Are There Real Interdecadal Variations in Marine Low Clouds?

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Abstract
The dominant interdecadal signal in normalized frequency of occurrence of cumulonimbus reported by volunteer observing ships is globally uniform over the period 1952–92 over all ocean areas between 40°S and 50°N. Globally uniform signals in both normalized frequency of occurrence and amount-when-present also dominate interdecadal variations for other low cloud types. This pattern is inconsistent with plausible physical mechanisms and is apparently due to slow changes in observational practice. Eight ocean weather ships with approximately fixed positions also reported gradual changes in low cloud occurrence frequencies between 1952 and 1969 that were similar in pattern for all eight ships, but for most cloud types these variation patterns differed markedly from those at nearly collocated volunteer observing ships. These apparently spurious variations make it difficult to identify real interdecadal variations in marine clouds from ship observations. However, over the tropical Indian Ocean and central and eastern Pacific Ocean, small but widespread decreases in stratocumulus frequency and increases in deep convective cloud frequency between 1955 and 1978, and 1979 and 1991 tend to be consistently related to changes in sea surface temperature and are likely to be real. Over the western Pacific Ocean, ship reports indicate an increase in the frequency of deep convective clouds between these two periods that is not consistently related to SST changes and is less likely to be real.

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