Description and assessment of the OSTIA reanalysis. - Met Office

Table 2.2: Input data differences between the reanalysis and operational OSTIA systems (see glossary
for acronyms)
Reanalysis
Operational
ATSR-1, ATSR-2,
sampled)
AATSR (not sub-
AATSR (subsampled)
AVHRR Pathfinder (not subsampled)
NOAA-18 AVHRR,
(subsampled)
MetOp AVHRR
No other satellite data types
ICOADS in situ, nighttime only
OSI-SAF reprocessed ice concentration
(SMMR and SSM/I)
SEVIRI, NAR, TMI, AMSR-E (subsampled)
In situ via GTS, 24 hrs
OSI-SAF operational ice concentration
(SSM/I)
in the higher latitudes than for the satellite data, although coverage close to the ice edge is poorer
(figure 2.3).
Figures 2.3(c) and 2.3(d) provide a comparison of the total number of observations for each grid
box for two different years: one year from before the introduction of ATSR series data to the reanalysis
(1990) and one afterwards (2004). The ATSR series data increases the number of observations
globally, but particularly in the equatorial regions and mid-latitudes. The number of observations at
higher latitudes is also increased, see for example the north Atlantic Ocean and Southern Ocean.
Figures 2.3(e) and 2.3(f) show the total number of in situ observations for 1985 and 2007 respectively.
These figures illustrate the improved global coverage of in situ observations at the end
of the reanalysis period compared to the start, when the number of observations in the northern
hemisphere was significantly greater than in the southern hemisphere. It is interesting to note the
change in the pattern of observations, from the straight lines characteristic of ship tracks in 1985
(figure 2.3(e)) to the curved lines of drifters following current structures in 2007 (figure 2.3(f)). It is
also notable that in 2007 the number of in situ observations in the Atlantic Ocean is greater than in
the Pacific Ocean (figure 2.3(f)). There is also a relatively large number of in situ observations in
2007 around the Antarctic Peninsula and in the Arctic north of Scandinavia (figure 2.3(f)), presumably
related to IPY (International Polar Year) activities at this time.
Figure 2.4 shows the total number of observations for each 1/4 o grid box by season for the
entire reanalysis period (1985-2007). There is a marked seasonal difference in the number of
observations in the polar regions as all observations under ice are masked out (section 2.2) and the
ice extent varies seasonally. The southern hemisphere region of multi-year ice in the Weddell Sea
can be seen in the southern hemisphere summer and autumn (December-February and March­
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