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In-Depth Information
SMOS observations. The following residual SSS anomaly can then be estimated from
SMOS temporal observations of salinity S(t, r) at point r following:
DS
ð
t
;
r
Þ¼
S
ð
t
;
r
Þ
S
o
ð
t
;
r
Þ~ð
t
;
r
Þr
S
ð
t
;
r
Þ
ð
3
Þ
According to the simplified salinity balance (Eq.
2
), a priori valid for the tropical Atlantic,
the resulting SSS anomaly given by Eq.
3
shall be strongly correlated with the net
freshwater flux forcing term. Examples for such SSS anomaly analysis are shown in
Fig.
16
for a selected point in the middle of the north tropical Atlantic (16N-35W). From
TRMM precipitation and OAFLUX daily evaporation fluxes, large-area P and E anomalies
were also evaluated:
DP
ð
t
;
r
Þ¼
P
ð
t
;
r
Þ
P
o
ð
t
;
r
Þ
DE
ð
t
;
r
Þ¼
E
ð
t
;
r
Þ
E
o
ð
t
;
r
Þ
where P
o
and E
o
are the local climate mean for the precipitation and evaporation.
As illustrated in Fig.
16
(middle right panel), very significant long-lived negative DS(t, r)
values are detected in SMOS anomalies at the selected point in the north tropical Atlantic during
September/October months (days 250-300) of 2010. Apparently, this happened just after a
strong positive anomaly in the precipitation rate as detected from TRMM during the passage of
the ITCZ in August (bottom right panel).
The spatiotemporal consistency between the large-area and large-amplitude S, P and E
anomalies can be further analyzed over all the tropical Atlantic. This is illustrated in
Fig.
17
for two selected months of 2010. The spatial distribution of the large-area and
long-lived (monthly averaged) SSS anomalies generally matches well the spatial patterns
for the large E-P anomalies. In particular, north-south oscillation in DS(t, r) around the
ITCZ (centered on 5N in March and 8N in July) follows the DE-DP(t, r) far from the
Amazon plume area, with negative DS(r, t) corresponding to positive DP(t, r) and positive
DS(r, t) found in region of positive DE(t, r). The average relationship between SMOS SSS
anomalies and the corresponding anomalies in the net atmospheric freshwater flux in the
tropical Atlantic (defined here by 5S-20N;75W-15E) was further evaluated over year
2010 by binning DS(t, r) values as function of DE-DP(t, r) as shown in Fig.
18
.
Despite a significant scatter in the data, the results clearly indicate the strong coherency
between SMOS SSS anomalies and the evaporation minus precipitation flux signal in the
tropical Atlantic. On average, SMOS SSS are thus systematically fresher than the SSS
climatology when precipitation rate exceed evaporation rate with respect climatological
means, and vice versa. As expected by the skin layer effects (Zhang and Zhang
2012
),
satellite SSS anomalies are weakly sensitive to excess evaporation showing an almost
constant value whatever positive values for DE-DP. Nevertheless, and as discussed in Sect.
4
, the average 0.3 salinity unit excess amplitude found for DS in evaporative zones is
significantly larger than the expected evaporation-induced effect on the satellite
*0.01 pss. The source for such observed signal amplitude is not yet understood. Other
physical processes, not yet well accounted for in the SSS retrieval algorithm, may sys-
tematically affect the L-band brightness temperature in strongly evaporative zone (e.g.,
skin effects in SST, badly accounted for roughness effects at low winds).
Nevertheless, Fig.
18
clearly shows that SSS anomalies become increasingly negative
as the precipitation anomalies progressively exceed the evaporation anomalies.
This shows that it is important to monitor SSS from space in the rainy regions as it
makes a good oceanic rain gauge for the changing water cycle (Cravatte et al.
2009
;Yu
2011
; Terray et al.
2011
), and therefore help to maintain a continuous observation network
