Geoscience Reference
In-Depth Information
they are associated with equatorial instability waves. SST fronts also
appear in wide regions of the North and South Pacific basins, in the
Southern Ocean and South Indian Ocean, exhibiting their
“universal”
distribution in the oceans.
However, studies carried out in 1980-1990 have shown that the
atmospheric flow is modified while passing above an SST front, even
if it has a weak intensity. In the case of a flow circulating from the
warm to the cold side, the boundary layer stabilizes (heat flux
decrease) and the wind slows due to the decrease in the layer
thickness. Conversely, if the wind blows from the cold to the warm
side, heat fluxes abruptly increase, leading to a deepening of the
boundary layer, associated with an increase of the average wind. On a
larger scale, a correlated decrease in surface pressure can be observed
[SMA
08]. Indeed, the oceanic structures have a long-life duration
(several weeks in the case of an eddy, months for currents (some are
permanent)). The influence of these fronts on the atmosphere, when
cumulated over the globe, is thought to have a significant impact: for
example, Bourras
et al.
[BOU
04] have shown that the local wind
beside an ocean vortex is disturbed as long as the ocean vortex
remains. The SST fronts are, therefore, detectable not only through the
difference in surface temperature, but also by the changes in surface
roughness associated with the wind, by altimetry (dynamic changes
associated with the sinking/rising of masses of water), and by changes
in chlorophyll levels at the surface. A complete review of the detection
of fronts and the oceanic mechanisms associated with them can be
found in [ROB
10].
Coupled phenomena between ocean and atmosphere implying an
SST front can be observed in all regions of the globe. The following
shows the importance of strong SST fronts.
However, even fronts of moderate-to-weak intensity (less than 3-
4°C) can play a role in the ocean-atmosphere coupling. For example,
in the Gulf of Guinea, in the boreal spring, the trade winds of the
Southern Hemisphere are pulsed by the Santa Helena high. These
wind bursts trigger a vertical mixing in the ocean, leading to the
formation of a seasonal cold tongue of fresh water. To the north of this
