Winds Connect Snow to Sea by Rebecca Lindsey February 21, 2006

 

At first, Joaquim Goés’ prediction sounds a little like the kind of folklore you might find in a farmer’s almanac: low-snow winters across Europe and Asia will be followed by summers during which the amount of fish food in the Arabian Sea will skyrocket.

Such a connection seems hard to believe; after all, thousands of miles and half a year separate the areas and the events. But in 2004, Goés, an oceanographer and remote-sensing expert from Bigelow Laboratory for Ocean Sciences in West Boothbay Harbor, Maine, and some of his colleagues put together a trail of evidence that they say leaves little doubt that the long-distance connection is real.

Anyone interested in understanding how global warming will matter on a regional level should be interested in the connection says Goés. Interested…and probably a little concerned.

Stumbling onto a Discovery

According to Goés, the relationship between global warming, declining snow cover in Eurasia, and the food chain in the Arabian Sea were not even on his mind when he stumbled onto the discovery. In fact, he explained, he was simply trying to correct a mistake he had made on a separate project—or what he thought was a mistake at the time.

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  Map of the Arabian Sea and surrounding land masses

“We were funded by NASA to come up with maps of nitrate concentrations in the world’s oceans based on satellite observations of chlorophyll and sea surface temperature,” he said. Nitrate is the major source of nitrogen for phytoplankton—the single-celled plants that are the foundation of the ocean food web—and knowing nitrate concentrations helps scientists model the ocean carbon cycle.

 

The Arabian Sea is connected to the Eurasian landmass by the winds of the Asian monsoon. Reduced snow cover over Eurasia to the north of India is strengthening the summer monsoon winds over the Arabian Sea. The strong winds bring nutrients to the sea surface, enhancing the growth of tiny marine plants called phytoplankton. (Natural Earth map courtesy Tom Patterson, U.S. National Park Service)

  Map of nitrate concentrations in the Arabian Sea during August of 2002

The problem, said Goés, was that after months of work on the project, it seemed like their mapping technique had a major bug: it didn’t seem to work for the Arabian Sea. According to the maps, nitrate amounts in the Arabian Sea during the summer months of July-August were way too high. Goés couldn’t find any estimates in the scientific literature that were as high as his team’s maps suggested. “So, we started this research as a way to backtrack from what we thought was a mistake in our technique for mapping nitrate.”

 

Goés and his team discovered surprisingly high concentrations of nitrate in the surface waters of the Arabian Sea. While trying to determine if his results were correct, Goés discovered a complex chain of cause and effect between climate change, Eurasian snow cover, and the plant life in the Arabian Sea. This map shows nitrate concentrations from August of 2002, with blue indicating low levels and light green signifying high levels. (Map by Robert Simmon, based on data provided by Helga do Rosario Gomes, Bigelow Laboratory for Ocean Sciences)

 

Tracking down a mistake

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To figure out what had gone wrong, Goés and his colleagues had to double check each piece of information that had gone into making the nitrate maps—like checking a string of holiday lights for the burned-out bulb that is shorting out the whole string.

The first “bulb” they checked was chlorophyll. “Since our maps relied on chlorophyll,” Goés explained, “the first thing we checked out was what the chlorophyll data showed. We wondered if it was also very high. We analyzed the chlorophyll observations collected by satellites over the Arabian Sea all the way back to about 1997, and we realized that there was over a 350 percent increase in chlorophyll concentration in the Arabian Sea in 7 years!”

   
  Maps of chlorophyll in the Arabian Sea from September 1997 and 2003
 

The chlorophyll signal detected by satellites comes from the phytoplankton that grow in the surface waters of the ocean. A 350 percent increase in chlorophyll meant that phytoplankton populations had skyrocketed in the previous 7 years.

