Overview of remote sensing, sea-level monitoring and numerical by vqx13199


									      9609 VOL II/III

  Overview of remote sensing, sea-level monitoring and numerical
   modeling needs and capabilities in Africa with emphasis on the
                        Western Indian Ocean


                            S. Ragoonaden

               A contribution to the GOOS-Africa meeting
                    (Nairobi, 19-23 November 2001)

October 2001


This paper provides an overview of the status of remote sensing, sea level and
modeling activities in Africa in general and Western Indian Ocean in particular.

Sea-level network has expanded significantly following the implementation of the
10-year TOGA programme. However, since its termination in 1994/1995, the
network has been degrading. A strategy should be developed to rehabilitate and
strengthen the network.

With regards to remote sensing, some development is taking place. But still much
remains to be done to develop the capabilities and enhance capacity building in the
region to ensure optimum utilisation of this modern technology for socio-economic

Modeling is still in its infancy. Products of interest to the region are mainly derived
from Global Numeral Models and are too coarse for practical application. Regional
models with higher resolution should be developed

An overview of remote sensing in the field of oceanography, sea level monitoring
             and modeling for coastal and marine environments

A1        Remote Sensing

Remote-sensing techniques are widely and increasingly being used to assess the status
of coastal and marine environment. It is also utilized as a basis for coastal and marine
planning and for the conservation, management, monitoring and evaluation of marine
resources. It enables a synoptic coverage of large area, which is repetitive, reliable
and available in real time for both operational, research and educational purposes.
Remote sensing and satellite oceanography have the capabilities to provide on a
synoptic scale, data and information for, inter alia, identification of potential fishing
areas, depicting large scale coastal and ocean processes, water quality monitoring and
pollution detection, climate variability and change studies and primary production.

    Some of the main satellites in orbit around the Earth are: -

•     The LANDSAT TM
•     The METEOSAT
•     NOAA
•     SPOT
•     ERS I and ERS II
•     Indian Remote Sensing Satellite (IRS)
•     Japanese Remote Sensing Satellite (JRS)

Satellites planned for launching are: -

•     JASON - I
•     METEOSAT second generation satellite (MSG)

The repetitivity and resolution of all satellites differ. Some examples are as follows

METEOSAT and NOAA                                 twice daily with ground
                                                  resolution of 1 km

LANDSAT T.M                                       twice monthly with ground
                                                  resolution of 10 m

SPOT, IRS, JRS                                    once weekly with ground
                                                  resolution of 10m

ERS 1 and 2                                       once every 30 or 35 days with ground
                                                  resolution of 12.5m

A2       Application of remote sensing

Some of the marine areas where remote sensing could be beneficially used are:

     •   Coastal zone management

         -    Monitoring coastal ecosystem such as coral reefs, lagoons, estuaries,
              wetlands, mangroves
         -    Monitoring of coastal geomorphology including beaches, sand dunes,
              and drainage.

     •   Marine resources

         -    Fishery potentials in deep sea fishing grounds
         -    Marine aquaculture
         -    Marine mineral resources

     • Ocean Monitoring

Basic observation and measurement from space include sea-surface temperature, sea
level topography, ocean colour, and roughness.

A2.1     Sea Surface Temperature

This is the most common and useful element measured by remote sensing for practical
application. Both polar orbiting (eg NOAA Satellite) and geo-stationary satellites can
monitor sea surface temperature.

Sea surface temperature charts used to be produced on a daily basis by Reunion within
the framework of the Indian Ocean Common Tuna project and distributed in real time
to all participating countries. The University of Cape Town prepares regularly SST
charts based on the AVHRR NOAA images and disseminates them to marine users
and fishing companies to enhance fish catch.

A2.2     Sea-level topography

Satellite altimetry - the measurement of sea surface height from space programme was
initiated in the 1970’s with the launching SEASAT altimeter satellites followed by
GEOSAT in 1985 and ERS-1.But sea-level measurement did not meet the
requirements for regional or global sea-level topography mapping. In the early
1990’s, microwaves with SAR (Synthetic Aperture Radar) came into use on satellites
of ERS 1 and 2.

The TOPEX - Poseidon launched in 1992 ushered in an entirely new era of satellite
altimetry, giving sea-level measurement accurate to 3-4 cm. The data have
demonstrated that very precise measurement of mean sea level can be made using
satellite altimeters.

