WP1: Physics (draft)
2.1. Physics in the OMZ of the ESP.
2.1.1 Mean oceanic circulation
The OMZ is embedded in the complex Eastern South Pacific Current System. On the large scale this system is
fed from all sides: the South, the North and the off-shore ocean. A rough representation of the water mass influx is the
following: In the south, large scale intermediate water masses, the Antarctic Intermediate Water (AAIW) and the shallower
Eastern South Pacific Intermediate Water (ESPIW; Figure 1) penetrates into the ESP. The ESPIW deepens westward and
northward from its outcrop region (around 32°S) reaching 150 m depth. To the North, the equatorial undercurrent brings
cold waters that spread in the ESP at the subsurface supposedly feeding the coastal currents. To the west the large scale
atmospheric gyre brings waters from the off-shore ocean at the average latitude of ~40°S (the so-called Western drift).
This surface waters split in a northward and southward branches as it arrives close to the Chilean coast.
Figure
1:
(left)
Schematics of the dominant
oceanographic features in the
South Eastern Pacific. The
picture on the right hand side
show the same schematics with
the OMZ superimposed (green
dots). Data are from the World
Ocean Atlas 2001 (annual
mean, z=300m and 400m).
The Northward branch forms the Humboldt current or Chile-Peru current (cf. Figure 2).
Figure 2: Surface circulation from satellite tracked drifters (1979-2004).
Closer to the coast, a strong off-shore current shear exists that is not discernable from altimetric data because of
the rather fine off-shore scale (~200 km):
(i) The CCC (Chile Coastal Current (CCC) and the Peru Coastal Current (PCC) flows northward (Strub et al.
1998) in the near surface layers (0-100 m, 20 cm s-1), with a 100 km width, and its maximum is located at the level of the
thermal front generated by the upwelling of cooler water at the coast (Smith, 1995). Its intensity is maximum in spring and
in summer when equatorward winds are strongest off central Chile. This current transports highly oxygenated surface
waters from the south .
ii) The PCCC (Peru-Chile Counter Current), a southward surface current, was observed offshore, 100-300 km
from the coast (Strub et al, 1995; 1998). Processes explaining the dynamics of this counter-current, of direction opposed
to the density gradient, are still badly known. The wind curl could force the current southwards, in agreement with
Sverdrup dynamics (Strub et al, 1998). The potential role of this current in advecting poorly oxygenated waters from the
equatorial area is unknown.
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iii) More offshore, the southern branch of the anticyclonic subtropical gyre of the south-eastern Pacific, named
the South Pacific Current, flows towards the American coast between 35°S and 48°S. It then separates in two branches,
one directed to the north, forming the Chile-Peru or Humboldt Current, a relatively broad (~1500 km), deep (700-1000 m)
and weak current (5-10 cm s-1) (Reid, 1997; Strub et al. 1998; Chaigneau and Pizarro, 2005a). The Chile-Peru Current,
which seems to delimit the western boundary of the OMZ, feeds the South Equatorial Current and also participates to an
anticyclonic recirculation cell (Chaigneau and Pizarro, 2005c).
iv) The Peru-Chile Undercurrent (PCU) is a well-defined, poleward, subsurface flow, which extends over the
continental shelf and slope off the west coast of South America with a core located between 100 and 300 m depth.
Figure 4 : Section verticale
de la concentration en O2 (gauche) et
du sous courant (droite) à 32.5°S
(Campagne WOCE)
The PCU is undoubtedly the current that is at the centre of the scientific questions addressed in this proposal.
First it is the one that literally connects the tropics with the high latitudes: The PCU transports warm, salty Equatorial
Subsurface Water from the eastern tropical Pacific to at least as far south as 48°S (Silva and Neshiba, 1979). This water
mass in turn is the main source for the coastal upwelling off Peru and northern Chile (e.g. Huyer et al., 1987). Second, it is
apparently associated with the oxygen minimum (Wyrtki, 1963), which makes it transporting low oxygenated waters from
equatorial regions into the SEP. Its dynamics is therefore a crucial issue for understanding the OMZ in the ESP.
