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By Chris Banks and Valborg Byfield

National Oceanography Centre, Southampton, U.K.
 

The Amazon is the second longest river in the world (after the Nile) and by far the largest when measured by water flow; it accounts for approximately one fifth of all the fresh water flowing into the ocean from the world's rivers. The Amazon river plume is clearly visible in satellite images - even a long way from the coast.

This lesson looks at the inflow of fresh water from the Amazon into the Atlantic. Its main focus is on the use of sea surface salinity data from the ESA Soil Moisture and Ocean Salinity (SMOS) mission. The SMOS data is compared with ocean colour data from the Medium Resolution Imaging Spectrometer (MERIS) onboard ESA's EnviSat satellite.

Bilko screenshot of SMOS data from the Amazon plume   MERIS image of the Amazon plume

Figure 1. The Amazon River Plume.
Left: Bilko display of SMOS sea surface salinity (SSS) showing low-salinity water as blue.
Right: MERIS true colour composite image showing the plume of muddy river water flowing into sea.

IMPORTANT NOTE: This lesson requires Bilko 3.4 from February 2013 or later, as earlier versions of the software can not open and display all data as described in the lesson.

Introduction

Routine measurement of sea surface salinity from space is relatively new, and began with the launch of the ESA Soil Moisture and Ocean Salinity (SMOS) satellite in November 2009. In June 2100, SMOS was joined in orbit by the joint NASA/Argentine Aquarius mission.

Until recently the only way to measure salinity was by direct sampling, using salinity sensors on ships or buoys. These cannot give the full overview possible from a satellite. Even at the surface, there were vast regions of the oceans that were rarely or never observed, and this led to an incomplete picture of global variation. The situation has improved markedly in the last decade thanks to the international Argo programme. Today, over 3000 Argo floats drift freely across the ocean. Every ten days each float measures salinity, temperature and depth in the top 2000 m and sends the signal back via satellite, but with Argo floats typically spaced more than 300 km from each other, in situ measurements of global ocean salinity are still limited. Routine measurement of sea surface salinity (SSS) from satellites is a significant improvement - the first-step towards providing global satellite SSS comparable to the sea surface temperature (SST) data that have been available from satellites for several decades.

One of the most obvious features when looking at SMOS data from the tropical Atlantic, is the input of freshwater from the Amazon River (~0.2 Sv ). This fresh water mixes with salty ocean water and creates a large 'plume' of low-salinity water, which is easy to detect.

The Amazon is responsible for delivering vast quantities of suspended sediment (sand, clay and soil particles) into the ocean. The river water also contains coloured dissolved organic matter (CDOM) from decomposing land plants. These two give the river water its murky, brown appearance. The sediment particles sink to the bottom relatively soon after entering the sea, but the CDOM remains and is slowly diluted as the river water mixes with surrounding ocean water.

The river also brings a steady supply of plant nutrients into the sea. These increase the growth of marine phytoplankton in areas influenced by the river plume. As a result the Amazon river-plume may sometimes be visible in maps of phytoplankton chlorophyll halfway across the Atlantic.

Lesson Overview

Aim and objectives

Using the Amazon river plume as an example, this lesson teaches how to use salinity data from SMOS with ocean colour data from MERIS to study the dynamics of river plumes and their influence on ocean the ocean.

At the end of the lesson you should be able to:

Lesson content

The lesson is divided into three main sections:

  1. Opening and examining SMOS data from the region of interest
  2. Opening and examining MERIS ocean colour data from the same region
  3. Using salinity and ocean colour data to study the mixing of river and ocean water

Data and tools for this lesson

All data used in the lessons were acquired over 6 days in September 2010. Both the salinity data from the Soil Moisture and Ocean Salinity Mission (SMOS) and the optical data from Envisat Medium Resolution Imaging Spectrometer (MERIS) are level 2 geophysical data products, provided in the original format, available to registered users from ESA on-line archives. All the data sets have been compressed, but do not require that you uncompress them before opening these in Bilko.

Files needed to complete the lesson activities

All data and tools needed to complete the lesson activities are listed below. We also provide additional data that may be used to complement or extend the lesson (see alternative and related data sets).

Data in their original format

These data files are in the format used by the data provider. Similar data sets are available from on-line archives, and may be displayed and processed in Bilko using tools and methodologies from this lesson.

SMOS Salinity Data:
MERIS Level 2 Data:

Supplementary and pre-processed data

Bilko tools

Formula documents:
Colour palettes:

Alternative and related data sets

The data sets listed below are not needed for the lesson activities, but are available from the LearnEO! data library for users who wish to extend the lesson activities. They may be displayed and processed in Bilko using tools and methodologies from the lesson, or slightly modified versions of these.

MERIS data

These MERIS images cover the lesson period, and were merged to create the temporal composites used in the lesson.

Downloading the lesson

The lesson downloads contain everything you need to complete the lesson. This includes the data and tools listed above, and three PDF documents:

To download the lesson and data you must be a registered user. If you have not yet done so you can register here. Once your registration is confirmed you can download the lesson after submitting your registered e-mail address below. This will take you to the download page where you can select the documents, tools and data you wish to download.

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