of the Philippine archipelago - Columbia University

14 Pages·2011·8.95 MB·English

PhiliPP iNe Strait S Dy NamicS e xPerimeNt oTh
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aBStract. Confined by the intricate configuration of the Philippine Archipelago,
forced by the monsoonal climate and tides, responding to the remote forcing from the
open Pacific and adjacent seas of Southeast Asia, the internal Philippine seas present a
challenging environment to both observe and model. The Philippine Straits Dynamics
Experiment (PhilEx) observations reported here provide a view of the regional
oceanography for specific periods. Interaction with the western Pacific occurs by way
of the shallow San Bernardino and Surigao straits. More significant interaction occurs
via Mindoro and Panay straits with the South China Sea, which is connected to the
open Pacific through Luzon Strait. The Mindoro/Panay throughflow reaches into the
Sulu Sea and adjacent Bohol and Sibuyan seas via the Verde Island Passage and Tablas
and Dipolog straits. The deep, isolated basins are ventilated by flow over confining
topographic sills that causes upward displacement of older resident water, made more
buoyant by vertical mixing, which is then exported to surrounding seas to close the
overturning circulation circuit.
iNtrODUctiON Tablas Strait. The South China Sea also
Following multiple pathways, waters of has access to the southern Sulu Sea via
the western Pacific enter the complex, Balabac Strait. The Sibutu Passage links
multidimensional array of seas and the southern Sulu Sea to the Sulawesi Sea.
straits that form the impressive archi- Once within the confines of the
pelago stretching some 3400 km from Philippine Archipelago, circulation and
Southeast Asia to Australia. The northern stratification are subjected to monsoonal
segment of this system is the Philippine winds that are textured by passages
Archipelago (Figure 1a), where the North between island morphology (Pullen
Equatorial Current bifurcation near 14°N et al., 2008, 2011; May et al., 2011),
(Nitani, 1972; Toole et al., 1990; Qiu and by sea-air heat and freshwater fluxes
Lukas, 1996; Qu and Lukas, 2003) forms including river outflow, and by regions
the western boundary for the equator- with strong tidal currents. Overflow
ward-flowing Mindanao Current and across < 500-m-deep topographic sills
the nascent poleward-flowing Kuroshio. ventilates the depths of isolated basins,
Pacific water seeps into the Sibuyan and the Sulu Sea, and the smaller Bohol
Bohol (Mindanao) seas by way of the and Sibuyan seas.
shallow San Bernardino and Surigao The Office of Naval Research spon-
straits, respectively, and in greater sored the Philippine Straits Dynamics
volume through the 2200-m-deep Luzon Experiment (PhilEx) with a goal
Strait into the South China Sea (Metzger of exploring the oceanography and
and Hurlburt, 1996, 2001; Centurioni dynamics in the narrow straits and deep
et al., 2004; Qu et al., 2006). From the basins of the Philippine Archipelago
South China Sea, the flow enters into the using both observations and model
Sulu Sea through Mindoro and Panay output. During PhilEx fieldwork,
straits, and eventually into the western conductivity, temperature, depth, and
Bohol (Mindanao) Sea through Dipolog dissolved oxygen measurements were
Strait, and perhaps into the Sibuyan Sea obtained, and lowered acoustic Doppler
by way of the Verde Island Passage and current profiler (CTD-O /LADCP)
2
Oceanography | march 2011 15
a b
118°E 120°E 122°E 124°E 126°E 119°E 120°E 121°E 122°E 123°E 124°E 125°E 126°E
16°N 100 16°N 100 Luzon
15004000 Pacific Ocean South China 100 100 100 Pacific Ocean
111024°°°NNN 4000Sou15t0h02000 10C00hina50 04S000e50P0aal1a0w0anMinSdt1r0o0aVrioet15152000r050d0000e120000 00Is115000lM0a10Lni0n05ud00d zoP510o0r0PaSo0n1T100s0Sa00tasrntabSara5ailg0it50a0y1b500i0e0ts0uLy4aa02S11001000Pm00Sn0000i00bSa o1o5S0u0enunDN5yeaa0t 0aehiaBypgn10010ao0r0oylo5s10g0005 1000S0V0tirSsaa100ei5yta010a010n001100CS01B00B0aaooS1nmh0h0e S0ooBoatllt ereSa2L00r055s0neie00t0yaatred1500iSnSouL1100G0001tr600e0000rigauy50060a0ti0ltfe150o0 42000000100 6000 111024°°°NNN 111190123°°°°°NNNNN 10P00aSl1a0ew0aan 1000 100 100 M10i00ndoro 41000010000100 Pan1Na00e0ygro1000s100100100MasbateC1e01000b1Bu00oB10h0ooh1o0l0 lSeLaeySteama1000r41000010100000 100 91111°0123N°°°°NNNN
100 12500000 4000 Sulu Sea 500 100 500 4000 Sulu Sea 1000 1000
8°N BSatrlaaibtac 100 1000 1000 100 1500 15000002000 Mindanao 15001000 8°N 8°N 4000 Reg10i0onal IOP100: Mar 2009Mindanao 8°N
6°N Borneo Sibutu500Passage S10u0lu24 00000A0rchip15el00ago100 Sulawe4s000i Sea 1400000 5001000 1002000 6°N 7°N 4000 AEJRoDxeipCngltiPoo: r nNMaatooolv orI/OyrDi:nP eJg:cus Jn21a0 0002n00 2700M70M8P Moorings 7°N
118°E 120°E 122°E 124°E 126°E 119°E 120°E 121°E 122°E 123°E 124°E 125°E 126°E
Figure 1. (a) Bathymetry of the seas and straits of the Philippine archipelago from Smith and Sandwell (1997) and http://topex.ucsd.edu/marine_topo.
