Geoscience Reference
In-Depth Information
key tropical oceans regions (e.g., northwestern tropical Atlantic, eastern and western Pacific
fresh pools, Bay of Bengal) is mostly controlled by salinity. In such freshwater pool regions, a
uniform density mixed layer is found to form the so-called Barrier Layers (BL) at shallower
depth than the uniform temperature layer. Because of stable halocline, the BL are acting to
inhibit surface cooling and vertical mixing under the action of surface wind stresses. Therefore,
there can be some feedback mechanisms between atmospheric, or terrestrial, freshwater fluxes
to the ocean and intense atmospheric processes. About 68 % of hurricanes that finally reached
category 4 and 5 have thus crossed the Amazon/Orinoco plume (Ffield
2007
) where the
presence of Barrier Layers can enhance their growth rate by 50 % (Balaguru et al.
2012
). Under
an intense hurricane, the halocline, which is above the thermocline, is first mixed. This produces
a SSS wake that is by a few pss saltier than initial SSS in the plume. By analyzing SMOS SSS
data before and after the passage of several intense hurricanes over the Amazon River plume in
2010 and 2011, SSS changes[1 pss over areas exceeding 10
5
km
2
were detected. These abrupt
changes have implications for SSS climate, since SSS is more long-lived and not damped like
SST. In addition, destruction of the BL is apparently associated with a decreased SST cooling in
the plume that, in turn, preserves higher SST and evaporation than outside the BL. This
difference in SST cooling is explained by additional work required to mix the BL. Thus, BL
leads to a reduction in hurricane-induced surface cooling that favors hurricane intensification,
as the resulting elevated SST and high evaporation enhance the hurricane's maximum potential
intensity. The geographic location and seasonality of the Amazon/Orinoco plume make hur-
ricane overpasses a frequent occurrence. Indeed, the expansion of the plume in August-
September coincides with the peak of the production of Cape Verde hurricanes, which includes
many of the most intense (category 4-5) hurricanes. Thus, the results presented here strongly
suggest that the role of the salinity stratification in mixed layer dynamics should be taken into
account when forecasting tropical cyclone growth over freshwater pools that are generating
thick BL (Amazon plume, Bay of Bengal, eastern and western Pacific fresh pools). The
availability of satellite SSS from Aquarius and SMOS along with in situ Argo measurements is
critical to making such model improvements practical.
Acknowledgments Work presented in this paper was partly done under ESA support in the context of the
development of the SMOS level 2/Expert Support Laboratory, ESA Support to Science Element SMOS ? SOS
and SMOS ? STORM projects. CNES also partly funded these activities in the frame of the Centre Aval de
Traitement des Donn
´
es SMOS (CATDS) and of the SMOS/ESA GLOSCAL Cal/Val projects. We are
indebted to Micha Rijkenberg (Koninklijk Nederlands Instituut voor Onderzoek der Zee Department of Bio-
logical Oceanography (BIO)) for giving us access to the Geotraces Leg 2 CTD data, to F. Gaillard (LPO/
IFREMER) for providing ISAS maps version 6 (see
http://wwz.ifremer.fr/lpo/SO-Argo/Products/Global-
Ocean-T-S
)
. Gilles Reverdin (LOCEAN/CNRS) is responsible for French salinity drifter deployment program;
he supervised the quality control of these drifter data set and is warmly acknowledged. These data are available
on
www.locean-ipsl.upmc.fr/smos/drifters
. We thank Nicolas Martin for data processing. We are also indebted
to J´romeVialard, Matthieu Lengaigne and Emmanuel Vincent for their very helpful comments about SSS
variability in the tropical Indian Ocean and about tropical cyclones interactions with the Amazon plume,
Thierry Delcroix about SSS variability in the tropical Pacific Ocean. SSM/I data have been processed by
Remote Sensing System (
www.ssmi.com
). The global ocean heat flux and evaporation products were provided
by the WHOI OAFlux project (
http://oaflux.whoi.edu
) funded by the NOAA Climate Observations and
Monitoring (COM) program. We thank the two anonymous referees and Doug Vandemark for their very useful
comments and detailed corrections, which were very constructive and which helped us to improve our paper.
References
Alory G, Maes C, Delcroix T, Reul N, Illig S (2012) Seasonal dynamics of sea surface salinity off Panama:
the far eastern Pacific fresh pool. J Geophys Res 117:C04028
