In the last newsletter, I talked about oceanic high-pressure systems and the types of weather they can produce, and I promised to expand the discussion this time to include the role they play in determining the tracks of tropical storms and hurricanes. The timing is appropriate as the 2010 hurricane season in the North Atlantic and the North Pacific has just recently gotten underway.
As most are aware, tropical storms and hurricanes tend to originate in the tropical latitudes of the North Atlantic and the North Pacific, mostly during the late summer and early fall. In the early stages of the life cycle of these systems, they tend to be steered by the low-level wind flow, and in these latitudes, this low-level wind flow is largely a result of the circulation around the oceanic subtropical high-pressure systems.
The isobars around oceanic subtropical highs are often not circular, but rather elongated in a more oval fashion with the longer axis of the oval in an east-west orientation. The result is not only the subtropical high center, but a ridge of high pressure which extends east and/or west from the center of the high. This orientation of the isobars leads to a wind flow generally from the east to the south of the subtropical high and its ridge(s). In some areas, it may be east-northeast, and in other areas east-southeast. These are the trade winds.
With tropical cyclones developing in these latitudes, and steered initially by the low level wind flow, it stands to reason that these systems will usually move from west to east in these areas. This is where the location and orientation of the oceanic subtropical highs come into play. Let’s look at that North Atlantic first.
Sometimes the North Atlantic subtropical high is centered in the western portion of the ocean, often in the vicinity of Bermuda (thus the popular term “Bermuda High”). If a ridge extends west from a Bermuda high well into the southeastern U.S., then the low level wind flow will remain generally from the east over the region south of the high and its ridge, and this will result in tropical storms and hurricanes which originate over the central tropical Atlantic tracking generally westward through the northern Caribbean, perhaps making landfall in central America, or maybe edging west-northwest into the southern Gulf of Mexico. If the western portion of the ridge is weaker and does not extend quite so far west, then the area where the isobars curve around the western end of the ridge will be located farther east, and in this region low level winds will be more from the southeast or even the south. This will be the region where tropical storms or hurricanes will begin to turn more to the northwest and north, or “recurve,” and a more eastward placement of this region will result in a greater threat for the eastern Gulf of Mexico and portions of the U.S. Gulf coast as systems move more northward from the western Caribbean region.
In the situation where the subtropical high center itself is located farther east in the Atlantic, and the western end of its ridge is therefore farther east as well, perhaps not extending into the continental U.S. at all, then systems will tend to turn toward the northwest and north even earlier, and could threaten portions of the U.S. east coast. If the subtropical high and its ridge are far enough east, then tropical systems will recurve well to the east of the U.S. coast and not threaten the coastal U.S. at all, but a threat to voyaging yachts in the Atlantic would still be present.
By examining the surface weather maps of the full Atlantic, it is fairly easy to identify the location of the oceanic subtropical high and the extent of a ridge extending to its west. This will provide a good general indication of where tropical storms or hurricanes may track. It is important to realize, though, that the strength, position, and extent of the subtropical high will change from day to day, so its position on any given day will not necessarily be reflective of how a tropical system might track several days ahead. Also, there are other factors which can influence the motion of tropical cyclones, particularly when they become very strong, but even so, having a good handle on the current and forecast position of the subtropical high will help in evaluating the general pattern of movement of tropical systems.
In the western north Pacific, similar conclusions can be drawn. This will help determine if tropical systems are likely to continue on a generally westward track in the tropical latitudes, threatening the Philippines or southeast Asia, or if a recurvature is likely increasing the threat to Taiwan, mainland China, and Japan. The possibility of systems recurving far enough east to miss all of the larger populated nations of eastern Asia is also there, but this would increase the threat to the many islands in that part of the Pacific.
In the south Pacific, a similar technique can be used with the subtropical high in that part of the world, helping to make a determination as to whether and when a tropical system will recurve toward the southwest and the south. This will dictate possible threats to Fiji, Tahiti, Tonga, and the many other islands and island groups in this area as well as western Australia and northern New Zealand.
Once again, the conclusion is that it is important to keep your eye on the oceanic subtropical high-pressure systems, as they have a great deal to do with weather patterns in areas where many people take ocean voyages.
About the Author:
Ken McKinley earned a bachelor’s degree in atmospheric science from Cornell University in 1980, and attended graduate school in meteorology at the Massachusetts Institute of Technology. After working as a meteorologist for nearly 10 years he founded his own meteorological consulting firm, Locus Weather, in Camden, Maine in 1991.
A large portion of his business at Locus Weather involves providing custom weather forecast services for oceangoing yachts, both racers and cruisers. Ken serves as an instructor for the Ocean Navigator School of Seamanship, and also as an adjunct instructor at the STAR Centers for Professional Maritime Officers in Dania, Fla., and Toledo, Ohio, and for MITAGS in Baltimore, Md. He has also taught meteorology at Maine Maritime Academy. He resides in Rockport, Maine, with his wife and two sons.