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Twenty‐first‐century climate change projections are uncertain, especially on regional scales. An important source of uncertainty is that climate models exhibit biases, which limits their ability to predict climate. One of the largest biases is the too warm sea surface temperature (SST) in the eastern tropical Atlantic (TA), reflecting deficient atmospheric and oceanic circulation. Here, we show that CO2‐forced TA‐sector climate changes simulated by state‐of‐the‐art climate models exhibit a strong mean‐state dependence. In particular, models simulating largest SST warming in the eastern TA, consistent with the warming observed since the mid‐20th century, typically exhibit a more realistic mean state than models simulating largest warming in the western TA. The former models exhibit a larger climate sensitivity, and predict stronger and in part qualitatively different climate changes over the TA sector, for example in precipitation. These findings may help to reducing uncertainty in TA‐climate change projections. Plain Language Summary: Twenty‐first‐century climate change projections are uncertain, especially on regional scales. An important source of uncertainty is that climate models exhibit biases, which limits their ability to predict climate. One of the largest biases is the too warm sea surface temperature in the eastern tropical Atlantic (TA), reflecting deficient atmospheric and oceanic circulation. Here, we show that CO2‐forced TA‐sector climate changes simulated by state‐of‐the‐art climate models exhibit a strong relationship to the quality of simulating the mean state. These findings may help to reducing uncertainty in climate change projections over the TA sector. Key Points: Climate projections for the tropical Atlantic sector depend on the quality of simulating present‐day conditionsLess biased climate models provide more reliable projectionsSpread in CO2‐forced climate changes over the Tropical Atlantic region [ABSTRACT FROM AUTHOR]

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 Niño/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 Niño/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 Niño/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. Key Points: Both ENSO variability and the annual cycle of SST in the eastern tropical Pacific tend to increase from the early to late interglacialsDirect radiation forcing and dynamical processes are underlying factors to intensify annual cycle of the EEP SST through the interglacialsStrengthening of the annual cycle of the EEP SST is in agreement with reconstructed seasonal range of the eastern tropical Pacific SST [ABSTRACT FROM AUTHOR]