6. Ocean Eddies: Lesson Overview

Aims and objectives

This lesson looks at some of the techniques currently used in the study of ocean eddies. Using eddies in the The Somali Current system as an example, the lesson demonstrates how Sea Surface Temperature (SST) data and Sea Surface Height (SSH) data may be used synergistically to provide information about current speeds and direction.

The data you will use is Level 2 data: geophysical parameters derived from the satellite measurements, but still in the satellite projection. In level 3 data sets Level 2 images have been resampled to a common grid, and merged to create a composite usually containing the mean of the images used to create the composite. Quality control of such data, for example by applying flags must be carried out before the data can be resampled in this way. The lesson shows how this may be done.

SST and chlorophyll concentrations from ocean colour are both commonly used to study productivity along ocean fronts and in eddies. ESA's Envisat has the advantage for studies such as this that altimeter data, sea surface temperature and ocean colour data were obtained from the same location at the same time. The final part of the lesson therefore looks at how ocean colour may be used in synergy with the other data sets.

By the end of this lesson you should be able to:

• apply land and cloud masks to SST data from AATSR,
• combine several AATSR images to create a gridded composite,
• open along-track Sea Surface Height data from RA-2,
• calculate the Sea Surface Height Anomaly (SSHA) from the RA-2 data,
• display RA-2 SSHA data over SST images.
• display and animate a sequence of stacked SSHA images
• display MERIS data
• interpret the SSHA, SST and MERIS images in the light of present-day knowledge about ocean eddies.

Lesson content

1. Preparing AATSR images for a composite
2. Creating an SST composite
3. Temporal resolution of composites
4. Calculating Sea Surface Height Anomaly (SSHA)
5. Viewing Maps of SSHA: The DUACS dataset
6. Comparing MERIS and AATSR data
7. Summary and conclusions

Images used in this lesson

• ATS_NR__2COUPA20040220_~.N1 Level 2 images showing sea surface temperature (SST), cloud top temperature and land temperature derived from measurements made by the AATSR sensor onboard ESA's Envisat satellite on 20th Februay, 2004.
• ATS_NR_200402**r.dat Nine AATSR images from February 2004, which have been masked and geocorrected in advance using Bilko. You will use these with the image above to create your own composites.
• RA2_GDR_2POF-P20040220_~.N1 Vector-data from the Radar Altimeter 2 (RA-2) onboard Envisat, obtained on February 20th, 2004, from the same orbit as the one containing the AATSR image above.
• msla_Jun&Jul03.nc and msla_Feb&Mar04.nc NetCDF files downloaded from AVISO's live access server. These contain maps of Sea Surface Height Anomaly (SSHA) created by combining data from all the satellite altimeters and interpolating between them. The two data sets are from June-July 2003 and February March 2004 respectively.
• MER_RR__2PRBCM20040220_~.N1 Level 2 images showing water leaving radiances, chlorophyll concentrations, yellow substance (CDOM), and suspended particulate concentration derived from measurements made by the MERIS sensor onboard ESA's Envisat satellite on 20th Februay, 2004.
• MER_RR__1PRBCM20040220_~.N1 MERIS Level 1 (top of atmosphere) radiances from which the Level 2 image has been calculated.
• MER_NR_200402**r.dat A series of MERIS Level 2 images from February 2004, which have been masked and geocorrected in advance using Bilko. You will use these with the image above to create your own composite.

Next: 6.1 Preparing AATSR images for a composite