The last deglacial warming is often attributed to retreating icebergs in the northern high latitudes due to increased summer insolation. However, studies show that Southern Hemisphere and Indo-Pacific Warm Pool (IPWP) sea surface temperature (SST) changes preceded those in the Northern Hemisphere and global ice volume variations, suggesting the Southern Hemisphere could directly induce global warming through its response to orbital forcing. Ocean-atmospheric teleconnections between the tropical Pacific and Southern Ocean significantly influence global climate. The IPWP, a major heat engine, and its thermal linkages with high southern latitudes during the last deglaciation remain unclear. El Niño-Southern Oscillation (ENSO) influences equatorial Pacific heat distribution and global climate teleconnections. Changes in the zonal equatorial temperature gradient across the Pacific are used as ENSO-like variation proxies. However, ENSO evolution since the last glacial epoch is debated, due to differing SST changes in the equatorial Pacific under varying hydrological conditions. Last Glacial Maximum (LGM) IPWP cooling ranges from 2–5°C, while eastern equatorial Pacific (EEP) cold tongue cooling ranges from 0.3–3.5°C. Discrepancies in previous ENSO studies, stemming from core locations distant from the central western Pacific warm pool (WPWP) or in areas impacted by coastal processes, lead to opposing assertions about La Niña-like or El Niño-like conditions during the last glacial period. This study aims to address these ambiguities by integrating published SST data (planktonic foraminiferal Mg/Ca and U37 alkenone index) over the last 30 kyr from the tropical and subtropical Pacific and eastern Indian Ocean. The study quantifies WPWP boundary variations, monitors ENSO-like transitions since the last glacial period, and spatially elucidates the tropical Pacific's thermal linkage to the Southern Ocean and their roles in deglacial warming. This is achieved by combining a new ocean heat content (OHC) record (from core KX22-4 in the WPWP) and published subsurface temperature (subT) records in the Pacific.

Previous research on the last deglaciation has focused on the role of northern high latitude ice sheets and their response to increasing summer insolation. However, evidence suggests that changes in the Southern Hemisphere, particularly in the Indo-Pacific Warm Pool (IPWP), played a significant role in initiating the deglacial warming. Studies have indicated that SST changes in the southern hemisphere preceded those in the northern hemisphere and global ice volume changes, suggesting a potential direct impact of Southern Hemisphere dynamics on global warming. The role of ocean-atmosphere teleconnections between the tropical Pacific and Southern Ocean in shaping global climate patterns has also been established. However, the specific mechanisms and the relative contributions of different regions, including the IPWP as the world's greatest heat engine, remain debated. Studies using geochemical proxies have estimated varying degrees of cooling in the IPWP during the Last Glacial Maximum (LGM), ranging from 2 to 5°C, compared to the Holocene. The changes in ENSO-like behavior throughout the glacial period is also understudied. Previous attempts to reconstruct ENSO-like patterns have yielded conflicting results, partly due to inconsistencies in proxy data and sampling locations. Some studies suggested a predominance of La Niña-like conditions during the glacial period, while others suggested El Niño-like conditions. These discrepancies highlight the need for a comprehensive study that considers both spatial and temporal variability across the IPWP and its connections with other ocean basins.

This study compiled 340 published SST datasets (planktonic foraminiferal Mg/Ca and U37 alkenone index) spanning the last 30 kyr from the tropical and subtropical Pacific Ocean and the eastern Indian Ocean. These data were used to gain a comprehensive understanding of the spatial and temporal variability in the region. The variations in the boundaries of the WPWP, the warmest region within the IPWP, were quantified by analyzing the temporal SST distribution. ENSO-like transitions since the last glacial period were monitored by comparing SST anomaly (SSTA) stacks between the warm pool core area and the EEP cold tongue. To investigate the thermal linkage between the tropical Pacific and the Southern Ocean, a new OHC record was generated (using Mg/Ca ratios of five planktonic foraminiferal species from core KX22-4 in the WPWP) and combined with seven published subsurface temperature (subT) records. The new OHC record from core KX22-4 provides crucial information on the heat content changes in the WPWP, which is a key region in regulating global climate. The integration of SST, subT and OHC data provides a multi-faceted approach to studying the thermal dynamics in the Indo-Pacific region and its link to the Southern Ocean. Spatial analysis of SST data allowed for quantification of the shifts in the WPWP boundaries over time. Temporal analysis of SSTA stacks provided insights into the prevalence of ENSO-like conditions during the glacial period and the deglaciation. The comparison of SST, subT and OHC records helped to elucidate the timing and magnitude of thermal changes in the different ocean layers and to understand the heat exchange processes between the tropical Pacific and the Southern Ocean. The data analysis methods included statistical analysis of SST and SSTA data, as well as the use of polynomial fitting to capture trends in SST and subT data. The use of multiple proxies (Mg/Ca, U37) ensured a more robust and comprehensive understanding of the region's thermal evolution. The study also utilized a transient simulation (TRACE) results which helps in better understanding of the changes.

