Circulation in the philippine archipelago simulated by 1/12° and 1/25° global HYCOM and EAS NCOM

Abstract

Three ocean models, 1/25° global HYbrid Coordinate Ocean Model (HYCOM), 1/12° global HYCOM, and EAS NCOM nested in global NCOM, were used to provide a global context for simulation of the circulation within the Philippine Archipelago as part of the Philippines Experiment (PhilEx). The Philippine Archipelago provides two significant secondary routes for both the Indonesian throughflow and the western boundary current of the Pacific northern tropical gyre. The deeper route enters the archipelago from the north through Mindoro Strait, after passing through Luzon Strait and the South China Sea. The second route enters directly from the Pacific via the shallow Surigao Strait and passes through Dipolog Strait downstream of the Bohol Sea. Both pathways exit via Sibutu Passage and the adjacent Sulu Archipelago along the southern edge of the Sulu Sea, and both are deeper than the pathway into the Indonesian Archipelago via Karimata Strait in the Java Sea. Within the Philippine Archipelago, these pathways make the dominant contribution to the mean circulation and much of its variability, while their outflow contributes to the flow through Makassar Strait, the primary conduit of the Indonesian throughflow, at all depths above the Sibutu Passage sill. Because of the narrow straits and small interior seas, the simulations are very sensitive to model resolution (4.4 km in 1/25° global HYCOM, 8.7 km in 1/12° global HYCOM, and 9.6 km in EAS NCOM in this latitude range) and to topographic errors, especially sill depths. The model simulations for 2004 and 2008 (the latter the central year of the PhilEx observational program) show extreme opposite anomalous years with anomalously strong southward Mindoro transport in 2004 and mean northward transport in 2008, but with little effect on the Surigao-Dipolog transport. Satellite altimetry verified the associated HYCOM sea surface height anomalies in the western tropical Pacific and the South China Sea during these extreme years. A 15-month (December 2007–March 2009) PhilEx mooring in Mindoro Strait measured velocity nearly top to bottom at a location close to the sill. The 1/12° global HYCOM, which showed the strongest flow above 200 m lay west of the mooring, was used to adjust a Mindoro transport estimate from the mooring data for cross-sectional distribution of the velocity, giving 0.24 Sv northward over the anomalous observational period. The results from the observational period were then used to adjust the 2004–2009 model transport, giving a mean of 0.95 Sv southward. The 1/25° global HYCOM simulated the observed four-layer flow in Dipolog Strait and the vigorous and persistent cyclonic gyre in the western Bohol Sea, observed during all four PhilEx cruises and in ocean color imagery. This gyre was poorly simulated by the two models with ~ 9 km resolution. Finally, a 1/12° global HYCOM simulation with tides generated the hydrostatic aspect of the internal tides within the Philippine Archipelago, including a strong internal tidal beam initiated at Sibutu Passage and observed crossing the Sulu Sea.

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2.
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3.
DATES
COVERED
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-
To)
4.
TITLE
AND
SUBTITLE
Circulation
in
the
Philippine
Archipelago
Simulated
by
1/12
degree
and
1/25
degree
Global
HYCOM
and
EAS
NCOM
5a.
CONTRACT
NUMBER
5b.
GRANT
NUMBER
5c.
PROGRAM
ELEMENT
NUMBER
0601153N
6.
AUTHOR(S)
Hrley
Hurlburt,
E.
Joseph
Metzger,
Janet
Sprintall,
Shelley
N.
Riedlinger,
Robert
A.
Arnone,
Toshiaki
Shinoda
and
Xiaobiao
Xu
5d.
PROJECT
NUMBER
5e.
TASK
NUMBER
5f.
WORK
UNIT
NUMBER
73-9255-10-5
7.
PERFORMING
ORGANIZATION
NAME(S)
AND
ADDRESS(ES)
Naval
Research
Laboratory
Oceanography
Division
Stennis
Space
Center,
MS
39529-5004
8.
PERFORMING
ORGANIZATION
REPORT
NUMBER
NRL/JA/7304--I0-04I9
9.
SPONSORING/MONITORING
AGENCY
NAME(S)
AND
ADDRESSIES)
Office
of
Naval
Research
800
N.
Quincy
St.
Arlington,
VA
22217-5660
10.
SPONSOR/MONITORS
ACRONYM(S)
ONR
11.
SPONSOR/MONITORS
REPORT
NUMBER(S)
12.
DISTRIBUTION/AVAILABILITY
STATEMENT
Approved
for
public
release,
distribution
is
unlimited.
13.
SUPPLEMENTARY
NOTES
20110224050
14.
ABSTRACT
In
PhilEx
1/12
and
1/25
world
ocean
models
provided
a
global
context
for
the
Philippine
Archipelago
circulation.
This
archipelago
provides
two
secondary
routes
for
both
the
Indonesian
throughflow
and
the
western
boundary
current
of
the
Pacific
northern
tropical
gyre.
These
enter
via
Mindoro
and
Surigao
Straits
and
exit
via
Sibutu
Passage.
The
outflow
continues
through
Makassar
Strait,
the
primary
conduit
of
the
Indonesian
throughflow,
at
all
depths
above
the
Sibutu
Passage
sill.
In
the
model
simulations
2004
and
2008,
2008
the
central
year
for
PhilEx
observations,
are
extreme
opposite
anomalous
years
with
strong
southward
Mindoro
transport
in
2004
and
mean
northward
transport
in
2008,
but
with
little
effect
on
the
second
route,
results
verified
using
satellite
altimetry.
A
PhilEx
mooring
in
Mindoro
Strait
and
1/12
global
HYCOM
were
used
to
estimate
mean
transports
of
0.24
Sv
northward
over
the
anomalous
observational
period
and
0.95
Sv
southward
over
2004-2009.
15.
SUBJECT
TERMS
Philippine
Archipelago
circulation,
Philippine
straits,
Mindoro
Strait
transport,
Indonesian
throughflow
16.
SECURITY
CLASSIFICATION
OF:
a.
REPORT
Unclassified
b.
ABSTRACT
Unclassified
c.
THIS
PAGE
Unclassified
17.
LIMITATION
OF
ABSTRACT
UL
18.
NUMBER
OF
PAGES
20
19a.
NAME
OF
RESPONSIBLE
PERSON
Harley
Hurlburt
19b.
TELEPHONE
NUMBER
Include
area
code)
228-688-4626
Standard
Form
298
(Rev.
8/98)
Prescribed
by
ANSI
Sid
Z39.18

PHILIPPINE
STRAITS
DYNAMICS
EXPERIMENT
Circulation
in
the
Philippine
Archipelago
Simulated
by
1/12°
and
1/25°
Global
HYCOM
and
EAS
NCOM
Q*E^
BY
HARLEY
E.
HURLBURT,
E.
JOSEPH
METZCER,
JANET
SPRINTALL,
SHELLEY
N.
RIEDLINCER,
ROBERT
A.
ARNONE,
TOSHIAKI
SHINODA,
AND
XIAOBIAO
XU
28
Oceanography
|
Vol.24,
No.
1

ABSTRACT.
Three
ocean
models,
1/25°
global
HYbrid
Coordinate
Ocean
Model
(HYCOM),
1/12°
global
HYCOM,
and
East
Asian
Seas
Navy
Coastal
Ocean
Model
(EAS
NCOM)
nested
in
global
NCOM,
were
used
to
provide
a
global
context
for
simulation
of
the
circulation
within
the
Philippine
Archipelago
as
part
of
the
Philippine
Straits
Dynamics
Experiment
(PhilEx).
The
Philippine
Archipelago
provides
two
significant
secondary
routes
for
both
the
Indonesian
throughflow
and
the
western
boundary
current
of
the
Pacific
northern
tropical
gyre.
The
deeper
route
enters
the
archipelago
from
the
north
through
Mindoro
Strait,
after
passing
through
Luzon
Strait
and
the
South
China
Sea.
The
second
route
enters
directly
from
the
Pacific
via
the
shallow
Surigao
Strait
and
passes
through
Dipolog
Strait
downstream
of
the
Bohol
Sea.
Both
pathways
exit
via
Sibutu
Passage
and
the
adjacent
Sulu
Archipelago
along
the
southern
edge
of
the
Sulu
Sea,
and
both
are
deeper
than
the
pathway
into
the
Indonesian
Archipelago
via
Karimata
Strait
in
the
Java
Sea.
Within
the
Philippine
Archipelago,
these
pathways
make
the
dominant
contribution
to
the
mean
circulation
and
much
of
its
variability,
while
their
outflow
contributes
to
the
flow
through
Makassar
Strait,
the
primary
conduit
of
the
Indonesian
throughflow,
at
all
depths
above
the
Sibutu
Passage
sill.
Because
of
the
narrow
straits
and
small
interior
seas,
the
simulations
are
very
sensitive
to
model
resolution
(4.4
km
in
1/25°
global
HYCOM,
8.7
km
in
1/12°
global
HYCOM,
and
9.6
km
in
EAS
NCOM
in
this
latitude
range)
and
to
topographic
errors,
especially
sill
depths.
The
model
simulations
for
2004
and
2008
(the
latter
the
central
year
of
the
PhilEx
observational
program)
show
extreme
opposite
anomalous
years
with
anomalously
strong
southward
Mindoro
transport
in
2004
and
mean
northward
transport
in
2008,
but
with
little
effect
on
the
Surigao-Dipolog
transport.
Satellite
altimetry
verified
the
associated
HYCOM
sea
surface
height
anomalies
in
the
western
tropical
Pacific
and
the
South
China
Sea
during
these
extreme
years.
A
15-month
(December
2007-March
2009)
PhilEx
mooring
in
Mindoro
Strait
measured
velocity
nearly
top
to
bottom
at
a
location
close
to
the
sill.
The
1/12°
global
HYCOM,
which
showed
the
strongest
flow
above
200
m
lay
west
of
the
mooring,
was
used
to
adjust
a
Mindoro
transport
estimate
from
the
mooring
data
for
cross-sectional
distribution
of
the
velocity,
giving
0.24
Sv
northward
over
the
anomalous
observational
period.
The
results
from
the
observational
period
were
then
used
to
adjust
the
2004-2009
model
transport,
giving
a
mean
of
0.95
Sv
southward.
The
1/25°
global
HYCOM
simulated
the
observed
four-layer
flow
in
Dipolog
Strait
and
the
vigorous
and
persistent
cyclonic
gyre
in
the
western
Bohol
Sea,
observed
during
all
four
PhilEx
cruises
and
in
ocean
color
imagery.
This
gyre
was
poorly
simulated
by
the
two
models
with
~
9
km
resolution.
Finally,
a
1/12°
global
HYCOM
simulation
with
tides
generated
the
hydrostatic
aspect
of
the
internal
tides
within
the
Philippine
Archipelago,
including
a
strong
internal
tidal
beam
initiated
at
Sibutu
Passage
and
observed
crossing
the
Sulu
Sea.
Oceanography
|
March
2011
29

