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Angélique
Melet
Physical Oceanographer
CONTACT
INFORMATION
Mail
: melet@hmg.inpg.fr
Address : LEGI, équipe MEOM, BP 53, 38041 Grenoble
cedex 9, France.
Tel
: (+33) 04 76 82 50 10
Fax : (+33) 04 76 82 52 71
ABOUT ME
Research interests (Among others !)
Physical
oceanography, ocean modeling, internal wave-driven mixing, southwestern tropical
Pacific, data assimilation
and parameter estimation
CV / Publications
Curriculum
Vitae / Communications and Publications
Solomon Sea research activities
I worked as a doctoral and
postdoctoral physical oceanographer in
the LEGI / MEOM team in Grenoble, France.
My research is focused on the
oceanic circulations in the Solomon Sea as part of the
CLIVAR / SPICE
and ANR / Solwaraprograms.
The western boundary currents transiting through the Solomon Sea
represent a major source of the Pacific Equatorial Undercurrent and of
the Equatorial Cold Tongue. Therefore, it was hypothesized that changes
on either the WBC transports or water masses characteristics occurring
in this region could impact on the equatorial mass field through
equatorial upwelling with potential consequences on the equatorial sea
surface temperature, air-sea interactions and ultimately on the Pacific
decadal climate variability such as the low-frequency modulation of
ENSO.
To study the Solomon Sea, I used a wide range of methodological approaches:
- Modeling : implementation of a high-resolution nested model to
characterize the circulations and water mass modifications in this
region of very intricate bathymetry using both an Eulerian and Lagrangian approach,
- Altimetry : specifically reprocessed along-track data adapted to coastal areas were used in
addition to standard gridded data to explore sea level and western
boundary currents in this region of high variability,
- Data assimilation : synthetic
glider data were assimilated to explore the potentiality of gliders to
control the thermohaline characteristics of the Solomon Sea and
estimate a key parameter of tidal mixing.
My Solomon Sea research activities are summarized below:
Thermocline
circulation in the Solomon Sea: A modeling study
In the southwest
Pacific, thermocline waters connecting the tropics to
the equator via western boundary currents (WBCs) transit through the
Solomon Sea. Despite its importance in feeding the Equatorial
Undercurrent (EUC) and its related potential influence on the low
frequency modulation of ENSO, the circulation inside the Solomon Sea is
poorly documented. A 1/12° model has been implemented to analyze the
mean and the seasonal variability of the Solomon Sea thermocline
circulation. The circulation involves an inflow from the open southern
Solomon Sea which is distributed via WBCs between the three north
exiting straits of the semi-closed Solomon Sea. The system of WBCs is
found to be complex. Its main feature, the New Guinea Coastal
Undercurrent, splits in two branches, one flowing through Vitiaz
Strait, the other one, the New Britain Coastal Undercurrent (NBCU),
exiting at Solomon Strait. East of the Solomon Sea, the encounter of
the South Equatorial Current (SEC) with the Solomon Islands forms a
previously-unknown current, which we call the Solomon Islands Coastal
Undercurrent (SICU). The NBCU, the SEC and the
SICU participate in the
feeding of the New Ireland Coastal Undercurrent (NICU) which
retroflects to the Equatorial Undercurrent, providing the most direct
western boundary EUC connection, particularly active in June-August.
The Solomon Sea WBC seasonal variability results from the combination
of equatorial dynamics, of remotely forced Rossby waves north of 10°S,
and of the spin up and down of the subtropical gyre as a response of
Rossby waves forced south of 10°S.
Solomon Sea
circulation vertically integrated
over the thermocline
(defined by the 24.0-26.5 isopycnal layer) from the 1/12° model
(1986-2004 mean). Units are m²/s. The main undercurrents are labeled:
NQC: North Queensland Current, NVJ: North Vanuatu Jet, SICU: Solomon
Islands Coastal Undercurrent, SEC: South Equatorial Current, NGCU: New
Guinea Coastal Undercurrent, NBCU: New Britain Coastal Undercurrent,
VS: Vitiaz Strait transport, SGU: St Georges Undercurrent, NICU: New
Ireland Coastal Undercurrent, EUC: Equatorial Undercurrent
The annual cycle
of the thermocline transport in the 1/12° model. The
figure is based on a 1-cpy harmonic analysis of the thermocline
circulation. The area of each vector indicates the magnitude of the
1-cpy harmonic, the direction points along the major axes of the
corresponding variance ellipse (i.e. the direction of maximum variance,
not the direction of mean flow a priori), and the color indicates the
month of maximum transport in the direction of the vector. The choice
of vector direction is arbitrary: each vector could be reversed and its
phase advanced by 6 months, to show the opposite phase of annual
anomalie
Variability in Solomon Sea circulation derived from altimeter sea level
data
The Solomon Sea
is a key region in the Pacific Ocean where equatorial
and subtropical circulations are connected. The region exhibits the
highest levels in sea level variability in the entire south tropical
Pacific Ocean. Altimeter data was utilized to explore sea level and
western boundary currents in this poorly understood portion of the
ocean. Since the geography of the region is extremely intricate, with
numerous islands and complex bathymetry, specifically reprocessed
along-track data in addition to standard gridded data were utilized in
this study. Sea level anomalies (SLA) in the Solomon Sea principally
evolve at seasonal and interannual time scales. The annual cycle is
phased by Rossby waves arriving in the Solomon Strait, whereas the
interannual signature corresponds to the basinscale ENSO mode. The
highest SLA variability are concentrated in the eastern Solomon Sea,
particularly at the mouth of the Solomon Strait, where they are
associated with a high eddy kinetic energy signal that was particularly
active during the phase transition during the 1997–1998 ENSO event.
