What determines the movement of tropical cyclones?
One look at the tracks of cyclones in the Australian region (see map) shows that cyclones can move in many different directions. The path of each storm varies considerably in response to the weather patterns occurring at the time. Tropical cyclones can be thought of as being steered by the surrounding environmental flow particularly in the middle parts of the atmosphere (from about 2 to 6 km above the surface). Typically there is a preferred movement to the west to southwest at speeds of 5 – 20 km/h especially when they are in the northern tropics. As they reach further south they are more likely to take a more southerly track and then once south of about 25S they typically are moving to the south southeast and accelerating. This was the case with Vance in the image below. Cyclone Lena and Rewa however followed very different and more erratic paths. The larger and more intense systems do influence the surrounding environment and in so doing affect their movement. Short-term fluctuation
Tropical cyclones can be thought of as being steered by the surrounding environmental flow throughout the depth of the troposphere (from the surface to about 12 km or 8 mi). Dr. Neil Frank, former director of the National Hurricane Center in Miami, used the analogy that the movement of hurricanes is like a leaf being steered by the currents in the stream, except that for a hurricane the stream has no set boundaries. In the tropical latitudes (typically equatorward of 20°-25°N or S), tropical cyclones usually move toward the west with a slight poleward component. This is because there exists an axis of high pressure called the subtropical ridge that extends east-west poleward of the storm. On the equatorward side of the subtropical ridge, general easterly winds prevail. However, if the subtropical ridge is weak – often times due to a trough in the jet stream – the tropical cyclone may turn poleward and then recurve back toward the east. On the poleward side of the subtropical ridge, wes
