocean control (OcnCTL)
simulation, we conduct an experiment by removing the orographic effect from the QuikSCAT
wind forcing (OcnNoTop). For simplicity, we define the orographic effect as the weighted July
wind velocity difference between the atmospher-
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Fig. 9. July–September mean stream function (contours at 0.5 Sv intervals) in the ocean model for (a) the OcnCTL and
OcnNoTop runs, with their difference shown in (c). (d) CTL-NoTop difference in surface wind stress (vectors in N m2 )
and its curl (contours at 1.0 107 N m3 intervals)
ic CTL and NoTop runs. The weights are (0.0,
0.5, 1.0, 1.0, 0.5, 0.0) for (May, June, July,
August, September, October) to mimic observations that the southwest wind jet begins to develop in June, is in full strength in July and August,
and weakens in September.
Figure 9 presents the three-month mean stream
function fields averaged for July, August, and
September (JAS). In the OcnCTL run, the simulated upper layer circulation displays a strong
anticyclonic gyre in the southern SCS, a weak
cyclonic gyre to the north, and an offshore jet
between the two gyres, which is in broad agree-
ment with observations of Fang et al. (2002). In
addition, two weak cyclonic centers are embedded in the northern gyre with one center just off
central Vietnam, and the other over the northeast
SCS around 20 N, 116 E (Fig. 9a).
With the orographic effect removed in the
OcnNoTop run, the northern cyclonic gyre retreats northeastward (Fig. 9b). The weak cyclonic recirculation off central Vietnam in the
OcnCTL is replaced by a weak anticyclone in the
OcnNoTop run. The offshore jet shows a meander pattern, which previous numerical studies
show is due to the north–south asymmetry in the
Orographic effects on SCS summer climate
wind stress forcing with a northeast tilted zerocurl line (Yasuda and Hanawa 1996). The orographic effect on the ocean circulation can be seen
more clearly in the OcnCTL minus OcnNoTop
stream function difference map (Fig. 9c), which
is dominated by negative stream function anomalies east of Central Vietnam lee of the Annams,
with weak positive anomalies south of the wind
jet. This anomalous circulation pattern is consistent with the anomalous wind curl that is much
larger to the north of the wind jet lee of the
Annam cordillera than to the south (Fig. 9d).
Thus, the cyclonic (re-)circulation north of the
wind jet owes its existence much to the orographic blockage by the Annams, which forces strong
positive wind curls there. The anticyclonic gyre
and recirculation, on the other hand, are only
weakly affected by the Annam orographic effect.
To the first order, the negative wind curls south of
the SCS wind jet are part of lager-scale atmospheric circulation rather than due to local orography, a result not obvious without numerical
experiments. Indeed, the southwesterly winds begin to transit to a southerly cross-equatorial wind
regime around 6–7 N in both observations and
the CTL simulation (Fig. 3).
5. Conclusions and discussion
A high-resolution regional atmospheric model is
used to study the effects of the Annam cordillera
on the summer climate of the South China Sea.
The model reproduces the salient features of SCS
summer climate as compared to QuikSCAT and
TRMM observations, including an intense wind
jet off the southeast coast of Vietnam, a precipitation band on the windward side of the Annam
mountain range, and a rainfall shadow on the
lee side. Our experiments with and without the
Annam cordillera demonstrate that these features
are indeed due to the orographic effects of the
mountain range. As the southwesterly winds impinge on the Annam range, the induced ascending motion promotes deep convection on the
windward side while the subsidence suppresses
convection on the lee side. At the southern tip of
the mountain range, the southwesterly winds
rush through to form a strong wind jet offshore.
Our results also show that the cold filament under the coast wind jet, which itself results from
orographic effect, is an additional factor help-
287
ing suppress atmospheric convection east of
Vietnam.
A 1.5-layer reduced-gravity ocean model is
also used to study the effect of orographicallyinduced wind changes on the SCS ocean circulation. As the ocean model is forced by monthly
mean climatological QuikSCAT winds, the simulated summer ocean circulation is characterized
by a strong anticyclonic gyre in the southern
SCS, a weaker cyclonic gyre in the northern
SCS, and a strong offshore ocean jet in between
off south Vietnam, in agreement with observations. With the orographic effects removed from
the QuikSCAT winds, the simulated ocean circulation changes markedly, with the northern
cyclonic gyre shifting northward. As a result,
the eastward ocean jet weakens east of south
Vietnam. Thus, the Annam cordillera exerts significant influences on the double-gyre circulation
in the summer SCS, especially the northern gyre
and the inter-gyre eastward offshore jet.
This study joins a growing body of literature
on coastal orography-induced air-sea interaction
phenomena (e.g., Xie et al. 2001, 2005; Chelton
et al. 2004). Owing to the increasing computing
power, some of these phenomena have been successfully simulated in numerical models, for example, west of Hawaii (Sakamoto et al. 2004;
Sasaki and Nonaka 2006) and Central America
(Sasai et al. 2007). Over the Asian summer monsoon region, narrow mountains such as the
Annam play an important role in the spatial distribution of convection (Chang et al. 2005; Xie
et al. 2006). On a smaller regional scale over the
SCS, the orographic effect of the Annams leaves
clear signatures in ocean circulation and SST. A
regional coupled ocean-atmosphere model (Xie
et al. 2007) is being developed to study the
ocean-atmosphere interaction over the SCS and
its effect on local and broad-scale monsoon.
Acknowledgments
We wish to thank Jan Hafner for archiving the TRMM-PR
and QuikSCAT data from Remote Sensing Systems’ Web
site and Y. Zhang for archiving the NCEP reanalysis dataset.
This work is supported by NSFC (40575045), 973 Program
(2006CB403607; 2004CB418304), Chinese Academy of
Sciences, NASA, and the Japan Agency for Marine-Earth
Science and Technology (JAMSTEC) through its sponsorship of International Pacific Research center at University
of Hawaii. IPRC publication 527 and SOEST publication
7473.
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