Coral windows onto seasonal climate variability in the northern Caribbean since 1479

Mean surface ocean conditions in the Caribbean were up to ∼2°C cooler than today at times during the Little Ice Age. The seasonal context for such mean state changes is important for determining the mechanisms involved. We reconstructed surface ocean conditions in southwest Puerto Rico at approximately monthly resolution over eight 4–12 year periods during the last ∼520 years to test if the seasonal cycles of temperature or salinity varied with mean state. We carried out paired analyses of Sr/Ca and δ18O for two coral cores. The δ18O data contained clear annual cycles and were significantly correlated to temperature during the 20th century calibration periods (1993–2004 and 1902–1912, r = 0.73). The Sr/Ca data contained high-frequency noise that obscured the seasonal cycles, although the centennial variability matched that of the coral δ18O, indicating a common forcing that is likely temperature. The seasonal coral δ18O amplitude averaged 0.60 ± 0.17‰, with none of the periods significantly different from the most recent. The simplest explanation is that the amplitudes of seasonal seawater δ18O and temperature variations were not different from today. Previous work in the southern Caribbean indicates that the Intertropical Convergence Zone was shifted southward or weaker during the Little Ice Age, and we speculate about how this could occur with no apparent affect on seasonality in the northern Caribbean.