Comment on “Circulation in the South China Sea during summer 2000 as obtained from observations and a generalized topography‐following ocean model” by H. Wang et al.

Abstract

[1] In a recent paper, Wang et al. [2004] discuss the summer circulation in the South China Sea (SCS) using observations and a generalized topography-following ocean model. Their paper appears to be important since it is based on the only intensive observation with basin-scale coverage of the SCS in late summer. However, their results from underway observations of acoustic Doppler current profiler (ADCP) are not properly processed and may lead to misunderstanding of summer circulations of the SCS. As principal investigators of the research project of the 2000 summer cruise, we would like to present our corrections to the ADCP measurements, which result in more convincing circulation fields, and to call for attentions of properly calibration while using shipboard ADCP measurements. [2] During the cruise, along-track current measurements were conducted using an RDI 150 kHz narrow band vesselmounted acoustic Doppler current profiler (ADCP). Navigation information was provided to an RDI DAS PC by the ship’s gyrocompass and a shipboard commercial GPS for in situ data acquisition. [3] The ADCP velocity vectors at depths of both 100 and 200 m presented in Figure 5 of Wang et al. [2004] look suspicious. If comparing them with ship tracks shown in Figure 1, one can easily see that almost all velocity vectors plotted are pointing toward the port side of the research vessel. This phenomenon suggests that there had been a misalignment of the transducer with respect to the ship’s centerline that caused wrong projection of the ship’s velocity on the ADCP and led to serious systematic error in the measured currents. Therefore additional calibration after the cruise is necessary [Joyce, 1989; Pollard and Read, 1989]. It is doubtful that such calibration had been included in Figure 5 of Wang et al. [2004]. [4] There are basically two types of errors in shipboard ADCP measurements: an alignment error in the Doppler transducer/ship gyro relative to the ‘‘true’’ heading (the misalignment angle a) and a sensitivity error of the acoustic system factor (the scaling factor 1 + b), where b is relatively small but a could cause severe problems to the measurements [Joyce, 1989; Pollard and Read, 1989]. As the major error source, an error in transducer alignment and ship gyrocompass (a) will produce an apparent athwartship velocity [Joyce et al., 1986] and normally cause a systematical ‘‘offset’’ error sideward of the ship track. [5] On the basis of physics given by Joyce [1989], Pollard and Read [1989] and Joyce et al. [1986], we calibrated the same ADCP data set used by Wang et al. [2004] accordingly. Using all available bottom tracking ADCP measurements from shallow sectors of the cruise, we first estimated these calibration coefficients with bottom tracking method developed by Joyce [1989], which yields a misalignment angle of 2.34 and a scaling factor of 1.0064. Since R/V Xiangyanghong 14 was steaming at 10 knots generally, measurement error raised by the misalignment is 0.20 m s 1 roughly in the cross-track direction, which is much greater than that caused by the scaling factor (roughly 0.03 m s ). [6] Calibrations were also applied for water track mode, because that bottom tracking was not available most of the time for the cruise. In the following analysis, we assume that drifting errors derived from the gyrocompass are random in the 40-day-long cruise; hence that misalignment are basically induced by installation alignment. [7] Following Joyce [1989], the water velocities in the ‘‘true’’ north and east (x, y) coordinate frame are