Sea Surface Temperature Reconstruction Research Articles

A Modeling Perspective on the Lingering Glacial Sea Surface Temperature Conundrum

AbstractThe strong cooling during the Last Glacial Maximum (LGM, 21 ka BP) provides a rigorous test of climate models' ability to simulate past and future climate changes. We force an atmospheric general circulation model with two recent global LGM sea surface temperature (SST) reconstructions, one suggesting a weak and the other a more pronounced cooling, and compare the simulated land surface temperatures (LSTs) to reconstructed data. Our results do not confirm either SST reconstruction. The cold SST data set leads to good agreement between simulated and observed LSTs at low latitudes, but is systematically too cold at mid‐latitudes. The opposite is true for the warm SST data set. Differences between the simulated LSTs are caused by varying land surface albedos, which is lower for the warmer SST reconstruction. The inconsistency between reconstructed and simulated climate points to a potentially significant bias in the proxy reconstructions and/or the climate sensitivity of current climate models.

New Southern Ocean transfer function for subsurface temperature prediction using radiolarian assemblages

Radiolarians are microzooplankton that produce siliceous shells that preserve well in sediments and allow for paleo-reconstructions. Previous studies have used them for sea surface temperature (SST) reconstructions. However, radiolarians peak in abundances between 100 and 400 m in the Southern Ocean (SO), suggesting that their assemblages are more representative of subsurface conditions. Here, we aim to develop a SO-wide transfer function (TF) for subsurface temperature reconstructions (subST) using the Southern Ocean RAdiolarian Dataset (SORAD), a compilation of data for about 240 radiolarian taxa in 228 surface sediment samples from the SO. This exhaustive dataset has been simplified using common TF criteria to minimize noise and optimize SORAD for subST prediction. Ordination tests and Q-mode Factor Analyses (QFA) were applied to the resulting training dataset, which includes 212 samples and 75 taxa (SORAD212_75). The results suggest that, out of six environmental variables, radiolarian assemblages in SORAD are mainly driven by summer temperatures at 200 m and that the first five factors of the QFA explained over 75% of SORAD212_75 variance. We applied common TF methods, IKM, MAT and Weighted-MAT (WMAT), to two data transformations (relative abundances of species and log-transformed). The six models of modern temperature predictions all show excellent performance over the −2 to 18 °C interval. To compare with previously published results generated with the same method, IKM% was applied to radiolarian census data of three cores from the Atlantic, Indian and Pacific sectors of the SO, thus calculating new subST reconstructions and testing the new TF performance.

Colpophyllia natans From Tobago, a Novel Paleoclimate Archive for Reconstructing Sea Surface Temperature in the Tropical Atlantic

AbstractMassive, long‐lived Siderastrea and Diploria corals are species commonly used for sea surface temperature (SST) reconstructions in the North Atlantic. However, they are rarely found to exceed 200 years in age. Thus, it is imperative to continuously develop alternative taxa for paleoreconstructions. Colpophyllia natans, a highly populous tropical North Atlantic coral, are known to grow large colonies, potentially containing environmental records spanning several hundreds of years. However, its low density and complicated architecture poses a challenge in extracting climate signals from this coral. This study presents the first monthly‐resolved climate calibration of Colpophyllia natans and validates its utility as a new paleoarchive, relative to Siderastrea siderea. Linear regressions of monthly and interannual coral Sr/Ca with instrumental SST reveal robust, significant relationships (p < 0.05), indicating that microsampling along a single thecal wall of C. natans allowed for robust climate reconstructions. Additionally, both corals capture similar SST variations (t‐test, p ≥ 0.05), which allowed for the generation of a single, composite interspecies SST record that correlates with instrumental SST even more strongly (p < 0.0001) than the individual corals. Mean annual and boreal summer interspecies SST correlate significantly with North Atlantic SST indices (p < 0.05), demonstrating the ability to capture regional, long‐term SST trends in the North Atlantic. Spatial correlation maps of boreal winter interspecies SST to instrumental SST and geopotential height anomalies reveal coherent spatial patterns linked to the North Atlantic Oscillation. Our findings suggest that Colpophyllia natans has enormous potential as a new paleoclimate archive for reconstructing temporal and spatial SST variability in the tropical Atlantic.