INTRODUCTION
In
the
Philippine
Straits
Dynamics
Experiment
(PhilEx),
global
ocean
models
with
resolutions
as
fine
as
1/25°
(4.4
km
over
the
latitude
range
0-11°
and
finer
at
higher
latitudes)
are
used
to
investigate
the
circulation
within
the
Philippine
Archipelago
in
a
global
context,
both
spatially
and
temporally.
These
models
also
provide
boundary
conditions
for
nested
regional
models
(e.g.,
Han
et
al.,
2009;
Arango
et
al.,
2011;
Lermusiauxet
al.,
2011),
with
nests
as
fine
as
~
1
km
feasible
with
boundary
conditions
directly
from
the
1/25°
global
model.
A
global
context
is
essential
because
the
Philippines
provide
secondary
pathways
for
the
Pacific
to
Indian
Ocean
throughflow
(Ilahude
and
Gordon,
1996;
Metzger
et
al.,
2010)
and
secondary
routes
to
close
the
northern
tropical
gyre,
which
spans
the
North
Pacific
between
the
North
Equatorial
Countercurrent
(NECC)
on
the
south
and
the
North
Equatorial
Current
(NEC)
on
the
north.
This
gyre
is
bounded
on
the
west
by
the
southward
Mindanao
Current
and
by
secondary
routes
through
the
Philippine
Archipelago
(Metzger
and
Hurlburt,
1996).
Metzger
et
al.
(2010)
show
outflow
from
Sibutu
Passage
feeding
both
into
the
Indonesian
throughflow
via
Makassar
Strait
and
into
the
Pacific
northern
tropical
gyre
via
the
NECC
in
a
1/12°
global
simula-
tion
(their
Figure
9a).
The
deepest
of
the
secondary
routes
surrounds
most
of
the
archipelago
via
Luzon
Strait
to
the
north
and
Mindoro
Strait
and
Sibutu
Passage
to
the
west.
Within
the
archipelago,
these
secondary
pathways
constitute
the
dominant
contribution
to
the
mean
circulation
and
are
responsible
for
much
of
its
variability.
Han
et
al.
(2009)
discuss
the
impacts
of
remote
and
local
forcing
on
the
seasonal
variability.
Realistic
modeling
of
the
circulation
within
the
Philippine
Archipelago
is
an
extreme
challenge
for
a
global
ocean
model
due
to
the
numerous
narrow
straits
and
small
interior
seas.
Accurate
modeling
of
flows
through
straits
requires
accurate
modeling
of
the
effects
of
hydraulic
control
and
appropriate
partitioning
between
geostrophic
and
hydraulic
control.
In
a
complex
archi-
pelago
like
the
Philippines,
the
challenge
is
increased
by
the
need
to
correctly
partition
the
flow
among
numerous
alternative
routes
throughout
the
archi-
pelago.
These
issues
make
simulations
in
this
region
particularly
sensitive
to
model
resolution,
to
errors
in
model
topography
and
atmospheric
forcing,
Harley
E.
Hurlburt
(harley.hurlburt@nrlssc.navy.mil)
is
Senior
Scientist
for
Ocean
Modeling
and
Prediction,
Oceanography
Division,
Naval
Research
Laboratory
(NRL),
Stennis
Space
Center,
MS,
USA.
E.Joseph
Metzger
is
Meteorologist,
Oceanography
Division,
NRL,
Stennis
Space
Center,
MS,
USA.
Janet
Sprintall
is
Research
Oceanographer,
Climate,
Atmospheric
Science,
and
Physical
Oceanography
Division,
Scripps
Institution
of
Oceanography,
Lajolla,
CA,
USA.
Shelley
N.
Riedlinger
is
Oceanographer,
Oceanography
Division,
NRL,
Stennis
Space
Center,
MS,
USA.
Robert
A.
Arnone
is
Head,
Ocean
Sciences
Branch,
Oceanography
Division,
NRL,
Stennis
Space
Center,
MS,
USA.
Toshiaki
Shinoda
is
Oceanographer,
Oceanography
Division,
NRL,
Stennis
Space
Center,
MS,
USA.
Xiaobiao
Xu
is
Research
Scientist,
Department
of
Marine
Sciences,
University
of
Southern
Mississippi,
Stennis
Space
Center,
MS,
USA.
and
to
model
numerics
and
physics.
Thus,
the
Philippine
Archipelago
poses
severe
tests
for
the
models,
tests
that
are
performed
using
data
from
the
PhilEx
field
program
and
other
sources.
In
turn,
the
models
are
used
to
help
interpret
the
data
and
their
ability
to
measure
observed
phenomena,
to
place
the
observations
within
the
context
of
the
larger-scale
circulation
and
its
temporal
variability
and
to
help
understand
the
dynamics
of
observed
phenomena.
IXPtRIMlNl
S
Philippine
Archipelago
circulation
is
investigated
using
1/12°
and
1/25°
global
simulations
by
the
HYbrid
Coordinate
Ocean
Model
(HYCOM;
Bleck,
2002)
and
data
assimilative
nowcasts
by
the
East
Asian
Seas
Navy
Coastal
Ocean
Model
(EAS
NCOM)
with
tides.
EAS
NCOM
(9.6-km
resolu-
tion
at
10°
latitude)
has
been
running
in
real
time
since
October
2003,
and
is
nested
in
global
NCOM
(Barron
et
al.,
2006),
which
has
19.2-km
resolu-
tion
at
10°
latitude.
Global
NCOM
is
an
operational
forecast
model
of
the
US
Navy
(without
tides)
that
assimilates
a
wide
variety
of
ocean
data.
Table
1
summarizes
the
characteristics
of
the
HYCOM
simulations
and
EAS
NCOM.
The
global
HYCOM
simulations
were
spun
up
for
10
years
after
initializa-
tion
from
the
Generalized
Digital
Environmental
Model
3
(GDEM3)
hydrographic
climatology
(Carnes,
2009)
and
forced
with
an
atmospheric
climatology
derived
from
the
European
Centre
for
Medium-Range
Weather
Forecasts
(ECMWF)
40-year
reanalysis
(ERA-40)
(Kallberg
et
al.,
2004;
HYCOM
Exps.
1/12°-18.0
and
l/25°-4.0).
The
simulations
were
then
continued
30
Oceanography
|
Vol.24,
No.
1

Table
1.
Ocean
model
experiments
Horizontal
—
•
Ocean
Model
txpenment
Resolution
Vertical
Atmospheric
Years
Number
1
at
10°N
a
Resolution
11
Forcing'
Used
Data
les
Assimilation
1/12°
global
HYCOM
18.0
8.7
km
32
coordinate
surfaces
ECMWF/
QuikSCAT
5-10
No
No
1/12°
global
HYCOM
18.2
8.7
km
32
coordinate
surfaces
NOGAPS/
ECMWF/
QuikSCAT
2003-2010
No
No
1/25°
global
HYCOM
4.0
4.4
km
32
coordinate
surfaces
ECMWF/
QuikSCAT
5-10
No
No
1/25°
global
HYCOM
4.1
&2
d
4.4
km
32
coordinate
surfaces
NOGAPS/
ECMWF/
QuikSCAT
2004-2009
No
No
1/12°
global
HYCOM
14.1&2
e
8.7
km
32
coordinate
surfaces
NOGAPS/
ECMWF
2004-2008
Yes
f
No
EAS
NCOM
—
9.6
km
40
levels
NOGAPS
2004-2009
Yes
f
Yes
'
Resolution
for
each
prognostic
variable.
For
HYCOM
the
nominal
resolution
in
degrees
is
the
equatorial
resolution,
which
is
.08°
»
1/12°
and
.04°
=
1/25°.
The
HYCOM
experiments
are
from
the
CLBa
series
and
all
experiments
use
topography
based
on
DBDB2
by
D.S.
Ko
(see
http://www7320.nrlssc.navy.mil/DBDB2_WWW).
b
HYCOM
has
a
hybrid
isopycnal/pressure
=
depth/terrain-following
vertical
coordinate.
NCOM
has
depth
coordinates
with
terrain-following
at
depths
shallower
than
137
m.
c
See
text.
6
Exp.
4.2
is
a
2005-2009
extension
of
4.1
with
changes
in
some
frictional
parameter
values
in
a
remote
area.
e
Exp.
14.2
is
a
one-month
(May
2004)
repeat
of
14.1
with
global
hourly
three-dimensional
output.
'
Eight
tidal
constituents.
interannually
using
archived
operational
forcing
from
the
Navy
Operational
Global
Atmospheric
Prediction
System
(NOGAPS)
(Rosmond
et
al.,
2002),
but
with
the
long-term
annual
mean
replaced
by
the
long-term
mean
from
ERA-40
(Exp.
1/12°-18.2
initial-
ized
from
18.0
and
Exp.
l/25°-4.18c2
from
4.0
[Table
1]).
In
most
of
the
experiments,
wind
speed
was
corrected
using
a
monthly
climatology
from
the
QuikSCAT
scatterometer
(Kara
et
al.,
2009).
Model
experiment
1/12°
global
HYCOM-14.1&.2
is
the
worlds
first
eddy-resolving
global
ocean
simulation
that
includes
both
the
atmospheri-
cally
forced
ocean
circulation
and
tides
(Arbic
et
al.,
2010).
The
1/25°
global
HYCOM
began
running
on
January
12,
2009,
and
has
the
highest
resolution
used
thus
far
in
a
global
ocean
general
circulation
model
(OGCM)
with
ther-
mohaline
dynamics
and
more
than
a
few
layers
in
the
vertical.
HYCOM
is
a
community
ocean
model
(http://www.hycom.org)
with
a
general-
ized
vertical
coordinate
because
no
single
coordinate
is
optimal
everywhere
in
the
global
ocean.
Isopycnal
(density-
tracking)
layers
are
best
in
the
deep
stratified
ocean,
pressure
levels
(nearly
fixed
depths)
provide
high
vertical
reso-
lution
in
the
mixed
layer,
and
o-levels
(terrain-following)
are
often
the
best
choice
in
coastal
regions
(Chassignet
et
al.,
2003).
The
generalized
vertical
coordinate
in
HYCOM
allows
a
combi-
nation
of
all
three
types
(and
others),
and
the
optimal
distribution
is
chosen
dynamically
at
every
time
step
using
the
layered
continuity
equation.
NCOM
is
a
depth
coordinate
ocean
model
with
a
terrain-following
coordinate
at
depths
shallower
than
137
m.
MEAN
CIRCULATION
SIMULATED
BY
1/12°
AND
1/25°
GLOBAL
HYCOM
AND
THE
IMPACT
OF
TOPOGRAPHIC
ERRORS
Figure
la,b
is
a
comparison
of
the
mean
currents
at
20-m
depth
and
strait
transports
from
1/12°
global
HYCOM-
18.2
with
those
from
1/25°
global
HYCOM-4.1&2,
and
Figure
led
is
a
comparison
of
their
respective
topogra-
phies
and
sill
depths.
Because
the
1/12°
topography
was
derived
from
the
1/25°,
they
demonstrate
close
agreement
in
deep
water,
although
numerous
hand
Oceanography
|
March
2011
31