Track data appear especially helpful for documenting the fine structure
of surface coastal currents. The annual variability of the boundary
currents that emerged from altimetry compared quite well with the
variability seen at the thermocline level, as based on numerical
simulations. At interannual time scales, western boundary current
transport anomalies counterbalance changes in western equatorial
Pacific warm water volume, confirming the phasing of South Pacific
western boundary currents to ENSO. Altimetry appears to be a valuable
source of information for variability in low latitude western boundary
currents and their associated transport in the South Pacific.
This figure shows the signature
for the 1997 El Niño in the Solomon Sea
with a large patch of negative SLA inducing northwestward surface
geostrophic velocity anomalies that increase the North Guinea Coastal
Current (NGCC) to compensate for the depletion of the warm water volume
in the western equatorial pacific. The low pass filtered (half power at
18 months) SLA anomalies during the 1997 El Niño averaged for the July
to December 1997 period are in gray shading. Superimposed are the
corresponding anomalies of surface geostrophic currents. The land and
the first 500 m oceanic depth are in black; the white line delineates
the coastline.
Equatorward pathways of Solomon Sea
water masses and their modifications
The Solomon Sea is a
key
region of the southwest Pacific Ocean,
connecting the thermocline subtropics to the equator via western
boundary currents (WBC). Modifications to water masses are thought to
occur in this region because of the significant mixing induced by
internal tides, eddies, and the WBCs. A high-resolution model
incorporating a tidal mixing parameterization was implemented to depict
and analyze water mass modifications and the Solomon Sea pathways to
the equator in a Lagrangian quantitative framework. The main routes
from the Solomon Sea to the Equatorial Pacific occur through the Vitiaz
and Solomon straits, in the thermocline and intermediate layers, and
mainly originate from the Solomon Sea south inflow and from the Solomon
Strait itself. Water mass modifications in the model are characterized
by a reduction of the vertical temperature and salinity gradients over
the water column: the high-salinity of upper thermocline water
(Subtropical Mode Water; STMW) is eroded and exported towards surface
and deeper layers, while a downward heat transfer occurs over the water
column. Consequently, the thermocline water temperature is cooled by
0.15 to 0.3°C from the Solomon Sea inflows to the equatorward outflows.
This temperature modification could weaken the STMW anomalies advected
by the subtropical cell and thereby diminish the potential influence of
these anomalies on the tropical climate. The Solomon Sea water mass
modifications can be partially explained (~ 60%) by strong diapycnal
mixing in the Solomon Sea. As for STMW, about a third of this mixing is
due to tidal mixing.
T-S diagrams
from the Coriolis database (dots) and from the model (line) upstream
(red, 147°E-153°E, 11.5°S-13°S) and downstream (blue, 143°E-147°E,
3°S-5°S) of the Solomon Sea. Only profiles flagged as "good data" were
extracted from the Coriolis database, from 01/01/1950 to 06/01/2010.
Streamfunction averaged for the surface
(SW, top left), upper
thermocline (TW, top right), lower thermocline (LTW, middle left),
intermediate (IW, middle right) and deep (DW, bottom) layers in Sv.
Contour spacing is 0.5 Sv except for DW (0.1 Sv). For each panel, the
total transports through the Vitiaz Strait (blue), St George's Channel
(green) and the Solomon Strait (red) are mentioned in the upper-right
corner box. The following percentages represent the part of the
transport that is achieved by the south inflow / Solomon Strait
pathways.
Potentiality
of glider data assimilation in the Solomon Sea: Control of the mass
field with some simple scenarios and estimation of the tidal mixing
parameter
Among
the recent ocean observing systems are the steerable underwater
gliders. Gliders were notably deployed in the Solomon Sea to improve
our knowledge of this potentially climatic important region. In this
study, we implemented an ocean observing system experiments methodology
to explore the potentialities of glider data assimilation to control
some characteristics of the ocean, chosen here to be thermohaline
misfits due to an erroneous tidal-mixing parameterization. A direct
exploration of several scenarios of deployment for the fleet of gliders
shows that their ability to control the Solomon Sea thermohaline
characteristics strongly depends on the design of the fleet. As for the
dimension of the array, a fairly good control of the Solomon Sea mass
field can be achieved with a somewhat unrealistic fleet of 50 gliders.
When the observational array is impaired by reducing its size to a more
realistic configuration of 10 gliders, the performance of the control
depends on the space and time distribution of the vehicles. It is
significantly improved when gliders trajectories are coordinated in
order to efficiently collect information-rich data. A substantial
control of the upper-thermocline salinity field, where the errors
reached a maximum, can be achieved in this case. Mass field errors are
further controlled by complementing glider data with synoptic sea
surface temperature data. As a complement, glider data assimilation was
used to directly correct the model instead of continuously
correcting
the state variables. To do so, the erroneous tidal mixing parameter is
estimated through assimilation of data provided by the 10 coordinated
gliders using an ensemble simulations method. This promising strategy
allows an accurate estimation of the parameter and therefore yields to
an efficient correction of the Solomon Sea thermohaline characteristics
errors.
Left:
Geographic
location of gliders as a function of time (colors refer to julian days)
for experiments assimilating data from 50 (top) and 10 (bottom)
gliders.
Right:
Salinity
(in psu) averaged over the thermocline (23.3-25.7 sigma layer) for
January 1993 for the true ocean (top-left), false ocean (bottom-left),
and theexperiments assimilating data from 50 (top-right) and 10
(bottom-right) gliders.
MY FILLING
CABINET