The water from the Atlantic and Pacific Oceans does mix to some extent, but the process is not instantaneous or uniform due to several factors:
Density Differences: The density of seawater varies based on temperature, salinity, and pressure. The Atlantic and Pacific Oceans have different circulation patterns and water masses with varying densities. The North Atlantic, for example, has a higher salinity and therefore higher density than the North Pacific. These density differences create stratification, making it more challenging for the waters to mix freely.
Ocean Currents: Both oceans have distinct circulation patterns driven by winds, temperature differences, and the Earth's rotation. Major ocean currents like the Gulf Stream in the Atlantic and the Kuroshio Current in the Pacific transport warm waters poleward, influencing the mixing of water masses across ocean basins.
Geographical Barriers: The narrow and shallow Isthmus of Panama connects North and South America, acting as a partial barrier between the Atlantic and Pacific Oceans. While some mixing occurs through this passage, it still limits the direct exchange of water between the two oceans.
Time Scales: Mixing between the oceans occurs over extended time scales due to the vastness of these bodies of water. While surface currents may interact more readily, deep ocean water exchange is a slower process that can take centuries or even millennia.
Despite these factors, it is important to note that water from the Atlantic and Pacific Oceans does mix on a global scale through ocean circulation patterns, including the thermohaline circulation. These large-scale movements slowly distribute heat, salt, and nutrients throughout the oceans, which have profound impacts on Earth's climate and marine ecosystems.