Circulation in the Philippine Archipelago Simulated by 1/12 degrees and 1/25 degrees Global

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Faculty Publications
3-1-2011
CCiirrccuullaattiioonn iinn tthhee PPhhiilliippppiinnee AArrcchhiippeellaaggoo SSiimmuullaatteedd bbyy 11//1122°° aanndd
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Harley E. Hurlbert
Stennis Space Center, [email protected]
E. Joe Metzger
Stennis Space Center, [email protected]
Janet Sprintall
Scripps Institution of Oceanography, [email protected]
Shelley Riedlinger
Stennis Space Center, [email protected]
Robert A. Arnone
Stennis Space Center, [email protected]
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Hurlbert, H. E., Metzger, E., Sprintall, J., Riedlinger, S., Arnone, R. A., Shinoda, T., Xu, X. (2011). Circulation in
the Philippine Archipelago Simulated by 1/12° and 1/25° Global HYCOM and EAS NCOM. Oceanography,
24(1), 28-47.
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AAuutthhoorrss
Harley E. Hurlbert, E. Joe Metzger, Janet Sprintall, Shelley Riedlinger, Robert A. Arnone, Toshiaki Shinoda,
and Xiaobiao Xu
This article is available at The Aquila Digital Community: https://aquila.usm.edu/fac_pubs/612
Oceanography
THE OffICIAl MAGAzINE Of THE OCEANOGRAPHY SOCIETY
CITATION
Hurlburt, H.E., E.J. Metzger, J. Sprintall, S.N. Riedlinger, R.A. Arnone, T. Shinoda, and X. Xu. 2011.
Circulation in the Philippine Archipelago simulated by 1/12° and 1/25° Global HYCOM and EAS
NCOM. Oceanography 24(1):28–47, doi:10.5670/oceanog.2011.02.
COPYRIGHT
This article has been published in Oceanography, Volume 24, Number 1, a quarterly journal of
The Oceanography Society. Copyright 2011 by The Oceanography Society. All rights reserved.
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DOwNlOADED fROM www.TOS.ORG/OCEANOGRAPHY
PhiliPPiNe StraitS DyNamicS exPerimeNt
circulation
in
Philippine archipelago
the

Simulated by 1/12° and 1/25° Global hycOm and eaS NcOm
By harley e. hurlBurt,
Zoom of the 2008 sea surface height Taiwan
Pacific Ocean
anomaly from Figure 3c. e. JOSePh metZGer,
Strait
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rOBert a. arNONe,

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Tablas
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Sulawesi
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t
ai
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r
Karimata a
s
s
Strait a
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Banda Sea
Java Sea
2288 OOcceeaannooggrraapphhyy || VVooll..2244,, NNoo..11
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 throughflow via Makassar Strait and and to model numerics and physics.
In the Philippine Straits Dynamics into the Pacific northern tropical gyre Thus, the Philippine Archipelago poses
Experiment (PhilEx), global ocean via the NECC in a 1/12° global simula- severe tests for the models, tests that are
models with resolutions as fine as 1/25° tion (their Figure 9a). The deepest of the performed using data from the PhilEx
(4.4 km over the latitude range 0–11° secondary routes surrounds most of the field program and other sources. In turn,
and finer at higher latitudes) are used archipelago via Luzon Strait to the north the models are used to help interpret
to investigate the circulation within and Mindoro Strait and Sibutu Passage the data and their ability to measure
the Philippine Archipelago in a global to the west. Within the archipelago, observed phenomena, to place the
context, both spatially and temporally. these secondary pathways constitute observations within the context of the
These models also provide boundary the dominant contribution to the mean larger-scale circulation and its temporal
conditions for nested regional models circulation and are responsible for much variability, and to help understand the
(e.g., Han et al., 2009; Arango et al., of its variability. Han et al. (2009) discuss dynamics of observed phenomena.
