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3. Uniform discontinuities - still there

Next, we should check to see if there are remaining "uniform discontinuities" in the data. Another benefit of IUK is that the "residual" field f it estimates (basically thedeviation between the measured T and one predicted from the estimated state of large-scale variations) is advantageous in identifying sudden shifts at a station.

A subjective investigation of f revealed evidentdiscontinuities at about eight stations. Two examples are shown in Figs. 4 and 5. Each of the evident discontinuities occurred at stations with tidal shifts (coinciding to within a month or two of that shift) or at "unidentifiable" stations, which supports the working assumption (1) that most instrument changes will have a detectable tidal component.

Fig. 4. Time series of IUK temperature residual at Yap Island. Dashed line shows time of tidal shift that had been previously estimated and removed prior to IUK analysis. This provides evidence of shift not only in tide but mean temperature at the same time. This shift corresponds with reported change in instrumentation (Gaffen et al., submitted).

Fig. 5. Time series of residual at Douala in Cameroon. This "unidentifiable" station has too few nighttime data to identify tidal shifts, but clearly shows an instrument change as did two other stations out of the 30 in this category.

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