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陳蕙雅思教材 - 雅思閱讀精選 (The Salinity of Ocean Waters)瀏覽數:45次
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。
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