HR: 16:25h
AN: GC54B-03    [Abstracts]
TI: Abrupt changes of ENSO variability due to orbital and millennial-scale climate change
AU: * Timmermann, A
EM: axel@hawaii.edu
AF: Axel Timmermann IPRC, SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, United States
AU: Lorenz, S
EM: stephan.lorenz@zmaw.de
AF: Stephan Lorenz Max Planck Institute of Meteorology, Bundesstr. 55, Hamburg, 20146, Germany
AU: Xie, S P
EM: xie@hawaii.edu
AF: Shang-Ping Xie IPRC, SOEST, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822, United States
AB: Paleo evidence from fossil corals and lake records suggests that ENSO is modulated on orbital timescales. The famous mid-Holocene ENSO suppression e.g. has been attributed to orbitally-induced background state and annual cycle changes. Using an accelerated orbitally-driven CGCM simulation representing the period from 142,000 years B.P (before present) to 22,900 years A.P. (after present), the fundamental mechanisms are explored that lead to the generation of precessional cycles in the tropics. Due to the mean seasonal cycle of cloudiness in the off- equatorial regions, an annual mean precessional signal of temperatures is generated outside the equator. The resulting meridional SST gradient in the eastern equatorial Pacific modulates the annual mean meridional asymmetry and hence the strength of the equatorial annual cycle. In turn, changes of the equatorial annual cycle trigger abrupt changes of ENSO variability via frequency entrainment, resulting in an anti-correlation between annual cycle strength and ENSO amplitude on precessional timescales. We furthermore demonstrate that a similar mechanism operates on millennial timescales. Changes of the Atlantic Meridional Overturning Circulation (AMOC), lead to changes of the meridional SST gradient in the eastern tropical Pacific, and hence a modulation of the strength of the annual cycle and via nonlinear frequency entrainment also of ENSO. Analysis of a multimodel-ensemble of waterhosing experiments, recently conducted as part of CMIP-2, reveals that a collapse of the AMOC leads to the disappearance of the equatorial Pacific annual cycle and an intensification of ENSO variability. Both, on orbital and millennial timescales, meridional SST gradients play a crucial role in modulating ENSO variability. Whether this new paradigm is also applicable to future greenhouse warming experiments is still an open question.
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 1620 Climate dynamics (0429, 3309)
DE: 4522 ENSO (4922)
DE: 4962 Thermohaline
SC: Global Environmental Change [GC]
MN: 2007 Fall Meeting