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Hurricane Season 2009: Anja (Southern Indian Ocean)
11.18.09
 
November 18, 2009

QuikScat measured sustained winds near 63 mph around Anja's center. > View larger image
NASA's QuikScat satellite captured Anja's winds on Nov. 18 at 0058 UTC, and its data was combined with other satellites to make this image. QuikScat measured sustained winds near 63 mph around Anja's center. At the time, Anja's minimum central pressure was 1000 millibars.
Credit: NASA/JWTC
NASA's QuikScat and Aqua Providing Important Data on Tropical Storm Anja

Anja has continued to weaken over the last 24 hours, and NASA's QuikScat satellite has confirmed that the once mighty Category 4 Cyclone is now a tropical storm in the southern Indian Ocean. Two instruments on NASA's Aqua satellite have also helped forecasters determine Anja's location and change of shape.

NASA's QuikScat satellite uses microwave technology to peer through a tropical cyclone's clouds, and actually read the speed of the rotating surface winds. In an overpass from space at 7:58 p.m. ET last night, November 18 (Nov. 19 at 0058 UTC), QuikScat noticed Anja's maximum sustained winds have dropped to 63 mph, making it a tropical storm.

Around that time, Anja's center was 560 miles east of Port Louis, Mauritius, near 22.4 degrees South latitude and 68.1 East longitude. Anja was moving south-southeast at 23 mph (20 knots). Anja is staying at sea and away from any landmasses, and poses no threat before it is expected to dissipate by the weekend.

Satellite imagery from NASA's Aqua satellite has helped forecasters determine where Anja's center is located, and how it has changed shape. Animated infrared imagery, such as that from NASA's Atmospheric Infrared Sounder (AIRS) instrument showed the system has become sheared and elongated toward the southeast. Another instrument on NASA's Aqua satellite, the Advanced Microwave Sounding Unit-B (AMSU-B), helped determine Anja's position using microwave technology when it flew overhead.

As Anja continues to weaken, its tropical storm-force winds don't have the extent they had yesterday. Today, November 18, tropical storm-force winds of 37 mph or higher extends out to 105 miles from Anja's center.

In addition to the wind shear that has been weakening Anja, sea surface temperatures are also decreasing as Anja continues to move toward the southeast. Sea surface temperatures are now below 26 degrees Celsius (78 degrees Fahrenheit) and are cooler farther southeast. To maintain intensity, tropical cyclones need sea surface temperatures of at least 80 degrees Fahrenheit.

Anja has now begun to speed up and is making a transition into an extra-tropical storm.

Text credit: Rob Gutro, NASA/Goddard Space Flight Center



November 17, 2009

ANJA had a well defined eye and was increasing in strength from a category 2 to 3 tropical cyclone. > View larger image
The TRMM satellite passed over Anja on November 15 at 0904 UTC. ANJA had a well defined eye and was increasing in strength from a category 2 to 3 tropical cyclone.
Credit: NASA/SSAI, Hal Pierce
TRMM data made it clear that upper level wind shear had caused Anja to weaken greatly. > View larger image
The TRMM satellite passed over Anja again on November 17 at 0850 UTC showing some heavy rain around the center. TRMM data made it clear that upper level wind shear had caused Anja to weaken greatly.
Credit: NASA/SSAI, Hal Pierce
Cyclone Anja Hits Wind Shear, Weakens Drastically

This morning, Cyclone Anja was a powerful Category 4 cyclone on the Saffir-Simpson scale. Wind shear has now giving Anja a strong "punch in the gut" as the storm has weakened to a Category 1 cyclone.

At 1500 UTC (10 a.m. ET) on November 17, Anja's maximum sustained winds were sustained at 75 knots (86 mph). Anja's center was located 715 miles east-northeast of La Reunion Island, near 18.3 South and 66.5 East. Anja is moving south-southwest at 12 mph. The storm seems to be spreading out as its weakening, as tropical storm force winds now extend out to 115 miles from the center (earlier today, they only extended 75 miles out from the center).

When tropical cyclone Anja formed on November 14, it was the first tropical cyclone in the southwestern Indian Ocean. That signaled the tapering off of northern hemisphere tropical cyclone activity and the start of tropical cyclone formation in the southern hemisphere. The Atlantic Ocean hurricane season officially ends at the end of November while the southwest Indian Ocean tropical cyclone season starts on November 1 and ends on April 30.

When the Tropical Rainfall Measuring Mission (TRMM) satellite passed over Anja on November 15 at 0904 UTC it had a well-defined eye and was increasing in strength from a category 2 tropical cyclone (on the Saffir-Simpson scale) to a powerful category 3 tropical cyclone later that day. TRMM is a joint mission between NASA and the Japanese space agency JAXA.