(b) conductivity, temperature, depth, and dissolved oxygen, and lowered acoustic Doppler current profiler (ctD-O/laDcP) stations obtained by the
2
four Philex cruises identified within the figure legend. Philex mooring positions are indicated.
stations were made during the explor- The hull-mounted shipboard 75- and SST, sea level, ocean color, and wind. The
atory cruise of June 2007 and two 150-kHz ADCP system measures circu- Panay Island-based high-frequency (HF)
regional Intensive Observational Period lation of the upper hundreds of meters radar array provides high-resolution sea
cruises in the winters of January 2008 of the water column along the ship surface current information in Panay
(IOP-08) and March 2009 (IOP-09; track. The cruise data provide nearly Strait. In situ measurements of ocean
Figure 1b). CTD casts were also under- synoptic views of regional water-column currents and properties are provided
taken during the Joint US/Philippines circulation and stratification. Moorings by towed instrumentation and free-
Cruise of November and December (Figure 1b) provide time series of the floating sensors, such as surface drifters
2007. All PhilEx cruises were conducted currents within major straits. Data from (Ohlmann, 2011), profilers (Girton,
from R/V Melville, whose underway sea MacLane Labs moored profilers (MMPs; 2011), and gliders. Biological parameters
surface water system provides sea surface Figure 1b) characterize the internal closely related to the ocean physical
temperature (SST) and salinity (SSS) at wave environments (Girton et al., 2011). processes are also a component of PhilEx
high resolution, as well as other meteo- Satellite remote sensing provides a (Cabrera et al., 2011; Jones et al., 2011).
rological and oceanographic parameters. variety of regional observations, such as Model studies (Hurlburt et al., 2011;
Arango et al., 2011; May et al., 2011)
Arnold L. Gordon ([email protected]) is Professor, Earth and Environmental complete the suite of methods employed
Sciences and Associate Director, Ocean and Climate Physics, Lamont-Doherty Earth by PhilEx researchers to investigate the
Observatory of Columbia University, Palisades, NY, USA. Janet Sprintall is Research oceanographic conditions and processes
Oceanographer, Scripps Institution of Oceanography, University of California, within the Philippine Archipelago.
San Diego, La Jolla, CA, USA. Amy Ffield is Senior Scientist, Earth & Space Research, The coupled ocean/atmosphere is
Upper Grandview, NY, USA. characterized by variability across a
16 Oceanography | Vol.24, No.1
wide range of spatial and temporal 2006.5 2007.0 2007.5 2008.0 2008.5 2009.0 2009.5
a) 10 10
scales. Beyond tidal forcing and daily 120˚ 123˚ 126˚
weather conditions are intraseasonal day) 89 1124˚˚ 89
m/
Madden Julian Oscillations, seasonal m 7 10˚ 7
monsoon forcing, and interannual ation ( 6 8˚ 6
El Niño-Southern Oscillation (ENSO), cipit 5 5
e
plus decadal and longer fluctuations. A Pr 4 4
P
recurring question arises when dealing PC 3 3
G
with observational data, which inher- 2 2
ently have gaps in spatial and temporal 1 1
d)
ctloioonvngeser aro-gft eet:hr hmeo e w“nc tltiiyrmep aircteeag”li aocronen, tdahnitedi oo onbfss t?eh rev a- b)Anomaly--21012 --21012 no 3 (dashe
2006.5 2007.0 2007.5 2008.0 2008.5 2009.0 2009.5 Ni
Observations can be “leveraged” by
Figure 2. (a) The averaged precipitation time series (green line), mean-annual time series
being used to evaluate model output,
(purple dashed line), and mean (thin horizontal black line) are shown for data from loca-
or directly assimilated into the models,
tions near/within the Philippine seas. (b) The anomaly time series (green=wet, yellow=dry)
thus allowing for a reliable model-based is shown, along with Nino3 (blue dashed line) for comparison. in (a), the four precipitation
data locations are indicated by red squares on the map inset. On both (a) and (b), the three
glimpse of this fuller spectrum of events.
