Double Trouble: twin tropical cyclones spin up in the Indian Ocean

There's double trouble in the Indian Ocean today, with a pair of counter-rotating tropical cyclones spinning away on either side of the Equator. In the Southern Hemisphere, we have Tropical Cyclone Jamala, a tropical storm with 45 mph winds that is headed southwards at 3 mph. Jamala is expected to intensify into a Category 1 storm over the weekend, but is not a threat to any land areas. In the Northern Hemisphere, it's Tropical Cyclone 01B, a tropical storm with 40 mph winds that is dumping heavy rains over western Sumatra. This storm much larger and far more dangerous than its Southern Hemisphere twin brother. Cyclone 01B is under high wind shear of 20 - 30 knots, which is keeping the system disorganized, with the heaviest thunderstorms displaced from the center of rotation. However, Cyclone 01B is expected to move generally northwestwards over the next few days through the Bay of Bengal, where wind shear is lower and ocean temperatures are an exceptionally warm 31°C (88°F). This is about 1°C warmer than average for this time of year. The official forecast brings the storm to Category 1 strength before landfall occurs in Myanmar just south of the border with Bangladesh early next week. Comparative model forecasts of Cyclone 01B from the GFS, ECMWF, UKMET, GEM, NAVGEM, and FIM models show poor agreement on the intensity of Cyclone 01B, though, and it is possible that wind shear will keep the storm disorganized until landfall, with heavy rain being the main threat.


Figure 1. Double trouble: Tropical Cyclone Jamala (lower) and Tropical Cyclone 01B (upper storm) spin on opposite sides of the Equator in this infrared satellite image taken at 12 UTC (7 am EDT) Friday May 10, 2012. Image credit: University of Wisconsin CIMSS.

Twin tropical cyclones: how unusual?
Twin cyclones forming on either side of the Equator are surprisingly common in the Indian Ocean. They tend to spin up in May and November--the months immediately preceding and following the annual monsoon (the monsoon dominates the circulation patterns during June - October, making tropical cyclones rare in the Northern Indian Ocean during those months.) The most recent occurrence of twin developments happened unusually late: the end of December 2011, when Tropical Cyclones Thane and Benilde developed. Both storms became mature hurricanes, with Thane hitting the southeast coast of India on December 30, making it the second latest landfalling cyclone in India (thanks go to Meteo-France's Philippe Caroff of RSMC La Reunion for this info.)

Usually, the twin storms spin up when the active phase of the Madden Julian Oscillation (MJO) is present. The MJO is a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. The area of increased thunderstorms associated with the MJO currently straddles the Equator in the Indian Ocean, and is helping boost thunderstorm activity in both Cyclone 01B and Tropical Cyclone Jamala. The strong MJO pulse coincides with something called a convectively coupled atmospheric Kelvin wave (CCKW), which is helping increase thunderstorm activity as well. The tutorial on Equatorial Waves in the COMET program's excellent Introduction to Tropical Meteorology, (free, but registration required) gives several examples of twin tropical cyclones forming on either side of the Equator. One of these cases occurred in November 2007, when the notorious Tropical Cyclone Sidr formed in the North Indian Ocean at the same time that Tropical Cyclone Lee-Ariel formed on the other side of the Equator. Sidr struck Bangladesh as a Category 4 storm, killing up to 10,000 people. Dr. Michael Ventrice of wsi.com gave me these examples of other twin Indian Ocean tropical cyclones that formed between 1990 - 2009:

1. mid Oct 1992
1. early November 1993 
2. late April 1994
3. mid Nov 1995
4. late April/early May 1995 
5. late October 1995
6. mid March 2000
7. early May 2002 (*Great Example, two sets of twins*) 
8. early Nov 2002
9. mid Dec 2002
10. early Dec 2003
11. early July 2004 
12. late April 2008
13. mid Oct 2008
14. mid Nov 2008
15. early Nov. 2009 

These twin cyclone events tend to  most commonly occur during strong MJO events over the Indian Ocean (strong equatorial lower tropospheric westerly winds), but it's not the only time that they can occur. To look at these years with MJO and Kelvin wave type filtered anomalies overlaid (contours), please visit http://www.atmos.albany.edu/student/ventrice/ncep/5S-5N/. Here, blue shaded TCs represent the tropical depression category forming south of the equator, whereas red TC symbolizes depressions that have formed north of the equator. 

It's worth mentioning that the active pulse of the Madden Julian Oscillation is expected to reach the Western Caribbean sometime May 22 - 26, and there will be a heightened chance of an early-season tropical storm forming in the Eastern Pacific and Western Caribbean during that time period.


Figure 2. The most spectacular case of twin cyclones in the Indian Ocean: "twin twins" on May 9, 2002. A strong MJO event superimposed with a strong convectively coupled atmospheric Kelvin wave passed the Indian Ocean and two sets of twins developed, the first over 60E on May 1-3, and the second pair of twins over 90E May 8-10. The eastern pair of twins are Tropical Cyclone Errol (Southern Hemisphere) and Tropical Cyclone 02B (Northern Hemisphere.) The western pair had quite a significant impact, with Tropical Cyclone Kesiny in the Southern Hemisphere making landfall in northern Madagascar as a hurricane, which was the first occurrence of a hurricane making landfall in the month of May in the Southwest Indian Ocean in at least 40 years (though Kesiny was followed the next year by Tropical Cyclone Manou, which hit the east coast of Madagascar at an even higher intensity.) Kesiny affected half a million people in Madagascar and caused numerous fatalities (at least 33), with people being drowned due to severe flooding in the aftermath of the TC-related rain event (Toamasina the major harbour of the country and main city of the east coast of the island received 891 mm (35.08") of rainfall in 3 days--although being quite far to the south from the landfalling point). At the same time, Tropical Cyclone 01A finished its life on the arid land of the Arabian Peninsula, also generating a severe rain event (unprecedented in at least 30 years), which caused devastation in the Dhofar region of western Oman. An observing station reported 251 mm (9.88") of rainfall (the equivalent of 4 to 5 years of average rainfall for this arid region). At least 9 fatalities were attributed to this unusual storm and rain event. Thanks go to Meteo-France's Philippe Caroff of RSMC La Reunion for this info and image.

Resources
Comparative model forecasts of Cyclone 01B from the GFS, ECMWF, UKMET, GEM, NAVGEM, and FIM models.

India Meteorological Department's tropical cyclone page


Tutorial on Equatorial Waves in the COMET program's Introduction to Tropical Meteorology, plus their case exercise built around the May 2002 "twin twins" case, for use in a tropical synoptic course.

Equatorial Rossby Waves and Twin Tropical Cyclogenesis

Dynamical Aspects of Twin Tropical Cyclones Associated with the Madden-Julian Oscillation

Shen, B.-W., W.-K. Tao, Y.-L. Lin, and A. Laing, 2012: Genesis of twin tropical cyclones as revealed by a global mesoscale model: The role of mixed Rossby-gravity waves. J. Geophys. Res., 117, D13114, doi:10.1029/2012JD017450


Have a great weekend, everyone!

Jeff Masters