A data-assimilated circulation model of the
Northwestern Atlantic Ocean using the new
mpi-version of the Princeton Ocean Model
(mpiPOM)
Fanghua Xu and Lie-Yauw Oey
Princeton University
OSM on Feb, 20, 2012
Goal:
Present mpiPOM: to analyze loop current shedding dynamics
Middle Atlantic Bight
Study Region
Xu & Oey, 2011: The Origin
of Along-Shelf Pressure
Gradient in the Middle
Atlantic Bight. J. Phys.
Oceanogr 41, 1720-1740
Highlights of Model Used in This Study
(http://www.aos.princeton.edu/WWWPUBLIC/PROFS/)
mpi-Princeton Ocean Model
(http://www.imedea.uib.es/users/toni/sbpom/)
Grids: horizontal: ~5km, vertical: 25 levels
CCMP (1988-2009) & NCEP/GFS wind (after 2009)
Data assimilation SSHA
A forecast experiment, 2011/07/01
Eddy shedding time
SSH
Black: model SSH=0; Pink: AVISO SSH=0
Forecast
2011/Jul/22
Observed
2011/Jul/22~29
A free model run, starting from
2011/05/16
Hindcast run
Free model run
Eddy shedding time
X Wind stress in Carib Sea
Yucatan transport
- Ci
03/26
1st order dynamics:
Qi < Ci = - βRo2, (Nof 1995 JPO)
can “peel” a portion of the Loop’s mass – i.e. a warm
eddy – westward.
Qi: Yucatan influx
Ci: the 1st mode baroclinic Rossby wave velocity
Ro = (g’h(t))1/2/f
05/16
07/08
There exists a close connection
between Yucatan transport and
winds in Caribbean and in GOM:
Chang & Oey, 2012, GRL, in press:
Why does the Loop Current tend
to shed more eddies in summer
and winter? Eda’s talk on
Thursday 14:30 session 30.
Chang & Oey, 2011, JPO
Free model run
Shedding
ssh (m)
Convergence divergence
Hindcast run
Loop Formation
Eddy shedding time
There is coupling with the deep layer; the coupling can be inferred from
basic mass conservation as indicated in the above sketch.
If Yucatan inflow and Rossby wave dynamics alone – it will take
roughly 2 months. i.e. mid-August – instead of Jul/08, for the Loop
to shed – the acceleration is through baroclinic instability & upper-lower coupling
BCI (m2 /s3) @ z=-750m
05/16 - 06/15
06/01 – 06/30
Growth rate 
1/40d ~ 1/10d
Oey, 2008, JPO
The Campeche Bank
 The
Loop Current eddy-shedding is primarily
due to an imbalance between YUC influx &
westward baroclinic Rossby wave, and
shedding is accelerated by baroclinic
instability and the upper-lower & east-west
coupling.
SSH
Latitude
ζ/f
Latitude
05/16
03/26
05/16
07/08
07/08
ζ/f : vorticity over the west Yucatan channel
Latitude: edge of Loop Currents
The decrease of ζ/f induces an anticyclonic circulation,
which retract the loop currents
Total transport in YUC, FL straits, and 90w
Comparison of model transports in middle
Atlantic with available observations
Comparison of 1-day average SSHA of hindcast
simulation with 1-day average of tide gauge data
1994
1994
2004
2004
Depth averaged currents
What drives the along-shelf circulation?
Momentum Balance:
x
  wind
gh

  bx  ru
x
o
Along-shelf pressure gradient (ASPG)
10-8~10-7 drives mean along-shelf flow
towards the southwest (Stommel and
Leetmaa,1972; Csanady, 1976; Lentz,
2008);
(Lentz, JPO, 2008)
Xu & Oey, JPO, 2011
Elevation
The origin of the mean along-shelf pressure
gradient (ASPG)
Distance from Cape Hatteras
ASPG×108 = RIV + (7×UA+1.3)×Hv(UA-UAcritical)
– WIND- GS
RIV=2; UAcritical=1; WIND=1; GS=6
Cross-shelf circulation
A schematic sketch of the cross-shelf section
circulation illustrating 3-layer structure from
mid- to outer shelves, and 2-layer from coast
to mid-shelf, as well as mid-shelf near-bottom
divergence, from Lentz [2008; his fig.2].
hcastT
Fig. Daily-averaged SSH (colors in m), surface currents (squiggly black lines) and wind stress
vectors (blue) shown for the 15th day of each month from Jan-Dec in 2010 for the hindcast
experiment with tides. Magenta contour=AVISO SSH=0 and white contour=200m isobath.
