Monitoring deep convection combining altimetry and
modelling: Application to the Labrador and Mediterranean Sea
Bouffard, J., IMEDEA, SPAIN; Herrmann, M., Météo-France / CNRM, FRANCE; Pascual,
A., IMEDEA , SPAIN; Beranger, K., ENSTA, FRANCE
Abstract
The center of the Labrador Sea and the North Western Mediterranean (NWM) are
characterized by weak stratification and, in winter exposed to intense buoyancy loss due to
atmospheric forcing generating open-sea convections. The Deep Convections (DC) is a keyprocess of the oceanic circulation, costly to monitor in situ and under the influence of climate
change. Our study is a first step toward monitoring DC combining remote-sensing and
models. In this respect, oceanic simulations of the Mediterranean and Labrador circulation
were performed respectively for the 1999–2007 and 1960-2001 period. DC are realistically
modelled, and the Sea Surface Elevation (SSE) is in agreement with altimetric data.
Numerical results show strong correlations (>0.8) between the annual DC characteristics and
the SSE. From that, we propose a simple method to monitor DC long-term evolution using
altimetry. Our method, applied to the longest available altimetric time-series, represents
correctly the variability of DC both in the NWM and Labrador Sea between 1994 and 2008.
1. Introduction
As climate warms, monitoring the health of
Deep Convection (DC) will be integral to
assessing the effects of climate change.
However, direct monitoring requires a
huge investment and can only provide
limited coverage in time and space.
Herrmann et al. (2009) studied the
feasibility of monitoring the interannual
variability and long-term evolution of DC
from space using satellite altimetry. The
authors focused on simulating circulation
in the Mediterranean Sea between 1999
and 2007. This work represented the first
step toward monitoring DC combining
remote-sensing and models. In this paper,
the authors suggested that:
(i)
(ii)
forming longer simulations
would help to reduce the
uncertainty associated with the
method
Broadening this method to
other DC sites of the world's
ocean would be of great interest
for establishing new climatic
indices
of
the
global
thermohaline circulation.
We have followed these requirements by
using oceanic simulations of Labrador
circulation for the 1960-2001 period.
Model results of interannual variability and
sea surface elevations are in good
agreement with altimetry data for the
1994-2001 period and indicated that sea
surface elevations were depressed in areas
where the model showed that deep waters
were forming. The degree of depression
varied from year to year, with lower sea
surface elevations corresponding to
locations where and times when the model
predicted stronger deep convection. On the
basis of this, we proposed that annual
characteristics of deep convection over the
Labrador Sea can be modelled using
altimetry data.
2. Data and model description
2.1 Altimetric data
In this study we have used standard multisatellite AVISO Sea Level Anomaly
(SLA). The corrected Sea Surface Height
(SSH) obtained for each mission has been
inter-calibrated with a global crossover
adjustment of the ERS-2, GFO and Jason-1
orbits,
using
the
more
precise
TOPEX/Poseidon data as a reference (Le
Traon and Ogor, 1998). Next, the data are
resampled every 7 km along the tracks
using cubic splines. A mean profile,
<SSH>, is removed from the individual
SSH measurement, yielding SLA, for the
different missions, where it is defined as
SLA = SSH - <SSH>. The mean profile
contains the geoid signal and the mean
dynamic topography over the averaging
period. For Jason-1 and ERS-2 a mean
profile calculated over a 7-year period
(1993-1999) is used. In terms of GFO, only
several months of data are available, and
for that reason, a specific processing is
applied in order to get mean profiles which
are consistent with Jason-1 and ERS mean
profiles. Finally, the SLA are smoothed
using a median and a Lanczos filter to
reduce noise measurement.
Whereas in Herrmann et al (2009), the
authors used a special high resolution
along track dataset (given that DC over the
Gulf of Lion occurs close to a coastal zone
over a small area) we use in this study
more standard gridded products, easier to
manipulate. The mapping method to
produce gridded SLA fields from alongtrack data is detailed in Le Traon et al.
