  Global atmospheric reanalyses have become a common tool for both the
validation of climate models and diagnostic studies, such as assessing climate
variability and long-term trends. Presently, the 20th Century Reanalysis
(20CR), which assimilates only surface pressure reports, sea-ice, and sea
surface temperature distributions, represents the longest global reanalysis
dataset available covering the period from 1871 to the present. Currently, the
20CR dataset is extensively used for the assessment of climate variability and
trends. Here, we compare the variability and long-term trends in Northeast
Atlantic storminess derived from 20CR and from observations. A well established
storm index derived from pressure observations over a relatively densely
monitored marine area is used. It is found that both, variability and long-term
trends derived from 20CR and from observations, are inconsistent. In
particular, both time series show opposing trends during the first half of the
20th century. Only for the more recent periods both storm indices share a
similar behavior. While the variability and long-term trend derived from the
observations are supported by a number of independent data and analyses, the
behavior shown by 20CR is quite different, indicating substantial
inhomogeneities in the reanalysis most likely caused by the increasing number
of observations assimilated into 20CR over time. The latter makes 20CR likely
unsuitable for the identification of trends in storminess in the earlier part
of the record at least over the Northeast Atlantic. Our results imply and
reconfirm previous findings that care is needed in general, when global
reanalyses are used to assess long-term changes.
