155mph tropical cyclone Chapala, the second strongest on record in the Arabian sea threatens Yemen with huge amounts of rain, winds and surging seas. Chapala is the strongest tropical cyclone ever in the fall monsoon retreat months when the Arabian sea waters used to have low heat content.
Tropical cyclone Chapala intensified explosively to 155mph, the strongest cyclone on record in the monsoon retreat season in the Arabian sea, fueled by the warmest waters on record for the season. Tropical cyclones were extremely rare in the Arabian sea but have become a threat since the turn of the millennium when the heat content of the north Indian ocean began to increase rapidly. The heat content in the monsoon retreat months used to be very low because strong southwest monsoon winds along the Somalian coast used to well up so much cool water that there wasn't enough heat stored in that sea to support tropical cyclone development. The Bay of Bengal, which is normally hot this time of year, has been the part of the north Indian ocean threatened by cyclones. This year, however, the oceanic heat content is unusually high in the Arabian sea. Almost 2 decades of intensified flow of Pacific ocean heat into the Indian ocean, combined with weakened cool water upwelling off of Somalia and intense atmospheric subsidence of hot dry air over Indonesia caused by the monster El Nino have led to the extraordinarily high ocean heat content.
North Indian Ocean heat content Oct. 29, 2005
In 2005, when AOML's ocean heat content data set began the waters off of Africa and the Arabia didn't have enough heat to support a hurricane strength cyclone in the monsoon retreat months.
The ever-warmer Indian Ocean
The Indian Ocean has been getting hotter. Recent research suggests that a significant fraction of the total ocean heat gain from climate change is finding its way to the Indian Ocean basin, driven by the global ocean circulation. The oceans to the northwest of Australia, a critical region for our rainfall, have been running at hottest-on-record for the past decade, and this record warmth has been almost basin-wide during the past three years.
Consistent with this trend, June, July and August 2015 were all record-warm months for the Indian Ocean, and this, in turn, is having an impact upon the climate, and the models we use to forecast it.
North Indian Ocean heat content Oct. 29, 2015
Indian ocean heat content 29Oct2015. High heat content is fueling the strongest Arabian sea tropical cyclone in history for the monsoon retreat season (fall).
After the super El Nino of 1997-1998 the trade wind intensity increased across the tropical Pacific ocean to much above normal speeds. These winds welled up cool water in the eastern Pacific and built up a very deep pool of very warm water in the western Pacific. The huge amount of heat stored in the ocean led to an apparent slow down in global warming. In fact, the oceans were heating up faster than ever. The strong winds pushed up sea level on the east coast of Indonesia driving much more warm water than usual from the tropical Pacific into the Indian Ocean. This huge increase in ocean heat in the Indian ocean has led to the intensification of tropical cyclones in the north Indian ocean.
Stronger than normal trade windssince 1998 have increased the through flow of tropical Pacific heat into the Indian ocean.
Black carbon and other pollution coming off the Asian continent has also played a major role by reducing cloud formation and warming the waters offshore of India. Pollution, however, has only been shown to lead to stronger cyclones in the "spring months" of monsoon advance. All the factors above led to the warmest temperatures in history over the Arabian sea. The unprecedented ocean heat is supporting this unprecedented tropical cyclone.
155mph Cyclone Chapala formed over record warm waters.
Yemen faces an unprecedented disaster. No recorded tropical storms have hit Yemen.
Strong Arabian Sea tropical cyclones are uncommon.
According to NOAA's Historical Hurricanes tool, there have only been six major Category 3 or stronger tropical cyclones recorded in the Arabian Sea (though accurate satellite records go back to just 1990.) The Arabian Sea doesn't get many tropical cyclones since it is small; furthermore, the Southwest Monsoon keeps the tropical cyclone season short, with a short season that lasts from May to early June before the monsoon arrives, then another short season in late October through November after the monsoon has departed. Strong Arabian Sea storms are rare due to high wind shear and copious dry air from the deserts of the Middle East, with just two Category 4 or 5 storms ever recorded--Gonu in 2007 and Phet in 2010. Both cyclones hit Oman after weakening below Category 4 strength. There are no recorded tropical storms to have hit Yemen, though the nation has been hit by two tropical depressions--Tropical Depression Keila in 2011, and Tropical Depression Three in 2008.
Tropical Cyclone Chapala is forecast by the Joint Typhoon Warning center to make landfall in Yemen at hurricane strength. No tropical cyclone has been recorded ever making landfall in Yemen.
NOAA's HWRF model predicts a maximum of 37.7 inches of rainfall, over 3 feet (or about 1 meter of rain) in the mountains of coastal Yemen. Yemen, which has been in the grip of a devastating drought now faces unprecedented floods. People will have no way to prepare and they will have no idea of what to do. War, drought and food shortages have left Yemen completely unable to respond if this intense tropical storm makes landfall as forecast since devastating flash flooding is likely on drought stressed soils which would wash away with the torrents pouring off the mountains.
Yemen is threatened by possibly as much as 3 feet of rain in the coastal mountains. There is no record of any tropical cyclone ever hitting Yemen but they are threatened by a cyclone forecast to landfall at hurricane strength.