edits
were
subsequently
applied
to
the
1/12°
topography
(Metzger
et
al.,
2010),
mainly
in
shallow
water
and
to
correct
sill
depths.
Edits
to
the
1/25°
topography
were
done
later,
and
fewer
were
made.
In
Figure
1,
some
of
the
sill
depths
in
the
two
models
are
in
good
agreement,
but
there
are
substantial
differences
in
the
Dipolog
Strait,
Surigao
Strait,
and
Sibutu
Passage
sill
depths
and
in
the
topography
along
the
entire
southern
archipelago
of
the
Sulu
Sea.
The
topography
of
this
archipelago
is
not
adequately
known,
nor
are
the
sill
depths
of
San
Bernardino
and
Surigao
straits.
Figure
la,b
depicts
two
main
routes
for
flow
through
the
Philippine
Archipelago:
a
deeper
pathway
from
the
South
China
Sea
via
Mindoro
Strait
to
outflow
through
Sibutu
Passage,
and
a
shallow
pathway
from
the
Pacific
via
Surigao
and
Dipolog
straits,
also
to
outflow
through
Sibutu
Passage.
With
the
resolution
increase
from
1/12°
to
1/25°,
a
very
large
increase
in
transport
is
seen
through
Mindoro
Strait
(where
the
topography
and
sills
depths
are
in
good
agreement)
and
through
Sibutu
Passage
(Figure
la.b).
In
contrast,
a
relatively
modest
increase
occurs
through
Surigao
and
Dipolog
(despite
the
very
large
Dipolog
sill
depth
error
and
a
1.8-fold
deeper
Surigao
sill
depth
in
1/25°
global
HYCOM).
These
results
indicate
that
the
mean
transport
of
the
Mindoro
to
Sibutu
pathway
is
largely
constrained
by
the
outflow
through
Sibutu
Passage,
while
the
transport
of
the
shallower
pathway
is
mainly
deter-
mined
by
the
inflow
through
Surigao.
Thus,
it
is
most
critical
to
improve
the
topography
and
sill
depths
of
Sibutu
15N-
a)1/12
0
c|lobalHYCOM-18.2|
'-•
\m\iii\iT%-
•
•
..'V
T
TE
hs.'.'A'/M
-0.70
•
:
P*
'/
W^~
*
15N
ION
120E
125E
d)1/25°B
asibutu^272ml
120E
125E
Figure
1.
(a,b)
Mean
currents
(m
s')
overlaid
on
speed
(in
color)
at
20-m
depth
in
and
around
the
Philippine
seas
from
(a)
1/12"
global
HYbrid
Coordinate
Ocean
Model
(HYCOM)-18.2
and(b)
1/2S"
global
HYCOM-4.1&2.
See
Table
1
and
related
discussion
in
the
"Ocean
Model
Experiments"
section.
The
2004-2009
mean
transport
through
straits
labeled
on
(a.b)
(in
Sv
=
10
6
m's
')
is
given
in
boxes
with
negative
values
for
southward
and
west
ward,
as
indicated
by
the
attached
arrows.
The
speed
contour
is
0.0S
m
s
'
and
the
reference
vector
is
0.5ms'
(upper
right
in
panel
b).
Every
second
(fourth)
vector
is
plotted
in
panel
a
(b).
(c,d)
Bottom
topography
for
(a)
1/12-
global
HYCOM
and(b)
1/25"
global
HYCOM
with
sill
depths
given
for
key
straits.
32
Oceanography
|
Vol.24,
No.
1

Passage
and
the
adjacent
passages
through
the
Sulu
Archipelago.
There
is
also
a
2.5-fold
increase
in
transport
through
the
very
shallow
San
Bernardino
Strait.
The
increased
transport
through
San
Bernardino
has
a
marked
impact
on
the
mean
flow
through
the
interior
passages
of
the
Philippines
between
Surigao
Strait
and
Tablas
Strait,
which
lies
just
east
of
Mindoro
(Figure
2a).
In
1/12°
(1/25°)
global
HYCOM,
there
is
a
net
transport
of
0.11
Sv
(0.04
Sv)
from
Surigao
to
Tablas
versus
0.13
Sv
(0.33
Sv)
from
San
Bernardino
to
Tablas.
In
1/12°
global
HYCOM,
both
of
the
straits
bordering
the
Pacific,
San
Bernardino
and
Surigao,
are
char-
acterized
by
choke
points
that
are
one
grid
point
wide
and
one
grid
point
long,
so
why
the
large
difference
in
the
trans-
ports
(Figure
la)?
An
explanation
based
solely
on
the
difference
in
sill
depths
(Figure
lc)
is
not
sufficient.
A
model
by
Mattsson
(1995;
a
barotropic
version
of
Equation
A5
in
Metzger
and
Hurlburt,
1996)
relates
transport
through
a
strait
(Q)
to
the
upstream-downstream
change
in
sea
surface
height
(ASSH)
based
on
a
combination
of
geostrophic
and
hydraulic
control,
the
latter
including
contributions
from
Bernoulli
setdown
and
bottom
friction.
The
relationship
between
Q
and
ASSH
is
given
entirely
in
terms
of
physical
and
geometric
parame-
ters.
The
mean
2004-2009
ASSH
is
8
cm
for
San
Bernardino
and
6
cm
for
Surigao.
Using
the
sill
depth
and
a
bottom
fric-
tion
coefficient
of
C
b
=
2.5
x
10
3
gives
close
agreement
at
San
Bernardino,
but
not
at
Surigao.
At
San
Bernardino,
the
depth
in
the
choke
point
is
29
m
and
the
sill
is
one
grid
point
upstream.
In
Figure
2.
(a-d)
Mean
meridi-
onal
velocity
cross
sections
at
1
r54'N,
the
latitude
of
the
PhilEx
Mindoro
mooring
(marked
with
a
vertical
line
on
the
cross
sections),
near
the
location
where
Mindoro
and
Tablas
straits
join
(dashed
line
on
Figure
1a).
The
labeled
transports
are
for
the
entire
cross
section
and
all
means
are
over
the
time
period
labeled
on
the
figure
panel,
(e-h)
Seasonal
means
over
2004-2009
for
JFM
(January-March,
winter),
AMJ
(spring),
JAS
(summer),
and
OND
(fall).
All
are
from
1/12*
global
HYCOM-18.2,
except
(c)
is
from
East
Asian
Seas
Navy
Coastal
Ocean
Model
(EAS
NCOM.)
120.5E
121E
121.5E
122E
120.5E
121E
121.5E
122E
-50
-40
-30
-20-10
0
10
20
30
40
50cm/s
Oceanography
|
March
2011
33

contrast,
at
Surigao
the
depth
in
the
choke
point
is
987
m
and
located
at
the
western
exit
point,
while
the
sill
is
near
the
eastern
entrance
in
a
relatively
broad
area
of
similar
depths.
At
the
Surigao
exit
point,
the
current
has
extended
down-
ward
through
the
mixed
layer
and
into
the
upper
thermocline
(~
120-m
deep).
When
a
depth
of
120
m
and
C
b
=
0
is
used
at
the
choke
point
in
Surigao,
the
ASSH
predicted
by
Equation
A5
is
in
close
agreement
with
the
6
cm
seen
in
the
model,
supporting
the
earlier
indica-
tion
that
Surigao
controls
the
transport
of
the
Surigao-Dipolog-Sibutu
route
for
flow
through
the
Philippine
Archipelago.
This
article
contains
all
the
information
used
in
the
barotropic
version
of
Equation
A5
in
Metzger
and
Hurlburt
(1996).
Additional
factors
also
have
an
impact
on
flow
through
the
straits,
such
as
differences
in
topographic
configura-
tion,
increased
numerical
accuracy
at
higher
resolution,
effects
of
tides
on
bottom
friction,
and
reduced
horizontal
friction
with
increased
resolution
(because
some
friction
parameters
scale
with
model
resolution).
The
outflow
from
the
interannual
HYCOM
simulations,
discussed
above,
contributes
to
the
transport
through
Makassar
Strait
at
all
depths
above
the
Sibutu
Passage
sill.
However,
despite
the
differences
in
Sibutu
Passage
transport
in
these
simulations,
the
transport
through
Makassar
Strait
is
nearly
the
same
(Table
2).
These
results
imply
either
a
variation
in
the
contribution
from
the
Mindanao
Current
(see
Figure
la)—the
case
in
these
simulations—or
a
direct
contribution
from
Sibutu
Passage
to
the
NECC,
as
seen
in
the
results
of
Metzger
et
al.
(2010).
The
impact
on
the
contribu-
tion
from
the
Mindanao
Current,
while
Makassar
transport
remains
unchanged,
suggests
an
indirect
contribution
to
the
NECC,
as
demonstrated
by
Metzger
and
Hurlburt
(1996,
their
Plate
2)
and
in
the
next
section.
Additionally,
Metzger
and
Hurlburt
(1996,
their
Table
3b)
performed
a
set
of
eight
global
ocean
Table
2.
Transports
(Sv)
through
straits,
model
2004-2009
vs.
observed
Transect
Orientation
EFJ
V
HYCOM
18.2
1/25"
HYCOM
4.1
and
4.2
Observed
Transport
EAS
NCOM
b
Mindoro
EW
-2.85
-0.70
-2.72
-0.95
c
Mindoro
overflow
d
EW
-0.08
-0.22
-0.21
-0.28
c
San
Bernardino
EW
-0.18
-0.13
-0.33
—
Surigao
EW
-1.45
-0.96
-1.15
—
Sibutu
EW
-4.81
1.71
-4.18
—
Tablas
EW
-0.51
-0.23
-0.36
—
Dipolog
NS
-1.10
-0.85
-1.11
—
Verde
Island
NS
-0.01
0.0
0.01
—
Balabac
NS
0.19
-0.08
-0.01
—
Luzon
NS
-4.92
-2.89
-5.17
-3.0
e
Taiwan
EW
1.40
1.60
1.74
1.8"
Karimata
EW
-0.50
-0.54
-0.61
-0.8*
Makassar
EW
-12.26
-14.00
-13.70
I1.6
h
'
The
transect
orientation
is
either
east-west
(EW)
or
north-south
(NS),
and
the
sign
convention
is
positive
northward/eastward
and
negative
southward/westward.
b
The
EAS
NCOM
mean
transport
is
computed
over
the
period
February
2004
through
December
2009.
c
See
Table
3.
d
Transport
below
350
m.
'
Qu
(2000),
based
on
hydrographic
data
down
to
400
db.
'
Wanget
al.
(2003),
based
on
2.5
years
(1999-2001)
of
shipboard
acoustic
Doppler
current
profiler
data.
8
Fangetal.
(2010),
extrapolated
estimate
based
on
11
months
(December
4,
2007-November
1,
2008)
of
mooring
data
h
Gordon
et
al.
(2008),
based
on
three
years
(2004-2006)
of
mooring
data.
34
Oceanography
|
Vol.24,
No.
1

