CRUISE PLAN

LADCP experiment

Since this experiment relies heavily on direct velocity measurements, we think it necessary to detail both these issues explicitly and outline how we plan to deal with them. Few experiments have been designed for LADCP sampling, so this program provides a unique opportunity to optimise data collection techniques specifically for direct velocity profiling. The LADCP is ideally suited to work in fast, narrow currents over steep topography.

INSTRUMENT/PROCESSING

The LADCP provides a full-depth profile of ocean current from a self-contained ADCP mounted on the CTD rosette. During processing, overlapping profiles of vertical shear of horizontal velocity are averaged and gridded, to form a full-depth shear profile. The shear profile is integrated vertically to obtain the baroclinic velocity and the resulting unknown integration constant is the depth-averaged or barotropic velocity. This barotropic component is then computed as the sum of the time-averaged, measured velocity and the ship drift (minus a small correction, less than 1 cm/sec, to account for a nonconstant fall rate) (Fischer and Visbeck, 1993; Firing, 1998). Errors in the baroclinic profile accumulate as 1/sqrt(N) where N is the number of samples (Firing and Gordon, 1990). This error translates to the lowest baroclinic mode and, for a cast of 2500 m depth, it is about 2.4 cm/sec (Beal and Bryden, 1999). The barotropic component is inherently more accurate, because the errors result from navigational inaccuracies alone. These are quite small with P-code GPS, about 1 cm/sec (2 to 4 cm/sec without). Comparisons with Pegasus suggest that the LADCP can measure the depth-averaged velocity to within 1 \cms (Hacker et al., 1996). The rms difference between Pegasus and LADCP absolute profiles are within the expected oceanic variability, 3-5 cm/sec (Send, 1994), due primarily to high frequency internal waves.

Because our measurements will be taken over steep topography where bottom velocities are high, we will use the new bottom track and inverse solutions method of Visbeck (2000) as a post-processing step in the production of final LADCP data. This method has the potential to further improve absolute velocities by constraining the shear profile, using the least squares method, to bottom tracked velocities, ships navigation and shipboard ADCP data.

In previous experiments the interference layer, which results from the previous ping reflecting off the bottom, has caused a large data gap in the LADCP profile, causing an uncertain velocity offset (several cm/sec) between the parts of the profile on either side of the gap. For this experiment bottom velocities will be greatly improved by using Chereskin's upgraded instrument which will ping asynchronously, thereby avoiding complete data loss in the interference layer. A second problem with data loss arises at the bottom of a CTD/LADCP cast, when the package is held 10 m above the sea bed for bottle sampling. At this distance the instrument is 'blind' since the blank after transmit is order 20 m, and a time gap in the data stream will result in an uncertainty in the absolute velocity. We propose to finish a cast at an optimum 80m above the sea bed, avoiding data loss altogether while maintaining a good return from the water column for profiling down to the sea bed.

AGEOSTROPHIC MOTION

Ageostrophic motions captured in LADCP measurements may represent large errors when trying to discern a mean circulation, and ordinarily remain unaccounted for. In this instance, we have estimated the magnitude of such motions using results from the ACE current time series at 32°. These energies translate to ageostrophic motions of amplitude up to 4 cm/sec close to the surface, assuming a pure sinusoidal wave, and of 3 cm/sec or less at the depths of the AUC. Since average velocities are over 10 cm/sec in the Undercurrent, and instantaneous velocities over 30 cm/sec, these oscillations will certainly not overcome the signal. In fact, they are of the order of the measurement accuracy. Low frequency modifications of the Agulhas Current, caused by meandering of the jet, are far more energetic than these inertial and tidal oscillations. They correspond to changes in velocity of 35 cm/sec (less than 5 cm/sec in the AUC), assuming sinusoidal variability. This variability will affect hydrographic and direct velocity measurements alike. The barotropic component of the tides will be estimated and removed from LADCP velocities using the Oregon State University global model of ocean tides (Egbert, 1994). The model output has been found to match closely with the barotropic component of velocity from LADCP (Firing, 1998) in regions of large tidal amplitude.

SHIPBOARD ADCP COMPLEMENT

Shipboard ADCP data can complement LADCP measurements in a number of ways, helping to reduce errors. First, it affords continuous resolution in the horizontal, where LADCP is limited by station spacing. Second, velocities at the surface of the ocean down to depths of 50 m are not measured with the lowering technique, due to the instrument's blank after transmit and this gap can be (partially) filled with ADCP data (Beal and Bryden, 1999). Third, comparisons of ADCP and LADCP over their common depth provides a good quality check for the lowered data and allows for quantification of some errors. Furthermore, ADCP velocities are essential for estimates of Ekman layer fluxes (Chereskin et al., 1997), which remain unresolved by either LADCP or geostrophy. Because ocean current is a small residual between the rotated ADCP measurement and the ship's velocity, the calculation is especially sensitive to errors in heading and transducer alignment. Heading bias errors, formerly the most limiting error in SADCP absolute velocity estimates, have by and large been eliminated by the use of GPS attitude measurements (King and Cooper, 1993; Griffiths,1994;Chereskin and Harris, 1997). The predicted error in ship-relative currents is less than 1 cm/sec for a 10-min average (Chereskin and Harding, 1993). Use of P-code GPS during WOCE resulted in an absolute velocity accuracy of 2 cm/sec for 10-min averages (Chereskin and Harris, 1997).

DIRECT VERSUS GEOSTROPHIC VELOCITIES

In regions with steep topography and narrow currents, direct measurements of velocity have an advantage over geostrophic velocities (Beal et al., 2000). The geostrophic method can significantly under-estimate the peak velocities and total transport of a slope current. This is due to the missing horizontal density gradient below the deepest common level of each station pair. This data gap is referred to as a 'bottom triangle' and for bottom-trapped currents, such as the AUC, it represents a significant portion of the flow. Another disadvantage of the geostrophic method in this case, is that it requires integration along pressure horizons. This tends to smear out and diminish the signature of a bottom-trapped current, which has largest velocities being observed successively deeper on each station. In the case of the Somali Deep Western Boundary Current, Beal et al. (2000) found that these two sampling effects meant that only 0.5 Sv of a directly estimated 5 Sv transport was captured by geostrophy. Thus, while the strength of the geostrophic method lies in capturing the net current over a region between station pairs, the LADCP can better resolve individual features of width not much greater than station-spacing, especially over topography.