PAMS 2011 Conference: Taipei, Taiwan
An Operational Ocean Model for the Northern South China Sea:
Forecasting Mesoscale Processes and Internal Tides
Authors: Dr. Mark Cobb, Dr. Andrea Mask, Dr. Chris DeHaan, Carl
Szczechowski, John Rogers-Cotrone, Lea Locke, and Dr. Charles Horton
Naval Oceanographic Office, Oceanographic Department
St
Stennis
i Space
S
Center,
C t MS 39522
Slide 1
Naval Oceanography
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Forecast Systems & Custom Products
Ocean Prediction Department CONOPS
Observations Ocean Models Forecasts
Satellite & In situ
Global – Regional Global Regional – Coastal Coastal – Port
14 km 3 km 300 m 5 m
‐ 3D Full Physics
‐ Assimilation
‐ Forecasts to 5 days
‐ Nesting & B.C.
NOGAPS/COAMPS
/
Oceanographers ‐ Interpret models & obs.
‐ Evaluate uncertainty
‐ Tailor forecasts & products to Navy mission.
US‐East
NCOM
Groton
Delft3D
Groton G
t
NCOM
Global NCOM
Slide 2
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Data Assimilation
Navy Coupled Ocean Data Assimilation System (NCODA)
• Today – MVOI Assimilation
• FY11 – transition to 3DVAR
– Improvements in
processing time but not skill
• FY14 – transition to 4DVAR
– Major improvement in
assimilation skill expected
– Need more computational
speed to implement
• Working on adding current observations to system
to system
• Cycling of yesterday’s 24‐hr model forecasts provides initial fields for OCNQC and MVOI (Multi‐Variant Optimal Interpolation). • Assimilation performed during model hindcast. Slide 3
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Global Ocean Modeling
Global Navy Coastal Ocean Model (G‐NCOM)
• Daily 3D Forecasts of ocean
– Temperature
– Salinity
– Currents
– Elevation (tides)
• Resolution – Horizontal grid: 1/8 deg – 40 vertical layers
40 vertical layers
– Including Arctic
• Forecast to 96hr @ 3hr increments
• Based on Princeton Ocean Model
• FNMOC NOGAPS atmosphere forcing
p
g
– Wind stress (momentum)
– Heat fluxes (IR/visible/sensible/evaporation)
• Tides from OSU (Egbert) model
• Coupled with Los Alamos CICE
• Assimilates – Sea surface temperature and elevation from satellites (SST / SSH)
– Temperature & salinity data from surface observations and profiles (CTD, floats, gliders, marine mammals)
lid
i
l)
– NCODA / MVOI assimilation scheme
– Employs insitu and synthetic profiles
• This is a deep water model
– To forecast mesoscale processes p
– Order ½ degree features
– Use with caution within 2‐3 points of land
Naval Oceanography
RE
G1
7
RE
G1
8
RE
G1
4
RE
G1
5
RE
G1
6
Temperature, Salinity, Elevation NRL Stennis graphics (past 30 days)
1/8 deg (~14 km / 7.5 nm)
Slide 4
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Regional Ocean Modeling
Regional Navy Coastal Ocean Model – R-NCOM
• Same structure / algorithms as
GNCOM
• Boundary Conditions provided by
GNCOM – 1 way nest
• FNMOC COAMPS forcing
• 3D Forecasts
– T, S, Currents, Elevation
– Resolution varies (~1 / 36 deg)
– Up to 55 vertical layers
– Forecast to 96hr @ 3hr
increments
• Assimilates data from
– Satellites (SST, SSH)
– insitu obs (XBTs, CTDs, floats,
buoys)
– Employs synthetic profiles
• Tides internal to model – internal tides
evident.