A2.3   Ocean-colour

Ocean-colour images can provide quantitative assessment of the chlorophyll - pigment
that gives an indication of the availability of food at the primary level and organism at
higher trophic level including fisheries. Suspended sediments can also be detected.

SEAWIFS sensors placed on the board the sea star satellite, launched in 1997 by
NASA, now provide ocean colour images on a routine basis. It enables mapping of
phytoplankton, pollution monitoring, algae bloom and ocean currents variability, to be
monitored. Data can be made available within two hours for operation use.

Some studies using ocean colour images have been done in Southern Indian Ocean,
which is usually described as a biological desert to identify seasonal and inter annual
changes in phytoplankton. It has been found that the region is not after all a desert.
Areas of relatively higher primary production were identified.

In particular, it was found that there is a strong seasonal relationship of the primary
production with sea surface temperature and sea surface height. Primary production is
observed to relatively increase during the winter month as a consequence of an
increase in the Southeast trade winds.

A2.4   Waves

Wave height can be computed from sea roughness observed from satellites. A
synoptic view of wave distribution over a basin wide area can thus be obtained.

A2.5   The PUMA project

The EUMETSAT series of geostationary meteosat satellites have provided images of
the Atlantic and Africa for years now. The Meteosat 5 was shifted on its orbit about
two years ago to a position in the Indian Ocean to ensure satellite weather coverage of
the Indian Ocean.

A new generation of satellites (Meteosat Second Generation) will be launched in 2002
to provide digitised images at fast speed and new products. Besides cloud images,
ocean parameters such as sea-surface temperature, waves, ocean winds will be made
available. It will have 12 channels for collection and transmission of data and
information. A pilot project involving 7 countries of the region will be implemented
at the beginning of 2003 with funding from the WMO Trust Fund. It will include the
acquisition of satellite receiver, their installation and operation and training of
technician for maintenance purposes. The project will be extended subsequently to
the remaining 46 African countries with funding from FED.

EUMETSAT has stressed that optimum use should be made of the data and products
by a maximum of users, both from the meteorological and oceanographic

A3     Remote Sensing Centres in the region

Two remote sensing receiving Ground station with processing facilities have been
established for Eastern and Southern African Region namely Nairobi (Kenya) and
Johannesburg (South-Africa).

                              GROUND STATIONS FOOTPRINTS

 Two stations with only satellite image processing facilities are located at Mauritius
and Reunion.
A3.1 Nairobi Remote Sensing Centre

The Regional Centre for mapping of resources for Development (RCMRD) is located
at Nairobi. Though marine resources are one of the services provided, emphasis is
usually on land resources.Services provided include physical planning, water
resources, agro-forestry and land use. It provides facilities for 18 countries in the

A3.2 Satellite Application Centre, Johannesburg

This centre is under the responsibility of the South-Africa Council for Science and
Industrial Research. Images, which can be downloaded, include SPOT, LANDSAT
and NOAA. Focus is usually on the mapping of land resources. Ocean products are
quite limited.

A3.3    Mauritius Satellite Image Processing Station

This station was established in collaboration with India. It is equipped with Digital
Image Processing and Visual Interpretation facilities. Appropriate hardware and
software are available for processing SPOT, IRS and LANDSAT images.

Images available for processing include SPOT images (1994, 1999 and 2000) and IRS
(1996 to 1999). Synthetic aperture images from RADARSAT (1998 and 1999) are
also available but no facilities at present exist for processing to derive ocean data
(waves, topography).

A receiving ground station is being planned during its second phase of development.

A3.4   Reunion remote-sensing station

Sea surface temperature charts using AVHRR Satellite images of NOAA used to be
produced in real time and distributed on a daily basis to participating countries within
the framework of the Indian Ocean Commission Tuna project. This programme
terminated in 1999.

A4     Airborne Imaging Spectrometric Imager

This equipment can be placed on board an aircraft to acquire several spectral bands of
imagery at 4m-ground resolution to map coastal lagoons and coral reef.

A project to map the coastal ecosystem of Mauritius using a digital approach
involving objective, multi-spectral classification was implemented in 1999. The
airborne package used was based on an Itd Instruments Ltd compact Airborne
Spectrographic Imager (CASI), which acquired multi-spectral imagery for processing.
One hundred and fifteen flight lines were acquired on eleven flights to cover the
whole coastal region of Mauritius.

A map of the Southwest coast of Mauritius with all the resources and ecosystem is
given below.

