Martina Hollstein

The tropical Indian and Pacific Oceans play a crucial role for global climate due to their extensive coverage across the low latitudes. However, our understanding of the regional sea surface temperature (SST) dynamics during major interglacial periods - when Earth’s climate was warmer than the preindustrial period - remains limited. Here, we compare proxy-based SST reconstructions from the low-latitude Indo-Pacific with Community Earth System Model (CESM) simulations for three key interglacial intervals: the mid-Holocene, Marine Isotope Stage (MIS) 5e, and MIS 11c. Proxy data show an overall warming during MIS 5e and MIS 11c relative to the preindustrial period. Moreover, smaller zonal SST gradients across the equatorial Pacific indicate a weakening of the Pacific Walker circulation. We attribute these findings primarily to extratropical warming and its influence on ocean circulation, particularly the Atlantic Meridional Overturning Circulation and shallow meridional overturning circulation cells. In contrast, CESM simulations indicate lower-than-preindustrial SSTs during MIS 5e and MIS 11c peaks and a larger zonal gradient. We perform individual forcing experiments to disentangle the roles of orbital forcing, greenhouse gas concentration and vegetation cover in shaping SST anomalies. We find that the implementation of paleo-vegetation in CESM reduces the discrepancies between proxy and model data. However, it does not improve the zonal SST gradients, suggesting that model representations of tropical ocean and climate dynamics remain insufficient. Possible causes for these shortcomings are discussed. Our findings underscore the need for refined model physics and improved paleo-proxy integration to better simulate tropical climate behavior during interglacials.

The tropical Indian and Pacific Oceans play a crucial role for global climate due to their extensive coverage across the low latitudes. However, our understanding of the regional sea surface temperature (SST) dynamics during major interglacial periods - when Earth’s climate was warmer than the preindustrial period - remains limited. Here, we compare proxy-based SST reconstructions from the low-latitude Indo-Pacific with Community Earth System Model (CESM) simulations for three key interglacial intervals: the mid-Holocene, Marine Isotope Stage (MIS) 5e, and MIS 11c. Proxy data show an overall warming during MIS 5e and MIS 11c relative to the preindustrial period. Moreover, smaller zonal SST gradients across the equatorial Pacific indicate a weakening of the Pacific Walker circulation. We attribute these findings primarily to extratropical warming and its influence on ocean circulation, particularly the Atlantic Meridional Overturning Circulation and shallow meridional overturning circulation cells. In contrast, CESM simulations indicate lower-than-preindustrial SSTs during MIS 5e and MIS 11c peaks and a larger zonal gradient. We perform individual forcing experiments to disentangle the roles of orbital forcing, greenhouse gas concentration and vegetation cover in shaping SST anomalies. We find that the implementation of paleo-vegetation in CESM reduces the discrepancies between proxy and model data. However, it does not improve the zonal SST gradients, suggesting that model representations of tropical ocean and climate dynamics remain insufficient. Possible causes for these shortcomings are discussed. Our findings underscore the need for refined model physics and improved paleo-proxy integration to better simulate tropical climate behavior during interglacials.

The tropical Indian and Pacific Oceans play a crucial role for global climate due to their extensive coverage across the low latitudes. However, our understanding of the regional sea surface temperature (SST) dynamics during major interglacial periods - when Earth’s climate was warmer than the preindustrial period - remains limited. Here, we compare proxy-based SST reconstructions from the low-latitude Indo-Pacific with Community Earth System Model (CESM) simulations for three key interglacial intervals: the mid-Holocene, Marine Isotope Stage (MIS) 5e, and MIS 11c. Proxy data show an overall warming during MIS 5e and MIS 11c relative to the preindustrial period. Moreover, smaller zonal SST gradients across the equatorial Pacific indicate a weakening of the Pacific Walker circulation. We attribute these findings primarily to extratropical warming and its influence on ocean circulation, particularly the Atlantic Meridional Overturning Circulation and shallow meridional overturning circulation cells. In contrast, CESM simulations indicate lower-than-preindustrial SSTs during MIS 5e and MIS 11c peaks and a larger zonal gradient. We perform individual forcing experiments to disentangle the roles of orbital forcing, greenhouse gas concentration and vegetation cover in shaping SST anomalies. We find that the implementation of paleo-vegetation in CESM reduces the discrepancies between proxy and model data. However, it does not improve the zonal SST gradients, suggesting that model representations of tropical ocean and climate dynamics remain insufficient. Possible causes for these shortcomings are discussed. Our findings underscore the need for refined model physics and improved paleo-proxy integration to better simulate tropical climate behavior during interglacials.