USEAST-NCOM
GNCOM
• Eventual transition to COAMPS-OS
(coupled atmosphere
atmosphere—ocean—waves)
ocean waves)
1/36 deg (3 km / 1.7 nm)
Slide 5
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Northern South China Sea (NSCS) R-NCOM
Model Overview
¾Domain Extent: 105oE -129.9oE, 14oN-23.96oN
¾3 km grid resolution
¾2 min bathymetry from Naval Research Laboratory (NRL) DBDB2_V30
Database
¾1 Hourly
H l 18 kkm FNMOC C
Coupled
l dO
Ocean/Atmospheric
/At
h i M
Mesoscale
l P
Prediction
di ti
System (COAMPS) wind fields (e.g. surface winds, total precip, radiation forcing)
¾Initialized from 1/8o G-NCOM
G NCOM on Oct 15
15, 2008
2008.
¾3 hourly baroclinic G-NCOM boundary conditions. Barotropic tides imposed at
the NSCS boundary.
boundary
¾42 Sigma levels (50 total). Sigma levels fully extended at ~ 1600 m depth.
Slide 6
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R-NCOM vs. G-NCOM Circulation
SSHA & Surface Currents
1/36o R-NCOM
Kuroshio Current
Black box indicates NSCS R-NCOM boundary
1/8o
G-NCOM
Slide 7
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Northern South China Sea: Regional Oceanography
(What is NAVO trying to forecast?)
¾Surface wind effects on mixed layer, radiation heat fluxes, diurnal cycle of heating and cooling, precipitation effects.
Mesoscale
M
l Circulation:
Ci
l ti
¾NE monsoon circulation: (i) Basin wide cyclonic gyre circulation pattern. (ii) Wind driven southward shelf break current.
¾Cyclonically propagating cold core eddies occur (i.e. Luzon cold eddy) during NE monsoon.
¾SE monsoon circulation: (i) Dual cyclonic/anti-cyclonic gyre circulation pattern (ii) Eddy generation (iii) Vietnam Jet.
¾NSCS typically
yp
y has a high
g thermocline ((relative to Philippine
pp
Sea)) and mixed layers
y
between 0-100 m.
¾Frontal structures due to eddies and currents.
¾Kuroshio current (KC) meanders into NSCS. This can generate eddies and filamentary structures in the vicinity of Luzon
Strait.
Internal Waves:
¾K1 and M2 are primary barotropic tides. M2 weakens away from Luzon whereas K1 intensifies (N to S).
¾Large (10-80 m) internal waves occur in vicinity of Luzon Strait. K1 internal waves dominate the interior of NSCS.
Mesoscale Circulation
Internal Waves
Sea Surface
Elevation (m)
Slide 8
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NE Monsoon Example:
Cold Core Eddy Generation near Luzon
Mean COAMPS Wind Stress
Mean Ekman pumping
Upwelling
region
Mean Temperature: 62 m
Model animation of mean temperature (3-day mean
intervals) at 62 m between Dec 31-Feb 23 clearly shows
f
formation
ti and
d cyclonic
l i propagation
ti off cold
ld core eddies
ddi
west of Luzon.
Other features:
¾Kuroshio intrusions across Luzon strait are observed.
¾Shelf break current is well established
¾Large warm core eddy in southeast corner of domain
Slide 9
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Validating NSCS R-NCOM Mesoscale
Circulation with Altimetry
¾24 hr mean model fields of sea surface height (SSH) are compared to previous week’s altimeter along track
sea surface height anomaly (SSHA) data. Altimeter data influences model via assimilation of synthetic
temperature-salinity profiles derived from SSHA and SST observations
observations.
¾Time scales of NSCS eddies (~ several weeks to months) permit valid comparisons to SSHA obs.
¾Cyclonic basin scale movement of cold core eddies
eddies, forecast by R-NCOM
R NCOM, is in reasonable agreement with
altimeter SSHA data.
¾Evolution of warm core/anti-cyclonic structure in SE corner of domain is also in agreement with SSHA obs.
