Earth and space science

Ocean Processes

La Nina

La Nina develops when the warmest region of the pacific shifts toward the western pacific. the strengthening temperature gradient increases pressure differences between the east and west, intensifying trade winds. these stronger winds enhances the westward movement of warm surface water causing the warm less dense water to accumulate in the western pacific deepening the thermocline in that region.

La Nina pt. 2

The thermocline is much shallower in the cooler eastern pacific. because the intensified trade winds pull surface water away from the south American coastline through friction, very strong upwelling occurs. cold nutrient rich water rises to the surface increasing biological productivity and supporting fisheries along the eastern pacific. Th e shift in ocean temperatures influence atmospheric circulation, creating wetter conditions in the western pacific and drier in the east.

El Nino

During El Nino events, the trade winds weaken or reverse. Warmer water that has built up in the west now moves back toward the east, assisted by gravity as it flows back down the gradient. The result is a flattening out of the sea level, causing warm waters to now pile up off the coast of South America. This increases the surface temps of the coastal waters. As warm water is less dense than cold water, it sits on top of the deeper, cooler water, pushing the thermocline down deeper.  

El Nino pt. 2

The depth of the thermocline prevents mixing of the surface and deep waters, so upper waters are now nutrient deficient. so waters are less biologically productive and fisheries collapse as a result of mass fish die off. The east now experiences heavy rain and flooding, and Australia and Indonesia in the west experience droughts. Tropical cyclones also shift towards the central pacific.

The bioloigcal pump

The physical pump

Upwelling

Downwelling