2.1.2 Oceanic variability:
Superimposed to the mean circulation, the SEP is characterized by a large variability at a broad band of
frequencies. First of all, as an extension of the equatorial wave guide, the coastal circulation is under the direct influence
of the tropical variability, which includes ENSO (El Niño Southern Oscillation). Atmospheric waves trapped by the Andes
chain propagate southward at a speed of approximately 900 km/day (Ruttlant and Garreau, 1995) and locally force
nearshore oceanic variability at 5 to 10 days time-scale (Shaffer et al, 1997). At lower frequency, oceanic equatorial
Kelvin waves transmit part of their energy to coastal waves when they reach the South American coast. Coastal waves
trapped by topography (Brink, 1982) propagate poleward at 3 m s-1 (Shaffer et al, 1997, Ulloa et al, 2001), on distances
reaching 15000 km. They induce a 50-day variability which influences the undercurrent flow and vertical structure,
managing to reverse the PCU flow northward at certain periods (Shaffer et al, 1997). In addition they induce vertical
displacements of the thermocline and nutricline, which involve fluctuations of temperature and of primary and secondary
productivities (Ulloa et al, 2001). This intraseasonal variability is reinforced during El Niño periods, when equatorial Kelvin
waves are more frequent and intense, and the thermocline is deeper (Ulloa et al, 2001).
On interannual scales, poleward propagation of sea level anomalies at speeds varying between 0.4 and 0.9 m s -1
(Enfield and Allen, 1980) was observed. On its passage, this signal generates westward propagation of Rossby waves
(Vega et al, 2003). Thus, there is a strong dynamical coupling between variability in the tropics and along the Peru-Chile
coasts, at interannual timescales. At higher frequencies, the important semiannual variability presented by the PCU near
30ºS has been also related to equatorial Kelvin waves reaching the South American coast (Pizarro et al., 2002). The
annual component of the Undercurrent variability is related to the alongshore wind stress integral.
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Figure XX: Low-passed time series of
zonal windsterss averaged between 2°S and 2°N
along the equatorial Pacific Ocean (a). Sea level
disturbances from a wind-forced, equatorial
Kelvin wave model (b). Low-passed sea level
anomalies obtained from satellite altimetry along
the equatorial Pacific (c), and at 30°S in the
eastern South Pacific (d). The straight slanted
line indicates the theoretical phase speed of the
first mode long Rossby wave (c = 0.02 cm s-1).
Longitude was reversed to illustrate the
relationship between equatorial and midlatitude
disturbances.
Equatorial
Anomalies
were
averaged between 2°S and 2°N. All time series
were low-passed to remove intraseasonal and
higher frequencies. Note the different color scales
in figures c and d (After Pizarro et al. (2002)).
The modulation of the coastal circulation by the coastal Kelvin wave (connected or not to the equatorial
variability) participates to the change in upwelling conditions along the coast. For example, the influence of the 1997-1998
ENSO on the dynamics upwelling in northern Chile (18°S-24°S) was observed by satellite (Carr et al., 2002). Coastal
upwelling continues, and even could be enhanced during El Niño, but upwelled waters are warmer and nutrient-poor
(Huyer et al., 1987). This impact of the coastal variability on the upwelling varies with latitudes because of the
heterogeneity of the mean stratification along the coast. North of 15°S, for instance, the upwelling is supplied by
equatorial subsurface water which is of high salinity and low oxygen content (Brainard and Mc Lain, 1987) and presents
the highest intensity in the months of May-June and August-September (Zuta, et. al., 1978; Zuta, 1988). Off the northern
part of Chile, upwelling is permanent but weak, reaching a maximum in southern winter and a minimum in southern
summer (Thomas, 1999; Blanco et al, 2001). Between 28°S and 32°S the seasonal cycle is weaker and less extended
offshore, in relation to the thin continental shelf. More to the south, upwelling is more intense and its seasonal cycle more
marked, with a maximum in southern summer. These different upwelling cells have supposedly a different response to
remote forcing originating from the equatorial region in particular with regards to the biogeochemical aspects.
The input of waters from the South is also a source of variability of the upwelling, supposedly at longer
timescales. Indeed the main sources of upwelled waters are the relatively cold and low salinity Sub-Antarctic Surface
Water (SASW), and the warmer and saltier Equatorial Sub-Surface Water (ESSW) which is also characterized by a very
low oxygen and high nutrients contents. These modal waters are subject to climate mode forcing at interannual to
interdecadal timescales.
