MERCATOR OCEAN JOURNA: 
SEPTEMBER 2021 
L3 and L4 NRT wind products have been complemented by 
equivalent L3 and L4 REP wind products. The covered time 
range stretches from 1992 to the year before present. 
Initially, the L3 REP product included collocated ECMWF 
ERA-Interim stress-equivalent model winds. Since ERA- 
Interim was discontinued in 2019 and users have switched 
to ERA5, ERA-Interim model winds in the L3 REP product 
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were replaced by ERA5 model winds in 2020. The L3 REP 
product update frequency was increased to quarterly in 
2020 and will be further increased to monthly in 2021, to 
include observations up to three months before present. 
The update frequency of L4 REP products will be increasec 
to at least 6&-monthlvy in 2021. 
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ıgure '1: Daily global number of scatterometer observations within the L3 NRT product from January 1% 2016 to March 31* 2021. The 
ıumber shown is the sum of all scatterometer datasets included in the product at each specific day. Comments indicate significant changes in 
chis number, caused by either introduction of new datasets or upstream data anomalies longer than 2 days 
1.2 Scatterometer versus ECMWF 
model surface winds 
Many marine forecasting centers use ECMWF-based wind 
inputs for ocean model foreing, including their associated 
biases. To characterize differences between observed and 
modelled surface wind fields, scatterometer observations 
\Metop-A ASCAT) were compared to the collocated 
ECMWF ERA-Interim and ERA5 wind fields in the L3 REP 
wind product (Belmonte Rivas and Stoffelen, 2019). The 
model mean zonal winds are larger than scatterometer 
winds, pointing to stronger subtropical easterlies and 
mid-latitude westerlies. On the other hand, the mean 
meridional model winds speeds are too low, resulting In 
reduced mean poleward flow at mid-latitudes and weaker 
trade winds in tropics (Figure 2a). The spatial distribution 
of the systematic model biases is similar for ERA-Interim 
and ERA5, but generally about 20% smaller in ERA®5. A 
zomparison of transient winds reveals that wind speec 
variability is significantly reduced in ERA models compared 
to scatterometer observations (Figure 2b). Inability of the 
atmospheric model to reproduce higher-frequency wind 
variability implies an underestimation of atmospheric 
foreing at the air-sea boundary. This has detrimenta: 
consequences for ocean forcing and the representation of 
alr-sea interaction in coupled models.