Pre-conditions for hurricanes
Several atmospheric ingredients must come together for a hurricane to form. There are perhaps seven atmospheric conditions which, if met, could cause a hurricane to form. A pre-existing disturbance, warm ocean water, low atmospheric stability, sufficient Coriolis force, moist mid- atmosphere, and upper atmosphere divergence are all important factors for hurricane formation. These conditions are discussed in greater detail in another article by Rich Johnson called How do hurricanes form? These factors are important in that tremendous amounts of heat energy are transported from the tropics northward to the higher latitudes. The hurricane is a large heat engine, where great amounts of heat are being produced from the process of latent heat of condensation. This occurs as water vapor is being evaporated from the ocean surface and condensed into cloud droplets.
Mechanisms to cause hurricane formation
If all of the pre-conditions are met, hurricane formation then becomes possible. There are several types of atmospheric disturbances that can cause a hurricane to develop. The most common mechanism to develop hurricanes is the monsoon trough. This is an extension of the Inter-Tropical Convergence Zone where cyclonic spin has developed. The ITCZ is a trough of low pressure created by the convergence of the northeast and southeast trade winds. This "trade wind" trough does not contain the spin to initiate hurricane formation. Hurricanes are caused mostly by the monsoon trough in six of the seven hurricane formation basins of the world. The north Atlantic basin is the exception.
In the north Atlantic hurricane basin, easterly waves are the primary cause of hurricane formation. These waves or troughs of low pressure form over north Africa then move off the African coast into the Atlantic. Sometimes a WADL or African squall line will act similar to an easterly wave and cause a hurricane to form.
Another cause of hurricane formation is from the Tropical Upper Tropospheric Trough (TUTT). The upper air low is cold in nature as compared to its surrounding environment. A typical low pressure center that forms a hurricane starts at the lower levels and is warm compared to its environment. If a TUTT low remains over the warm ocean waters for several days, sometimes it gradually warms and takes on tropical characteristics. At this point the low "builds" down to the surface from the upper atmosphere. The "cold" low pressure system is transformed into a "warm" one and tropical development starts.
Another cause of hurricane development is from a front which stalls over tropical waters. Inherently, a front has a cyclonic spin associated with it. If winds aloft become favorable with little wind shear, showers and storms could become more numerous and cause a hurricane to form.
Lastly, a mesoscale convective system or MCS can sometimes be the cause of tropical cyclone development. These organized clusters of storms can move off a continental landmass and drift over warm waters. If they already contain a small vortex in the cluster, this may make to region even more favorable for tropical cyclone development. Again, by far, easterly waves cause most hurricanes to form in the Atlantic Ocean. Research has shown about 60% of hurricanes and 85% of major hurricanes form from easterly waves. Otherwise, most hurricanes are caused by the monsoon trough around the globe.