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Velocity

Near surface temperature and velocity structure

The ARC was crossed on four occasions during the cruise. During the southern part of leg a, the first crossing of the ARC occurred near 39°S 23°E, with the current setting in a northeastward direction. The second crossing occurred near 30°E of leg b along the 40°S parallel. The current flowed southeastward into the first crest leeward of the Agulhas Plateau. Slightly farther east, near 31.5°E, the section crosses the current returning out of the crest and setting northeastward. The final crossing of the current occured during the subsequent northbound leg c. Directly adjacent to the last crossing, the return flow from crest C1 swings towards the north, crossing this section obliquely at an angle of 40°.

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ADCP sections of legs a, b and c (top to bottom), combined with XBT sections for leg a and c. ADCP velocities are assigned to the center depth for each bin, i.e. to 350m for the 325-375m bin. Please note that the x-axes are scaled to be approximately equidistant for the three subplots.

After adjustment for oblique current crossings, the four sections suggest surface speeds decreasing from greater than 2m/s to above 0.8 m/s while progressing downward, while the currents width at half maximum (HMW) increases slightly from around 60km to around 75km. Velocities in the current core at the bottom of the ADCP abservations (350m depth) decrease from above 1.2m/s to less than 0.8m/s for the section farthest east.

The deep velocity structure

An average deep velocity can be derived directly from the RAFOS float velocity information, provided it is possible to determine which floats are trapped in the current. Additionally, if a float's lateral position relative to the current's center is known, one can even extract an averaged velocity profile. Following the method developed by Hendry (1988) and Bower and Rossby (1989) for the Gulf Stream, we map the floats' lateral position in the curren tbased on a model of the ARC thermal structure. A double tangent hyperbolic function was fitted to the XBT section of the first ARC crossing, which produced a generic temperature field. Daily float pressure and temperature measurements were then used to map the corresponding float cross-current position.

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XBT-based temperature section (white lines and lables) superposed on model temperature field (black lines and labels with background shading). The figure enlarges the ARC segment of the temperature section a.

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Float location within ARC, mapped onto model temperature field. Isopycnal floats (black dots) follow closely the isothermal structure (which is assumed to be close to the isopycnal structure). The two groups of floats ballasted for s_q=26.8 and 27.2, respectively, can clearly be distinguished. Isobaric floats (white dots) follow the isopycnals to a lesser extent.

To estimate the ARC transport, float and ADCP velocities were combined into a single generic velocity section. The mapped float data were binned and averaged in 50 m (vertical) by 5 km (lateral) boxes. Aligning the maximum ADCP velocity with the bin of maximum (averaged) float velocities in the horizontal, a velocity map was generated which subsequently was objectively mapped. The ADCP section used herein is the first ARC crossing during leg a, upstream of the Agulhas Plateau. This section most likely represents an upper limit for ARC velocities, but is favorable over the other section due to its near perpendicularity to the ARC, hence giving a good estimate of the current's width. Integration of mapped velocities for a region ± 40km from the velocity maximum and for a mapped speed > 0 cm s^-1 (negative speeds are a relict of the objective mapping procedure) result in a transport of 61 Sv for the upper 1350m. The stability of this estimate was tested using various vertical (200 m – 1000 m) and lateral (7.5 km to 40 km) correlation scales in the objective mapping.

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Objectively mapped lateral section of ARC velocities, based on ADCP and RAFOS float data.