These CASI images are proving very valuable for coastal resources management and

A5     Proposal

Enough attention has been given for the generation of regional ocean products from
ocean satellites. It is proposed that one Centre be upgraded or established in the region
to receive and process images and prepare oceans products for dissemination to
countries in the region for national adaptation.

B1     Sea level monitoring

Tide gauge stations were mainly established in the early days on the coast of Africa
for navigation purposes to monitor tidal variability. Records show that sea level
measurement started as early as 1926 at Durban, Mombassa (1932) and Port Louis
(1942). However, as the tidal range was quite small around Africa and the tidal
regime was well documented, maintaining the sea-level station was given low priority.
The number of stations decreased gradually to reach only a few in the 1970’s and early
1980’s. A revival in interest in sea level data took place in the mid-1980’s with the
implementation of the 10-year WMO/IOC Tropical Ocean Global Atmosphere
programme in 1985 and the establishment, at the same time, of the Global sea-level
observing system (GLOSS). Policy-makers became then conscious of global warming
and the need to monitor sea-level rise. Within the framework of GLOSS, 300 sites
were identified around the globe and plans to upgrade existing stations and install new
ones were initiated. In the western Indian Ocean, the number of stations increased
from a few in the early 1980’s to nearly 20 in the second half of 1980’s with financial
support in the context of the TOGA programme and technical assistance from the
TOGA Sea-Level Centre, University of Hawaii.

B2     Sea-level measurement and monitoring in the region

The most common type of gauges installed in the region is the stilling well float
gauge. Most of the stations established in the region in 1986 within the framework of
the TOGA programme are of this type. Some acoustic gauges have also been set up.
For instance, in Nigeria, a Next Generation Water level Measurement System
(NGWLMS), which also monitors some meteorological and other oceanographic
parameters, is now operating. Other type of gauges, such as the pneumatic bubbler
system and the Direct reading systems, are few.

B3     Scientific and practical application of sea-level data

The technology for the installation of sea-level gauge is quite simple but the amount
of information, which can be derived from sea-level data, is immense. This is one of
the Oceanographic programmes which are within the capabilities of developing
countries and which most developing countries in African could participate.
Applications of sea-level data include: -

B3.1   Sea-level rise

Accelerated sea-level rise is usually regarded as the most certain consequence of
global warming, with much adverse implication for the coastal zone. Rising sea-level
erode beaches by increasing offshore and longshore loss of sediment, directly
inundating marshes and other low-lying lands, increasing the salinity of estuaries and
aquifers, raising coastal water tables and exacerbating coastal flooding and storm
damage. Beach erosion is the most obvious problem of rising sea level. Small islands
and river deltas are the most vulnerable to this problem.

Assessment of economic impacts (see Table1) as a consequence of a 1m rise has been
made for a few countries in the region.
                                        Table I
 Country          People         %        Capital     %     Land loss         %
                 affected                values loss         (sq.km)
Mauritius          6000         1             -      -          10           0.5

Nigeria               .32 m       4         18,000         5.2      18,600           2

Senegal              11,000       >1         700           14        6100           3.1

Recent analyses have shown that

•   Sea level has risen by over 120 m since the last glacial maximum about 20,000
    years ago with a rapid rise between 15000 and 6000 years ago at an average rate of
•   Sea level increased at an average rate of 0.5 mm/yr over the last 6000 years and at
    an average rate of 0.1 to 0.2 mm/yr over the last 3000 years.
•   Based on very-long tide gauge data, the rate of global mean sea level rise during
    the 20th century is in the range of 1.0 to 2.0 mm/yr with a central value of 1.5

Various models output has shown a projection of global sea-level rise of 0.12 to 0.88
m for 2000 to 2100 with a central value of 0.49m. However, models have shown that
regional variation will be substantially different from the global average sea-level rise.
At national and local levels, the average rise could also be different as a consequence
of local land movement.

B3.2      Sea-level rise in the region

A long-term series of sea-level data, usually more than 20 years, is required in order to
identify trends. Very few stations in the region have such a long record. Studies
carried out on available data in Mauritius have shown a sea level rise of 1.7mm/yr
since 1986, which is comparable to the global sea-level rise.

B3.3      Need of long-term series of data in the region

Studies have shown that average shoreline change rate is about 150 times the sea level
rise rate. Hence sea level monitoring will provide valuable data to conduct research
on beach erosion. In order to identify local, national and regional trend in sea level, it
is important that a national and regional network of tide gauges be established and
maintained to obtain long-term series of continuous sea-level data in the region.

