MERCATOR OCEAN JOURNA: 
SEPTEMBER 2021 
sea surface temperature and sea ice concentration 
observations). Its main purpose was to improve ocean 
circulation at meso scale, surface currents and also large- 
scale biases and interannual variability. To reach this goal, 
the following main updates were introduced in the system: 
correction of atmospheric forcing fields at large-scale 
with satellite data, 
addition of freshwater runoff from ice sheets melting 
to river runoffs, 
addition of a time varying global average steric effect 
to the model sea level, 
improvement of the Mean Dynamic Topography used 
for altimeter data assimilation taking into account the 
last version of the GOCE geoid, 
introduction of an adaptive tuning on some of the 
observational errors, 
- addition of a dynamic height criteria to the quality 
control of the assimilated temperature and salinity 
ertical profiles, 
assimilation of satellite sea ice concentrations, and 
assimilation of climatological temperature ana 
salinity in the deep ocean below 2000 m to prevent 
drifts In those very sparsely observed depths. 
The most satisfying outcome, illustrated in Lellouche et al., 
(2018), was the great improvement of models’ accuracy for 
water masses and in particular the salinity property with a 
decrease of the global RMS error from 0.1 psu. 
Moreover, the Surface and Merged Ocean Currents (SMOC) 
product was developed, based on the GLO12v3 system, 
specifically for surface drift applications. It includes wave 
(Stokes drift) and tidal currents in addition to physical 
system ocean currents. Figure 1 shows the reduction of 
Lagrangian forecast errors of 18.7% on average for the 
Jlobal area. Locally, and especially in large-scale winc 
circulations, improvements are much larger and can react 
up to 200% (like in the Antaretic Circumpolar Current). 
B0°N | 
50°N | 
40°N 
- 
20°N 
0° 
20°S 
10°5 
50°5 
30°S 
| Ai 
180°W 
m 
120°W 60°W 0° 
S60°E 
120°E 
30°N 
50°N 
40°N 
20°N 
20°5 
10°5 
50°5 
B0°S — 
180°W 
FE 
1L120°W 
SAW 
9° 
an’ 
120°FE 
0 8 16 24 32 
. 1— —————— ' 
40 48 56 64 72 80 
(km) 
— 
18 
180 
Figure 1: Illustration of error reduction using SMOC, a surface current product containing effects of waves and tides. Maps illustrate error (in 
km) for a 72 h advection using the standard surface current (top panel) and the SMOC total current (bottom panel). More precisely, the map of 
separation distance compares 72 hours of Lagrangian forecasts with drifting buoys from the Global Drifter Program. Numerical trajectories 
were computed with u0: physical model currents alone (top panel), compared to u_total, the SMOC total current (bottom panel). The results 
were averaged per ?° boxes