Why do some parts of the ocean turn over in spring while others not?

Spring overturns are driven by the combination of surface heat loss, wind mixing, and freshwater input. These force denser surface water to sink and bring deeper water to the surface, mixing the upper layers of the ocean.

Seasonal Changes in Surface Heat Loss and Solar Radiation:

- In spring and autumn, the Northern Hemisphere experiences a transition from winter to summer and vice versa. These seasonal changes in atmospheric conditions lead to significant variations in surface heat loss and solar radiation received.

- During winter, surface heat loss is maximum, causing the surface water to cool and become denser. This creates an unstable stratification, where denser water lies above less dense water.

- In spring, as the weather warms up, surface heat loss decreases, and solar radiation increases. The heating and reduced cooling lead to a gradual increase in surface water density.

- However, in certain ocean basins or regions, the combination of heat loss, wind mixing, and freshwater input may not be sufficient to overcome the density difference between surface and deep waters. This results in the persistence of a stable density stratification, preventing complete vertical mixing.

- As a result, only those regions where the surface water density becomes similar to the density of deeper water experience spring overturns, mixing the upper layers and driving nutrient-rich water to the surface.

Regions of Spring Overturns:

- Coastal Upwelling: Spring overturns are often associated with coastal upwelling events. As winds blow over the ocean's surface, they generate coastal currents that pull cold, nutrient-rich water upward. This upwelling process can occur year-round in some regions (like the California Current) but is particularly pronounced in spring due to increased wind speeds.

- Mid-Latitude Ocean Basins: In certain mid-latitude ocean basins, such as the North Atlantic and North Pacific, spring overturns occur in localized areas driven by specific atmospheric conditions and wind patterns.

Regions without Spring Overturns:

- Deep, Stratified Oceans: In regions with deep, highly stratified oceans, such as the tropics and the Mediterranean Sea, the density difference between surface and deep waters is more significant. This strong stratification prevents complete vertical mixing, even with seasonal changes.

- Polar Regions: In the Arctic and Antarctic regions, the surface water remains cold and dense throughout the year due to the persistent cold climate. This prevents significant surface warming and density changes during spring, resulting in limited overturns.

In summary, the occurrence of spring overturns in certain parts of the ocean depends on a delicate balance of surface heat loss, wind mixing, freshwater input, and the existing density stratification. Where these conditions are met, spring overturns can occur, driving nutrient-rich water to the surface and supporting marine productivity. However, in regions where these conditions are not favorable, spring overturns do not take place, and the water column remains stratified.