2.1.3 Key atmospheric features in the Eastern South Pacific
The dominant large scale atmospheric patterns in the ESP
are schematised on figure XX: The subtropical gyre forms the high
pressure zone with a centre of action near (120°W; 30°S). Near the
equator, the associated surface circulation links to the equatorial
trade winds and inter tropical convergence zone (ITCZ). Near the
Chile coast, it connects with the coastal jets that feed the Northern
tip of Chile and Peru with surface dry air. On the other hand, the
South American Monsoon (SAM) brings wet air from inland in the
high atmosphere. The resulting vertical gradient in humidity and the
subsidence over the cool waters participates to the formation of the
Strato Cumulus Cloud (Scu) deck (Ref) that limits the warming of
surface waters through solar radiation forcing.
Figure XX: Large scale atmospheric features (after René Garreaud’s presentation at the CLIVAR Workshop in Concepcion
(October 2005)).
The study of the atmospheric circulation in the ESP is not an objective of PRIMO. This is being addressed in the
VOCALS (VAMOS Ocean Cloud Atmosphere Land Study) program (http://www.ofps.ucar.edu/vocals/). However
considering the active ocean-atmosphere coupling in the ESP, the atmospheric mean circulation and variability needs to
be a major concern of PRIMO, in particular with regards to the modelling activities. Of particular interest is the impact of
mesoscale atmospheric activity in the regional ocean models in order to estimate the off-shore transport of water mass
properties by oceanic mesoscale activity (see section XX -modelling). PRIMO will be also instrumental in connecting the
‘atmosphere’ and ‘ocean’ communities interested in this region. In particular, coastal cruises to be performed in 2007 in
the frame of VOCALS are to be supported by PRIMO (see section – Implementation Plan – VOCAL Costeros).
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2.1.4 Scientific context
Most of the scientific questions related to the physics and addressed in this proposal are orientated towards the
understanding of the OMZ dynamics. As a first step, considering O 2 as a passive tracer, the objective in the physics
perspective, is then to quantify the transport and energy fluxes by the large scale ocean circulation and variability
(advection) and the meso-scale activity. The diffusion and mixing processes may of course modify the pattern of O 2
concentration associated to transport and they will be also studied within PRIMO. Due the close interrelationship between
the large scale circulation and variability and the eddy activity in this region, we rely to a large extent to the information
provided by the regional model that will be developed for PRIMO (see section XX - modelling).
In the following, we list the most relevant issues for understanding the oceanic circulation and variability in the
South Eastern Pacific and to which PRIMO will help resolving.
Large scale:
The South Eastern Pacific is fed from the north by waters of the EUC. From model studies, it is believed that the
EUC when reaching the eastern tropical Pacific split into two branches: one reaching the Ecuadorian coast, the other
going south before the Galapagos (~91°W). The later branch brings poorly oxygenated waters in the South Eastern
Pacific which potentially ‘feeds’ the OMZ. The PUC that connects the EUC to the coastal system is also a source of low
oxygenated waters. What is then the transport associated to these two branches of the EUC? What is also the impact of
the bifurcation of the EUC on the distribution of waters in the ESP?
From the South, isopycnal northward advection of intermediate waters is also a process that may constrains the
characteristics of the OMZ. In particular, the Northward extension of the salinity minimum at subsurface (Figure XX) may
impact the mixing processes at the southern tip the OMZ and limits the poleward extension of the OMZ.
Figure XX: Distribution of salinity (a) and dissolved oxygen
concentration (b) along the WOCE-P19 section off the SouthAmerican coast. STSW : Subtropical Surface Water ; SASW :
Subantarctic Surface Water ; ESPIW : Eastern South Pacific
Intermediate Water ; ESSW : Equatorial Subsurface Water; AAIW :
Antarctic Intermediate Water ; PDW : Pacific Deep Water.
Along with the issue of water mass transformation and formation, the question will be also addressed in PRIMO
from the modelling perspective.