2011; Lermusiaux et al., 2011), with the impacts of remote and local forcing
nests as fine as ~ 1 km feasible with on the seasonal variability. OceaN mODel exPerimeNtS
boundary conditions directly from the Realistic modeling of the circulation Philippine Archipelago circulation
1/25° global model. A global context is within the Philippine Archipelago is an is investigated using 1/12° and 1/25°
essential because the Philippines provide extreme challenge for a global ocean global simulations by the HYbrid
secondary pathways for the Pacific to model due to the numerous narrow Coordinate Ocean Model (HYCOM;
Indian Ocean throughflow (Ilahude and straits and small interior seas. Accurate Bleck, 2002) and data assimilative
Gordon, 1996; Metzger et al., 2010) and modeling of flows through straits nowcasts by the East Asian Seas Navy
secondary routes to close the northern requires accurate modeling of the effects Coastal Ocean Model (EAS NCOM)
tropical gyre, which spans the North of hydraulic control and appropriate with tides. EAS NCOM (9.6-km resolu-
Pacific between the North Equatorial partitioning between geostrophic and tion at 10° latitude) has been running
Countercurrent (NECC) on the south hydraulic control. In a complex archi- in real time since October 2003, and
and the North Equatorial Current (NEC) pelago like the Philippines, the challenge is nested in global NCOM (Barron
on the north. This gyre is bounded on is increased by the need to correctly et al., 2006), which has 19.2-km resolu-
the west by the southward Mindanao partition the flow among numerous tion at 10° latitude. Global NCOM is
Current and by secondary routes alternative routes throughout the archi- an operational forecast model of the
through the Philippine Archipelago pelago. These issues make simulations US Navy (without tides) that assimilates
(Metzger and Hurlburt, 1996). Metzger in this region particularly sensitive to a wide variety of ocean data. Table 1
et al. (2010) show outflow from Sibutu model resolution, to errors in model summarizes the characteristics of the
Passage feeding both into the Indonesian topography and atmospheric forcing, HYCOM simulations and EAS NCOM.
The global HYCOM simulations were
Harley E. Hurlburt ([email protected]) is Senior Scientist for Ocean spun up for 10 years after initializa-
Modeling and Prediction, Oceanography Division, Naval Research Laboratory (NRL), tion from the Generalized Digital
Stennis Space Center, MS, USA. E. Joseph Metzger is Meteorologist, Oceanography Environmental Model 3 (GDEM3)
Division, NRL, Stennis Space Center, MS, USA. Janet Sprintall is Research Oceanographer, hydrographic climatology (Carnes,
Climate, Atmospheric Science, and Physical Oceanography Division, Scripps Institution of 2009) and forced with an atmospheric
Oceanography, La Jolla, CA, USA. Shelley N. Riedlinger is Oceanographer, Oceanography climatology derived from the European
Division, NRL, Stennis Space Center, MS, USA. Robert A. Arnone is Head, Ocean Sciences Centre for Medium-Range Weather
Branch, Oceanography Division, NRL, Stennis Space Center, MS, USA. Toshiaki Shinoda is Forecasts (ECMWF) 40-year reanalysis
Oceanographer, Oceanography Division, NRL, Stennis Space Center, MS, USA. Xiaobiao Xu (ERA-40) (Kållberg et al., 2004; HYCOM
is Research Scientist, Department of Marine Sciences, University of Southern Mississippi, Exps. 1/12°–18.0 and 1/25°–4.0). The
Stennis Space Center, MS, USA. simulations were then continued
30 Oceanography | Vol.24, No.1
table 1. Ocean model experiments
Horizontal
Experiment Resolution Vertical Atmospheric Years Data
Ocean Model Numbera at 10°Na Resolutionb Forcingc Used Tides Assimilation
32 coordinate ecmWF/
1/12° global hycOm 18.0 8.7 km 5–10 No No
surfaces QuikScat
NOGaPS/
32 coordinate
1/12° global hycOm 18.2 8.7 km ecmWF/ 2003–2010 No No
surfaces
QuikScat
32 coordinate ecmWF/
1/25° global hycOm 4.0 4.4 km 5–10 No No
surfaces QuikScat
NOGaPS/
32 coordinate
1/25° global hycOm 4.1&2d 4.4 km ecmWF/ 2004–2009 No No
surfaces
QuikScat
32 coordinate NOGaPS/
1/12° global hycOm 14.1&2e 8.7 km 2004–2008 yesf No
surfaces ecmWF
eaS NcOm — 9.6 km 40 levels NOGaPS 2004–2009 yesf yes
a 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 GlBa 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.
d 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.
f eight tidal constituents.
interannually using archived operational used thus far in a global ocean general layered continuity equation. NCOM is
forcing from the Navy Operational circulation model (OGCM) with ther- a depth coordinate ocean model with a
Global Atmospheric Prediction System mohaline dynamics and more than a few terrain-following coordinate at depths
(NOGAPS) (Rosmond et al., 2002), layers in the vertical. shallower than 137 m.
