Abstract
The mid-Holocene (MH), a period about 6,000 years ago, provides an opportunity to understand climate change in response to orbital forcing change. Numerical model simulation is an effective tool through which we can study the climate change in the MH, although the climate in the MH can be partly inferred from proxy data. As the Paleoclimate Model Inter-comparison Project phase 4 (PMIP4) recently released the latest simulations for different past climate scenarios, we investigated tropical climate changes, including both the basic state and interannual variability, and tried to find out whether the PMIP phase 3 (PMIP3) and PMIP4 results can be reconciled. Almost all the modelling results show that the inter-hemisphere contrast was enhanced over the tropical Pacific, with warmer and wetter condition on the northern side of the equator and an intensified cross-equatorial flow in the MH than at present, and the annual cycle of the sea-surface temperature (SST) during the MH was reduced. Such background mean state changes arose from the seasonal changes of the solar incident radiation in the MH. In addition to the consistent changes in background mean state changes, some El Niño-Southern Oscillation (ENSO) features, such as the seasonal phase locking feature and periodicity, show consistent changes across the PMIP3 and PMIP4 models, that is, both suites of models exhibit no marked difference in the MH with respect to present-day simulations. In contrast, the modeling results show only agreement on the sign of the ENSO amplitude change (i.e., decrease in the ENSO amplitude in the MH), while the range of reduction varied with model and region. Additionally, the occurrence probability of central Pacific El Niño events increases in the MH, whereas the significance is quite marginal. The modeled changes in the mean state and ENSO serve as a test bed for studying tropical climate system’s response to natural warming, which may provide some insights into understanding climate changes in response to the current anthropogenic warming.
Highlights
In the mid-Holocene (MH) of ~6,000 years before present, the climate system differed greatly with respect to the present-day climate
As the Paleoclimate Model Inter-comparison Project phase 4 (PMIP4) recently released the latest simulations for different past climate scenarios, we investigated tropical climate changes, including both the basic state and interannual variability, and tried to find out whether the PMIP phase 3 (PMIP3) and PMIP4 results can be reconciled
We examine the annual cycle of sea-surface temperature (SST) in the equatorial (5oS–5oN) Pacific derived from the model ensemble mean (MME) of the PMIP3 and PMIP4 models
Summary
In the mid-Holocene (MH) of ~6,000 years before present, the climate system differed greatly with respect to the present-day climate. LH and SW are the mean latent heat flux and the FIGURE 4 | Plots for MME of SST annual cycle over the equatorial Pacific (averaged between 5oS and 5oN) derived from PMIP3 (left column) and PMIP4 (right column) models. Previous studies (e.g., An and Choi 2013, 2014) pointed out that the annual cycle amplitude of SST is largely determined by the second and third terms on the right-hand side of the FIGURE 5 | MME of mean insolation at the top of the atmosphere over the tropical Pacific (averaged between 120oE and 160oE) from PMIP3 (left column) and PMIP4 (right column) models. The difference plots between MH and PI simulations (Figures 7C,F) show an obvious decrease in the interannual variation of SST anomalies over the central-eastern equatorial Pacific, indicating a reduced ENSO activities during the MH. The model results show a larger occurrence of CP El Niño in the MH than in the PI, though the significance is quite marginal
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