Mercator Ocean newsletter 05

The MERCATOR
Quarterly newsletter
#5 - April 2002 - page 1
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Implementation of the high-resolution MERCATOR model
for the North Atlantic and Mediterranean regions
by Laure Siefridt, Yann Drillet, Romain Bourdallé-Badie, Karine Béranger, Claude Talandier and Eric Greiner
The PAM system, introduction
The PAM system, experiments and validation
Editorial
Dear Mercatorians,
The time has come to talk about the second MERCATOR
prototype.
2002 marks a significant development with the
commissioning of the PAM model for operational
service. PAM (for Prototype Atlantique Méditerrannée)
offers a description of the North Atlantic and
Mediterranean regions with very high horizontal
resolution. This issue of the newsletter describes how it
is being implemented.
In addition, since the winter is now over, we’ll be
discussing winter convection with a comparison of
winter 2000/2001 and winter 20001/2002.
Naturally we have not forgotten our quiz, which has
been slipped in between the two articles.
Now that we’ve filled you in, sit back, put your feet up
and have a good read!
Contents
Implementation of the high resolution
MERCATOR model for the North Atlantic and
Mediterranean regions
The PAM system, introduction
The PAM system, experiments and
validation
General circulation
The Carribean and the Gulf of Mexico
The Gulf Stream
Circulation in the North; deep convection
and ocean bottom waters
The Northeastern Atlantic; Mediterranean
waters and the Azores front
Mediterranean circulation
Conclusion
References
Quizz
Winter convection: comparative test
Introduction
Labrador sea
Irminger sea and Iceland sea
Rockall-Faroe
Conclusion
Notebook
The first MERCATOR analysis and forecasting
prototype, PSY1-v1 has been providing near- real-time
forecasts since January 2001. Its two-week forecasts,
which assimilate analyses of recent past events, are
available on Wednesday afternoons.
The goal of this first stage was to demonstrate
MERCATOR’s effectiveness for routine, near-real-time
production.
PSY1-v1 was already a very ambitious project. The
new PSY2-v1 prototype will be using a no-nonsense
numerical ocean model offering refined resolution
(~6km) combined with high-quality ocean dynamics. It
will thus be possible to reveal a new range of
phenomena. As a prelude to real-time use, here is a
glimpse of the model’s asymptotic behaviour (spin-up).
The model, named Prototype Atlantique-Méditerranée
(PAM), provides a description of the North Atlantic
(from 9 to 70°N) and Mediterranean regions, with very
high horizontal resolution (a rotated Mercator grid at
1/12° for the Atlantic and 1/16° for the Mediterranean,
i.e. 5 to 7 km). The vertical resolution ranges from 6m
on the surface to 200 m at the bottom of the
Mediterranean and 300 m for the Atlantic. The model’s
bathymetry comes from the Smith and Sandwell data
base, 1997.
It uses the OPA, 8.1 version, numerical code, yielding
vertical Z coordinates, including a rigid lid assumption,
a diagnostic ice condition and a turbulent kinetic energy
mixing system with a 1.5 closure scheme. The PAM
modelling project drew on both the expertise gained
during the CLIPPER research project for high-resolution
modelling of the Atlantic and several tools which were
developed during this project (NATL code at 1/3°,
DIAGOFF diagnostics software, etc).

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Implementation of a high-resolution MERCATOR model for the North Atlantic and Mediterranean
regions (continued)
General circulation
PAM-05 simulates spin-up, in other words the initial
rotation of water masses. The experiment is initialised
from a stable state using as input, temperature and
salinity conditions taken from Reynaud climatology in
the Atlantic, produced for the CLIPPER project, and taken
from the MODB in the Mediterranean. The northern and
southern boundaries of the domain are of the ‘buffer
zone’ type, in other words the model’s temperature
and salinity variables are tied to climatological,
seasonal variables. A lateral free slip is imposed on
land boundaries. Diffusion phenomena are
parameterized by means of biharmonic operators
whose coefficients are set to –9e9 and –3e9 m²/s² for
the dynamics and tracers respectively. Atmospheric
forcing is based on operational analyses from the
European Centre for Medium-Range Weather Forecasts
(ECMWF),
from March 1998 to February 1999.
The atmospheric forcing is applied as a daily constant,
except for the wind which is linearly interpolated for
each time step. A boundary limit of 40 W/m² is set for
the surface at the weekly Reynolds temperature and
for the climatological seasonal salinity value to
compensate for possible forcing defects and to prevent
too great an error occurring in the model.
This article contains a scientific analysis of the long
spin-up of the PAM model (for a total period of 11
years). We compare this with simulations done using
other model configurations (CLIPPER 1/6° ATL6-V4 and
MNATL 1/3° with a southern boundary at 90 North
and 20° South).
The general circulation obtained after a simulation of 8
years of (figure 1) is fairly realistic. A map of mean sea
surface heights is used to reveal the various principal
fronts and currents. We can see the subtropical Atlantic
gyre made up of the northern equatorial current, along
which tropical waters are heated while moving
westward, of the Gulf Stream which carries warm
water towards the North and East, of the Azores
current which continues directly Eastward and the
Canary Islands’ current which flows along the African
coast towards the South. We can also see eddies
forming in the Yucatan straits from the Carribean
current and breaking off in the Gulf of Mexico. As for
the subpolar gyre, it consists of the Gulf Stream and its
north-western extension, of the Norway and Greenland
currents and of the Irminger and Labrador currents,
which bring very cold water to the boundary where the
Gulf Stream separates from the coast.
Figure 2: Barotropic current function calculated during
the eighth year of PAM-05 simulation. Position of
sections mentioned in the article.
Figure 1: Mean height of sea surface (in m) calculated
during the eight years of PAM-05 simulation.
The Gulf Stream transport at 55° West (145 Sverdrups
towards the East through section 3) and the transport
in the Straits of Gibraltar (0.9Sv) are fairly realistic.
Some transports are too weak, as for the Florida
current (25Sv instead of the 30 to 35 Sv observed) and
for the subpolar gyre.
The underestimation of transport in the Florida straits
is a problem shared by the CLIPPER range of
experiments (1°, 1/3°, 1/6°), compounded by other
perturbing factors such as the proximity of the buffer
zone, the small extent of the domain along the
Meridian and the specific forcing applied, which
corresponds to a year of low convection and hence a
weak thermohaline circulation.

