陳蕙雅思教材 - 雅思閱讀精選 (The Salinity of Ocean Waters)

The Salinity of OceanWaters

If the salinity of oceanwaters is analyzed, it is found to vary only

slightly from place toplace. Nevertheless,         some of thesesmall changes are

important. There arethree basic processes that

cause a change in oceanicsalinity.

One of these is thesubtraction of water from the ocean by means of

evaporation - conversionof liquid water to        water vapor. Inthis manner the

salinity is increased,since the salts stay behind. If this is carried to

the      extreme, of course, white crystals of saltwould be left behind.

The  opposite of evaporation is  precipitation,such  as rain, by which

water is  added to the ocean. Here the ocean is being diluted so that the

salinity is decreased.This may occur in areas of high rainfall  or  in  

coastal  regions   where  rivers  flow into  the  ocean. Thus  salinity

may  be increased by the subtraction of water byevaporation, or    decreased

by the addition of freshwater by precipitation or runoff.

Normally, in tropicalregions where the sun is very strong, the ocean

salinity is somewhathigher than it is in   other parts of theworld where

there is not as muchevaporation. Similarly, in coastal regions where

rivers dilute   the sea, salinity is somewhat lower than in other oceanic

areas.

A third process by whichsalinity may be altered is associated with the

formation and melting ofsea ice. When sea       water is frozen,the dissolved

materials are leftbehind. In this manner, sea water directly beneath

freshly formed seaice  has a  higher salinity than it didbefore the  ice

appeared. Of course, whenthis ice melts, it will tend to          decreasethe

salinity of thesurrounding water. In the Weddell SeaAntarctica, the

densest water in theoceans is formed as a result of this freezing process,  

which increases thesalinity of cold water. This heavy water sinks and is

found in the deeperportions of the          oceans of theworld.

海水鹽度 如果我們分析海水的鹽度，會發現地區間只有輕微的變化，然而有些小的變化是重要的。

導致海洋的鹽度變化的基本過程有三個，其中之一是通過蒸發的方式即把液態水轉化為水蒸 氣來減少海洋中的水分。  這樣由於鹽留了下來，所以鹽度增大。  

當然，如果這種方式走向 極端，將會餘下白色的鹽晶體。  與蒸發相反的是降水，如降雨，由此水被加入海中，海水 被稀釋，從而鹽度降低。  

這種情形會發生在大量降雨的地區，或江河入海岸處。  因此，鹽 度通過蒸發減少水分而上升或通過降水或徑流增加淡水成分而下降。  一般來說，在陽光很

強烈的熱帶地區，海水的鹽度略高於世界上其它沒有熱帶那樣多的蒸發的地區。  同理，在 江河稀釋海水的海岸地帶，海水鹽度略低於其它海區。

第三個可以變更鹽度的過程與海洋 中冰的形成和融化有關。  海水凍結時，溶於其中的物質被留了下來。  這樣，在新形成的海

水冰面的正下方的海水比在冰形成之前有更高的鹽度。  當然，當冰融化的時候，會降低周 圍水中的鹽度。  

在南極洲邊緣的威德爾海中，結冰過程增加低溫海水的鹽度，從而形成了 濃度最大的海水。  這些大密度的海水下沉，可以在世界海洋的深水域發現。