Thursday, November 26, 2009

Intertropical Convergence Zone

The Intertropical Convergence Zone is an area near the Equator that circumnavigates the globe. Most of it lies between 10ºN and 10ºS. The trade winds, or the winds near the Equator, converge on this zone. Since the winds are rotated in different directions by the Coriolis Effect, no low pressure systems, and therefore no tropical cyclones, can form too near the Equator.

The Intertropical Convergence Zone over the East Pacific. Although no low pressure systems can form in the ITCZ, it is constantly marked by thunderstorm activity.

However, the boundary of the ITCZ, or the ITCZ Axis, is always shifting, and a tropical system can sometimes, but very rarely, from below 5ºN or above 5ºS. A notable example is Typhoon Vamei, which was a Western Pacific cyclone that reached tropical storm strength at a mere 1.5ºN in December of 2001, which is the closest formation to the Equator on record. Also notable is 2004's Cyclone Agni, which formed farther from the Equator than Vamei and moved towards it, eventually reaching the most southerly point of 0.7ºN before turning back northward. Claims on this system are disputed because it was not officially tracked until a few days after the record was set but a fair amount of evidence supports that Agni does indeed hold the record for closest cyclone to the Equator.

Cyclone Agni at record latitude, a mere 45 miles from the Equator.

Although the winds over the ITCZ hinder tropical cyclones from forming within the region, they do provide an important factor for tropical cyclone development: tropical waves. A vast majority of tropical cyclones form from tropical waves, which are areas of convection that typically move west over the ITCZ. After the wave moves northward, it then can develop a low pressure system, and eventually become a tropical system. Therefore, the ITCZ is an important factor in tropical cyclone development, even if tropical cyclones can't form in the area itself.

A map of the formation and progression of tropical waves before becoming tropical cyclones.

Sunday, November 8, 2009

Hurricane Ida (2009)

Storm Active: November 4-10

On November 2, a low pressure system formed from a tropical wave along the Intertropical Convergence Zone. The low was situated in the extreme southwestern Caribbean, off the coast of Nicaragua. The low was nearly stationary for the next two days and it began to gather cloud cover and tropical characteristics. On November 4, Tropical Depression Eleven formed with 35 mph winds and a pressure of 1006 millibars. Later that afternoon, the winds near the center of circulation jumped to 60 mph and the system was named Tropical Storm Ida. Then, as it moved northwest, Ida reached a minimal hurricane strength of 75 mph winds and a 987 millibar pressure before making landfall in Nicaragua in the morning of November 5. As it encountered the mountainous terrain of the region, Ida promptly lost hurricane status, and it steadily weakened, becoming a tropical depression by late on November 5. During the morning of November 6, Ida crossed into Honduras, still maintaining tropical depression strength. Finally, later on November 6, Ida reemerged into the northwest Caribbean Sea. Ida regained tropical storm strength on November 7. During the day, the deep moisture and warm water of the region fed Ida, and it rapidly strengthened once again. By the evening of November 7, Ida was approaching hurricane strength, and was continuing north, towards the Gulf of Mexico. Ida then became a Category 2 hurricane with 100 mph winds and a pressure of 976 millibars. It brushed past the Yucatan Peninsula, causing tropical storm force winds and rain, but a majority of the wind was on the east side of the system due to a strong ridge of high pressure over the Bahamas. The pressure difference caused sustained winds of over 30 mph to rip through the Florida Keys and the surrounding region, despite the fact that the circulation was still fairly far away. In addition, rain from Ida extended farther to the southeast, reaching the Cayman Islands and beyond. Later on November 8, Ida reached its peak intensity of 105 mph winds and a pressure of 976 millibars. Then as Ida entered the northern Gulf of Mexico, it encountered cooler water, and weakened once again. By the afternoon of November 9, Ida had become a tropical storm once again, and was already battering the coasts of Alabama, Louisiana and the Florida Panhandle with high surf and tropical storm force winds and rain. Overnight, Ida weakened further and began its extratropical transition. By the morning of November 10, the center of Ida was sill offshore, but all convection associated with the system had moved to the north over much of the southeast U.S., producing rain and thunderstorms, including up to six inches of rain locally in parts of Florida. As Ida made landfall in Alabama, it promptly weakened to a tropical depression and became extratropical. It then combined with a westward moving cold front and another low pressure system approaching North Carolina from the east to bring a huge rain and wind event to the entire U.S. east coast. In addition, a very strong high pressure system was situated over Maine (it was a powerful one, with a pressure exceeding 1035 millibars) causing east-to-west wind to bring surf and tropical storm force gusts to much of the coast. The high pressure also blocked the system, and it moved very slowly. Rain and wind continued for the next three days and some areas accumulated over 10 inches of rain. By November 13, the low pressure system had moved up to the coast of New Jersey and was finally weakening. The system sped off to the northeast and left the U.S. on November 14. Ida was a notable storm in El Salvador because it contributed to a mudslide that killed 124 people, but Ida also became a powerful and dangerous nor'easter after becoming extratropical, killing an additional 10 people. Ida also caused $2.15 million in damage.

Ida strengthening as it enters the Gulf of Mexico.

Track of Ida.