Excellent  video (<5 min) explaining gyres, the Coriolis effect, and thermohaline circulation as well as the the Global Conveyor Belt.

Recommend for middle school and up.

Excellent  video (4 min) explaining "oceanic circulation is driven by differences in temperature and salinity and by wind currents, and oceanic circulation has a major impact on temperature and precipitation in coastal areas."

Recommend for high school and up.

Short article with embedded video (3 min and excellent) and images explaining changes to the ocean currents in the Gulf of Maine (Massachusetts up to Nova Scotia). 

Recommend for middle school and up.

Excellent article and graphics explaining the "global conveyor belt" and scientists' thoughts on how changing water density is affect the flow of the current. 

Recommend for high school and up.

Short article explaining thermohaline circulation. Has a 3D visualization of ocean currents in the North Atlantic Ocean . 

Recommend for high school and up.

Current scientific thought on this topic.  

Recommend for high school and up.