Chapter 16 — FUNDAMENTALS OF OCEANOGRAPHY 



Table 16-1„ — Seawater survival times 



Water 

 (°F) 



Temp 



Exhausted or 

 Unconscious 



E 

 S 



xpected 

 urvival Time 



32.5 







Less than 

 15 minutes 



Less than 15 to 

 45 minutes 



32.5 



- 40 







15-30 minutes 



30 



-90 minutes 



40.0 



- 50 







30-60 minutes 



1 



- 3 hours 



50.0 



- 60 







1 - 2 hours 



1 



- 6 hours 



60.0 



- 70 







2-7 hours 



2 



- 40 hours 



70.0 



- 80 



.0 



3-12 hours 



3 



hours - indef 





> 80 



.0 



indefinite 



i n 



def i ni te 



209.376 



Mixed layer depth (MLD) varies most on 

 a seasonal cycle; not because of the season 

 itself, but because of weather patterns which 

 are associated with seasons. These weather 

 patterns are the primary cause of "mechanical 

 mixing." 



Mechanical Mixing 



Mechanical mixing is the indirect result of 

 wind stress on the surface of the water. With 

 the exception of areas where frequent advection 

 of different water types occurs, mechanical 

 mixing is the major process in formation of 

 the mixed layer during the spring, summer, 

 and fall. Mechanical mixing falls into three 

 classifications: Advection, convergence and 

 divergence. 



ADVECTION. — In areas where vertical 

 boundaries exist, such as the boundary between 

 a cold current and a warm current, tongue-like 

 protrusions of warm water under cold water 

 or cold tongues over warm water create unstable 

 water conditions. 



known as "upwelling". This upwelling of water 

 which is always associated with divergence is 

 a form of vertical mixing which decreases the 

 depth of the mixed layer. Convergence of water 

 occurs when wind causes surface water to pile 

 up in an area. This piling up of water causes 

 a sinking action which increases the depth of 

 the mixed layer. Convergence of water normally 

 occurs in areas of anti-cyclonic wind circu- 

 lation while divergence is associated with 

 cyclonic wind circulation. Convergence and 

 divergence also occur where water masses or 

 currents converge or diverge. In winter, 

 mechanical mixing has little or no effect on 

 the mixed layer since all mixing is caused by 

 density increases in the surface layer. This 

 type of mixing is referred to as "Instability 

 Mixing". 



INSTABILITY MIXING. — Instability mixing 

 occurs when the uppermost layer of water 

 becomes more dense than the underlying water, 

 causing it to sink. This condition is set up by 

 two processes called conduction and evaporation: 



DIVERGENCE AND CONVERGENCE. — Di- 

 vergence occurs where the wind blows surface 

 water out of an area, which happens with an 

 off-shore wind. This diverging water is 

 replaced by water from below through a process 



CONDUCTION. — This is a process where 

 one surface comes in contact with another and 

 an energy transfer is made. In winter, cold air 

 masses invade the warmer ocean areas causing 

 the surface of the sea to cool. This cooling 



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