Tropical vs. Polar Ocean Salinity: It's Not Just Temperature

The idea that tropical oceans are uniformly saltier than polar oceans is a myth. Salinity is governed by the local balance of evaporation (E), precipitation (P), river runoff (R), ice processes, and advection—not latitude alone.

Why Tropics Can Be Saltier

• High E-P: Intense sun + trade winds → strong evaporation. Subtropical gyres (e.g., Sargasso Sea, ~37 PSU) are salinity maxima.
• Low rainfall zones: Eastern subtropical Pacific/Atlantic under subtropical highs.

Why Tropics Can Be Fresher

• ITCZ rainfall: Equatorial band (5°N–5°S) gets 3–5 m/yr rain → surface salinity <34 PSU (e.g., Western Pacific Warm Pool).
• River plumes: Amazon, Congo, Orinoco, Ganges-Brahmaputra create vast low-salinity lenses.

Why Poles Can Be Saltier

• Brine rejection: Sea ice formation expels salt, creating dense, saline shelf water (Antarctic Bottom Water ~34.7 PSU).
• Low precipitation: Polar deserts (Antarctica interior, Arctic Basin) get <200 mm/yr.

Why Poles Can Be Fresher

• Ice melt: Summer meltwater + glacial runoff freshens surface (Arctic <30 PSU near coasts).
• River discharge: Ob, Yenisey, Lena, Mackenzie pour fresh water into Arctic.

Enclosed Basins: The Real Extremes

Red Sea (~41 PSU), Persian Gulf (~40 PSU), Mediterranean (~38.5 PSU)—high E-P, restricted exchange. Baltic Sea (~7 PSU), Black Sea (~18 PSU)—high R, restricted exchange.

Conclusion: Salinity is a budget variable, not a latitudinal one. The global pattern reflects atmospheric circulation (Hadley/Ferrel cells), ocean currents, and geography—not a simple tropical>polar rule.