242
M. Krug et al.
2002). Since the early 1990s, there have been a minimum of two altimeters in space.
The AVISO data presented in this chapter merges SSH observations collected from
the Jason-1, Jason-2 and Envisat altimeters. Ascending and descending tracks from
these 3 altimeters over a period of 7 days have been plotted in Fig. 12.4 to illustrate
the level of spatial and temporal smoothing required to produce maps of the oceanic
surface circulation from multiple altimeters.
Like microwave radiometers, observations from radar altimeters suffer from
contamination near the coast. Land contamination directly corrupts altimeter measurement, resulting in a loss of data near the coast. Inaccuracies in the wet
tropospheric correction also restrict the use of altimetry near the coast. Some altimeters rely on an on-board microwave radiometer to correct for the presence of
water vapour in the atmosphere. Since microwave radiometers typically have a footprint size of 30 km and can not accurately measure within 1.5 foot print size from the
coast, accurate observations from altimeters can not be obtained within about 50 km
of the coast (Vignudelli et al. 2011); models can be used to fill this gap but usually
lack the shorter spatial scales of variability of water vapour.
Additional challenges faced by altimeters in the coastal regions are their inability
to resolve high frequency signals from tidal or atmospheric forcing in the coastal
regions. Finally, altimeters measure SSH variations in reference to a rough approximation of the Earth’s surface, called the reference ellipsoid. To study ocean
circulation from altimetry, it is necessary to refer SSH measurements to the geoid
rather than to the reference ellipsoid. Recent measurements of the Earth gravity field
from the highly successful Gravity Recovery and Climate Experiment (GRACE)
and Gravity field and steady-state Ocean Circulation Explorer (GOCE) missions
(and their combination) are able to resolve the geoid over length scales of the order
of 100 km; Janji´ c et al. 2012), while global gravity models resolve the geoid with
a resolution of a few 100 km (Rio and Hernandez 2004). Assuming the geoid is
stationary, the time varying part of the ocean circulation can be reproduced by subtracting the mean SSH and working with height anomalies. However this procedure
also removes the MDT which has a strong signature in western boundary current
regions such as the Agulhas Current (Byrne and McClean 2008). The CNES-CLS09
MDT of Rio et al. (2011) which is integrated in the AVISO MADT-DT dataset presented here, is a hybrid MDT. It makes use of extended datasets of drifting buoy
velocities (1993–2008) and dynamic heights (1993–2007) and adequately captures
the time-averaged circulation of the Agulhas Current (Rouault et al. 2010).
The Agulhas Current region is a challenging region for space-based observations
of SSH and SST. Cloud contamination and inadequate cloud masking procedures
severely impact on the quality and density of IR SST observations over the Agulhas
Current, particularly in the northern Agulhas region. Microwave radiometers provide
a good alternative to IR sensors in the southern Agulhas and the Retroflection area
but are not able to image most of the northern Agulhas Current. The proximity of the
Agulhas Current to the coast from Durban to Port Elizabeth and its fairly invariant
path (Gründlingh 1983) also limit the use of merged altimetry products in the northern
Agulhas Current region. In Sect. 4, we demonstrate how despite their limitations,
altimetry and SST observations can still be combined to improve our understanding
of the northern Agulhas Current dynamics.
M. Krug et al.
2002). Since the early 1990s, there have been a minimum of two altimeters in space.
The AVISO data presented in this chapter merges SSH observations collected from
the Jason-1, Jason-2 and Envisat altimeters. Ascending and descending tracks from
these 3 altimeters over a period of 7 days have been plotted in Fig. 12.4 to illustrate
the level of spatial and temporal smoothing required to produce maps of the oceanic
surface circulation from multiple altimeters.
Like microwave radiometers, observations from radar altimeters suffer from
contamination near the coast. Land contamination directly corrupts altimeter measurement, resulting in a loss of data near the coast. Inaccuracies in the wet
tropospheric correction also restrict the use of altimetry near the coast. Some altimeters rely on an on-board microwave radiometer to correct for the presence of
water vapour in the atmosphere. Since microwave radiometers typically have a footprint size of 30 km and can not accurately measure within 1.5 foot print size from the
coast, accurate observations from altimeters can not be obtained within about 50 km
of the coast (Vignudelli et al. 2011); models can be used to fill this gap but usually
lack the shorter spatial scales of variability of water vapour.
Additional challenges faced by altimeters in the coastal regions are their inability
to resolve high frequency signals from tidal or atmospheric forcing in the coastal
regions. Finally, altimeters measure SSH variations in reference to a rough approximation of the Earth’s surface, called the reference ellipsoid. To study ocean
circulation from altimetry, it is necessary to refer SSH measurements to the geoid
rather than to the reference ellipsoid. Recent measurements of the Earth gravity field
from the highly successful Gravity Recovery and Climate Experiment (GRACE)
and Gravity field and steady-state Ocean Circulation Explorer (GOCE) missions
(and their combination) are able to resolve the geoid over length scales of the order
of 100 km; Janji´ c et al. 2012), while global gravity models resolve the geoid with
a resolution of a few 100 km (Rio and Hernandez 2004). Assuming the geoid is
stationary, the time varying part of the ocean circulation can be reproduced by subtracting the mean SSH and working with height anomalies. However this procedure
also removes the MDT which has a strong signature in western boundary current
regions such as the Agulhas Current (Byrne and McClean 2008). The CNES-CLS09
MDT of Rio et al. (2011) which is integrated in the AVISO MADT-DT dataset presented here, is a hybrid MDT. It makes use of extended datasets of drifting buoy
velocities (1993–2008) and dynamic heights (1993–2007) and adequately captures
the time-averaged circulation of the Agulhas Current (Rouault et al. 2010).
The Agulhas Current region is a challenging region for space-based observations
of SSH and SST. Cloud contamination and inadequate cloud masking procedures
severely impact on the quality and density of IR SST observations over the Agulhas
Current, particularly in the northern Agulhas region. Microwave radiometers provide
a good alternative to IR sensors in the southern Agulhas and the Retroflection area
but are not able to image most of the northern Agulhas Current. The proximity of the
Agulhas Current to the coast from Durban to Port Elizabeth and its fairly invariant
path (Gründlingh 1983) also limit the use of merged altimetry products in the northern
Agulhas Current region. In Sect. 4, we demonstrate how despite their limitations,
altimetry and SST observations can still be combined to improve our understanding
of the northern Agulhas Current dynamics.