Impact of oceanic gateway and CO2 changes on the East Asian summer monsoon during the mid-pliocene in a coupled general circulation model

The Pliocene epoch from about 5.3 million to about 2.6 million years before present is the most recent period of sustained global warmth similar to the near future projections. The restriction of the Indonesian Passages, the closure of the Panama Seaway and declining atmospheric CO 2 -concentration are suggested to have caused the global climate evolution to the present-day condition. Here, we present the Pliocene sea surface temperature (SST) reconstructions along with sensitivity experimental results from a coupled General Circulation Model. We find that, in terms of SST, simulated model sensitivity to CO 2 is in good agreement with the Pliocene reconstructions in most regions except the North Atlantic and Arctic. This suggests the necessity for improved boundary conditions and a possible underestimation of internal climate feedback at the high-latitudes. The responses of East Asian Summer Monsoon (EASM) to the oceanic gateway and CO 2 are investigated. Influences of declining CO 2 on the EASM are more prominent. An intensification of the intensity of EASM by ∼50% is simulated in response to the declining CO 2 , which is largely attributed to the strengthened land-ocean thermal contrast, while the precipitation decreases by ∼4.8%. In contrast, the restriction of two seaway changes only drives relatively weak changes with respect to wind speed and precipitation. A water vapor budget analysis suggests that the reduced atmospheric moisture content due to decreasing CO 2 significantly contributes to precipitation response.

Empirical evidence for multidecadal scale global atmospheric electric circuit modulation by the El Niño-Southern Oscillation

The El Niño-Southern Oscillation (ENSO) modifies precipitation patterns across the planet. Charge separation in disturbed weather and thunderstorms drives the global atmospheric electric circuit (GEC), hence ENSO-induced precipitation changes are anticipated to affect the global circuit. By analysing historical atmospheric electricity data using a new data processing procedure based on the Carnegie curve, signals correlated with ENSO sea surface temperature (SST) anomalies are revealed. These demonstrate a persistent ENSO-GEC relationship for the majority of the twentieth century, in potential gradient data from Lerwick, Shetland and Watheroo, W. Australia. The recovered data is weighted towards the first half of the UTC day, giving a GEC sensitivity up to ∼5% °C−1 of SST anomaly in the Niño 3.4 and 4 regions of the Pacific Ocean. Transferring ENSO variability by electrical means represents an unexplored teleconnection, for example, through proposed GEC effects on stratiform cloud microphysics. The strong ENSO-GEC relationship also provides a quality test for historical atmospheric electricity data, and encourages their use in reducing SST reconstruction uncertainties.

A Pliocene Precipitation Isotope Proxy-Model Comparison Assessing the Hydrological Fingerprints of Sea Surface Temperature Gradients.

The Pliocene offers insights into future climate, with near-modern atmospheric pCO2 and global mean surface temperature estimated to be 3-4°C above pre-industrial. However, the hydrological response differs between future global warming and early Pliocene climate model simulations. This discrepancy results from the use of reduced meridional and zonal sea surface temperature (SST) gradients, based on foraminiferal Mg/Ca and Alkenone proxy evidence, to force the early Pliocene simulation. Subsequent, SST reconstructions based on the organic proxy TEX86, have found warmer temperatures in the warm pool, bringing the magnitude of the gradient reductions into dispute. We design an independent test of Pliocene SST scenarios and their hydrological cycle "fingerprints." We use an isotope-enabled General Circulation Model, iCAM5, to model the distribution of water isotopes in precipitation in response to four climatological SST and sea-ice fields representing modern, abrupt 4×CO2, late Pliocene and early Pliocene climates. We conduct a proxy-model comparison with all the available precipitation isotope proxy data, and we identify target regions that carry precipitation isotopic fingerprints of SST gradients as priorities for additional proxy reconstructions. We identify two regions with distinct precipitation isotope (D/H) fingerprints resulting from reduced SST gradients: the Maritime Continent (D-enriched due to reduced convective rainfall) and the Sahel (wetter, more deep convection, D-depleted). The proxy-model comparison using available plant wax reconstructions, mostly from Africa, is promising but inconclusive. Additional proxy reconstructions are needed in both target regions and in much of the world for significant tests of SST scenarios and dynamical linkages to the hydrological cycle.

