Geoscience Reference
In-Depth Information
5 Fresh Pool Interactions with Wind-driven Processes
In this section, two specific SMOS observation cases of wind-driven phenomena are
presented. The first example illustrates the erosion of the Far Eastern Pacific Fresh Pool by
the gap-wind-driven Panama upwelling processes, whereas the second focuses on the salty
wake left behind hurricanes after their passing over the Amazon-Orinoco river plumes.
5.1 An Example of Fresh Pool Erosion by Wind-driven Upwelling
The eastern tropical Pacific Ocean between about 120W and South America is unique in
many respects. Lying in an environment predominantly influenced by the south and
northeastern trades and the doldrums, and seasonally affected by the winds from the
Caribbean, this region is characterized by complicated and large seasonal variations in the
wind field, current pattern, temperature and salinity structure.
The region exhibiting the lowest SSS of the tropical Pacific Ocean, the Eastern Pacific
Fresh Pool (EPFP), is found between the warm pool characterized by a mean sea SST
greater than 28 C centered on 15N along the coast of Central America and the cold and
fresh equatorial region, with SSS values lower than 33 pss off the Panama isthmus and
lower than 34 pss extending as far as 130W from the equator to 15N (Fig.
24
).
The EPFP reflects both the conditions of excess precipitation over evaporation beneath
the ITCZ and inputs of freshwater from the Andes and Caribbean regions (Benway and
Mix
2004
). Analysis of a recent gridded in situ SSS product (Delcroix et al.
2011
) points
out that interannual variations are relatively weak in the EPFP but that seasonal variations
are the strongest within the tropical Pacific. Large-scale analysis suggests that the SSS
seasonal balance is mostly driven by precipitation in the part of the EPFP covered by the
ITCZ, but more complex in the far east as advection and entrainment become important
processes (Bingham et al.
2010
; Alory et al.
2012
).
By focusing on seasonal SSS variations along a well-sampled Voluntary Observing Ship
(VOS) line from Panama to Tahiti, Alory et al. (
2012
) recently showed that this fresh pool
dynamically responds to strong regional ocean-atmosphere-land interactions. First,
monsoon rains (and associated river runoff) give birth to the fresh pool in the Panama
Bight during summer and fall. Second, strong currents driven by topography-induced
winds extend the pool westward in winter, while it eventually disappears by mixing with
upwelled saltier waters to the east. These dynamic features also generate steep SSS fronts
at the edges of the fresh pool (sometimes larger than *4 pss/ of longitude at the eastern
edges).
These SSS fronts and the amplitude of their seasonal cycle are large enough to be
detected by the new SMOS satellite mission. Compared to in situ data, SMOS satellite data
provide a more homogeneous coverage with finer spatial resolution. Examples of SMOS
SSS maps averaged over 10 days and centered at selected dates in December 2010,
February and April 2011 are presented in Fig.
24
. Remarkably, all the major features
observed with in situ VOS data as detailed in Alory et al. (
2012
) are well reproduced in the
SMOS analysis, notably the westward expansion of the fresh pool (SSS\33 pss) from 85
W in December to 95W in April, the steep SSS front east of the 32 pss isohaline and SSS
minimum of 28 pss in the Panama Bight in December, and the strong SSS increase to
around 35 pss in the Panama Bight in April. Moreover, SSS changes occurring between
December and April are qualitatively consistent with the expected effects of winter cli-
matological currents, including the Panama Bight upwelling.
