One of the keys to understanding ocean currents, and the "ocean conveyor belt" is realizing how temperature and salinity affect the density of water - referred to as thermohaline circulation. As water gets colder and/or saltier, it tends to sink (density current). The lab-activity shown in the video is one of our favorites at Helena High School. We call it "The Briny Deep". Students start with 800 ml of room temperature water in the tilted box, then mix various types of water (salty, cold, hot), make predictions, and then pour.
Credit for the demo goes to WARD's Science. The activity shown in the video is part of a kit that we purchased from WARD's, called "Exploring Convection". The kit also includes a second lab-activity that helps students understand the role convection plays in causing wind. Both labs come with great handouts that guide students through the activities and then follow up with questions that help them understand real-world applications.