Philex cruise periods are indicated by vertical red bars. The precipitation data are from the
In addition to a change in the winds, GPcP Satellite-Gauge (SG), One-Degree Daily (1DD), Version 1.1 data set that is produced
the monsoon brings a change in precipi- by optimally merging estimates computed from microwave, infrared, and sounder data, and
precipitation gauge analyses from October 1996 to June 2009 (http://lwf.ncdc.noaa.gov/oa/
tation to the Philippine Archipelago,
wmo/wdcamet-ncdc.html#version2).
which is expected to be reflected in
surface layer thermohaline seasonal
stratification. Although there are local-
ized variations due to the interaction provide an introduction to the regional patterns within the major Philippine
of the wind with orographic lifting setting in order to place in context the basins of the Sulu and Bohol seas, and
processes and subsequent river runoff, information presented in the collec- to provide schematic overviews of the
in general, over the ocean waters, the tion of studies described in this special circulation to serve as a guide to the more
winter monsoon brings a time of less PhilEx issue of Oceanography. Each topic detailed studies that are found in this
rainfall and the summer more rainfall included in this regional introduction issue of Oceanography or are to follow.
(Figure 2a). However, comparison of the has been covered by PhilEx researchers
actual rainfall for a specific year to the (e.g., published to date are Han et al., reGiONal StratiFicatiON
average annual cycle reveals precipita- 2008; Rypina et al., 2010; Tessler et al., aND circUlatiON
tion anomalies (Figure 2b), which may 2010), is presented in this special issue the Sea Surface layer
be expected to induce anomalies in the of Oceanography, or will be further Surface-layer circulation observed by
surface layer thermohaline stratifica- developed in future publications. Here, the shipboard ADCP (Figure 3a,b)
tion. During the eight months prior to we use data from the regional PhilEx for IOP-08 and IOP-09 brings out the
the June 2007 exploratory cruise, there hydrographic cruises and moored time complexity of the pattern of surface
was an anomalous dry period, an accu- series measurements to describe the currents and the relationship to SST.
mulative response to the El Niño of the flow pattern and water mass distribu- The direct connection of the
preceding year, whereas the IOP-08 and tion within the Philippine Archipelago. Philippine seas to the western Pacific is
IOP-09 cruises were conducted during Our goal is to determine the potentially through San Bernardino Strait, with a sill
anomalously wet periods. important first-order processes that depth of 92 m near 12°47'N, 124°14'E,
The objective of this paper is to might lead to observed circulation and Surigao Strait, with a sill depth
Oceanography | march 2011 17
a
119°E 120°E 121°E 122°E 123°E 124°E 125°E 126°E of 58 m near 10°17'N, 125°50'E (both
14°N 14°N
Luzon
topographic estimates are from http://
119°E 120°E Cool S1S2T1°E 122°E 123°E 124°E 125°E 126°E
14°N 14°N
Luzon topex.ucsd.edu/marine_topo; Smith
Cyclonic
eddy Cool SST
13°N 13°N and Sandwell, 1997). A comparison of
111232°°°NNN CSSyecdCCldoSSynic AAnntticicyycclolonnicic eeddddCyyMMoCiionnyldd cSoolorrSoonTic eddy TTIIssaallbabalnlnaaddss MMaassbbaattee SSttccrruuoorrnnrrggee nnttiittddssSSaaaallmmaarr 111232°°°NNN twPvhiaigetcoh itfirihonceu rtdsmhe meposiheicx atsisltni rangane,i t wews nitatovhtie trrpho-oancstom soliubefm nltehtn ueo ppfw wreoesfiltleliern ng
Cool SST Panay
Leyte of subsill-depth western Pacific water
11°N 11°N
Cyclonic eddy
Panay entering into the confines of the straits.
