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Characteristics of the seasonal and interannual sea surface temperature variability in the eastern equatorial Pacific (EEP) over last two interglacials, the Holocene and Eemian, are analyzed using transient climate simulations with the Kiel Climate Model. There is a tendency toward a strengthening of the seasonal as well as the El Nino/Southern Oscillation (ENSO)-related variability from the early to the late interglacials. The weaker EEP sea surface temperature annual cycle during the early interglacials is mainly a result of insolation-forced cooling during its warm phase and dynamically induced warming during its cold phase. Enhanced convection over northern South America weakens northeasterlies in the EEP leading to weaker equatorial upwelling, deeper thermocline and subsequent warming in this region. We show that a negative ENSO modulation of the annual cycle operates only on short timescales and does not affect their evolutions on orbital time scales where both ENSO and annual cycle show similar tendencies to increase.

Plain Language Summary Although the Sun crosses the equator twice a year, the sea surface temperature (SST) in the eastern equatorial Pacific (EEP) exhibits a distinct annual cycle. Therefore, in addition to the direct solar forcing, coupled ocean-atmosphere processes must also be taken into consideration to explain the observed annual cycle of the EEP SST. The annual cycle is also an important factor for the seasonal phase locking of the El Nino/Southern Oscillation. The aim of this study is to identify dominant factors affecting the annual cycle and interannual variability in the EEP on geological time scales. For this purpose we analyze transient climate simulations of the last two interglacials, the Holocene (9,500-0 BP) and the Eemian (126,000-115,000 BP). We find that both El Nino/Southern Oscillation variability and the annual cycle of SST in the EEP tend to increase from the early to late interglacials. We will show that local radiative forcing and reorganized atmospheric circulation are dominant factors to modify the annual cycle. However, a relative contribution of these factors may vary within the year. Our research demonstrates that the mechanism of the present-day annual cycle should be updated when considering long-term variations of climate states on orbital time scales.