Just like convection in air, when denser water sinks, its space is filled by less dense water moving in. This creates convection currents that move enormous amounts of water in the depths of the ocean.
The way to tell them apart from each other lies in the speed of the wind. When the speed of the wind is between 39 mph to 73 mph, the tropical depression developed (low pressured areas in the ocean that have the power to grow stronger) is classified as a tropical storm. Hurricanes are more intense and are formed over oceans with the wind speed reaching at least 74 mph. Hurricanes also have a noticeable eye in the center of the storm and tropical storms don't have any at all. Basically, a hurricane is a more intense version of a tropical storm that appear to be smaller compared to a tropical storm.
Hope I answered this correctly and hope it helps! :)
