Winter monsoon variability accompanies Xisha SST–North China hydroclimate teleconnection reversal during the Medieval climate anomaly

Winter sea surface temperature (SST) in the South China Sea is a precursor of North China hydroclimate, but the relationship’s long-term stability remains uncertain. Using monthly resolved Xisha coral Sr/Ca-SST and three hydroclimate reconstructions analyzed at native temporal resolution, we find that the Medieval Climate Anomaly (MCA, 1149–1205 CE) is the only sampled interval with a cross-proxy-consistent positive relationship. The annually resolved Zheng record remains positive after detrending and autocorrelation correction ( r = 0.476, p adj = 0.043), whereas non-MCA relationships are predominantly negative or near zero. East Asian winter monsoon (EAWM)-related variability is largest during the MCA among the six intervals, yet its association with teleconnection strength is weak ( r = 0.52, p = 0.29), arguing against a universal threshold. Independent central-Pacific coral evidence indicates altered tropical-Pacific variability. Because winter-to-summer SST memory persists, we interpret the MCA reversal as downstream atmospheric reorganization, with EAWM variability as a possible preconditioning factor.