Abstract
Abstract. Using a coupled atmosphere-ocean model we analyze the responses of the mean climate and interannual variations in the tropical Pacific to the changes in insolation during the early and mid-Holocene, with experiments in which only the variations of Earth's orbital configuration are considered. We first discuss common features of the Early and mid-Holocene climates compared to the pre-industrial conditions. In particular, an equatorial annual mean cooling that has a "U" shape across the tropical Pacific is simulated, whereas the ocean heat content is enhanced in the western tropical Pacific and decreased in the east. Similarly, the seasonality is enhanced in the west and reduced in the east. We show that the seasonality of the insolation forcing, the cloud radiative forcing and ocean dynamics all contribute to increasing these east–west contrasts. ENSO variability is reduced in the early Holocene and increases towards present-day conditions. Obliquity alone does not affect ENSO characteristics in the model. The reduction of ENSO magnitude results from the relationship between changes in seasonality, which involves wave propagation along the thermocline, and the timing of the development of ENSO anomalies. All these effects are larger in the Early Holocene compared to the mid-Holocene. Despite a one-month difference in the insolation forcing and corresponding response of SST, winds and thermocline depth between these two periods, the timing and changes in the east–west temperature and heat content gradients are similar. We suggest that it explains why the timing of development of ENSO is quite similar between these two climates and does not reflect the differences in the seasonal timing.
Highlights
The El Nino-Southern Oscillation (ENSO) phenomenon is the most important manifestation of short-term climate variability, resulting from air–sea interactions in the tropical Pacific (Philander, 1990)
The changes of tropical Pacific climate over the mid- to late Holocene is largely induced by orbitally driven changes in the seasonal cycle of solar radiation in the tropics
The model simulates a smaller reduction in ENSO amplitude of around 10 %, and it simulates a slight shift to longer period variability and a weakening of ENSO phase-locking to the seasonal cycle in the mid-Holocene
Summary
The El Nino-Southern Oscillation (ENSO) phenomenon is the most important manifestation of short-term climate variability, resulting from air–sea interactions in the tropical Pacific (Philander, 1990). Analysis of the instrumental records shows that each event has its own characteristics They show that ENSO frequency increased in the last years of the twenty century, with the maximum SST anomalies located in the middle of the basin but not at the east coast as in the traditional view (Latif et al, 1997; Kao and Yu, 2009; Kug et al, 2009; Yeh et al, 2009). These changes appear to be connected to shifts in mean climate, raising the possibility that ENSO might undergo discernible changes in response to anthropogenic driven warming.
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