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South Asia's Deadly Rains of 2015; Tropical Storm Watch in Hawaii

By: Bob Henson and Jeff Masters 3:55 PM GMT on August 11, 2015

A multi-week series of disastrous rains, floods, and mudslides has taken more than 400 lives and affected millions in a broad swath from Pakistan to Vietnam over the last several weeks. This is the height of the South Asia and Southeast Asia monsoon season, when life-giving rains sweep from the Indian Ocean across India and neighboring countries after the parched, scorching conditions that typify spring. Monsoon-related floods often produce hundreds of deaths across India each year. This year’s Indian monsoon has brought an distinct patchwork of impacts. Although several regions have been affected by torrential downpours, others are wrestling with unusually dry conditions, in a monsoon that’s actually been skimpy for the nation as a whole.

Figure 1. An aerial view shows floodwaters inundating houses and vegetation in Kalay, upper Myanmar's Sagaing region, on August 3, 2015. Image credit: Ye Aung Thu/AFP/Getty Images.

The most widespread flood impacts have been in the Bay of Bengal region, from far eastern India across parts of Pakistan and Myanmar. Three weeks of flooding across five provinces of Pakistan have killed at least 151 people, and the government of Myanmar is seeking international assistance, as more than a million acres of farmland have been inundated and at least 46 people have died. In the South Bengal state of eastern India, floods have taken at least 83 lives, and more than 300,000 homes have been destroyed or seriously damaged. Further west, in the central state of Madhya Pradesh, flash flooding was the apparent cause of two train derailments that led to at least 29 deaths.

Figure 2. Two Indian passenger trains lay next to each other following a derailment after flash floods struck a bridge outside the town of Harda in Madhya Pradesh state on August 5, 2015. At least 27 people died as a result of the derailment. Image credit: STR/AFP/Getty Images.

The worst of the rains were associated with Tropical Cyclone Komen. A weak but wet system, with top 3-minute sustained winds of only 45 mph, Komen moved inland over Bangladesh on July 29 but left behind a remnant low that drifted west near the India-Bangladesh border last weekend, according to weather.com. Moist southwesterly flow on Komen’s east flank led to huge rainfall totals, especially along the hilly southeast coast of Bangladesh, where several locations reported between 40” and 50” for the 10-day period from July 24 to August 2.

A separate area of monsoon-related flooding has affected more than a million people in Pakistan and far western India. At least 166 deaths have been reported in Pakistan, where catastrophic flooding in 2010--the nation’s worst natural disaster on record--caused more than 1,700 deaths and left some 11 million people homeless.

Figure 3. A Pakistani resident drinks water from a hand-pump at a flooded area in Nowshera district, in Khyber Pakhtunkhwa province, on August 3, 2015. Image credit: A. Majeed/AFP/Getty Images.

India’s monsoon falling short of average
Despite the flooding disasters scattered across parts of India, the nation is actually seeing a mix of drought and deluge this summer, leaning toward the dry side. The total amount of rainfall in the 2015 monsoon season to date across India (June 1 - August 11) has been only 91% of the long-term average. The nationally weighted rainfall total for the Jun 1 - Aug 11 period was 501.8 mm (19.76”), compared to a typical value to date of 553.1 mm (21.78”). The 91% value is only a slight improvement over the start-of-monsoon outlook from the India Meteorological Department (IMD), which called for 88% of average over the entire monsoon season (June-September). The driest areas have been concentrated along the nation’s northern tier, from Punjab to Assam states (20% to 40% below average) and in the southwest peninsula (20% to more than 50% below average). The central state of Telangana is making contingency plans for drought response should the second half of monsoon season turn out as disappointing as the first half did. In a mid-monsoon update on August 3, the IMD held to its 88% outlook. Such a deficit can cause severe stress on agriculture and the power grid, which relies heavily on hydroelectric power. However, a 12% reduction in rains would not rank in the top five for worst monsoons on record (see below).

