Why Spring Overturn Occurs in Some Ocean Areas but Not Others

Spring overturn (vertical mixing) is driven by surface heat loss, wind mixing, and freshwater input, which make surface water dense enough to sink, bringing deep water up.

Where It Happens

• Coastal upwelling zones (e.g., California Current) – wind‑driven upwelling peaks in spring.
• Mid‑latitude basins (North Atlantic, North Pacific) – localized overturns from specific atmospheric conditions.

Where It Doesn't

• Deep, highly stratified oceans (tropics, Mediterranean) – strong density gradient prevents mixing.
• Polar regions – surface stays cold/dense year‑round, limiting spring warming.

Thus, overturn occurs only where seasonal forcing overcomes existing stratification.