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In-Depth Information
large as about 20 m, then the fresh layer will potentially inhibit vertical mixing signifi-
cantly. As the freshwater surface layer (halocline) of the Amazon and Orinoco river plumes
is warmer than the water below (Ffield
2007
), salinity stratification acts to reduce the depth
of vertical mixing and thus sea surface cooling. The reduced cooling amplitude in the wake
of hurricanes passing over the Amazon and Orinoco river plumes, associated with thick
barrier layer (BL) effects, might be an important mechanism in favor of hurricane inten-
sification in that region. Similar impact of barrier layers on TC-induced sea surface cooling
has been recently evidenced for several case studies such as in the tropical Atlantic
(Balaguru et al.
2012
), in the Bay of Bengal (Yu and McPhaden
2011
; Neetu et al.
2012
)
and in the tropical Northwest Pacific (Wang et al.
2011
).
New insight into the interactions between such extreme atmospheric events and large-
scale fresh pools at the ocean surface has been gained from the satellite-based SSS
observations as recently reported by Grodsky et al. (
2012
). They used data from the
Aquarius/SAC-D and SMOS satellites to help elucidate the ocean response to hurricane
Katia, which crossed the Amazon plume in early fall 2011. As illustrated in their paper, the
Katia passage left a 1.5 pss high haline wake covering [10
5
km
2
(in its impact on density,
the equivalent of a 3.5 C cooling) due to mixing of the shallow BL.
As illustrated in Fig.
25
, very similar observations were also detected from SMOS data
alone during the passage of the Category 4 hurricane Igor over the river plume in 2010.
The data evidence an erosion of the thin northern reach of the plume fresh surface layer by
Igor hurricane-induced mixing, covering an area of *89,000 km
2
located on the storm
right-hand side, where SSS increases by *1 practical salinity scale while SST cools by
2-3 C (not shown). On the left side of the storm, much smaller SSS and SST changes are
detected after the storm passage. The strong SSS increase in the hurricane wake within the
plume is explained by the erosion of the BL. This is supported by Argo profiles collected
within the plume (see Grodsky et al.
2012
). Mixed layer salinity is lower by 2-4 pss than
the water beneath. The shallow haline stratification is destroyed by hurricane-forced
entrainment which is stronger on the right side of hurricane eye (Price
2009
). It results in a
strong SSS signal. Although the hurricane strengthened further along the trajectory, the
SSS change is much weaker there corresponding to weak vertical salinity stratification
outside the plume.
As further discussed in Grodsky et al.
2012
, the fresh (more buoyant) BL limits the
turbulent mixing and then the SST cooling in the plume, and thus preserved higher SST
and freshwater evaporation than outside. Combined with SST, the new satellite SSS data
thus provide a new and better tool to monitor the plume extent and quantify the upper
ocean responses to tropical cyclones with important implications for hurricane forecasting.
6 Conclusions and Perspectives
The ocean is the primary return conduit for water transported by the atmosphere. It is the
dominant element of the global water cycle, and clearly one of the most important com-
ponents of the climate system, with more than 1,100 times the heat capacity of the
atmosphere. Two new satellite sensors, the ESA SMOS and the NASA Aquarius SAC-D
missions, are now providing the first space-borne measurements of the SSS. Synergetic
analyses of the new surface salinity data sets together with sea surface temperature,
dynamic height and surface geostrophic currents from altimetry, near-surface wind, ocean
color, in situ observations, and rainfall estimates will certainly help clarify the freshwater
budget in key oceanic tropical areas.
