KnowHOW team explains: Tropical oceans are an almost transparent azure blue compared to grey and murky sea water in temperate regions.

This is because the tropical sea water lacks nutrients which severely limits the growth of aquatic plants (phytoplanktons), and hence the rest of the local food chain. The tropical sun makes the surface layer warm but deeper waters remain cold.

So the water does not form a smooth temperature gradient (difference). It divides into sharp delineations of temperature called thermoclines. These layers can have a consistent temperature for many metres and then change by several degrees within the space of a few millimetres. Here there is also often a change in the direction of the current setting up strong permanent thermoclines that block the circulation which would otherwise bring nutrients from the depths up to the sunlit layer.

n temperate oceans the thermocline is not permanent. During colder times of the year, the surface water is just as cold as the depths and the thermocline vanishes altogether. Nutrient-laden water from the bottom can then come to the surface, replenishing the nutrient supply and allowing the aquatic plant organisms or phytoplankton (and all that feeds on them) to grow to such numbers as to make the ocean that murky grey.

The question was sent by Arun Sen from Murshidabad