Oceanography Seminar

January 23, 3:00pm - 4:15pm
Mānoa Campus, MSB 100

Kim Cobb
Associate Professor
Earth and Atmospheric Sciences Georgia Tech

“Internal versus forced variations in paleo-ENSO: implications for 21st century ENSO”

Abstract: Projections of future ENSO activity under continued greenhouse forcing are highly uncertain (e.g. Collins et al., 2010). In part, the relatively short observational record of ENSO makes it hard to detect trends in the presence of appreciable natural decadal-scale variability in ENSO properties. As such, there is no way to discriminate among the widely divergent projections of 21st century ENSO simulated by coupled climate models. In this study, we present a reconstruction of ENSO variability over the last 7,000 years in the form of decades-long fossil coral oxygen isotopic records from the central equatorial Pacific. We use a suite of 20th century coral records collected from these sites to calibrate our ENSO proxy records against 20th century instrumental ENSO records. We find that the range of internal variability implied by the new reconstruction is much larger than that observed over the 20th century, in line with previous modeling (Wittenberg, 2009) and paleoclimate (Cobb et al., 2003) studies. In fact, our data imply that the oft-cited reduction in mid-Holocene ENSO variance (e.g. Clement et al., 2000; Tudhope et al., 2001) was much smaller than resolved by models (e.g. Clement et al., 2000; Otto-Bliesner et al., 2003; Timmermann et al., 2007) or paleoclimate data from the western (Tudhope et al., 2011; McGregor, H. et al., 2013) or eastern (Koutavas et al., 2006, Koutavas and Joanides 2012) tropical Pacific, if it occurred at all. Indeed, the only statistically robust change in ENSO variance occurs during the 20th century, which contains higher ENSO variance than the preceding centuries' worth of data (Cobb et al., 2013). Such results lend support to similar conclusions reached in a recent synthesis of ENSO reconstructions (McGregor, S. et al., 2013) - that late 20th century ENSO variance is anomalously high with respect to the last six centuries. Taken together, such studies provide empirical evidence for a strengthening of ENSO in response to anthropogenic greenhouse forcing. If confirmed by additional paleo-ENSO studies, this observation provides a key constraint for climate model simulations of past and future ENSO.

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Oceanography, Mānoa Campus

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