B3.4   Extreme events monitoring

Extreme weather events and storm surges are expected to become more frequent with
global warming for coastal development and management purposes. It will be
essential to monitor these events to obtain statistics on extreme water levels for
coastal engineering works.

B3.5   Reference for baseline

Mean sea level over a long period of time is usually used as the zero national datum
level in land surveying and as reference in hydrographic charts. This datum, in
principle, is recalculated every thirty years to identify any changes in MSL.

Many coastal countries and small Island States are currently undertaking studies to
demarcate their territorial waters and Exclusive Economic Zone limit within the
framework of UNCLOS. The lowest water tide obtained from a sea-level record is
usually used as the baseline for the purpose.

B3.6   Navigation purposes

Software is now available to compute tidal prediction. However, tidal data in some
cases, 13 months of continuous hourly data are required to calculate the main
harmonics. Tidal predictions are used by a wide range of marine users including
fishermen, public, shipping and harbour communities.

B3.7   Coastal and Ocean processes

Monthly, seasonal and inter-annual changes in sea level provide important indication
about variation in coastal and ocean processes. Sea-level data can be used also for
ocean circulation studies across narrow straits, wide straits, basin-sections and “choke
points”. For instance a network of tide gauges along Western Madagascar and the
Mozambique coast can provide valuable data to study the ocean current variability in
the Mozambique Channel.

B3.8   Monitoring of Indian Ocean - Dipole

Recent studies have shown the existence of a long-term phenomenon in the equatorial
Indian Ocean which has become known as the “Indian Ocean Dipole”. In IOD events,
rainfall swaps from one side of the Indian Ocean to the other with winds in the central
equatorial feeding moisture towards the enhanced rain. Both wind and rain anomalies
correlate well with the IOD index which has been defined as the difference between
sea surface temperatures (SST) anomalies on either side of the Indian Ocean.

Analyses suggest that an initial cooling of the Eastern Indian Ocean set up an East to
West SST gradient that drives the near equatorial anomalies easterlies. In turn these
winds change the sea surface height (SSH) to tilt upwards to the west. Relaxation of

the SSH anomalies in the form of westward propagating and downwelling ocean
Rossby waves, depress the thermocline in the west and enhance the western Indian
Ocean thermal capacity.

Consequently monitoring sea-level changes on the Western Indian Ocean can provide
the clue to the formation of the Indian Ocean Dipole and its forecasts.

B4     The status of sea-level network in Western Indian (WIO) Ocean

The focus will be on the status in the WIO as Aarup and el will be providing some
comprehensive information in their position paper to the meeting on the status of
GLOSS in Africa with much emphasis on West Africa. Before 1985, very few
stations were operational in the WIO. The number increased gradually with the
implementation of the sea-level programme in the context of TOGA and GLOSS.

The Gloss Implementation plan has identified 23 stations in the Western Indian
Ocean. In addition to these, a regional workshop (Mombassa, Kenya, 1991)
recommended additional stations for regional and national purposes. 32 Non-Gloss
stations were thus identified. The status is as follows: -

          Type                    Gloss Stations            Non-Gloss Station
Operational                             13                        11
Non-operational                         10                        21

Most of the stations are of floating type (Leopald Stevens or OTT R 16). The South-
African Stations have changed to SRD Acoustic gauges in 1996. Some stations have
been upgraded to Hander Encorder, which transmit 15 min sea-level data to the
University of Hawaii via Meteosat and GOES. (Eg Mauritius and Seychelles)

The UHSLC is responsible for the collection of hourly sea-level data and PSMSL for
monthly data. Some stations are submitting regular appropriate sea-level data to these

B5     Upgrading of African sea-level network and proposed future activities

The sea-level network, which expanded significantly with financial and technical
assistance through the TOGA programme (1985-1994), is gradually degrading.The
reasons are many-folds: -
• At the end of the TOGA programme in 1994/1995 financial and technical support
    was no longer made available for spare parts and consumables (eg paper rolls,
    batteries). Subsequently, upkeep of stations was not maintained properly due also
    to lack of well-trained technicians.
• Most of the sea-level stations have now become obsolete and spare parts are no
    longer available to maintain them in good working conditions.
• Around 1998, NOAA discontinued its assistance in the installation and
    maintenance of gauges outside the US
• Some stations were located in high-energy coastal region and were destroyed
    during storms.