Evaluation of the distributions of hydroxylated glycerol dibiphytanyl glycerol tetraethers (GDGTs) in Holocene Baltic Sea sediments for reconstruction of sea surface temperature: the effect of changing salinity

Abstract. Hydroxylated glycerol dibiphytanyl glycerol tetraethers (OH-GDGTs) produced by both marine and freshwater thaumarchaea are increasingly used for the reconstruction of past sea surface temperature (SST). They occur throughout the modern Baltic Sea, but it is unknown if OH-GDGTs can be used for assessing past SST in this area, where salinity has changed considerably over the Holocene. Three commonly applied OH-GDGT proxies for SST reconstruction, i.e., the OH-GDGT%, RI-OH, and RI-OH′ indices, were tested using a thaumarchaeotal culture enriched from the Baltic Sea grown at 4 and 22 ∘C, as well as 12 surface sediments from the Baltic Sea and the adjacent Skagerrak. In the culture experiments all three proxies showed the expected response with the rise in temperature, but their absolute values were not always in line with existing marine core-top calibrations, especially for the OH-GDGT% index. Of the two proxies based on the distribution of OH-GDGTs, the RI-OH index shows no increase with increasing mean annual SST, whilst the RI-OH′ index shows only a linear correlation with SST when the sediments from the Bothnian Sea and Bothnian Bay area, with a reduced salinity and increased lateral sediment influx, are omitted from the dataset. Two sedimentary Holocene records from the Arkona and Gotland basins were studied, the latter in high resolution. In the brackish phase of the Baltic Sea (the Littorina Sea stage), the RI-OH′ index shows a good correlation with the TEX86L, an established temperature proxy in the Baltic Sea, and can be used to identify important climatic events. However, during the preceding Ancylus Lake phase the RI-OH′ (and RI-OH) index records values that are far too high, resulting in anomalously high SST estimates. This is probably because freshwater thaumarchaea adjust their OH-GDGTs differently, as has been shown for thaumarchaea in Lake Lugano's water column. In the Littorina Sea, Ancylus Lake, and Yoldia Lake phases of the Baltic basin, the record of the RI-OH′ index thus most likely reflects changes in both temperature and salinity. Overall, our study indicates that a reduced salinity increases the values of the RI-OH′ (and RI-OH) indices substantially, and this should be considered when applying these proxies in other settings.

Linking North Atlantic and Alpine Last Glacial Maximum climates via a high-resolution pollen-based subarctic forest steppe record

During the last glaciation the Eurasian continent was impacted by the global reorganization of atmospheric circulation caused by North Atlantic events, through changes in moisture storm tracks modulated by orography. Our research explores the linkages between climate evolution in the North Atlantic and in the Alps between 30.6 and 12.5 ka, This period, centered on the Last Glacial Maximum, experienced the most recent phase of maximum global ice volume, corresponding to a millennia-long phase of sea-level lowstand, minima in reconstructed sea surface temperatures and isotopic values from polar ice. We present a high-resolution multiproxy record from Lake Fimon at the southern Alpine foothills, where a larch forest-steppe persisted throughout the coldest spells. We analyse the fine climate structure of the MIS 3–2 transition, of the Alpine Last Glacial Maximum stepped by HS2, and provide an unprecedented fine resolution and accurate 14C dating of the Last Glacial Maximum and the early Alpine Lateglacial. We reconstructed quantitative climate parameters based on sensitive pollen descriptors, and derived co-registered sedimentary proxies for dust flux. Reconstructed series suggest that extreme continental spells in the Alps and cold boreal winters match the timing of Heinrich Stadials 2 and 1, episodes of iceberg discharge and release of rock fragments (Ice Rafted Debris) in the North Atlantic. The initiation of the LGM at 27.3 ka is marked by a 4 °C drop in July temperatures, likely responsible for the onset of glacial build-up in the Southern Alpine piedmont. The first extreme continental spell coincides with the British-Irish-sourced IRD event at 26.2 ka and with glacier retreat in the Southern Alps, persisting in the subsequent HS2. Late Glacial glacier collapse and extreme drought occurred during the first half of the HS1, 17.4–16.2 ka. The comparison of the results obtained at Lake Fimon with other terrestrial Eurasian records and with recalibrated marine chronologies from the North Atlantic suggests in-phase reactions of the British-Irish Ice Sheet and of Alpine glaciers and ecosystems to changes in the Atlantic Meridional Circulation. Asymmetric ecoclimatic effects produced by North Atlantic events at the windward and leeward sides of the Alps confirm a southern provenance of moist air masses throughout the LGM.

