365 Days of Climate Awareness 117 – Upwelling

 

Upwelling, where winds pull deeper ocean water to the surface, is entirely a function of Coriolis-related Ekman transport, where surface water moves away at a 90° angle from the wind (to the right in the northern hemisphere; to the left in the southern). Winds along the coastline or in a hurricane push water away, forcing deeper water to take its place. The opposite process, downwelling, also occurs but mostly along the coast, as high-pressure storm systems analogous to hurricanes don’t exist.

Coastal upwelling (northern hemisphere).

When wind moves along the coast in the northern hemisphere, so that land is to the left and ocean to the right along its path of travel, it forces water along the shore to move away through Ekman transport from the shoreline. But due to the continuity of mass—more water must take the place of the water pushed away—the only water available is from deeper down. The deeper water is colder and, because mineral and dead organic matter slowly settles toward the sea floor over time, nutrient-rich. For this reason, coastal areas on the planet where wind patterns consistently produce conditions of upwelling are known as rich fisheries.

Coastal downwelling (northern hemisphere).

This is also why El Niño events destroy the fish catch along the west coast of South America. The steady influx of warm water from the western Pacific shuts off wind-driven upwelling. The warmer water is relatively depleted of oxygen and can’t sustain fish and other species nearly as well as the colder water which winds pulled up from deeper down, leading to die-offs and depressed catches.

Global upwelling zones.

Hurricanes are tremendous sources of upwelling, and tend to leave a trail of cool, well-mixed water in their path. As the hurricane winds blow violently around the storm center in cyclonic (same rotational sense as the planet: counterclockwise in the north, clockwise in the south), the tremendous wind stress they create on the ocean surface drives water to the right (left) away from the storm on all sides. This creates a tremendous, continual pump of deeper water up into the storm center as it moves (and is a main reason why hurricanes tend to die as they move north: colder upwelled water gradually cuts off the upward circulation of air in the storm).

Rotational sense (direction) of each hemisphere, and why hurricanes spin in different directions.

Downwelling occurs mostly along the coast, when the wind blows such that, in the north, open water is to its left as it goes, land to right (and opposite in the south). High-pressure atmospheric systems do produce winds which blow in an anticyclonic (clockwise in the north, counterclockwise in the south) sense, but they tend to be far gentler than the violent low-pressure storm systems, and do not produce such large effects in the ocean.

Upwelling pump of a hurricane.

Tomorrow: ocean gyres.

Be brave, and be well.