simulations
with
a
wide
range
of
trans-
ports
(0
to
12
Sv)
through
Sibutu
Passage
and
through
Karimata
Strait
in
the
western
Java
Sea.
In
their
simulations,
the
transport
of
Pacific
Ocean
to
Indian
Ocean
throughflow
was
very
insensitive
to
the
transports
through
these
straits,
and
the
Makassar
transport
was
insensi-
tive
to
Sibutu
Passage
transport.
All
of
their
simulations
used
the
same
monthly
global
wind
stress
climatology,
except
that
the
wind
stress
was
zeroed
over
the
Philippine
Archipelago,
the
South
China
Sea,
and
northeast
of
Luzon
Strait
in
two
of
the
simulations.
MINDORO
STRAIT:
THE
DEEPEST
CONNECTION
TO
THE
PHILIPPINE
SEAS
Mindoro
Strait
is
the
deepest
passage
connecting
the
interior
seas
of
the
Philippine
Archipelago
to
the
large-
scale
ocean
circulation
(Figure
1)
and
is
predominantly
an
inflow
pathway.
Figure
2
depicts
cross
sections
of
meridi-
onal
velocity
through
Mindoro
and
Tablas
straits
near
the
location
where
they
join
(section
marked
by
the
dashed
line
on
Figure
la).
The
latitude
of
the
cross
section
coincides
with
that
of
a
PhilEx
mooring
near
the
Mindoro
sill,
which
is
depicted
by
a
vertical
line
in
Figure
2.
Strong
seasonal
and
interannual
variabilities
of
comparable
amplitude
are
evident
in
the
meridi-
onal
velocity,
with
variability
seen
at
all
depths,
but
with
the
largest
variability
located
in
and
above
the
thermocline
(Figure
2).
The
extreme
opposite
trans-
ports
during
the
seasonal
cycle
occur
during
the
monsoon
transition
seasons
with
the
largest
southward
transport
occurring
during
the
boreal
fall
onset
of
the
northeast
monsoon
(Figure
2h)
and
mean
northward
transport
during
the
spring
transition
to
the
southwest
monsoon
(Figure
2f).
Weaker
southward
transport
occurs
during
the
winter
peak
of
the
northeast
monsoon
(Figure
2e),
and
mean
transport
is
very
low
during
the
summer
southwest
monsoon
sensitive
indicator
of
past
La
Nina
events
(Hanley
et
al.,
2003).
The
National
Climatic
Data
Center
(http://www.ncdc.
noaa.gov)
shows
a
positive
Southern
Oscillation
Index
during
nearly
all
of
2008
(an
indication
of
La
Nina),
but
the
NOAA
Climate
Prediction
Center
REALISTIC
MODELING
OF
THE
CIRCULATION
WITHIN
THE
PHILIPPINE
ARCHIPELAGO
IS
AN
EXTREME
CHALLENGE
FOR
A
GLOBAL
OCEAN
MODEL
DUE
TO
THE
NUMEROUS
NARROW
STRAITS
AND
SMALL
INTERIOR
SEAS.
(Figure
2g),
similar
to
the
seasonal
cycle
reported
by
Han
et
al.
(2009),
who
focused
on
the
upper
40
m
and
found
that,
below
the
Ekman
layer,
the
seasonal
cycle
of
the
Mindoro-Sibutu
pathway
is
driven
largely
by
remote
forcing.
The
central
year
for
PhilEx
measure-
ments
was
2008,
but
in
1/12°
global
HYCOM-18.2,
2008
is
a
very
anomalous
year
(Figure
2d)
with
flow
through
Mindoro
Strait
that
is
much
like
the
spring
mean
(Figure
2f),
including
similar
mean
northward
transport
through
Mindoro
plus
Tablas.
In
contrast,
2004
is
an
extreme
opposite
anomalous
year
(Figure
2b),
much
like
the
fall
mean
(Figure
2h),
the
season
with
the
largest
southward
transport.
In
the
model,
the
periods
of
the
anomalies
are
well
captured
by
the
two
calendar
years.
Neither
year
is
a
traditional
El
Nino
or
La
Nina
year
based
on
the
Japan
Meteorological
Agency
(JMA)
index,
which
has
been
a
particularly
(http://www.cpc.noaa.gov)
reported
La
Nina
conditions
only
from
late
2007
to
May
2008.
However,
2004
has
been
identified
as
an
El
Nino
Modoki
(pseudo-El
Nino)
year,
where
the
primary
warm
sea
surface
temperature
(SST)
anomaly
is
located
in
the
central
equatorial
Pacific,
flanked
by
cool
anom-
alies
at
the
eastern
and
western
ends
of
the
tropical
Pacific
(Ashok
et
al.,
2007).
Figure
3
presents
2004
and
2008
comparisons
between
1/12°
global
HYCOM-18.2
sea
surface
height
(SSH)
anomalies
and
those
observed
by
satellite
altimetry
(on
a
1°
grid
from
Aviso,
2010).
Both
the
model
and
the
altimetry
show
the
opposite
anoma-
lies
of
2004
and
2008
spanning
the
domain
depicted
in
Figure
3,
essentially
a
strengthening
(weakening)
of
the
northern
tropical
gyre
in
the
western
Pacific
in
2004
(2008),
but
with
the
anomalies
extending
northward
to
the
entrance
of
Luzon
Strait
into
the
South
Oceanography
|
March
2011
35

China
Sea,
well
north
of
the
northern
tropical
gyre
and
the
North
Equatorial
Current,
the
strength
of
which
is
not
substantially
impacted
by
the
anomalies.
A
strong
gradient
in
SSH
anomaly
is
seen
at
the
northern
edge,
part
of
which
extends
into
the
South
China
Sea
and
the
Philippine
Archipelago.
Associated
with
the
gradient
in
SSH
anomaly
is
an
anomaly
in
Luzon
Strait
transport
and
downstream,
a
transport
anomaly
entering
the
Philippine
Archipelago
via
Mindoro
Strait.
The
2004
anomaly
increases
the
southward
transport
entering
the
Philippine
Archipelago
through
Mindoro
from
the
2004-2009
mean
of
0.70
Sv
to
2.41
Sv
and
the
southward
transport
exiting
through
Sibutu
Passage
from
1.71
to
3.39
Sv.
The
increases
are
1.71
Sv
through
Mindoro
and
1.68
Sv
through
Sibutu.
The
Sibutu
outflow
transport
minus
the
Mindoro
inflow
gives
the
combined
contribution
to
Sibutu
Passage
outflow
from
other
straits,
which
is
1.01
Sv
in
the
2004-2009
mean
and
0.98
Sv
in
the
2004
mean.
Thus,
the
combined
contribution
to
the
2004
Sibutu
outflow
anomaly
from
the
other
straits
(labeled
on
Figure
1)
is
minimal.
The
largest
of
these
contri-
butions
to
Sibutu
Passage
transport
is
the
direct
inflow
from
the
Mindanao
Current
through
Surigao
Strait,
which
has
large
seasonal
variability
but
little
interannual
variability
in
the
yearly
means.
The
transport
through
Mindoro
Strait
in
2008
demonstrates
a
1.66
Sv
anomaly
in
the
opposite
direction
from
2004
(from
0.70
Sv
southward
in
the
mean
to
0.96
Sv
northward).
Since
Sibutu
Passage
transport
changes
from
a
mean
of
1.71
Sv
southward
to
0.07
Sv
northward
in
2008,
the
outflow
through
Mindoro
is
fed
largely
by
inflow
through
Surigao
Strait
via
Dipolog
Strait.
Consistent
with
these
results,
the
SSH
at
the
two
ends
of
Surigao
Strait
rises
or
falls
in
tandem
by
5-6
cm
from
the
2004-2009
mean
during
each
of
the
anomalous
years,
and
the
upstream-
downstream
ASSH
values
for
the
mean
and
the
two
anomalous
years
are
the
same
within
1
cm;
the
situation
is
similar
for
San
Bernardino
Strait.
During
PhilEx,
a
15-month
time
series
of
velocity
versus
depth
was
obtained
from
the
mooring
in
Mindoro
Strait.
The
measurements
cover
nearly
the
full
water
column
in
the
deepest
part
of
the
strait
near
the
sill
(vertical
lines
in
Figure
2).
These
measurements
allow
the
first
estimate
of
transport
through
Mindoro
Strait
based
on
in
situ
observations.
Because
there
was
only
one
mooring
in
the
strait,
results
from
1/12°
global
HYCOM-18.2
are
used
in
combination
with
the
data
to
better
include
the
impact
of
the
cross-strait
flow
structure.
Additionally,
the
model
l/12°globalHYCOM18.2
1°
Aviso
2
ON
HOE
120E
130E
140E
150E
160E
170E
180W
170W
-.2
.1
0.
0E
120E
130E
140E
150E
160E
170E
180W
170W
0.2
m
0.1
Figure
3.
2004
(a,b)
and
2008
(c,d)
mean
sea
surface
height
(SSH)
anomalies
from
(a,c)
1/12°
global
HYCOM-18.2
with
respect
to
a
2004-2009
mean
and
(b,d)
1"
Aviso
analyses
of
altimeter
data
with
respect
to
a
2002-2008
mean.
The
contour
interval
is
0.02
m.
36
Oceanography
|
Vol.24,
No.
1

results
allow
an
extension
of
the
mean
transport
estimate
obtained
over
the
highly
anomalous
PhilEx
time
period,
centered
on
2008,
to
a
more
representa-
tive,
longer
mean
over
2004-2009.
Figure
4
presents
comparisons
between
the
meridional
velocity
measurements
and
transports
from
the
mooring
and
HYCOM-18.2.
Figure
4a,b
is
a
direct
comparison
of
daily-averaged
velocity
versus
depth
over
the
December
22,2007
to
March
18,
2009
time
period
of
the
observations,
and
Figure
4c
shows
the
mean
and
variability
versus
depth.
Depiction
of
the
two
time
series
versus
depth
reveals
very
similar
vertical
structure
and
temporal
variability,
including
individual
events
such
as
the
upward-propagating
northward
anomaly
in
early
2008.
The
largest
seasonal
variability
is
seen
in
and
-1
0
8
-0.6
-0.4
-0.2
0
0.2
0.4
0
6
0.8
1
m/s
41
H
—
2
T
—
'
c
g
1
to
ML
\
•
-2
Y
I
.
•
i
v.
4
2004
2005
2006
2007
2009
Figure
4.
Mindoro
Strait
comparisons
of
daily
mean
meridional
velocity
versus
depth
(negative
southward)
over
the
observational
period
with
dates
labeled
at
the
beginning
of
each
month
or
year,
from
(a)
the
mooring
and
(b)
1/12"
global
HYCOM-18.2
(in
ms'with
a
0.1
m
s
'
contour
interval),
(c)
Mooring
and
model
means
(solid
lines)
and
standard
deviations
(dashed)
of
meridional
velocity
component
versus
depth.
Transport
(in
Sv)
versus
time
over
(d)
2004-2009
(monthly
means
with
a
1-2-1
filter,
one-year
running
means,
and
2004-2009
mean
transports)
from
(H)
(red)
1/12"
global
HYCOM-18.2
and
(T
c
)
(green)
combined
mooring
and
1/12°
HYCOM-18.2
estimates
and
(e)
the
December
22,
2007-March
18,
2009
period
of
the
observations.
Daily
and
observing
period
mean
transports
M
0
(black)
are
estimated
from
the
mooring
alone,
M
H
(blue)
from
a
collocated
HYCOM-18.2
mooring,
H
(red)
from
HYCOM-18.2,
and
T
c
(green),
from
mooring
and
HYCOM-
18.2
combined.
See
Table
3
and
related
discussion.
Jan08
Mar
Jan09
Mar
Oceanography
|
March
2011
37

