Current Fronts

An example of a current front is the boundary between the cold ocean water (dark blue) when it floods twice a day through the entrance (green fingers) to San Francisco Bay and pushes back the warmer, older water (light blue).
Water in the Bay is warmer than the ocean because much of the bay is shallow (average depth of about 3 m, 10 feet) and a few hours of sun is sufficient to warm this relatively thin layer. The water in the Bay has less salt than the ocean because the Sacramento River empties into the Bay. Fresher, warmer water is lighter than salty, cold water.

The color of the ocean water often contrasts with the Bay water, particularly in the Spring, when the River discharges a large amount of silt laden water.

What we see from above is the boundary or front between the two kinds of water. The front moves into the Bay, which is what we expect since the ocean water is displacing Bay water. However, you don't actually see the water currents, just the movement of the front. The actual currents are complex and best seen in a side view, as if you were a diver submerged at the front.

A convergent front
In the side view of the front, on the left, we are moving along with the front, so it appears fixed in the middle of view.

The boundary is not vertical, but inclined (the angle can be more extreme than indicated here). As the dense cold water pushes under the warm water it draws the warm water along with it. Thus, on both sides of the front the surface current is moving toward and thus converging at the front, as indicated by the red arrows.

Along the bottom the warm, and perhaps the cold water also, are moving away from the front. Of course if we average the velocity (relative to the bottom now) of the current from surface to bottom it must flow toward the right, the direction the front is moving.

What happens at a convergent front?

Floating trash collects at a convergent front. Pieces of wood, feathers, Styrofoam and everything else (called flotsam by sailors) on the surface is carried to the front and collects there if it can't be sucked down by the current. Thus, a line of trash on the surface is a hint that a convergent front is under the line.
Foam collects at and is created by a convergent front. The water of the Bay is actually a thin soup, since it contains a rich variety of dissolved and particulate organic material. The concentration at the air-water surface is higher than the average, and a unique class of molecules concentrates there. This material, proteins, lipids, and carbohydrates, is swept to the front and forms foam with entrapped air due to turbulence as water is drawn down into the front.
Plankton collect below a convergent front. Many zooplankton, for example copepods, maintain a fairly constant depth, by swimming up if they sink, and down if they come too close to the surface. As they are swept to a front they swim against the local downward current which then reduces their horizontal movement. As a result, they concentrate at the front [reference].
Fish that eat zooplankton know they can get them at a front and go there if they are hungry (fish are hungry most of the time).

Birds that eat fish know there is good fishing at a front and often congregate there.

Fishermen that are smart know that fronts are often good places to find fish.

Current fronts are where the action is.