(1998). This mapping technique consists in
a sub-optimal space-time objective
analysis that takes into account along-track
correlated errors. For each grid point to be
estimated with the objective analysis
scheme, data are selected in a temporal
sub-domain with typical radii of 10-50
days (timescale of oceanic signal). In the
case of delayed-time maps, this data
selection implies considering a centered
time window of along-track data taking
into account both past and future
measurements. Delayed-time maps are
produced every week on a 1/3° Mercator
projection grid.
2.2 Model characteristics (section
adapted from E. Vidal master report)
In this study we have also used the
ORCA025-G70 numerical simulation
developed in the framework of the
DRAKKAR project (Barnier et al., 2006)
aiming at the study of ocean variability
under realistic atmospheric conditions
(from ECMWF/ERA40) over the last half
century (1958-2004). The project has built
a hierarchy of numerical model
configurations from global to regional
scale, each based on the NEMO2
modelling system, which presently
includes the latest version of the primitive
equation, free surface ocean circulation
code OPA9 coupled to the multi layered
sea-ice code LIM2. The NEMO code is a
new version of the OPA primitive
equation, z-level, ocean circulation model.
The model simulates the evolution of
temperature, salinity, velocity, sea surface
height, sea-ice characteristics, and oceanic
concentrations of tracers (CFC11 and C14)
(Barnier et al., 2007).
The ORCA-R025 G70 simulation uses a
global
configuration
of
NEMO
implemented on an ORCA grid at 1/4o
resolution (eddy permitting but not eddyresolving). In an ORCA grid, effective
resolution gets finer with increasing
latitudes. Grid, masking, and initial
conditions are inherited from the global
configuration
of
the
operational
oceanography centre MERCATOR-Ocean.
Bathymetry is derived from the ETOPO2
bathymetry file
of
the
National
Geophysical Data Centre and the GEBCO
for shelf areas. Initial conditions for
temperature and salinity were derived from
the NODC World Ocean Atlas data set for
middle and low latitudes. This model has
been compared to other models at similar
resolution and has generally performed
better. In addition the ORCA-R025
(hereafter ORCA) solution is often
comparable to (or even better than)
solutions obtained at 1/6 o or 1/10 o
resolution in some aspects concerning
mean flow patterns and distribution of
EKE (eddy kinetic energy) (Barnier et al.,
2006).
3. Analysis of altimetric signals
In this study, our main hypothesis lies in
the fact that there should have a close
relationship between the SSE variability
and the DC characteristics at the center of
the Labrador Sea. In this respect, we firstly
proceed to a statistical analysis of the
altimetric signals.
During summer, the altimetric SLA are
globally positive and show across shore
slopes: Positive sea surface anomaly at the
center of the Labrador sea are observed
whereas the values are lower close to the
coast. In a geostrophic hypothesis, it means
that the cyclonic circulation globally
decelerate during the summer (June-July)
toward the mean cyclonic circulation. In
winter, the conclusion are opposite with a
negative across shore slope associated with
acceleration of the cyclonic gyre.
When we look at the SLA standard
deviation (STD) it turns out that in
summer, the maximum of sea surface
variability occurs in the center of the
Labrador (std twice more than along the
coast). This is essentially due to a seasurface interranual signals more intense in
the Labrador basin center. In order to both
analyse the spatial structures and the
associated to the temporal signals we
compute a Singular Value Decomposition
(SVD) of the altimetric SLA.
Figure 1: altimetric SVD (mode 1); top : spatial
structure (with box of DC area inside the
polygon) and corresponding geostrophic
velocity field (black arrows); bottom: associated
time series (raw signal in blue, smoothed signal
in red and derive in black)
The first mode of SVD is significant
(>23%) and shows a strong gradient
between the coast and the centre of the
Labrador Sea. The associated times series
is essentially seasonal and interranual (see
red curve). It is also marked by a tendency
which corresponds to a global increase of
the sea surface over the 15 studied years
(mainly due to the global warming).