On the large scale, the mean circulation may not be the only contributor to low oxygenated waters. The variability
associated to long-wavelength extra-tropical Rossby waves may also participate to the transport of O 2 through anomalous
off-shore advection. The Peru-Chile coast indeed radiate Rossby waves that can propagate as far as ~1000 km off-shore
(see Figure XXd). The off-shore extension and wave properties are to a large extent constrained by the stratification and
Coriolis parameters. The latitudinal heterogeneity of the wave properties may explain some features of the OMZ
distribution. PRIMO will explore such possibility based on observations (satellite data and historical in situ data) analyses
and model experiment studies. Of particular interest will be the control of the Rossby wave activity and properties by the
equatorial variability at the main timescales of variability associated to biogeochemical processes to take place in the
OMZ (see section XX – BGC). In particular, within PRIMO, we propose to estimate the off-shore and downward energy
flux associated to the extra-tropical Rossby wave at seasonal and intraseasonal timescales.
Regional scale:
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On the regional scale (typically oceanic variability associated to spatial scale < 100 km), the coastal trapped
activity is a major concern of PRIMO considering, first, that it links the study region with the equatorial variability and,
second, that it may have a strong impact on the meso-scale variability (vortex and filaments) through its impact on the
baroclinic instability. Of particular interest within PRIMO is the study of the vertical structure of the coastal trapped waves
and its link with the equatorial Kelvin wave. From observations and model experiments, on-going studies will be pursued
in order to document the propagating characteristics of the equatorial Kelvin and coastal-trapped waves with regards to
the most energetic baroclinic modes. The model platform to study these processes (see section XX - Modelling) will be
used for the interpretation of the data collected during PRIMO (cruise + moorings). These studies should allow to identify
the main mechanisms associated to linear dynamics, a prerequisite to address the issues related to meso-scale activity
and diffusion/mixing.
As mentioned above, the dynamical adjustment of the coastal circulation operates to a large extent through the
turbulent flow. Of particular interest within PRIMO is how the mesoscale turbulent flow relates to the general circulation.
The former has typical Lagrangian time and length scales of around 3-6 days and 30-40 km respectively, with an
elongation in the zonal direction (Chaigneau and Pizarro, 2005a). Energetic mesoscale eddies have been observed in the
region from both hydrographic data (Blanco et al., 2001) and satellite measurements (Hormazabal et al., 2004). They
have a clear signature on the eddy kinetic energy (EKE) calculated from altimeter measurements (Hormazabal et al.,
2004) and satellite tracked drifters (Chaigneau and Pizarro, 2005a), with enhanced levels of EKE near the coast. off
central Chile, between 29°S and 39°S, there is a high eddy kinetic energy level associated to strong but variable
equatorward wind stress. Between 19°S and 29°S, off northern Chile, the circulation is characterized by low eddy kinetic
energy associated to weak but persistent equatorward wind stress forcing. Hormazabal et al. (2004) suggest that the lowfrequency mesoscale variability off central Chile is mainly associated with baroclinic instability of coastal currents and
westward propagation of Rossby waves from the eastern boundary and is strongly modulated over El Niño/La Niña
cycles.
Because mesoscale eddies play an important role in transferring heat and salt away from their region of
formation (Wunsch, 1999; Jayne and Marotzke, 2002), it is believed that mesocale activity is tighly linked to the OMZ
structuring. It is however not clear how its activity relates to the main currents and waves activity. PRIMO will provide
material from measurements and models to address this question and document processes associated with mesoscale
eddies that are relevant to the understanding of the OMZ dynamics (through in particular, the estimate of eddy transports
and lateral diffusion).
2.1.5 Benefits from PRIMO
The data collected during PRIMO will serve as a support to address the scientific issues mentioned above. The
modelling component of PRIMO (see section XX) associated to the ‘historical’ data set and the global and regional
observing systems is the other major support of the proposed tasks from the physics point of you. We detail in the
following the specific benefits from PRIMO for the physics and its expected contribution to on-going projects.
Specific benefits:
-
-
-
Deployment of a deep mooring in the northern part of the region: The mooring to be installed 6 month
prior to the cruise is seen as a major contribution to the observing system in the region. The data
collected with this mooring will allow to monitor the coastal variability associated to equatorial forcing
and to study the sub-monthly variability prior to the cruise. Along with the equatorial Pacific observing
system (TAO array), this mooring is the main indicator of the poleward energy influx entering the
studied region.