 

Between 1997 and 2003, chlorophyll concentrations in the Arabian Sea rose 350 percent. In the maps above, high chlorophyll concentrations are yellow, while low concentrations are dark blue. (Maps by Robert Simmon, based on data provided by the NASA GSFC Ocean Color Team)

This graph shows seasonal cycles of chlorophyll (milligrams per cubic meter) in the Arabian Sea. Dark green segments show the concentrations measured during the heart of the summer monsoon. (Graph by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)

Graph of chlorophyll concentration in the Arabian Sea from 1997 to 2004

Goés and his colleagues were intrigued. “That discovery got us looking for causes,” he explained. The first thing they checked was sea surface temperatures because in many parts of the ocean, phytoplankton growth rates are linked to water temperatures. Cool waters are often more productive than warm waters. Phytoplankton grow in the surface waters of the ocean where there is light for photosynthesis, but with so much life concentrated in the surface waters, nutrients eventually become scarce. Growth subsides until a fresh source of nutrients arrives. Often the fresh source of nutrients is water rising up from the depths of the ocean, where the bodies of millions of sea creatures and all the nutrients they contained accumulate over time. Because the water is from deep in the ocean, it is also cold. Pockets of cool sea surface temperatures show where deep water has welled up to the surface.

   
Graph of sea surface temperature in the Arabian Sea from 1997 to 2004

“We went back and looked at the sea surface temperature data for the same 7-year period, and we discovered that temperatures had gotten cooler,” he said. The cool water was especially pronounced along the coast of Somalia, a well-known area of coastal upwelling. The unusually cool temperatures in that part of the sea suggested that upwelling of deep water had increased. Now Goés needed an explanation for the increased upwelling.

 

Chlorophyll concentrations in the Arabian Sea increased while sea surface temperatures decreased. This graph shows seasonal cycles in temperatures in the Arabian Sea from 1997 to mid-2004. Dark segments mark the observations collected during the heart of the summer monsoon. Falling temperatures indicate more upwelling of cold, deep water. (Graph by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)

 

Explaining Upwelling

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“At this point,” explained Goés, “we backtracked to winds as an explanation for the increased upwelling and cool sea surface temperatures.” Why are the winds important? Goés provides a mini-lecture on the climate of Southwest Asia. In Southwest Asia, the cycle between the winter and summer monsoon dominates the climate. In the summer, the winds blow from the ocean toward the land, bringing heavy rains to India, Pakistan, Bangladesh, and Myanmar among other countries. In the winter, the direction of the winds reverses, with winds blowing from land to sea. The seasonal reversal of the monsoon has dramatic effects on the Arabian Sea because the sea is landlocked to the north, isolating it from large-scale ocean circulation patterns.

“When the Southwest Asian Monsoon is underway in June, July, and August,” explained Goés, “the winds blow from the southwest across the Arabian Sea, and the surface waters are pushed toward Asia. Cold water wells up along the coast of Somalia and the Arabian Peninsula to replace waters pushed ashore.” The most likely explanation for the cool sea surface temperatures off Somalia and the Arabian Peninsula that Goés and his colleagues had discovered was that upwelling had increased. The most likely explanation for increased upwelling, they reasoned, was a stronger Southwest Monsoon.

   
  Maps of winds during the winter and summer monsoons.
 

To verify what was behind the upwelling, Goés worked with Prasad Thoppil, an ocean circulation modeler from the Naval Postgraduate School in Monterey, California. Using observations of wind speed and direction, they reconstructed maps of surface winds over the Arabian Sea over the same 7-year span. “We confirmed that the winds during the southwest phase of the monsoon had definitely picked up,” said Goés.

 

In the summer, strong and steady monsoon winds drive the surface waters in the western Arabian Sea northeast. In response, cold water wells up from the ocean depths. These maps show sea surface temperatures (color) and winds (arrows). (Maps by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)

Graph of wind stress in the Arabian Sea from 1997 to 2004

Goés could have stopped hunting for an explanation at this point. After all, he had only started checking this scientific “string of lights” to find the burned-out bulb responsible for shorting out his global nitrate maps. A strengthening of the summer monsoon and a dramatic increase in plant productivity would have been an adequate explanation for why nitrate concentrations in his maps were so much higher than anything published before; his maps were unusual because the conditions in the Arabian Sea were unusual.

But by now, Goés was tangled in the web of relationships that connect the land to the ocean to the atmosphere in Southwest Asia. Each question he asked and answered led to another, and like most scientists, Goés just couldn’t stop asking himself “Well, now what could be causing that?” If phytoplankton increased between 1997 and 2004 because upwelling had increased, and if upwelling had increased because monsoon winds had picked up, then the obvious question to Goés was why had the winds picked up?

