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Excerpt From Reply version 1 (lines 63-96) on purported GMT bug:

R10 also offer an explanation for the incorrect processing of the GKSS field in the M07 paper by claiming that a "bug" exists in the Generic Mapping Tools (GMT) software (Wessel and Smith 1991). This purported bug produces erroneous fields when the GMT surface function, which fits a continuous curved surface to randomly-spaced data, is employed using its default tension setting. If this observation is correct, it would be a valuable piece of information for a wide community of GMT users. Nevertheless, we cannot confirm any signs of such a bug in our own experiments with the GMT surface function and the peculiar nature of the error affecting the M07 processed GKSS field — namely the selective smoothing of a single hemisphere — makes the claim by R10 seem untenable. In fact, our own experiments provide a simpler and more plausible explanation.

We illustrate our findings using the GKSS annual surface temperature mean from 1880–1990 C.E., in keeping with S10. Figure 1a shows this field averaged by S10 onto a 5° spatial grid, but still in its native longitude range (0°–360°). For simple illustration purposes, we apply the GMT surface function to the field shown in Figure 1a using a default tension setting (tension = 0; this setting can range from 0 to 1), which yields the correctly gridded version of the field shown in Figure 1c with the longitudinal range changed to -180°–180° (note that we do not endorse the use of the surface function for the purpose of regridding fields in general, but we consider it here because it is at the heart of the M07 regridding procedure). This result was accomplished using a flag -fg in the call of the surface function to ensure that the spatial grid was interpreted as geographic coordinates and not as regular numbers. If the latter interpretation is made due to the absence of the -fg flag, however, the surface function will regard the input points with longitudes <0° as unavailable because the input data range from 0° to 360°. Consequently, the Western Hemisphere (WH) will be interpolated with a continuous curved surface anchored only by the points on its eastern boundary. The resulting field is shown in Figure 1d and has a striking resemblance to the M07 product shown in Figure 1b (Figure 2 replots Figures 1b and 1d over the range 0°-360° and clearly illustrates, in both cases, the effect of the anchoring of the WH on its eastern boundary and the discontinuity of the boundary at 180°). Furthermore, we find no evidence to support the dependence of these results on the tension setting of the surface function. In Figures 1e and 1f, we plot correct and incorrect results for a prescribed tension setting (0.5) that do not differ from their respective counterparts using the default tension in any substantial way. These findings thus suggest a misuse of the GMT surface function by M07 as the origin of the errors in the regridded GKSS field, rather than the existence of a hypothetical bug that only occurs at the default tension setting and only affects one hemisphere.

Excerpt from Reply version 5 (lines 49-64) on Niño3 validation statistics:

Regarding the M07 Niño3 assessment statistics, R12 point to two papers in review (Emile-Geay et al. 2012a,b) that seek to reconstruct the Niño3 index by applying RegEM-TTLS to an expanded data set tailored for tropical Pacific sea surface temperature reconstructions. These papers only reconstruct the Niño3 index; they do not perform a hemispheric or global CFR. Testing the performance of RegEM-TTLS for global CFRs was the motivation of M07, who used reconstruction skill from the Niño3 region as a spatial validation measure. Mann et al. (2009a) and Mann et al. (2009b) subsequently used RegEM-TTLS to derive real-world global CFRs, from which Niño3 indices were derived and used to infer ocean-atmosphere dynamics or to make quantitative calculations of Atlantic hurricane counts over the last millennium. More recent efforts to reconstruct the Niño3 index exclusively, without reconstructing the entire global field, are therefore not relevant to the way in which the Niño3 index was used in M07. Despite these distinctions and the importance of the Niño3 validation statistics in previous papers, no subsequent publications, including the present R12 comment, have corrected the erroneous statistics from M07. One consequence of this omission was a confusing disparity between the Niño3 reconstruction skill in the M07 CCSM1.4 and ECHO-G experiments prior to the publication of S10.