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regions (continued)
On a meridian section south of Greenland the mean
transport for a simulation of ten years was evaluated at
30 Sv for the PAM05 simulation, 30Sv for the MNATL
1/3° simulation and 36Sv for the ATL6-V5 simulation.
This could be due to the lager extension of the spatial
domain.
The mixing layer (figure 3) shows fairly realistic values
even though they are too deep in the convection
zones, which is common for OPA code simulations.
The convection is much too deep for the Labrador sea
and the Irminger sea. Nevertheless, in 1998 there was
little convection in these two regions of deep water
formation, due to a much greater net heat flux during
that winter. On the other hand, in the PAM spin-up
simulation, the convection is established in two stages
and it would appear that forcing by daily wind, which is
particularly changeable and high in energy, overcomes
the stabilising effect of heat flux. The combination of
forcing and of the poor restratification during the
summer had the effect of making the maximum value
for the depth of the mixing layer deeper (figure 4),
winter after winter.
As for the Mediterranean, some density profiles for the
initialising climatology data were unstable and thus
contributed to this over-estimated convection. The
values reached in the Gulf of Lions correspond to
observed maxima. Nevertheless these maxima have
never been observed for several years in a row. The
formation of deep water in the Gulf of Lions is thus
abnormally intense. In the Eastern Mediterranean the
depths are no more realistic. The problem has since
been resolved by rigorous checking and adjusting of
MODB climatological data (Beranger K, 2001).
Figure 3: Mean depth (m) of lixing layer from 25th
to
29th
March during the 9th
year of PAM-05 simulation.
Figure 4: Maximum depth of the mixing layer in the
North Atlantic during spin-up.

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regions (continued)
Studies of processes using a simplified configuration
(CLIPPER works) have shown that the TKE convection
scheme probably does not set up deep convection
quickly enough, whence a lack of time for
restratification of waters as indicated by the
climatology. The KPP convection scheme appears to
give more realistic results in this case (Barnier B,
Brasseur P, 2001). This scheme, with a realistic
configuration, will thus be tested soon by the Mercator
and Coriolis Mission group for priority application to the
PAM model in 2002.
The turbulent kinetic energy (figure 5) calculated for
the eighth year of simulation shows values which are
realistic on the whole. For instance, the maximum
values in the Gulf Stream are of the order of 4500
cm²/s². A comparison with Topex/ERS values reveals
losses which are too high at the place where the Gulf
Stream separates from the cost, an anaemic North
Atlantic drift and a lack of relationship with the Azores
current. The two maxima of the Gulf Stream are not
shown. Finally there is no sign of a signal in the buffer
zone up to 20°N.
Figure 5: Turbulent kinetic surface energy in cm²/s²,
calculated for the 2nd
year of PAM-05 spin-up
simulation.
Figure 6: turbulent kinetic surface energy in cm²/s²
calculated for the 2nd
year of spin-up simulation with
smoothed bathymetric data.
The meridian current function (figure 7) calculates
transport integrated by latitude strip and characterised
by the thermohaline circulation of the model. This is in
response to thermal and freshwater forcing as well as to
the transformation of water masses in the ocean. The
North Atlantic is the region on Earth with the highest
rate of formation of deep waters. The intensity and
depth of meridian transport show the different routes
taken by deep waters, the areas where basins spill
over and also the driving, plunging effect of the
subpolar gyre and in particular of the Labrador and
Irminger seas.
As mentioned earlier, the mean annual meridian
transport of the model is fairly low (less than 12 Sv).
Figure 7: Meridian transport (Sv) as a function of depth
for the 8th
year of PAM-05 SIMULATION