Whereas the signals at the coast are
relatively flat, the signal become twice at
the centre of the basin which implies i) a
marked seasonal anomaly slope inversions
in the across shore direction and ii) a
general decrease of this gradient over the
last 15 years.
In a geostrophic hypothesis, this gradient
inversions mainly corresponds to seasonal
and interranual cyclonic circulation
variabilities. In winter, the cyclonic
circulation accelerates (positive anomaly
slope) whereas in summer this circulation
decreases (negative anomaly slope).
Häkkinnen and Rhines (2004) have
previously shown, with satellite and in-situ
data, a decrease of the whole Subpolar
North Atlantic circulation. The analysis of
local surface forcing suggested that a weak
thermohaline forcing in the 1990s allows
the decay of the domed structure of
subpolar ispycnals and weakening of
circulation.
Here, we also observe such a decline of the
geostrophic circulation at the Labrador
basin scale from the beginning of 90’s until
2004-2005. We however note that from
2005 to 2008 the situation become
different, there is a change in the sea
surface height tendency which implies
mainly a new acceleration of the local
cyclonic circulation.
Now, we are going to check if those interannual variabilities are related with the
variability of DC in the center of the Gyre.
For this, we use long-term realistic
simulations from ORCA (see section 2.2).
analysis between SLAbox,summer ( averaged
SLA inside the box during summer) and
respectively Dmax,winter (Averaged depth of
winter convection, inside the same box).
The temporal lag between the SLA
signature (in summer) and the depth of DC
(in winter) has to be investigated (on going
work).
When the modelled SLA long-term
tendency is removed (no influence on the
DC variability) we obtain a correlation
coefficient of −0.88 (with Significant
Level > 0.99). The model therefore shows
that there is a strong linear relationship
between the winter sea surface decrease
and the convection characteristics.
Based on these results, we propose and test
the method described in Hermann et al.
(2008) to monitor DC interannual
variability and long term evolution using
altimetry data by applying the regression
indicators to the whole altimetric series of
SLAbox,summer to estimate the interannual
variability of winter DC in the Labrador
Sea from 1993 to 2008.
5. First Results and conclusion
4. Analysis of model outputs and
deep convection monitoring
DC
interannual
variabilities
are
realistically modelled with the ORCA
model from 1961 to 2001 (ERA40 forcing
period), and the SSE is in agreement with
altimetry data for the period 1993-2001.
Like over the Mediterranean Sea,
numerical results show strong relationships
between the annual DC characteristics and
the SLA signals inside the DC area (see the
box in figure 1). Stronger convection
events lead to more intense cyclonic
circulation. This suggests that there is a
relationship between the annual DC
characteristics and the indicator of the
intensity of the sea surface deepening. We
use the model results to establish this
relationship, performing linear regression
The regression is then applied to the
longest available altimetry series (see
figure 2)
Figure 2: time evolution of spatial-averaged
SLA*(-1) in summer (from 1993 to 2008). In red:
raw signals, in black: smoothed signals
Like shown with in situ data (from Argo
and AR7W survey database) in Yashayaev
et al. (2009), winter convection of 2008
was the largest since the begin of nineties,
this fact is clearly observed in altimetry in
which the spatial-averaged SLA are very
low (see figure 2) and associated with a
strong vorticity (not shown, std > 0.9/s
only for the years 1994 and 2008).
Yashayaev (2009) shows an absence of
major DC in recent years until 2008, the
upper and intermediate layers of the
Labrador Sea have been steadily warming
since 1994 with the exception of a brief
and small interruption in 2000 associated
with a relative small DC that has also been
well reproduced with our methods (see
figure 2).
As a conclusion, the first results are
encouraging and in agreement with in-situ
observations. Like for the Mediterranean
Sea, the methods allow to represents
correctly the interannual variability of DC
in the Labrador Sea between 1993 and
2008. This suggests that altimetry can be
used in complement to models and in-situ
data to provide an index of long-term DC
intensity over different areas of the world.
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