First of all, mixing in the ocean isn’t as simplistic as two fluids with different properties forming one single fluid. In a fluid system as big as the ocean, there are different ‘length-scales’ associated with mixing. That is, the amount of mixing depends on the scale or size of the water body being referred to. It’s not two oceans meeting, its glacial melt water meeting the off shore waters of gulf of Alaska. The reason for this strange phenomenon is due to the difference of water density, temperature and salinity of the glacial melt water and off shore waters of gulf of Alaska, making it difficult to mix.
“What? How can they have different densities if they’re both waters?” – The difference in their densities is a function of varying salinity concentration and levels of temperature.
Density is expressed as mass over volume:
Density is directly proportional to mass. And when you put more salt in water, the density of the water increases because the mass of the water increases. The greater the salinity, the greater its mass. There is mixing going on at the interface between the two oceans. Notice their differences in color? (Sky blue color is typical of a shallow tropical shelf while deeper water may look dark blue, but if there are considerable blooms of phytoplankton, it may look green.)
However, the rate of mixing isn’t strong enough to completely homogenize both water bodies together. In a fluid system, mixing as big as the ocean has volume-scale and time-scale associated with it. The duration and amount of mixing depends on how big is the water body. Given by Prandtl’s mixing length hypothesis:
rate of mixing = (volume scale)^2/(time scale)
Where the rate of mixing is directly proportional to the square of the volume scale over time scale, implying to consider the size of the fluid parcels in a given time. If we are going to mix about 1 cubic meter volume of water, or even less, then the mixing rate will be accordingly shorter. When you increase the volume, it will also increase the duration to homogenize both the water masses. Therefore, completely mixing of larger water bodies such as oceans with different chemical and physical properties will take longer time. But they do mix, that’s the point.