...One specific problem that has been identified in the
40-yr European Centre for Medium-Range Weather
Forecasts (ECMWF) Re-Analysis (ERA-40) product is
a shift of temperature bias in 1997. The ECMWF Web
site (
www.ecmwf.int/research/era/Data_Services/section3.
html) states that:
"a problem of concern is
cold bias in the lower troposphere
(below about 500 hPa) over the ice-covered oceans
in both the Arctic and the Antarctic . . . These polar cold
biases arise from the assimilation of HIRS [High Resolution
Infrared Radiation Sounder] radiances. Changes to
the thinning, channel-selection and quality control of the
infrared data that were introduced for analyses from 1997
onwards to reduce tropical precipitation bias have also
virtually eliminated the cold polar biases."
The lower-tropospheric cold bias in ERA-40 was first
documented by Bromwich et al. (2002). Its elimination
in 1997 is mentioned by Bromwich and Wang (2005) and
is discussed in more detail by Bromwich et al. (2007).
The problem is specifically mentioned in the ERA-40
documentation (Uppala et al. 2005). However, despite
this ‘‘known’’ shortcoming, ERA-40 has been recently
used to assess Arctic temperature trends and their vertical
structure...
...We stress that it is not our intention to evaluate the
conclusions of all the numerous studies that have used
ERA-40 in the Arctic, many of which likely were, and
remain, valid. However, here we briefly mention two
examples to help illustrate our concerns with ERA-40
temperature trends and the conclusions derived from
them.
...In the case of Yang et al. (2010), their analysis
of TMT trends was undertaken with MSU, ERA-40,
and NNR data—all of which were in broad agreement.
We also find agreement between datasets for the TMT
trends (Fig. 1) and therefore have no reason to question
the validity of their results pertaining to the evolution
of TMT temperature.
...While our results reiterate problems with using reanalyses
to examine trends (Bengtsson et al. 2004; Sterl
2004; Thorne and Vose 2010), they do not imply that all
reanalyses are unsuitable for studying Arctic temperature
trends and their vertical structure. Indeed, we have
shown that ERA-40 is unique among a series of alternative
reanalyses in displaying statistically significant errors.
We conclude that with the exception of ERA-40,
current reanalyses are in broad-scale agreement with
observed Arctic temperature change and encourage their
discerning use instead of ERA-40, and in conjunction
with observations where possible.
