The hydrological cycle and ocean circulation of the Maritime Continent in the Pliocene: results from PlioMIP2

Abstract

Abstract. The Maritime Continent (MC) forms the western boundary of the tropical Pacific Ocean, and relatively small changes in this region can impact the climate locally and remotely. In the mid-Piacenzian warm period of the Pliocene (mPWP; 3.264 to 3.025 Ma) atmospheric CO2 concentrations were ∼ 400 ppm, and the subaerial Sunda and Sahul shelves made the land–sea distribution of the MC different to today. Topographic changes and elevated levels of CO2, combined with other forcings, are therefore expected to have driven a substantial climate signal in the MC region at this time. By using the results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2), we study the mean climatic features of the MC in the mPWP and changes in Indonesian Throughflow (ITF) with respect to the preindustrial. Results show a warmer and wetter mPWP climate of the MC and lower sea surface salinity in the surrounding ocean compared with the preindustrial. Furthermore, we quantify the volume transfer through the ITF; although the ITF may be expected to be hindered by the subaerial shelves, 10 out of 15 models show an increased volume transport compared with the preindustrial. In order to avoid undue influence from closely related models that are present in the PlioMIP2 ensemble, we introduce a new metric, the multi-cluster mean (MCM), which is based on cluster analysis of the individual models. We study the effect that the choice of MCM versus the more traditional analysis of multi-model mean (MMM) and individual models has on the discrepancy between model results and data. We find that models, which reproduce modern MC climate well, are not always good at simulating the mPWP climate anomaly of the MC. By comparing with individual models, the MMM and MCM reproduce the preindustrial sea surface temperature (SST) of the reanalysis better than most individual models and produce less discrepancy with reconstructed sea surface temperature anomalies (SSTA) than most individual models in the MC. In addition, the clusters reveal spatial signals that are not captured by the MMM, so that the MCM provides us with a new way to explore the results from model ensembles that include similar models.

Similar North Pacific variability despite suppressed El Niño variability in the warm mid-Pliocene climate

“…In the last part of this Discussion, we present a synthesis of the results presented in this paper on the mid-Pliocene changes in North Pacific atmosphere variability, combined with other published results using PlioMIP2 data, specifically regarding tropical-North Pacific variability (Oldeman et al, 2021;Pontes et al, 2022;Canal-Solis et al, 2024) as well as Indo-Pacific tropical mean climate (Han et al, 2021;Ren et al, 2023;Zhang et al, 2024). Figure 8 presents a summary view of the interactions of mid-Pliocene tropical and extratropical variability and mean state changes, according to the PlioMIP2 multimodel mean.…”

“…Examining this step is relevant considering the substantial changes in Indo-Pacific mean hydrological cycle in the PlioMIP2 (e.g. Han et al, 2021;Ren et al, 2023;Zhang et al, 2024). So, we also consider the regression between the Niño3.4 index and tropical precipitation, and the regression between WEP precipitation and North Pacific SLP anomalies.…”

“…In the tropical Indo-Pacific mean climate, the rising branch of the Pacific Walker circulation (PWC) is shifted westwards, both in the annual mean (Han et al, 2021) and in boreal winter (Zhang et al, 2024). Furthermore, the Indian Ocean, the Maritime Continent and the WEP warm, but the Indian Ocean warms more than the WEP (Ren et al, 2023). We hypothesize that the westward shift of the PWC, the Indo-Pacific warming asymmetry and the ENSO suppression are related, and we find that there is indeed a significant ensemble correlation between the change of these three variables (Supplementary Material Figure S7).…”

“…A hemispheric energy asymmetry shifts the Hadley circulation and ITCZ northwards (Han et al, 2021;Zhang et al, 2024). The Pacific Walker circulation is shifted westward (Han et al, 2021;Zhang et al, 2024), which is associated with warmer and wetter conditions over the Indian Ocean and Maritime Continent (Ren et al, 2023).…”