Former Indian President Pranab Mukherjee once famously described the South Asian monsoon as “the real finance minister of India.”
Countries across South Asia set their economic calendars by the drenching rain that begins nearly always on June 1 and continues through September. The arrival is front-page news throughout the region.
As the monsoon makes its journey from its birthplace in the central Indian Ocean, it douses a swath that extends from Pakistan to the Philippines with nearly two billion people in its path. It is lethal in its intensity. The flooding it caused this season had killed more than 1,000 people by August. In 2013, more than 5,700 people were killed by monsoon floods in northeast India. There, two cities in Meghalaya state vie for the title of wettest place on Earth, each usually receiving more than 1,100 centimeters (430 inches) of rain a year.
The monsoon delivers 75 percent of India’s annual rainfall, which makes or breaks the country’s agricultural sector. Deviations from the norm can cost billions of dollars.
As if to prove the point, this year’s monsoon rains did not turn off as expected. They lingered through September and caused widespread damage to the Indian onion crop–so much so that Prime Minister Narendra Modi declared a ban on all exports of the staple at the beginning of October to combat runaway prices. Truckloads of onions were stopped at the border. Onion hoarding was outlawed.
Jennifer MacKinnon, Drew Lucas, and Amy Waterhouse, physical oceanographers at Scripps Institution of Oceanography at the University of California San Diego, have been studying the monsoon’s ocean physics for several years. Two of them, Lucas and MacKinnon, arrived in Chennai in July to begin a new round of field research just as the city of seven million people nearly ran out of water.
“It’s because the monsoon was late,” said MacKinnon.
Scientists have been attempting to predict monsoons since the 1870s after a drought-triggered famine took the lives of an estimated five million Indians. But despite more than a century of forecast advances, they still don’t know enough to predict when exactly monsoons will come, whether they will be delayed, diminished, diverted, or disrupted.
The relentless superimposition of global warming renders whatever fundamentals scientists do know subject to change. Another wild card is air pollution. While global warming would generally be expected to make monsoons stronger, recent research suggests coal burning is actually stifling the monsoon in some regions of China.
On July 6, Lucas, MacKinnon, other researchers, students, and engineers from the Multiscale Ocean Dynamics group at Scripps Oceanography pushed off from the dock in Chennai aboard Scripps Oceanography research vessel Sally Ride. They were among an international team of oceanographers and meteorologists who took part in an Office of Naval Research-funded project called MISO-BoB (Monsoon Intra-seasonal Oscillations in the Tropical Indian Ocean and the Bay of Bengal). The endeavor includes scientists from India, as well as U.S. institutions such as Woods Hole Oceanographic Institution, Oregon State University, the University of Massachusetts Dartmouth, University of Notre Dame, and other research centers.
The broad details of monsoon variability are reasonably well-understood, but a big unknown is what is happening in the Bay of Bengal as the ocean and atmosphere mingle during monsoon season. MISO-BoB participants consider this study at the cutting-edge of research on how ocean-atmosphere interaction shapes the monsoon, a project that has a potential payoff for more than just the people living in its path.
“This research will enable better predictive skill at a variety of lead times, from hours to weeks and a better understanding of the importance of ocean observations in providing that predictive skill,” said ONR program manager Scott Harper. “Understanding the maritime environment is critical for safe and efficient naval operations around the globe, and this research should help support that by improving the Navy’s ability to forecast the environment in the future.”
Physical oceanographers are only one group at Scripps Oceanography actively studying the monsoon. Other researchers are examining its links to global weather phenomena, its origins, and its effects on civilization throughout history.
Notably, upcoming work by archaeologist Jade d’Alpoim Guedes, who has a joint appointment at Scripps Oceanography and the UC San Diego Department of Anthropology, suggests that the dominance of the monsoon over South Asian agriculture has existed for several thousand years. The very distribution of inhabited areas in modern-day India bears the imprint of its direct influence.