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regions (continued)
The Carribean and the Gulf of Mexico
Eddies are created in water carried by the Brazil current in the straits of Yucatan and cross the Gulf of Mexico to
the coast. Their characteristics are satisfactorily modelled by PAM and in particular with a frequency of one to two
eddies per year. It would appear that the blocking of the eddies which has been observed in CLIPPER simulations
but also in those of POP (Smith, R. et al,1999) after a few years of inter-annual simulations are due to insufficient
representation of the West Indies arc. The blocking occurs simultaneously with a transport inversion in the
Windward Passage and a sudden decrease in transport in the straits of Florida. The problem is not found in PAM.
This is due to the 1998 forcing which stands out as the
convection was low in the Labrador and Irminger seas
that year. However, Boning et al. (1995), suggested,
on the basis of numerical studies, that transport of
water mass in the subpolar gyre and the production of
North Atlantic deep waters are controlled more by the
flow of water from the seas around Iceland and
Scotland. These two regions had normal convection in
1998. It is thus not easy to predict the contributions of
convection and the Gulf Stream. To estimate the
effective impact of 1998 forcings on meridian
transport, we used the MNATL-9N model configuration
with a resolution of 1/3° and a boundary at 9°N. Using
this configuration which is similar to PAM, but which
requires less computing time, we did two simulations
while changing the atmospheric forcing value. In this
way we confirmed
that the 1998 forcing gives a transport which is lower
by 2 Sv than that obtained using forcing values
deduced from 15 years of ERA-15 re-analyses.
The 1998 forcing thus clearly has a moderating effect
on meridian transport. We also looked for other
explanations, such as the position of the buffer zone at
9°N or the feeding of the Gulf Stream from the
Carribean. The ATL6-V5 simulation, with a resolution of
1/6° and a southern boundary located at 75°S showed
transport of more than 14 Sv, with a transport
maximum at a depth of 1000m between 30°N and 40°
N as against 25°N and 45°N in PAM-05. The transport
maximum in the deep circulation around 4500 m was
given at about 15°N in ATL6-V4 and at about 30°N in
PAM-05. This last difference in the vertical structure of
meridian transport can be explained by the southern
boundary at 9°N.
Cable measurements of Gulf Stream transport off the
coast of Florida give an annual mean of about 32 Sv
with a strong inter-annual variability, whereas 1998
appeared rather to be a year of intense transport.
The circulation in the Carribean was quite realistic in
PAM05 but showed particularly low transport values
(figure 9) ; it lacked about 5Sv when entering the West
Indies arc and almost 7 Sv when leaving the straits of
Florida.
The 24.9 Sv of PAM-05 may be compared to the 27 Sv
of MICOM 1/12° (Chassignet et al., United States), and
to the 26.7 Sv of POP 1/10° (Bryan et al, 1998, United
States) before the blocking of the eddy of the Gulf of
Mexico and around 26 Sv in ATL6-V5 1/6. It should be
noted that PAM bathymetric data is obtained by simple
interpolation on the grid of Smith and Sandwell’s data
base model, whose spatial resolution is 2 minutes. A
test of the sensitivity of bathymetric data to smoothing
(3 successive Shapiro filters) was made to test the
impact of the crude PAM bathymetric data. This test
yielded very positive results, since PAM transport in the
Florida straits increased up to 26.6 Sv. Moreover, the
simulation indicated
Figure 8: Transport between the Carribean islands
during the 8th year of PAM-05 simulation.
better penetration of the North Atlantic drift (figure 6).
Furthermore, inter-annual simulations revealed
transport which exceeded 27 Sv. However, while this
made it possible to compare PAM’s efficiency to that of
other simulations, the numerical representation of this
transport and the complex geophysical processes which
regulate it, remain an obstacle.

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regions (continued)
The Gulf Stream
The behaviour of the Gulf Stream (figure 10) has been improved in comparison with the MNATL simulations at
1/3° (figure 9) and those of CLIPPER at 1/6°, and it describes a fairly realistic trajectory after leaving the coast.
The meso-scale activity seems to be satisfactory and shows detailed structures, with a realistic number of hot
eddies in the North and cold ones in the South dissipating at times of approximately year.
However, even though the permanent eddy simulated in ATL6-V5 has disappeared, the separation from the coast
has not been entirely revealed. The current seems to be too stuck to the coast and separated too far North. The
resolution achieved does not appears to have solved all problems encountered when modelling the separation
process.
Figure 9: Behaviour of the Gulf Stream at Cape
Hatteras determined by MNATL simulations at 1/3º.
Mean for the period between 15th
and 19th
December
of the 8th
year of simulation.
Figure 10: Behaviour of the Gulf Stream at Cape
Hatteras determined by PAM-05 simulations . Mean for
the period between 15th
and 19th
December of the 8th
year of simulation.
The vertical sections (figure 11) indicate a good
vertical penetration of the Gulf Stream and mean
speeds of the order of 20 cm/s, with, however, strong
attenuation of the vertical temperature gradients
during the simulation.
Figure 11: Annual mean vertical section (8th
year of
PAM-05 spin-up) at 55°W; superimposing of zonal
speeds (m/s) and temperature curves (°C).

