Study: Sea level changes may affect tropical rainfall patterns

How will global warming affect global atmospheric circulation and precipitation patterns? Photo courtesy NASA’s Blue Marble collection.

At lower sea levels, exposed land masses could affect convection

By Summit Voice

FRISCO — Even though scientists are continuously fine-tuning their global warming models, climate change is likely to dish up some big surprises in the decades ahead.

In one recent study, researchers with the University of Hawaii and Woods Hole Oceanographic Institution found evidence that past changes in sea level rise had a somewhat unexpected influence across the center of the Indo-Pacific warm pool — a vast region of warm ocean waters in the western Pacific region that is the main source of heat and moisture to Earth’s atmosphere.

By studying proxy records that show rainfall patterns, Pedro DiNezio and Jessica Tierney discovered that the area was much drier about 26,000 to 19,000 years ago during the ice age, while wetter conditions prevailed on either side of the warm pool, extending as far as East Africa.

The primary cause for these conditions during glacial times was lower sea levels, which exposed the now-submerged Sunda Shelf as dry land and connected what are now Indonesian islands into one large land mass that affected tropical circulation and convection patterns.

The  clues included charcoal from fires, and evidence of more sand dune activity and desiccated lakes, all indicating drier conditions, and evidence for higher lake levels and more pollen, indicating wetter conditions. The researchers also looked at records of seafloor sediments containing preserved shells of dead marine organisms; the shells contain higher or lower levels of a heavier isotope of oxygen, depending on the relative salinity of surface waters when the organisms were alive (less salty waters indicate more rainfall over the ocean).

They then compared this evidence with results from 12 different mathematical climate models that simulate Earth’s climate, which incorporate basic laws of physics, chemistry, and fluid dynamics surrounding air-sea-land-ice interactions.The idea is that the ice age provides a great test “to evaluate numerical models’ ability to simulate climates radically different from the present one,” the scientists said.

Only one model, developed by the Hadley Centre for Climate Prediction and Research in the England, reproduced the rainfall patterns they found from the geological evidence: a pattern of strong, widespread dry conditions over Indonesia, Southeast Asia and northern Australia, wetter conditions in eastern Africa, saltier waters (less rainfall) in the eastern Indian Ocean and Bay of Bengal and less salty waters (more rainfall) in the Arabian Sea and the western Pacific.

Climate scientists think that the main weakness of the models is their limited ability to simulate convection, the vertical air motions that lift humid air into the atmosphere. Differences in the way each model simulates convection may explain why model results for the glacial period are so different and don’t match the proxy evidence.

“The good news is, the Hadley model combined with the geological evidence show a pathway to improve our ability to simulate and predict tropical rainfall in the future,” Tierney said. “The more we study the mechanisms that governed tropical climate in the past, the better we can predict the climate changes that will affect the billions of people that live in this vast region of the world.”


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