11°N Palawan 1 m/s Cebu SLtecryuoternrge ntitdsal 11°N The currents within San Bernardino
10°N Palawan 1 m/s Negros CebuBohol Stcruornrge ntitdsal 10°N and Surigao straits were the strongest
10°N Sulu Sea 10°N observed during all the PhilEx cruises.
BBoohhooll Jet
Negros In each strait, the average speed of the
9°N Sulu Sea 9°N
27.2 27.6 28.0 28.4 Bohol Jet upper 50 m for the one- to two-day
Iligan Bay Mindanao
9°N 27.2 R082 7 S.6ST NB15208:. 025-55m 28.4 Cool SST Eddy 9°N ship occupation was about 0.5 m s-1
Iligan Bay
119°E 120°E 121°E 122°E 123°E 1E24d°dEy 125°EMindanao126°E from the western Pacific to the interior
R08 SST NB150: 25-55m Cool SST
119°E 120°E 121°E 122°E 123°E 124°E 125°E 126°E seas. However, the strong tidal currents
14°N b 119°E 120°E Luz1o2n1°E 122°E 123°E 124°E 125°E 126°1E4°N
obscure the nontidal flow and there-
119°E 120°E Cool 1S2S1T°E 122°E 123°E 124°E 125°E 126°E
14°N 14°N
Luzon fore prohibit meaningful estimation of
Cool SST
13°N 13°N the nontidal throughflow, other than
SCS Mindoro Strong tidal
currents suggesting that it is likely that the direc-
13°N 13°N
111212°°°NNN SCS SSuurfrafaccee f lfoloww t oto S CSMindoro PTTIIassaanllbabaalnlnyaaddss MMaassbbaattee StcruornrgLee ntyittdSSseaaammlaarr 111212°°°NNN tPcioholnThiuli mopefpn sti hnhteieepm itbnhpotreeaorrruaidotg urAhr flsDeeo/aCswsaP. l wi naanitsdy i npwtroaot fiethrle-es
S
C
S Panay do not show clear continuity of the
11°N Palawan Cebu Leyte Strong tidal 11°N San Bernardino water into the Sibuyan
10°N Palawan 1 m/s Negros CebuBohol Stcruornrge ntitdsal 10°N or Camotes seas. In contrast, the Surigao
1 m/s currents Strait characteristics do intrude into the
10°N 10°N
BBoohholol Jet
Negros Bohol Sea within a well-defined surface
9°N 9°N
Bohol Jet current across the northern Bohol Sea,
Iligan Bay Mindanao
9°N Eddy 9°N linking the Surigao Strait throughflow
Sulu Sea
Cool SST Iligan Bay
8°N Eddy Mindanao 8°N to Dipolog Strait with export into the
Sulu Sea 27.C2ool SS27T.6 28.0 28.4 Sulu Sea. South of this “Bohol Jet” is a
8°N 8°N
27.2 R092 7 S.6ST OS15208:. 023-55m 28.4 cyclonic circulation feature that we call
7°N 7°N
the Iligan Bay Eddy, found on all of the
R09 SST OS150: 23-55m
7°N 7°N PhilEx cruises, near 124°E (Figure 3).
119°E 120°E 121°E 122°E 123°E 124°E 125°E 126°E Generation of the Iligan Bay Eddy
is not coupled to a sea surface ocean
119°E 120°E 121°E 122°E 123°E 124°E 125°E 126°E
Figure 3. current vectors from the shipboard-mounted 150-Khz aDcP system within color signal, as may be expected from
the 25- to 55-m layer color coded by sea surface temperature (SSt). a 1 m s-1 arrow is upwelling usually associated with a
given for scale. ScS is the South china Sea. Various features are shown in italics. (a) Philex
cyclonic circulation pattern (Cabrera
regional cruise January 2008. (b) Philex regional cruise march 2009.
