Why do tropical cyclones develop and how do they move?

I find observing the evolution of tropical cyclone development in action on satellite imagery mesmerizing. Mature hurricanes are the epitome of beauty, power, and symmetry nature may possess occupying vast scales. Fantastic, but how do we get a tropical cyclone on the planet in the first place and what makes them move? Time for some fundamentals!

There are a few key environmental factors that make tropical cyclone formation and spin-up ideal. First, got to have warm ocean waters, specifically near 80°F (26°C) extending down to around 200 feet (61 meters) below the surface. Deep warm waters provide the ongoing evaporation necessary to increase ambient water vapor in the air above. Water vapor carries vital latent heat energy that eventually can get released into the atmosphere upon phase changing to visible cloud material (liquid condensation), which is made possible by rising motion and then cooling of moisture laden air masses after achieving higher altitudes. The convective process for replenishing moisture and heat energy through the tropospheric column is essential to support and sustain tropical thunderstorm activity associated with tropical cyclones.

Coupled with increasing atmospheric instability from excessive latent condensation heat release, a second necessary requirement is actually weak wind shear acting on a developing tropical disturbance. Too strong a difference in vertical wind speed between the surface through the top of the troposphere prevents clusters of unsettled tropical thunderstorm activity from organizing into a coherent single tropical cyclone. Generally, a threshold of greater than 20 mph has been linked to being hostile for cyclone development.

Likely the most striking feature of tropical cyclones is of course the spin, counterclockwise (clockwise) in the Northern Hemisphere (Southern Hemisphere). If you recall, Earth's rotation induces the Coriolis effect, which happens to be instrumental for tropical cyclone development by imparting the necessary cyclonic rotation to facilitate broad-scale thunderstorm banding and overall organization around a center of lowest surface pressure (for mature cyclones this is the cloud free "eye" portion of the storm). As a quick refresher, the Coriolis effect in the Northern Hemisphere (Southern Hemisphere) forces traveling air parcels moving in any direction to become deflected to the right (left) of the intended straight line target (i.e., air wants to move directly from high pressure to low pressure). As air moves towards a developing tropical disturbance (an area of relative low pressure) from all sides, the net result is a counterclockwise (clockwise) rotation when observing a cyclone from above in the Northern Hemisphere (Southern Hemisphere). Without the Coriolis effect, hurricanes would not develop. This is why tropical cyclones have not been observed to develop along the equator where the Coriolis effect is null on our planet.

Alright, we now have sufficiently warm sea and sub-sea surface temperatures, weak vertical wind shear, and an existing tropical disturbance north (or south) of the equator to allow the Coriolis effect to instigate rotation and help organize convective thunderstorms. A hurricane soon forms, but how and why does the tropical system move?

Well, geographically speaking, the ideal overlap of very warm sea waters, tropical air masses, and exposure to the Coriolis effect puts you in the prevailing easterly "trade winds" of the planet bounded between 30 degrees north and south latitude (but not inclusive of the equator). The persistent easterly flow in these latitudinal belts above and below the equator would then tend to carry a developing tropical cyclone on a westwards trajectory, hence, for example, why meteorologists at the NOAA National Hurricane Center are watching the evolution of any tropical disturbances as far away as offshore of Africa as they could pose an eventual risk on the other side of the Atlantic Basin for places like the Caribbean, Bahamas, and eventually the United States.

We can get more detailed on how hurricanes move, though. The Coriolis effect also imparts a general westwards momentum on tropical systems due to the principle of conservation of absolute vorticity for air masses. In other words, air masses have a certain "spin" or vorticity value associated with them. Strong low pressures have high positive vorticity, for example. Because the Earth is rotating, the Coriolis effect is a contributing factor to determining an absolute vorticity balance. Importantly, the Coriolis effect vorticity contribution progressively increases (decreases) poleward (equatorward). Therefore, in order to conserve absolute vorticity, air moving around a circulation must increase or decrease in "relative vorticity" to compensate for a varying Coriolis effect contribution.

Considering Northern Hemisphere tropical cyclone activity, what's happening here is the air mass rotating southwards on the western side of a spinning tropical cyclone has inherent greater relative vorticity arriving from higher latitudes; however, the opposite is occurring on the eastern side of the cyclone as air is being transported northwards leaving lower latitudes having less relative vorticity from a diminished Coriolis effect contribution. Importantly, there's a natural tendency for tropical cyclone circulations to keep being drawn towards regions of highest relative vorticity (i.e., found on the western side of the storm's circulation). In other words, tropical disturbances favor a west or northwest trajectory after spin-up. This process is called the "beta effect" and has a lot of significance for hurricane forecasting purposes! You likely have already seen the beta effect in action when shown a projected hurricane forecast map indicating an initial westward cyclone track that starts to bend more northwestwards with time.

Finally, placement and progression of upper level large-scale areas of high pressure (anticyclones/ridges) and low pressure (troughs) greatly impact where a hurricane may get steered to or if a natural blocking pattern against a potentially devastating landfall is a possibility. For example, a costly environmental and economic disaster from a major hurricane landfall on the United States eastern coastline may simply be spared by a timely low pressure trough passing in the Westerlies (~30-60 degrees north and south latitude). The accompanying strong southwesterly flow can halt, weaken, or deflect a hurricane back out to the open waters of the Atlantic Ocean. In lieu of a blocking pattern or encountering a region of greater vertical wind shear, tropical cyclone activity starts to weaken substantially either by moving too far northwards over cooler sea waters or moving inland, especially upon interacting with mountainous terrain that slows and disrupts the symmetry of the circulation via frictional forces.

Yes, there's a lot to consider when monitoring and tracking tropical cyclones!

-JWM

Data Source: College of DuPage