The deep CTDs to be realized during PRIMO in the northern part are believed to be essential to the
understanding of the bifurcation of the EUC waters masses.
The deep CTDs will also help to better document the vertical structure of stratification which is
unknown in that region. The only deep profiles are from the WOCE sections. These profiles will be
used in particular to estimate the vertical modes structure to be used in the coastal regional linear
models for the process studies (see section XX – Modelling).
The ADCP measurements will help documenting the vertical structure of the coastal currents and
complement existing data sets.
Contribution to on-going projects:
-
-
A large effort as been devoted in recent years to the understanding of the transmission of the
equatorial signal along the coats of Peru and Chile. PRIMO will benefit to a large extent to the national
initiatives on this matter. At the same time, PRIMO, as an extensive and comprehensive regional
program, is seen as a major support to the on-going observational and modelling studies in providing
a detailled snapshot of the circulation off Peru and Northern Chile. The modelling activities of PRIMO
to meet the biogeochemical objectives (see section XX – BGC) should also be a contribution to the
current modelling projects, in particular from an intercomparative perspective. (Ole?)
VOCALS Costero (air-sea coupling at meso-scale)
Support to the modelling activity (model validation and tuning): specify national projects:
5
o
-
in France: Mercator, INTERUP (intercomparison project of the Eastern Boundary systems to
quantify the coastal and off-shore transport from a modelling perspective)
…
6
PHYSICAL MODELLING SECTION (Old document – To be updated)
2.3.
Modelling
Studies that use sophisticated numerical models of the climate system have become a common tool to improve our
understanding of the spatial and temporal implications of global warming (IPCC, 2001). Estimating the biological response
to anthropogenic forcing and quantifying the contribution of the biological pump on the carbon budget is an important
component of these models. However the resolution of these large-scale climate models is inadequate to resolve and
predict the coastal ecosystem changes, which to this say remain a big uncertainties of the climate change problem.
2.3.1.
Dynamical modelling
Dynamical modelling will cover several space-time scales, from intraseasonal to interannual, and from the large
to the mesoscale, with a hierarchy of models. One of the objectives of the PRIMO program is to understand the dynamical
processes which control interannual, seasonal and intraseasonal variability of the OMZ in the coastal and offshore areas
of Peru and Chile. The coastal variability influences the Peru-Chile Undercurrent (PCU) poleward transport of low
oxygenated waters of equatorial origin. The dynamic large scale variability also may impact on the structure of the OMZ.
Figure XX: Imbrication of the different dynamical models developped for the PRIMO
project.
Process-oriented studies with simple models
Simple process models will be used to investigate equatorial- subtropics connections and wave radiations from
coastal regions. It is well known that equatorial variability propagates as coastal trapped waves along the coasts (Brink,
1982, Shaffer et al., 1997) and that coastal sea level anomalies generate Rossby waves which propagate westward.
Several simple linear models have been used to study this variability at long time scales. These models correctly
7
reproduce interannual variability related to propagation of sea level anomalies along the coast (Clarke and Van Gorder,
1991, Pizarro et al., 2001) and offshore (Vega et al., 2003). Both models are able to describe the vertical structure
variability of the Rossby wave and of the coastal trapped Peru-Chile Undercurrent given a background stratification but
they do not provide a description of the mean state, and remain very sensitive to simplified parameterizations. These
simplified, process-oriented approaches will address specific question oriented to better understand the link between the
variability of the tropical Pacific and the variability of Peru-Chile Current System, and their impact on the OMZ spatial and
temporal variability. They will be useful as benchmark for interpreting more complex model simulations, and data.
