A Century of Observed Temperature Change in the Indian Ocean

The Indian Ocean is warming rapidly, with widespread effects on regional weather and global climate. Sea‐surface temperature records indicate this warming trend extends back to the beginning of the 20th century, however the lack of a similarly long instrumental record of interior ocean temperatures leaves uncertainty around the subsurface trends. Here we utilize unique temperature observations from three historical German oceanographic expeditions of the late 19th and early 20th centuries: SMS Gazelle (1874–1876), Valdivia (1898–1899), and SMS Planet (1906–1907). These observations reveal a mean 20th century ocean warming that extends over the upper 750 m, and a spatial pattern of subsurface warming and cooling consistent with a 1°–2° southward shift of the southern subtropical gyre. These interior changes occurred largely over the last half of the 20th century, providing observational evidence for the acceleration of a multidecadal trend in subsurface Indian Ocean temperature. Plain Language Summary The Indian Ocean is warming rapidly, with far reaching effects on weather and climate. Sea‐surface temperature records suggest this warming trend extends over the 20th century, however, similar long records of subsurface temperatures have not been available. Here we extend the observational record back more than a century using data from 3 historical oceanographic expeditions. These observations reveal a mean 20th century Indian Ocean warming that extends down to 750 m depth, as well as deep cooling in the subtropics. This provides evidence for the existence of a multidecadal trend in subsurface Indian Ocean temperatures that has accelerated over the last half of the 20th century. Key Points Historical observations of subsurface Indian Ocean temperature are recovered from expeditions in the late 19th and early 20th century Indian Ocean warming over the 20th century extends to 750 m depth Pattern of temperature change is consistent with surface warming and a poleward shift of the gyre over the last half of the 20th century