Dec 15
15, 2008
Jan 15
15, 2009
F b1
Feb
1, 2009
Feb 22
22, 2009
March 1
1, 2009
April 8
8, 2009
Slide 10
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Validating NSCS R-NCOM Mesoscale Circulation
NE Monsoon: January 2010 K10 SST Data + Altimetry
NCOM
7 day SST composite
Slide 11
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Validating NSCS R-NCOM Mesoscale Circulation
SW Monsoon: August 2009 K10 SST Data + Altimetry
NCOM
7 day SST composite
Slide 12
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Luzon Transit: Ship of Opportunity
Sept 10-11, 2009
¾A ship of opportunity can be an excellent way of
obtaining a transect of observations with relatively
high spatial resolution (~ 20 km intervals).
¾Large change in thermocline depth from the
Philippine Sea to the NSCS is reproduced quite well
by R-NCOM model.
¾Signature of internal waves is apparent in
isotherms of obs as well as R-NCOM.
118
120
122
124
126
Ship transit
Slide 13
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NSCS R-NCOM vs. Insitu Observations
¾Profiling floats (temperature and salinity data) provided extensive coverage of NSCS model domain during Jan (09
NE monsoon) and Aug (09 SW monsoon).
¾R-NCOM is in reasonable agreement
g
with T-S data from floats spanning
p
g the model domain during
g NE and SW
M
Monsoons
(black
(bl k circles-red
i l
d centers).
t
)
¾Model produces the correct transition from Philippine Sea to NSCS with regard to thermocline and T-S curves.
¾NCOM sonic layer depths (SLD) (first max of sound speed profile) are in good agreement with SLD derived from
observations (all NSCS float data are used). Model tends to over-predict depth of shallow SLD.
Locations
Temperature
T-S Diagram
Sonic Layer Depth
Slide 14
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Forecasting Internal Waves within the NSCS
¾R-NCOM barotropic tides generate baroclinic hydrostatic internal waves at the Luzon Strait. Note:
RNCOM cannot simulate non-linear internal waves.
¾This example shows internal waves, generated by a sill at 800 m depth, along a longitudinal
transect at 20 deg north. Internal waves propagate away from Luzon Strait. 3-Hourly model output.
¾Power spectrum analysis of sea glider results indicate that K1 baroclinic internal tides dominate
SW of Luzon Strait as expected from other studies of this region
region.
¾R-NCOM results are in fair to good agreement with observed internal wave power spectrum
Slide 15
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NSCS R-NCOM vs. Glider Observations
Virtual Mooring:
Mooring April 5-12, 2009
Depth vs. Temperature (oC)
R-NCOM Sea Surface Height (m)
Model SSH indicates
diurnal tidal signal in
vicinity of glider.
Glider performed
Glid
f
d 3-hourly
3h
l
dives at the same
approximate location for 1
week.
T-S Diagram
OBS
R-NCOM
G-NCOM
G
NCOM
R-NCOM
R
NCOM
Depth vs. temperature
plots indicate reasonable
agreement between the
model
d l and
d observations
b
ti
with regard to phase and
amplitude of internal
waves.
T-S diagram reveals a RNCOM bias towards
slightly higher salinity at
temperatures > 20oC/100C/100
150m
OBS
Slide 16
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NSCS R-NCOM vs. Glider Observations: Power Spectra
April 5-12, 2009
¾Fourier analysis of glider and R-NCOM temperature time series at model output depths
¾Overall good agreement between model and observations
observations.
¾ Majority of R-NCOM baroclinic energy is centered around the thermocline depth (100-200 m) at the
diurnal tidal frequency.
¾R-NCOM under-predicts surface energy (< 70 m) in the diurnal frequency band. Precise reason for this
is not clear. Low frequency surface energy may be due to synoptic weather events.
Slide 17
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NSCS R-NCOM vs. Glider Observations:
Power Spectra at 100 m:
m April 5-12, 2009
¾R-NCOM has a diurnal and semi-diurnal
peak in the power spectrum.
¾Observations clearly show a diurnal
peak in the power spectrum.
spectrum
25 hours
12.5 hours
¾Observations indicate a step or small
peak at frequencies between diurnal and
semi-diurnal frequencies. No well defined
semi-diurnal peak. Possibly due to 3-hour
glider dive interval.
