Of all the weather systems on Earth, tropical cyclones produce some of the most intense and damaging weather. Known as Hurricanes in the Atlantic, Typhoons in the Pacific and Cyclones in the Indian Ocean, these storm systems often make the news when they make landfall in populated areas, and sometimes cause widespread devastation to homes, infrastructure and crops.
The largest losses of human life due to weather events are also associated with these systems, with death tolls sometimes into the 100’s and even 1000’s. Usually the majority of deaths occur, not whilst the system is actively affecting a region which may be for periods of less than a day, but in the aftermath as flooding and damage to the local infrastructure lead to disease, drowning and lack of a clean water supply. Figure 1 shows a satellite image of Hurricane Katrina which hit the Gulf coast of the USA near New Orleans in August 2005. This storm led to an estimated 1800 deaths mostly through disease and dehydration in the days following the storm.
Figure 1: Satellite image of Hurricane Katrina over the Gulf of Mexico, August 28th, 2005. © NASA’s Earth Observatory website
Whilst intense storms are very common throughout the Tropics, tropical cyclones need a very specific set of ingredients in order to form. These are:
• Warm ocean temperatures, greater than 26°C to a depth of at least 60m. Tropical cyclones only form over oceans, never over land, as they need sources of both heat and moisture. Even if the surface waters are very warm, the strong winds in a developing cyclone cause mixing of the surface layer of the ocean, bringing water up from below the surface. If this water is very much cooler than the surface, the supply of heat to the system will be reduced.
• Latitudes greater than 5° north or south of the equator. Cyclones are rotating systems and so only form in regions with a sufficient component of the Earth’s rotation about the local vertical (the Coriolis Effect) . This is zero on the equator itself and only becomes large enough to generate rotation within a weather system poleward of 5° of latitude.
• No large changes in wind speed and direction with height (known in meteorological terms as low wind shear). Cyclones rely on the development of tall columns of convective cloud which extend of the order of 10 km in the vertical. If the wind is changing too much with height, these columns of cloud cannot form through a great enough depth of the atmosphere as the tops will be constantly blown away.
• Lots of moisture through the depth of the atmosphere. As well as large wind shear, a dry atmosphere can also act to prevent deep columns of cloud forming.
• A pre-existing disturbance in the atmosphere. Tropical cyclones do not form spontaneously from nothing. There needs to be an area of enhanced thunderstorm activity which can act as a focus for the development of the cyclone in the presence of all the other conditions listed above. Forecasting the formation of tropical cyclones relies on early identification of these pre-existing disturbances and then recognising which ones will amplify into full-blown cyclones.
The development of tropical cyclones is a complex process and is still the subject of much research. In simple terms, a cyclone acts like an engine. It converts the energy available from a warm ocean surface into strong vertical air currents and horizontal wind speeds via the evaporation of warm water from the ocean surface and the subsequent condensation of this water vapour in deep columns of cloud around the centre of the storm. This deep cloud often forms an almost circular ring called the eyewall around the very centre of the storm which is itself free from clouds. Thankfully cyclones are rather inefficient engines, converting less than 10% of the available heat energy from condensation in the clouds into the mechanical energy of the motion of the winds.
There is an important positive feedback mechanism which allows cyclones to develop into intense systems. As the system starts to form, evaporation from the ocean surface acts as the source of heat and moisture for the formation of deep clouds. As these clouds intensify, strong rising currents of air in the eyewall around the storm centre. Near the surface, air is drawn into the centre of the storm to replace the rising air. This inrushing air near the surface results in strong winds which increase the evaporation from the ocean surface. This evaporation provides more heat and moisture to the clouds making the rising air currents within them stronger and thus intensifying the surface winds even further. As the system develops the Earth’s rotation acts on the inrushing winds, deflecting them into a pattern that rotates about the centre of the storm, spiralling in towards the centre.
A result of the mechanism for development described here is that the most severe weather, the heaviest rain and strongest winds, is strongly focused in the centre of the cyclone. In Figure 2 the diameter of the almost circular region of cloud associated with a Hurricane is about 800km. However, the most damaging winds and heaviest rain are all concentrated within the innermost 200km of the storm
Figure 2: Diagram of a tropical cyclone.© NASA
The decay of a tropical cyclone usually occurs when the source of energy to the storm, the warm ocean surface, is removed. This may be due to the cyclone moving over land or into an ocean region with lower surface temperatures. An increase in vertical wind shear can also bring about the decay of a tropical cyclone.
The Met Office website has an informative section on tropical cyclones. This includes information on how they are named and categorised, maps of the tracks of all cyclones since 1992, notes on how the storms are forecast and links to information on how climate change may affect the formation of these systems in the future.
The Hurricane Research Division of the US National Oceanic and Atmospheric Administration (NOAA) also has a very informative FAQ on the subject.
All of the systems described in this and the two previous Steps, have emphasised the greater intensity of the weather patterns in the Tropics which is a response to the large amounts of energy received from the Sun at tropical latitudes.
The strong coupling between the warmest ocean surface temperatures and heaviest rainfall rates is a striking feature of the tropical climate.
El Niño is an excellent example of the coupling between tropical rainfall amounts and ocean temperatures. Shifts in the pattern of temperatures in the equatorial Pacific Ocean lead to very large changes in weather patterns, not just in the tropical Pacific but right around the world.
Monsoon climates involve distinct seasonal shifts in wind direction, which are accompanied by large changes in rainfall amounts between dry and wet seasons.
The regions which experience monsoons are landmasses situated on the poleward side of very warm equatorial oceans. Rapid heating of the landmasses during the spring and early summer exaggerates the passage of the seasonal cycle in these areas.
Tropical cyclones are highly energetic weather systems which can lead to large amounts of damage and even loss of life if they make landfall.
Cyclones convert heat energy from the warm ocean surface into mechanical energy of strong vertical and horizontal winds through a positive feedback mechanism. This involves evaporation of water from the ocean surface, condensation within clouds and strong rising motion within the clouds that in turn leads to stronger surface winds and more evaporation.