Mechanisms Reducing ENSO Amplitude and Asymmetry via an Enhanced Seasonal Cycle in the Mid-Holocene

Tomoki Iwakiri Atmosphere and Ocean Research Institute, The University of Tokyo, Tokyo, Japan

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Masahiro Watanabe Atmosphere and Ocean Research Institute, The University of Tokyo, Tokyo, Japan

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Abstract

Paleo proxy records have suggested that El Niño–Southern Oscillation (ENSO) variability during the mid-Holocene [8200 to 4200 years ago (8.2–4.2 ka)] was weaker than during the instrumental periods, but the mechanisms remain unclear. We examined processes of ENSO suppression using a coupled general circulation model (CGCM) that simulates ENSO amplitude and skewness under the present climate reasonably well. Two long simulations were performed: one using the preindustrial condition (CTRL) and the other using the 8-ka insolation having a greater seasonal cycle (MH8K). Consistent with proxy records and previous modeling studies, the ENSO amplitude weakened by 20% in MH8K compared to CTRL, mainly because of reduced thermocline feedback during the mature and decay phases. The weak thermocline feedback, likely a result of the loose equatorial thermocline in the mid-Holocene, suppresses the occurrence of extreme El Niño events and consequently explains the reduction in both ENSO amplitude and asymmetry. In MH8K, strengthened trade winds over the western-central Pacific Ocean act to cool the surface via evaporation while warmer water in the southern subtropical Pacific is transported beneath the equatorial thermocline, both contributing to diffuse the thermocline. Multimodel simulations for the mid-Holocene showed mean state changes and ENSO weakening similar to MH8K, but most models did not show reduced ENSO skewness, probably because of the failure in reproducing extreme El Niño events under the present climate.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: T. Iwakiri, iwakiri@aori.u-tokyo.ac.jp

Abstract

Paleo proxy records have suggested that El Niño–Southern Oscillation (ENSO) variability during the mid-Holocene [8200 to 4200 years ago (8.2–4.2 ka)] was weaker than during the instrumental periods, but the mechanisms remain unclear. We examined processes of ENSO suppression using a coupled general circulation model (CGCM) that simulates ENSO amplitude and skewness under the present climate reasonably well. Two long simulations were performed: one using the preindustrial condition (CTRL) and the other using the 8-ka insolation having a greater seasonal cycle (MH8K). Consistent with proxy records and previous modeling studies, the ENSO amplitude weakened by 20% in MH8K compared to CTRL, mainly because of reduced thermocline feedback during the mature and decay phases. The weak thermocline feedback, likely a result of the loose equatorial thermocline in the mid-Holocene, suppresses the occurrence of extreme El Niño events and consequently explains the reduction in both ENSO amplitude and asymmetry. In MH8K, strengthened trade winds over the western-central Pacific Ocean act to cool the surface via evaporation while warmer water in the southern subtropical Pacific is transported beneath the equatorial thermocline, both contributing to diffuse the thermocline. Multimodel simulations for the mid-Holocene showed mean state changes and ENSO weakening similar to MH8K, but most models did not show reduced ENSO skewness, probably because of the failure in reproducing extreme El Niño events under the present climate.

© 2019 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: T. Iwakiri, iwakiri@aori.u-tokyo.ac.jp
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