Frequency of different types of El Niño events under global warming

AbstractThe El Niño–Southern Oscillation (ENSO) is the dominant natural climate pattern that influences the global climate on subdecadal timescales. Thus, a better understanding of the impacts of global warming on the characteristics of ENSO events offers large socioeconomic benefits. Changes in the frequency of the eastern Pacific (EP), central Pacific (CP) and total El Niño events are analysed in the future period (2051–2100) under the two shared socioeconomic pathways (SSPs) scenarios (i.e., SSP2‐4.5 and SSP5‐8.5) compared to the historical period (1951–2000) using outputs of eight models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). Types of El Niño events are diagnosed based on pattern correlation coefficients (PCCs) between monthly sea surface temperature (SST) anomalies and the first two leading empirical orthogonal function (EOF) modes of SST anomalies in the tropical Pacific. Based on the ensemble of models, the number of total El Niño events decreases by 26 and 16% under the SSP2‐4.5 and SSP5‐8.5 scenarios, respectively, in the future compared to the historical period. A smaller decrease in the number of total El Niño events under the faster warming rate of the SSP5‐8.5 scenario suggests that the response of ENSO dynamics to global warming is not linear. Analysis of the Extended Reconstructed Sea Surface Temperature, version 5 (ERSSTv5) dataset during the period 1951–2020 indicates that the ratio of the number of CP El Niño to EP El Niño has substantially increased over the past two decades. Nevertheless, under both the SSP2‐4.5 and SSP5‐8.5 scenarios in the future period, the ratio of the number of CP El Niño to EP El Niño does not change considerably compared to that in the historical period. This suggests that the recent increase in the frequency of CP El Niño could be a result of multidecadal variations rather than anthropogenic global warming.

Distribution characteristics of terrestrial and marine lipid biomarkers in surface sediment and their implication for the provenance and palaeoceanographic application in the northern South China Sea

Multi biolipids, including plant wax, algal biomarkers, and glycerol dialkyl glycerol tetraethers (GDGTs), were studied in surface sediments of the northern South China Sea (SCS). Unusual elevated contents of odd n-alkanes (C27, C29, C31) and even n-alkanols (C22, C24, C26) derived from terrestrial plant wax in the outer shelf areas indicated the strong influence of deep-water counter current in redistributing and deposition of Taiwan Island sourced organic matter (OM) in the northern SCS. The spatial distribution of dinosterol and brassicasterol (corresponding to dinoflagellates and diatoms of high competitive advantage for nutrients), respectively, displayed a high abundance near the Pearl River Estuary (PRE), southwest off Taiwan Island, and southeast off Hainan Island, in contrast to the relatively high biomass of C37-alkenones, C30-diol/ketol and isoprenoid GDGTs (iGDGTs) in the outer shelf oligotrophic areas, indicating their feasibility in marine productivity reconstruction. The observed high contents of alkenones than brassicasterol in the sediment, albeit the overall dominance of diatoms in the SCS surface water, showed differences in the degradation and/or preservation efficiency of different organic lipids in sediments. The influences of season, source organisms, and different regression methods on the sea surface temperatures (SST) reconstruction were also evaluated in the region extending from shelf to deep water. The reconstructed SSTs based on U37K' mainly reflected spring SSTs, indicating the influence of growing seasons on the source organism. The reconstructed TEX86 SST corresponded more with the mean annual SST in the northern SCS, mainly in the deep-water regions. Changes in the total composition of the archaeal community and population is suggested to be the major factor causing the unusually low and wide range of TEX86 SSTs in the nearshore areas. The combination of the two organic paleothermometers offers complementary information on SST developments in the northern SCS. Multi-biomarkers study in the present work indicates the feasibility of lipids in paleoecology and paleoclimate reconstruction. However, care must be taken in palaeoceanographic reconstruction due to complex hydrological and biological influence, i.e., the deep lateral current on the continental matter transportation, the differential degradation of various organic lipids, and the different seasonal and/or source organism community bias on the organic thermometer in the study area.

