Statistical constraints on El Niño Southern Oscillation reconstructions using individual foraminifera: A sensitivity analysis

Abstract

[1] Recent investigations of submillennial paleoceanographic variability have attempted to resolve high-frequency climate signals such as the El Niño Southern Oscillation (ENSO) using the population statistics of individual planktic foraminiferal δ 18 O analyses. This approach is complicated by the relatively short lifespan of individual foraminifers (~2-4 weeks) compared to the time represented by a typical marine sediment sample (~decades to millennia). Here, we investigate the uncertainty associated with individual foraminiferal analyses (IFA) through simulations on forward modeled δ 18 O carbonate . First, focusing on the Niño3.4 region of the tropical Pacific Ocean, a bootstrap Monte Carlo algorithm is developed to constrain the uncertainty on IFA-statistics. Subsequently, to test the sensitivity of IFA to changes in seasonal cycle amplitude, ENSO amplitude, and ENSO frequency, synthetic time series of δ 18 O carbonate with differing variability are constructed and tested with our algorithm. The probabilities of the IFA technique in detecting changes in ENSO amplitude and seasonal cycle amplitude (or a combination of both) for the surface ocean and thermocline at different locations in the tropical Pacific are quantified. We find that the uncertainty in the standard deviation is smaller than the range, that the IFA-signal is insensitive to ENSO frequency, and at certain locations the seasonal cycle may dominate ENSO. IFA sensitivity towards ENSO is highest at the central equatorial Pacific surface ocean and the eastern equatorial Pacific (EEP) thermocline whereas sensitivity towards the seasonal cycle is highest at the EEP surface ocean. Our results suggest that rigorous uncertainty quantification should become standard practice for accurately interpreting IFA-data.

El Niño evolution during the Holocene revealed by a biomarker rain gauge in the Galápagos Islands

“…First, the oscillatory nature of ENSO between El Niño and La Niña conditions (Clement et al, 1999) can dampen the signal of inter-annual climate extremes in slowly accumulating sedimentary records since the two opposing modes tend to have counteracting effects on sea surface temperature (SST) reconstructions that average over decades to millennia. Second, it can be difficult to attribute a sedimentary signal unambiguously to one climate phenomenon or the other in locations influenced by both ENSO and seasonal migrations of the Intertropical Convergence Zone (ITCZ) (Braconnot et al, 2012;Koutavas and Joanides, 2012;Leduc et al, 2009aLeduc et al, , 2009bThirumalai et al, 2013). Corals have sub-annual resolution adequate to reconstruct ENSO, but as they are short lived the most comprehensive composite coral record to date is too under-sampled to document the complete Holocene time period .…”

A Review of Paleo El Niño-Southern Oscillation

“…However, this representation is likely to depend on location, time period and the inferred variable. In fact, statistical and modeling studies suggest that changes in the total variance during the LGM can be very different from changes at the interannual time scale, especially over the eastern equatorial Pacific [56,125] where the annual cycle is prominent [126].…”

“…First, as discussed above, information from reconstructions using individual foraminifera analysis is inconclusive and these reconstructions are usually subject to complicated uncertainties. For example, changes in total variance from individual foraminifera analysis could reflect changes in the annual cycle [56,125], vertical migration of the foraminifera [56], variations in the isotope composition of seawater [54], intershell variability and changes in seasonality of sedimentation rate [130]. Second, all climate models have to employ lots of parameterizations and they all have systematic biases in simulating the mean climate in the tropics and the extratropics, for example, the double ITCZ problem and the bias in simulating stratiform clouds in the subtropics [99].…”

Paleo Agulhas rings enter the subtropical gyre during the penultimate deglaciation

“…Recently, paleoceanographic research has started to employ the oxygen isotope composition (δ 18 O) of individual foraminifera, to address questions related to variability. For instance, Billups and Spero (1995) and Ganssen et al (2011) used single-shell analyses to unravel the range of hydrographic conditions in the equatorial Atlantic and in the Arabian Sea, respectively, while Koutavas et al (2006) and Leduc et al (2009) paleoceanographic variability originating from ENSO, were recently provided by Thirumalai et al (2013).…”