There is no doubt that a strategy should be developed to maintain and upgrade existing
GLOSS stations, replace old and obsolete instrument and establish new stations which
are of national and regional importance. Measurement of meteorological parameters
such as atmospheric pressure, air temperature, humidity, winds and rainfall and other
oceanographic parameters such as sea-surface temperature, salinity, and currents
should be incorporated in the system. For sustainability, it is important to ensure
commitment of financial support over a long time period for capacity building in term
of provision of new equipment and spare parts, training of human resources for
installation and maintenance so that at the end of the project, countries could become
self-dependent in the maintenance of the station.

B6      Caribbean and South-Pacific sea-level projects example

The design and establishment of sea-level climate monitoring is one of the four
regional components of the Caribbean Planning and adaptation to Global Climate
Change Project (CPACCP) which receive funding to the tune of US $6.7 m in grant
from GEF. This component involves the establishment of state of the art telemetry
sea level and meteorological monitoring network in countries participating in
CPACCP and to develop the region’s capacity to take charge of the network over the
course of the project. It includes upgrading of existing historic sites and installation of
new sites. A training and technology transfer program has been designed to ensure
that participating regional and national institutions acquire the capacity to co-ordinate
the network, support data analysis and dissemination and maintain its operational
integrity during and after the project. A similar programme has also been proposed
for the South Pacific Ocean region.

B7      Recommendations

In order to develop and strengthen the sea-level network in the region, the following
recommendations are proposed: -

•    Formulate a project proposal on the development and strengthening of sea level
     network in Africa for submission to UNDP-GEF for funding.

This will be in line with one of the recommendations of the Conference of Parties
(COP). The Global Climate of Observing System (GCOS) has already taken the
initiative by organising regional workshops (Samoa, 2000 and Kisumu, Kenya, 2001)
to address the issue of systematic observation within the framework of the UNFCCC.
This proposal is also one of the recommendations of the Western Indian Ocean
Marine Application Project (WIOMAP), which is a regional contribution to GOOS.

•    Investigate the possibility of preparing monthly sea-level anomaly charts of the
     Western Indian Ocean using data from the sea-level network.

 It is recalled that monthly sea level topographic anomaly charts of the Tropical
Pacific Ocean region are now produced routinely to monitor sea-level changes in the
region. This is a valuable predictor of the EL NINO and LA-NINA phenomena. It is
admitted that it is still premature to envisage the implementation of this task in the

immediate as a prerequisite is a fully operational network of sea-level gauges which is
not yet in place until the first above recommendation is implemented.

C1      Modeling and forecasting system

Modeling in oceanography is an important tool to visualize the evolution of a system
on a large scale. The advantage of modeling is that few data are needed for validation
and future temporal and spatial evolution of a system can be computed.

C2      Modeling in the region

A numerical storm surge model developed by Meteo France is operation since 1996 at
La Reunion. An oil dispersion model has also been adapted for the region of the
South-West Indian Ocean.

The South-African Weather Bureau maintains an ETA model for coastal wind
generation, which run on a mainframe Gray SW1. The Bureau used to operate a
regional wave-model adapted from a global wave-model from Germany. Owing to
lack of human and financial resources, its operation has been discontinued.

C3      Products from numerical models

The following data and products from Global Numerical models output, of relevance
to the Indian Ocean, are available through Internet and the WMO Global
Telecommunication System (GTS).

•    Daily actual wave chart (height, direction and period) and its prognosis up to 96

•    Monthly sea-surface temperature (SST) charts and anomalies. Monthly SST
     forecast is available up to three months. Daily SST charts processed from NOAA
     images can also be obtained from Analysis centres in South Africa and Reunion
     on a pre-arranged basis

•    Surface ocean winds and prognosis charts up to 120 hours
•    Monthly sea-level pressure charts and monthly forecast up to three months
•    Monthly sea level topography and SLT anomaly charts as computed from satellite
     altimetry data.

C4      Proposal

Most of the numerical products available for the region are from Global Numerical
Models output from Global Centres. No operational regional oceanographic models,
which generate data and products on a regular basis, are currently available.

It is proposed that regional models be developed and specialised regional centres be
established to prepare such products as coastal and ocean waves, SST, ocean wind,
ocean colour, currents, and their forecast to be distributed to participating countries for
national adaptation.

Capacity building in modeling is still very rudimentary in the region and should be
enhanced to develop capabilities for active participation of countries in this area.


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