et al., 2011). The Bohol Jet enters the
18 Oceanography | Vol.24, No.1
Sulu Sea, though its path once within weak and toward the South China Sea. summer SST was about 2° warmer than
the Sulu Sea is not clear. Relatively cool In March 2009, the surface flow in in January 2008 or March 2009. The
SST is observed south of the Bohol Jet Panay and Mindoro straits, with less SSS range between cruises amounts to
extension into the eastern Sulu Sea, a mesoscale activity, was also toward 1.0 psu, with the lowest SSS in January
consequence of upwelling along the the South China Sea. In January 2008, 2008. The highest SSS is in June 2007, a
Zamboanga coast (Villanoy et al., 2011). there was a strong cyclonic eddy in the consequence of the normal dry season
Sulu Sea surface layer circulation South China Sea adjacent to Mindoro compounded by the drier conditions
(Figure 3) displays much mesoscale
activity at horizontal scales of ~ 100 km,
with varied SST and SSS, rather than a
clear basin-scale gyre. However, these “
Philex DiD mUch tO aDVaNce OUr
synoptic “snapshots” offered by the
shipboard ADCP mapping do not allow UNDerStaNDiNG the WaterS OF the
for conclusions about the general circu- PhiliPPiNe archiPelaGO.
”
lation pattern of the Sulu Sea as each
cruise covered different segments of the
this sea. The warmer SST observed in
March 2009 relative to January 2008 may
be partly the normal seasonal signal, Strait (Pullen et al., 2008). The surface of an El Niño (Figure 2). The lowest
though in 2009 the survey extended current in Tablas Strait displayed a weak SSS was observed in January 2008 (as
further to the south, and 2008 tended cyclone (note westward flow at 121.8°E well as during the Joint Cruise of late
to have stronger winds and surface between Mindoro Island and Tablas 2007), a consequence of the phasing out
currents. The January 2008 surface Island, Figure 3), though the tendency of the previous wet season, whereas in
circulation of the Sulu Sea east of 120°E in March 2009 was for flow out of the March 2009, nearly two more months
is cyclonic, with northward flow along Sibuyan Sea into the Panay-Mindoro into the dry season, SSS was slightly
the eastern boundary that continues corridor. The throughflow of the rela- more elevated. The lowest SSSs (< 33.4)
into Panay Strait and southward flow tively cool SST of the Verde Island are observed in the Bohol Sea and the
west of 121.3°E. The Sulu circulation was Passage was weak, slightly toward the South China Sea entrance to Mindoro
anticyclonic in June 2007 (not shown). west in January 2008, and toward the Strait during the winter regional
The March 2009 cruise data covers east in March 2009. cruises, perhaps a consequence of the
different parts of the Sulu Sea so that delayed river runoff from their respec-
direct comparison with January 2008 Water-column Stratification tive larger neighboring landmasses of
is not practical, though again we find Upper 400-m Profiles Mindanao and Luzon. The Sibuyan
northward flow in the eastern Sulu Sea With descent into the water column, and Camotes seas’ SSSs are between
and within Panay Strait, albeit relatively the warm, low-salinity surface water 33.4 and 33.6. The seasonal influence
subdued relative to January 2008. above the top of the thermocline at determined by comparing the salinity
In Mindoro and Panay straits, the 50–70 m rapidly gives way to cooler, differences between the PhilEx cruises
January 2008 currents form energetic saltier water at 200 m (Figure 4, upper is found to reach to about 130 m, into
eddies in response to complex wind panels). The potential temperature (θ) the mid thermocline.
stress curl (Rypina et al., 2010; May drops by ~ 10°C over only 100 m, The relatively warm thermocline data
et al., 2011; Pullen et al., 2011), though from 70 to 170 m, coinciding with an (Figure 4) are from the western Pacific
on average, the surface currents are intense pycnocline in which density adjacent to San Bernardino and Surigao
directed toward the South China Sea. increases by nearly three sigma-0 units, straits. In the salinity profiles, these
In June 2007, the surface flow was from 22.5 to 25.2. The June 2007 early stations display a pronounced salinity
Oceanography | march 2011 19
a
Potential Temperature (oC) Salinity maximum (s-max) in the 75–250 m,
8 12 16 20 24 28 32 33.2 33.6 34.0 34.4 34.8
0 0 16° to 28°C interval that marks North
50 50 Pacific Subtropical Water. Although
db) 110500 T h erm oclin e 110500 tShuer isghaaol lsotwra Sitasn b lBoecrkn sa-rmdianxo w anatde r,
ure ( 200 Pacific 200 North Pacific Subtropical Water has
s
s
Pre 250 250 access to the South China Sea via
Luzon Strait. However, at the South
300 Expo-07: Jun 2007 300
IOP-08: Jan 2008 China Sea entrance to Mindoro Strait,
350 IOP-09: Mar 2009 350
the pronounced s-max core in the
400 400
20 21 σ 20° to 28°C range is greatly attenuated
0
30 Surface layer 22 with only a weak s-max near 28°C
26 σ
0
23 as observed in the June 2007 cruise
25 Pacific IOP-08 12
oe (C) SuSb-tmroapxica l24 380-440 m 26.4 dflaetxau, roer n ae taerm 2p5°eCra itnu rdea/staa lfirnoitmy (tThe/S )
ntial Temperatur 1250 South China Sea 2256 SoutPh4 aC8n0ha-iyn5 a8SS 0iSl8u lme0lua0 S-1e2a00 m 12680.8 raThe dgeei oepnpraeorlc 2ess0-sm0e8sa xaw nnidteh a2irn0 1 0t69h° ecC rS u(oFiusietghsu, Craenh 4din) .a