¾Primary internal tide peak occurs at ~25
hours in both model and observations.
50 h
hours
10 hours
¾R-NCOM has more power at diurnal and
semi-diurnal peaks than observations.
Perhaps due to lack of non-linear
dissipation mechanisms
mechanisms.
Slide 18
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Summary and Conclusions
3 km R-NCOM model demonstrates ability to forecast (96 hrs) 3-D mesoscale
circulation and internal wave environment of NSCS.
NSCS
Overall good agreement between R-NCOM NSCS model and un-assimilated insitu
observations.
Because observations are assimilated through NCODA MVOI process, directly or via
synthetic profiles, model is pushed towards obs at their locations. Slow moving gliders
will rapidly remove model bias in their vicinity.
Note: MVOI assimilation process can create temporary shocks to the system!
Assimilation
A
i il ti off synthetic
th ti profiles
fil derived
d i d from
f
SSHA and
d SST strongly
t
l iinfluence
fl
proper
location of mesoscale eddies. Model will diverge from reality without remotely sensed
observations (that span the domain) after several weeks.
Model over-predicts energy in diurnal and semi-diurnal bands, but is in reasonable
agreement with amplitude and phase of observed internal waves.
Future
F
t
virtual
i t l mooring
i studies
t di with
ith gliders
lid
should
h ld be
b 2
2-3
3 weeks
k att lleastt ffor proper
resolution of internal wave power spectrum.
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Slide 19
References
•
Barron, C., A.B. Kara, H. E. Hurlburt, C. Rowley, L. Smedstad, JAOT, 21, 1876-1893 (2004).
•
Chao, S-Y, D-S Ko, R-C Lien, P-T Shaw, Assessing the West Ridge of Luzon Strait as an Internal
Wave Mediator, J. Ocn, 63, 897-911 (2007).
•
Hu, J., H. Kawamura, H. hong, Y. Qi, A Review of the Currents in the South China Sea: Seasonal
Circulation, South China Sea Warm Current and Kuroshio Intrusion, J. Ocn, 56 , 607-624 (2000).
•
Jan, S., C-S Chern, J. Wang,
g S-Y Chao, Generation of diurnal K1 internal tide in the Luzon Strait
and its influence on the surface tide in the South China Sea, JGR, 112, C06019 (2007).
•
Jia, Y., Q. Liu, Eddy Shedding from the Kuroshio Bend at Luzon Strait, J. Ocn, 60, 1063-1069 (2004).
•
Qu T
Qu,
T., Upper-Layer
Upper Layer Circulation in the South China Sea
Sea, JPO
JPO, 30
30, pp
pp.1450
1450-1460
1460 (2000).
(2000)
•
Qu, T., Y.T. Song, T. Yamagata, An introduction to the South China Sea throughflow: Its dynamics,
variability, and application for the climate, Dyn. Atmos. and Ocns, 47, 3-14 (2009).
•
Wang, G.,
W
G Dake
D k Chen,
Ch
Jilan
Jil Su,
S Mesoscale
M
l eddies
ddi in
i the
th South
S th China
Chi Sea
S observed
b
d with
ith altimeter
lti t
data, Geo. Res. Lett, 30, 21, 2121 (2003).
Slide 20
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References
•
Wang, G., Dake Chen, Jilan Su, Generation and life cycle of the dipole in the South China Sea
summer circulation, JGR, 111, C06002 (2006).
•
Yang,
g, H.,, Q. Liu,, Forced Rossby
y wave in the northern South China Sea,, Deep-Sea
p
Res. 1,, 50,,
917-926 (2003).
Slide 21
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Backup Slides
Slide 22
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NSCS R-NCOM vs. Glider observations
Feb 8-18, 2009
Slide 23
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NSCS R-NCOM vs. Glider observations
Feb 8-18, 2009
Slide 24
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Slide 25
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