but with the long-term annual mean HYCOM is a community ocean model
replaced by the long-term mean from (http://www.hycom.org) with a general- meaN circulatiON
ERA-40 (Exp. 1/12°–18.2 initial- ized vertical coordinate because no SimulateD By 1/12° aND
ized from 18.0 and Exp. 1/25°–4.1&2 single coordinate is optimal everywhere 1/25° GlOBal hycOm
from 4.0 [Table 1]). In most of the in the global ocean. Isopycnal (density- aND the imPact OF
experiments, wind speed was corrected tracking) layers are best in the deep tOPOGraPhic errOrS
using a monthly climatology from the stratified ocean, pressure levels (nearly Figure 1a,b is a comparison of the
QuikSCAT scatterometer (Kara et al., fixed depths) provide high vertical reso- mean currents at 20-m depth and strait
2009). Model experiment 1/12° global lution in the mixed layer, and σ-levels transports from 1/12° global HYCOM-
HYCOM-14.1&.2 is the world’s first (terrain-following) are often the best 18.2 with those from 1/25° global
eddy-resolving global ocean simulation choice in coastal regions (Chassignet HYCOM-4.1&2, and Figure 1c,d is a
that includes both the atmospheri- et al., 2003). The generalized vertical comparison of their respective topogra-
cally forced ocean circulation and tides coordinate in HYCOM allows a combi- phies and sill depths. Because the 1/12°
(Arbic et al., 2010). The 1/25° global nation of all three types (and others), topography was derived from the 1/25°,
HYCOM began running on January 12, and the optimal distribution is chosen they demonstrate close agreement in
2009, and has the highest resolution dynamically at every time step using the deep water, although numerous hand
Oceanography | march 2011 31
edits were subsequently applied to the and Surigao straits. Passage (Figure 1a,b). In contrast,
1/12° topography (Metzger et al., 2010), Figure 1a,b depicts two main a relatively modest increase occurs
mainly in shallow water and to correct routes for flow through the Philippine through Surigao and Dipolog (despite
sill depths. Edits to the 1/25° topography Archipelago: a deeper pathway from the very large Dipolog sill depth error
were done later, and fewer were made. the South China Sea via Mindoro Strait and a 1.8-fold deeper Surigao sill depth
In Figure 1, some of the sill depths in the to outflow through Sibutu Passage, and in 1/25° global HYCOM). These results
two models are in good agreement, but a shallow pathway from the Pacific via indicate that the mean transport of the
there are substantial differences in the Surigao and Dipolog straits, also to Mindoro to Sibutu pathway is largely
Dipolog Strait, Surigao Strait, and Sibutu outflow through Sibutu Passage. With constrained by the outflow through
Passage sill depths and in the topography the resolution increase from 1/12° to Sibutu Passage, while the transport of
along the entire southern archipelago 1/25°, a very large increase in transport the shallower pathway is mainly deter-
of the Sulu Sea. The topography of this is seen through Mindoro Strait (where mined by the inflow through Surigao.
archipelago is not adequately known, the topography and sills depths are in Thus, it is most critical to improve the
nor are the sill depths of San Bernardino good agreement) and through Sibutu topography and sill depths of Sibutu
15N Figure 1. (a,b) mean currents
a) 1/12° global HYCOM-18.2 b) 1/25° global HYCOM-04.1&2
0 0.1 0.2 0.3 0.4 0.5 m/s (m s-1) overlaid on speed
(in color) at 20-m depth in
and around the Philippine
. seas from (a) 1/12° global
hybrid coordinate Ocean
-0.13 -0.33 model (hycOm)-18.2
-0.70 -2.72
and (b) 1/25° global
hycOm-4.1&2. See table 1
10N
and related discussion in the
“Ocean model experiments”
-0.96 -1.15
-0.08 -0.01 section. The 2004–2009
-0.85 Mindanao -1.11 mean transport through
Current Bohol Sea straits labeled on (a,b) (in
Sv = 106 m3 s-1) is given in
-1.71 -4.18
boxes with negative values
for southward and west-
ward, as indicated by the
15N
attached arrows. The speed
South 5 00 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 m contour is 0.05 m s-1 and the
China reference vector is 0.5 m s-1
San Bernadino: 23 m Verde Island San Bernadino: 35 m
Sea (upper right in panel b).
every second (fourth) vector
is plotted in panel a (b).
Surigao: 63 m Surigao: 113 m (c,d) Bottom topography
Mindoro: 442 m Mindoro: 446 m
for (a) 1/12° global hycOm
and (b) 1/25° global hycOm
10N
with sill depths given
for key straits.
Balabac Sulu Sea
Dipolog: 451 m Dipolog: 228 m
c) 1/12° Sibutu: 246 m d) 1/25° Sibutu: 272 m
5N
120E 125E 120E 125E
32 Oceanography | Vol.24, No.1
Passage and the adjacent passages In 1/12° global HYCOM, both on a combination of geostrophic and
through the Sulu Archipelago. There of the straits bordering the Pacific, hydraulic control, the latter including
is also a 2.5-fold increase in transport San Bernardino and Surigao, are char- contributions from Bernoulli setdown
through the very shallow San Bernardino acterized by choke points that are one and bottom friction. The relationship
Strait. The increased transport through grid point wide and one grid point long, between Q and ΔSSH is given entirely in
San Bernardino has a marked impact so why the large difference in the trans- terms of physical and geometric parame-
on the mean flow through the interior ports (Figure 1a)? An explanation based ters. The mean 2004–2009 ΔSSH is 8 cm
passages of the Philippines between solely on the difference in sill depths for San Bernardino and 6 cm for Surigao.
