I have decided to post my third blog entry in the past 24 hours. Each of these entries are focued on three of the most influential atmospheric and climate signals that impact serasonal activity, as well as any perspective hurrcane landfalls. That being said, the following is simply a repost of my April 19th weather 101 series on the "Bermuda High" from last year. As time allows, I will be providing subsequent blog entries all tailored to helping all of us better forecast what may transpire during the upcoming 2007 Atlantic hurricane season. Thanks again for taking the time to read and/or post in my blogs. I look forward to talking with each of you very soon. In the meantime, I want to wish everyone a great Easter weekend to come.)
WHAT IS THE BERMUDA HIGH?
First of all, it derives its name by its very distinct character. Specifically it is a very large area of high atmospheric pressure that sets itself up and becomes firmly entrenched over the Sub-Tropical Atlantic Ocean. Generally, the center of this high pressure area tends to shrink in size (decrease in its intensity) and moves eastward towards the Azores Islands, where it is affectionately referred to as the "Azores High" as well, near the very end of one hurricane season and throughout the winter months. However, the center of this most influential weather system begins to shift back towards the west and intensify (expands in size), as we approach the beginning of a new hurricane season, and as a result plays a very significant role in both hurricane formation, intensity, and their eventual tracks. Generally speaking, it is a semi-permanent area of sub-tropical High Pressure that migrates between about 30 degrees and 40 degrees North Latitude depending on the season. Specifically, it is located further South and East during the winter and early spring closer to the Azores (why it is often called the Azores High as previously mentioned), and moves more to the North and West as we get into the the late spring through the summer and fall months.
HOW DOES IT AFFECT HURRICANE FORMATION AND INTENSITY?
First, we need to understand that high atmospheric pressure pushes air down towards the surface which in turn causes the air to become drier and much warmer. Since this air is now drier and warmer, it suppresses the formation of clouds (which form from moist air rising off the water). Since there are fewer clouds, the sun is able to more directly heat the oceans surface which in turn allows the water temperatures to increase. Naturally, the intensity of the Bermuda High has a big impact on sea surface temperatures in the main breeding grounds for tropical cyclone activity know as the inter-tropical convergence zone or ITCZ. This region is where the vast majority of tropical waves move off the west coast of Africa between about 10 degrees and 30 degrees North latitude. Furthermore, this area of most favorable atmospheric conditions for tropical cyclone genesis extends westward all the way to Central America.
By the time we reach the peak months of hurricane season that begins in August, the Bermuda High being a weather system that stirs the air around the North Atlantic in a huge clockwise circle, has helped create very favorable atmospheric conditions in this region by allowing the waters to bake beneath the relatively clear skies. Consequently, increased sea surface temperatures in this region helps to provide extra fuel for the hurricane wanna-bees moving from east to west off Africa in the general direction of the east and northeastern Caribbean. As a result, the eventual strength of the Bermuda High plays a surprisingly significant role in how much heat energy will be available for both cyclogenesis as well as further intensification after it actually occurs.
HOW DOES IT AFFECT HURRICANE TRACKS?
As we learned in the previous section, this huge area of High Pressure spins the air over the Atlantic in a giant clockwise circle (image a clock). As also alluded to in the previous section, most tropical systems move off Africa in a general westerly direction being steered by the currents of air on the very southern most edge of the Bermuda High. As they move further westward, the strength, size, and location of the Bermuda High determines where they will ultimately go. In most cases, the clockwise circulation will steer storms more Northwards as they begin to reach the western edges of the Bermuda High and push them out to sea before they can impact the U.S. East Coast. However, it is important to realize that this High pressure system isn't stationary, for it is a weather system like any other and can shift more east, west, north, or south and does so in relation to the effects of other storm systems like frontal systems moving off the U.S. Coast which usually acts to weaken the Highs influence and causes it to migrate farther east. In other instances, the Bermuda High can set itself up farther west than normal where the southern and western edges of its clockwise circulation can allow hurricanes to move farther south and west before they make their eventual and inevitable northward turn. This was the specific pattern that was in place during the peak months of the 2004 hurricane season when a record four hurricanes directly impacted Florida. This is also the scenario that makes the Southeast coast of Florida and the Gulf Coast States most vulnerable. In contrast, if the Bermuda High is set up more towards the east during the peak months of the hurricane season, the East Coast of the U.S. is more vulnerable as was the case in 1996 when North Carolina was struck by two hurricanes in the same season and was also the last time a major hurricane made landfall farther north than Ft. Pierce, Fl.
During the hurricane seasons of 2000-2002, the Bermuda High was much weaker and located farther East than normal which acted to deflect all possible hurricanes well out to sea during those three consecutive seasons when each had well above normal tropical cyclone activity, with only hurricane Lili making a landfall in October of 2002 as a category one in La. Overall, it is when the Bermuda High is displaced from its normal location more South and Westward, that it can stretch its most Western edges close to and even over the East Coast States of the U.S. When this particular set-up occurs, the U.S. coastline is left completely vulnerable to hurricanes. This is because a tropical cyclone is a relatively intense low pressure system that cannot penetrate an area of strong high pressure (which is most intense during the summer months coinciding with our hurricane season) and must go around it. As a result, this most unique High Pressure system acts as a buffer and doesn't allow the Northward turn to take place until it impacts some portion of the U.S. Coastline, or keeps hurricane tracks even as far south and west as Central America. To reiterate, the specific location, size, and intensity of the Bermuda High will ultimately determine if and where the next hurricane will strike.
Updated: 02:57 GMT le 06 avril 2007
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