12 Observing the Agulhas Current With Sea Surface Temperature . . .
237
uniform grid and can be downloaded from the Remote Sensing Systems website
4 .
In this study, the AMSR-E data was flagged to only retain valid geophysical data
(values above 250).
In this paper, the circulation in the Agulhas Current region is depicted using
merged maps of the absolute geostrophic currents from the AVISO Maps of Absolute
Dynamic Topography—Delayed Time (MADT-DT) product. The AVISO merged
altimetry datasets are produced by Ssalto/Duacs and distributed by AVISO
5 , with
support from the French Centre National d’Etudes Spatiales (CNES). The MADTDT product combines sea level anomaly signals from the Ocean Surface Topography
Mission (OSTM)/Jason-2, Jason-1 and Envisat altimeters to the Mean Dynamic
Topography (MDT) of Rio et al. (2011). The AVISO MADT-DT product is provided
on a rectilinear grid with a spatial resolution of 1/3
◦ and combines SSH observations
collated by altimeters over a period of 1 week.
12.3 Some Challenges of SST and Altimetry Observations
in the Agulhas Current Region
Over the Agulhas Current core, about 5 times as much water vapour is transferred
to the atmosphere in comparisons to neighbouring waters (Rouault et al. 2000).
The Agulhas Retroflection (Fig. 12.1) also constitutes one of the most significant
region of heat flux loss globally (Lutjeharms 2006). Cloud contaminations can induce significant data loss in IR SST imagery as they obscure the ocean surface,
absorb surface-leaving radiation, and re-emit this energy at lower temperatures. Most
datasets of IR derived SST routinely available have been subjected to automatic cloud
screening procedures to remove contaminated data. Clouds in IR radiometry are associated with colder and more reflective signals. Cloudy regions in IR images also
display more spatial variance. Cloud detection algorithms usually involve masking
pixels in the SST image which are significantly cooler than either neighboring pixels and/or the climatological value (Robinson 2004). In regions of strong thermal
gradients, automatic cloud screening procedures often result in the loss of good
geophysical data.
Monthly climatologies of the number of unclouded SST observations in the Agulhas Current region were derived from the Pathfinder v5.2, Reprocessed MODIS
and OSI-SAF SST datasets to illustrate the importance of cloud contamination in
the Agulhas Current region. Figure 12.2 shows the average number of valid SST observations for the summer and winter seasons and for all 3 daytime IR datasets. The
Agulhas Current core and the Retroflection are poorly sampled by IR SST sensors
due to high level of evaporation (Rouault et al. 2000). The northern Agulhas is one
of the region most affected by cloud contamination. In the northern Agulhas, topographic steering confines the location of the current to the continental shelf break
4 See http://www.remss.com/
5 At http://www.aviso.oceanobs.com/duacs
237
uniform grid and can be downloaded from the Remote Sensing Systems website
4 .
In this study, the AMSR-E data was flagged to only retain valid geophysical data
(values above 250).
In this paper, the circulation in the Agulhas Current region is depicted using
merged maps of the absolute geostrophic currents from the AVISO Maps of Absolute
Dynamic Topography—Delayed Time (MADT-DT) product. The AVISO merged
altimetry datasets are produced by Ssalto/Duacs and distributed by AVISO
5 , with
support from the French Centre National d’Etudes Spatiales (CNES). The MADTDT product combines sea level anomaly signals from the Ocean Surface Topography
Mission (OSTM)/Jason-2, Jason-1 and Envisat altimeters to the Mean Dynamic
Topography (MDT) of Rio et al. (2011). The AVISO MADT-DT product is provided
on a rectilinear grid with a spatial resolution of 1/3
◦ and combines SSH observations
collated by altimeters over a period of 1 week.
12.3 Some Challenges of SST and Altimetry Observations
in the Agulhas Current Region
Over the Agulhas Current core, about 5 times as much water vapour is transferred
to the atmosphere in comparisons to neighbouring waters (Rouault et al. 2000).
The Agulhas Retroflection (Fig. 12.1) also constitutes one of the most significant
region of heat flux loss globally (Lutjeharms 2006). Cloud contaminations can induce significant data loss in IR SST imagery as they obscure the ocean surface,
absorb surface-leaving radiation, and re-emit this energy at lower temperatures. Most
datasets of IR derived SST routinely available have been subjected to automatic cloud
screening procedures to remove contaminated data. Clouds in IR radiometry are associated with colder and more reflective signals. Cloudy regions in IR images also
display more spatial variance. Cloud detection algorithms usually involve masking
pixels in the SST image which are significantly cooler than either neighboring pixels and/or the climatological value (Robinson 2004). In regions of strong thermal
gradients, automatic cloud screening procedures often result in the loss of good
geophysical data.
Monthly climatologies of the number of unclouded SST observations in the Agulhas Current region were derived from the Pathfinder v5.2, Reprocessed MODIS
and OSI-SAF SST datasets to illustrate the importance of cloud contamination in
the Agulhas Current region. Figure 12.2 shows the average number of valid SST observations for the summer and winter seasons and for all 3 daytime IR datasets. The
Agulhas Current core and the Retroflection are poorly sampled by IR SST sensors
due to high level of evaporation (Rouault et al. 2000). The northern Agulhas is one
of the region most affected by cloud contamination. In the northern Agulhas, topographic steering confines the location of the current to the continental shelf break
4 See http://www.remss.com/
5 At http://www.aviso.oceanobs.com/duacs