above
the
thermocline
(top
~
140
m)
with
northward
flow
except
from
mid-
November
to
mid-March,
a
longer
period
of
northward
flow
than
in
other
years,
based
on
the
Mindoro
transport
time
series
in
Figure
4d.
A
sharper
thermocline
is
seen
in
the
model.
Below
the
thermocline,
the
flow
is
largely
southward
and
dominated
by
the
event
time
scale.
The
stronger
bottom-trapped
southward
flow
depicts
the
sill
overflow.
In
the
mean
(Figure
4c),
both
the
model
and
observations
show
northward
flow
above
~
140
m,
more
strongly
northward
in
the
model.
The
bottom-
trapped
southward
flow
is
stronger
in
the
observations
than
in
the
model
simulation,
which
does
not
include
tides.
The
vertical
means
are
southward
and
in
close
agreement,
4.7
cm
s
'
observed
versus
4.4
cm
s'
1
in
the
model
simula-
tion.
Both
the
model
and
observations
demonstrate
enhanced
variability
just
above
the
thermocline.
the
model
mooring.
Over
the
deploy-
ment
time
period,
the
mooring
data
are
dominated
by
stronger
southward
flows
in
the
intermediate
and
overflow
depths,
so
the
total
mooring
transport
is
M
()
=
0.055
Sv
southward
compared
to
M
H
=
0.20
Sv
northward
in
the
1/12°
global
HYCOM-18.2
simulation
(Table
3).
However,
in
the
models
and
in
repeat
acoustic
Doppler
current
profiler
(ADCP)
transects
across
the
strait
(Gordon
et
al.,
2011),
the
strongest
flow
occurs
west
of
the
mooring
loca-
tion
(e.g.,
Figure
2).
Thus,
assuming
the
single
mooring
velocity
is
representative
across
half
the
strait
width
may
not
be
correct.
Hence,
the
observation-based
estimate
(M„)
was
adjusted
using
the
difference
between
the
HYCOM-18.2
full
transect
estimate
(H)
with
model
cross-passage
velocity
structure,
and
the
HYCOM-18.2
mooring
estimate
(M
H
),
both
over
the
observational
period.
This
results
in
a
combined
estimate
using
observations
and
the
model
of
T
c
=
M
0
+
(H
-
M
H
)
=
0.24
Sv
northward
over
the
December
22,
2007-March
18,
2009
time
span
of
the
observations
(Table
3).
As
evident
in
the
velocity
fields
(Figure
4a,b),
high
variability
is
found
on
every
time
scale
resolved
in
the
trans-
port
time
series
of
both
the
observed
and
model
estimates
(Figure
4e).
All
estimates
give
northward
transport
from
mid
March
through
October
2008
(Figure
4e).
For
the
2004-2009
transport
estimate
(T
c
=
0.95
Sv
southward),
it
was
assumed
that
the
difference
between
the
observations
and
the
model
(M
()
-
M
M
)
remained
the
same
as
found
during
the
observational
deployment
period,
because
a
linear
regression
analysis
between
H
and
(M
()
-
M
u
)
showed
no
relationship.
The
resulting
interannual
transport
time
series
clearly
shows
oppo-
site
transport
anomalies
in
2004
and
ESTIMATION
OF
MINDORO
STRAIT
TRANSPORT
Table
3
provides
Mindoro
Strait
trans-
port
estimates,
Figure
4e
presents
daily
transport
time
series
over
the
period
of
the
observations,
and
Figure
4d
shows
interannual
time
series.
Transport
estimates
based
on
the
vertical
profiles
of
velocity
from
the
observations
(M„
in
Table
3)
and
from
1/12°
global
HYCOM-18.2
(M
M
)
were
obtained
by
assuming
that
the
observed
velocity
at
the
mooring
location
is
representa-
tive
of
the
flow
over
half
the
channel
width
(centered
near
the
mooring)
and
tapers
to
zero
at
the
side
walls,
using
the
SRTM30_PLUS
topography
(Becker
et
al.,
2009)
with
the
mooring
observa-
tions
and
the
HYCOM
topography
with
Table
3.
Mindoro
Strait
mean
transport
estimates
Overflow
-1
Total
Transport
Transport
Transport
Estimate
Source
Symbol
Estimate
(Sv)
Estimate
(Sv)
Estimates
over
the
time
period
of
the
observations
(December
2007-March
2009)
Mooring
alone
M
0
-0.055
-0.24
Collocated
HYCOM-18.2
mooring
M
H
0.20
-0.14
1/12°
global
HYCOM-18.2
H
0.49
-0.18
Combined
mooring/HYCOM-18.2
T
c
0.24
b
Estimates
over
2004-2009
1/12°
global
HYCOM-18.2
H
-0.70
-0.22
Combined
mooring/HYCOM-18.2
%
-0.95
-0.28
The
sign
convention
is
positive
northward
and
negative
southward.
A
linear
regression
analysis
for
both
the
total
transport
and
overflow
transport
showed
no
relationship
between
H
and
M
0
-
M
H
,
so
M
0
-
M
H
was
treated
as
a
constant
when
calculating
mean
transports.
a
Transport
below
350
m.
b
Omitted
because
the
deep
channel
is
so
narrow
there
is
no
need
for
a
model-based
adjustment
for
cross-sectional
velocity
structure
and
T
e
for
the
overflow
was
estimated
from
M„
+
(H
-
H).
38
Oceanography
\
Vol.24,
No.
1

2008,
plus
large
seasonal
variability
with
seasonal
transport
reversals
evident
in
all
but
2004
(Figure
4d).
In
addition
to
the
estimate
of
total
Mindoro
Strait
transport,
separate
esti-
mates
were
made
for
the
deep
overflow
clearly
evident
in
Figure
4a-c.
These
esti-
mates
were
defined
as
transports
deeper
than
350
m.
Because
the
strait
is
narrow
at
these
depths,
no
model-based
adjust-
ment
for
cross-sectional
velocity
struc-
ture
was
used.
Table
3
gives
the
overflow
transports
based
on
the
mooring
data
and
1/12°
global
HYCOM-18.2
for
the
observation
period
and
2004-2009.
Table
2
gives
the
overflow
transports
for
additional
model
experiments.
At
0.28
Sv,
the
overflow
contributes
29%
of
the
estimated
2004-2009
mean
south-
ward
transport
through
Mindoro
Strait.
For
comparison,
Qu
and
Song
(2009)
found
a
"zero-order"
estimate
of
2.4
Sv
for
Mindoro
Strait
transport
over
the
period
2004-2007,
which
they
describe
as
providing
"a
useful
upper
bound
of
strait
transport
based
solely
on
satellite
observations."
Yaremchuk
et
al.
(2009)
found
a
Mindoro
Strait
transport
of
1.5
±
0.4
Sv,
using
an
inverse
modeling
approach.
The
transport
was
obtained
from
an
optimized
solution
to
a
4Vi
layer
reduced-gravity
model
of
the
South
China
Sea
that
best
fit
the
temperature,
salinity,
and
mixed
layer
thickness
of
an
ocean
climatology.
Estimates
from
relatively
high
resolution
global
ocean
models
include
~
1.3
Sv
from
a
1/10°
OfES
simulation
(Qu
et
al.,
2006;
Qu
and
Song,
2009)
and
1.77
Sv
from
a
1/6°
MOM2
simulation
(Fang
et
al.,
2005).
All
of
the
estimates
are
southward
transports.
THE
BOHOL
SEA
AND
DIPOLOG
STRAIT
The
Bohol
Sea
inflow
through
the
shallow
Surigao
Strait
and
outflow
through
Dipolog
Strait
form
part
of
a
second
and
shallower
major
pathway
connecting
the
circulation
within
the
Philippine
Archipelago
to
the
large-scale
ocean
circulation
(Figure
1),
but
one
that
is
deeper
than
the
pathway
feeding
into
the
Indonesian
Archipelago
via
the
Karimata
Strait
connection
to
the
Java
Sea.
As
discussed
earlier,
the
larger
sill
topography
error
in
1/25°
global
HYCOM-4.1&2
has
less
impact
on
the
transport
along
this
pathway.
Dipolog
Strait
is
the
deepest
of
the
five
straits
connecting
to
the
Bohol
Sea,
but
it
is
much
shallower
than
the
~
1500-m
depth
of
the
sea
itself.
The
shallow
surface
jet
and
isolation
of
the
deep
basin
result
in
a
four-layer
flow
through
the
strait.
The
four-layer
flow
structure
in
Dipolog
Strait
was
observed
in
PhilEx
cruise
conduc-
tivity,
temperature,
depth
(CTD)/lowered
ADCP
surveys
(Gordon
et
al.,
2011)
and
in
mooring
velocity
measurements
made
by
J.
Sprintall.
It
is
also
depicted
in
1/25°
global
HYCOM-4.1&2,
which
has
a
weak
lower
overturning
cell
and
a
sill
depth
half
that
observed
(Figure
5a,
with
a
vertical
line
at
the
mooring
location).
10QL
200
h
.c
300
a)
2004-2009
mean
1/25*
global
HYCOM-4.2
Dipolog
Strait
M
,
•
•
_1
I
I
1
1
1
I
L
100
200
300
8.5N
9.0N
9.5N
Figure
5.
Dipolog
Strait
2004-2009
mean
zonal
velocity
cross
sections
at
123°22'E,
the
longitude
of
the
PhilEx
mooring
(marked
with
a
vertical
line)
that
is
located
near
the
sill,
from
(a)
1/25°
global
HYCOM-4.2
and
(b)
EAS
NCOM.
Velocity
has
0.05
m
s
'
contour
intervals
with
blue
westward
and
yellow-red
eastward.
Oceanography
|
March
2011
39

