Jim Gower

Ocean Color Satellites Show Extensive Lines of Floating Sargassum in the Gulf of Mexico

We present satellite imagery that is interpreted as showing extensive lines of floating Sargassum in the western Gulf of Mexico in the summer of 2005. In spite of frequent reports of floating weed covering extended areas in different parts of the worlds ocean, this appears to be the first observation of Sargassum from space. Satellite observations were made with the Medium Resolution Imaging Spectrometer (MERIS) on the Envisat satellite launched by the European Space Agency, and subsequently with the Moderate Resolution Imaging Spectroradiometer (MODIS) launched on both the Terra and Aqua satellites by the National Aeronautics and Space Administration. Both instruments cover wide swaths, providing near-daily images. Both have optical spectral bands in the range 670 to 750 nm, which detect the chlorophyll red-edge characteristic of land and marine vegetation, but only MERIS has a band at 709 nm, which was critical to the initial discovery. The combined satellite data from both sensors show the seasonal cycle of weed density in different areas of the Gulf. A wider ranging study is now needed to map its occurrence in other areas, including the Sargasso Sea (named for the weed, but not so far covered in our survey). The satellite observations suggest that Sargassum biomass is greater than previously estimated, and hence plays a more important part in oceanic productivity

Jason 1 detects the 26 December 2004 tsunami

Satellite altimeters have detected mid-ocean tsunami waves only once in the past, with relatively small amplitude [Okal et al., 1999], according to published reports. The 26 December 2004 tsunami changes this picture dramatically. This event was the strongest to have occurred since satellite altimetry started in the 1970s. Instruments now in orbit can measure sea surface height with sufficient resolution (a few centimeters for the average of a 5-km circle of ocean), but measure only along their tracks, and cannot provide a full picture of an event. Also, they are unlikely to be optimally placed for early detection near the source earthquake. By chance, however, Jason 1 was in the right place at the right time.

Observation of chlorophyll fluorescence in west coast waters of Canada using the MODIS satellite sensor

The first observations of chlorophyll fluorescence from space for the west coast of Canada, using the U.S. moderate resolution imaging spectrometer (MODIS), show that the signals should provide a useful new tool for studying chlorophyll biomass and primary productivity. We compare MODIS fluorescence and sea-viewing wide field-of-view sensor (SeaWiFS) chlorophyll data, using a simple theoretical model of the expected variation of fluorescence emission with variations in chlorophyll concentration. The results show good agreement with the model and appear to allow separation of water masses according to fluorescence yield. As additional MODIS data come available it will be possible to study data for the full range of seasonal conditions. Additional data from the European medium-resolution imaging spectrometer (MERIS) sensor are available from 2002.

Global monitoring of plankton blooms using MERIS MCI

The MERIS maximum chlorophyll index (MCI), measuring the radiance peak at 709 nm in water‐leaving radiance, indicates the presence of a high surface concentration of chlorophyll a against a scattering background. The index is high in ‘red tide’ conditions (intense, visible, surface, plankton blooms) and is raised when aquatic vegetation is present. A bloom search based on the MCI has resulted in the detection of a variety of events in Canadian, Antarctic and other waters round the world, as well as detection of extensive areas of pelagic vegetation (Sargassum spp.), previously unreported in the scientific literature. Since 1 June 2006, global MCI composite images, at a spatial resolution of 5 km, are being produced daily from all MERIS (daylight) passes of reduced resolution (RR) data. The global composites significantly increase the area now being searched for events, although the reduced spatial resolution may cause smaller events to be missed. This paper describes the composites and gives examples of plankton bloom events that they have detected. It also shows how the composites are affected by the South Atlantic anomaly (SAA), where cosmic rays impact the detectors of the MERIS instrument.

Validation of chlorophyll fluorescence derived from MERIS on the west coast of Canada

MERIS on Envisat provides images of above‐atmosphere spectral radiance in 15 bands of the baseline spectral band‐set. This set includes three bands centered at 665, 681 and 709 nm, placed to measure the fluorescence emission from chlorophyll a in marine and fresh water phytoplankton at 685 nm, stimulated by ambient sun and sky light. We compare the observed fluorescence signal levels with measurements of extracted chlorophyll from research cruises and buoy data during 2002 and 2003, and with satellite estimates of chlorophyll from the blue to green ratio observed by MERIS. We present an average relation between fluorescence and surface chlorophyll concentration based on a simple model accounting for absorption of stimulating and emitted radiation by chlorophyll pigments, which gives a first‐order fit to the observations.