"Humans in monsoon India have adapted their farming and pastoral strategies to spatial patterning in the monsoon,” Guedes said. “In the past, when rivers moved and available sources for irrigation shifted, human settlement shifted accordingly and people focused more on pastoralism as a strategy in increasingly arid areas."
Every year in the Bay of Bengal there is a buildup of heat as the sun’s path moves north approaching the vernal equinox. The Himalayas act like a wall to keep the heat confined to the Indian subcontinent. The land heats up faster than the ocean, causing winds that had been blowing from the northeast reverse direction and blow from the southwest.
Major rivers laden with early monsoon season rains such as the Ganges, the Brahmaputra-Jamuna, the Godavari, the Irrawaddy, and several lesser rivers spread a sheet of fresh water over the surface of the Bay of Bengal. That fresh water combines with the rainwater falling on the surface of the bay. Together they act like a lid on top of seawater, but it is not at all uniform in thickness. Instead the fresh water layer is thick in some places but thin in others. Rivers of fresh water swirl through the saltwater of the bay in filaments stretching for hundreds of miles. Those alternating tendrils of fresh- and saltwater translate information to the atmosphere centimeters and kilometers above the ocean surface.
The top layers of the Bay of Bengal are kinetic. They are permeated by swirling masses of ocean called mesoscale eddies that are 200-250 kilometers (125-155 miles) in diameter. They “stir the oceanic layers like swirling the batter of a layered cake,” said Amit Tandon of UMass, who was the U.S. lead on MISO-BoB.
All this happens, with variation, every year and creates a storm system that produces rain that works its way north from the bay in waves. Tandon compares it to a battery being discharged then needing time to recharge before starting over again. The rain stops when the battery recharges.
“This layering setup by the eddies also influences the communication between the ocean and atmosphere, which in turn affects the monsoon. We know that the air-sea interaction at large scales matters but at what horizontal scales does the air-sea interaction affect the monsoons? This remains a puzzle,” Tandon said.
That interplay is not the only factor. The monsoon also appears to be influenced by ocean conditions happening far away. For instance, the total summer rainfall over India tends to decrease in years when El Niño dominates with an irregularly warm Pacific Ocean at the equator. Scripps climate scientist Shang-Ping Xie determined earlier this year that the monsoon is delayed, sometimes as much as two weeks, in summers following El Niño.
“The atmosphere connects the tropical Indo-Pacific oceans,” Xie said. “The global-scale ocean-atmosphere coupling drives the monsoon to vary from one year to another.”
And then the monsoon goes on to radiate its own influence worldwide, reversing the flow of the current off Somalia, altering circulation in the Arabian Sea and helping control the cycle of El Niño and La Niña that in turn influences the next monsoon.
For the oceanographers, MISO-BoB was a chance to do fundamental research involving direct observation of a phenomenon. In this case, the task played to Scripps’s strength of measuring small-scale ocean physics where answers have the potential to provide great societal value.
Part of MISO-BoB’s reason for being came from another experiment earlier in the decade in which Lucas, MacKinnon, and Waterhouse also participated. That one, called the Air-Sea Interaction Research Initiative, established that surface water divides itself into layers distinguished from each other by their relative saltiness. That layering, or stratification of salinity, has the effect of keeping warm water under the surface, where sometimes it remains until a cyclone or other major storm churns it up to the surface. The cyclone uses the heat as fuel to become a monster storm.
Still, a mystery remained on how the heat is used to turn on and shut off the monsoon throughout the season, the oscillations referenced in MISO-BoB’s name. Researchers believe that the air-sea interface, in areas oceanographers call boundary layers, holds the answer.
“We presume that the missing parts are the things happening on scales smaller than what is represented in forecasts,” said Lucas.
There were two MISO-BoB expeditions this summer. They were part of the sixth and final monsoon season collectively documented by the two field projects. A hallmark of both was the deployment of some of the most sophisticated instruments ever used in oceanography. Many were “homegrown” at Scripps, said Lucas.
