The hydrological cycle of the Maritime Continent in the mid-Pliocene: results from PlioMIP2

The Maritime Continent (MC) forms the western boundary of the tropical Pacific Ocean, and relatively small changes in this region could impact the climate locally and remotely. In the mid-Pliocene (3.264 Ma and 3.025 Ma BP), atmospheric CO₂ concentrations were ∼ 400 ppm, and the subaerial Sunda and Sahul shelves made the land-sea distribution of the MC different to today. These two effects, combined with other forcings, are therefore expected to have driven a substantial climate signal in MC region. By using the results the Pliocene Model Intercomparison Project phase 2 (PlioMIP2) we study the mean climatic features of the MC in the mid-Pliocene, and the changes in Indonesian Throughflow (ITF). Results show a warmer and wetter climate of the MC and lower sea surface salinity. Another feature is enhanced easterlies over the western tropical Pacific Ocean which may result from the westward shift of the Pacific Walker Circulation and the intensified Southern Hemisphere Subtropical High pressure system. Furthermore, we quantify the volume transfer through the ITF; although the ITF may be expected to be hindered by the subaerial shelves, 7 out of 9 models show an increased volume transport.
In order to avoid undue influence from closely-related models, we introduce a new metric - the multi-cluster mean (MCM), based on cluster analysis of the individual models. We compare the MCM to the more traditional multi-model mean (MMM) and individual models by quantifying the discrepancy between model results and reconstructed proxy data. The result shows that the MCM can reproduce sea surface temperature better than more than half of the PlioMIP2 models in both the pre-industrial experiment and the mid-Pliocene experiment. The clusters include spatial signals that are not captured by the MMM, so the MCM  provides us with a new way to illustrate the results from multiple models.