The tropical Indian and Pacific Oceans play a crucial role for global climate due to their extensive coverage across the low latitudes. However, our understanding of the regional sea surface temperature (SST) dynamics during major interglacial periods - when Earth’s climate was warmer than the preindustrial period - remains limited. Here, we compare proxy-based SST reconstructions from the low-latitude Indo-Pacific with Community Earth System Model (CESM) simulations for three key interglacial intervals: the mid-Holocene, Marine Isotope Stage (MIS) 5e, and MIS 11c. Proxy data show an overall warming during MIS 5e and MIS 11c relative to the preindustrial period. Moreover, smaller zonal SST gradients across the equatorial Pacific indicate a weakening of the Pacific Walker circulation. We attribute these findings primarily to extratropical warming and its influence on ocean circulation, particularly the Atlantic Meridional Overturning Circulation and shallow meridional overturning circulation cells. In contrast, CESM simulations indicate lower-than-preindustrial SSTs during MIS 5e and MIS 11c peaks and a larger zonal gradient. We perform individual forcing experiments to disentangle the roles of orbital forcing, greenhouse gas concentration and vegetation cover in shaping SST anomalies. We find that the implementation of paleo-vegetation in CESM reduces the discrepancies between proxy and model data. However, it does not improve the zonal SST gradients, suggesting that model representations of tropical ocean and climate dynamics remain insufficient. Possible causes for these shortcomings are discussed. Our findings underscore the need for refined model physics and improved paleo-proxy integration to better simulate tropical climate behavior during interglacials.

The tropical Indian and Pacific Oceans play a crucial role for global climate due to their extensive coverage across the low latitudes. However, our understanding of the regional sea surface temperature (SST) dynamics during major interglacial periods - when Earth’s climate was warmer than the preindustrial period - remains limited. Here, we compare proxy-based SST reconstructions from the low-latitude Indo-Pacific with Community Earth System Model (CESM) simulations for three key interglacial intervals: the mid-Holocene, Marine Isotope Stage (MIS) 5e, and MIS 11c. Proxy data show an overall warming during MIS 5e and MIS 11c relative to the preindustrial period. Moreover, smaller zonal SST gradients across the equatorial Pacific indicate a weakening of the Pacific Walker circulation. We attribute these findings primarily to extratropical warming and its influence on ocean circulation, particularly the Atlantic Meridional Overturning Circulation and shallow meridional overturning circulation cells. In contrast, CESM simulations indicate lower-than-preindustrial SSTs during MIS 5e and MIS 11c peaks and a larger zonal gradient. We perform individual forcing experiments to disentangle the roles of orbital forcing, greenhouse gas concentration and vegetation cover in shaping SST anomalies. We find that the implementation of paleo-vegetation in CESM reduces the discrepancies between proxy and model data. However, it does not improve the zonal SST gradients, suggesting that model representations of tropical ocean and climate dynamics remain insufficient. Possible causes for these shortcomings are discussed. Our findings underscore the need for refined model physics and improved paleo-proxy integration to better simulate tropical climate behavior during interglacials.

The Australian–Indonesian monsoon (AIM) is the manifestation of seasonal rainfall change and wind reversal over northern Australia and the Maritime Continent, within a region of extreme land-sea fragmentation. Past AIM variability and change have been related to large, covarying shifts in the boundary conditions such as the atmospheric CO2, Northern Hemisphere ice sheets, orbital insolation, and sea-level variations. This review synthesizes what is currently known about AIM evolution and mechanisms, emphasizing the Quaternary where proxy evidence, dating control, and modeling experiments are most abundant. We assess how model hierarchies have been used to separate rainfall and circulation responses, diagnose the role of SST pattern changes and inter-hemispheric energy balance, and show whether shelf exposure should be treated as a first-order monsoon control or a modest boundary condition adjustment. We argue that progress now hinges more on explicit quantification of data uncertainty and proxy-aware evaluation, as well as using isotope-enabled transient simulations and proxy system models that compare models to what proxies record rather than to precipitation alone.