 

The force of wind (measured in newtons per square meter) on the surface of the Arabian Sea during the height of the summer monsoon (dark segments) has increased since 1997. The increased winds drive the upwelling of cool, nutrient-rich water, which in turn encourages the growth of phytoplankton. Knowing this, Goés was still curious: why was the wind getting stronger? (Graph by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)

 

Linking Snow Cover and the Monsoon

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To answer this question, Goés contacted John Fasullo, a scientist who suggested that the best description for what he does is “climate diagnostician,” someone who studies the underlying causes of climate phenomena. Fasullo specializes in the underlying causes of variability in the monsoon.

The Southwest Asian Monsoon is one facet of the larger Asian Monsoon. The basic cause of the Asian monsoon is that the Eurasian landmass and the Indian Ocean warm and cool at different rates through the seasons. In the summer, the land surface gets hotter than the ocean. Hot air above the land surface rises, and moist air from over the ocean flows in to replace it. In the winter, the opposite occurs. The air over land gets much colder and denser than the air over the ocean. Warmer air over the ocean rises, and cold air from the north blows in to replace it. Because the monsoon is driven by land-ocean temperature differences, anything that increases that difference could, in theory, strengthen the monsoon.

   
  Maps of winds and snow cover over South Asia and the Arabian Sea in Winter and Summer
 

“Joaquim contacted me because of a paper I had published in 2004 in which I supported the existence of a link between the strength of the Asian monsoon and the amount of snow cover in Europe and Asia,” explained Fasullo. The basic idea is that before summer’s heat can kick the monsoon into high gear, it must first melt the winter snow cover and evaporate the resulting soil moisture.

“The idea that there is a link between snow cover and the monsoon has been around since the 1800s,” said Fasullo. “But the initial theory,” proposed in 1884 by a scientist named H.F. Blandford, “didn’t specifically include the Arabian Sea,” he added. “It was much more general; it basically said the less snow cover there was over the Himalaya, the sooner you would have warming in the spring and the warmer it would get over the summer. Therefore, snow cover would influence both the onset and intensity of the monsoon.”

“The trouble was the theory wasn’t supported by extensive observational studies of the modern satellite era. Scientists who looked for evidence of the relationship between snow cover and the strength of the monsoon couldn’t find it, or when they did, their methods were quite questionable,” Fasullo said. “In 2004, my colleagues and I published the results of an analysis we did of satellite-derived snow cover in Eurasia since 1967. We discovered that while El Niño-La Niña cycles had the biggest influence on the strength of the Asian Monsoon, Eurasian snow cover did play a major role in the monsoon intensity in non-El Niño/ La Niña years.”

 

Differences in temperature between land and ocean drive the Asian monsoon. During the winter in South Asia (upper), warm air over the Arabian Sea rises, drawing cold air from the land to the north. In summer (lower), after much of the snow melts, the land warms more than the ocean, and the wind direction reverses. (Maps by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)

Graph of Change in Himlayan Snow Cover, 1972 through 2003

That paper established a general relationship between snow cover in Eurasia and the entire Asian monsoon from around 1970 to the present. Goés wanted something more specific. He wanted Fasullo to investigate whether snow cover changes in Eurasia since 1997 could be linked to the intensification of the Southwest Monsoon winds over the Arabian Sea. “We looked at changes in satellite observations of snow cover going back to 1967,” explained Fasullo, “and there is a very clear decline in snow cover in Eurasia since 1997—the rapid decline is certainly unique in the data record. We corroborated this decline by comparing the snow cover data with an independent data set—station observations of air temperature. The trend is verified from that data: air temperatures are going up considerably, and snow cover is going down.”

