174
M.-H. Rio
Another application of getting absolute estimates of the ocean dynamic topography is the estimation and monitoring of the ocean mass transports all around the
globe.
In areas where the ocean circulation is known to be mostly barotropic, the absolute dynamic topography values obtained adding altimetric Sea Level Anomalies
to the Mean Dynamic Topography, can be used to compute the surface geostrophic
current transport, that, multiplicated by the ocean depth, results in an estimate of the
volume transport. This approximation was made by Saraceno et al. (2009) to compute the time series of the Zapiola anticyclonic flow transport over the 1993–2007
period. They found a mean transport value for the full time period of 50 Sv when
using the Rio et al. (2005) MDT, compared to 40 Sv with the Maximenko and Niiler
(2005) MDT. Using model outputs, Volkov and Fu (2008) found a mean value of
77.5 Sv for the 1993–2006 period.
In areas where the ocean circulation is both barotropic and baroclinic, this
approximation is not anymore valid. However, provided the vertical structure of
the ocean density is known, the geostrophic surface velocity field inferred from altimetric absolute dynamic topography can be used as reference level velocity so as to
reconstruct, through the thermal wind equation, the 3D structure of the geostrophic
flow (Hunegnaw et al., 2009).
Last but not least, a key application of higher resolution MDT is the assimilation
of altimetric anomalies into operational ocean forecasting systems, whose development has been greatly accelerated in the last decade thanks to international projects
as GODAE, MERSEA, or MyOcean.
Studies have been carried out to quantify the impact of using an observed Mean
Dynamic Topography (as opposed to the model mean) to assimilate altimetric
anomalies. For example, a twin experiment was done by the MERCATOR team,
covering 8 months (starting in September 2001) to compare the model outputs (analyses and forecasts) using the model MDT (Reference run) or an observed, Combined
MDT (CMDT run). The Reference MDT is derived from the mean sea surface height
from a forced model run (i.e. no assimilation) covering January 1992–December
1995. The CMDT was computed by (Rio and Hernandez, 2004) through a combination of in-situ data, altimetry and a geoid model. The study showed the strong impact
of a more realistic MDT for getting improved analysis and forecasts. Figure 10.8
shows the Mean Eddy Kinetic Energy (EKE) for the Reference run, the CMDT run,
and from observed drifters in the Gulf stream region. The CMDT enhances eddy
activity suggesting that constraining the modeled mean current to be in the correct
place, allows the model to generate eddies that are more consistent with assimilated
sea level anomalies, thereby enhancing EKE.
A similar experiment was done in the framework of the European GOCINA
(Geoid an Ocean CIrculation in the North Atlantic) project: a specific Mean
Dynamic Topography was computed for the North-East Atlantic region, combining
information from ocean general circulation models, altimetry and gravimetry, and
studies were carried out in three European operational forecasting systems (FOAM,
TOPAZ, MERCATOR) to quantify the impact of using this new MDT when assimilating altimetric anomalies. It was shown that the use of the improved MDT led to
M.-H. Rio
Another application of getting absolute estimates of the ocean dynamic topography is the estimation and monitoring of the ocean mass transports all around the
globe.
In areas where the ocean circulation is known to be mostly barotropic, the absolute dynamic topography values obtained adding altimetric Sea Level Anomalies
to the Mean Dynamic Topography, can be used to compute the surface geostrophic
current transport, that, multiplicated by the ocean depth, results in an estimate of the
volume transport. This approximation was made by Saraceno et al. (2009) to compute the time series of the Zapiola anticyclonic flow transport over the 1993–2007
period. They found a mean transport value for the full time period of 50 Sv when
using the Rio et al. (2005) MDT, compared to 40 Sv with the Maximenko and Niiler
(2005) MDT. Using model outputs, Volkov and Fu (2008) found a mean value of
77.5 Sv for the 1993–2006 period.
In areas where the ocean circulation is both barotropic and baroclinic, this
approximation is not anymore valid. However, provided the vertical structure of
the ocean density is known, the geostrophic surface velocity field inferred from altimetric absolute dynamic topography can be used as reference level velocity so as to
reconstruct, through the thermal wind equation, the 3D structure of the geostrophic
flow (Hunegnaw et al., 2009).
Last but not least, a key application of higher resolution MDT is the assimilation
of altimetric anomalies into operational ocean forecasting systems, whose development has been greatly accelerated in the last decade thanks to international projects
as GODAE, MERSEA, or MyOcean.
Studies have been carried out to quantify the impact of using an observed Mean
Dynamic Topography (as opposed to the model mean) to assimilate altimetric
anomalies. For example, a twin experiment was done by the MERCATOR team,
covering 8 months (starting in September 2001) to compare the model outputs (analyses and forecasts) using the model MDT (Reference run) or an observed, Combined
MDT (CMDT run). The Reference MDT is derived from the mean sea surface height
from a forced model run (i.e. no assimilation) covering January 1992–December
1995. The CMDT was computed by (Rio and Hernandez, 2004) through a combination of in-situ data, altimetry and a geoid model. The study showed the strong impact
of a more realistic MDT for getting improved analysis and forecasts. Figure 10.8
shows the Mean Eddy Kinetic Energy (EKE) for the Reference run, the CMDT run,
and from observed drifters in the Gulf stream region. The CMDT enhances eddy
activity suggesting that constraining the modeled mean current to be in the correct
place, allows the model to generate eddies that are more consistent with assimilated
sea level anomalies, thereby enhancing EKE.
A similar experiment was done in the framework of the European GOCINA
(Geoid an Ocean CIrculation in the North Atlantic) project: a specific Mean
Dynamic Topography was computed for the North-East Atlantic region, combining
information from ocean general circulation models, altimetry and gravimetry, and
studies were carried out in three European operational forecasting systems (FOAM,
TOPAZ, MERCATOR) to quantify the impact of using this new MDT when assimilating altimetric anomalies. It was shown that the use of the improved MDT led to