The 1991 North Indian Ocean Cyclone season was the period in which tropical cyclones formed to the north of the equator in the Indian Ocean. During the season tropical cyclones were monitored by the India Meteorological Department (IMD) and the Joint Typhoon Warning Center. The IMD assigned all depressions that it monitored with BOB followed by a number in numerical order. The JTWC also assigned a number and either the letter A or B depending on where the depression was when the first advisory was issued.
During the year there were eight depressions that were monitored by the IMD while the JTWC monitored four during the year of which one was not monitored by the IMD. The first cyclone of the year formed on January 17 and had little effect on ships that were moving through the Arabian sea to take part in the Gulf War. The deadliest cyclone during the year was Super Cyclonic Storm BOB 01 which killed over 138,000 people.
During 1991 the India Meteorological Department officially monitored eight cyclonic disturbances, which included five depressions and three cyclonic storms. For the sixth and final year in a row, there was no cyclonic storms officially monitored within the Arabian Sea. However, the United States Joint Typhoon Warning Center unofficially monitored a tropical storm within the Arabian Sea, during January that posed a direct threat to Coalition forces in the buildup to the Gulf war. The first official system of the season was the strongest, deadliest, and most damaging system of the season
During January 14, the JTWC started to monitor an area of convection that had developed within the near equatorial trough of low pressure, about 900 km (560 mi) to the southeast of Colombo, Sri Lanka. Over the next couple of days the disturbance moved towards the northwest under the subtropical ridge of pressure, before early on January 17, the JTWC noticed a steady increase in deep convection and issued a tropical cyclone formation alert. Later that day the JTWC initiated advisories as the disturbance had intensified into a tropical storm and designated it as Cyclone 01A. As the Cyclone was suffering from being in an area of strong vertical wind shear it was not able to intensify past minimal tropical storm strength of 65 km/h, (40 mph). During the next couple of days, strong upper level winds stripped deep convection away from the center with the JTWC downgrading the cyclone to a tropical depression early on January 19. The remaining low level circulation center slowly dissipated with the JTWC issuing their final advisory early the next day.
The cyclone posed a direct threat to Coalition forces, which were operating in the Arabian Sea, Persian Gulf and the Red Sea, in the buildup to the Gulf war. However it was not a significant factor in the buildup to the Gulf war and due to it low latitude track and weak intensity it had little effect on ships steaming to the Middle East. As a result of the cyclone remaining out to sea there was no reported impact to land.
BOB 01, tracked by the Joint Typhoon Warning Center as 02B, hit Bangladesh on April 29 as a strong cyclone. It brought a tremendous storm surge and massive wind damage, resulting in the deaths of over 138,000 people. BOB 01 was the deadliest cyclone on Earth since the 1970 Bhola Cyclone. It also destroyed an estimated 1 million homes, leaving as many as 10 million people (a substantial portion of the country's population) homeless.
On May 30, a depression formed in the Bay of Bengal. It moved north-northeastward and strengthened to a 60 mph tropical storm on June 2. BOB 02 affected the same region as the super cyclonic storm nearly a month earlier. Although the storm disrupted relief efforts, as a result of well-executed warnings, it caused no reported fatalities. BOB 02 dissipated inland the next day.
Early on August 22, reported that Depression BOB 04 had formed about 160 km (100 mi), to the southeast of Balasore, India. As the depression moved towards northwest, it failed to intensify any further. The depression made landfall in the Indian state of Orissa later that day and maintained its identity until it weakened into a low pressure area early on August 26. The depression's windspeeds were estimated to have peaked at 45 km/h (30 mph), whilst the lowest pressure recorded was 992 hPa.
Depression BOB 05 developed in the Bay of Bengal on September 21. It struck Andhra Pradesh and dissipated the following day.
Depression BOB 06 developed in the Bay of Bengal on October 12. It struck Bangladesh and dissipated on October 14.
Depression BOB 07 developed in the Bay of Bengal on October 28. It struck Tamil Nadu and dissipated on October 30.
The final storm of the season, which formed in the eastern Bay of Bengal on November 9, hit eastern India as a 45 mph tropical storm on the 15th. It dissipated the next day over the country.
The Cyclonic storm brought torrential rain and flash floods across southern India when it made landfall. Twenty-four-hour rainfall totaled 480 millimetres (19 in) at Karaikal. As a direct result of the flooding at least 40 people died.
During July 27, a depression developed over the northern Bay of Bengal to the east of the Indian state of Odisha. Over the next day the system moved westwards and developed into a Deep Depression, as it made landfall near Badudebpur in Odisha with estimated winds of 55 km/h (35 mph). The system dissipated over East Rajasthan during July 31, as it enhanced monsoon activities over northern and eastern India.