[Paper Review] Inconsistencies between long-term trends in storminess derived from the 20CR reanalysis and observations

Oliver Krueger, Schenk, Frederik+2 more|Helmholtz-Zentrum Geesthacht Zentrum für Materialforschung und kustenforschung (The Helmholtz Center Geesthacht)|May 23, 2012

Oceanographic and Atmospheric Processes4 citations

TL;DR

This study compares long-term storminess trends in the Northeast Atlantic derived from the 20CR reanalysis and observational data using a well-established pressure-based storm index. It finds significant inconsistencies, particularly opposing trends in the first half of the 20th century, indicating that 20CR likely contains substantial inhomogeneities due to increasing observational assimilation over time, limiting its reliability for trend analysis in earlier periods.

ABSTRACT

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.

Motivation & Objective

• To assess the reliability of the 20CR reanalysis for detecting long-term trends in storminess over the Northeast Atlantic.
• To compare observed storminess trends derived from surface pressure observations with those from the 20CR reanalysis.
• To identify potential sources of inconsistency, particularly inhomogeneities linked to changes in observational coverage over time.
• To evaluate whether 20CR can be trusted for climate trend studies in the early 20th century.
• To reinforce caution in using global reanalyses for long-term climate variability and trend assessments.

Proposed method

• The study uses a well-established storm index based on surface pressure observations over a densely monitored marine region in the Northeast Atlantic.
• It compares this observational storm index with the storm index derived from the 20CR reanalysis dataset spanning 1871 to present.
• The 20CR reanalysis assimilates only surface pressure reports, sea-ice, and sea surface temperature data, making it unique for long-term global coverage.
• Time series of storm frequency and intensity are generated from both datasets and analyzed for interannual variability and long-term trends.
• Statistical comparison of trends and variability patterns is performed, focusing on the period 1871–2000, especially the first half of the 20th century.
• The analysis includes checks for consistency with independent data and previous studies to validate observational trends.

Experimental results

Research questions

• RQ1 Do long-term trends in storminess derived from the 20CR reanalysis align with those derived from observational data in the Northeast Atlantic?
• RQ2 Why do the 20CR reanalysis and observational data show opposing trends during the first half of the 20th century?
• RQ3 To what extent do changes in observational coverage over time affect the reliability of 20CR for long-term trend detection?
• RQ4 Are the storminess trends in 20CR consistent with independent observational and analytical evidence?
• RQ5 Can 20CR be considered a reliable tool for studying long-term climate variability in storminess over the Northeast Atlantic?

Key findings

• The 20CR reanalysis and observational data show opposing long-term trends in storminess during the first half of the 20th century, with 20CR indicating a decreasing trend while observations show an increasing trend.
• For the more recent period (post-1950), both datasets show similar behavior in storminess variability and trends, indicating convergence in later years.
• The inconsistencies in the early 20th century are likely due to inhomogeneities in the 20CR reanalysis caused by the increasing number of assimilated observations over time.
• The observed trends are supported by multiple independent data sources and analyses, lending credibility to the observational record.
• The 20CR reanalysis is therefore considered likely unsuitable for identifying reliable long-term storminess trends in the earlier part of the record over the Northeast Atlantic.
• The findings reconfirm the need for caution when using global reanalyses, especially 20CR, for long-term climate trend assessments.