Surigao Strait and Tablas Strait, which (Figure 1c) is not sufficient. A model by Using the sill depth and a bottom fric-
lies just east of Mindoro (Figure 2a). In Mattsson (1995; a barotropic version of tion coefficient of C = 2.5 x 10-3 gives
b
1/12° (1/25°) global HYCOM, there is a Equation A5 in Metzger and Hurlburt, close agreement at San Bernardino, but
net transport of 0.11 Sv (0.04 Sv) from 1996) relates transport through a strait not at Surigao. At San Bernardino, the
Surigao to Tablas versus 0.13 Sv (0.33 Sv) (Q) to the upstream-downstream change depth in the choke point is 29 m and
from San Bernardino to Tablas. in sea surface height (ΔSSH) based the sill is one grid point upstream. In
Figure 2. (a–d) mean meridi-
onal velocity cross sections
100
at 11°54’N, the latitude of
200 the Philex mindoro mooring
(marked with a vertical line
300 Mindoro Tablas on the cross sections), near
Strait Strait the location where mindoro
400
and tablas straits join (dashed
a) 2004-2009 mean -0.93 Sv b) 2004 mean -2.58 Sv
121E 121E line on Figure 1a). The labeled
transports are for the entire
cross section and all means are
100
over the time period labeled on
200 the figure panel. (e–h) Seasonal
means over 2004–2009 for JFm
300 (January–march, winter), amJ
(spring), JaS (summer), and
400
OND (fall). all are from 1/12°
c) 2008 mean -1.63 Sv d) 2008 mean 0.72 Sv
121E global hycOm-18.2, except
(c) is from east asian Seas
Navy coastal Ocean model
100
(eaS NcOm.)
200
300
400
e) 2004-2009 JFM mean -1.68 Sv f) 2004-2009 AMJ mean 0.81 Sv
121E 121E
100
200
300
400

g) 2004-2009 JAS mean -0.12 Sv h) 2004-2009 OND mean -2.70 Sv
121E 121E
120.5E 121E 121.5E 122E 120.5E 121E 121.5E 122E
-50 -40 -30 -20 -10 0 10 20 30 40 50 cm/s
Oceanography | march 2011 33
contrast, at Surigao the depth in the used in the barotropic version of in these simulations, the transport
choke point is 987 m and located at the Equation A5 in Metzger and Hurlburt through Makassar Strait is nearly the
western exit point, while the sill is near (1996). Additional factors also have an same (Table 2). These results imply either
the eastern entrance in a relatively broad impact on flow through the straits, such a variation in the contribution from the
area of similar depths. At the Surigao exit as differences in topographic configura- Mindanao Current (see Figure 1a)—the
point, the current has extended down- tion, increased numerical accuracy at case in these simulations—or a direct
ward through the mixed layer and into higher resolution, effects of tides on contribution from Sibutu Passage to the
the upper thermocline (~ 120-m deep). bottom friction, and reduced horizontal NECC, as seen in the results of Metzger
When a depth of 120 m and C = 0 is friction with increased resolution et al. (2010). The impact on the contribu-
b
used at the choke point in Surigao, the (because some friction parameters scale tion from the Mindanao Current, while
ΔSSH predicted by Equation A5 is in with model resolution). Makassar transport remains unchanged,
close agreement with the 6 cm seen in The outflow from the interannual suggests an indirect contribution to the
the model, supporting the earlier indica- HYCOM simulations, discussed above, NECC, as demonstrated by Metzger and
tion that Surigao controls the transport contributes to the transport through Hurlburt (1996, their Plate 2) and in
of the Surigao-Dipolog-Sibutu route for Makassar Strait at all depths above the the next section. Additionally, Metzger
flow through the Philippine Archipelago. Sibutu Passage sill. However, despite the and Hurlburt (1996, their Table 3b)
This article contains all the information differences in Sibutu Passage transport performed a set of eight global ocean
table 2. transports (Sv) through straits, model 2004–2009 vs. observed
Transect 1/12° HYCOM 1/25° HYCOM Observed
Strait Orientationa EAS NCOMb 18.2 4.1 and 4.2 Transport
mindoro eW –2.85 –0.70 –2.72 –0.95c
mindoro overflowd eW –0.08 –0.22 –0.21 –0.28c
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 –3.0e
taiwan eW 1.40 1.60 1.74 1.8f
karimata eW –0.50 –0.54 –0.61 –0.8g
makassar eW –12.26 –14.00 –13.70 –11.6h
a 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.
e Qu (2000), based on hydrographic data down to 400 db.
f Wang et al. (2003), based on 2.5 years (1999–2001) of shipboard acoustic Doppler current profiler data.
g Fang et al. (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