The TRMM satellite passed over Anja again on November 17 at 0850 UTC. The data from TRMM made it clear that upper level wind shear (winds blowing at different directions in the atmosphere that can tear a storm apart) had caused Anja to weaken greatly from its earlier maximum category 4 intensity. Anja no longer had a well defined eye but the TRMM derived rainfall analysis showed there was still very heavy rainfall in an area near Anja's center.

Anja is predicted to continue weakening and move harmlessly toward the southeast over the open waters of the south Indian Ocean.

Text credit: Rob Gutro, NASA/Goddard Space Flight Center



AIRS image of Anja > View larger image
NASA's Aqua satellite captured cold (purple) thunderstorm cloud tops surrounding Anja's center in this infrared image November 17 at 1:30 p.m. local time (0930 UTC).The highest clouds and strongest thunderstorms appear to be on the north and west side of the storm.
Credit: NASA JPL, Ed Olsen
AIRS image of Anja > View larger image
NASA's Aqua satellite caught a visible image of Cyclone Anja early this morning (ET) when it reached Category 4 Cyclone status in the Southern Indian Ocean.
Credit: NASA JPL, Ed Olsen
Cyclone Anja Now a Category 4 Storm in Southern Indian Ocean

Overnight Cyclone Anja continued on its power trip. Yesterday, November 16, Anja was a Category 3 Cyclone. Today, Anja has reached Category 4 status on the Saffir-Simpson scale as the storm's maximum sustained winds have increased to 115 knots (132 mph).

NASA's Aqua satellite makes daily overpasses from space and has observed the towering thunderstorms as the cloud top temperatures have grown colder. The latest satellite imagery from the Atmospheric Infrared Sounder (AIRS) instrument on Aqua taken at 0930 UTC (4:30 a.m. ET), revealed that cloud top temperatures were colder than minus 63 degrees Fahrenheit! That puts cloud tops near the top of the troposphere, meaning the cyclone has some very powerful thunderstorms in it.

AIRS data is also coupled with data from the Advanced Microwave Sounding Unit (AMSU) that flies with AIRS on Aqua to create microwave images of storms. The AMSU image uses the radiances of the 89 GHz channel, and it revealed cold areas that indicated precipitation or ice in the cloud tops.

At 10 p.m. ET on November 16 (0300 UTC November 17), Anja was located about 715 miles east-northeast of La Reunion Island, near 16.4S and 67.3E. Reunion Island is located east of Madagascar in the Indian Ocean. The nearest island is Mauritius which is 120 miles to the northwest. Anja was moving toward the south-southwest near 13 mph.

Anja is a compact storm, with hurricane-force winds extending only 25 miles from the center, and tropical storm-force winds extending only 75 miles from the center.

Anja is forecast to move southeast and weaken in the next day or two.

Text credit: Rob Gutro, NASA/Goddard Space Flight Center



November 16, 2009

Cyclone Anja in the southern Indian Ocean on November 16 at 1 a.m. ET (0600 UTC). > View larger image
NASA's MODIS instrument on the Terra satellite captured Cyclone Anja in the southern Indian Ocean on November 16 at 1 a.m. ET (0600 UTC).
Credit: NASA, MODIS Rapid Response
NASA's Terra Satellite Spots Tropical Cyclone Anja, the First of the Southern Season

NASA's Terra satellite captured a stunning image of Anja, the first tropical cyclone of the southern Hemisphere cyclone season. When Anja formed on Saturday, November 14, in the Southern Indian Ocean, about 330 miles south-southwest of Diego Garcia it was designated Tropical Cyclone 01S ("S" for south). By Sunday, November 15, 01S had strengthened into a tropical storm and was named Anja.

On Monday, November 16, Anja had grown into a Category Three cyclone on the Saffir-Simpson Scale, packing maximum sustained winds near 105 knots (120 mph). NASA's Terra satellite flew over Anja, and the Moderate Imaging Spectroradiometer (MODIS) instrument captured an image of the storm that showed a cloud-filled eye in the center of circulation. Anja was located 795 miles east-northeast of Port Louis, Mauritius, near 13.4 North and 69.3 East. Anja was moving southwest at 4 mph.

Anja was a medium-sized cyclone with tropical storm-force winds extending out to 100 miles from its center, and cyclone (or hurricane)-force winds out to 45 miles from its center. Anja was generating waves as high as 24 feet, and is not threatening any landmasses.

Forecasters at the U.S. Navy's Joint Typhoon Warning Center expect that Anja will move south over the next couple of days and weaken under wind shear.

Text credit: Rob Gutro, NASA/Goddard Space Flight Center