“ENSO-Modoki” on the rise under global warming conditions? A view from the past

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Recent observational studies have shown that aside of the “classical” El Niño Southern Oscillation (ENSO), which is characterized by anomalous sea surface temperature (SST) warming and thermocline deepening either in the Eastern Equatorial Pacific (EEP) or Western Equatorial Pacific (WEP), a third type of ENSO has become more prominent. During these events SST anomalies are located in the Central Equatorial Pacific (CEP) leading to alternate ENSO states coined “El Niño/La Niña Modoki” (or “Central Pacific ENSO”). During El Niño Modoki events, warming and thermocline deepening occurs in the CEP, while the cooling is centered in the CEP and WEP (vice versa for La Niña Modoki events). This leads to twin Walker circulation cells with their common updraft branch centered over the CEP causing anomalously arid conditions in both tropical South America and the western Pacific realm. Related global consequences include a higher frequency of hurricanes hitting the Gulf of Mexico as well increased warming in the subpolar realms of both hemispheres potentially accelerating high latitude ice sheet demise.To shed light onto the potential further evolution of ENSO Modoki, the proposed project aims to provide insights from a paleo-perspective. This will be done via assessing the occurrence of ENSO Modoki-like climatic states during varying climatic background states regarding (i) pCO2, (ii) global ice volume and (iii) insolation, factors with a well-documented impact on classical ENSO as well as ENSO Modoki dynamics. For this purpose, we will generate high-resolution proxy records of sea surface temperature (SST) and subsurface temperature (subT) variability on ODP/IODP Sites from the WEP (Site U1488), CEP (Site 871), and EEP (Site 846) covering 300 kyr-long time slices with distinctly different climatic background conditions: (i) the mid-Pleistocene to Holocene (0–300 ka) with relatively low pCO2 concentrations and high Northern Hemisphere ice volume, and (ii) the Early Pliocene (4.85–5.15 Ma) with pCO2 conditions similar to modern and largely absent Northern Hemisphere ice sheets.These SST and subT records will be based on Mg/Ca analysis of surface and deep dwelling planktic foraminifera as well as alkenone-based SST estimates (depending upon the method used on each site during previous studies to ensure consistency). The SST records will be used to compute an index of the past El Nino Modoki variability while the subT data will inform about associated thermocline variability. In the context with published reference records for pCO2, ice volume, and insolation our proxy data will be evaluated utilizing statistical methods (linear models). The results of this project will provide new insights into the drivers of El Nino Modoki variability and aid the predictions of its variability in response to future global warming.