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    Short note: time series of Drake Passage Oscillation Index (DPOI) from 1952 to 1988

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    M. Naganobu and K. Kutsuwada (Japan)
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    Naganobu et al. (1999) had assessed variability in krill recruitment and density with hypothesized environmental factors; strength of westerly winds (westerlies) determined from sea-level pressure differences across the Drake Passage, sea ice cover and chlorophyll-a in the Antarctic Peninsula area during 1982-1998. They found significant correlations between krill recruitment and those factors. The westerlies were especially regarded as a key environmental index. Fluctuations in the westerlies across the Drake Passage were referred to as the Drake Passage Oscillation Index (DPOI).
    We planned to extend time series of DPOI using historical data. We searched the historical data and found the time series since the 1950s until 1988 at the Web sites of the Carbon Dioxide Information Analysis Centre for Rio Gallegos and since the 1940s until 1998 at British Antarctic Survey for Esperanza. Here we calculated a time series of DPOI from 1952 to 1988 at this stage. Time series since 1988 will be soon calculated after obtaining appropriate data.
    The total number of monthly data used from 1952 to 1988 was 420. The mean, median, and mode were 13.6, 13.8, and 14.0 hPa, respectively. The maximum, minimum, and range were 27.5, -6.4, and 33.9 hPa, respectively. The standard deviation was 6.2. Linear regression declined as a whole from 1952 to 1988. A time series of 3-month running mean suggested considerable seasonal variability of climate. A time series of 12-month running mean indicated various yearly changes without seasonal variability. High DPOI periods, not less than 16 hPa, were mostly observed in the period before 1964 and only in 1973 and 1986-88 after 1964. Low DPOI periods, less than 14 hPa, were continued longer after 1964. A low DPOI less than 10 hPa appeared in 1967 and 1980. Intervals between the years of low DPOI were generally observed for around 3 years except 6 years between 1958 and 1964.