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