Study tracks El Niño shifts
Climate change will likely subject many low-lying Pacific island nations to more extreme fluctuations in sea level from year to year, in synch with more intense El Niño cycles. Some years, high sea level will lead to bigger floods, while in other years, big drops in sea level will leave coral reefs exposed, according to researchers based in Hawaii and Australia.
“Our results are consistent with previous findings that showed the atmospheric effects of both El Niño and La Niña are likely to become stronger and more common in a future warmer climate,” said Wenju Cai, an El Niño expert with CSIRO in Australia.
“The possibility of more frequent flooding in some areas and sea level drops in others would have severe consequences for the vulnerable coastlines of Pacific islands,” said Matthew Widlansky, with the International Pacific Research Center, University of Hawai’i at Mānoa.
With modeling, the scientists projected behavioral changes of the El Niño phenomenon and its characteristic Pacific wind response. During El Niño, warm water and high sea levels shift eastward, leaving in their wake low sea levels in the western Pacific. Scientists have already shown that this east-west seesaw is often followed six months to a year later by a similar north-south sea level seesaw with water levels dropping by up to one foot in the Southern Hemisphere.
Such sea level drops expose shallow marine ecosystems in South Pacific Islands, causing massive coral die-offs with a foul smelling tide called taimasa (pronounced [kai’ ma’sa]) by Samoans.
The modeling showed that, by the end of this century, intensified wind impacts of strong El Niño and La Niña events are likely to double the frequency of extreme sea level occurrences, especially in the tropical southwestern Pacific.
“From our previous work, we know that toward the end of a very strong El Niño event, the tide-gauge measurements around Guam quickly return to normal reflecting the east-west seesaw, but those near Samoa continue to drop as a result of the lagging north-south seesaw,” said Widlansky. “During these strong events, the summer rainband over Samoa, called the South Pacific Convergence Zone, shifts toward the equator and alters the trade winds and ocean currents which in turn change the sea level.”
“The next logical step in our work was to understand how future changes in winds, projected by most climate models, will impact the interannual swings in sea level,” said Axel Timmermann, with the International Pacific Research Center, University of Hawai’i at Mānoa. “We noted a trend in greater variability and were surprised at first to find not only more frequent and prolonged drops in sea level, but also more frequent high sea level events. This will further increase the risk of coastal inundations.”
The authors hope that better predictability of not only rising sea levels, but also the sea level fluctuations examined in this study, will aid Pacific Island communities in adapting to the impacts of climate change as well as shorter-term climate events such as the ongoing 2015 El Niño.