The
four-layer
flow
occurs
because
an
inertial
westward
surface
jet
with
strong
vertical
shear
at
its
base
entrains
water
from
below
that
is
fed
by
eastward
flow
from
the
Sulu
Sea.
The
deeper
overturning
cell
is
driven
by
vertical
mixing
in
the
Bohol
Sea,
thought
to
be
largely
due
to
internal
tides.
Tides
are
not
included
in
the
simulation
depicted
in
the
Dipolog
Strait
section
from
EAS
NCOM
with
tides
(Figure
5b).
EAS
NCOM
includes
data
assimilation
(but
not
assimilation
of
PhilEx
data),
and
it
has
a
shallow
bias
in
Dipolog
sill
depth
similar
to
Figure
5a.
While
EAS
NCOM
has
accurate
temperatures
at
all
depths
in
the
Sulu
Sea
in
comparison
to
unas-
similated
PhilEx
CTD
profiles,
it
has
a
...THE
PHILIPPINE
ARCHIPELAGO
POSES
SEVERE
TESTS
FOR
THE
MODELS,
TESTS
THAT
ARE
PERFORMED
USING
DATA
FROM
THE
PHILEX
FIELD
PROGRAM
AND
OTHER
SOURCES.
in
Figure
5a,
although
the
K-Profile
Parameterization
(KPP)
vertical
mixing
scheme
of
Large
et
al.
(1994)
contains
a
simple
parameterization
of
the
effects
of
internal
wave
breaking
and
vertical
shear,
and
a
contribution
from
tides
is
included
in
the
parameterization
of
bottom
friction.
The
vertical
mixing
reduces
the
Bohol
Sea
density
below
the
sill
depth
and
creates
a
pressure
gradient
that
drives
denser
Sulu
Sea
water
into
the
Bohol
Sea
just
above
the
sill,
which
is
compensated
by
the
westward
outflow
just
above
(based
on
an
explanation
of
the
four-layer
flow
by
Gordon
et
al.,
2011).
The
1/12°
global
HYCOM-18.2
does
not
have
the
shallow
sill
depth
bias
in
Dipolog
(Figure
lc),
but
it
does
not
exhibit
the
deeper
cell
due
to
a
thick
model
layer
straddling
the
sill
(not
shown).
The
four-layer
flow
is
not
present
cold
deep
bias
of
~
6°C
in
the
Bohol
Sea,
making
it
colder
than
deep
tempera-
tures
in
the
Sulu
Sea
and
suggesting
the
possibility
of
westward
bottom
flow
through
Dipolog.
Such
flow
is
not
seen
in
Figure
5b
because
the
shallow
Dipolog
Strait
sill
in
EAS
NCOM
is
shallower
than
the
deep
temperature
bias.
The
data
assimilation
projects
surface
data
downward
via
synthetic
temperature
and
salinity
profiles
based
on
statistics
of
the
historical
hydrographic
data
base
(Fox
et
al.,
2002).
Due
to
a
lack
of
Bohol
Sea
historical
data,
the
Bohol
Sea
synthetics
are
contaminated
by
data
from
the
Pacific
and
the
Sulu
Sea.
The
HYCOM
simulations
(Table
1)
did
not
have
data
assimilation
and
were
initialized
from
GDEM3
hydrographic
climatology
(Carnes,
2009),
which
has
realistic
deep
temperatures
in
the
Bohol
Sea.
A
small
gap
in
the
westward
surface
jet
is
evident
near
the
mooring
loca-
tion
in
Dipolog
Strait
(Figure
5a).
The
mooring
lies
in
the
lee
of
a
small
rise
capped
by
Silino
Island,
a
feature
not
present
in
EAS
NCOM.
In
HYCOM,
the
gap
in
the
westward
flow
results
in
a
large
meridional
gradient
in
the
surface
jet
at
the
location
of
the
mooring,
making
it
difficult
to
use
the
model
and
mooring
data
to
estimate
Dipolog
Strait
transport,
as
was
done
at
Mindoro
Strait,
even
if
the
model
sill
depth
error
was
corrected.
Instead,
hull-mounted
ADCP
measurements
from
a
PhilEx
cruise
(Figure
6a)
are
compared
with
contemporaneous
March
1-8,
2009
means
from
the
three
model
experi-
ments
depicted
in
Figure
6:
(b)
1/12°
global
HYCOM-18.2,
(c)
EAS
NCOM,
and
(d)
1/25°
global
HYCOM-4.2.
In
each
case,
the
current
vectors
are
plotted
at
the
resolution
of
the
model.
All
of
the
models
simulate
the
westward
surface
jet
across
the
northern
Bohol
Sea
with
flow
passing
on
both
sides
of
Siquijor
Island.
At
21-m
depth,
the
observed
mean
speed
of
this
jet
(black
vectors
in
Figure
6a)
is
0.56
m
s
'
between
the
inflow
from
Surigao
Strait
near
125°24'E
and
the
Dipolog
Strait
outflow
near
123°E.
Using
the
Figure
6
model
currents
at
20-m
depth
interpolated
to
the
observation
locations,
a
few
of
which
lay
outside
the
model
jets,
the
model
mean
speeds
and
vector
correlations
with
the
observations
are
0.32
m
s
'
and
0.68
for
1/12°
global
HYCOM-18.2,0.45
m
s
'
and
0.80
for
1/25°
global
HYCOM-4.2,
and
0.46
m
s
'
and
0.76
for
EAS
NCOM.
In
the
ADCP
data,
the
jet
is
robustly
evident
at
81-m
depth
and
weakly
evident
at
101
m,
corroborating
the
sharp
gradient
in
velocity
at
the
base
of
the
current
simulated
by
1/25°
HYCOM-4.1&2
40
Oceanography
|
Vol.24,
No.
1

(Figure
5a),
but
at
a
slightly
shallower
depth
in
the
observations.
Similar
results
for
the
depth
structure
were
obtained
from
other
PhilEx
cruises.
Additionally,
the
ADCP
observations
corroborate
the
robust
transport
through
Surigao
Strait
simulated
by
the
models.
The
hull-mounted
ADCP
measure-
ments
also
depicted
a
robust
cyclonic
gyre
in
the
western
Bohol
Sea
during
every
PhilEx
cruise.
The
mean
speed
of
the
measured
gyre
currents
at
21-m
depth
was
0.34
m
s
'
in
June
2007,
0.26
m
s
'
in
December
2007,
0.25
m
s
'
in
January
2008,
and
0.36
m
s"
1
in
March
2009.
Current
vectors
measured
at
21-m
depth
during
March
1-8,
2009,
provide
a
striking
depiction
of
this
gyre
adjacent
to
the
main
westward
current
through
the
Bohol
Sea
(Figure
6a).
In
Figure
6,
only
1/25°
HYCOM
simulates
a
robust
cyclonic
gyre
(0.36
m
s
'
mean
speed
observed
[red
vectors
in
Figure
6a]
versus
0.14
m
s
'
and
a
vector
correlation
of
0.48
at
corresponding
locations
in
the
model).
At
times,
1/25°
HYCOM
even
simulates
the
small
anticyclonic
gyre
observed
south
of
Siquijor
Island.
EAS
NCOM
simulates
an
anticyclonic
gyre
in
the
Bohol
Sea
and
1/12°
HYCOM-18.2
simulates
no
gyre,
only
coastal
upwelling
in
the
two
southern
bays
(current
vectors
emanating
from
a
boundary).
Coastal
upwelling
and
downwelling
(current
vectors
terminating
near
a
boundary)
are
evident
in
the
other
two
simulations
as
well.
A
cyclonic
gyre
in
the
western
Bohol
Sea
is
present
at
20-m
depth
in
the
six-year
mean
from
1/25°
global
HYCOM-4.18c2
(Figure
lb),
but
not
at
that
depth
in
the
mean
from
1/12°
global
HYCOM-18.2
(Figure
la),
although
it
is
present
below
~
50
m.
In
the
ADCP
measurements,
this
gyre
extends
to
a
depth
of-
100
m
versus
~
120
m
in
1/12°
and
1/25°
global
HYCOM
simulations.
Ocean
color
is
also
useful
in
evalu-
ating
the
performance
of
ocean
models,
and,
in
turn,
the
models
are
useful
in
identifying
ocean
circulation
features
depicted
in
surface
chlorophyll
products
(Chassignet
et
al.,
2005;
Shriver
et
al.,
2007).
Ocean
color
can
depict
both
near-surface
circulation
features
and
areas
of
upwelling
and
downwelling.
A
persistent
cyclonic
gyre
in
the
Bohol
Sea
is
observed
in
ocean
color
(Cabrera
et
al.,
2011)
and
in
ADCP
velocities,
and
it
is
simulated
by
models
with
sufficient
resolution.
In
Figure
7,
mean
values
of
chlorophyll
observed
by
the
Moderate
Resolution
Imaging
Spectroradiometer
(MODIS)
satellite
(at
1-km
resolution)
in
March
2007
(Figure
7a)
are
compared
with
March
2007
mean
currents
at
20-m
depth
from
(b)
1/12°
global
123E
124E
125E
1
1
1
•
0
0
1
0
>
0
3
0.4
0.5
m/s
124E
125E
Figure
6.
(a)
ADCP
velocity
vectors
at
21-m
depth
observed
in
the
western
Bohol
Sea
during
March
1-8,
2009
versus
March
1-8,
2009
mean
currents
at
20
m
overlaid
on
speed
from
(b)
1/12"
global
HYCOM-18.2,
(c)
EAS
NCOM,
and
(d)
1/25°
global
HYCOM-4.2.
The
reference
vector
for
velocity
is
0.5
m
s
'
and
mean
speed
is
contoured
at
0.05
m
s'
1
intervals.
The
black
line
in
(d)
defines
the
location
of
the
cross
section
used
in
Figure
5,
and
the
black
dot
denotes
the
mooring
location.
The
observed
mean
speed
of
the
westward
jet
(black
vectors
in
panel
a)
is
0.56
m
s'\
and
within
the
cyclonic
gyre
(red
vectors)
it
is
0.36
m
s
',
both
at
21
m
depth.
See
text
for
model-data
comparisons.
To
avoid
a
biased
comparison,
the
observational
result
was
chosen
before
the
contemporaneous
model
results
were
extracted,
a
procedure
also
used
for
Figure
7.
Oceanography
|
March
2011
41

ION
8.4N
ION
8N-
|c)
EAS
NCOM
f
Macajalar
|d)
1/25°
global
HYCOM-04.21
^
123E
124E
12SE
124E
125E
TM
0.1
0.2
0.3
0.4
0.5
m/i
Figure
7.
Same
as
Figure
6
but
for
March
2007
means,
and
panel
(a)
is
replaced
by
chlorophyll
concentration
in
mg
m
!
.
HYCOM-18.2,
(c)
EAS
NCOM,
and
(d)
1/25°
global
HYCOM-4.2.
Chlorophyll
concentrations
charac-
terize
the
cyclonic
gyre
as
an
oligotro-
phic
center.
Elevated
concentrations
occur
along
frontal
boundaries
and
in
upwelling
centers
at
the
southern
bound-
aries.
The
results
from
the
three
models
are
similar
to
corresponding
results
in
early
March
2009
(Figure
6),
but
differ
in
detail.
Again,
all
three
models
simulate
the
westward
jet,
depicted
as
a
band
of
relatively
high
chlorophyll
(Figure
7a).
At
the
southern
edge,
the
low-
chlorophyll
Bohol
Sea
gyre
is
marked
by
a
zonal
band
of
slightly
elevated
chlorophyll
across
Iligan
Bay
near
8.4°N.
A
narrow
plume
of
low-chlorophyll
water
from
the
gyre
enters
the
westward
jet
between
Silino
and
Mindanao
islands
(see
Figure
6
for
locations).
The
posi
tion
of
the
cyclonic
gyre
in
1/25°
global
HYCOM-4.2
is
in
close
agreement
with
ocean
color
(Figure
7a,d),
including
a
portion
of
the
jet
from
the
western
edge
of
the
gyre
passing
south
of
Silino
Island,
a
jet
also
seen
south
of
a
gap
in
the
five-
year
mean
velocity
cross
section
simu-
lated
by
1/25°
HYCOM
(Figure
5a).
As
in
Figure
6,
the
other
two
models
were
not
successful
in
depicting
the
cyclonic
gyre
(Figure
7b,c).
Parts
of
two
small
eddies
are
also
indicated
by
low
chlorophyll,
one
in
the
northeast
corner,
the
other
east
of
Camiguin
Island.
EAS
NCOM
and
1/25°
HYCOM
depict
an
anticyclonic
eddy
in
the
northeast
corner
and
1/25°
HYCOM
a
cyclonic
eddy
east
of
Camiguin
Island.
In
Iligan
Bay,
south
of
the
main
gyre,
the
chlorophyll
is
relatively
high
on
the
east
side
and
low
on
the
west
side,
except
for
very
high
chlorophyll
at
the
mouth
of
an
estuary
in
the
southwestern
corner.
This
pattern
is
consistent
with
coastal
upwelling
on
the
east
side
and
down-
welling
to
the
west,
as
seen
in
the
1/12°
and
1/25°
global
HYCOM
simulations.
Additionally,
the
ocean
color
indicates
upwelling
in
Macajalar
Bay,
again
seen
in
the
two
HYCOM
simulations.
In
the
ocean
color,
there
is
evidence
of
relatively
strong
upwelling
on
the
west
side
of
Camiguin
Island
and
on
both
sides
of
the
strait
between
Camiguin
and
Mindanao
islands.
The
resulting
high
chlorophyll
is
advected
to
the
north-
west
by
a
westward
jet
exiting
through
this
strait,
a
feature
depicted
only
in
EAS
NCOM
(Figure
7c).
A
very
narrow
band
of
low
chlorophyll
is
depicted
along
the
southeastern
coast
of
Bohol
Island,
consistent
with
the
downwelling
simulated
by
1/25°
HYCOM
and
seen
to
a
lesser
extent
in
EAS
NCOM.
Very
high
chlorophyll
is
seen
in
Dipolog
Strait
along
the
coast
of
Mindanao.
It
emanates
from
Dapitan
Bay,
possibly
due
to
riverine
outflow.
This
outflow
plume
is
advected
downstream
by
a
westward
coastal
current
that
passes
between
the
islands
of
Silino
and
Mindanao,
a
current
depicted
in
1/12°
and
1/25°
global
HYCOM.
The
1/25°
global
HYCOM
simulates
a
Bohol
Sea
cyclonic
gyre
in
every
42
Oceanography
|
Vol.24,
No.
1

