The current reconstructions of the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) primarily rely on marine sediment proxies. However, the limited resolution of these records makes it difficult to precisely understand the short‐term dynamics of the PDO and AMO, and consequently, their influence on global climate. This study unveils two new Holocene reconstructions of AMO and PDO derived from a large‐scale compilation of speleothem isotope records (δ18Os) from Chinese monsoon region, in which spatial patterns of summer rainfall are generally accepted as good indicators of internal variability. Principal component analysis applied to Chinese δ18Os records identifies the first principal component as representative of Holocene Asian monsoon (AM) variation. This data‐driven approach reveals a significant shift in hydroclimatic conditions after 8.2 ka, potentially providing valuable insights into the underlying forcing mechanisms driving these changes. The remanent components, which show spatial rainfall patterns, are effectively validated through independent records of AMO and PDO derived from the previous paleo‐proxy based reconstructions and model simulations. The present reconstructions suggest a significantly enhanced stability of the AMO and PDO in the middle Holocene, which may explain the concurrent more stable and optimum climate observed in the AM region. These findings also imply that the Holocene AM intensity recorded in Chinese cave records is largely controlled by external forcing, whereas the regionally heterogenous rainfall is regulated by internal variability. The successful attempt on the Holocene AMO and PDO reconstructions indicates an effective isolation between different modes of climate variability from paleoclimate records.

Holocene Variability of the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation Inferred From Chinese Speleothem δ18O Records

17O‐excess is a relatively new water isotope parameter that could potentially provide useful information about the hydrological cycle. Previous works focusing on 17O‐excess in polar regions suggest that it primarily tracks moisture source relative humidity, but little is known about how to interpret 17O‐excess data in lower latitudes. Here we present quasi‐hourly triple oxygen isotope data of precipitation collected from two tropical cyclones in Texas and Louisiana in 2020 to understand the impacts of environmental and meteorological processes on the 17O‐excess of low‐to mid‐latitude precipitation. We find that at both hourly timescales and the event scale, 17O‐excess is strongly correlated to changes in on‐site rainfall intensity and relative humidity, which is consistent with the theory that the isotopic fractionation associated with rain re‐evaporation lowers the 17O‐excess of the remaining droplet. In addition, although evaporative conditions at the moisture source region may also influence 17O‐excess of water vapor transported to the precipitation site, their impacts are likely overprinted by the post‐condensation rain re‐evaporation processes. Our results thus suggest that 17O‐excess can be used as a proxy for local rather than source region evaporative conditions during tropical cyclones.

17O‐Excess in Tropical Cyclones Reflects Local Rain Re‐Evaporation More Than Moisture Source Conditions

The oxygen isotope compositions (δ18O) of stalagmites have been widely used to reconstruct summer monsoon variability, which is primarily controlled by the solar radiation on orbital to seasonal timescales. However, the relationship between solar activity and the monsoon system remains unclear. This study is based on a 13-year, from 2010 to 2022, monitoring campaign of the δ18O values of precipitation, cave drips and modern speleothems in Jiguan Cave, central China. By analyzing moisture circulation and the local meteoric water line (LMWL) under different ENSO (El Niño Southern Oscillation) events, it was found that on seasonal–interannual timescales, the precipitation δ18O values reflect the atmospheric moisture circulation. In addition, we used cross-wavelet transform analysis to investigate the periodic relationship between the sunspot number (SSN) and the Asian Summer Monsoon (ASM) since solar activity is one of the most important external influences on the ASM. The stalagmite δ18O data for our study areas reflect the solar activity-induced decadal-scale variations in the local precipitation. The key factors related to the δ18O value include the driving mechanism of the solar activity and ENSO-related sea surface temperature (SST) anomalies. We speculate that changes in solar activity cause anomalies in the tropical SST gradient, which affects central China's hydrological cycle by changing the variability of ENSO and the Indian Ocean Dipole (IOD) and ultimately causes the variations in stalagmites δ18O records. 

New insights into potential effects of solar activity on the oxygen isotope composition of modern speleothems: Observations from Jiguan Cave, central China II

The interhemispheric anti-phase variation of monsoonal precipitation at the millennial timescale has been attributed to the latitudinal movement of the Intertropical Convergence Zone (ITCZ). However, a debate persists regarding the lead-lag or synchronous relationship between the Asian Summer Monsoon (ASM) and South American Summer Monsoon (SASM) on the onset and termination timings of the Heinrich stadials, hindering a deeper understanding of their underlying driving mechanisms. Here, we presented a new stalagmite δ18O record from Xianyun Cave, southeastern China. This record, with ∼5-year temporal resolution and an averaged uncertainty of ∼60 years on U–Th dates, spans Heinrich Stadial 2 (HS2) and represents East Asian Summer Monsoon (EASM) intensity variations. Based on our record, we further divided the evolution of the EASM during HS2 into three phases: an onset phase (HS2-I, 24,450–24,260 yr BP) and a termination phase (HS2-III, 23,950–23,630 yr BP), sandwiching a phase with relatively intensified monsoon (HS2-II, 24,260–23,950 yr BP). During the onset and termination of HS2, synchronous transition in monsoon records from both hemispheres was observed, driven by the large-scale meridional movement of the ITCZ. In HS2-I, a “Large excursion” observed in both tropical ISM and SASM records was also evidenced in the Xianyun record, suggesting an extension of the “Tropical atmospheric seesaw” to broader regions near tropical moisture sources. In phase II, our record precisely characterizes a centennial-scale strengthening of the EASM coinciding with a weakening SASM, while the Indian Summer Monsoon (ISM) showed no significant changes. The decoupling of the ISM and EASM may be related to the distinct responses of the regional ITCZ to El Niño-Southern Oscillation variabilities. Our study highlights the significant role of the tropical ocean-atmosphere coupling systems in the synchronized or divergent evolution of global monsoons within Heinrich stadials. 

