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Lecture

Vertical Circulation Upwelling and Downwelling

Evidence of upwelling along the southern California coast.
Evidence of upwelling along the southern California coast. Colors indicate levels of productivity, with oranges and reds indicating higher productivity. Productivity is simply a measure of how many marine plants, the base of oceanic food chains, are found in the waters. (Images provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE)
Up to this point we have been discussing the horizontal movement of water through surface circulation. Now, lets explore how water moves vertically either from the deep ocean to the surface through upwelling or from the surface to deeper parts of the ocean through downwelling.

Upwelling occurs where water from the deeper parts of the ocean is allowed to travel upwards to the surface. We know from our discussion of seawater that deep ocean water is generally very cold and nutrient rich. Therefore, in areas of upwelling we often find very productive waters like on the California coast in the image to the right.

We find upwelling along the equator due to diverging surface waters, around Antarctica between the East Wind Drift and West Wind drift, and along coastlines where the wind blows in such a way that Ekman Transport pushes water away from the coast. Your book provides explanations of each of these processes and several others that cause upwelling.

Downwelling is the vertical movement of surface water to deeper parts of the ocean.  Surface water is generally fairly warm and low in nutrients, but carries an abundance of dissolved oxygen to deeper waters. Dissolved oxygen is critical for living organisms in the deep ocean. Downwelling occurs in areas where waters converge and “pile up” such as in the North Atlantic where several surface currents meet. We also find downwelling along the outermost boundary of the Southern Ocean where cold Antarctic water sinks below warmer South Pacific and South Atlantic waters. Additionally, we find downwelling on some coastlines where the wind blows in a direction that causes Ekman transport to move water toward the coast.  Your book provides explanations of each of these processes that cause downwelling.

This figure below shows how Ekman transport, aided by the wind and the coriolis effect, creates areas of upwelling and downwelling in coastal locations of the Southern hemisphere. Recall that Ekman transport moves water to the LEFT of the wind direction in the Southern Hemisphere. How would these images look different if these coasts were located in the Northern Hemisphere?

illustration of how Ekman transport creates areas of upwelling and downwelling

 

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