Why Tropical Cyclones Move the Way They Do

Every year, violent storms rip across tropical waters, prompting vessels to deviate from their planned routes. As the storms approach land, coastal residents evacuate inland to safety. These storms, called hurricanes, typhoons, or tropical cyclones depending on the basin in which they form, pose interesting challenges to forecasters, particularly when it comes to the storm’s intensity and movement. Tropical cyclones (the generic term) move much like a leaf floating in a stream, drifting where the current pushes it. However, the atmosphere does not have a fixed boundary like a stream. Free-flowing air lacks stream banks, and receiving energy from the sun on a tilted, rotating planet means the air is in constant motion. This influences how “leaves” such as tropical cyclones (TCs) are steered in the atmospheric flow. The conditions for TC formation (see our article on TC basics here) limit development to specific areas over the ocean, where they typically move along similar tracks to those shown below.

Each ocean basin has a semi-permanent area of high pressure near the center of the basin due to complex interactions of the sun heating the Earth, rising and sinking air, and the Earth’s rotation. These high pressure areas meander throughout the year but remain over the oceans. In the North Atlantic, the semi-permanent high pressure area is often called the “Bermuda” or “Azores” high, depending on which island group the center of high pressure is closest to.

In the Northern Hemisphere, winds flow clockwise around the high pressure center, while in the Southern Hemisphere, the winds blow the opposite direction. The result is that in both hemispheres, winds move from east to west near the equator, producing the easterly trade winds. When a TC forms, usually in or near the trade winds, the storm moves generally east to west. After a while, the trend is to then turn to the right in the Northern Hemisphere or left in the Southern Hemisphere, and move toward the poles. What causes TCs to change direction like that?

The aforementioned semi-permanent high pressure areas strongly influence how TCs move. The position and strength of the center of high pressure depends on season and the mid-latitude storm track. As an example, the above map shows the track for Hurricane Ophelia in the North Atlantic for 2011. Notice that initially the storm moved mostly west to west-northwestward. This was along the southern edge of the Bermuda high in easterly winds. On 26 and 27 October, Ophelia weakened and meandered northeast of the Leeward Islands due to a cut-off low pressure area near the Azores weakening the steering winds south of high pressure. Starting on the 29th, Ophelia headed more northwesterly to northerly in a process called “recurving.” Recurving is caused by a weakening or shifting eastward of high pressure, usually due to an approaching trough.

To see the location of troughs, meteorologists often refer to 500 millibar (mb) maps, which show how high above sea level the 500 mb pressure surface is in the atmosphere, in units of tens of meters. Warm air is taller than cold air due to thermal expansion. With this in mind, the 500 mb level in the atmosphere is useful for seeing the location of ridges (high pressure, with warm air near the center of the ridge) and troughs (low pressure and cold air near the middle of the trough), as well as the winds that steer surface features like low pressure areas, including TCs. Dashed lines indicate the centers of troughs. Winds are shown with barbs. Wind barbs are like arrows, pointing in the direction of the wind, and the barb pattern is the wind speed. Short lines are 5 knots, long are 10 knots, and flags are 50 knots. To know how fast the winds are at a barb, simply add up all the lines and flags on the barb.

The above example is the 500 mb map spanning the North Atlantic for 30 September, 2011. The low pressure circled in blue is Ophelia - at this time a borderline Category 2 hurricane (83-95 knots). The trough over eastern North America shifted the Bermuda high to the east, causing the movement of Ophelia to track more northwestward along the western edge of the high. While the winds steering Ophelia are weak at this moment, stronger southwesterly winds are approaching with the trough.

By 02 October, 2011, 0000Z, Ophelia reached its peak intensity, a category 4 storm with estimated winds of 120 knots (138 mph). The trough drew closer from the northwest, with increasing southerly to southwesterly winds. This caused Ophelia to recurve and accelerate to the north/northeast. This is common - recurving storms tend to increase their forward speed as they are picked up by faster winds along troughs.

Over the next 24 hours, Ophelia moved quickly north-northeastward toward Nova Scotia and weakened to a tropical storm due to increased wind shear and cooler sea surface temperatures. Ophelia would dissipate on 04 October.

The map below compares the paths of 2023’s Hurricane Lee to Hurricane Dennis from 2005, illustrating the wide range of paths TCs can take depending on the strength of high pressure. For Hurricane Dennis, the Bermuda high stayed firmly in place over the western North Atlantic, which pushed the storm farther west and recurved Dennis into the Gulf Coast. For Lee, a trough approached from the west (similar to Ophelia) so that the storm recurved farther east. It is important to note that the northeastern US and eastern Canada still felt the effects of high winds, rough seas, and heavy rain, even though the center of Lee remained well out to sea for most of its life.

During any given year, a host of factors contribute to location and strength of the semi-permanent high pressure areas. For the North Atlantic, factors include El Ni?o/La Ni?a, the North Atlantic Oscillation, and variations in sea surface temperatures. Some years, many TCs take paths similar to Lee (like in 2023). Other times, more TCs move like Dennis. Most years, TCs take paths that are a mix of the two regimes, shifting with the season. TCs tend to recurve farther east during the early summer and autumn due to a weaker Bermuda high with more troughs passing over North America and the North Atlantic. In the height of summer, the Bermuda high is strongest, and tends to steer TCs on a more westerly track toward land.

One of the most challenging aspects of TC forecasting is determining when a storm will recurve. As troughs approach from the west and high pressure weakens, the forward motion of TCs often slows or stalls. The TC track could loop across itself for a day or more before it finally turns northward and accelerates (see inset for the Ophelia track). It is imperative that ships at sea receive up-to-date forecasts when in the vicinity of TCs (tropical cyclone avoidance is one of the many services we offer at AMI).

TC forecasting is a complicated task, requiring regular updates from data collected by reconnaissance aircraft, satellite imagery, and weather models. A slight change in high pressure location and trough movement could drastically change where a TC will be in the coming 24 hours and beyond.