Question: Steering of Tropical Systems

[Jerry Prsha]

I'm trying to get a handle on what factor steer tropical depressions and hurricanes into the path that they eventually take.

It seems that most of the Atlantic systems that drift off Africa end up turning North and churn out to sea but some migrate straight westward.

I'm currently watching a depression west of the Leeward islands as it slowly ramps up to tropical storm level. Examining some historical data has made me think that the track is very independent and not respectful of any topographical objects.

I've not seen this topic before so I hope someone can give me an idea on where to look for steering mechanisms. I've not paid all that much attention to this in the past so I'm a complete neophyte... Nothing is too basic to explain to me.

 

[Frank Kienast]

Winds aloft created by upper level pressure differences steer hurricanes. In the late summer and early fall, there is usually a high pressure center near Bermuda (the "Bermuda High"). If this high is of normal strength, hurricanes approaching from the southeast will rotate clockwise around it, curving off to the east instead of hitting the US east coast. However, if the Bermuda high is stronger/larger than usual, the path around it will take hurricanes on a more westerly track, causing them to make landfall on the US east coast.

Lots more to it than that (which I'm sure others with more knowledge than me can add), but that is the overall idea.

 

[cdcollura]

Good day all,

I posted this on my web site and it has a pretty interesting explanation on WHY such a busy season this year is actually causing the US to be affected LESS from hurricanes (so far)...

...

The Atlantic hurricane season of 2010 has been a very active season so far (as of mid September 2010); however, none of the most violent storms have affected land. The above-normal sea surface temperature anomaly in the far eastern Atlantic is producing more hurricanes, but fortunately the development so-far-out east is actually causing the Eastern US to be spared, with hurricane Earl being the closest shave of one of these missing-the-US storms.

In the image below, two powerful hurricanes (Igor and Julia) are raging in the far tropical Atlantic, far from any land. The image to the right is a close-up of the eye of Hurricane Igor at maximum intensity on September 13. None of these storms were affecting anyone at this time; with fish, birds, and whales most likely the only "beings" experiencing the fury of these hurricanes! Even ships do not travel through that part of the ocean very often.

During the Cape Verde season (September), tropical waves normally move off Africa and do not develop into a close circulation until making their way across the ocean (near the Windward Islands). An "open" tropical wave, like a wave in a river going across the current, can propagate THROUGH a trough / weakness in the steering flow (still move WEST although the trade wind flow is SE).

Tropical cyclones that develop RIGHT OFF the African coast, often gain latitude (they do NOT move due WEST, but more WNW). This is because a CLOSED low always moves with the flow it’s embedded in (if the trade winds are SE nearing a trough, the low moves NW, not WEST like an "open" tropical wave would). By the time these systems reach the western Atlantic (near longitude 60 to 65 west or so), they have already missed the northern Antilles / Puerto Rico, and are more susceptible to be picked up by a stronger trough and re-curve into the westerlies and into the North Atlantic. Sometimes the tiny island of Bermuda is affected by these, but a DIRECT hit there is rare.

 

[L.B. LaForce]

Not a real Hurricane expert here, but 500mb level winds are key to the steering of tropical systems. Like Chris said, none this year that have formed off the coast of Africa/Verde islands have hit land in the U.S., and this trend will likely continue until the sub tropical ridge over the SE United States diminishes, or a maritime high positions itself over Newfoundland area.

The sinking motion cause by the subtropical ridge is not letting the hurricanes get very close to the U.S. coast, and as they skirt northward, they get picked up by the westerlies, and carried back out to sea.