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2. Irregular sampling treatment

Changes in sampling rate, or accidental interactions between sampling availability and natural variations, can also produce spurious trends at stations that do not report frequently and consistently.

The "iterative universal kriging" (IUK) method has recently been introduced (Sherwood, 2001) for coping with this situation and obtaining unbiased estimates of trends given the available data. This method was used on the adjusted data.

The usefulness of this method is apparent from Fig. 3, which compares the trends found by standard regression at each station against those obtained from IUK.

Fig. 3. Map of 50 hPa trend, standard (top) and IUK(bottom). Note the greater agreement among different stations in the deep tropics with IUK. Model simulated temperatures there vary by less than a degree zonally (Steve Pawson, pers. comm.) Most of the remaining anomalous tides in the lower panel are probably due to instrument problems not expressed in the tides.

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