As humans warm the planet through the emission of heat-trapping gases, we expect weather to change. Some ways it has changed are clear and measurable. For instance, heat waves and droughts are setting in faster and are more severe. We are also seeing more intense precipitation events that lead to more flooding.
But what about storms? We know that hotter ocean waters add fuel to storms, particularly typhoons and hurricanes. That tends to make them stronger. Also, the added heat increases rainfall and the rising seas make us more vulnerable to storm surge. But it isn’t this straightforward. Hurricanes need the right conditions to form and there is evidence that those conditions will become less likely. So, the general rule of thumb is, there may be fewer typhoons and hurricanes, but they will become more intense.
But that is the future, what has the past looked like? A recent paper has just been published which looks at this issue. Now, with good satellite and measurement coverage, we have a good sense of past storm trends. Are we in a hurricane drought or are there plenty of storms occurring now? To answer this, it depends on how you define hurricanes and their strength. Currently, storms are binned to 5 knot wind speeds (grouped together). However, the wind speeds are uncertain to 10 knots. So there is a good deal of uncertainty of the actual strength of a specific storm.
If we look at landfall hurricanes in the USA, the last major hurricane was Wilma in 2005. That hurricane had winds above 96 knots. The 9-year drought for hurricanes of this strength is pretty rare. But what if we define major hurricanes differently, using different thresholds? Then the drought becomes less significant. Furthermore, with an even better measure (pressure) the current hurricane drought disappears because Irene and Sandy made landfall in 2011 and 2012 and had very low pressures.
So, what measure do we use, wind speed or pressure? And what levels do we define as major events in our categories? It seems that using the current somewhat arbitrary wind speed is not wise. Under that scheme, both Sandy and Irene are not significant events, even though they caused almost $100 billion in damages. The authors of the above-referenced paper write that even though both pressure and velocity differ in what they represent, both are valid metrics for the intensity of a hurricane. In fact, pressure is usually a better indicator of economic damage than is velocity.
As a further note, while the wind speed is a measure of the air velocity of a hurricane, the square of the speed gives you information about the pressure exerted on surfaces, and the cube of the speed gives you the power of the wind flow. In fact, when I design wind turbines, I routinely use wind speed cubed to estimate available wind power. The term Accumulated Cyclone Energy is a measure of the pressure exerted by wind (velocity squared). The term Power Dissipation Index is a measure of the flow power of wind (velocity cubed).
To me, (I study wind power and fluid mechanics but I am not a hurricane expert), the best measure of a hurricane/cyclone’s strength would be the power dissipation along with measures of the size of the storm and the precipitation. All of these factors work together to lead to economic damage. Perhaps the central pressure in the storm is the best single indicator of these multiple parameters. The authors give convincing evidence that pressure is a better yardstick than just velocity. When we use their yardstick, we see a continuous population of land-fall storms in the USA. The so-called “drought” has disappeared.