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Natural Climate Variability on Decade-to-Century Time Scales
box if the existing anomaly was missing or differed from it by more than 2.25°C. This criterion was chosen empirically following careful tests on monthly 5° latitude × longitude fields taken from a range of years since 1860 and covering the entire annual cycle (Colman, 1992).
Next, for each box, the average anomaly for the previous and the subsequent months was calculated, if both were available. This average was then substituted in the box if the existing anomaly was still missing or differed from it by more than 2.25°C.
Processes b and c were carried out three times altogether. The substitution of missing data greatly augmented the global coverage in data-sparse years while maintaining spatial coherence (compare Figures 1a and 1b in the color well). The effects in recent years were greatest along the boundaries between well-sampled areas and major data voids, e.g., in the Southern Ocean.
4. Further Enhancement
Where data were still missing for a 5° latitude X longitude box, a search was made up to 5 months backward and forward to find the nearest anomalies. If both anomalies were available for months - 1 and + 1, their average was substituted for the missing value. Otherwise, any available anomaly an observed n months before (n negative) or after (n positive) the target month was multiplied by a reduction factor 0.6|n|. The search was continued with increasing |n| until the sum of the reduction factors used (Sdn0.6|n| where dn = 0 for missing data, 1 for available data) reached 0.6; note that both anomalies were used when available from equidistant months. The average of the reduced or "muted" anomalies (p-1Sandn0.6|n|), where p is the number of anomalies used, was substituted for the missing value. The empirically chosen reduction factors are consistent with the global annual average of the monthly lag correlations presented in Bottomley et al. (1990), but no geographical or seasonal variation has been allowed.
A further spatial quality control was carried out. This was designed to reduce any grid-scale incoherence introduced by (a) above, especially where anomalies were rapidly changing in time or were much larger than the newly introduced muted anomalies. The procedure corresponded to item (b) in the section on Filling and Quality Control, but as few as two neighboring anomalies were used, and no missing boxes were substituted. If only a single neighboring anomaly was available, the mean of it and the anomaly being checked was used in the same way. Isolated 5° anomalies exceeding ± 2.25°C were reduced to ± 2.25°C.
The step-by-step effects of the "filling" and enhancement stages on sparse data can be seen by comparing Figures 1a, 1b, and 1c for January 1878. For recent years with far more data, the effects were much smaller.
5. Conversion from 5° to 1° Resolution and to Absolute SST Values
This step was an essential preparation for the incorporation of sea-ice fields as well as for the Laplacian interpolation (see below), in which it was necessary to preserve climatological gradients of SST. The 5° resolution monthly anomalies output after the "further enhancement" described in the previous subsection were added to the Bottomley et al. (1990) globally complete 1° resolution monthly climatological SST for 1951 to 1980. This climatology was assigned to 1° boxes in 5° areas without anomalies.
6. Sea Ice
The sources of sea-ice data are listed in Table 1. The NOAA analyses from 1973 onward are largely satellite based (Ropelewski, 1990). Note that published manuscript climatologies were used for earlier times, so that the same calendar-monthly ice cover was used in successive years, as opposed to the use of observed, interannually changing ice cover for more recent times, i.e., 1953 onward for the Arctic and
TABLE 1 Sources of Sea-ice Data
Up to 1943
German 1919-1943 climatology (Deutsches Hydrographisches Institute, 1950)
Interpolation to recent climatology (1953-1982)
Observed data provided by J. Walsh (Walsh, 1978)
Observed data provided by NOAA (Walsh, 1991)
Up to 1939
German 1929-1939 climatology (Deutsches Hydrographisches Institute, 1950)