Understanding the different trends of precipitation in different regions over monsoonal East Asia is critical to water resource management and environment/ecology planning. The recent two decades have witnessed increasing studies concerning about the spatiotemporal rainfall heterogeneity over East Asia monsoon areas, an example can be the widely reported tripolar and/or bipolar rainfall heterogenous patterns identified in modern meteorological records. Yet little is known whether this recently observed rainfall heterogeneity also exists during the historical periods when human activity was weak and the Earth's climate was dominated by natural forcing. The causes of the recently observed rainfall heterogeneity are also rather debatable. Among the variable potential forcing candidates, the Western Pacific Subtropical High (WPSH) has been widely recognized as a major one due to its anticyclone feature. The central area controlled by WPSH generally receives less precipitation due to airmass subsidence from the descending motion, while more frontal precipitation occurs around its edge areas due to strong airmass uprising from the ascending motion; the migration of WPSH may thus lead to spatiotemporal rainfall heterogeneity. However, although the modern WPSH behaviors have been well studied, few is known about the historical WPSH behaviors, the driving forcing, and their role in regulating regional hydroclimates.

         To address this gap, we developed, compiled and assimilated geological precipitation records over different regions in East Asia of the last millennium, examined the simulated climatic changes, and diagnosed the meridional and zonal behavior of WPSH and its role in regulating regional hydroclimatic changes. Our results suggest: (1) the WPSH shapes the observed spatial rainfall heterogeneity over East Asia not only in the recent anthropogenic warming epoch, but also during the past millennium, resulting in decadal/multidecadal to centennial heterogenous hydroclimatic pattern over central to southwest China as compared with those over southeastern Asia and north China. (2) the external solar forcing plays an important role in modulating WPSH behaviors on different timescales. The above finding has been published on Communications Earth & Environment (https://doi.org/10.1038/s43247-024-01660-9).

           This is a multidisciplinary study: we developed a well-dated high-resolution timeseries of δ¹⁸O in peat cellulose and used it as a proxy of monsoon precipitation. The studied peatland is called Douhu peatland, a small alpine peatland in southeastern China (Fig.1), and it is quite difficult to climb to the mountain top with sediment coring equipment (Fig.2). This remote condition also implies less human disturbance with the peat deposits. There are numerous biogeological proxy records over monsoonal East Asia, we collected those with accurate dating, and clear climatic significance. To evaluate the correlations between different records, we further assimilated the paleoclimatic data inferred from biogeological indices and/or extracted from climatic simulations. Climatic simulations in different climatic models were examined, and it is interesting to find that the ACCESS-ESM1-5 model has high performance in simulating precipitation over the south, central, and north China (as compared with the biogeological records), and both the observed and simulated south and north China precipitation synchronize with total solar irradiance (TSI). We thus further compared the difference in climatic status between high TSI and low TSI episodes of the last millennium in the ACCESS-ESM1-5 simulations. The high and lower TSI intervals are defined by low ¹⁰Be concentration in Antarctica ice cores and decreased observed sunspots, respectively. The results suggested that the intensified solar activity can: (1) increase the sea surface temperature (SST) and convection over the Indo-western Pacific warm pool and enhance the Hadley circulation over the western Pacific, and (2) lead to bigger increases in north Pacific SSTs, and higher temperature gradient between the north and south hemispheres, which combinedly lead to enhanced and northward and westward extension of WPSH, resulting in the observed decadal to centennial tripolar rainfall patterns.

         The results of this study further imply that other atmospheric circulations, like the El Niño Southern Oscillations, Pacific Decadal Oscillations, etc., although they can certainly influence East Asia precipitation, they may hardly directly generate the observed heterogeneity. In other words, without the anticyclone effect of WPSH, there would be likely no decadal to centennial rainfall heterogeneity over East Asia as observed both in biogeological and meteorological records. Providing this mechanism stands, the WPSH would be intensified and northwardly and westwardly extended if the earth continues warming (analogous to an enhanced solar activity scenario), and would lead to lower decadal to centennial natural water availability over central to southwestern China, which is crucial to water management, and industrial/agricultural planning, as well as environment/ecology protection.

Figure 1. Shown are photography of Douhu peatland studied in this work. Lake Douhu is an alpine lake located in southeast China (left, viewed from Google Earth); while the Douhu peatland is a wetland connected to Lake Douhu but at a slightly higher elevation (right, viewed from a drone).

Figure 2. Shown are group members transporting the sediment coring equipment to the Douhu alpine peatland.