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regions (continued)
Circulation in the North; deep convection and ocean bottom waters
The study of a section 1 in the Labrador sea (from 63°W, 55°N to 47°W, 61°N, figures 12 and13) yields a
scenario for the development of convection and the formation of deep waters. Indeed, as of the first year of
simulation, we observe homogenisation of water in a column of almost 1000m in depth. The vertical surface
gradients contract significantly from 0 to 50m as compared with 100 m for the initialisation state in the open
Labrador sea. Apart from the restratification problem which has already been mentioned, as related to the TKE
scheme, the cause is the Westward return current of the subpolar gyre which drives relatively hot and saline
water from the North East Atlantic. This subsurface current rapidly penetrates the Icelandic sea from 200 to
2000m in depth and then invades the Labrador sea from the surface to a depth of 1000m. During simulation the
current stabilised at between 700 and 2500 m in the Labrador sea. Over 8 years the salinity thus increased to the
maximum of 0.15 PSU at 500m and up to 1 PSU at 100m in the Westward return current.
For a section at 53°N (not shown), while the characteristics appear to have been relatively well reproduced to the
East of the mid-Atlantic chain, a homogenisation of the waters at depths between 400 and 2500m may be seen
on the Western half. The gradients are confined near the surface, while at 35° West, a vein of saline bottom
water has disappeared.
As for section 4 at 53°W (figures 14 and 15) and 64°W (not shown), the error after 8 years seems to be slight.
However we can see heating and a significant excess of salt of intermediary waters at a depth of between 800
and 1500 m (increase of 1 PSU at 900 m in depth at 30°N, 53°W, as well as at 30° N, 64°W). The Gulf Stream
appears to be stuck to the coast and may again be seen at about 41°N in the form of a salt maximum between
50m and 400m. This growth of the Gulf Stream pushes slightly saline surface waters at depths of between 100
and 500 m from Labrador (around 45°N) towards the North. On the whole the spatial structure and variability are
still comparable with those of sections covered during the WOCE (World Circulation Experiment) campaigns.
Figure 12: Vertical salinity section in the Labrador Sea
(from 63°W, 55°N to 47°W, 61°N), with Reynaud
climatology.
Figure 13: Vertical salinity section in the Labrador Sea
(from 63°W, 55°N to 47°W, 61°N), after one year of
PAM-05 simulation.
Figure 14: Vertical section of annual mean salinity at
53°W using Reynaud climatology.
Figure 15: Vertical section of annual mean salinity at
53°W during the 8th

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regions (continued)
The Northeastern Atlantic; Mediterranean waters and the Azores front
Figure 16: Vertical salinity sections in the Gulf of Cadiz at 8.3°W using Reynaud climatology (top left), measured
during the Semane mission (top right), simulated by CLIPPER 1/6° (ATL6-V5, bottom left) and by PAM (PAM-05,
bottom right).
In collaboration with the LPO at Brest (Talandier C,
Theetten Set Gaillard F, 2001), data from the Cambios,
Arcane, Semane and Chaos campaigns were compared
for the third and eighth year of simulation.
Vertical sections of observations at 8.3° West (figure
16) revealed two Mediterranean water
veins (750m at 1200m), for which PAM only represents
the upper vein, stuck closely to the coast and for
which, on the contrary, only the lower vein was
revealed using Reynaud climatology. It would appear
that the representation of the upper vein in PAM is due
to a good representation of the bathymetric ridge in
the Gulf of Cadiz.
In ATL6-V5 1/6, due to the existence of an open
boundary in the Gulf of Cadiz, the sections are close to
Reynaud climatology.
Finally, the temperature and salinity sections at 40°N
(not shown here) indicate too strong a penetration of
Mediterrean water towards the West, in PAM. The
thickness of the thread of water is however comparable
to that of the Arcane campaign. Theta/S diagrams
plotted as a comparison with the Semane campaign in
particular, confirm these results; only the uppper
Mediterranean water vein may be seen in PAM while
the ATL6-V5 represents only the lower vein. Significant
drifts of water masses have thus been identified and
we can see a tendency towards homogenisation of the
water
masses, as well as to heating and increased salinity of
the waters between 500 and 1000 m. It should be
noted that this marked tendency of MNATL inter-annual
simulations is however much less significant with PAM.
The main problem is the poor representation of basin
outflow in models with z coordinates. Mediterranean
waters, in spite of the diffusion coefficients used and
attempts to adjust bathymetric data (CLIPPER works),
do not plunge to the right depth. A solution might be to
parameterize the bottom boundary layer as suggested
by Beckmand and Dörsher and currently tested in
CLIPPER configurations, but also to use ‘partial steps’
which are better for representing model's bathymetry
in the last layer of the model.

