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July 17, 2005

Ask Bryan: What steers hurricanes?

Thanks for the in-depth explanation. Feel like tackling another? What atmospheric features "steer" hurricanes on their path and determine their ultimate track? What causes them to curve and sometimes make drastic changes in direction, like Jeanne's loop-the-loop from last year?
x_DaveMan_x | Homepage | 07.16.05 - 10:57 pm | #

This question is much simpler than eyewall replacement cycles so I would be happy to answer it for you. Hurricanes tracks are influenced by the surrounding environmental winds within the troposphere. Typically in an environment where a hurricane exists, these winds do not change very much with height. If the winds were to change speed or direction with height, we would refer to this as wind shear.

If you think of a hurricane as a fluid mass, you can quickly understand how wind shear will disrupt the storm formation. If you take this fluid mass and blow on it at a uniform speed and direction, it will simply float intact in the direction of the flow. However, if you blow on this fluid mass at speeds or directions within the mass, it will break apart. This is why if an updraft in an area of convection is blown at a different speed or direction as it goes up, the updraft will not be able to sustain itself long enough to organize and develop.

Now that we know that hurricanes exist almost exclusively in areas of vertically nearly uniform flow, called low-shear environments, we can understand how hurricanes are blown across the globe by the general atmospheric flow. When the flow becomes highly sheared, the hurricane is typically ripped apart and weakens. Take a look at the diagram below to see what the current mid-level flow looks like around Hurricane Emily.

If you look carefully, you can see the wind flow from the east at 15 knots where Emily just came from and the flow from the ESE over the Yucatan blowing in the direction that Emily is being taken. For help understanding the wind barbs on this map, check here. However, what causes this global flow to change and cause a hurricane like Jeanne to loop is a different question completely. In more extreme circumstances, tropical systems like Ivan march clear across the entire Atlantic Ocean, miraculously dodge ever island they come across, hit Alabama, travel back into the open Atlantic, cross Florida, enter the Gulf again, and all of the sudden strike Texas as a tropical storm. Yeah, don't get me started on that one. They claim the vorticity maxima was intact the entire time. Humm, oh, back to topic.

In mid-latitudes the prevailing winds blow out of the west while near the equator they blow from out of the east. In between these two wind belts, tropical systems often find themselves under the influence of weak steering currents that change based on the progression and strength of systems to the north and south. As a result, tropical cyclones generally move very slowly, often under 10 mph, and can wander greatly. If the upper-level pattern shift during the lifetime of a tropical cyclone, the storm can find itself under a completely different steering regime, often causing the storm to reverse course and loop around. This situation can be quite common and occurs most frequently around 30 degrees latitude, roughly around New Orleans and Jacksonville. For a quick example of how this can happen check out the two maps below.

Take a look at this map and notice how over the central Gulf Coast, the winds are mostly out of the south. Also take note of the shortwave trough and associated vorticity maxima (I don't expect you to understand what this is, just understand that it will move) over Idaho and Montana. There a quick "explanation" of shortwaves, but it really doesn't simply things too much.

Now progress forward 24 hours and note how the winds over the coast are now out of the west and just off-shore they are out of the east. Also, the shortwave has now moved over to the Dakotas. Believe it or not, it is the progression of this shortwave far to the north that has altered the steering current over the Gulf Coast. It is this turning of the winds that allows slow moving tropical systems to loop around.
Tracking God's Fury:
Tropical Storm Maria
Tropical Storm Nate
Hurricane Ophelia
  • None
  • --
    Ask Bryan:
    What steers hurricanes?
    What is eyewall replacement?
    Jordan Golson
    Bryan Woods