Satellite altimetry dedicated to coastal ocean.
Satellite altimetry is limited near the coastlines due to land contamination in the altimetry and radiometric footprints (10 km and 50 km of footprint diameter, respectively) but also to inaccurate geophysical corrections. Despite this, the altimetric measurements are present and may contain useful information for coastal studies.
This data use case proposes to give some instructions on how to use the Coastal along-track Delayed Time Sea Level Anomaly (CoastalDT-SLA)products distributed by Aviso over the South African coasts, in the Agulhas current.
We will show that more altimetry data are recovered in coastal areas and that this high resolution products allow retrieving small scale oceanic structures, like the Natal Pulse signature. Later, some comparisons with independent data (SAR and in situ measurements: current meters, tide gauges data) will show the good correlation.
The Jason-2 Coastal along-track Delayed Time Sea Level Anomaly (CoastalDT-SLA) product over the Agulhas area is used here.
The CoastalDT-SLA are Level-3 products, conceived as an evolution of the Jason-2 Level-2 coastal products, commonly called PISTACH products. The Level-2 products rather address altimetry experts and are quite difficult to be used by non-expert users.
The CoastalDT-SLA products are easier-to-use and offer high resolution (20-Hz sampling rate) sea surface height on reference tracks. Differently from Level-2 products, the CoastalDT-SLA products are only computed over specific ocean areas (see details on the Aviso website).
Download CoastalDT-SLA data files from the Aviso FTP server for the Agulhas area. An authentication is required to access products. It can be obtained by filling in the form on Aviso and by selecting the CoastalDT-SLA product on the second page. Once connected to the server, files are in the sub-directories named /experimental/coastal_dt_sla_j2.
Ssalto/Duacs Along-Track Sea level anomalies are used here for comparison with the CoastalDT-SLA products. They are also distributed by Aviso and can be downloaded from the same authenticated ftp server with the same login/password (see details on the Aviso website and in the dedicated User Manual). Once connected to the server, files are in sub-directories named /global/dt/upd/sla/j2_cf and files are tipically named dt_upd_global_j2_sla_vxxc_20090805_20090812_20110329.nc.gz
We use the Broadview Radar Altimetry Toolbox to observe the data and do some computation.
For the CoastalDT-SLA product, three Jason-2 passes are used in this data use case:
Ssalto/Duacs Along-track Sea level anomalies used for comparison with the CoastalDT-SLA products contain all the available passes during a 7-days period. The downloaded time series streches out from the cycle_011 (October 2008) to the cycle_110 (2011-07-08). Although BRAT can handle numerous files, it appeared preferable in terms of efficiency for BRAT to perform an extraction of the files based on a portion of the given track. We performed this extraction with the following script, based on the use of the “ncdump”, “ncgen” and “ncea” commands. The latter needs the installation of NCO (NetCDF operators) on the computer. The NCO homepage contains more information. Here, the extraction is done between -45°S and -33°S.
fig 1: Jason-2 ground tracks over the Agulhas area included in the CoastalDT-SLA product.
More altimetry data close to the coasts
Once the relevant files are all downloaded, create a dedicated workspace in BRAT and then, create two datasets in the “Dataset” tab:
- a first one, named “agulhas_coastal_096” includes the CoastalDT-SLA product with only one file: coastal_dt_sla_j2_agulhas_096.nc. It contains all the time serie between 2008-07-17 to 2011-07-07.
- the second, named “agulhas_duacs_096” includes the 101 Duacs SLA files between 2008-10-18 to 2011-07-08.
On “Operations” tab, we create one operation for each dataset, CoastalDT-SLA and Duacs-SLA, in order to plot an hovmöller diagram, i.e a latitude-cycle diagram. These operations are respectively named “Op_agulhas_coastal_096” and “Op_agulhas_duacs_096”.
- Operation “Op_agulhas_coastal_096”: in “Operations” tab, click on “New” button to define a new operation, give the choosen operation name in the “Operation name” box.
In Data expression, drag and drop the variables “cycle” in “X”, the latitude in “Y” and the “SLA_Filtered_41pts_MLE4” in “Data”.
The spatial resolution for this dataset is ~1/106° in latitude. But as the resolution is set to 1/3°, by default, in BRAT, we have to change it to reflect the native resolution of the CoastalDT-SLA product. In “Set Resolution/Filter” (still in the Operations menu on the bottom right), the “Step” in Y resolution sets to 1/99° (note that a such small spatial resolution as 1/106° doesn’t work).
- Operation “Op_agulhas_duacs_096”: The previous Operation can be copied to create the operation corresponding to the “agulhas_duacs_096” dataset. To do that, select the previous operation (“Op_agulhas_coastal_096”) and click on the “Duplicate” button. Change the name to refer to the new operation name (“Op_agulhas_duacs_096”) and be aware to well select the corresponding Dataset with the corresponding Operation.
- In Data expression, drag and drop the variables “cycle” in “X”, the latitude in “Y” and the “SLA” in “Data”. Since the file resolution is different, change the “Step” in Y resolution at 1/20°.
Once each Operation has been defined, click on the “Execute” button.
Once the Operations finished, define one View per Operation in the “Views” tab:
- View “Vi_agulhas_coastal_096”: Drag and drog the corresponding operation “Op_agulhas_coastal_096” from the list of operations in the “Available” box, on the left, to the “Selected” box on the right. Finally, click on “Execute”. For comparison between both dataset, the SLA limits are set to -0.9 and 0.9 m in the “Range” boxes.
