−85 −80 −75 −70 −65 −60 −55
−70
−65
−60
−55
−50
−45
−40
Depth [m]
−8000
−7000
−6000
−5000
−4000
−3000
−2000
−1000
FES M
Finite Element Southern Ocean Model
Nov Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov Jan Mar
−0.08
−0.06
−0.04
−0.02 0 0.02 0.04 0.06
pressure [dBar]
OBP anomalies at 1.42° E/50.25° S
2003 2004 2005
| | |
Nov Jan Mar May Jul Sep Nov Jan Mar May Jul Sep Nov Jan Mar
−0.06
−0.04
−0.02 0 0.02 0.04 0.06 0.08
pressure [dBar]
OBP anomalies at 7.08° E/44.66° S
2003 2004 2005
| | |
model observation
model observation
−1.65 −1.6 −1.55 −1.5 −1.45 120
130 140 150 160 170 180
ssh [m]
transport [Sv]
South (66.75°W,68.25°S)
−1.3 −1.2 −1.1 −1 −0.9
120 130 140 150 160 170 180
ssh [m]
transport [Sv]
Centre (66.75°W,60.75°S)
−0.1 −0.05 0 0.05 0.1
120 130 140 150 160 170 180
ssh [m]
transport [Sv]
North (66.75°W,54.75°S)
−1.6 −1.55 −1.5 −1.45 −1.4 120
130 140 150 160 170 180
ssh [m]
transport [Sv]
Difference South−North
−150 −100 −50 0 50 100 150
−80
−60
−40
−20 0 20 40 60 80
−8000
−7000
−6000
−5000
−4000
−3000
−2000
−1000
Introduction
For the validation of the GRACE mission that aims at a more accurate determination of the geoid an investigation of ocean bottom pressure (OBP) anomalies and circulation in the South Atlantic is performed.
For this purpose we use bottom pressure data recorded by Pressure Inverted Echo Sounders (PIES), radar altimeter and gravity data pro- vided by satellite geodesy and utilize the 3D finite element global cou- pled sea ice−ocean model (FESOM) which has recently been developed at the AWI.
Validation OBP: Model vs. PIES
Comparison of simulated and observed monthly mean ocean bottom pressure time series at locations on the northern slope of the Atlantic- Indian Ridge (top) and in the Cape Basin (bottom) shows that patterns of variability are well captured in the ACC region, although amplitudes are underestimated. Further north, in the region of influence of the Agulhas Current, deviation becomes more expressed due to eddies not captured by the model at 1.5˚resolution. This calls for a local refine- ment of the computional grid, which can be easily accomplished due to the finite element approach.
Concerning the GRACE mission similar comparisons are performed with GRACE and PIES data (see Poster A. Macrander, Session OS12/
G2).
Conclusions
• Vertically integrated velocity anomalies in the Southern Ocean are strongly correlated with OBP anomalies (not with SSH).
• OBP at either coast of Drake Passage seems to be a good measure of ACC transport (better than SSH gradient).
• The model underestimates OBP variability.
A global finite element ocean model:
Circulation and bottom pressure anomalies in the South Atlantic
Carmen Böning, Ralph Timmermann, Sergey Danilov, Jens Schröter, and Olaf Boebel
Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (cboening@awi-bremerhaven.de)
Characteristic Quantities for Drake Passage Transport
OBP/Barotropic Velocity Anomalies
180oW 120oW 60oW 0o 60oE 120oE 180oW 80oS
60oS 40oS 20oS 0o
Pressure [dBar]
−0.025 −0.02 −0.015 −0.01 −0.005 0 0.005 0.01 0.015 0.02 0.025
Barotropic Velocity, OBP, and SSH Anomalies
Scatter plots of ACC transport through Drake Passage vs. ocean bottom pres- sure (top) and sea surface height (bottom) indicate that OBP records at the coasts of Drake Passage give a better measure of ACC transport than SSH gra- dients. This complements findings of Hughes et al. (1999) who demonstrated a close correlation to OBP records only on the southern coast.
Outlook
Model results (shown are typical examples of monthly means) indicate that anomalies of vertically integrated velocities are correlated to bottom pressure anomalies much stronger than to SSH anomalies. Correlation between SSH anomalies and velocities is strongest at the 100 m level, i.e.
immediately beneath the Ekman layer.
427 427.05 427.1 427.15 120
130 140 150 160 170 180
pressure [dBar]
transport [Sv]
South (66.75°W,68.25°S)
3553.253553.3753553.5 120
130 140 150 160 170 180
pressure [dBar]
transport [Sv]
Centre (66.75°W,60.75°S)
1008.9 1009 1009.1 1009.2 120
130 140 150 160 170 180
pressure [dBar]
transport [Sv]
North (66.75°W,54.75°S)
−582.1 −582 −581.9 −581.8 120
130 140 150 160 170 180
pressure [dBar]
transport [Sv]
Difference South−North
Reference
Hughes, C. W., M. P. Meredith and K. Heywood, 1999: Wind-Driven Transport Fluctuations through Drake Passage: A Southern Mode.J. Phys. Oceanogr.,29, 1971- 1992
SSH/Velocity at 100m Anomalies
180oW 120oW 60oW 0o 60oE 120oE 180oW 80oS
60oS 40oS 20oS 0o
Height [m]
−0.04 −0.03 −0.02 −0.01 0 0.01 0.02 0.03 0.04
SSH/Barotropic Velocity Anomalies
180oW 120oW 60oW 0o 60oE 120oE 180oW 80oS
60oS 40oS 20oS 0o
Height [m]
−0.04 −0.03 −0.02 −0.01 0 0.01 0.02 0.03 0.04
In order to consider mesoscale processes the model will be run on a grid with a res- olution of 1/4˚ in the South Atlantic in the nearest future.
Model results will be compared to PIES and SSH data and used to validate GRACE gravity anomaly measurements.
Depth [m]
