Tropical cyclones are incredibly powerful and dangerous natural phenomena. Every year, coastlines all across the globe are impacted by these tempests. When the storms move toward land, heavy rain, strong wind, storm surge and tornadoes all contribute to their destructive power. Depending on location and strength, tropical cyclones are called tropical depressions, tropical storms, hurricanes, cyclones, typhoons, super typhoons, etc. Wave-riders have a strange relationship with tropical cyclones. We know their power but enjoy their wave energy. Let’s look at the cyclogenesis of these storms.
The sun warms the tropical Ocean and delivers radiant energy to its surface. Wind blows over the water, it evaporates and ascends energetically into the atmosphere. The rising moisture condenses into towering thunderheads. Air rushes upward and the atmospheric pressure in the center drops. This is a thunderstorm.
An organized group of thunderstorms that persists for 24 hours is called a tropical disturbance. When winds exceed 30mph, it becomes a tropical depression. The Earth’s rotation (coriolis effect) drives wind around the warm core of the storm. For the storm to continue to strengthen, it must remain over warm water and encounter minimal wind shear. This is when vertical winds slant the storm, dispersing the heat over a larger area, degrading the storm. Without wind shear, the cyclone remains upright and continues to develop.
When winds reach 39mph, the cyclone becomes a tropical storm and meteorologists give it a name. At 74mph, the storm becomes a hurricane. Hurricanes are characterized by a defined eye and strong low pressure in the center. Around the eye is the eye wall, an area of intense thunderstorms and the storm’s strongest wind. The Saffir- Simpson Scale rates hurricanes based on wind speed. From Category 1, 74-95mph to Category 5, >155mph. Category 3, 4 and 5 storms are deemed major hurricanes with the most dangerous conditions.
A tropical cyclone is a heat engine, fueled by the temperature gradient between the warm Ocean surface and the cooler upper atmosphere. Heat becomes motion as the warm, moist air rushes upward and is replaced by surrounding air.
There are many meteorological resources devoted to studying and predicting the path that a hurricane will take. There are lives and communities at stake. Wave-riders also take a particular interest in hurricane forecasting because the path of storm is a very important aspect of it’s swell generation. Although, tropical cyclones have strong winds, they lack the size and fetch of their extratropical cousins that generate 15-20+ second groundswell. Hurricane force winds might extend 100 miles from the eye and gale force winds another 300 miles beyond that. For ideal swell impact, a tropical cyclone will slowly move toward land, strengthening as it moves but turn away or dissipate before making landfall.
North America is impacted by two distinct tropical cyclone regions: the North Atlantic and Northeast Pacific. Interestingly, it is hypothesized that cyclones in both originate from the same phenomena. African easterly waves are disturbances in jet stream flowing off Saharan Africa. They can develop into tropical storms in the warm Atlantic or continue across the Caribbean and Central America without development. Reaching the tropical Pacific, the disturbance can develop into a tropical cyclone using the warm water off the coast of Mexico and Central America as fuel.
NOAA Hurricane Research Division