Figure 4. Cumulative rainfall from June 1 to August 11 across India has produced a patchwork of above- and below-average accumulations compared to a typical year. Within each state, the average rainfall from June 1 to August 11 is shown at bottom, in millimeters, with the actual 2015 total at top and the percentage anomaly following in parentheses. A total of 16 states have deficient rainfall, while 5 have an excess and 15 are near normal. The nationwide total rainfall for the period was 91% of average. Image credit: India Meteorological Department.

El Niño and the Indian monsoon
In recent decades, El Niño has been closely associated with deficient monsoon rainfall over India, so this year’s underwhelming rainfall is not a total surprise. But the relationship isn’t iron-clad. Monsoon rainfall was 2% above average in 1997 even as a strong El Niño very similar to the current one was building, and rainfall deficits are possible even without El Niño. El Niño Modoki, the type where warming is focused in the central tropical Pacific rather than toward the east, tends to be more effective at suppressing the monsoon than a classic east-Pacific El Niño. The monsoon is also influenced by the Indian Ocean Dipole, measured by the east-west difference in sea-surface temperatures across the Indian Ocean. A positive IOD event tends to enhance moisture in the southwesterly flow over India that brings monsoon rain.

If the 2015 monsoon does end up falling short of the norm, it will be for the second consecutive year, as the monsoon of 2014 produced only 88% of average rainfall. Since most significant El Niño events last just a year, it’s unusual to have two low-rainfall years in a row. The last time two consecutive Indian monsoons saw below-average rain was during the two-year El Niño event of 1986 and 1987. There’s also marked multi-decadal variability in monsoon rains, according to the Indian Institute of Tropical Meteorology. From 1921 to 1964, only three monsoon seasons produced less than 90% of average rainfall. Over the following period, 1965 - 1989, 10 of 24 years fell short of the 90% mark. The most devastating monsoon rainfall deficits since modern records began were as follows:

1) 1877, -33%
2) 1899, -29%
3) 1918, -25%
4) 1972, -24%
5) 2009, -22%

Averaged across the globe, the planet’s major monsoons appear to be collectively producing more rain in recent decades. A 2014 study in the Bulletin of the American Meteorological Society found evidence for increased global monsoon precipitation between 1979 and 2011, but with substantial year-to-year variability in the mix. Theory, modeling, and observations all support the general trend toward intensified precipitation events in many areas, together with exacerbated impacts when drought does strike (the “wet get wetter, dry get drier” concept). For more on the dynamics that drive the Indian monsoon, and its relationship to climate change in India, see this 2013 post from Jeff Masters.

Bob Henson

Tropical Storm Watch posted for Hawaii's Big Island for Hurricane Hilda
In the Eastern Pacific, a Tropical Storm Watch has been issued for the Big Island of Hawaii as Hurricane Hilda heads northwest at 6 mph towards Hawaii. Hilda is under high wind shear of 25 - 35 knots, and the shear will increase to 30 - 40 knots by Wednesday. Although Hilda has remained remarkably intact in the face of this high shear, the storm's increasingly degraded appearance on Tuesday morning gives me confidence that the increasing shear will cause the storm to unravel rather quickly. I expect that Hilda will weaken to a tropical depression by Wednesday evening; the Tuesday morning runs of our two most reliable models for predicting hurricane tracks, the European and GFS models, both showed Hilda weakening to a tropical depression before reaching Hawaii on Thursday. However, even if Hilda dissipates before reaching Hawaii, it will still be capable of bringing dangerous flooding rains to the islands, particularly to the Big Island. Wunderblogger Steve Gregory has a more detailed look at Hilda in his Tuesday afternoon post.

Figure 5. Latest satellite image of Hilda.

In the Western Pacific, the European and GFS models predict that twin tropical storms will form in the waters midway between Hawaii and the Philippines' Luzon Island this weekend. Both of the these storms will have the potential to cause trouble for Asia late next week.

Jeff Masters

Hurricane Flood Extreme Weather

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.