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Interannual modulation of seasonalscale variability in alongshore flow off Chile

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Title: Interannual modulation of seasonalscale variability in alongshore flow off Chile


1
Interannual modulation of seasonal-scale
variability in alongshore flow off Chile
UNIVERSIDAD DE CONCEPCION
Samuel Hormazábal Department of
Geophysics Faculty of Physics and Mathematics
University of Concepción e-mail
sam_at_profc.udec.cl Grant from FONDECYT N1040618
2
Introduction
The coastal ocean off Peru and Chile is
recognized as one of the most biologically
productivity regions of the global ocean. Several
important factors support this high marine
production. These include a persistent poleward
undercurrent which carries nutrient-rich water
poleward and a persistent, wind-driven upwelling
which lifts this nutrient-rich water towards the
surface.
Here I study the structure and origin of low
frequency variability (from intraseasonal to
interannual periods) off the west coast of South
America, and discuss some impacts of these
physical variability.
3
Observations
About a decade of Current observations off
Chile at Continental Shelf (CSM) site
Sea level along the Peru-Chile coast
Satellite wind stress (Weekly ERS1-2 and daily
QuikSCAT, grid resolution of 10 x 10)
4
Coastal Time Series
5
Wavelet Power Spectrum
Intraseasonal Variability (30-90 days)
  • Strong energy enhancement during the 1991-92 and
    1997-98 El Niño event.
  • core of 50 days (1991-92)
  • core of 70 and 40 days (1997-98)

Seasonal and longer periods
The PCU is strongly modulated over El Niño and La
Niña cycles -Strong semiannual variability during
El Niño -Annual variability dominates during La
Niña
6
Wavelet Phase Angle
From the general solution for Kelvin waves, the
alongshore flow is given by 
where g is gravity, h is sea surface displacement
and c is the speed of long gravity waves.
From the Rossby wave solution, the flow at the
coast is given by
where f is the Coriolis parameter and b is the
latitudinal gradient of f.
7
Origin of Intraseasonal Variability
IS poleward (equatorward) wind anomalies off
Chile are related to IS westerly (easterly) wind
anomalies in the western or central tropical
Pacific
The correlation structure may be associated with
two atmospheric teleconnection pattern
  • Equatorial Kelvin waves, and
  • Rossby waves

8
Model Setup
Using the baroclinic normal mode to describe the
vertical structure of the wind forced response,
simple, linear, wind driven ocean models of
equatorial Kelvin waves (Kessler and McPhaden,
1995) and coastal trapped waves (Brink and
Chapman, 1987) were combined.
Equatorial Kelvin wave model Forced by weekly
satellite wind stress data Wind integration along
the baroclinic Kelvin wave characteristic
Coastal trapped wave model Forced by weekly
satellite wind stress data 11 segments between
5S and 35S, bottom topography, stratification,
and bottom friction
9
Forcing of Intraseasonal Periods
IS variability, particularly during summer and
during El Niño events, arrives as free, oceanic,
coastal trapped waves via oceanic, equatorial
Kelvin waves, forced by tropical Pacific winds
South of 20S, some oceanic intraseasonal
variability is forced by local winds associated
with the atmospheric teleconnections
10
Remote Forcing of Seasonal and Longer Periods
Rossby wave solution for the flow at the coast is
given by
where n indicates the mode in question.
As suggested from wavelet analysis local wind
forcing may be needed to explain such variability
during cold periods
Warm periods, rms ratio 0.78 and r0.56 (r0.9
for a lag of 27 day) Cold periods, r 0.14
11
Rossby Waves effects
12
Minimum Oxygen Zone Variability
13
Coastal Sections
14
Thermocline variability
15
Anchovy Distribution
16
Conclusions
Results show that wind-forced, equatorial Kelvin
waves and local wind forcing are strongly related
to the variability in the eastern boundary
current system off the west coast of South America
Intraseasonal variability (30-90 days) observed
in the study region may be explained by the
superposition of two distinct modes of response
to ocean-atmosphere equatorial dynamics. The
oceanic pathway via equatorial Kelvin waves and
coastal-trapped waves, and the atmospheric
teleconnection via atmospheric equatorial Kelvin
waves and Rossby waves.
17
Conclusions
Annual and semiannual variability of coastal
currents may be explained to a large degree by
Rossby waves forced by equatorial Kelvin waves
and by local winds. Strong (weak) semiannual
Rossby waves are forced by equatorial Kelvin
waves during El Niño (La Niña) periods. Strong
annual variability of coastal currents is found
mainly during la Niña periods in response to
remote and local winds forcing.
18
Conclusions
The oxygen minimum zone dynamics is strongly
affected by Rossby waves.
In the north zone of Chile, the vertical
distribution of anchovy is strongly affected by
remotely forced Rossby waves.
19
OMZ Thickness
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