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southwestward jet-like current in its path with a dipole of Ekman pumping/eddies on its
flanks. As a result, upwelling in the Panama Bight brings cold and salty waters to the
surface that erode the fresh pool on its eastern side while surface currents stretch the pool
westward.
Interestingly, SMOS data are also able to detect other meso-scale features in the region
around the fresh pool such as the near-equatorial SSS front or the local SSS maximum in
the Costa Rica dome.
Therefore, SMOS SSS data will help in exploring qualitatively the seasonal dynamics of
the fresh pools from their birth to their final erosion by wind-driven and turbulent processes
(surface current stirring and wind-driven upwelling). Quantifying the relative contribution
of the different mechanisms on SSS variations would require a model-based synergetic
data analysis scheme to establish the mixed layer salt budget. Also, the regional occurrence
of SSS fronts and barrier layers (de Boyer Mont´gut et al.
2007
) suggests, by analogy with
the western tropical Pacific, a link between surface and subsurface salinity which could
give additional value to the satellite SSS data (Maes
2008
; Bosc et al.
2009
). As barrier
layers can play an active role on the tropical climate (e.g., Maes et al.
2002
,
2005
),
studying their impacts in the region seems worthwhile. This could be done through
regional modeling combined with the analysis of subsurface/surface in situ and satellite
data. Also, interannual variations of the fresh pool, even if quantitatively smaller than its
seasonal variations, need further investigation as ENSO is a strong climate driver in the
eastern Pacific. Now that 3 years of SMOS data are available, such type of analysis can be
initiated.
5.2 Fresh Pool Interactions with Tropical Cyclones
Because of the buoyant plume of freshwater that forms in the Atlantic due to discharge
from the Amazon and Orinoco rivers, the northwestern tropical Atlantic is a region where
the salt-driven upper ocean stratification may significantly impact ocean-atmosphere
interactions under tropical Cyclones. The spreading of the Amazon-Orinoco River plume
exhibits a seasonal cycle coinciding with the Atlantic hurricane season (1 June-30
November) with river influenced minimum salinities observed farthest eastward and
northwestward during the height of the hurricane season (mid-August to mid-October). As
shown by Ffield (
2007
), for the 1960-2000 time period, 60 and 68 % of all category 4 and
5 hurricanes, respectively, passed directly over the plume region, revealing that the most
destructive hurricanes may be influenced by plume-atmosphere interaction just prior to
reaching the Caribbean. Historical in situ data reveal that average ocean surface temper-
atures first encountered by tropical cyclones moving westward between 12 and 20Nis
only 26 C, but upon reaching the northern reaches of the Amazon-Orinoco River plume
(e.g., see Fig.
25
), the average SST encountered by tropical cyclones are 2 C warmer.
These warm ocean surface temperatures may play a role in hurricane maintenance and
intensification since hurricanes can only form in extensive ocean areas with a surface
temperature greater than 25.5 deg C (Dare and McBride
2011
). In addition, as shown by
Ffield (
2007
), the buoyant, and therefore stable, 10- to 60-m-thick layer of the plume can
mask the presence and influence of other ocean processes and features just below the
plume, in particular cool (during hurricane season) surface temperatures carried by NBC
rings. After shedding from the NBC retroflection, the 300-500-km-diameter anticyclonic
(clockwise) NBC rings pass northwestward through the Amazon-Orinoco River plume
toward the Caribbean. The limited observations reveal that at times the cool upper-layer
temperatures of the NBC rings are exposed to the atmosphere, while at other times, they