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regions (continued)
Mediterranean circulation
Figure 17: Horizontal section at 1000m in the salinity
field in March of the 8th
Figure 18: Vertical section through the Meddies as
observed during March of the 8th
year of PAM-05
simulation.
In spite of their poor vertical location (800m instead of
1000m), the PAM Meddies do represent satisfactory
horizontal and vertical structures of these ‘lenses’ of
warm, saline water coming from the Mediterranean
and with a realistic frequency of occurrence.
A technical solution which has given interesting results
consists in restoring the mass towards the climatology
in the Gulf of Cadix. The restoring term increases
linearly from 500 to 1000m in depth then remains
constant until the bottom is reached and is applied to
a half circle with a radius of 3° centred at 8°W. This
makes it possible to restitute realistic amounts of
Mediterranean waters with satisfactory characteristics,
without significantly inhibiting the variability of the
system in this region.
Finally the Azores front transport has been estimated
on the PAM rotated grid using a broken lines method
across a section at 24°W. The Azores current, whose
main transport is restricted to the first 800 metres,
has thus been estimated at an average of almost 5Sv
(and up to 15 Sv) which should be compared with the
Semaphore measurements of 17 Sv. This current has
been located too far North in PAM on this longitude.
Finally, while the Azores front has been correctly
identified to the East of the basin, it does not link up
well with the Gulf Stream.
It should be remembered that convection in the
Mediterranean (figure 3) is significant (and reaches the
bottom) in the Adriatic sea and that it is too deep in
the Levantine basin in which it exceeds 1000m instead
of the 400m as observed. This convection is of course
dependent on the particularly cold and windy forcing
but also on the initial unstable conditions used in PAM-
05. Waters with too much salt thus enter the
Tyrrhenian basin. Consequently we shall not dwell any
longer on the biases and drifts observed in
temperature and salinity in the PAM-05 simulation. We
shall instead describe the general circulation, while
nevertheless noting that the target resolution of 1/16°
is sufficient
for correctly simulating some well-known
characteristics of Mediterranean circulation.
Interesting circulation characteristics are represented
for the Mediterranean. The coastal currents, transports
in straits and the seasonal cycle are fairly realistic. On
the whole (figure 19), the currents in the Western
Mediterranean are well represented. The anti-cyclonic
eddy of the Alboran sea is shown. The Almeria-Oran
front is, however, not represented. The Algerian
current is well stuck to the coast and its meanders
produce anti-cyclonic eddies. It then forks satisfactorily
in the straits of Sicily.

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regions (continued)
Figure 19: Kinetic surface energy in cm²/s² and transports in the various straits in the Mediterranean domain
during the 8th
The liguro-provençal current is too wide and too stable,
which may be related to too great a transport of water
in the Corsican channel (reaching 2.4 Sv in winter) and
hence too much Levantine intermediary water, LIW (1
Sv) formed in the Eastern Mediterranean and entering
into the Western basin.
In the Eastern Mediterranean, the winter circulation
shows a current of Atlantic water which is on the whole
stuck to the coast at 22°E, which appears to match
recent satellite observations but which contradicts
previous circulation schemes plotted from in situ
measurements. This may be partly due to the
representation of the deep convection in the Levantine
basin and in particular the lack of the Rhodes eddy.
This lack is itself probably intensified by the relatively
strong ECMWF 1998-99 winds and relaxation to the
weekly Reynolds temperature (resolution of 1°). One
should however note that the representation of the
mesoscale structures which have been observed such
as the Shik-Mona anti-cyclonic eddy at 34°E and the
cyclonic eddy in the Adriatique.
The circulations which have already been described,
change in summer, both in position and in intensity.
The circulation intensifies in the Aegean sea with an
anti-cyclonic circulation to the North in the Dardanelles
as well as in the Western edge current. These results
fit Kourafalou’s simulation of the Aegean sea
(Communication, EGS, 2001), performed as part of the
MFSPP project. Likewise, in the Adriatic, one can see
the cyclonic circulation of the Southern eddy and the
intensification of the Western edge current.
As far as the transports (figure 18) are concerned, one
may note in particular a good exchange of water in the
Straits of Gibraltar with more than 0.8 Sv whereas
bibliographical values give between 0.5 and 1.5 Sv .
These should be compared with the estimates of 0.8 Sv
during the Canigo campaign (La Fuente et al). In the
Sicilian straits, more than 1 Sv have been simulated
with peaks at 2 Sv. Finally, the transports in the straits
of Bonifacio and Otranto show very clear seasonal
cycles, but the values in the straits of Corsica are a bit
too high (1.1 Sv).

