Today we get a look inside how Tropical Cyclones (tropical storms and hurricanes) develop and track.
Most Tropical Cyclones form from disturbances in the easterly trade winds known as Tropical Waves. They are so called because many originate in the Tropics. A ‘wave’ in this sense is an area of slightly lower pressure due to a slight wind shift. This slight wind shift creates surface convergence which helps showers and thunderstorms or convection develop lowering surface pressures. If the atmospheric and oceanic conditions are right – warm seas (>80F/26C), humid atmosphere, and light upper atmospheric winds – the slight wind shift can be amplified into a broad area of rotation by the convective activity and a tropical low forms.
Continued convection near the center of this broad area of rotation helps to lower pressures further. At the surface, air or wind tends to flow towards low pressure and lower pressures typically translate to faster winds. As the surface pressures fall and winds increase, the broad area of rotation can become better defined. If shower and thunderstorm activity appears to be sustained near the center of this low a Tropical Depression has formed and gets a number identifier.
Now that this is a consolidated cyclone – Tropical Depression – convective activity is enhanced not only by convergence but also through upper divergence. Convergence at the surface has created thunderstorms so tall that they’ve reached the upper limit of the troposphere and cannot rise any further. Air cools as it rises and will continue to rise until it is cooler than surrounding air (eg. at the top of the troposphere where the surrounding air is often warmer than air rising from the surface). Because these thunderstorms can no longer rise, they spread out (diverge) at this level. This divergence at the tops of the troposphere creates a void allowing more air from below to rise – and lowering surface pressures further. When pressures fall enough to allow winds of more than 34kts/39mph, the National Hurricane Center redefines the Tropical Cyclone as a Tropical Storm and it gets a name.
If atmospheric winds above the surface do not interrupt this process, convective activity will continue to intensify and organize around the center of low pressure that is our Tropical Storm. Bands of showers and thunderstorms begin to form and they rotate in towards (converge on) the center of the storm. These bands can wrap around the center forming a ring of very intense convection called an eye wall. When this process begins the Tropical Storm’s winds are likely exceeding 64kts/74mph and the National Hurricane Center can define the storm as a Hurricane.
Essentially Tropical Cyclones are heat engines, and they are pretty efficient! They transport energy from the surface to the upper atmosphere in the form of latent heat. Additionally, this energy is transported from the tropics to the poles as these cyclones are pulled towards the north. A Hurricane can significantly cool the sea surface beneath it (via evaporation and mixing through wave action), leaving a wake of cooler waters where it tracked, this can hinder further development if the Hurricane moves too slowly. Where warm water extends to a great depth and atmospheric conditions are near ideal, hurricanes continue to strengthen allowing winds to reach 96kts/111mph and they are called major hurricanes in reference to the amount of damage they are capable of based on the Saffir-Simpson hurricane wind scale.
It is still not well understood what exact conditions are necessary/ideal for Tropical Cyclone intensification and this is an area of forecasting that is lackluster. However, track forecasting has become remarkably accurate in the last few years. Tropical Cyclones that form early in the season tend to be confined to the Southwestern Atlantic (including the Caribbean and Gulf of Mexico) and they track northwards because of frontal systems that are still able to dig that far south. However, later in the season (towards the peak in activity) Tropical Cyclones can form in the central Atlantic where they are trapped beneath the dominating Bermuda-Azores high pressure system. This system pushes tropical cyclones west and west-northwestward into the Western Atlantic. Here the ridge, associated with the Bermuda-Azores high, is weaker and storms can be pulled northwards through the Caribbean and Gulf of Mexico towards the Southeastern United States or through the Western Atlantic towards Bermuda. As the season comes to an end, frontal systems can once again reach the tropical Atlantic and pull systems easily northwards from their birthplace that is again confined to the Southwest Atlantic. See a more detailed description of common tracks on this National Hurricane Center Page.
Sometimes these frontal systems get cut off from the mid-latitude westerlies and get stuck in the tropics and sub-tropics. When this happens they can start to take on the convective processes associated with tropical cyclones and lose their frontal characteristics and Sub-tropical Cyclones form. If the cyclone is sustained wholly by these tropical processes, it becomes a Tropical Cyclone – another, less common way for them to develop.Sub-tropical Cyclones rarely reach hurricane intensity without becoming tropical because weaker non-tropical processes compete with the tropical ones, moderating the cyclone’s intensity somewhat – however some have reached hurricane intensity like Sub-tropical storm Karen (2001). This Sub-tropical transformation process is more common early and late in the season, but could occur at any time in the year – even outside the season.
Additionally, when a Tropical Cyclone moves out of the tropics into the mid-latitude westerlies, they begin lose the tropical processes and take on frontal characteristics and become Post-tropical Cyclones. These can be just as dangerous as Tropical Cyclones because of their tropical origins but they are meteorologically different. Instead of relying on surface convergence, convection, and upper divergence these systems rely on a temperature and moisture difference between to adjacent air masses. These air masses meet and form fronts – the different densities of the air masses cause the warmer/more humid (less dense) one to rise above the cooler/less humid(more dense) one allowing convection. For reference, Hurricane Sandy (2012) became Post-tropical before hitting New Jersey. Post-tropical Cyclones are typically a threat to Maritime Canada, the Azores, and Europe as they track eastwards across the North Atlantic.
Cyclones can be very unpredictable, especially in terms of intensity and when they are slow moving. If a storm threatens, please stay informed by reading updated from the Bermuda Weather Service, including their Warnings, and Tropicals pages. Additional official information can be found on the Emergency Measures Organization page and the National Hurricane Center‘s page. Check out some Hurricane Awareness Week information provided by the Bermuda Weather Service.