Hurricane Dynamics

Hurricanes or tropical cyclones are essentially low pressure systems which spin counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Because the Earth is spherical, objects on the surface of the Earth closer to the poles experience a stronger Coriolis force than objects closer to the equator. This Coriolis force is an effect of the conservation of angular momentum which causes a deflection towards the right in the Northern Hemisphere and towards the left in the Southern Hemisphere. At the equator, the Coriolis effect is zero. While tropical cyclones usually do not form in the mid-latitudes with colder sea surface temperatures, they also do not form right at the equator because some amount of the Coriolis force is necessary for the tropical cyclones to spin up.

At the center of every tropical cyclone is a low pressure center. Because of the relatively higher pressure surrounding the tropical cyclone, the high pressure forces winds inward toward the low pressure center. These winds, moving from the surrounding high pressure to the low pressure, are turned toward the right in the Northern Hemisphere due to the Coriolis effect. This rightward turn of the winds moving toward the low pressure center causes a counter-clockwise rotation of tropical cyclones in the Northern Hemisphere and a clockwise rotation of tropical cyclones in the Southern Hemisphere. The low pressure center therefore develops a counter-clockwise, closed circulation around the lower pressures. Once a clear circulation of winds flowing counter-clockwise all the way around a low pressure center is observed, the storm is referred to as a tropical cyclone. This low pressure center with a closed, counter-clockwise circulation is the fundamental and necessary condition for a storm system to be referred to as a tropical cyclone in the Northern Hemisphere.

The strong, counter-clockwise rotation within tropical cyclones in the Northern Hemisphere advects higher amounts of potential vorticity from the equator towards the North Pole. This is due to the fact that objects on the Earth’s surface are spinning slightly faster closer to the equator as opposed to closer towards the poles. The resulting advection of potential vorticity causes tropical cyclones to slowly move poleward and westward relative to the motion they would have if no advection of potential vorticity was occurring. For example, in the Northern Hemisphere, hurricanes within the Atlantic basin tend to drift toward the northwest while they are in the tropics due to this effect. This induced movement of tropical cyclones is referred to as “Beta Drift” because the strength of the Coriolis effect is often referred to by the Greek letter Beta. For an in-depth look at how this Beta Drift motion is created, see the Tweet/imagery below.

After tropical cyclones have drifted toward the northwest for a certain amount of time, they will begin to be affected by the mid-latitude jet stream, which flows from west to east in both the Northern and the Southern hemispheres. This will eventually overcome the westward component of the beta drift, causing the tropical cyclone to be deflected toward the east once it reaches the mid-latitudes. This results in the common “C” shape of the tracks of tropical cyclones in the Atlantic basin as they develop in the tropics west of Africa, move northwest towards the Caribbean and the United States, and then eventually get swept toward the northeast by the mid-latitude jet stream.

The Beta Drift effect discussed above is strongest closer to the surface, where the winds within a tropical cyclone are the strongest. Therefore, the low-level circulation, or LLC, induces a stronger Beta Drift closer to the ground level than the mid-level circulation, or MLC. This difference in the strength of the Beta Drift causes the lower levels of the storm to drift further toward the northwest than the mid-levels or upper-levels of the storm. Because of the difference in magnitude of the Beta Drift with height, the tropical cyclone becomes tilted as though a small amount of vertical wind shear were acting on it. When the LLC and the MLC are not aligned within a tropical cyclone, strengthening is typically limited because of the displacement within the convection ongoing in the eyewall surrounding the eye. This is a major reason for large amounts of wind shear being detrimental to the structure and strength of a tropical cyclone. The difference in Beta Drift between the LLC and the MLC results in a small amount of “Beta Shear,” which causes the LLC and the MLC to be misaligned.

A small amount of southeasterly shear (vertical wind shear from the southeast) will combat this “Beta Shear” and realign the LLC and the MLC within a tropical cyclone. Therefore, in the Atlantic basin, small amounts of southeasterly shear often result in more well-defined eyes within tropical cyclones. These picturesque tropical cyclones with little to no clouds in the eye and a strong, well-defined, and symmetric eyewall are often referred to as annular tropical cyclones. Annular hurricanes often lack strong outer rainbands and appear as more of a “donut” shape on satellite imagery.