Although the hurricane season doesn’t end until 30th November, the hurricane season this year has presented an unheard of number of storms when based against the numbers of storms recorded in each hurricane season in the past, recorded since 1950. In the last month alone, five hurricanes have occurred, which represents a frequency of hurricanes never before heard of in that space of time. This is especially true as four of these storms formed in the space of just twenty days. Indeed, so far this season there have already been eleven named storms, six of which have been hurricanes.
With just over nine weeks left to go in the season, the weather forecasts of Weather Services International which predicted at least eighteen named storms this season, mean that whilst America hasn’t been hit by a major storm yet this season, vigilance still needs to be exercised, particularly as historically, the strongest storms hit in September. With Tropical Storm Lisa forming the twelfth storm of the season, still blowing more than three hundred miles off the Cape Verde Islands, the season doesn’t look like letting up.
The reason for this high number of tropical storms is partly due to the high temperature of the Atlantic Ocean this year. Warm ocean waters meeting warm moist air are what cause hurricanes to form, which is why hurricanes occur around the tropics. As the warm ocean waters evaporate, the water vapour meets cooler air high above, condensing to form clouds. As the water vapour condenses into cloud, the warmth released from the water vapour as it condensates causes cool air above the forming clouds to be heated also, so that the cool air moves higher, allowing more and more warm, humid air to form at higher altitudes as more and more water vapour evaporates from the ocean. This in turn creates more and more storm cloud.
As the evaporation of the warm water also causes the ocean temperature to drop, the movement of heat into the atmosphere with cool water below creates wind patterns which circle, forming the spiral pattern of hurricane clouds. As these spinning winds converge with the winds of the storm above, this causes even more water vapour and so moist, warm air to evaporate, building the storm, whilst also building the wind speeds, as the strong winds at high altitude pull more and more heat upwards, away from the centre of the storm. As the air at high altitude is also at high pressure, this causes the warm air to move outwards away from the centre of the storm, creating the huge spiralling storm clouds that distinguish hurricanes.
As can be seen, the driving force behind hurricanes is the meeting of warm moist air with warm waters, as more and more heat is sucked up into the atmosphere as the warm ocean waters evaporate. As the waters of the Atlantic have been particularly warm this year, this is what is causing the high frequency of tropical storms and hurricanes, and for a prolonged period, as a higher temperature allows more heat to be absorbed by these storms for a greater part of the hurricane season.
This year’s storm season is also being affected by the phenomenon known as La Nina. This is the opposite of El Nino which drags warm water to the Eastern Pacific, preventing the rise of cool waters from the deep ocean along the west coast of South America, and leads to high precipitation storms. La Nina works as the counterpart to El Nino. La Nina causes a reduction in high altitude winds, which impede the formation of the storm clouds which turn into hurricanes. As such, less of the storm clouds are being dispersed before they can turn into tropical storms or hurricanes.
As such, the reason why the 2010 hurricane season has produced more storms than usual is the combination of record high sea temperatures in the Atlantic, combined with the effects of La Nina, which impedes the high altitude winds which help to scatter the storm clouds which turn into tropical storms or hurricanes.
References
http://www.nola.com/hurricane/index.ssf/2010/09/tropical_storm_lisa_is_on_a_sl.html
http://articles.sun-sentinel.com/2010-09-21/news/fl-hurricane-season-midpoint-20100920_1_storm-season-large-scale-atmospheric-condition-storm-formation
