Mid to late 20th century freshening of the western tropical South Atlantic triggered by southward migration of the Intertropical Convergence Zone

In the tropical Atlantic Ocean, the Intertropical Convergence Zone (ITCZ) is an important climate feature controlled by the interhemispheric sea surface temperature (SST) gradient, and greatly influences rainfall patterns over the adjacent continents. To better understand ITCZ dynamics in the context of past and future climate change, long-term oceanic records are needed, but observational data are limited in temporal extent. Shallow water corals provide seasonally-resolved archives of climate variability over the tropical ocean. Here we present seasonally-resolved records of stable oxygen (delta O-18) and carbon (delta C-13) isotope values of a Siderastrea stellata coral from northeastern Brazil (Maracajau, similar to 5 degrees S). We show that the long-term trends in the record of coral delta O-18 values are not primarily driven by SST but by hydrological changes at the sea surface. Combining the record of coral delta O-18 values with instrumental SST, we present the first reconstruction of seawater delta O-18 changes (delta O-18(seawater)) in the western tropical South Atlantic back to the early 20th century, a parameter that is related to changes in sea surface salinity. The reconstructed delta O-18(seawater) changes indicate a prominent freshening between the mid-1940's and mid-1970's, which coincides with a weakening of the Atlantic interhemispheric SST gradient during this time interval. Our results suggest that the weakened Atlantic SST gradient resulted in a southward shift of the thermal equator that was accompanied by a southward migration of the ITCZ, resulting in freshening of the western tropical South Atlantic during the mid to late 20th century.

Automated summary

The study focused on the isotopic records of a Siderastrea stellata coral from northeastern Brazil, showing that long-term trends are primarily driven by hydrological changes at the sea surface. It also presented the first reconstruction of seawater delta O-18 changes in the western tropical South Atlantic, revealing a prominent freshening trend during the mid to late 20th century.