Global reanalyses use fixed numerical weather prediction models and data
assimilation schemes to produce gridded fields over time periods suitable for climate
research. Reanalyses are particularly useful over polar regions, providing a coherent
representation of weather and climate where relatively short temporal spans of data
records and areas of sparse observations exist. However, caution must be exercised when
using reanalyses to study climate trends, as output is sensitive to changes of the observing
system and how observations are processed (Bengtsson et al. 2004a,b; Sterl 2004; Thorne
and Vose 2010; Screen and Simmonds 2011). Such changes result in erroneous trends,
particularly over Southern Hemisphere polar regions (e.g., Hines et al. 2000; Marshall
and Harangozo 2000; Marshall 2002), limiting the viability of reanalysis products in
these regions to the post-1978 modern satellite era (e.g., Bromwich and Fogt 2004;
Renwick 2004; Trenberth et al. 2005; Bromwich et al. 2007). There are also substantial
differences between reanalyses, based on different models and parameterizations,
observations, and data assimilation systems (e.g., Bromwich and Fogt 2004; Bromwich et
al. 2007; Walsh et al. 2009; Screen and Simmonds 2011; Bromwich et al. 2011). In this
study we use three reanalyes, described below, for the 32-year period from 1979-2010.
The European Centre for Medium-Range Weather Forecasts (ECMWF) “Interim”
Reanalysis (ERA-Interim, Dee et al. 2011) supersedes the ERA-40 reanalysis (Uppala et
al. 2005), and improves upon ERA-40 in several regards (see Dee et al. 2011). ERAInterim uses a 12-hourly 4D-Var data assimilation system, and also uses an automated
satellite radiance variational bias correction scheme (Dee and Uppala 2009). The
observational sources for polar regions are listed in Andersson (2007) and Dee et al.
(2011). ERA-Interim features spectral T255 (~0.7°) horizontal resolution and 60 vertical
levels. Output on a regular 512x256 0.7° Gaussian grid from the National Center for
Atmospheric Research Data Support Section (NCAR DSS) is used in this study.
The National Centers for Environmental Prediction (NCEP) Climate Forecast
System Reanalysis (CFSR, Saha et al. 2010) is a coupled atmosphere-ocean-land surfacesea ice model that supersedes the NCEP/Department of Energy Atmospheric Model
Intercomparison Project 2 reanalysis (Kanamitsu et al. 2002). CFSR uses a 3D-Var
gridpoint statistical interpolation (GSI) data assimilation system (Kleist et al. 2009), and
ingests a wide array of satellite observations in radiance form. The CFSR atmospheric
component features spectral T382 (~0.31°) horizontal resolution with 64 vertical levels.
Output on a 720x361 0.5° latitude/longitude grid from the NCAR DSS is used here.
The Japan Meteorological Agency (JMA) 25-year reanalysis (JRA-25, Onogi et
al. 2007) uses the JMA numerical weather prediction and data assimilation systems.
JRA-25 uses a 3D-Var data assimilation scheme that ingests satellite radiances and
features spectral T106 (~1.125°) horizontal resolution with 40 vertical levels. Output on
a regular 320x160 1.125° Gaussian grid from the NCAR DSS is used in this study.
The previously described Polar Marine (EM) climate classification from Shear
(1964) was applied to each gridpoint of 2-m temperature from the three reanalyses. The
union of the regions of warmest month mean temperature greater than 32°F but less than
50°Fand coldest month mean temperature greater than 20°F represent EM climate. In
addition to 2-m temperature, precipitation, mean sea-level pressure, sea-surface
temperature, sea ice fraction, 10-m wind, and total cloud fraction are also analyzed to
provide a more complete description of EM climate. While all three reanalyses are used
to characterize the spatial distribution of EM area, to simplify the analysis, we solely use
ERA-Interim for the detailed description of EM climate. ERA-Interim is the only
reanalysis to use 4D-Var data assimilation, and it along with its predecessor ERA-40
compare favorably against other global reanalyses in both the Northern Hemisphere (e.g.,
Bromwich and Wang 2005; Bromwich et al. 2007; Walsh et al. 2009; Screen and
Simmonds 2011) and Southern Hemisphere (Bromwich and Fogt 2004; Monaghan et al.
2006; Bromwich et al. 2011; Hodges et al. 2011). The primary findings of this study are
not critically dependent upon which reanalysis is used for detailed analysis.
Figure Xa shows EM area for all three reanalyses over the Northern Hemisphere.
The area east of Newfoundland extending south of Greenland, across Iceland, and over
the Norwegian and Barents Seas matches well between all three reanalyses. The second
area over the Bering Sea also shows general agreement, although the JRA-25 and CFSR
areas are slightly larger than ERA-Interim. There are also scattered small EM regions
along the southern Alaska and western Canadian coastlines. Additional small highaltitude mid-latitude regions primarily show up in CFSR, likely due to the enhanced
horizontal resolution compared to the other reanalyses. The two large-scale EM areas in
Fig. Xa are along the primary high-latitude storm tracks (e.g., Hoskins and Hodges 2002),
where warm and moist air is advected into these areas from the south. Notice the eastern
offset of EM areas from the Canadian, Greenland, and Siberian coasts, where continental
effects prevent establishment of EM climate until a marine influence dominates offshore.
Figure Xb shows EM area for the Southern Hemisphere from all three reanalyses.
The Southern Hemisphere contains 90% of global EM climate area. EM area exists over
much of the Southern Ocean, and farther equatorward in the Southern Hemisphere
compared to the Northern Hemisphere. Differences between reanalyses are again small,
with CFSR extending slightly farther south, ERA-Interim extending slightly north, and
some discrepancies over southern Chile. CFSR also identifies EM over southwestern
New Zealand. The Southern Hemisphere EM area generally follows the Southern
Hemisphere storm track, which dips poleward from the south Atlantic eastward to
regions south of New Zealand and into the south Pacific. However, the EM area occurs
on the northern edge of the primary circumpolar storm track (e.g., Simmonds et al. 2003;
Hoskins and Hodges 2005), where equatorward incursions of Antarctic airmasses allow
for establishment of EM climate in otherwise marine environments.
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