COG3

Post-doctoral Fellow, Singapore-MIT Alliance for Research and Technology.

Title: "Coral records of central tropical Pacific sea-surface temperature and salinity variability over the 20th century."

Abstract: Accurate forecasts of future regional temperature and rainfall patterns in many regions largely depend on characterizing anthropogenic trends in tropical Pacific climate. However, strong interannual to decadal-scale tropical Pacific climate variability, combined with sparse spatial and temporal coverage of instrumental climate datasets in this region, have obscured potential anthropogenic climate signals in the tropical Pacific. Existing coral oxygen isotopic (delta18O) records from the central tropical Pacific exhibit late 20th century trends towards depleted coral delta18O values suggesting warming and/or freshening conditions that may be unprecedented over the last millennium. Here, I present sea-surface temperature (SST) and salinity proxy records that span over the 20th century using modern corals from the central tropical Pacific via coral Sr/Ca and delta18O of seawater (delta18Osw), respectively. First, I investigate the spatial and temporal character of the late 20th century SST and salinity trends in the central tropical Pacific, as revealed by corals from Palmyra (6ºN, 162ºW), Fanning (4ºN, 159ºW) and Christmas (2ºN, 157ºW) Islands. The late 20th century SST proxy records exhibit equatorially-enhanced warming trends, in line with GCM responses to increased atmospheric CO2. Freshening trends revealed by the salinity proxy records are larger at those sites most affected by the Inter-Tropical Convergence Zone (ITCZ), suggesting a strengthening and/or an equatorward shift of the ITCZ, also consistent with GCM trends under anthropogenic forcing. Next, I characterize the signatures of natural and anthropogenic variability in central tropical Pacific SST and salinity over the course of the 20th century using century-long coral proxy records from Palmyra. On interannual timescales, the SST proxy record is sensitive to both eastern and central tropical Pacific flavors of El Niño-Southern Oscillation (ENSO). The salinity proxy record tracks eastern Pacific ENSO events but is poorly correlated to central Pacific ENSO – the result of profound differences in precipitation and ocean advection that occur during the two types of ENSO. On decadal timescales, the coral SST proxy record is significantly correlated to the North Pacific Gyre Oscillation (NPGO), the second principal component (PC) of SST variability in the North Pacific whose spatial pattern resembles central Pacific mode, suggesting that strong dynamical links exist between the central tropical Pacific and the North Pacific. The salinity proxy record is significantly correlated to the Pacific Decadal Oscillation (PDO), the first PC of North Pacific SST whose spatial pattern resembles the eastern Pacific mode, but poorly correlated to the NPGO, suggesting that different mechanisms control central tropical Pacific SST and salinity variability on decadal timescales. However, the most striking feature of the salinity proxy record is a prominent late 20th century freshening trend that is likely related to anthropogenic climate change. Taken together, the proxy records reveal that SST and salinity variations are largely decoupled on decadal and secular timescales, and that anthropogenic hydrological trends may be more detectable than SST trends in the tropical Pacific.