ARABIAN SEA BIOLUMINESCENCE
Reflectance and Contrast
The Arabian Sea is one of the richest areas in the
world for marine bioluminescence. It is known to
appear with the onset of the southwest and northeast
Reports indicate that there is no correlation between
this phenomena and meteorological conditions.
LEARNING OBJECTIVES: Recognize the six
factors affecting underwater visibility.
Compare water transparency in various parts of
the North Atlantic ocean.
Visibility in seawater is restricted in a manner
somewhat similar to the restriction of visibility in the
atmosphere. The restriction in seawater differs from
that in the atmosphere primarily because of scattering
(predominant in coastal waters) and absorption
(predominant in deep, clear ocean waters).
FACTORS AFFECTING UNDERWATER
Underwater visibility depends primarily upon the
transparency of the water, reflectance and contrast,
water color, sea state, incident illumination, and optical
The term transparency is often thought of as that
property of water that permits light of different
wavelengths to be transmitted; transparency is
sometimes measured as the percent of radiation
penetrating a path length of 1 meter. However, the most
commonly used definition and measurement of
transparency, as applied to underwater visibility, is the
average depth below sea surface at which a Secchi disc
(white disc) first disappears and then reappears at the
surface to an observer who successively lowers and
raises the disc.
The degree to which seawater becomes transparent
is a function of the combined effects of scattering and
absorption of light by the water surface, suspended,
organic and inorganic particulate matter, dissolved
substances, plankton, and the waters molecular
For a target to be visible, it must contrast with its
Deep (clear) water is very transparent to the blue
portion of the light spectrum and less transparent to the
green, yellow, red, and violet portions. In the more
turbid coastal waters, green and yellow light penetrates
to greater depths than does blue.
Irregular sea surfaces affect visibility in several
ways. Variable refraction results in a reduction of the
contrast of a target. Winds that barely ruffle the surface
reduce contrast of a target by as much as 40 percent.
The amount of incident illumination, as determined
by cloud coverage and the sun above the horizon, is a
definite consideration in underwater visibility.
The optical image of a target can be due to its own
light, to reflected light, or to its being silhouetted against
an illuminated background.
GEOGRAPHIC VARIATION OF
Figure 9-14 depicts the Seawater transparency of the
North Atlantic. Figure 9-14 also shows that deep North
Atlantic waters range in transparency from
approximate y 50 feet off the continental slope to over
115 feet in the Sargasso Sea.
EFFECTS OF OCEAN FRONTS,
EDDIES, AND UPWELLING
LEARNING OBJECTIVES Define oceanic
fronts, eddies, and upwelling. Recognize
typical locations of oceanic fronts, eddies, and
upwelling in the Pacific and Atlantic oceans.
Be familiar with the effects of oceanic fronts,
eddies, and upwelling on acoustics. Recognize
oceanic front, eddy, and upwelling locations
using satellite data.