greta leber rosenstiel school of marine and atmospheric science student seminar presentation:...
TRANSCRIPT
HYDROGRAPHIC SURVEY OF A SOLITARY
MEANDER IN THE AGULHAS CURRENT
Greta Leber
Rosenstiel School of Marine and Atmospheric Science
Student Seminar Presentation: December 1, 2010
Advisor: Dr. Lisa Beal
BACKGROUND: The Agulhas System
Image Courtesy Dr. Lisa Beal
BACKGROUND: Description of the ACT Project
Average Vertical Spacing: 2mAverage Horizontal Spacing: 23km
Lowered Acoustic Doppler Current Profiler (LADCP) with Conductivity-
Temperature-Depth (CTD)
Line 1: LADCP + CTD During Deployment: Less
Synoptic: April 7-15
Steamed back for a continuous underway
shipboard ADCP section
Line 2: LADCP + CTD More Synoptic: April 17-
19
Image Courtesy Dr. Lisa Beal
BACKGROUND: Non-meander Phase
Images Courtesy Dr. Lisa Beal
Agulhas Undercurrent Experiment East London line
February-March 2003
BACKGROUND: Solitary Meander Phase
Solitary Meander Characteristics (de Ruijter et al, 1999):
• Downstream velocity ranges from 5 (North) to 20 (South) km day-1, • Diameter ranges from 30-200 km•Period between 50 and 150 days.
Marine Remote Sensing Unit; http://www.afro-sea.org.za/mrsu/flags.php
BACKGROUND: Solitary Meander during ACT
April 7
April 12
April 15
April 17
April 18
April 19
Line 2
Line 1
Diameter: ~100km Velocity: ~15 km day-1 downstream
SSALTO/ Data Unification and Altimeter Combination System (DUACS) Real-Time AVISO MADT Product
ANALYSIS: Direct Velocities (LADCP)Line 1 Line 2
ANALYSIS: Cumulative Transport Across Lines
Line 1 Line 2
Northeastward (Undercurrent) 68.3 19.2
Southwestward (Agulhas) 128.2 129.1
Total (Southwestward) 59.9 109.8
ANALYSIS: Calculating Geostrophic FlowNeed a ZVS… commonly:But, we know that the ZVS is V-shaped, so use ZVS from LADCP data:
Still no Northeastward
(Barotropic Feature)!
ANALYSIS: Calculating Geostrophic Flow SOLUTION:
Demean the Geostrophic Velocity and add Geostrophic Barotropic component from LADCP.
ANALYSIS: Ageostrophic FlowLine 1 Line 2
SUMMARY AND FUTURE WORK
During the passage of a Solitary Meander, the northward flow strengthens and extends to the surface as a barotropic feature.
Geostrophic velocity is highly sensitive to the choice of a ZVS, with comparison to LADCP barotropic component giving the best results.
Ageostrophic flows in the Agulhas are concentrated in the surface layer.
Next Steps (with the hydrographic data): Investigate structure of ageostrophic flow (Cylostrophic?
Ekman? Data artifact?). Continue evaluating meander properties (vorticity, mixing,
etc.).
ACKNOWLEDGEMENTS
My advisor, Dr. Lisa Beal. The crew of the R.V. Knorr Funded by NSF
THANK YOU!
BACKGROUND: Description of the ACT Project
Images Courtesy Dr. Lisa Beal
MOTIVATION: Why Study the Agulhas?
Largest WBC in the world (Bryden and Beal,
2005) Large heat and salt transports.
Important inter-ocean link in the Thermohaline Circulation as the so-called ‘warm-water
route’ first described by Gordon, 1986. Upstream dynamics have
been linked to frequency of ring-shedding events (Lutjeharms and van Ballegooyen, 1988; van Leeuwen et al, 2000).
Zahn, 2009
SSALTO/ Data Unification and Altimeter Combination System (DUACS) Real-Time AVISO MADT Product