The suite of instruments included the “Fast-CTD,” a device that measures water conductivity (a proxy for salinity) and temperature at various depths. CTDs have been an oceanographic mainstay for decades but none work like the Fast-CTD. It descends and surfaces at a rate of 10 to 12 knots as the ship travels at four knots. That’s six times faster than a typical CTD deployment speed.
The MISO-BoB Fast-CTD was deployed to a depth of 200 meters (650 feet), unlike a typical CTD that measures depths up to several thousand feet. The Fast-CTD, however, can measure more of the ocean, in this case taking 12,000 full profiles over six weeks at sea and covering 1,500 kilometers (930 miles) of the Bay of Bengal. The winch capable of working at that speed without cables getting tangled or snapping was itself a feat of engineering delivered by specialists like Michael Goldin, who have spent 30 years or more at Scripps solving such problems.
“Simply put, you can't just go out and buy systems like that,” Lucas said.
Added to that was the Wirewalker, a device invented at Scripps for measuring ocean variability, including turbulence, ocean currents, and the vertical structure of the light field –those top several meters of the ocean where sunlight reaches. It makes the measurements powered entirely by the ocean’s own surface waves.
MacKinnon used a meter developed in collaboration with Scripps oceanographer Matthew Alford that records turbulence as it sinks in the water the way the needle on a turntable turns bumps and grooves into sound. Buoys, gliders, and drifters from multiple universities added to the suite of data-gathering instruments. Scientists with India’s Ministry of Earth Sciences launched sondes – devices to measure atmospheric conditions – from several coastal stations around the Bay of Bengal. Key to the experiment was that the aforementioned measurements of ocean and atmosphere were done simultaneously. The scientists were eavesdropping on both parts of a dialogue.
MISO-BoB scientists thus have confidence that the mass of data coming from the Bay of Bengal can give modelers enough solid data to set guidelines for the range of possible dynamics of the upper ocean. Those guidelines would then become variables fed into simulations. The simulations would then lead to forecasts hopefully accurate enough to let people in India know that there might be onion-threatening rains late in the season.
“The measurements, aided by satellite data and numerical model experiments, will help us to understand the feedbacks between the river-dominated upper layer of the Bay of Bengal and the monsoon atmosphere,” said MISO-BoB participant Debasis Sengupta, an oceanographer at the Indian Institute of Science in Bangalore. “Better knowledge of air-sea interaction will eventually translate to improved predictive models of the ocean, and of monsoon weather systems and intra-seasonal oscillations.”
As for the Scripps scientists, they believe that MISO-BoB generated a “transformative dataset,” as Lucas put it. Researchers will now spend the next two to three years analyzing reams of measurement data and releasing findings. The specialized instruments invented at Scripps labs have become part of the toolkit that oceanographers can carry to other parts of the world to estimate completely different phenomena such as how fast Arctic sea ice is going to melt.
The multi-institutional and technologically advanced mission paid dividends not only in terms of data collected but in the experience received by a new generation of Indian oceanographers. ASIRI and MISO-BoB both featured summer courses and student exchanges, said Waterhouse.
“Being at sea with Indian and Sri Lankan researchers, learning from each other and collaborating on this important project has been one of the most rewarding experiences,” she said.
In part because of MISO-BoB, Scripps is now in talks to create an agreement for further collaborative research with the Indian Ministry of Earth Sciences.
“For a country with long coastlines impacted strongly by monsoons from the Bay of Bengal, the number of junior scientists in India trained in observational physical oceanography and coupled ocean-atmospheric boundary layers tools has typically been very small,” said MISO-BoB participant R. Venkatesan of India’s National Institute of Ocean Technology. “Such collaborative work and this resource-sharing model is very much required to continue both for scientific and societal needs. This Indo-U.S. collaboration is a success story that can be showcased to the world.”
– Robert Monroe
Related Image Gallery: MISO-BoB July 2019 cruise