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 Anglique Melet

  Physical Oceanographer

   
 

    CONTACT INFORMATION

mail
Mail     : melet@hmg.inpg.fr

Address   : LEGI, quipe 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 10S, and of the spin up and down of the subtropical gyre as a response of Rossby waves forced south of 10S.

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 m/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 m/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 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.

SLA and geostrophic surface currents during the 1997 El Nino

This figure shows the signature for the 1997 El Nio 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 Nio 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

T-S diagrams from the Coriolis database (dots) and from the model (line) upstream (red, 147E-153E 11.5S-13S) and downstream (blue, 143E-147E 3S-5S) of the Solomon Sea
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 0.3C 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, 147E-153E, 11.5S-13S) and downstream (blue, 143E-147E, 3S-5S) of the Solomon Sea. Only profiles flagged as "good data" were extracted from the Coriolis database, from 01/01/1950 to 06/01/2010.

















pathways





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.glider positions
salinity field in the thermocline

       


  

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.










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