Tropical Cyclones are not unusual in the North Indian Ocean, though the bifurcation produced by the Indian subcontinent limits their potential for development.
As we saw with Hurricane Patricia, however, under the right circumstances, a modest tropical system can become very powerful in a day or so.
Tropical Cyclone Chapala has become the equivalent of a category five hurricane and is taking an unusual path towards the Arabian Peninsula.
It will strike Yemen Sunday night as the equivalent of a major hurricane.
Cyclone Categories In The Indian Ocean
The World Meteorological Organization (WMO) needs to get all the parties affected by tropical storms together and settle on a standardized system for categorizing these storms. As it is, we have ‘hurricanes’ in the north Atlantic and eastern north Pacific, ‘typhoons’ in the western north Pacific, and ‘tropical cyclones’ in the north Indian Ocean. There are different names for southern hemisphere cyclones. Worse, the strength categories vary from sea to shining sea.
Decoded will use the local designations, but also mention the hurricane equivalents.
In the Indian Ocean, the categories are as follows:
- Depression: Winds are up to 30 miles per hour.
- Deep Depression: Winds are 31 to 37 miles per hour.
- Cyclonic Storm: Winds are 38 to 53 miles per hour.
- Severe Cyclonic Storm: Winds are 54 to 73 miles per hour.
- Very Severe Cyclonic Storm: Winds are 74 to 104 miles per hour.
- Extremely Severe Cyclonic Storm: Winds are 105 to 134 miles per hour.
- Super Cyclonic Storm: Winds are over 134 miles per hour.
To make matters worse, the organizations that use the different categories have different definitions of ‘sustained wind,’ which is the value on which the designation is based. In the Indian Ocean, the sustained wind is taken as a three-minute interval wind. In the Atlantic, it’s ten minutes. For simplicity, we will ignore the difference, which is minor.
Thankfully, all jurisdictional entities agree that the wind measurement should be taken ten meters above the surface.
North Indian Ocean Cyclones And Their Paths
The majority of north Indian Ocean tropical cyclones form in the Bay of Bengal, east of India, and never visit the Arabian Sea to the west. A fair number of storms form in the eastern Arabian Sea and come ashore in india.
It is rare for a tropical cyclone, particularly one with the strength of Chapala, to move west towards the Arabian Peninsula. The few storms that have affected the area have moved towards Oman; a storm hitting Yemen at this intensity is unprecedented.
Previous Western Arabian Sea Record
Cyclone Gonu caused widespread damage to Oman and the United Arab Emirates in 2007. Fifty people died and damage was estimated at $4.2 billion dollars. Yet when Gonu made landfall it was just the equivalent of a tropical storm. The region is normally dry and peacefully calm, though hot. Gonu dropped up to two feet of rain, several years’ worth, near the coast, and flooding was extensive as the remnants of the storm moved through Iran and Pakistan. The maximum winds in Gonu were 150 miles per hour over the ocean and 70 miles per hour at landfall.
Chapala is taking a track farther south than Gonu, actually in the never-before-seen direction of Yemen.
The previously strongest cyclone to hit Yemen produced paltry 35 mile per hour winds — not even tropical storm strength. Compared to the previous record, the expected 120 mile-per-hour winds from Chapala are off the chart.
The Outlook For Chapala
Super Cyclone Chapala has already attained the equivalent of category five status, with top winds of 175 miles per hour.
It is forecast to retain category three strength until landfall on Sunday. A storm surge of up to 12 feet is expected, and rainfall could total over eight years’ worth (a year’s worth is only a couple of inches).
After passing through Yemen, the remnants of Chapala will bring heavy rains to Saudi Arabia.
It will be a new experience for the desert nation.
Does Global Warming Have Anything To Do With This Storm?
2015 is on track to be the warmest year since meteorologists began keeping records in 1880.
Conditions are exacerbated by a powerful El Niño, which resulted in record numbers of hurricanes in the eastern and central Pacific and a record number of Super-Typhoons in the western Pacific. The warmer water certainly contributed to the explosive intensification of last week’s most-powerful-hurricane-ever Patricia, which struck Mexico.
As the oceans warm, more — and more powerful — tropical cyclones are a logical corollary, though feedback mechanisms could create conditions which inhibit tropical cyclone formation in some places. Such conditions existed this summer in the Atlantic Ocean, where vertical wind shear kept tropical cyclone numbers below average.
The weak tropical season in the Atlantic and the strong ones in the Pacific and Indian Ocean show that the effects of global warming will not be distributed evenly across the globe. One place’s feast could be another’s famine.
The unusual paths of Patricia and Chapala indicate that relatively minor changes in water and air temperatures can have a significant effect on atmospheric circulation. Some parts of the globe may have to adjust to radical changes in their weather.