Naval aircraft that will operate from aircraft carriers not equipped with catapults will use short takeoff and vertical landing (STOVL).
Carriers intended for STOVL aircraft may have "ski jump" ramps, as used on current British, Indian, Russian and Spanish vessels. By making a short takeoff roll up a ramp, the aircraft can take off with more energy and a heavier load.
U.S. amphibious warfare ships that carry U.S. Marine Corps STOVL aircraft, such as the current AV-8B Harrier II and future Harrier (fighter), do not use ski jumps. The principal reason is that this gives more deck space for VTOL helicopters and MV-22 Osprey tilt-rotor aircraft.
Different kinds of interservice politics may be involved in not having the ski jump. From a cynical Marine perspective, it gives them confidence that the Navy will not take its amphibious assault ships as general-purpose aircraft carriers. From a cynical Air Force perspective, the Navy will not be able to have more decks that can handle the higher-performance F-35C Lightning II.