Pollen records of the Little Ice Age humidity flip in the middle Yangtze River catchment

High-resolution precipitation reconstructions of the Little Ice Age (LIA) are fundamental for investigating the driving mechanisms behind hydroclimatic changes, which would provide reference for predicting precipitation change in the future. However, current precipitation reconstructions mainly focus on mean conditions during the whole LIA, little attention was paid to internal changes. Here, we present a high-resolution precipitation reconstruction based on pollen records from the middle reaches of the Yangtze River, spanning from 1300 to 2010 CE. In this study, the LIA was defined as the period of 1300-1870 CE based on PC2 value and pollen assemblage, manifest as cold interval with internal fluctuations (early-LIA: 1300-1340 CE, mid-LIA: 1340-1720 CE and late-LIA: 1720-1870 CE). Negative PC1 value during the early-LIA and mid-LIA indicated dry condition, while positive PC1 value recorded wet late-LIA. Cold interval in the early-LIA increased effective moisture by reducing evaporation, which may contribute to high content pollen concentrations and wetland herbs pollen percentage. Dry condition in the mid-LIA probably induced by warm interval by increasing evaporation and less precipitation. During the late-LIA, high content pollen concentrations and more fine-grained sediment indicate there was standing water in the bog, which was induced by heavy precipitation and cold interval. Moisture variation recorded by PC1 value and grain size distribution indicated humidity-flip between the mid-LIA and late-LIA. This humidity-flip is also supported by the frequency of flood disasters reported in historical documents. Precipitation changes during the LIA could be associated with East Asian Summer Monsoon (EASM) and El Niño-Southern Oscillation (ENSO) variability. The dry early-LIA and mid-LIA caused by the weaker EASM and more La Niña-type conditions, during which the Western Pacific Subtropical High (WPSH) weakened and retreated northeastward, leading to less Mei-yu precipitation in the Yangtze River basin. Intensified EASM and more El Niño-type events during the late-LIA lead to longer Mei-yu season along the Yangtze River and more precipitation.