- View “Vi_agulhas_duacs_096”: Do the same for the operation “Op_agulhas_duacs_096”.
|fig 2: Latitude-time (cycle) diagram of Sea Level Anomalies, from -35°S up to the nearest point to the coast, for the Jason-2 pass #096. LEFT: CoastalDT-SLA product between August 2008 (cycle 1) and July 2011 (cycle 110). RIGHT: Ssalto/Duacs Along-track product between October 2008 (cycle 11) and July 2011 (cycle 110). The vertical white areas represent missing data.|
The northern limit of the CoastalDT-SLA product reaches -33.32°S whereas it is -33.42°S for the Ssalto/Duacs Along-track product.
Thanks to CoastalDT-SLA product, a better along-track sampling is obtained and more data are recovered in coastal areas compared to the Ssalto/Duacs Along-track product, by roughly 11 km.
Signature of small structures
The aim is to plot the geostrophic currents with hovmoeller diagrams for several passes and retrieve the small oceanic structures near the coast.
Create new datasets in the “Dataset” tab, one per pass:
- The previous dataset defined for the CoastalDT-SLA product, pass #096 is also used here : “agulhas_coastal_096”. It includes the CoastalDT-SLA product with only one file: coastal_dt_sla_j2_agulhas_096.nc. It contains all the time serie between 2008-07-17 to 2011-07-07.
- On the basis of this dataset, do the same for the following passes by creating the datasets “agulhas_coastal_020” and “agulhas_coastal_172”.
|On “Operations” tab, we create a first operation for a given dataset, by filtering the variable SLA. Then, this operation is exported and integrated in a new dataset. A second operation will compute the geostrophic currents from the filtering SLA which will be plotted as an hovmoeller diagram, i.e a latitude-cycle diagram. These steps are detailed below for only one pass. They will be done again for all the passes in order to obtain one plot hovmoeller diagram for each pass.
Once the Operation finished, define a View in the “Views” tab:
fig.3: Latitude-time (cycle) diagram of coastal filtered geostrophic velocity anomalies, perpendicular to the track, up to the nearest point to the coast, for the Jason-2 passes #020 (bottom), #096 (middle) and #172 (top), between August 2008 (cycle 1) and July 2011 (cycle 110). The vertical white areas represent missing data.
The Agulhas current is a major western boundary current flowing in the southwestern Indian Ocean. Near the South African coasts, it follows the narrow continental shelf where is characterized by stable flow conditions. Nevertheless, some variability patterns occurs during the “Natal Pulses”, some solitary cold cyclonic meanders. These meanders are generated at the Natal Bight (off Durban), close to the coast, and move downstream along the Agulhas Current by going away from the coast.
Natal pulses can be identified with satellite altimetry as depressions in the sea surface topography (see also negative sea level anomalies on fig.2, left) and as negative flow anomalies in the geostrophic velocity anomalies, describing southwestward flow anomalies (fig.3). Evidence of such variability are several times plotted on the latitude-time diagrams, in the northern extremes of each figure: around -35.5°S for pass #020, around -34.5°S for pass #096 and around -33.3°S for pass #172. The Natal Pulse is well observed either on the western part of the area (on the pass #020) when it comes off the coasts, or in the eastern part (pass #172) when it flows closer to the coasts.
To assess the quality of the CoastalDT-SLA product, some comparisons with external data have been done (note that these figures have not been plotted with BRAT).
The small scale structures (50 km wavelength) observed with the CoastalDT-SLA product are confirmed by the SAR currents. The coastal anomalies retrieved with CoastalDT-SLA in the last 50 km are confirmed by SAR data.
fig. 4: Comparison of velocity anomalies during cycle 65 (April 2010) between SAR projected velocities and CoastalDT-SLA product, Jason-2 pass #096 (note that the coast is on the left).
The strong negative current anomaly (~ -34.5°S) occured during a Natal Pulse event is coherent in position and amplitude between the CoastalDT-SLA product and ADCP in situmeasurements. The same Natal Pulse event is also observed with the SAR currents the same day (see fig.4).
fig. 5: Comparison of velocity anomalies between ADCP projected velocities (middle) and CoastalDT-SLA product, Jason-2 pass #096 (top and bottom) on April 2010 (note that the coast is on the right).
fig. 6: Correlation between the altimeter data (Jason-2 pass #248) and tide gauge measurements done in Durban (see details on the tide gauge identification, on the Aviso website).
- Other Data Use Case using CoastalDT-SLA products: The Florida Keys by using coastal dedicated products
- User Handbook: CoastalDT-SLA User Manual
- S.Labroue et al.: Level-3 PISTACH Products for Coastal Studies (pdf, Presentation at the San Diego Coastalt meeting, 2011)
- C.Dufau et al.: The PISTACH product: altimeter data for coastal ocean, Presentation at the 2012 EGU meeting (EGU2012-12649).
- J.Tournadre et al.: Agulhas Current Estimation from High Resolution Altimetry (pdf, Presentation at the San Diego Coastalt meeting, 2011)
- F.Collard et al.: Surface Current in the Coastal Region of the Agulhas Current. (pdf, Presentation at the San Diego Coastalt meeting, 2011)