HR: 13:40h
AN: PP53A-01    [Abstracts]
TI: ENSO response to radiative forcing over the Holocene: a model perspective
AU: * Emile-Geay, J B
EM: julieneg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-8000 United States
AU: Cane, M A
EM: mcane@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-8000 United States
AU: Mann, M E
EM: mann@virginia.edu
AF: Department of Environmental Science, University of Virginia, Clark Hall, Charlottesville, VA 22903 United States
AU: Seager, R
EM: seager@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964-8000 United States
AB: Using a model of intermediate complexity, we simulate the response of the El Niño-Southerm Oscillation system (ENSO) over the Holocene. Solar forcing is reconstructed from radiocarbon production rate data (Bond et al, 2001), using various scaling factors to account for the conflicting estimates of solar irradiance variability since the Maunder Minimum (1645 to 1715 AD). As reviewed in Frölich and Lean (2004), these estimates of the lowpass-filtered difference range from 0.06% to 0.5% of the current mean solar irradiance of S_o=1367 \unittwo{W}{}{m}{-2}. We therefore consider these two extreme cases, along with the intermediate - widely used - case of 0.2% × S_o. We show that for a large forcing (the 0.5% case), the smoothed east-west SST gradient along the equator responds remarkably linearly to irradiance forcing, with a phase lag less than a year. For the intermediate case of 0.2% × S_o, the response is weaker, but still noticeable. On the contrary, the 0.05%× S_o case shows no significant variability above that inherent to the model's chaotic behavior. Wavelet analysis suggest a statistically-significant enhancement of the centennial to millenial-scale ENSO variability for moderate-to-strong irradiance forcing, but not in the weak forcing case. Orbitally-driven insolation forcing is found to add almost linearly to solar irradiance. However, the latter has a disproportionately large influence on tropical SSTs in view of the large peak-to-peak orbital forcing anomalies, (1 vs 50\unittwo{W}{}{m}{-2}). We attribute this to the near-cancellation of orbital anomalies over a given year, during times where eccentricity is close to today (e.g. the Holocene). In contrast, irradiance anomalies typically persist for several decades, thereby strongly influencing the ENSO statistics in spite of their weak amplitude. Given the central importance of ENSO in the climate system, the results suggest a mechanism whereby long-term solar irradiance variability can act as a driver of natural climate change. This mechanism does not rely on planetary wave radiation from the stratosphere to the troposphere.
DE: 1620 Climate dynamics (0429, 3309)
DE: 4215 Climate and interannual variability (1616, 1635, 3305, 3309, 4513)
DE: 4922 El Nino (4522)
DE: 7538 Solar irradiance
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005