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regions (continued)
Conclusion
The primary aim of the simulation was to prove the feasibility of high-resolution forecasting for operational
applications. The first immediate consequence of this was a few restrictions, such as for the domain (9°N for the
Southern boundary) and for the spin-up (started in 1998).
Once the configuration had been validated and a certain number of resolution parameters calibrated, experiments
were conducted with the model at 1/3° to quantify the impact of the design choices, which were, a priori,
restrictive. Since the choices proved to be relevant, the long simulation was thus undertaken.
Analysis of its results revealed a certain number of errors common to all simulations done with the OPA model,
such as the insufficient plunging of Mediterranean waters in the Atlantic and the non re-stratification of Labrador
waters after the strong winter convection. It also lacked a few specific phenomena such as the transport in the
straits of Florida and the weak meridian current. Finally, while the separation of the Gulf Stream was improved in
comparison with CLIPPER simulations, it appears that the resolution is still not high enough to accurately
reproduce this significant characteristic of Atlantic circulation.
Some partial solutions had been defined during simulation of sensitivity, such as the smoothing of the
model's bathymetry and the use of a three year mean for the forcing in spin-up mode. Other adjustments were
made such as the initial conditions in the Mediterranean, which were stabilised and the introduction in the Gulf of
Cadiz of a zone tied to the climatology in order to better project the outflow of Mediterranean waters.
It was thus possible to obtain state of the art
simulations of results of models with vertical z
coordinates. Finally new paramaterisations were defined
and validated by the research community involved in
Mercator and it will now be possible to apply these to
the PAM model, for instance the parameterising of the
bottom boundary layer and the KPP vertical mixing.
In addition to the necessary adjustments, the current
PAM-05 simulation has a few very realistic
characteristics, which were not in the initial state, such
as the level of turbulent kinetic energy, the African
coast upwelling and the Cape Ghir (Morocco) threads,
the structure of the Meddies and more generally, the
meso-scale activity, which was well established
everywhere and to the East of the mid-Atlantic chain
right to the bottom of the Bay of Biscay. The most
striking of these is the turbulence of mesoscale
structures, typically with vorticity threads (figure 20)
escaping from the eddies. While this is nothing new for
regional simulations, we are in fact entering a new era
for large scale oceanic simulation.
Figure 20: Relative surface vorticity in the PAM-05
simulation.

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regions (continued)
References:
Barnier B, Brasseur P (2001): Modélisation de la couche de mélange océanique et assimilation de donnéees
spatiales au moyen du filtre SEEK dans la configuration Atlantique Nord au 1/3 NATL3. Rapport final avril 2001
Béranger K. (2002): Modélisation aux équations primitives à très haute résolution de la circulation générale de la
Méditerranée. Rapport annuel janvier 2002.
Madec G., Delecluse P., Imbard M., Lévy C. (1998) OPA8.1 ocean generalcirculation model reference manual,
Notes du pôle de modélisation IPSL, 11
Michel S, Messager C (2000): DIAGOFF programme de diagnostics oof-line pour le projet CLIPPER. Manuel
d'utilisation juin 2000.
Reynaud T., Legrand P., Mercier H., Barnier B. (1998) A new analysis of hydrographic data in the Atlantic and its
application to an inverse modelling study, International WOCE Newsletters, 32
Smith, R. D., M. E. Maltrud, F. O. Bryan, and M. W. Hecht, 1999: Numerical Simulation of the North Atlantic
Ocean at 1/10º, submitted to J. Phys.Oceanogr.
Smith W. , Sandwell D. (1997) Global sea floor topography from satellite altimetry and ship depth soundings,
Science, 277
Talandier C, Theetten S, Gaillard F (2001): Validation de PAM-05 en Atlantique Nord-Est. Rapport final juin 2001
Treguier et al. (2001): An eddy permitting model of the Atlantic circulation Evaluating open boundary conditions.
J. Geophys. Res., in press, August 2001
- Quizz -
This is a map of sea level. The geometrical structure is almost perfect. Who created it?
Answer on the following page

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- Answer to the quizz -
The North Equatorial counter current, which moves Eastwards between 4°N and 9°N, depending on the position of
the ITCZ (inter tropical convergence zone), touches the South Equatorial current which moves Westwards around
the equator:
Map showing temperature and current at 100m, on 13-2-2002
In the springtime the Counter-Current is weak and the
Equatorial Current destabilizes it. This creates
meanders with a more or less organised structure. In
some years this structure is almost perfect, as was the
case in 1993 and now in 2002. It is then possible to
observe the meanders from the Brazilian coast to the
African coast, by means of satellite altimetry. To add to
this, here is the vertical temperature section seen by
MERCATOR:
Sea level, analysis on 13-2-2002
Vertical temperature section at 7°N30', analysis on 13-
2-2002
This analysis is similar to that observed in the past
(CITHER 1 sea campaign). The only criticism is that the
thermocline is not pinched enough (the vertical
gradient is too diffuse).
In passing, this date is not only noteworthy for the
meanders of the North Equatorial Counter Current,
since we can also see a clear eddy activity in the Gulf
of Mexico as well as in the region of the Gulf Stream,
with very distinct cold rings (cyclonic eddies) shown in
blue in the figure. The North Atlantic Current has not
been neglected either. Finally, we can even see small
anti-cyclonic structures in the Labrador Sea. This
extremely contrasted circulation thus marks a very
good year; a case study worth remembering.