seasonal
mean,
most
strongly
in
the
winter
mean
(January-March),
succes-
sively
weaker
in
spring
and
fall,
and
only
weakly
in
summer.
The
1/12°
HYCOM-
18.2
simulated
a
mean
cyclonic
gyre
every
year
during
winter
except
2009
(e.g.,
Figure
6b),
every
year
during
spring,
but
not
during
the
last
half
of
the
year.
During
winter,
EAS
NCOM
alternates
between
states
with
a
cyclonic
gyre,
an
anticyclonic
gyre,
and
no
gyre.
Over
2004-2009,
EAS
NCOM
simulated
a
realistic
cyclonic
gyre
in
only
three
seasonal
means
(-10
cm
s"
1
in
spring
2006,2008,
and
2009)
and
one
annual
mean
(~
5
cm
s
'
in
2008).
The
westward
jet
along
the
northern
boundary,
its
curvature
to
the
southwest,
and
the
basin
depth
(which
reduces
the
impact
of
bottom
friction)
are
conducive
to
generation
of
positive
relative
vorticity
and
formation
of
a
cyclonic
gyre
within
the
western
Bohol
Sea.
An
additional
mechanism,
proposed
by
Cabrera
et
al.
(2011),
is
based
on
the
upwelling
that
occurs
when
the
Bohol
Sea
subsurface
inflow
is
entrained
into
the
westward
surface
jet
above
(Figure
5).
This
upwelling
stretches
the
water
column,
thus
requiring
cyclonic
rotation
in
order
to
conserve
potential
vorticity.
However,
the
conditions
are
not
always
sufficient
for
these
mechanisms
to
work,
as
evidenced
in
Figures
6
and
7.
What
additional
factors
might
be
associated
with
the
gyre
formation
and
its
variation
in
strength
in
the
model
simulations?
The
largest
impact
comes
from
doubling
the
model
resolution
from
~
9
km
to
4.4
km.
Three
other
mechanisms
are
identified
that
clearly
have
an
influence:
(1)
Dipolog
Strait
transport,
which
affects
the
strength
of
the
mechanisms
described
above,
(2)
wind
stress
curl
over
the
Bohol
Sea,
and
(3)
nondeter-
ministic
variability
due
to
flow
instabili-
ties.
A
preconditioning
impact
from
the
previous
season
was
not
identified,
nor
was
there
any
evidence
of
an
indepen-
dent
impact
from
the
latitude
and
angle
of
the
surface
jet
entry
into
the
western
Bohol
Sea
north
and
south
of
Camiguin
Island.
Evidence
of
an
impact
is
greatest
for
the
strength
of
the
Dipolog
Strait
transport.
In
1/25°
HYCOM-4.0,
the
seasonal
variability
in
the
strength
of
the
Dipolog
Strait
transport
and
the
strength
of
the
Bohol
Sea
cyclonic
gyre
vary
in
tandem
with
the
greatest
strength
in
winter
and
successively
weaker
strength
in
spring,
fall,
and
summer.
In
1/12°
HYCOM-18.2,
the
Bohol
Sea
cyclonic
gyre
is
simulated
only
during
the
two
seasons
with
the
greatest
Dipolog
Strait
transport,
but
those
two
seasons
are
reversed
from
1/25°
HYCOM-4.0
with
the
strength
in
spring
greater
than
in
winter
for
both
transport
and
gyre
strength.
The
mean
wind
stress
curl
in
the
Bohol
Sea
is
positive
in
three
seasons,
strongest
in
winter
and
successively
weaker
in
fall
and
spring,
but
it
is
nega-
tive
in
summer,
an
ordering
different
from
that
of
the
cyclonic
gyre
and
Dipolog
transport
strength.
However,
the
interannual
variability
of
seasonal
means
in
winter,
the
season
with
the
strongest
positive
wind
stress
curl,
did
show
a
stronger
gyre
in
the
years
with
the
stron-
gest
wind
stress
curl,
independent
of
the
relative
jet
strength.
The
climatologically
forced
simulations
demonstrated
nonde-
terministic
variability
in
jet
transport
and
gyre
strength,
and
nondeterministic
variability
in
gyre
strength
that
at
times
was
independent
of
jet
transport.
Such
variability
in
gyre
strength
is
also
evident
in
interannual
simulations.
The
Regional
Ocean
Modeling
System
(ROMS)
simulations
of
Han
et
al.
(2009)
have
5-km
resolution,
but
depict
the
Bohol
Sea
gyre
less
robustly
than
1/25°
global
HYCOM;
for
example,
it
is
evident
in
their
2006
annual
mean,
but
not
in
the
means
for
2004
and
2005,
the
other
years
depicted.
It
is
also
evident
in
June
of
2006
and
2007,
but
is
much
weaker
than
observed
in
the
January
2008
PhilEx
ADCP
data,
and
not
evident
in
December
2005,
January
2006,
and
August
2006,
the
other
months
depicted.
In
discussing
the
dynamics,
the
authors
place
greater
emphasis
on
the
impacts
of
Ekman
transport
and
wind
stress
curl,
which
are
less
evident
in
the
HYCOM
simulations.
SIMULATION
OF
TIDES
IN
THE
PHILIPPINE
ARCHIPELAGO
The
l/12°HYCOM-14.1&2isthe
first
eddy-resolving
global
ocean
simulation
(e.g.,
one
like
1/12°
global
HYCOM-18.2)
that
also
includes
external
and
internal
tides
(Arbic
et
al.,
2010).
As
illustrated
by
the
sunglint
imagery
in
Figure
8a,
internal
tides
can
have
a
strong
surface
signature
in
the
interior
Philippine
seas.
Figure
8b
shows
a
snapshot
of
the
steric
SSH
anomaly
with
respect
to
a
25-hour
mean
from
1/12°
global
HYCOM-14.2,
a
month-long
repeat
of
HYCOM-14.1
with
global
hourly
three-dimensional
output
for
May
2004.
While
not
contemporaneous
with
the
sunglint
observation,
HYCOM-14.2
has
a
clear
representation
of
this
surface
signal
that
is
generated
in
the
same
location
and
further
indicates
that
internal
tides
are
ubiquitous
within
the
Philippine
seas.
Jackson
et
al.
(2011)
find
internal
waves
observed
by
satellite
imagery
and
Girton
Oceanography
\
March
2011
43

Figure
8.
(a)
Moderate
Resolution
Imaging
Spectroradiometer
(MODIS)
true
color
sunglint
image
of
the
western
Sulu
Sea
on
April
8,
2003.
Note
the
SSH
signature
of
the
internal
tides
that
are
generated
in
Sibutu
Passage
and
propagate
at
speeds
of
~
2Vi
m
s
'.
(b)
Steric
SSH
anomaly
(in
m)from
a
25-hour
average
centered
on
May
15,
2004,12Zfrom
1/12°
global
HYCOM-14.2
with
tidal
forcing
for
the
area
in
and
around
the
Philippine
seas.
The
black
box
outlines
the
region
of
the
MODIS
image.
While
not
contemporaneous,
global
HYCOM
has
a
similar
SSH
signature
of
the
internal
tides,
but
without
the
soliton
packets,
because
solitons
have
nonhydrostatic
physics
and
HYCOM
is
a
hydrostatic
model.
Also
note
the
strong
internal
tidal
signatures
in
Mindoro
Strait
and
the
Bohol
Sea,
both
focus
areas
for
the
PhilEx
Intensive
Observational
Period
cruises.
et
al.
(2011)
by
in
situ
data
in
several
areas
within
the
Philippine
seas,
and,
like
Apel
et
al.
(1985),
internal
tidal
beams,
generated
in
Sibutu
Passage,
that
propagate
across
the
Sulu
Sea
(depicted
in
Figure
8a,b).
In
HYCOM-14.2
the
maximum
peak
to
peak
amplitude
of
this
internal
tidal
beam
is
0.18
m
in
the
steric
SSH
and
~
40
m
in
the
pycnocline
after
separation
from
Sibutu
Passage.
The
propagation
speed
of
the
simulated
internal
tides
(2.5
m
s
')
in
the
Sulu
Sea
is
close
to
the
observations
of
internal
tides
with
similar
amplitude
(2.4
m
s"
1
;
Apel
et
al.,
1985).
Because
HYCOM
is
a
hydrostatic
model,
it
simulates
only
the
hydrostatic
physics
of
internal
tides,
not
the
nonhydrostatic
physics
required
for
simulation
of
the
soliton
packets
gener-
ated
by
tidal
cycles
in
the
Sulu
Sea
(Apel
et
al.,
1985)
and
visible
in
the
sunglint
imagery.
Theoretical
work
by
St.
Laurent
and
Garrett
(2002)
suggests
that
tidal
beams
are
primarily
low
vertical
mode
waves
capable
of
propagating
for
distances
of
0(1000
km)
with
dissipation
occurring
due
to
critical
slope
interac-
tions
and
bottom
scattering.
These
mechanisms
cause
enhanced
mixing,
especially
near
the
bottom.
Tidal
beams
propagating
>
1000
km
occur
in
large
ocean
basins
in
HYCOM-14.1&2.
SUM
i
(.
ONCI
LJSIONS
The
circulation
within
the
Philippine
Archipelago
is
an
integral
component
of
the
large-scale
ocean
circulation
in
a
region
of
interbasin
exchange.
In
that
role,
it
provides
two
signifi-
cant
secondary
routes
for
both
the
Indonesian
throughflow
and
the
western
boundary
currents
that
close
the
Pacific
northern
tropical
gyre
in
addition
to
the
Mindanao
Current.
The
deeper
route
enters
the
archipelago
from
the
north
through
Mindoro
Strait,
after
passing
through
Luzon
Strait
and
the
South
China
Sea.
The
second
route
is
very
shallow
and
enters
directly
from
the
Pacific
via
Surigao
Strait
and
44
Oceanography
|
Vol.24,
No.
1

