Salau, Opeyemi R. S. (2012) El Niño/Southern Oscillation during the Holocene and Eemian Warm Periods. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, XII, 105 pp.

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Abstract

El Niño/Southern Oscillation (ENSO) is a natural interannual climate fluctuation originating in the Tropical Pacific Ocean, which affects climate worldwide through atmospheric teleconnections. The sensitivity of ENSO phenomenon to changes in the mean Tropical Pacific climate is investigated with a coupled atmosphere-ocean-sea ice general circulation model (AOGCM), the Kiel Climate Model (KCM). Different mean climate states of the two most recent interglacial warm periods, the Holocene (last 10 kyr BP) and the Eemian (126–115 kyr BP), were generated by changing the orbital parameters that determine the latitudinal and seasonal distribution of the top of atmosphere (TOA) solar insolation which was the major driver of both climates. The orbital induced insolation changes throughout the Eemian are about twice as large as the changes throughout the Holocene. This is due to the higher values of eccentricity which is about twice the value in the Holocene but both exhibits a largely similar seasonal and latitudinal structure of the TOA insolation. Changes in all other climate forcings such as variations in greenhouse gas concentration and continental ice volume are considered to be of minor importance during the periods of interest (Holocene, Eemian) and have been neglected for the benefit of comparability. The simulated annual mean TOA insolation and the corresponding sea surface temperature (SST) increases over time in the Tropical Pacific in both the Holocene and the Eemian. The resulting ENSO amplitude is positively correlated with both the Equatorial Pacific SST and the Equatorial zonal SST contrast. The latter is controlled by the upwelling-induced dampening of the SST changes in the Eastern Equatorial Pacific (EEP), and by the vertical ocean dynamical heating and zonal heat transport convergence in the Western Equatorial Pacific (WEP). The ENSO amplitude also correlates positively with the seasonal SST amplitude in the EEP and negatively with the strength of the easterly Trades over the Equatorial Pacific. Only minor variations in the upper ocean heat content of the Tropical Pacific are simulated, indicating that the origin of the ENSO changes is primarily linked to surface layer processes. The enhanced SST gradient in conjunction with weakened trades suggests the direct radiative forcing effects played a role in addition to air-sea interactions. However, the ENSO period is rather stable and stays within 3-4 years. Enhanced ENSO amplitude is simulated during the late-Holocene, in agreement with paleo proxy records. The tight positive correlation (r=0.89) between the ENSO strength and the Western Pacific Warm Pool (WPWP) SST suggests that the latter may provide an indirect measure of the ENSO amplitude from proxy data that cannot explicitly resolve interannual variability.

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