Comment on “Current system east of the Ryukyu Islands” by A. Nagano et al.

[1] In a recent paper, Nagano et al. [2007] (hereafter N2007) discussed the variation of current structure and volume transport of the Ryukyu Current System (RCS) east of the Ryukyu Islands using an inverse technique with hydrographic-section data collected from three cruises. Their paper appears to be important for understanding this western boundary current, since it is based on repeated hydrographic surveys in the region east of the Ryukyu Islands. But acquiring meaningful results from inverse calculations requires great care. Fiadeiro and Veronis [1982, p. 160] say ‘‘Because inverse analysis always supplies a solution, it appeared that the assumed reference level matters less in inverse theory than in the hydrographer’s [level-of-no-motion] approach. Actual computations have shown that is not true. A bad assumption leads to bad results. The ‘advantage’ of inverse theory is that one sees how bad the results are.’’ The inverse technique minimizes the velocity at the reference level, and that point is indeed raised in the final paragraph of N2007, which used the reference level of 100 dbar. To determine whether this shallow reference level is an appropriate condition or not, one should investigate the effect of choosing other reference levels, in particular deeper levels. In this note, we perform two inverse calculations to calculate the current east of the Ryukyu Islands from the same data as one of the N2007 cruises but using a reference level of 2000 dbar. The resulting RCS circulation field is, we believe, more realistic than that of N2007. [2] Three hydrographic surveys were carried out during May–June, September and October 2002 along three lines (AE, E, OS) east of the Ryukyu Islands (Figure 1). May– June and September cruises used XCTD/XBT casts along the E line, but the October cruise collected CTD casts along all three lines. Surprisingly, absolute geostrophic velocity sections, determined by N2007 with a 100-dbar reference level, revealed large values up to 30–40 cm s 1 at 1000 dbar, the deepest level shown (Figures 2, 3b and 4c in N2007). We have calculated full-depth absolute geostrophic velocity sections using the inverse results of N2007 and plotted them to 3000 dbar (Figures 2a, 2b, 3a, 3b, 4a and 4b). Figures 2b, 3b and 4b show strong deep currents with maximum speeds of 20–50 cm s 1 on each line. In each case, note the contour lines of velocity are nearly vertical and strong currents extend to the bottom. For example, a current velocity of 57 cm s 1 touches the bottom near 3100 dbar in the AE section. Is the computed strong deepcurrent field real, or is it an artifact of the shallow reference level? We address this question by carrying out the inverse calculation using a reference level of 2000 dbar instead of 100 dbar, and compare results using the two reference levels. We present results for the October cruise data only, because the XBT/XCTD casts reached only 1000 dbar; also the CTD measurements provide more accuracy in the inverse calculations.