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The role of each glacial forcing and associated mechanisms was assessed by quantifying the agreement between the simulated changes with two multiproxy reconstructions. We assessed agreement between simulated rainfall responses (Table 1) against the updated version of our synthesis of LGM hydroclimate described in section S1. As this reconstruction is categorical, we estimated the model-proxy agreement using the weighted Cohen’s κ statistic, following previous work (10). Cohen’s κ ranged from κ = 1, if a simulated pattern is in complete agreement with the proxies, to κ = 0, if the agreement could be expected entirely by chance. The weighted Cohen’s κ uses a weight matrix that penalizes models for a total miss (e.g., drier when it should be wetter) more than a near miss or partial agreement (e.g., drier when it should be no change). A weight of 0.5 is given to these cases. We explored the sensitivity of the κ values by varying the thresholds of rainfall, over which we placed the model output into the same categories of change assigned to the proxies. We consider that the model-proxy agreement is robust when 2σ statistically significant κ values are obtained for large range of dry-wet thresholds used to categorize the model simulated changes. The maximum Cohen’s κ values for each climate response are reported in Fig. 4A along with their 2σ confidence intervals.

Approach used to compute the climate response to different LGM boundary conditions. First column identifiers are used throughout the study to identify each climate response. The third column is the operation between climatologies used to compute each climate response. See tables S3 and S4 for a list of simulations. PI, preindustrial; GHG, greenhouse gases; ITF, Indonesia throughflow; SL, sea level; MC, Maritime Continent.

We also assessed the agreement between simulated SST changes and a new compilation of LGM temperature anomalies across the IPWP derived from sedimentary alkenone, Mg/Ca, and δ18O data. As this reconstruction is quantitative, we simply calculated the pattern correlation between proxy and simulated values at the core sites. We applied different temperature calibrations to the alkenone and Mg/Ca data to explore the sensitivity of the model-proxy agreement to calibration choice. Most calibrations show the highest significant correlations with the simulated SST changes in response to either full sea level boundary conditions or sea level plus ice sheet albedo (fig. S5). The minimum-maximum range of r values computed from this distribution of SST reconstruction is reported as uncertainty of the pattern correlation. Further details are available in sections S2 and S6.

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