e
Pot 10 27 Sea that attenuate the North Pacific
Pacific
NPIW
6 subtropical s-max are beyond the
34.42 34.44 34.46 34.48 34.50
5
28 Salinity scope of this paper.
A salinity minimum (s-min) is
33.2 33.6 34.0 34.4 34.8
Salinity observed from 350 to 600 m in the
western Pacific stations, marking
b
North Pacific Intermediate Water,
which also has access to the South
400 400
Sea Sea China Sea via Luzon Strait. An
Pressure (db) 11826000000 Panay Strait Tablas Strait Sibuyan Sea S. Sibuyan Sea Bohol Sea Camotes S. Sibuyan SeaCamotes buyan Sea 11826000000 anapSsttnerertoadaeu vrn c(P iTu1tdaue0aenrst°seesaC l tdy(eh Firsbse ti eu grootatub v iasrstelteser.i., rfll 4lI2v o,tv0 e wliid1oss 0 wiitibn)nhe.lt ieroThMs ls wet-iheftnma e dθtp ie/SonarSur n otlheu la )t
Si Bohol Sea further brings out the stratification
Sulu
Sea
2000 2000 features of the Philippine waters,
Sulu Sea
particularly the “gap” between the
9 10 11 12 13 14 0.0 0.4 0.8 1.2 1.6
Potential Temperature (oC) Oxygen (ml/L) western North Pacific saline subtrop-
Figure 4. ctD temperature and salinity obtained on the June 2007 Philex exploratory cruise, ical water and fresher thermocline
the January 2008 regional cruise, and the march 2009 regional cruise. (upper panels) Potential water of the Philippine waters, as
temperature and salinity profiles with pressure (depth) for the upper 400 m for the three
well as the attenuated North Pacific
Philex cruises. (lower panels) Potential temperature and salinity scatter plot. The lower right
panel shows the 6° to 13°c strata. Intermediate Water s-min in Mindoro
and Panay straits.
Two CTD stations (Figure 4, upper
panels) in the southern Sulu Sea from
20 Oceanography | Vol.24, No.1
March 2009 show relatively warm, salty minimum. For example, the Bohol Sea of ~ 500 m. The oxygen minimum of
water between 125 and 150 m, marking oxygen minimum near 12°C is observed the Sibuyan Sea serves as the overflow
the trough of solitons observed in that spreading near 300 m throughout water source for the South Sibuyan Sea,
area (Jackson et al., 2011). the Sulu Sea, with traces entering which, with further oxygen consump-
into Panay Strait. tion, accounts for the near zero oxygen
Deep Basin Ventilation The effective sill depths are found by (< 0.3 ml l-1) of the southern Sibuyan Sea
The Philippine seas are composed of matching the bottom temperature with bottom water.
numerous deep basins isolated from one the temperature profile of the external The coldest deep basin is that of
another by topographic barriers. There is source water. The shallower the sill, the the Sulu Sea. The source of deep Sulu
the open deep Pacific Ocean to the east; warmer the basin waters. The relation- Sea water is generally considered to be
there are the relatively large seas to the ship of source sill depth θ/S to deep South China Sea water entering through
west and south of the Philippines (the basin water θ/S depends on the mixing Mindoro Strait (Broecker et al., 1986).
Sulu, South China, and Sulawesi seas);
and there are the smaller interior seas,
most notably the Bohol and Sibuyan
seas, and the still smaller Visayan and
“
Camotes seas (Figure 1). Below roughly
[Philex] POiNtS the Way tOWarD FUrther,
500 m, these seas have marked differ-
mOre qUaNtitatiVe reSearch aND illUStrateS the
ences in θ (and S) and oxygen values
from each other and from the source NeeD FOr hiGh SPatial aND temPOral reSOlUtiON
water column of the open western North iN BOth OBSerVatiONS aND mODeliNG.