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Winter convection: comparative test
by E. Greiner, with G. Reverdin
Introduction
Note
In this study ‘MERCATOR analysis’ means analysis which is done on day D for day D-14. It
covers the most observations and is therefore a true ‘re-analysis’. However, in order not to
delay publication of this newsletter, fields for the end of March are considered to be day D
analyses.
The situation shows a Gulf Stream front which is more north-west of 60°W during the winter of 2002. The
subtropical gyre is less intense in the winter of 2002 as shown by the negative difference in sea level. The sea
level of the Atlantic façade is clearly higher during the winter of 2002 to the East of 30°W, from Portugal to the
Faroe islands. The sea level around Norway and the Denmark straits is also higher. The level on the Western
coast of Greenland is much higher. On the other hand, the level of the Labrador sea is lower during the winter of
2002. If we accept the PSY1-v1 analyses, this information on sea level would result in the following surface
currents:
Analysed surface currents: JFM 2002 and JFM 2001
The positions of the Irminger and Islande moorings are shown
For the second winter in a row, the PSY1-v1, the first
analysis and forecast system for the North Atlantic, at
1/3° has been operated in near-real-time. What has
been happening in this new year in the deep
convection regions? This new study was designed to
gather the most recent information (observations and
model results) on this complex topic in order to
determine a few lines for future investigation.
The study began with a map showing changes in sea
level between the winters of 2001 and 2002:
Sea level analysed by MERCATOR:
differences Jan-Fev-Mar 2002 - JFM 2001

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Winter convection: comparative test (continued)
The Azores Current is less intense than in 2002. In
particular, we can see that the cyclonic subpolar gyre
appears to have been split in two around the
Reykjanes Dorsal to the South-West of Ireland. The
Western part consists of the Irminger Current, the
Greenland Current and the Labrador Current. There is
no clear connection with the Eastern part of the dorsal,
as was the case in 2001 by intermediary of the North
Atlantic Current and the Icelandic Current. A first
analysis would appear to suggest that there is more
convection in the West of the dorsal in 2002, with
particular intense Greenland and Labrador currents.
Concerning surface temperatures, here are the
changes found in Reynolds' daily analyses:
Reynolds analyses of surface temperature:
difference JFM 2002 - JFM 2001
MERCATOR analyses of surface temperature:
difference JFM 2002 - JFM 2001
In fact if we smooth the changes in sea level, the
results clearly restitute the change in surface
temperature for the Gulf Stream, the subtropical gyre
and the Atlantic façade. This is normal to the extent
that warmer water is less dense and this expansion
causes a rise in water level: this is what is known as
the ‘steric effect’. This relationship is less clear for the
Norway sea and the region close to the Denmark strait
and the effect is opposite to that found for the
Labrador sea.
The PSY1-v1 analysed surface temperature agrees with
the Reynolds analysis for most points. This is not
surprising to the extent that the Reynolds data are
assimilated in PSY1-v1 in the form of a correction for
heat flux. Even the negative anomalies on either side
of Iceland are to be seen. On the other hand there is
complete disagreement between them for the Labrador
sea. PSY1-v1 follows the satellite altimetry of
TOPEX/POSEIDON-ERS-2 and ignores the Reynolds
surface temperature.

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Labrador sea
If we compare the maximum depths of the mixing layer (the maximum height of homogeneous water from the
surface) it may be seen that in the Labrador sea, convection appears to be as active as in 2001, but with a
different geographic distribution. Moreover, we can see that the convection appears to be weaker in 2002 off the
western continental shelf as well as in the Irminger sea.
Depth of the mixing layer:
maximum during JFM 2002 and JFM 2001
The relevance of MERCATOR analyses in this region is
in question. However, if we compare both analyses
month by month, we can see that the MERCATOR
analyses clearly fluctuate, with small, highly energetic
structures. Inversely, there are no small structures in
Reynold’s analyses, which vary little from one month to
another. It should be remembered that there are not
very many satellite observations for this region in
winter due to the snow cover. Nor are there many in
situ observations and these are localized on the
outskirts of the region. It is thus possible that Reynolds
only partly captures the convective episodes. In this
case, Reynolds data does not change much between
January and February in the Labrador sea, whereas
MERCATOR shows a sudden convective event. More
precisely, the convection ‘explodes’ between the 23rd
and the 28th
January. It may then be seen that the
convection centres turn in a cyclonic direction, driven
by an active Labrador Current: analysed surface currents and depth of the mixing
layer on 6-2-2002

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To determine what (may have) occurred in the Labrador sea around 56°W-58.5°N, we can look at the density
profiles for the begining of January and the end of March. When the convection ends we see that the water
around 300m is lighter than it was at the beginning of January:
Potential density profiles in the Labrador convection zone:
beginning of January 2002 and end of March 2002
According to the MERCATOR analyses, the southern branch of the Labrador Current (which leaves the coast south
of Cape Desolation and approximately follows the 3000m isobath around 61°N) is more intense in 2002 than it
was in 2001. It interacts with the convection centre by carrying waters which are slightly less cold. This
phenomenon was not apparent in 2001:
Potential density profiles in the Labrador convection zone:
beginning January 2001 and end of March 2001

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The T-S diagrams (potential temperature and density) for this zone can also be compared at the beginning and
end of convection for the two years:
T-S diagram in the Labrador convection zone:
T-S diagram inthe Labrador convection zone:
Finally it may seen that in spite of the different scenarios, the results in terms of water masses are quite similar.

