| 18 Aug 2026
Sustained Late-Nineteenth-Century Monsoon Drought in Seoul and the Indian Ocean–North Atlantic Forcing of Its Circulation Weakening
Abstract. The Cheugugi rain-gauge record of Seoul, Republic of Korea (1777–1907), is among the world's longest instrumental daily precipitation records and a rare quantitative gauge of the pre-industrial East Asian monsoon. Here, we reconcile it against the original chronicle and use it to document, and attribute, a severe sustained rainy-season drought from 1881 to 1907: rainy-season (June–August, JJA) precipitation fell ~27 % and annual totals 28 % (with a Pettitt change point in 1880, p ≈ 0.01), with the contribution of days >100 mm falling by ~43 %. Three distinct record channels – the gauge, a separate documentary reconstruction, and qualitative rain reports in the Seungjeongwon Ilgi – registered the same shift, arguing against any decay in observational practice. In the Twentieth Century Reanalysis, the unconstrained precipitation field did not resolve the drought, whereas its pressure-constrained circulation registered a weakened monsoon low. Pacemaker experiments (10 members each) were conducted to identify the oceanic driver, prescribing the observed sea-surface-temperature anomalies over the tropical Pacific, North Atlantic, and Indian Ocean individually and globally. The historical forcing produced a significant weakening of the low-level monsoon circulation over Korea: a nearly 8 m depression of the JJA 850 hPa geopotential height (p < 0.001, the most robust signal), weakened southerly inflow, and significant reduction in the vertically integrated moisture supply. The geopotential-height weakening was driven principally by the Indian Ocean, the largest single-basin contributor, together with the North Atlantic, and not by the tropical Pacific despite its El Niño-like state. The reconciled record supplies a pre-industrial monsoon baseline and, with the experiments, attributes the monsoon-circulation weakening that accompanied the historical Korean drought to Indian Ocean and North Atlantic cooling. Future work should disentangle the tropical-cyclone contribution to the heavy-rainfall deficit and confirm the attribution with additional models.
This manuscript reconciles three independent reconstructions of Seoul's Cheugugi rain-gauge record (1777-1907) into a new daily dataset (Cheugugi-Seoul v2), documents a severe and sustained rainy-season drought from 1881-1907, and uses a set of SST-forced pacemaker experiments to attribute the associated weakening of the low-level monsoon circulation over Korea principally to Indian Ocean and North Atlantic cooling, with a comparatively minor role for the tropical Pacific despite an El Nino-like SST pattern during the drought epoch. The dataset reconciliation is a valuable contribution in its own right, and the study is commendably transparent about its own limitations, in particular the displaced summer rain band of the atmospheric model used for attribution. I recommend publication after the points below are addressed; none of them require new experiments.
General comments
The three-channel argument against observational-practice decay (Sect. 3.2) is a convincing way to rule out the most obvious alternative explanation for the drought signal. The use of 20CRv3 in Sect. 3.3, exploiting the fact that only pressure is assimilated over this period, is a defensible and instructive way to interrogate an early reanalysis. The pacemaker experimental design (Sect. 2.2, 3.5) is sound, and the additivity check (TP+NA+IO vs. GOGA, Fig. 5) is a welcome addition. My main concerns are (i) an inconsistency between diagnostic fields in the basin attribution that is not fully reconciled, (ii) the strength of the title/abstract claim relative to the single-model basis of the attribution, and (iii) some statistical details that are currently confined to the Supplement and would strengthen the main text if summarized there.
Specific comments
1. Basin attribution is inconsistent across diagnostic fields (Sect. 3.5, p. 8). The headline finding that the Indian Ocean and North Atlantic, not the tropical Pacific, drove the circulation weakening is based on the 850-hPa geopotential height field, described as having "the highest signal-to-noise ratio." However, the same section reports that for the 850-hPa meridional wind and the vertically integrated moisture flux, no single basin was more than marginally significant and the tropical Pacific contributed comparably. Since these latter fields most directly govern moisture delivery to Korea, I would ask the author to discuss explicitly why the basin ranking differs across fields, rather than noting the discrepancy in passing.
2. The title and abstract state the attribution in fairly unconditional terms, while the Conclusions appropriately note that the result "rests on a single T42 model whose southward-displaced rain band makes local precipitation unreliable" and that confirmation with additional models is warranted. I suggest bringing this caveat forward into the abstract, or softening the title/abstract phrasing, so readers who do not reach the Conclusions are not left with an overstated impression of certainty.
3. Statistical multiplicity across the many significance tests (Pettitt, block-bootstrap, permutation, Welch t-tests across five experiments and multiple fields, Spearman rank correlation, CSI, Fisher's exact test, superposed epoch analysis) is not discussed. Please clarify whether multiple-comparison correction was applied across the basin x field Welch t-tests, and whether the p<0.05 stippling in Figs. 3, 4, and 6 reflects pointwise or field significance.
4. The attribution diagnostics are computed over a fairly large "Korea box" (34-43N, 124-130E) relative to the single point-station record (Seoul, ~37.5N, 127E) that motivates the study. Some justification for this domain choice, or a sensitivity check with a tighter box, would strengthen the link between the box-mean diagnostics and the Seoul record.
5. The reversal of the conventional El Nino-driven explanation for Late Victorian droughts is central to the paper's novelty, but rests on a short (27-year) epoch (r = -0.07 for Nino3.4-Seoul correlation; p ~ 0.19 for the Fisher's exact test). A brief discussion of statistical power at this sample size would help readers weigh how strongly "not significant" argues against an El Nino role.
6. The block-bootstrap methodology and treatment of serial autocorrelation (Text S2) materially affect the robustness claims for a strongly autocorrelated, multidecadal series. Please summarize the approach briefly in the main text.
Technical corrections
- p. 3 (Sect. 2.2): the claim that the model's summer rain band sits "~7 degrees too far south" has no citation or supporting figure. Please clarify whether this is an established bias from prior work or a diagnostic finding of this study.
- p. 11 (Discussion): please clarify whether the comparison to tree-ring drought atlases (Cook et al., 2010) was checked directly against the atlas data, or is cited from the literature.
- Please confirm that all cross-references to Supplementary Tables/Text (S1-S8, Table S1-S5) are consistent with the final Supplement.