”
Pacific. The deep Sulu Sea has a potential
temperature of 9.9°C, the deep Bohol Sea
11.6°C, and the Sibuyan Sea 10.4°C; the
southern Sibuyan Sea is slightly warmer
at 10.7°C, with the warmest isolated
basin; and the Camotes Sea has a bottom environment at the controlling sill and However, the deep salty bottom water
potential temperature of 13.2°C. the mixing/entrainment environment of the Sulu Sea does not match a sole
These marked property differences of the descending plume: the effective South China Sea source (Figure 4, lower
are a product of sill depths of the topo- sill depth is less deep than the deepest right panel). Quadfasel et al. (1990)
graphic barriers to the neighboring passage as determined by sonic surveys. recognized that the density of the
seas. The isolated deep basins are The warmest deep basin is the interior South China Sea source cannot reach
ventilated by spillover at the topo- Camotes Sea, with a controlling sill the bottom of the Sulu Sea unless there
graphic barriers that then descend to depth of less than 300 m. The oxygen is substantial addition of suspended
the depths, replacing resident water of the Camotes deep water is near sediment to make for a denser blend.
made less dense by vertical mixing. The 0 ml l-1, indicating slow ventilation rela- Based on evidence from sedimentary
resident water is lifted upward by denser tive to the oxygen consumption. The records of the Sulu Sea, Quadfasel et al.
overflow water, and is subsequently oxygen levels of the bottom water of (1990) suggest that episodic turbidity
exported to the surrounding seas to the southern Sibuyan Sea are also near currents from the South China Sea at
close the overturning circulation cell. 0 ml l-1. They are slightly warmer than intervals of several decades (on average
As these waters are reduced in oxygen the bottom water of the main Sibuyan 50 years) may have played an important
by the rain of organic material from Sea. The South Sibuyan Sea has an effec- role in plunging dense water toward the
the sea surface, their export to neigh- tive sill depth of ~ 400 m, while the main bottom of the Sulu Sea. However, this
boring seas can be traced as an oxygen Sibuyan Sea has an effective sill depth still would not explain the salty bottom
Oceanography | march 2011 21
water of the Sulu Sea. While Sulu Sea ventilation to time SerieS
PhilEx observations of stratification ~ 1250 m is drawn from the 570-m-deep The time series of moored velocity obser-
and currents between June 2007 and northern sill in Panay Strait, we specu- vations provide a longer-term context
March 2009 reveal a strong overflow late that the deeper Sulu water may be for the “archipelago-scale” fieldwork
between 400- to 570-m depth from derived from the Sulawesi Sea to the undertaken as part of PhilEx (Figure 1),
Panay Strait into the Sulu Sea (Tessler south by way of the Sibutu Passage. The such as the synoptic shipborne flow and
et al., 2010). The overflow water is Sibutu Passage sill is around 350 m, but property measurements of the regional
derived from approximately 400-m deep delivers denser water into the Sulu Sea survey described above, and the short-
in the South China Sea. Sulu Sea strati- than does the South China Sea. This term drifter deployments (Ohlmann
fication indicates that the overflow does is primarily because of the shallower et al., 2011). The high-resolution time
not descend below 1250 m in the Sulu pycnocline of the Sulawesi Sea compared series data from the moorings are also
Sea, but rather settles above high-salinity to that found in the South China Sea, used as an important metric to test the
deep water. The mean observed overflow and the strong tidal heaving of the Sibutu veracity of numerical models in the
transport at the sill is 0.32 Sv, with a Passage pycnocline that can lead to Philippine region (e.g., Hurlburt et al.,
residence time of 11 years in the affected rather startling solitons within the Sulu 2011; Pullen et al., 2011).
Sulu layer from 575 to 1250 m. Sea (Apel et al., 1985). The current measurements from
the PhilEx moorings (Figure 5) in
Mindoro, Panay, Tablas, and Dipolog
straits (see Figure 1 for locations) reveal
Along-Strait Velocity 150 m
much variability and baroclinity over
0.4 the ~ 15-month deployment period.