The tropical Indian and Pacific Oceans play a crucial role for global climate due to their extensive coverage across the low latitudes. However, our understanding of the regional sea surface temperature (SST) dynamics during major interglacial periods - when Earth’s climate was warmer than the preindustrial period - remains limited. Here, we compare proxy-based SST reconstructions from the low-latitude Indo-Pacific with Community Earth System Model (CESM) simulations for three key interglacial intervals: the mid-Holocene, Marine Isotope Stage (MIS) 5e, and MIS 11c. Proxy data show an overall warming during MIS 5e and MIS 11c relative to the preindustrial period. Moreover, smaller zonal SST gradients across the equatorial Pacific indicate a weakening of the Pacific Walker circulation. We attribute these findings primarily to extratropical warming and its influence on ocean circulation, particularly the Atlantic Meridional Overturning Circulation and shallow meridional overturning circulation cells. In contrast, CESM simulations indicate lower-than-preindustrial SSTs during MIS 5e and MIS 11c peaks and a larger zonal gradient. We perform individual forcing experiments to disentangle the roles of orbital forcing, greenhouse gas concentration and vegetation cover in shaping SST anomalies. We find that the implementation of paleo-vegetation in CESM reduces the discrepancies between proxy and model data. However, it does not improve the zonal SST gradients, suggesting that model representations of tropical ocean and climate dynamics remain insufficient. Possible causes for these shortcomings are discussed. Our findings underscore the need for refined model physics and improved paleo-proxy integration to better simulate tropical climate behavior during interglacials.

The low-latitude flow of water masses from the Pacific to the Indian Ocean, the Indonesian Throughflow (ITF), is a choke point of the surface ocean return flow of the ocean conveyor belt. Even though the significance of the ITF for the modern global ocean circulation and climate has long been established, little is known about the hemispheric origin of the water masses contributing to its overall transport in the past. Here, we take advantage of the distinctly different isotopic composition of subsurface nitrate in the Northern and Southern Hemisphere source waters to document the admixture of these waters in the ITF through time. Our record of bulk sedimentary δ ¹⁵ N from the Banda Sea, at the heart of the ITF, shows that Southern Hemisphere-sourced subsurface waters contributed significantly to the total ITF transport during the last 800,000 years. Because Southern Ocean processes ultimately set the biogeochemical source signature of the Southern Hemisphere endmember, the Banda Sea record implies an important conduit by which high southern latitude climate and ocean variability is transmitted into the global ocean.

The Indo-Pacific Warm Pool (IPWP) holds the largest warm water body on Earth. With sea surface temperatures (SST) above 28°C, it promotes deep atmospheric convection in the rising limb of the Hadley and Walker circulation cells, and is a major source of heat and moisture to the global atmosphere with far-reaching climate impacts. Spatiotemporal changes in SST influence the location and strength of atmospheric convection and thus the atmospheric circulation. Despite its importance for the global climate, long-term SST variability across the IPWP is not well understood yet. Compilations of proxy-based reconstructions of SST during previous interglacials combined with climate models provide ideal means to study the SST variability in response to varying astronomical forcing. Hitherto, global compilations of interglacial SST anomalies and data-model comparisons have mostly focused on the Holocene and the Last Interglacial (LIG) period. The available studies reveal a striking mismatch between proxy-derived and modelled SST anomalies across low latitudes. However, the data coverage across the low-latitude Indo-Pacific is poor with little to no data from the IPWP. Here, we compare a proxy network of SST anomalies from the low-latitude Indo-Pacific during Holocene, LIG and MIS 11 time slices to the output of CESM 1.2 climate model simulations. We find large discrepancies between the proxy network and CESM output, concerning the magnitude and pattern of SST change including zonal gradients across the tropical Pacific. For instance, proxy data indicate highest SSTs during the LIG, with a slight warming as compared to the preindustrial reference period, while CESM indicates lowest SSTs during the LIG. By performing individual forcing experiments with CESM, we disentangle the roles of astronomical forcing, greenhouse gas concentration and vegetation cover in shaping interglacial tropical SST patterns. In particular, we find that an expanded Northern Hemisphere vegetation cover during interglacials mitigates model-data discrepancies in IPWP temperatures.

Sea surface temperature (SST) across the Coral Sea is tightly coupled to the regional and larger‐scale ocean circulation and climate. Continuous records that reflect past changes in Coral Sea SST in high resolution are missing, however. Here, we present Mg/Ca‐ and alkenone‐based SST reconstructions from the northwestern Coral Sea that cover the past 130 kyr. Our SST estimates vary in line with southern hemisphere high latitude climate variability, linked to atmospheric CO₂. Combining the newly generated with published records, we find that the SST changes across the Coral Sea show a large spatial heterogeneity during the last glacial‐interglacial cycle. Particularly prior to 60 ka, precession exerts a discernible influence on the meridional SST gradients across the Coral Sea, as well as between the Coral Sea and southern Western Pacific Warm Pool. We posit that these changes are linked to the regional trade winds and the South Pacific subtropical gyre circulation, and/or to changes in the El Niño Southern Oscillation.