Stimulating investment in pearl farming in ... - World Fish Center

Water Temperature and Cyclone Frequency in the Pacific: Implications for Pearl Farming Final, June 2008variation in measurement error from one observation to the next, depending on the methodsemployed (Metoffice UK, undated). Measurements taken from the intake of ships, forexample, are not consistent, because the depth of the water intake and where thetemperature is measured vary from vessel to vessel.An alternative remote method of SST measurement based on data collected by infra-redradiometers on board satellites has been in operation since 1978. One of the principalinstruments used to measure SST from space is the Advanced Very High ResolutionRadiometer (AVHRR) carried on board the polar-orbiting satellites operated by the NationalOceanic and Atmospheric Agency (NOAA) (NASA Oceanography 2005). The AVHRR hasan array of five small sensors that record the amount of visible and infrared radiationreflected and/or emitted from the Earth's surface. This instrument takes thousands ofmeasurements of broad swaths of Earth each day and night, resulting in concurrent data setsof the entire planet (National Atlas of the United States 2007). Two other satellite-bornesensors, the Moderate-Resolution Imaging Spectroradiometer (MODIS) and AdvancedMicrowave Scanning Radiometer for EOS (AMSR-E) are also commonly used to measureSST (NASA Oceanography 2005). MODIS, like AVHRR, uses a radiometer to measure thethermal infrared radiation given off by the surface of the ocean, but is sensitive to fivedifferent wavelengths of radiation and can therefore provide more detailed SSTmeasurements than AVHHR (NASA Oceanography 2005). The MODIS sensor has aviewing swath width of 2,330 km and scans the entire surface of Earth every one to twodays. Its detectors measure 36 spectral bands between 0.405 and 14.385 µm and acquire dataat three spatial resolutions: 250 m, 500 m, and 1,000 m (NASA Jet Propulsion Laboratory2007). AMSR-E is a passive microwave radiometer that measures radiation from the oceansurface. The measurements derived from satellite-borne sensors are corrected for the effectsof atmospheric water vapour which reduce the strength of the signal emitted and arecalibrated and quality controlled against SST data collected from ships and drifting buoys,which are co-located in both space and time.Satellite-borne sensors provide SST data over much large spatial scales than traditionalmethodologies, but there are a number of problems associated with their use. First, infra-redsensors take radiation measurements from the top "skin" of the ocean, a layer less than 0.1mm thick, rather than bulk SSTs (Metoffice UK, undated). The temperature of the “skin” isusually lower than the bulk SST, because of the loss of radiant and latent heat throughevaporation and mixing. The “skin” temperature also varies during the day, because ofdiurnal solar heating at the sea surface. This problem can be overcome by restricting data tonight-time measurements. Long-term trends in SST derived from AVHRR and MODISsensors also tend to have a “fair weather” bias, because these instruments cannot seethrough cloud cover (NASA Oceanography 2005). SST data may consequently be sparse insome seasons or in some regions of the world. The AMSR-E sensor, however, is able tomeasure radiation from the ocean surface through most types of cloud cover. Variousalgorithms have been developed to correct for signal attenuation by atmospheric effects.Despite these limitations, a number of SST data products have been derived from satellitebasedsensors, including near real-time images, contour charts, analysed fields, SSTanomalies, maps of coral bleaching hot spots and charts showing degree heating weeks, anindicator of the accumulation of thermal stress that coral reefs have experienced over thepast 12 weeks. The products that are available depend on the type of sensor used and thetype and version of the algorithms used to process the data. They also vary in terms of thetime span covered and their spatial resolution on the ground. SST data derived from theNASA MODIS sensors on board the Terra and Aqua satellites, for example, are availablefrom February 2000 and from July 2002 to present, respectively. The data from bothThe Ecology Lab Pty Ltd – Marine and Freshwater Studies Page 3