Sea Level Rise in Indian Ocean

Indian Ocean sea level rise threatens millions -study

Wed Jul 14, 2010 7:02am GMT

  • Parts of Indian Ocean could rise more than average -study
  • Changes in wind patterns could also affect monsoons
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By David Fogarty, Climate Change Correspondent, Asia

SINGAPORE, July 14 (Reuters) - Sea levels are rising unevenly in the Indian Ocean, placing millions at risk along low-lying coastlines in Bangladesh, Indonesia and Sri Lanka, scientists say in a study.

Researchers from the University of Colorado and the National Center for Atmospheric Research say the rising sea levels are caused in part by climate change and are triggered by warming seas and changes to atmospheric circulation patterns.

In his acceptance speech for the Nobel Peace Prize last year, U.S. President Barack Obama warned that if the world does nothing to confront climate change, "we will face more drought, famine and mass displacement that will fuel more conflict for decades".

The authors of the latest study say higher seas could exacerbate monsoon flooding, placing crops, homes and livelihoods at greater risk. They argue a better understanding of the changes are needed to improve risk assessment planning for the future.

Sea levels in general are rising globally by about 3 mm (0.1181 inch) a year. Scientists blame rising temperatures caused by the growing amounts of greenhouse gases, such as carbon dioxide from burning fossil fuels, that trap heat in the atmosphere.

Oceans are absorbing a large part of this extra heat, causing them to expand and sea levels to rise. Warmer temperatures are also causing glaciers and parts of the ice blanketing Greenland and West Antarctica to melt.

The team of researchers in their study used long-term tide gauge data, satellite observations and computer climate models to build a picture of sea level rises in the Indian Ocean since the 1960s.

They found that sea-level rise is particularly high along the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra and Java and that these areas could suffer rises greater than the global average.

But they also found that sea levels are falling in other areas. The study indicated that the Seychelles Islands and Zanzibar off Tanzania's coast show the largest sea-level drop.

WARM POOL

"Global sea level patterns are not geographically uniform," said co-author Gerald Meehl of NCAR in Boulder, Colorado.

The study is published in the latest issue of the journal Nature Geoscience.

A key player in the process is the Indo-Pacific warm pool, a large oval-shaped area spanning the tropical oceans from the east coast of Africa to the International Date Line in the Pacific.

The pool has warmed by about 0.5 degrees Celsius (1 degree Fahrenheit) over the past 50 years, primarily because of mankind's greenhouse gas emissions. The warmer water has strengthened two major atmospheric circulation patterns that have a major impact on sea levels.

"Our new results show that human-caused atmosphericoceanic circulation changes over the Indian Ocean, which have not been studied previously,contribute to the regional variability of sea-level change," the researchers say in the study.

The two main wind patterns in the region are the Hadley and Walker circulations.

In the Hadley circulation, air currents rise above strongly heated tropical waters near the equator and flow poleward at upper levels, then sink to the ocean in the subtropics and cause surface air to flow back toward the equator.

The Walker circulation causes air to rise and flow westward at upper levels, sink to the surface and then flow eastward back toward the Indo-Pacific warm pool.

Strengthening of these two patterns could have far-reaching impacts on AsianAustralian monsoons, Indonesian floods and drought in Africa, the study says. (Editing by Jeremy Laurence)

Indian Ocean sea level rise threatens coastal areas

July 13, 2010
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BOULDER—Indian Ocean sea levels are rising unevenly and threatening residents in some densely populated coastal areas and islands, a new study concludes. The study, led by scientists at the University of Colorado at Boulder (CU) and the National Center for Atmospheric Research (NCAR), finds that the sea level rise is at least partly a result of climate change.

Sea level rise is particularly high along the coastlines of the Bay of Bengal, the Arabian Sea, Sri Lanka, Sumatra, and Java, the authors found. The rise—which may aggravate monsoon flooding in Bangladesh and India—could have future impacts on both regional and global climate
The key player in the process is the Indo-Pacific warm pool, an enormous, bathtub-shaped area spanning a region of the tropical oceans from the east coast of Africa to the International Date Line in the Pacific. The warm pool has heated by about 1 degree Fahrenheit, or 0.5 degrees Celsius, in the past 50 years, primarily because of human-generated emissions of greenhouses gases.

"Our results from this study imply that if future anthropogenic warming effects in the Indo-Pacific warm pool dominate natural variability, mid-ocean islands such as the Mascarenhas Archipelago, coasts of Indonesia, Sumatra, and the north Indian Ocean may experience significantly more sea level rise than the global average," says lead author Weiqing Han of CU's atmospheric and oceanic sciences department.

While a number of areas in the Indian Ocean region are experiencing sea level rise, sea level is lowering in other areas. The study indicates that the Seychelles Islands and the island of Zanzibar off Tanzania's coast show the largest sea level drop.

"Global sea level patterns are not geographically uniform," says NCAR scientist Gerald Meehl, a co-author. "Sea level rise in some areas correlates with sea level fall in other areas."

The new study was published this week in Nature Geoscience. Funding came from the National Science Foundation, NCAR's sponsor, as well as the Department of Energy (DOE) and NASA.

Wind and sea level
The patterns of sea level change are driven by the combined enhancement of two primary atmospheric wind patterns, known as the Hadley circulation and the Walker circulation. The Hadley circulation in the Indian Ocean is dominated by air currents rising above strongly heated tropical waters near the equator and flowing poleward at upper levels, then sinking to the ocean in the subtropics and causing surface air to flow back toward the equator.

The Indian Ocean's Walker circulation causes air to rise and flow westward at upper levels, sink to the surface and then flow eastward back toward the Indo-Pacific warm pool.

"The combined enhancement of the Hadley and Walker circulation forms a distinct surface wind pattern that drives specific sea level patterns," Han says.

In the Nature Geoscience article, the authors write, "Our new results show that human-caused changes of atmospheric and oceanic circulation over the Indian Ocean region—which have not been studied previously—are the major cause for the regional variability of sea level change."

The new study indicates that in order to anticipate global sea level change, researchers also need to know the specifics of regional sea level changes.

"It is important for us to understand the regional changes of the sea level, which will have effects on coastal and island regions," says NCAR scientist Aixue Hu.

The research team used several sophisticated ocean and climate models for the study, including the Parallel Ocean Program—the ocean component of the widely used Community Climate System Model, which is supported by NCAR and DOE. In addition, the team used a wind-driven linear ocean model for the study.

The complex circulation patterns in the Indian Ocean may also affect precipitation by forcing even more atmospheric air than normal down to the surface in Indian Ocean subtropical regions, Han speculates.

"This may favor a weakening of atmospheric convection in subtropics, which may increase rainfall in the eastern tropical regions of the Indian Ocean and drought in the western equatorial Indian Ocean region, including east Africa," Han says.

About the article

Title:
Indian Ocean Sea Level Change in a Warming Climate

Authors:
Weiqing Han, Gerald Meehl, Balaji Rajagopalan, John Fasullo, Aixue Hu, Jialin Lin, William Large, Jih-wang Wang, Xiao-Wei Quan, Laurie Trenary, Alan Wallcraft, Toshiaki Shinoda, and Stephen Yeager

Publication:
Nature Geoscience, July 11, 2010