9th Nov, 2008

Supported by satellite – Sunday 9th November 2008

The measurements we make on the ship are not the only data that we use to improve our understanding of Atlantic Ocean Dynamics. The Royal Research Ship James Clark Ross receives satellite images each day from the National Earth Observation Data Archive and Analysis Service (NEODAAS) at Plymouth Marine Laboratory (PML).

High above our heads, circulating in the space, NASA and European Space Agency satellites carry sensors that scan the earth and continually measure the surface properties of the ocean. The main data that they collect, which we receive on the ship, are Sea Surface Temperature (SST) and Ocean Colour (Chl-a). Sea surface temperature is detected by infra-red sensors AVHRR (Advanced Very High Resolution Radiometer) on board the NASA satellite NOAA and NASA’s MODIS (Moderate-resolution Imaging Spectroradiometer) SST. Ocean Colour is literally ‘the colour of the ocean’ derived from radiometers, such as MODIS (Moderate-resolution Imaging Spectroradiometer) and MERIS (Medium-resolution Imaging Spectrometer), which measure the reflection of light from the top 5m of the ocean and convert it into Chlorophyll-a concentration, the photosynthetic active pigment of phytoplankton (the marine algae). These data are received by Dundee Satellite Receiving station, decoded and transferred to NEODAAS at PML, who process them into mapped images at 9, 4, 1 km or 500 and 300 meter resolution.

Sea surface temperature and ocean colour (Fig1)

Sea surface temperature and ocean colour (Fig1)

During AMT18, we received some spectacular satellite imagery from NEODAAS. These AVHRR SST and MODIS-aqua Chl-a images (Figure 1) show the transition from colder water (<14°C) with relatively high Chl-a (>1mg m-3) in the North Atlantic to the warmer (>24°C) sub-tropical Atlantic water off the North coast of Africa with lower Chla (<0.07 mg m-3). Further South, MODIS-aqua SST showed warmer water (>29°C) at the tropical equatorial front between 5 & 10°N and equatorial upwelling south of 0°N indicated by the out cropping of water <25°C (Figure 2).

Sea surface temperature (Fig 2)

Sea surface temperature (Fig2)

In the southern Gyre, MODIS-aqua SST & Chl-a images indicated warmer water (>25°C) in the Southern Gyre and low Chla (<0.06 mg m-3) and colder water (<23 °C) with higher Chla (>0.07 mg m-3) to the south of the Gyre (Figure 3).

 

Sea surface temperature (Fig3)

Sea surface temperature (Fig3)

In the South Atlantic, MODIS-aqua SST image showed the transition from warmer (>20°C) sub-tropical Atlantic water between 30 & 40°S to the cold (~2°C) Antarctic overflow water south of 49°S (Figure 4a). By comparison, MODIS-aqua Chl-a showed filaments of phytoplankton emanating from the Patagonian Shelf and the coast of South America (Figure 4b). This is partly due to calcite shelled phytoplankton known as Coccolithophores which can be detected easily as they are highly reflective, as seen in this image of sea surface reflection (Figure 4c). This image can then be processed into a calcite image, the Coccolithophores then show up as light green swirls entrained in ocean eddies off the Patagonian shelf, between Argentina and South Georgia as can be seen in this MODIS-aqua Calcite image (Figure 4d).


(Fig4a, b, c & d)

(Fig4a, b, c & d)

Other data can be calculated from the SST, Chl-a and Sea Surface Reflection data. The Remote Sensing Group at PML develop algorithms and models to derive more interesting parameters from the standard satellite data. They have algorithms capable of estimating phytoplankton primary production, or the amount of carbon fixed by the marine algae which is useful for climate change or fisheries studies. This image for example shows the primary production in the southern gyre where it is low (<200 mgC m-2 d-1) to the coast of South America where it increases to >400 mgC m-2 d-1 (Figure 5).


Primary production (Fig5)

Primary production (Fig5)

Thanks to Rory Hutson, Jane Netting, Uvindu Perera and Peter Miller for Satellite Imagery Support during AMT18.

Posted by: Kelly-Marie Davidson

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