Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 14. syyskuuta 2011 klo 13:53 (GMT)
A year after enduring the most devastating flooding in its history, Pakistan is again experiencing historic floods. An unusually heavy and late-lasting monsoon has brought torrential rains to Pakistan's southeast Sindh Province, which borders India to the east and the Arabian Sea to the south, and includes Pakistan's largest city, Karachi. The heavy rains began in the 2nd week of August, and have continued into the 2nd week of September, accumulating to 2 1/2 times more than average. According to Dr. Qamar-uz-Zaman Chaudhry, Pakistan's Federal Advisor on Climate Affairs, this is the highest 4-week monsoon rainfall total ever recorded in Sindh province, amounting to more than 37 million acre feet of water, "which is unimaginable." The "unimaginable" rains occurred after a 12-month period where the province received no rain and was under severe drought conditions. At least 226 people have been killed in the new flooding, 1.2 million houses have been damaged or destroyed, and 280,000 people made homeless. There were already 1 million people needing food aid and 800,000 families without permanent shelter due to last year's floods, making this year's renewed flooding particularly disruptive. According to the India Meteorological Department, by September 1, the monsoon usually begins to retreat from northwest India and southeastern Pakistan. That hasn't happened this year, and the monsoon rains are forecast to continue at least for the remainder of this week--well into the 3rd week of September. This very unusual monsoon season also started a week earlier than normal.
Figure 1. Rainfall during the 2011 monsoon season has accumulated to 8 to 12 inches above average over portions of Pakistan's Sindh Province. Image credit: Pakistan Meteorological Department. Before and after satellite images of the flood are available at NASA Earth Observatory.
Is there a climate change connection?
Last year, heavy monsoon rains were enhanced by a very unusual jet stream configuration that brought cool air and rain-bearing low pressure systems to northern Pakistan. The great floods of 2011--rightfully called Pakistan's Katrina--submerged one fifth of the country, killing 1985 people, leaving 11 million homeless, and doing a record $9.5 billion in damage. This year, the monsoon weather patterns were much different, but also highly unusual, resulting in yet another great flood in Pakistan. In an interview with dawn.com, Dr. Qamar-uz-Zaman Chaudhry, Pakistan's Federal Advisor on Climate Affairs, stated: "...climate change has become a reality for Pakistan. Clearly, Pakistan is heading for an increased frequency and intensity of extreme weather events which included frequent floods and droughts, and the need of the hour is to plan for the future changes." These events are in line with international climate change projections, he said.
Figure 2. Evacuations in Pakistan's Sindh Province during the summer 2011 floods. Image credit: Pakistan Meteorological Department.
A skeptic of Dr. Qamar's arguments might point to the fact that monsoon rainfall in neighboring India was not all that unusual in either 2010 or 2011, and that major monsoon flooding disasters in back-to-back years in Pakistan were probably just bad luck. However, the monsoon in India and Pakistan has undeniably changed in recent decades. In a study published in Science in 2006, Goswami et al. found that the level of heavy rainfall activity in the monsoon over India had more than doubled in the 50 years since the 1950s, leading to an increased disaster potential from heavy flooding. Moderate and weak rain events decreased over the past 50 years, leaving the total amount of rain deposited by the monsoon roughly constant. The authors commented, "These findings are in tune with model projections and some observations that indicate an increase in heavy rain events and a decrease in weak events under global warming scenarios." A warming climate loads the dice in favor of heavier extreme precipitation events. This occurs because more water vapor can evaporate into a warmer atmosphere, increasing the chances of record heavy downpours. In addition, heavy downpours preferentially occur during thunderstorms, and a warmer climate produces a longer period of time during the year when thunderstorms can occur, giving more opportunities for heavy rainfall events. During August 2011, ocean temperatures in the Arabian Sea off the coast of Pakistan, in the region between 15°N - 25°N, 60°E - 70°E, were 0.8°C (1.4°F) above average, according to an analysis I did of the HADSST2 dataset. This was the 5th highest such value in over 100 years of record keeping. During the July 2010 monsoon, this region of ocean was 1.1°C (2.0°F) above average, the warmest July ocean temperatures on record. The extra heat in the ocean the past two summers have undoubtedly contributed to the high rainfall totals in Pakistan by allowing more water vapor to evaporate into the air. Thus, we should expect to see an increased number of disastrous monsoon floods in coming decades as the climate continues to warm and the oceans off the coasts of India and Pakistan heat up. Since the population continues to increase at a rapid rate in the region, death tolls from monsoon flooding disasters are likely to climb dramatically in coming decades. Another concern is that climate change might lead to more failures of the monsoon--years when the rains are far below normal, leading to widespread drought and crop failures. This is a more dangerous scenario, since historically, droughts have been much more deadly than floods in Asia. Failure of the monsoon rains typically occur during El Niño years, so if climate change increases the frequency of El Niño, we might see an increase in the failure of the monsoon rains. So far, climate models are unclear on how climate change might affect El Niño, so we don't know how great a concern future failures of the monsoon might be.
