Imagery from the Moderate Resolution Imaging
Spectroradiometer (MODIS) was chosen as the most
suitable data source for this project. MODIS data has
become a key source in providing global environmental
data for a vast range of scientific and monitoring
applications. MODIS data is freely available in a wide
variety of formats and temporal bins. Its time-series
spanning several years and ongoing supply of data makes it
suitable for temporal investigations.
The MODIS sensor encompasses atmospheric, land and
ocean imaging in a single instrument. There are two
processes such as surface water movement (i.e. currents and
eddies) are indirectly detected through the effects they have
on SST or Ocean Colour. Satellite imagery can also show
evidence of oceanic events, such
as upwellings, which occur through the water column from
depths greater than ones where satellite sensors can directly
detect. The MODIS SST and Ocean Colour data have
proven to be reliable (EOS Aqua in particular) and widely
used in oceanographic research. The combination of near
global coverage and relatively high resolution has made
MODIS a valuable source of environmental data.
SST indicates the temperature of the very thin surface layer
of water (less than 1mm thick). It is often slightly cooler
than the ‘bulk’ sea surface temperature, which is taken from
the top several meters of ocean surface. Satellite SST is
essentially measuring the temperature at the transition point
between the ocean and atmosphere, which makes it very
(A)
(B)
Figure 1, June 2004 data, (A) Original and (B) Linear Least
Squares residuals
of the major challenges to CSAT accuracy is situations
where non-phytoplankton contaminates in the ocean alter
pixel values. These contaminates are often suspended
particulates or dissolved organic matter (CDOM). CDOM is
essentially made up of disturbed substrate and/or detritus
resulting from a myriad of shelf/coastal processes. In order
to minimise large regions of no data in the final results,
efforts have been made to fill cloud cover pixels from the
average value of nearby valid pixels. A neighbourhood
MODIS instruments on board the Earth Observing System
(EOS) satellites, which are aimed at furthering knowledge
of global dynamics and process. The EOS satellites are in
circular, sun-synchronous, near-polar orbits approximately
705km from the Earth's surface. Almost all of the Earth is
covered every 1-2 days. The MODIS instrument has a
swath at 10km along track and 2330km across track. There
are 36 spectral bands covering wavelengths of 0.4um to
14.5um.
The SST and Ocean Colour data, available through
MODIS, are two of the key oceanic properties that enable
global insight into oceanic surface processes. Many of the
important in oceanic/atmospheric transfer research. SST has
become one of the most important climate and
oceanographic variables available (Kapalan 1998).
Initially, SST was processed in its raw form using the
degree Celsius values. Data in this form are largely affected
by the first order latitudinal trend of the Earth’s temperature
gradients. While this approach eventually provided useful
results in identifying the latitudinal gradients in SST, it did
not satisfy the effort to identify oceanographic features such
as currents, eddies, etc. that may have proved more useful
and relevant at a regional scope. A simple approach to
removing the first order latitudinal trend was by obtaining
the residuals from a Least Squares Linear Regression fit
between raw temperature values and latitude. These SST
residuals enhanced clarification of SST water bodies and
allowed interpretation without the influence of latitude
(Figure 1).
A key goal of ocean colour sensing is the determination of
phytoplankton chlorophyll-a concentration in the near
surface of ocean waters (Yoder and Kennelly 2006).
Phytoplankton are microscopic plants and principle
photosynthetic organisms in the oceans which form the
basis for many food webs and primary production (Yoder
and Kennelly 2003). The concentration of the active
vi photosynthetic agent, chlorophyll-a, in phytoplankton is
derived from ocean colour values giving a proxy for the
level of primary biological activity (Ryther and Yentsch
1957).
Ocean colour data has been collected via a variety of
satellite sensors since the late 1970s. Over this time the
accuracy and limitations of ocean colour data has been
determined and examined widely. Ocean colour is detected
in the visible wavelength spectrum, with the concentration
of chlorophyll-a derived from the green and blue spectrum
bands (Robinson 2004, Yoder and Kennelly 2006). As with
g most remotely sensed properties, there are contaminates
,| which decrease the accuracy of detected pixel values. Since
"ocean colour is detected in the visible spectrum, cloud
cover plays a major contaminate role with sometime vast
areas completely masked. Sun glint from waves is also
another source of reflectance contamination. Probably one
interpolation from the averaged value of valid pixels in a
nine pixel radius (—42km) was found suitable to fill small to
medium sized regions of cloud cover, while maintaining an
indicative pixel value reflecting an appropriate influence of
its surrounds.
A monthly time-series of MODIS satellite imagery from
July 2002 to June 2006 was used in this study. The
standardised principle component analysis (sPCA) was used