Near Earth’s equator, southern winds meet northern winds in a rainy ring around the globe known as the Intertropical Convergence Zone (ITCZ). The ITCZ follows the Sun, migrating slightly north or south depending on the season. New research by Brocas et al. examines clues from ancient corals to reveal how seasonal ITCZ shifts under warmer climate conditions brought heavier summer rainfall to the southern Caribbean Sea.
Previous research has explored the effects of ITCZ migration on long timescales, including associated changes in the Atlantic Meridional Overturning Circulation, a system of currents that contributes to global circulation of ocean waters. However, it is unclear how global warming might affect seasonal ITCZ migrations, as well as the resulting regional weather effects, such as drought or heavy precipitation.
To explore the potential effects of climate change, the researchers turned to the last interglacial period (LIG), when global temperatures were warmer than they are today. Fossilized corals from that period hold records of seawater temperature and salinity in the form of distinct ratios between different elements and isotopes. These records allow scientists to estimate the amount of freshwater in the ocean at a given time, which corresponds to rainfall.
To reconstruct month-to-month rainfall rates during the LIG, the researchers analyzed oxygen isotope ratios in fossil corals from the southern Caribbean island of Bonaire. They compared these results with seasonal rainfall patterns during modern, middle to late Holocene, and late LIG climates, which had previously been reconstructed using coral records from the same island.
The analysis revealed that oxygen isotope ratios varied more strongly between seasons during the mid-LIG (124,000 to 126,000 years ago) than today, reflecting greater seasonal variability in rainfall amounts. This increased rainfall seasonality accompanied increased seasonal variation in sea surface temperature that is associated with seasonal differences in the amount of sunlight that reaches the Caribbean.
The researchers also performed climate model simulations for the mid-LIG, which confirmed that increased summer rainfall in the southern Caribbean could be responsible for the summertime changes in oxygen isotope ratios seen in the fossil corals. If that’s the case, these results would suggest a different regional climate from today’s, which is semiarid and features a minor rainy season in wintertime.
Overall, these findings suggest that the ITCZ expanded northward into the southern Caribbean Sea during the mid-LIG. According to the authors, these results highlight the need to consider regional scales when examining ancient rainfall and other climate patterns. (Paleoceanography and Paleoclimatology, https://doi.org/10.1002/2017PA003216, 2018)
—Sarah Stanley, Freelance Writer