(A) SSHA
(B) (Ug,Vg)
Fig. 4 Comparison between AVISO and modeled (a) SSHA (anomaly) and (b) surface geostrophic velocity
(Ug,Vg). The “(a)” panels on left show top: spatial correlation coefficient between the model and AVISO SSHA’s
in the region north of 23oN and west of 84oW, and over water region deeper than 500 m in the Gulf of Mexico for
time period Jan-Dec/2010; and bottom: the corresponding RMS error for the same region. The “(b)” panels on
right compare the corresponding (Ug,Vg): top & middle are magnitude and angle of the complex correlation and
bottom is the RMS error. In each case, the 2010-mean’s are also shown. See Table1 for experiment names.
R=Correlation; S=95% Significance
Mean
R /S
R /S
(Sv) Jan-Dec Apr-Dec
Obs.
30.5
-
-
Exp.1
31.2
.32/.16
.52/.17
Exp.2
30.7
.34/.12
.57/.12
Exp.3
30.2
.39/.14
.71/.19
Exp.4
30.9
.38/.13
.60/.13
Exp.1= hcastNoT
Exp.2= hcastT
Exp.3= hcast.horcon0p2
Exp.4= hcastT.horcon0p2
Fig. 5 Comparison between modeled transport at the Straits of Florida off the eastern coast of Florida between
West Palm Beach, FL to Eight Mile Rock, Grand Bahama Island and cable-estimated observed transport
(http://www.aoml.noaa.gov/phod/floridacurrent/data_access.php) for the time period Jan-Dec/2010. Note that
model was not forced by wind from Jan/01 through Mar/15. The Table on left shows the 2010-mean’s for
observed and modeled transports, and their zero-lag correlations and 95% significances computed for the entire
2010 from Jan-Dec (3rd column in the Table) and for Apr-Dec only (4th column in the Table).
Comparison of hindcast simulation with drifters
Comparison of hindcast simulation with ADCP currents
Goal:
Present mpiPOM: to analyze loop current shedding dynamics
Xu and Oey, 2011,JPO
Middle Atlantic Bight
Data assimilation:
SST, ADCP, Drf, &T/S
IC
Grids
wind
mpiPOM
BC
Highlights of the mpi-version of the Princeton Ocean Model
tides
River
output
Highlights of the mpi-version of the Princeton Ocean Model
Princeton’s Circulation & Wave Model of the Northwestern Atlantic Ocean
(http://www.aos.princeton.edu/WWWPUBLIC/PROFS/)
Model engine: mpi-Princeton Ocean Model – 3D
currents, T/S, SSH, turbulence KE (TKE) & length w/wavebreaking and Stokes drift TKE production; waveenhanced BBL; 4th-order pressure grad scheme;
Grids: horizontal: ~5km, vertical: 25 levels;
Topography: Etopo2+NOS digitized map on shelves;
Open BC@55W: Transport and climatological T/S +
radiation & FRS [Oey & Chen, 1992a,b; JGR];
Surface fluxes:
CCMP wind (1987-2009); NCEP/GFS 2010-present;
supplemented w/hurricane winds from AOML’s HRD;
Heat & salt fluxes: monthly climatology or daily NCEP;
51 Rivers: daily, 34 in GOM, 17 east coast;
Data assimilation: SSHA, SST, Drifters, bred-ensemble;
Tides: M2, S2, K1 & O1;
Wind-waves, w/option to couple with currents;
Inundation
Nesting