Final answer:
High latitude waters lack a pycnocline due to uniform water density created by similar temperatures and reduced salinity differences, while low latitude waters have distinct pycnoclines due to greater temperature and salinity variations with depth.
Step-by-step explanation:
In the ocean, the density of water varies with depth due to changes in temperature and salinity. Density differences create a layer known as the pycnocline, which is a rapid change in density with depth. In high latitudes, such as polar regions, the surface waters are cooled to a temperature that is nearly the same as deeper waters, and the influence of freshwater from melting ice reduces salinity differences. This results in a more uniform water density with depth and generally lacks a pronounced pycnocline. In contrast, low latitude (tropical) waters experience a stronger sun, which increases surface water temperatures and evaporation rates, leading to higher salinity and temperature differences between surface water and deeper layers, creating a distinct pycnocline.
Pressure in a fluid varies directly with depth due to the weight of the fluid above. In the oceans, the deeper you go, the higher the pressure. This pressure affects the density of sea water, but in high latitudes, density is more homogeneous due to similar temperatures and salinity values from top to bottom, so there is less variation in pressure with depth compared to low latitudes where a pycnocline exists.