Since 1995, Gray and others have been saying that conditions are ripe for a return to more active hurricane seasons, a periodic shift that could last two or three decades. He pointed to known historical cycles to buttress his argument that nature is at a turning point. In between, from the early 1930s through the late '60s, more active seasons were the rule. While many factors control the number of hurricanes in a given season, Gray has said the long-term trend is ruled by a conveyor belt of sorts that moves Atlantic Ocean water northward from near the Caribbean to an area east of Greenland. There, the current sinks deep, moves southward and flows into the southern Atlantic Ocean and beyond.
Recent measurements show that the water in the conveyor belt contains more salt than normal.
This increases the water's density, researchers explain, causing it to sink to greater depths. This in turn increases the flow of deep water southward, which forces an increase in the flow of warm, tropical surface water northward. Thus, the northern Atlantic becomes warmer than normal, providing the heat needed to make hurricanes into powerful killers. In the shorter term, La Ni a gets some of the blame. During an El Ni o event, energy from warm water in the Pacific is transferred high into the atmosphere and funneled, in the form of strong winds, eastward, where it settles over hurricane-forming regions of the Atlantic, thousands of miles away.