Coupled atmosphere-ice-ocean dynamics during Heinrich Stadial 2

Millennial-scale climate fluctuations during the last glacial period, characterized by alternating cold-dry stadials and warm-humid interstadials, are documented in Greenland ice core records as Dansgaard-Oeschger (D-O) oscillations. These oscillations are often linked to major iceberg discharge events, known as Heinrich events, resulting in substantial ice-rafted debris deposition in the North Atlantic. Six such Heinrich events have been identified over the last 60,000 years, significantly impacting marine environments and extending beyond the duration of the corresponding Greenland stadials. Mid-to-low latitude monsoon systems also show marked responses to these events, with weakening of the Asian summer monsoon (ASM) during the Asian Heinrich Periods (AHPs) and strengthening of the South American summer monsoon (SASM) during the South American Heinrich Periods (SAHPs). These shifts are associated with the southward displacement of the Intertropical Convergence Zone (ITCZ). The Atlantic Meridional Overturning Circulation (AMOC) is believed to have weakened or even shut down during Heinrich Stadials (HSs), leading to reduced northward oceanic heat transport, resulting in Greenland cooling and Antarctic warming (the bipolar seesaw). Studies suggest signal propagation from high latitudes to the tropics and then to the southern high latitudes, although the precise phasing and the role of tropical and Southern Hemisphere hydroclimatic variations remain uncertain, especially during the Last Glacial Maximum (LGM) due to the large age uncertainties in Greenland ice core records and a lack of high-resolution, absolutely dated low-latitude proxy records. This study aims to address these limitations by providing high-precision speleothem δ¹⁸O records from monsoon regions to refine the chronology of HS2 and investigate the coupled atmosphere-ice-ocean dynamics during this key millennial-scale event.

Previous research has extensively investigated the dynamics of Dansgaard-Oeschger oscillations and Heinrich events. Studies have highlighted the impact of these events on various climate proxies, including Greenland ice cores, marine sediments, and low-latitude monsoon records. The bipolar seesaw, characterized by anti-phased temperature changes between Greenland and Antarctica, has been a central concept in understanding these events. However, the exact mechanisms driving the bipolar seesaw and the role of low-latitude climate systems remain debated. Several studies have investigated the role of AMOC changes, freshwater forcing, and atmospheric teleconnections in shaping these millennial-scale climate shifts. There's a growing recognition of the influence of low-latitude hydroclimate dynamics, especially concerning the roles of the Amazon River runoff and associated sea-surface salinity anomalies in influencing the AMOC and terminating Heinrich events. However, precise chronological frameworks and high-resolution data, particularly from low latitudes during the LGM, are crucial for a more comprehensive understanding of the coupled atmosphere-ice-ocean dynamics during these events.

This study utilizes nine precisely dated speleothem δ¹⁸O records from the ASM and SASM domains spanning 27-22 ka BP, encompassing HS2. Two records from Cherrapunji Cave (Northeast India) were used to construct a composite record with high temporal resolution (~4 years) and precision (<50 years), achieved by combining annual lamina counting and ²³⁰Th dating. The age model was refined by comparing it with the Greenland dust flux record (reflected by ice-core [Ca²⁺]). Six additional speleothem δ¹⁸O records from caves across India, China, and Brazil were included. Precise ²³⁰Th dates and StalAge algorithm were used for age modeling of these speleothems. A total of ~2810 stable oxygen isotope data were obtained, processed using various age modeling and change point detection methods such as Ramp-fitting and BREAKFIT to determine key transition timings within the records. Greenland ice-core [Ca²⁺] records were used as a proxy for mid-latitude westerly winds and hydroclimate conditions in Asian dust source regions. The correlation between Greenland ice-core [Ca²⁺] and the Cherrapunji δ¹⁸O record provided the basis for adjusting the GICC05 timescale. Similarly, the Antarctic ice-core chronology was refined by correlating with volcanic events and radiocarbon ages. The improved chronologies allowed for detailed comparisons and analyses of phase relationships between various proxy records from both hemispheres, including monsoon records, ice-core δ¹⁸O and CH₄ records, and marine records. Statistical methods were employed to quantify uncertainties and test the robustness of the results.

The study reveals several key findings. Firstly, the Greenland and Antarctic ice-core chronologies were refined using high-precision speleothem data, suggesting adjustments of +320 and +400 years respectively. These adjustments are supported by volcanic evidence and radiocarbon ages. A synchronous onset of HS2 is observed globally. The high-resolution speleothem records clearly show a centennial-scale “tropical atmospheric seesaw”, characterized by synchronous but anti-phase changes in monsoon hydroclimate in the two hemispheres during the early stage of HS2. This phenomenon is superimposed on the conventional bipolar seesaw, highlighting the unique influence of low-latitude hydroclimate. The study also observed a much earlier shift in the South American monsoon at the termination of HS2 compared to the Asian monsoon, suggesting a possible lead role of low-latitude hydroclimate changes in the termination dynamics. The abrupt nature of the “tropical atmospheric seesaw” is especially evident in a “large excursion” event within stage III, marked by a rapid oxygen isotope shift within decades, as indicated by lamina counting. This implies the presence of a tipping point in the tropical climate system. Analysis of the timing of change points between various records reveals sub-centennial level synchronicity in high- and low-latitude atmospheric changes. It also indicated the importance of the AMOC strength in the coupled atmospheric changes. Finally, the study shows that reduced freshwater input from the Amazon River during the HS2 termination, potentially induced by long-term drying in the SASM domain, likely contributed to the AMOC resumption, eventually leading to warming in the North Atlantic region and triggering the eventual termination of AHP2. This emphasizes the active roles of both atmospheric and oceanic processes in the tropics and Southern Hemisphere during millennial-scale event terminations.

The findings significantly advance our understanding of millennial-scale climate events. The refined chronologies substantially improve the precision of inter-hemispheric comparisons, reducing uncertainties in the timing of key climate shifts. The identification of the “tropical atmospheric seesaw” challenges the traditional bipolar seesaw concept by highlighting the active role of tropical hydroclimate dynamics. The observed lead-lag relationship between the South American and Asian monsoons during HS2 termination emphasizes the importance of low-latitude processes in driving the transitions out of glacial periods. The results suggest that the tropical atmospheric system amplifies disturbances from higher latitudes and plays an important role in triggering rapid climate shifts. These findings call for a more integrated framework incorporating both high and low latitude dynamics and the varying boundary conditions when modeling abrupt climate changes like Heinrich events. The findings have major implications for improving climate models that aim to simulate rapid climate changes, particularly those related to the AMOC.

This study provides a significantly improved chronological framework for Heinrich Stadial 2 and highlights the active role of low-latitude hydroclimate in the coupled atmosphere-ice-ocean system. The discovery of the “tropical atmospheric seesaw” challenges conventional views of interhemispheric climate coupling and calls for a more comprehensive understanding of tropical-extratropical interactions. Future research should focus on further refining the chronologies of other Heinrich events and using advanced modeling techniques to simulate the observed tropical atmospheric seesaw, exploring its mechanisms and potential implications for future climate change.

While this study greatly improves chronological precision, the age models are still subject to uncertainties associated with both the speleothem and ice-core dating methods. The interpretation of speleothem δ¹⁸O records relies on assumptions about the relationship between isotopic composition and monsoon intensity, and additional research could help refine these interpretations. The study primarily focuses on HS2, and further research is needed to extend these findings to other Heinrich events and investigate potential regional variations in response patterns. Additionally, while the study suggests a causal link between South American monsoon drying and AMOC resumption, further research is needed to fully confirm this relationship and quantify the magnitude of the effect.