Sea-level rise is particularly high along the coastlines of the Bay of Bengal and the Arabian Sea, as well as the islands of 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 Indo-Pacific warm pool. Credit: NASA. Click to enlarge.

The key player in the process is the Indo-Pacific warm pool, an enormous, bathtub-shaped area spanning a huge area of the tropical oceans stretching from the east coast of Africa east 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 in 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.

—Weiqing Han, CU, lead author

While several areas in the Indian Ocean region are experiencing sea-level rise, sea level is lowering in other areas. The study indicated that the Seychelles Islands and Zanzibar off Tanzania’s coastline show the largest sea level drop.

Global sea-level patterns are not geographically uniform. Sea-level rise in some areas correlates with sea-level fall in other areas.

—NCAR scientist Gerald Meehl, co-author

Funding for the research came from the National Science Foundation (NSF), NCAR’s sponsor, as well as the Department of Energy and NASA.

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 form a distinct surface wind pattern that drives specific sea-level patterns, according to Han.

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.

—Han et al.

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

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 the US Department of Energy (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 down to the surface in Indian Ocean subtropical regions than normal, 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.

—Weiqing Han

Resources

• Weiqing Han et al. (2010) Patterns of Indian Ocean sea-level change in a warming climate. Nature Geoscience doi: 10.1038/ngeo901