The study revealed that during the Last Glacial Maximum (LGM), El Niño-like conditions prevailed in the Indo-Pacific Warm Pool (IPWP), characterized by a smaller zonal sea surface temperature (SST) difference between the WPWP and the EEP compared to modern conditions. This asymmetry in cooling is potentially related to inhibited upwelling in the EEP due to a deeper thermocline and reduced Walker circulation. During the deglaciation, a shift to La Niña-like conditions occurred, marked by more intense warming in the WPWP than in the EEP. This resulted in a larger zonal SST contrast. The WPWP reached 28°C around 14 ka during the Bølling-Allerød warm period and remained above 28°C after the Younger Dryas cold event, correlating with the precession minimum. The WPWP OHC also peaked around this time. The intensified deglacial warming in the WPWP might be due to the presence of a barrier layer, which restricts the entrainment of cold thermocline water into the mixed layer, leading to surface heat buildup. Analysis of Mg/Ca-SST and residual seawater δ18O data from core KX22-4 suggests that the barrier layer played a crucial role in the prevalence of La Niña-like conditions during the deglaciation. The study also highlights the oceanic coupling between the IPWP and the Southern Ocean during the deglaciation. Subsurface temperatures (subT) in the eastern WPWP rose earlier than SST, correlating with South Pacific subT and orbital precession. This suggests that heat from the Southern Ocean, transported northward through shallow meridional overturning circulation/subtropical overturning cells, contributed to the warming of the WPWP. The rapid warming in the subsurface layer and reduced vertical temperature gradient in the WPWP facilitated the intense deglacial SST increase and a La Niña-like mode. This enhanced heat transport to the high latitudes, invigorated the Walker circulation, and facilitated the out-gassing of CO2 from the EEP, accelerating global warming. The study found a strong correlation between the rise in WPWP subT, OHC, and ENSO-like variations with the decrease in the precession parameter during the deglaciation. The study points to the importance of considering subsurface thermal dynamics and Southern Ocean influence when modeling and understanding past and future climate changes in the Indo-Pacific region.

The findings of this study significantly advance our understanding of the complex interplay between the IPWP, Southern Ocean, and global climate change during the last deglaciation. The observed shift from El Niño-like conditions during the LGM to La Niña-like conditions during the deglaciation highlights the dynamic nature of the tropical Pacific and its response to changing boundary conditions. The demonstration of the crucial role played by the Southern Ocean in supplying heat to the IPWP offers a new perspective on the mechanisms driving deglacial warming. This emphasizes the importance of incorporating accurate representations of Southern Ocean dynamics in climate models, particularly for projections of future warming. The findings suggest that the thermal coupling between the IPWP and the Southern Ocean is a critical factor that needs to be considered in understanding past climate change and making predictions for the future. The identified correlation between precession cycles and ENSO-like variations points to the importance of long-term orbital forcing in shaping the climate of the Indo-Pacific region. The study also underscores the impact of barrier layers in modulating the surface heat content of the WPWP, which has implications for understanding ENSO variability and its contribution to global climate patterns. These findings suggest the need for more comprehensive climate models that accurately represent the complex interactions and feedbacks within the coupled system.

This study provides compelling evidence for the crucial role of the Indo-Pacific Warm Pool (IPWP) and its thermal coupling with the Southern Ocean in driving deglacial warming. The shift from El Niño-like to La Niña-like conditions, the earlier subsurface warming in the WPWP linked to Southern Ocean heat input, and the influence of orbital precession are major contributions to our understanding of climate dynamics. Future research should focus on improving the resolution of climate models to fully capture the intricate interactions within this system, including better representation of barrier layer dynamics and Southern Ocean heat transport mechanisms. This will enable more accurate predictions of future climate change in the Indo-Pacific region and globally.

The study relies heavily on proxy data, which can have inherent uncertainties and limitations. Although the authors used multiple proxies and a large dataset, variations in proxy resolution and potential biases need to be considered. The spatial coverage of the dataset, while extensive, may not fully capture the complexities of smaller-scale processes or regional variations within the IPWP. The study focuses primarily on oceanic processes and teleconnections, and it would be beneficial to incorporate atmospheric data and models for a more comprehensive understanding of the coupled system. Additionally, the interpretations of ENSO-like conditions based on zonal SST gradients need further validation. Future research incorporating more detailed data and sophisticated modeling techniques could improve our understanding of the dynamic interactions driving climate change in the study region.