The global ocean is filled with water whose average density is 1.03 g/cm3. The density is not uniform: it varies in the vertical, with denser water always below lighter water. The temperature, the quantity of dissolved salts (also known as salinity), and the pressure to which a parcel of seawater is exposed determine its density. 75% of the water in the oceans has temperatures ranging between 0° and 6°C; the average temperature is 3.5°C. Water at depth and near the poles is cold. It gets warmer towards the surface and towards the equator (Fig. 12). Surface temperature in the Bay of Bengal is usually between 22°C and 31ºC. It is cooler by 1-2ºC in the Arabian Sea. This difference has major implications for the atmosphere above the two basins.
Salinity is measured as the ratio of weight of dissolved salts to total weight; the ratio is usually expressed as parts per thousand (ppt). 75% of seawater has a salinity ranging between 34-35 ppt (Fig. 13). The average salinity in the
oceans is 34.7 ppt, i.e., on an average there is 34.7 g of salt in every kg of seawater.
Salinity near the surface in the northern Bay of Bengal can be as low as 31 ppt (Fig. 14) because the bay receives lots of freshwater in the form of rain and from runoff of surrounding rivers (Ganga, Brahmaputra, Irrawaddy, Godavari, and others). If all the freshwater that the bay receives during a year is accumulated and spread uniformly over its entire surface, it would form a layer over a metre thick. Salinity near the surface in the Arabian Sea is much higher than in the Bay of Bengal because evaporation over the Arabian Sea is much greater and it receives relatively less river runoff.
We live at 1 atmosphere pressure. In the ocean, pressure increases by 1 atmosphere for every 10 m increase in depth. This means that the pressure at the bottom of the ocean of an average depth of 3700 m will be 370 atmospheres. Pressure has an influence on physical (density), chemical, and biological (decomposition of shells) properties of the ocean.