The 2011 North Indian Ocean cyclone season was the least active tropical cyclone season in the North Indian Ocean since 1993. Only two cyclonic storms formed, below the average of four to six. The North Indian Ocean cyclone season has no official bounds, but cyclones tend to form between April and December, with peaks in May and November. These dates conventionally delimit the period of each year when most tropical cyclones form in the northern Indian Ocean. The scope of this article is limited to the Indian Ocean in the Northern Hemisphere, east of the Horn of Africa and west of the Malay Peninsula. There are two main seas in the North Indian Ocean — the Arabian Sea to the west of the Indian subcontinent, abbreviated ARB by the India Meteorological Department (IMD); and the Bay of Bengal to the east, abbreviated BOB by the IMD.
This is the first season to have only two named storms since the 1993 North Indian Ocean cyclone season. However, multiple Depressions along with Cyclonic Storm Keila and Very Severe Cyclonic Storm Thane wreaked damage worth at least US$1.64 million and killing some 360 people overall. It is believed that La Niña, a quasiperiodic climate pattern that causes a rise in surface pressure over the Indian Ocean and makes the region drier is the main cause behind the below normal activity in the basin.
Season summary
This season, 9 depressions developed out of low-pressure areas, with six intensifying further into deep depressions, two deep depressions have developed into cyclonic storms, and one cyclonic storm intensified into a severe cyclonic storm. The first depression of the season developed on February 2 about to the east of Colombo, Sri Lanka. The depression brought isolated rainfall to parts of Sri Lanka, while remaining near stationary before weakening into an area of low pressure early the next day.
Depression ARB 01 formed in early June near India, before Deep Depression BOB 02 formed a few days later. Land Depression 01 formed on July 22, and dissipated a day later. Depression BOB 03 formed on September 22, and soon made landfall on India. Depression BOB 03 dissipated the next day, on September 23. October was a much more active month, as Deep Depression BOB 04 and Deep Depression ARB 02 both formed, during this period of time. Then Cyclonic Storm Keila formed in November and came ashore in Oman, before Depression ARB03 formed and dissipated near the Oman coast.
Systems
Depression BOB 01
On February 2, the IMD upgraded an area of low pressure, located approximately 100 km southeast of Pottuvil, Sri Lanka, to a depression, giving it the designation "BOB 01." The convection in the system gradually increased and the system drifted towards land. Early on the next day, the IMD downgraded the system into a remnant low because of its proximity to land and weakened.
Following catastrophic floods in December 2010 and January 2011, the depression brought additional rainfall to Sri Lanka. The subsequent floods and mudslides killed 18 people and affected nearly 1.2 million. Numerous roads were washed away as reservoirs across the island overflowed their banks and inundated surrounding communities. In the wake of the floods, the Government of Sri Lanka allocated 33 billion Sri Lankan rupee ($287 million US$) for rehabilitation.
Depression ARB 01
In early June, a low-pressure area formed over the Arabian Sea. The low-pressure area remained stationary and became more well marked. On June 11, the IMD upgraded the area of low pressure to a depression giving it the designation "ARB 01". The same day, the JTWC designated the system as Tropical Cyclone 01A. At that time it was located approximately northwest of Mumbai, India and southeast of Veraval, Gujarat. Later, on June 12, IMD reported that the depression had crossed the Saurashtra coast of India about 25 km east of Diu. Later on the same day, the IMD reported that the depression had weakened into a well-marked low-pressure area in their last bulletin for the system.