Development of Algorithms for Remote Sensing of Trichodesmium Blooms

Trichodesmium is a major component of the global carbon cycle, but because of its sporadic occurrence it is extremely difficult to study by conventional shipboard methods. Information on the variability and spatial extent of this cyanobacterium is essential for calculation of its contribution to carbon and nitrogen fluxes. Intense surface blooms of Trichodesmium have been observed in satellite imagery from the Coastal Zone Color Scanner and in color photography from the space shuttle, but such reports are rare. To date it is difficult to differentiate Trichodesmium from other species by remote sensing measurements alone.

Satellite images suggest a new Sargassum source region in 2011

In the summer of 2011, a major ‘Sargassum event’ brought large amounts of seaweed onto the beaches of the islands of the eastern Caribbean with significant effects on local tourism. We present satellite observations showing that the event had its origin north of the mouth of the Amazon in an area not previously associated with Sargassum growth. A significant concentration of Sargassum was detected in April, when it was centred at about 7° N latitude and 45° W longitude. By July it had spread to the coast of Africa in the east and to the Lesser Antilles and the Caribbean in the west. We have previously used images from MERIS (Medium Resolution Imaging Spectrometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) to show the value of satellite observations in tracking patterns of Sargassum. For the years 2003–2010, we were able to determine the seasonal distribution over the range of 20°–40° N latitude and 100°–40° W longitude covering the ‘Sargasso Sea’ region of the North Atlantic and the Gulf of Mexico. In 2011, satellite data showed a large shift in the distribution, whose cause is unclear.

The 26 December 2004 tsunami measured by satellite altimetry

The 26 December 2004 magnitude 9 earthquake off Sumatra provided the first examples of travelling tsunami waves in mid‐ocean clearly detected by satellite altimetry. The earthquake was the largest since satellite altimetry started in the 1970s and gave peak‐to‐trough wave heights in mid‐ocean of over a metre. The tsunami was detected by three of the four altimeters presently giving sea surface height information. Each detected the spreading front twice, as it moved south‐westwards into the Indian Ocean and as it moved northwards into the Bay of Bengal. They also detected the disturbed region closer to the epicentre that expands with the slower velocities of higher‐frequency waves. Although the plate rupture is estimated to extend over about 1300 km in a north/south direction, the satellite observations appear consistent with a smaller generation area towards the south of this rupture zone. Fronts observed in the Indian Ocean show a positive first crest. Those observed in the Bay of Bengal are of smaller amplitude and appear to show a first negative first crest (trough). The structure in the Indian Ocean front observed by Jason‐1 suggests the possible presence of a shorter‐wavelength negative component superimposed on the positive crest.

Global remote sensing of Trichodesmium

The maximum chlorophyll index (MCI) from the Medium Resolution Imaging Spectrometer (MERIS) satellite imager gives a robust indicator of the presence of a variety of floating slicks, near-surface vegetation, and intense surface plankton blooms. The index responds to the presence of surface slicks of Trichodesmium due to the ‘red edge’ increase in radiance with increasing wavelength near 700 nm. Global composite images of this index can be used to map the distribution of surface slicks of Trichodesmium, showing seasonal and long-term variations. The MCI also responds to the ‘red edge’ in the spectral signature of Sargassum. The two species share some areas of common occurrence and it is important to distinguish between them. We have developed spectral techniques of distinguishing between these two, and for discriminating a variety of other confusing targets that occur in different areas. We feel that MERIS MCI can be a useful tool in monitoring global Trichodesmium spatial distribution, and its short- and long-term variation.

U.S. warning system detected the Sumatra Tsunami

Although the December 2004 great Sumatra earthquake and the resulting tsunami were very distant from the northeast Pacific Ocean, the U.S. Deep-ocean Assessment and Reporting of Tsunamis (DART) array in the northeast Pacific successfully demonstrated high sensitivity and provides useful data for understanding the propagation of the tsunami. At the time of the tsunami, the Pacific DART network already was one of the most sophisticated tsunami detection systems in operation.The network, which then consisted of eight stations (seven U.S. and one Chilean), now consists of 11 stations (10 U.S. and one Chilean) [Gonzalez et al., 2005] (Figure l). Each station is equipped with a bottom pressure recorder (BPR) transmitting data acoustically from the ocean bottom to a surface buoy, which then passes the data to tsunami warning centers and other land stations by satellite communication links.