Synchronous anti-phase variations between the East Asian summer monsoon and South America Summer Monsoon during Heinrich stadial 2

Southwest China has experienced a series of record-breaking droughts over the last two decades, threatening regional ecosystems and exerting socioeconomic effects. Recent studies have focused on the drivers of specific drought events that seasonally vary in occurrence and duration, and a better understanding of regional paleo-records may be key to understanding their underlying mechanisms. Here, we present records of dry and wet alterations from 4730 yr B.P. based on multi-proxy stalagmite records. In particular, we report δ13C records at a five-year-resolution, and trace element (Mg, Sr, and Ba) to Ca ratios at a 25-year-resolution for stalagmite HD12 from Dark Cave, southwest China. Statistically significant correlations between δ13C and Mg/Ca suggest that they are controlled by a common factor, possibly related to local hydrological variations associated with the monsoon circulation. Long-term trends in the Sr/Ca and Ba/Ca ratios may be related to temperature- and/or precipitation-controlled growth rates. The first principal component (multi-proxy PC1) of δ13C and Mg/Ca can be used as the hydrological index in southwest China, indicating high-frequency oscillations on the millennial to multidecadal scales, with four prominent dry periods at 4255 ± 21 to 3430 ± 11, 2605 ± 12 to 2105 ± 9, 1755 ± 11 to 1255 ± 13, and 705 ± 8 to 405 ± 7 yr B.P. This is in general agreement with the monsoon index reconstructed from the detrended and z-scored cave δ18O PC1 in southwest China, indicating the sensitivity of regional hydrological response to monsoon intensity. The dry conditions of southwest China on the millennial-centennial scale may be related to solar forcing and the North Atlantic Oscillation owing to their influence on the strength and position of the westerlies and the intertropical convergence zone. Additionally, the El Niño Southern Oscillation could also affect water vapor transport to southwest China by influencing the strength and position of the Walker circulation and Western Pacific Subtropical High. Moreover, the dry periods of southwest China over the past 4730 years are closely related to the warming of the sea surface temperature of the tropical warm pool on multidecadal scales. Such warming may be detrimental to the transport of water vapor from the North Western Pacific and Indian Ocean to southwest China, resulting in a dry regional environment. 

Hydroclimate variability over the past 4730 years based on multi-proxy stalagmite records from southwest China

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

Typhoons produce a large amount of rainfall with a highly depleted isotopic composition. However, the mechanisms of isotope fractionation during typhoons remain poorly understood. Here, we present the hourly resolution isotope composition of rainfall during Typhoon Lupit (2021) in Fuzhou, on the southeast coast of China, to better understand these processes. Our results showed that the rainfall of Typhoon Lupit had significant fluctuations in δ18O values, ranging from −20.1‰ to −6.3‰, and exhibited a three‐stage variation with distinctly negative δ18O values in the second stage. In the first stage of Lupit, water vapour originated primarily from the nearby sea area, which did not undergo sustained convective activity, resulting in relatively enriched δ18O values. In the second stage, the δ18O values were extremely negative mainly because of the accumulated convective activity and continuous precipitation as they travelled towards the centre of the typhoon. In the third stage, the mixing of more localized and nearby sea water vapour caused an increase in the δ18O values. In summary, the three‐stage characteristics of Typhoon Lupit were largely caused by the integration of accumulated convective activity and moisture sources. Our findings reveal how upstream rainout affects the isotopic behaviour of rainfall and may have implications for understanding the dynamics of typhoon rainfall isotopes.

Variations of stable isotope in Typhoon Lupit (2021) rainfall and its response to upstream convective processes

Strong link between Asian summer monsoon and westerlies for the past 4750 years

Millennial-scale East Asian summer monsoon hydroclimate variability during 47–39 kyr B.P. inferred from a stalagmite IRMsoft-flux record in Southeastern China

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Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

Variations of stable isotope in Typhoon Lupit (2021) rainfall and its response to upstream convective processes

Strong link between Asian summer monsoon and westerlies for the past 4750 years

Hydroclimate variability over the past 4730 years based on multi-proxy stalagmite records from southwest China

Millennial-scale East Asian summer monsoon hydroclimate variability during 47–39 kyr B.P. inferred from a stalagmite IRMsoft-flux record in Southeastern China

Kui Gao

Author Tools

Chat about Author

Papers

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

Variations of stable isotope in Typhoon Lupit (2021) rainfall and its response to upstream convective processes

Strong link between Asian summer monsoon and westerlies for the past 4750 years

Hydroclimate variability over the past 4730 years based on multi-proxy stalagmite records from southwest China

Millennial-scale East Asian summer monsoon hydroclimate variability during 47–39 kyr B.P. inferred from a stalagmite IRMsoft-flux record in Southeastern China

Strong link between Asian summer monsoon and westerlies for the past 4750 years

Hydroclimate variability over the past 4730 years based on multi-proxy stalagmite records from southwest China