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 monsoons. Reports indicate that there is no correlation between this phenomena and meteorological conditions. UNDERWATER VISIBILITY 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 VISIBILITY Underwater  visibility  depends  primarily  upon  the transparency of the water, reflectance and contrast, water color, sea state, incident illumination, and optical image. Transparency 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   water’s   molecular structure. For a target to be visible, it must contrast with its background. Water Color 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. Sea State 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. Incident Illumination The amount of incident illumination, as determined by cloud coverage and the sun above the horizon, is a definite  consideration  in  underwater  visibility. Optical  Image 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 TRANSPARENCY 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. 9-14