The impact of the mean, large and mesoscale, interannual to intraseasonal circulation on the OMZ
One of the objectives of the PRIMO program is to study the mean and variability of the nearshore along-shore
and cross-shore advection, surface and subsurface mixing by eddies,a s to infer their impact on the mean structure and
variability of the OMZ. To attain this goal, a primitive equation model (hereafter PE) will be used.. Using such a model
makes it possible to highlight the potential non linear coupling between the coastal waves, and between the mean and
mesoscale circulation. It allows to study the impact of the intra-seasonal, seasonal and interannual fluctuations of the
atmospheric fluxes on the physical processes that influence the OMZ structure. It gives access to the mean circulation
associated to the horizontal currents and upwelling cells, to the water mass characteristics whose origin is fundamental
for the understanding of biogeochemical processes in the SEP.
Within the PRIMO program, the goal is to model the coastal region dynamics at a high resolution (~5 km), in
order to correctly resolve the cross-shore structure of the surface and alongshore subsurface currents .
Regional dynamical modelling in the SEP will be performed using the realistic numerical simulations performed
over the global ocean. The global model ORCA, developed at LODyC, with resolutions 2° and 0.5°, represents realistic
interannual ENSO-related variability (Lengaigne et al., 2001). This model plays a key role in the modelling of the SEP
since it can provide initial conditions and high frequency (~daily) open boundary conditions to the regional model. Several
interannual simulations, with different horizontal grid resolutions from the global ORCA model, will be available during the
PRIMO program to initialise and force at its open boundaries the regional SEP model (either ORCA 2x2°, or 2°x0.5°, or by
the 0.25°x 0.25° POG of the MERCATOR operational model).
An interannual regional simulation at 1/3° resolution, initialized with the ORCA 2x2° global simulation output, is
already available and is currently analysed. To model the coastal regions at a high resolution (~5 km), ROMS (Regional
Ocean Model System), will be embedded in the regional model. Its parameterizations include a free surface, topography
following "s"-coordinates, and several formulations of the open boundary conditions to be specified offshore.
Two regional model embedments will be performed:
i)
the first regional model (A) of 1/9° (~ 12 km) resolution will be implemented over the region 5°S-40°S,
90°W-70°W, and forced by ORCA 0.5° outputs. This model will represent the regional circulation in
the SEP, its connection with the large scale gyres, and with the equatorial undercurrent.
ii)
a second model (B) of 1/18° (~ 6 km) will be implemented in the near-shore area (within 100 nm),
5°S-40°S, in order to focus on the PCU and upwelling processes. The forcing of model (B) by model
(A) will be performed using the AGRIF software, which enables efficient and straightforward
downscaling from one model (A) to the other (B) (Debreu and Blayo, 1999).
The model at 1/9° will be run over several years in order to study the interannual to intraseasonal variability of
the large scale and mesoscale circulations near the Peruvian and Chilean coasts. Focus will be given on the mean
structure and variability of the PCU, interactions of the PCU with the coastally trapped waves, the offshore propagating
Rossby waves and the off-shore mesoscale circulation. The impact of these dynamical processes on the structure of the
OMZ will be studied in several numerical experiments:
i)
iii)
experiments with passive tracers and/or numerical lagrangian floats simulating suboxic waters of
equatorial origin will be performed, to study the respective roles of coastal waves, horizontal and
vertical advection, mixing by mesoscale eddies, on the OMZ spatial and temporal variability.
the influence of the shallow salinity layer (Schneider et al., 2003) on the stratification near the coast
of Chile and its influence on the OMZ will be investigated. This water mass being formed south of the
OMZ location, it will be introduced as a forcing at the south open boundary of the model in a
sensitivity experiment.
The technical set-up of models (A) and (B), the forcing of model (B) by model (A) with the AGRIF software will be
achieved by V. Echevin (IRD/LOCEAN), in collaboration with P. Penven and P. Marchesiello (IRD/IDYLE, Brest), and L.
Debreu (LMC, Grenoble). The technical work will be supported by a MERCATOR project in 2003-2004 (P.I.: V. Echevin).
The design and analysis of the numerical experiments will be carried out by V. Echevin, B. Dewitte (IRD/LEGOS), M.
Ramos and O. Pizarro (COPAS, Chile) for the Chilean area, P. Penven and P. Marchesiello, I. Puillat (LOCEAN/LEGOS)
and J. Pasappera and A. Ingunza from the IMARPE modelling team for the Peruvian area (model simulations can be
done with workstations both in France and in Peru, and shared via internet).