 

Across Eurasia, snow cover (red line) has dropped significantly since the late 1990s. Warming temperatures are most likely to blame. Lack of snow allows the land surface to heat up more in the summer, and the widening temperature contrast between land and ocean in the summer strengthened the monsoon. (Graph by Robert Simmon, based on data provided by the Bigelow Laboratory for Ocean Sciences)

 

Causes and Consequences

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Step by step, Goés and his team traced their way backward from what they thought was a mistake to a surprising discovery of how complexly connected Earth’s land surface, oceans, atmosphere and living creatures are—declining snow cover and a hotter Eurasian land mass; greater temperature difference between land and ocean; stronger monsoon winds across the Arabian Sea; greater upwelling of deep, nutrient-rich water; and bigger blooms of phytoplankton. The intricacy is amazing.

According to Goés, more than one link in that chain of climate cause-and-effect could make problems for people. On the one hand, increasing phytoplankton populations might help fisheries. But too much phytoplankton can have a down side, especially in the Arabian Sea. When phytoplankton die, bacteria decompose them, using up oxygen in the water. “The Arabian Sea already has a large zone of oxygen-poor water because it is landlocked to the north,” explains Goés. It doesn’t benefit from large-scale ocean circulation patterns that disperse oxygen-depleted water. In this already oxygen-poor environment, Goés suspects that such huge increases in phytoplankton could be catastrophic.

“Just recently [October 2005] we have received messages from our colleagues in Oman saying there are reports of massive fish deaths in the area. In many cases, these deaths have been preceded by mass strandings of fish and other marine animals closely following major blooms,” Goés said. One reasonable hypothesis, he says, is that the animals are tying to escape suffocation in the oxygen-depleted waters that result from such massive phytoplankton blooms and their resulting decay.

Oxygen-poor waters are a problem for a second reason: they create more habitat for a kind of bacteria that metabolizes nitrate in the water and produces nitrous oxide—laughing gas—in the process. Unfortunately, there is nothing funny about possible increases of nitrous oxide. Nitrous oxide is a powerful greenhouse gas; its heat-trapping potential is about 300 times greater than carbon dioxide. According to measurements published by scientists in the journal Nature in 2000, the amount of nitrous oxide coming from the western Indian Ocean has been increasing as the sea’s low-oxygen zone increases.

Goés is also concerned about the impact of an intensifying monsoon on regional rainfall. “This past summer,” said Goés, whose family is originally from the state of Goa in southwestern India, “the city of Mumbai, on India’s western coast, experienced its worst flood disaster on record.” At least a thousand people died, and much of the city was underwater for days. “We speculate that the runaway decline in Eurasian snow cover since 1997 may be strengthening Southwest monsoon rainfall in regions where the rains are strongly tied to the Arabian Sea winds, such as along the western coast of India,” said Goés. This could mean more intense rain and more frequent floods in Southwest Asia.

   
  Photograph of relief workers after Mumbai floods in 2005.
 

The most immediate threat of course, is the snow cover decline itself. “Many people in India, Pakistan, and China depend on snow melt for water, and it [snow] is in decline. If glaciers continue to melt at the present rate, many high-altitude lakes could start overflowing,” Goés explained. The melting of glaciers and warming of the ground can destabilize mountain slopes and trigger dangerous landslides. Eventually drought could prevail in regions where snow and glaciers are unable to persist.

 

Stronger summer monsoons may mean more rain for parts of India—perhaps much more rain. On July 26, 2005, the coastal city of Mumbai endured a record 24-hour rainfall of 94.4 centimeters (37.2 inches). In this photograph, people lift bottled water over a flooded street. (Photograph copyright Soumik Kar)

  Photograph of glaciers in the Karakorum
 

As a remote-sensing oceanographer, this study was particularly satisfying to Goés. He hopes it will inspire others to look for ways to use satellite observations to help predict how climate change like global warming will be felt at a regional level. “This study is a concrete example of what satellite data can do to help predict regional climate change. Without satellite data, we probably never would have been able to establish these relationships and trends,” said Goés. “But because we were able to cross-check so many different types of data, we can be much more certain these connections are real.”

 

Declining snow cover and retreating glaciers in the Himalaya may reduce fresh water supplies in India, Pakistan, and Tibet. Mountain glaciers will likely continue to retreat, and their loss could destabilize mountain slopes. This photograph shows summer melt ponds on the Vigne Glacier in the Concordia region of Pakistan. (Photograph copyright Kelly Cheng)