passes
through
Dipolog
Strait
down-
stream.
Though
shallow,
the
second
route
is
deeper
than
the
pathway
entering
the
Indonesian
Archipelago
via
Karimata
Strait
in
the
western
Java
Sea.
Both
routes
through
the
Philippine
Archipelago
exit
at
the
southern
end
via
Sibutu
Passage
and
the
adjacent
Sulu
Archipelago.
Within
the
Philippines,
these
secondary
pathways
are
the
domi-
nant
contribution
to
the
mean
circula-
tion
and
much
of
its
variability.
In
all
of
the
1/12°
and
1/25°
global
HYCOM
simulations
the
outflow
from
Sibutu
Passage
contributes
to
the
southward
flow
through
Makassar
Strait
at
all
depths
above
the
Sibutu
Passage
sill,
but
despite
their
differences
in
Sibutu
Passage
transport,
all
simulate
nearly
the
same
transport
through
Makassar
Strait,
which
carries
the
largest
contribution
to
the
Indonesian
throughflow.
The
1/12°
and
1/25°
global
HYCOM
simulations
and
archived
real-time
data-
assimilative
nowcasts
from
EAS
NCOM,
nested
in
global
NCOM,
were
used
to
study
the
circulation
in
the
Philippine
Archipelago
within
the
context
of
global
ocean
circulation.
However,
because
the
straits
are
narrow
and
shallow
and
the
interior
seas
are
small,
the
simulations
are
very
sensitive
to
model
resolution
and
to
the
accuracy
of
the
topography
and
sill
depths
within
the
narrow
straits.
In
some
cases
this
results
in
serious
simulation
errors,
such
as
the
transport
of
the
Mindoro
to
Sibutu
route
in
some
simulations
and
the
Bohol
Sea
circula-
tion
in
all
but
the
1/25°
model.
Precise
topography
and
sill
depths
are
required
to
accurately
simulate
the
effects
of
hydraulic
control
and
parti-
tioning
of
the
effects
of
hydraulic
and
geostrophic
control
on
the
flows
through
straits.
Results
from
theory
relating
the
pressure
head
and
the
transport
through
narrow
choke
points
in
Surigao
and
San
Bernardino
Straits
were
used
to
explain
the
much
larger
model
trans-
port
through
Surigao
despite
similar
widths
at
a
choke
point,
similar
pressure
heads,
and
shallow
sill
depths
that
did
not
explain
the
difference.
The
results
show
the
transport
at
San
Bernardino
is
constrained
by
the
sill
depth,
but
the
transport
through
Surigao
is
not
because
the
choke
point
at
the
outflow
is
deep
and
the
upstream
shallow
inflow
is
broad.
Instead,
the
transport
and
the
pressure
head
are
reconciled
by
using
the
thermocline
depth
in
the
Bohol
Sea
(the
depth
of
the
modeled
Surigao
outflow)
and
excluding
the
effect
of
bottom
fric-
central
year
of
the
PhilEx
observational
program,
were
extreme
opposite
anoma-
lous
years,
highlighted
by
anomalously
strong
southward
flow
through
Mindoro
in
2004
and
mean
northward
flow
in
and
above
the
thermocline
during
2008.
Associated
opposite
SSH
anomalies
in
the
western
tropical
Pacific
were
veri-
fied
by
satellite
altimetry.
The
northern
edge
of
both
anomalies
was
located
at
the
latitude
of
Luzon
Strait
and
was
characterized
by
gradients
in
SSH
that
fed
into
the
South
China
Sea
and
down-
stream
into
the
Philippine
Archipelago
via
Mindoro
Strait.
The
SSH
anomalies
had
little
effect
on
the
transport
of
the
Surigao-Dipolog
route,
which
demon-
strated
strong
seasonal
variability
but
weak
interannual
variability.
At
Surigao,
...THE
MODELS
ARE
USED
TO
HELP
INTERPRET
THE
DATA
AND
THEIR
ABILITY
TO
MEASURE
OBSERVED
PHENOMENA,
TO
PLACE
THE
OBSERVATIONS
WITHIN
THE
CONTEXT
OF
THE
LARGER-SCALE
CIRCULATION
AND
ITS
TEMPORAL
VARIABILITY,
AND
TO
HELP
UNDERSTAND
THE
DYNAMICS
OF
OBSERVED
PHENOMENA.
tion.
The
robust
Surigao
transport
and
the
~
100-m
depth
structure
of
the
west-
ward
Surigao-Dipolog
surface-trapped
jet
are
supported
by
the
hull-mounted
ADCP
data
collected
at
multiple
loca-
tions
along
the
jet
during
each
of
the
four
PhilEx
cruise
periods.
The
global
simulations
demonstrate
that
2004
and
2008,
the
latter
the
the
SSH
associated
with
the
anoma-
lies
at
each
end
of
the
strait
varied
in
tandem,
so
the
yearly
mean
pressure
head
remained
nearly
constant.
Thus,
the
inflow
through
Mindoro
Strait
was
found
to
be
the
primary
external
source
of
interannual
variability
in
Philippine
Archipelago
circulation.
The
December
22,
2007
to
March
18,
Oceanography
|
March
2011
45

2009
data
from
a
single
PhilEx
mooring
in
Mindoro
Strait
in
combination
with
HYCOM
simulation
results
allow
the
first
estimate
of
transport
through
Mindoro
Strait
using
in
situ
data.
The
model
helped
extend
the
data
across
the
strait
and
beyond
the
anomalous
period
of
the
observations,
giving
a
mean
transport
of
0.24
Sv
northward
during
the
observation
period
and
a
mean
of
0.95
Sv
southward
over
2004-2009,
with
the
deep
overflow
contributing
0.28
Sv
(29%)
of
this
transport.
The
1/25°
global
HYCOM
simulates
the
observed
four-layer
flow
through
Dipolog
Strait.
The
upper
cell
is
driven
by
entrainment
into
the
westward
Surigao-Dipolog
surface
jet,
which
has
high
vertical
shear
at
the
base,
and
the
lower
cell
by
vertical
mixing
in
the
Bohol
Sea.
All
four
of
the
PhilEx
cruises
observed
a
robust
cyclonic
gyre
in
the
Bohol
Sea
using
velocity
measurements
from
a
hull-mounted
ADCP.
This
gyre
is
well-simulated
in
1/25°
HYCOM
with
4.4-km
resolution
in
comparison
to
the
cruise
current
measurements
and
to
ocean
color
imagery,
but
poorly
simu-
lated
in
the
two
models
with
~
9-km
resolution.
The
1/25°
HYCOM
is
the
first
global
ocean
model
with
such
fine
hori-
zontal
resolution
and
more
than
a
few
layers
in
the
vertical.
The
Philippine
Archipelago
is
a
region
with
strong
internal
tides.
The
1/12°
HYCOM
was
used
in
the
first
eddy-resolving
global
ocean
simula-
tion
with
both
atmospherically
forced
circulation
and
tides.
The
model
demonstrated
the
ability
to
realisti-
cally
simulate
the
hydrostatic
aspect
of
internal
tides
within
the
Philippine
Archipelago,
including
the
strong
tidal
beam
observed
crossing
the
Sulu
Sea,
but
not
the
associated
soliton
packets,
which
are
nonhydrostatic.
A
real-time
data
assimilative
global
ocean
prediction
system,
based
on
1/12°
global
HYCOM,
has
been
running
in
real
time
or
near-real
time
at
the
Naval
Oceanographic
Office
since
December
22,2006
(Hurlburt
et
al.,
2008;
Metzger
et
al.,
2008;
Chassignet
et
al.,
2009).
Real-time
and
archived
results
are
available
at
http://www.
hycom.org.
The
HYCOM
system
was
used
to
provide
nowcasts,
forecasts,
and
boundary
conditions
for
some
of
the
regional
models
and
prediction
systems
run
as
part
of
PhilEx.
A
global
ocean
prediction
system,
based
on
1/25°
global
HYCOM
with
tides,
is
planned
for
real-
time
operation
starting
in
2012.
At
this
resolution,
a
global
ocean
prediction
system
can
directly
provide
boundary
conditions
to
nested
relocatable
models
with
~
1-km
resolution
anywhere
in
the
world,
a
goal
of
US
Navy
operational
ocean
prediction.
Knowledge
gained
in
this
study
is
being
used
to
improve
the
performance
of
subsequent
model
simu-
lations
covering
this
region.
ACKNOWLFDCFMPN1S
This
work
was
sponsored
by
the
Office
of
Naval
Research
under
6.1
program
element
601153N,
primarily
via
the
Philippine
Straits
Dynamics
Experiment
(PhilEx),
a
6.1
Directed
Research
Initiative,
but
also
via
the
6.1
projects
"Dynamics
of
the
Indonesian
through-
flow
(ITF)
and
its
remote
impact"
and
"Global
remote
littoral
forcing
via
deep
water
pathways"
and
by
the
6.2
project
"Full
column
mixing
for
numerical
ocean
models"
(program
element
602435N).
The
PhilEx
DRI
effort
of).
Sprintall
was
supported
by
ONR
Award
N00014-06-1-0690.
The
computational
effort
was
supported
by
the
US
Defense
Department
High
Performance
Computing
Modernization
Program
via
grants
of
challenge
and
non-challenge
computer
time.
We
thank
PhilEx
participant
Chris
Jackson
for
providing
the
MODIS
sunglint
image
used
in
Figure
8.
This
is
contribution
NRL/JA/7304-10-419
and
has
been
approved
for
public
release.
KZ
Apel,
J.R.,
J.R.
Holbrook,
A.K.
Liu,
and
J.J.
Tsai.
1985.
The
Sulu
Sea
internal
soliton
experiment,
lournal
of
Physical
Oceanography
15:1,625-1,651.
Arango,
H.G.,
J.C.
Levin,
E.N.
Curchitser.
B.
Zhang,
A.M.
Moore,
W.
Han,
A.L.
Gordon,
CM.
Lee,
and
J.B.
Girton.
2011.
Development
of
a
hindcast/forecast
model
for
the
Philippine
Archipelago.
Oceanography
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Arbic.
B.K,
A.J.
Wallcraft,
and
E.J.
Metzger.
2010.
Concurrent
simulation
of
the
eddying
general
circulation
and
tides
in
a
global
ocean
model.
Ocean
Modelling
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Ashok.
K.,
S.K.
Behera,
S.A.
Rao,
H.
Weng,
and
T.
Yamagata.
2007.
El
Nino
Modoki
and
its
possible
teleconnection.
Journal
of
Geophysical
Research
112,
Cl
1007.
doi:10.1029/2006IC003798.
Aviso
(Archiving,
Validation
and
Interpretation
of
Satellite
Oceanographic
data).
2010.
DT
CorSSH
and
DT
SLA
Product
Handbook.
CLS-DOS-NT-08.341,
Edition
1.7.
Available
online
at:
http://www.aviso.oceanobs.com/
fileadmin/documents/data/tools/hdbk_dt_
corssh_dt_sla.pdf
(accessed
December
7,
2010).
Barren,
C.N.,
A.B.
Kara,
P.).
Martin.
R.C
Rhodes,
and
L.F.
Smedstad.
2006.
Formulation,
implementation
and
examination
of
vertical
coordinate
choices
in
the
global
Navy
Coastal
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