At 150 m, the strong southward flows
0.2
s) at Mindoro and Panay are evident
m/ 0 as two intraseasonal (30–60 day)
y (
cit−0.2 pulses during the northeast monsoon
o
el (December–February). During the
v
−0.4
southwest monsoon (April–October)
when the flow is mainly northward at
−0.6
Mindoro Panay Tablas Dipolog 150 m in Mindoro and Panay, the flow
−0.8
20 08 F M A M J J A S O N D 20 09 F M is more southward at Tablas. Some of
this southward Tablas flow is probably
Along-Strait Velocity 420 m
siphoned off to contribute to the stronger
0.4
northward flow observed at Mindoro
0.2 compared to Panay during this period.
s) The along-strait flow at 150 m in Dipolog
m/ 0
y ( Strait exhibits more of a short-period
cit−0.2 (15–20 day) signal and is primarily
o
el eastward. At 300-m depth, the flow in
v
−0.4
all four passages is also characterized by
−0.6 short-period variability (not shown). At
420 m, there is high coherence between
−0.8
20 08 F M A M J J A S O N D 20 09 F M the consistently strong southward flow
through Mindoro and Panay straits.
Figure 5. along-strait velocity at 150 m and 420 m measured at the moorings
shown in Figure 1. At this depth, the flow is near bottom
22 Oceanography | Vol.24, No.1
in Mindoro Strait and often slightly is of prime importance in ventilating (based on ADCP and CTD data) in these
stronger than that observed at Panay the subsurface layers of the Bohol Sea. channels is negligible, but this channel
Strait, although the deeper, near-bottom The eastern end of the Bohol Sea is may be an effective way to deliver river
flow in Panay Strait (~520 m) can connected to the Pacific Ocean across runoff from the islands of the central
reach velocities of > 1 m s-1. This strong the broad, shallow Leyte Sea through Philippines, and reduce the SSS of the
benthic overflow contributes to the the 58-m-deep Surigao Strait, where northern Bohol Sea.
ventilation of the deep Sulu Sea (Tessler there appears to be a small net flow of The LADCP profiles reveal the
et al., 2010). In Dipolog Strait, the near- surface water into the eastern Bohol Sea. highly layered circulation profile within
bottom 420-m flow is strongly eastward As described above, Surigao inflow Dipolog Strait (Figure 6), with two layers
from the Sulu Sea into the Bohol Sea, streams across the northern Bohol Sea of inflow into the Bohol Sea and two
although as in the shallower layers, the to be exported into the Sulu Sea through outflow layers. Near the topographic sill
flow here is also characterized by strong Dipolog Strait. The northern Bohol Sea where the PhilEx mooring was sited, the
15–20-day variability. is connected to San Bernardino Strait by flow into the Bohol Sea occurs within the
way of a 330-km-long narrow channel thermocline from roughly 80 to 200 m
BOhOl Sea that runs through the Camotes Sea to and in the benthic layer overflow at the
With a sill depth of 504 m, the enter the Bohol Sea both to the east and Dipolog sill. The 150-m and 420-m time
47-km-wide (as measured between the west of Bohol Island. This channel has series (Figure 5), which show eastward
100-m isobaths) Dipolog Strait between an 18-m-deep constriction to the north- flow, are consistent with the LADCP
Negros and Mindanao separates the east of Bohol Island and a 3-km-wide, data. The two layers exported into the
Bohol Sea from the Sulu Sea and is the though deep (280 m), channel to the Sulu Sea consist of the surface water
deepest connection of the Bohol Sea to northwest of Bohol Island. PhilEx obser- of the upper 50 m and a second layer
surrounding seas. The Dipolog Strait vations indicate that the throughflow centered at 300 m. The inflow/outflow
North South North South
Westward Eastward 54 55 56 57 5854 55 56 57 58
0 0 0
Towards Sulu Towards South
Towards North
Out of Bohol Sea
Mainly northern side of strait
100 into Bohol Sea 100 Towards Bohol 100
200 200 200
Towards Sulu
m) m)
Depth (300 Dipolog Strait Throughflow Depth ( 300 300
Out of Bohol Sea
400 400 Towards Bohol 400
123°00'E 123°30'E
9°00'N
500 Bohol 500 500
Overflow into Bohol Sea Dipolog LADCP Sulu Dipolog Bohol
U V
8°30'N
600
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0 10 20 30 40 0 10 20 30 40
U (m/sec) Distance (km) m/sec
-0.4 0.0 0.4
Figure 6. regional 2008 laDcP data from within Dipolog Strait. (left) laDcP profiles of the zonal flow within the deep channel shown in the
map insert. The position of the moorings is shown as a yellow triangle. (right) The laDcP section across Dipolog Strait showing zonal and
meridional currents.
Oceanography | march 2011 23