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Irminger sea and Iceland sea
One could say the same thing about the two (fictitious) MERCATOR moorings. The first, located to the East of the
Irminger Sea on the Reykjanes dorsal, yields a maximum temperature at about 150 m at the beginning of
January 2002. A similar maximum is found for the second mooring located to the South of Iceland, West of the
Rockall reef . These warmer waters are transported by the North Atlantic Current from the South-East, whereas it
was weaker and located to the East of Rockall-Faroe at the beginning of 2001. These differences in the currents
are less apparent in February to the East of the mid-Atlantic dorsal and, at the end of March, the temperature
profiles for 2001 and 2002 are quite similar.
Temperature time series
(Irmigner sea at top, Icelandic sea at bottom):
JFM 2002 and JFM 2001

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Rockall-Faroe
For the Atlantic façade, the MERCATOR analyses, Reynolds fields and altimeter observations all agree. It is
possible to compare the density profiles for the beginning of January and the end of March in the Rockall-Faroe
region (at about 12°W-59°N).
Potential density profiles in the Rockall-Faroe convection zone:
Potential density profiles in the Rockall-Faroe convection zone:

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Here it is clear that the convection was not as deep in 2002 as in 2001. The T-S diagrams clearly show that there
was more lighter water in 2002. Nevertheless it is not very different to 2001 in terms of water mass.
T-S diagram in the Rockall-Faroe convection zone:
T-S diagram in the Rockall-Faroe convection zone:

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Conclusion
More generally speaking the winter of 2000-2001 was marked by a slightly negative NAO* index (see a
discussion of its impact on winter convection in the Quarterly Newsletter #1). However the ‘90s are noteworthy in
particular for their very positive indices. At the end of December 2001, the NAO index was even clearly negative.
However, since January 2002, positive values have again been noted (with the magnitude varying according to
the calculation method used). On average, the winter of 2001-2002 had a slight NAO+
. The NAO thus played a
moderate driving role on convection in 2002. It would appear that oceanic advection played a more significant
role in 2002 than in 2001 in mixing water masses. There was less formation of deep water in 2002 but it was
nevertheless comparable to 2001.
NAO: The North Atlantic Oscillation is the dominant mode of climatic variability in the
Northern Atlantic. This type of variability is characterised by a reinforcing of the Azores
anticyclone and the Icelandic depression. The Westerly Wind is reinforced thus causing a
warmer and more humid land mass in Northern Europe than on average. At sea there is
intense convection in all zones (Labrador, Irminger, Iceland). The intensity of this
fluctuation can be measured using fairly similar indices, throughout the winter months. The
simplest index measures the difference in atmospheric pressure between the Azores and
Iceland. The negative phase of the NAO is relatively symmetrical with opposite effects (A
colder and dryer Northern Europe). On land, the Loire marks the transition latitude and the
effects are felt as far as Russia and the Eastern Mediterranean.

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- Notebook -
Recommended reading
Mercator gets special coverage in:
Ludions et satellites sondent les caprices
de l'océan (Cartesian diver device and
satellites probe the ocean’s quirks)
Le Monde - 14.03.2002.
"The profession of oceanic forecaster remains to
be invented"... is Pierre Bahurel’s conclusion to
this article describing the indispensable
components of operational oceanography.
Feature article: Mettez vous aux courants
(Go with the flow)
Voiles et voiliers - n° 373 mars 2002.
MERCATOR, which provides surface current
forecasts each week, has been included in this
very comprehensive article on ocean currents.
Editorial board
Corinne Guiose
Eric Greiner
Authors
Article 1: Laure Siefridt, Yann Drillet, Romain
Bourdallé-Badie, Karine Béranger, Claude Talandier
and Eric Greiner
Article 2: E. Greiner
Article 3: E. Greiner, with G. Reverdin
Acknowledgements
We would like to thank all the authors who contributed
to this newsletter and in particular, Gilles Reverdin for
his very welcome assistance.
Next issue: July 2002
Now that we have discussed the PAM model, the
time has come to deal with the second MERCATOR
prototype, PSY-2 and to compare it with the first
prototype, which has been in operation since 17th
January 2001.
Which new fields will be described ? What will the
resolution be? What will PSY-2 contribute? The
answers to all of these questions may be found in
Newsletter #6!
There is no longer any doubt that we have entered
the new era of large scale ocean simulation.
Address
Please send us your comments to the following
e-mail address:
webmaster@mercator.com.fr

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