Models results will be validated with available data analysed as mentioned above,) for the Peruvian area. It
includes, depending of the scales of interest, in situ data and satellite data.
Specific modelling objectives for the PRIMO cruise:
The PRIMO cruise (see 2.3) will document the following two types of dynamic scales :
- The large scale quasi-synoptic structure of the Peru-Chile undercurrent to give access to the mean PCU. Its offshore
extension, its variations in depth and intensity as functions of topography and latitude, have only been measured locally,
from regular cruises at certain fixed latitudes (20.5°S, offshore Iquique, 30°S, offshore Coquimbo). For the first time, these
8
parameters will be measured on a broad meridional extension (10°S-37°S) during the PRIMO cruise. The numerical
model will be run for the PRIMO cruise period. It will be initialised and forced by outputs of the eddy permitting POG
(ORCA 0.25°) MERCATOR model in which satellite and in situ data will be routinely assimilated in 2005. Initialisation of
the model will be performed using a 3D variational initialisation method (Auclair et al., 2000) and implemented for regional
and coastal models in the EC project MFSTEP. Intraseasonal variability produced by coastal trapped waves has been so
far relatively well reproduced by the SEP regional model. The intraseasonal signal will be extracted from the model
outputs and withdrawn from observations after the cruise.
-The meso-scale circulation:
A numerical simulation with the high resolution 1/18° model will provide a three dimensional description of the
meso-scale dynamics in the upwelling cells targeted for the cruise biological fixed stations. Numerical simulations will be
performed to reproduce the annual cycle and then will focus on the cruise period. Sensitivity experiments focusing on the
atmospheric forcing provided by different atmospheric models (NCEP, ECMWF, MM5 (WRF), see next section) or
blended from satellite observations (Chao et al., 2002) will be performed.
- The atmospheric forcing:
Considering the oceanic spatio-temporal scales to be investigated, it is necessary to test the sensitivity of the
regional circulation to the atmospheric forcing. Also, high resolution model in the upwelling region need high resolution
forcing (Capet et al., 2004). This will be done through the use of regional high-resolution atmospheric model outputs in
collaboration with the Universidad de Chile (J. Ruttlant, P. Garreaud) and the Instituto Geofisico del Peru (IGP; Yamina
Silva). Over the last 5 years, the Department of Geophysics (DGF) of University of Chile has been using the PSU-NCAR
mesoscale model (MM5) in studies dealing with weather and climate of South America and the adjacent Pacific ocean;
the IGP is also running MM5 since February 1998. Specifically, MM5 has been used to understand the regional circulation
and precipitation over the tropical Andes (Garreaud, 1999), the atmosphere-topography interaction in central Chile
(Garreaud, 1998; Rondanelli et al., 2003), and the origin and evolution of coastally trapped lows (Garreaud and Rutllant,
2003).
Regarding the PRIMO program, a two-way interaction between the modeling and the observational components
is foreseen. Given the general lack of atmospheric data off the Chile-Peru coast, the PRIMO cruise data, combined with
coastal and island data will provide a unique dataset to validate a number of mesoscale features that so far have only
been documented in regional simulations using MM5. Moreover, MM5 results provide a high-resolution, physically
consistent "data" set, which has been useful in the description of the regional circulations (e.g., ocean surface winds,
cloud cover). Furthermore, the model simulations provide fields that are difficult to measure (e.g., vertical velocity, surface
fluxes), but are key elements for specifying consistent air-sea interactions.
In addition to the operation of MM5 in “research mode” for diagnosis studies, MM5 will be run in real time to
provide relevant guidance during the PRIMO cruise. Currently, at DGF, it is run in real-time, with one cycle per day
(initialized at 0000 UTC) and integrated for the next 72 hr. The current configuration features 3 nested domains, with the
finest one (15 km resolution) centered over central Chile. This inner domain, will be displaced and duplicated during the
PRIMO cruise to the coast of northern Chile – southern Peru. At IGP, MM5 is running operationally for 60 hr forecast with
3 nested domains: for south America with 54 km, Peru with 18 km and Lima with 6 km resolution. This task: J. Ruttlant, R.
Garreaud, B. Dewitte, Lionel Renault (PhD Student - CNES-IFREMER grant starting in November 2005).
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