Goswami, et al., 2006, " Increasing Trend of Extreme Rain Events Over India in a Warming Environment", Science, 1 December 2006:Vol. 314. no. 5804, pp. 1442 - 1445 DOI: 10.1126/science.1132027
Tropical Storm Maria headed towards a brush with Bermuda
Tropical Storm Maria is finally pulling away from Puerto Rico, and is headed north-northwest towards a brush with Bermuda, which will occur Thursday morning. Wind shear has fallen about 5 knots since yesterday, and is now a moderate 10 - 15 knots. This reduction in shear has allowed Maria a strengthen some, and satellite loops show the storm has more heavy thunderstorms that are better organized. The storm's surface circulation is still exposed on the storm's west side, though, and Maria does not have anything close to a complete eyewall built. Maria passed near NOAA buoy 41046 this morning, which reported a 1-hour period of sustained winds of 38 mph, gusting to 45 mph. An outer spiral band of Maria is just beginning to appear on Bermuda radar.
Figure 3. Morning satellite image of Tropical Storm Maria.
Forecast for Maria
A trough of low pressure moving off the U.S. East Coast a predicted to turn Maria to the north-northeast by early Thursday, and accelerate the storm past Bermuda. The trough will also bring high wind shear of 20 - 30 knots beginning late tonight, which gives Maria just a short window of opportunity to intensify today. NHC gave Maria a 32% chance of reaching hurricane strength by Thursday in their 5 am EDT wind probability forecast. Intensification will be hampered by the fact that Maria will be passing over the cold water wake left by Hurricane Katia today. On Thursday morning, Maria will be making its closest approach to Bermuda. Bermuda will see an 8-hour period of sustained winds in the 25 - 35 mph range, accompanied by heavy rain squalls, beginning near 4 am local time on Thursday. Bermuda may experience a few hours where the wind rises above tropical storm force, 39 mph, near 8 am local time Thursday. Occasional rain squalls are expected to bring 1 - 3 inches of rain to the islands. Most of the models show that Maria will brush or strike Newfoundland, Canada on Friday afternoon. Most of the storm's high winds will be on the right side, and Maria will be weakening rapidly then, so I'm not expecting the storm will do much wind damage. Heavy rains could bring minor to moderate flooding to the eastern portion of the island.
Elsewhere in the tropics
Even the busiest of hurricane seasons have lulls, and we're hitting one this week during what is traditionally the busiest week of hurricane season. The models are backing off this morning on the development of a tropical depression or strong tropical disturbance late this week off the coast of Africa. The NOGAPS model continues to predict the Western Caribbean could see the development of a tropical depression 6 -7 days from now, but the other models are showing little support for this idea.
The Climate Reality Project
Those of you who like Al Gore's efforts to promote climate change awareness and solutions may be interested in checking out his latest effort tonight at 7 pm local time, in all 24 of the world's time zones, via climaterealityproject.org. It's a live streaming multimedia presentation created by Al Gore and delivered once per hour by 24 different presenters for 24 hours, representing every time zone around the globe.
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