Short essay on The Vertical Temperature Distribution in Adjacent Seas

It has already been noted that there is a steady decrease in temperature with increasing depth in the open oceans. But in the adjacent seas that are connected with the open oceans over shallow sills the temperature below a certain depth is constant irrespective of the actual depth. According to Defant, there are two types of the adjacent seas in respect of their temperature stratification:

(i) The adjacent seas, in which the deep layers of water show the lowest surface temperature recorded during the coldest month of the year. The Mediterranean Sea and the Red Sea fall into this category.

(ii) The adjacent seas, where the surface temperature throughout the year is very high. The thermal structure of the sea below the sill depth is determined by the temperature at the sill depth. Sill depth has been defined as “the maximum depth of an entrance from the open ocean to a basin”. It is also called the threshold depth of that entrance.

Here the word ‘entrance’ signifies a depression in the barrier which separates the basin from the open ocean. It is not necessary that the whole or a part of the barrier should rise above the sea level. In this case only oceanic water can reach the deeper layers below the sill depth.

According to Defant, the sinking of oceanic water into the deeper layers of such enclosed seas produces a potential temperature extending to the bottom that is determined by the potential temperature of the open ocean at the level of the sill.

The temperature of a water mass after being moved adiabatically to the surface is known as the Potential Temperature. In the second group are included the Australian-Asiatic deep sea basins beneath the depths of the sills over which they are connected with the Pacific Ocean or with other basins lying nearby.

The following table gives an accurate and detailed picture of the conditions in the above mentioned basins i.e. Australian-Asiatic Basins.

Similar as above have been prepared for the vertical distribution of temperature and salinity in the American Mediterranean, in the adjacent seas at higher latitudes, and in the Polar Regions.

Such tables have been based on the observations made by various oceanographic expeditions organised from time to time. However, the limited space available in a text book as this does not allow the full discussion of the temperature conditions in the oceans and adjacent seas.