Geochemical significance of Acropora death assemblages in the northern South China Sea: Implications for environmental reconstruction using branching corals

The geochemistry preserved in coral skeletons provides access to pre-instrumental records of environmental changes. While a variety of proxies have been established for coral paleoclimatology, their applications to the use of Acropora to generate longer-term reconstructions have been studied less. Here, we examine the geochemical proxies (i.e., Sr/Ca, Li/Mg, δ18O, δ13C, δ11B, and B/Ca) of dead Acropora assemblages collected from a fringing reef off Hainan Island in the northern South China Sea. These samples have been precisely dated using UTh isotopes and record reef development episodes since the mid-Holocene, allowing us to assess their potential as paleoclimate archives. The sea surface temperature (SST) trend reconstructed by Sr/Ca and Li/Mg exhibits better consistency with each other, and they have recorded the SST shifts around 5–4 ka BP and the subdued variability during the Medieval Climate Anomaly (MCA), whereas the δ18O-SST record exhibits less clear variations over the past 7000 years. However, the universal Li/Mg-SST calibration tends to underestimate the SST reconstruction from tropical corals, highlighting the importance of using a site- and species-specific calibration of the Li/Mg-SST. Boron systematics are used to reconstruct the carbonate chemistry of coral calcifying fluid (CF), which reveals significant differences between the ancient and modern corals. The pH of the coral CF (pHcf) is significantly lower in the modern Acropora compared to the ancient corals, with a mean difference of ~0.08 pH, corroborating the pronounced influence of ocean acidification on the coral CF chemistry. The dissolved inorganic carbon of the coral CF (DICcf) is also lower for modern Acropora, and this decreasing trend seems to have persisted over the past 7000 years. In addition, the skeletal δ13C is closely related to the CF carbonate chemistry, highlighting the intrinsic relationship between the coral internal carbon pool used for calcification and the up-regulation of the pHcf.

A new global surface temperature reconstruction for the Last Glacial Maximum

Abstract. We present a new reconstruction of surface air temperature and sea surface temperature for the Last Glacial Maximum. The method blends model fields and sparse proxy-based point estimates through a data assimilation approach. Our reconstruction updates that of Annan and Hargreaves (2013), using the full range of general circulation model (GCM) simulations which contributed to three generations of the PMIP database, three major compilations of gridded sea surface temperature (SST) and surface air temperature (SAT) estimates from proxy data, and an improved methodology based on an ensemble Kalman filter. Our reconstruction has a global annual mean surface air temperature anomaly of -4.5±0.9 ∘C relative to the pre-industrial climate. This is slightly colder than the previous estimate of Annan and Hargreaves (2013), with an upwards revision on the uncertainty due to different methodological assumptions. It is, however, substantially less cold than the recent reconstruction of Tierney et al. (2020). We show that the main reason for this discrepancy is in the choice of prior. We recommend the use of the multi-model ensemble of opportunity as potentially offering a credible prior, but it is important that the range of models included in the PMIP ensembles represent the main sources of uncertainty as realistically and comprehensively as practicable if they are to be used in this way.

Millennial-scale hydroclimate changes in Indian monsoon realm during the last deglaciation

In the Indian monsoon realm, spatial homogeneity of hydroclimate response to glacial abrupt climate events in the North Atlantic Ocean and its underlying mechanism are contentious. Here we reconstruct sea surface temperature (SST) and seawater oxygen isotope (δ18Osw) in the central Andaman Sea, which span over the past 26,000 years, to investigate the potential driving mechanism of millennial-scale climate changes in tropical Indian Ocean. Our results demonstrate that millennial-scale SST and δ18Osw changes during the last deglaciation are associated with changes in Asian monsoon system which follows North Atlantic climate changes. In particular, during the transition from BØlling/AllerØd (B/A) to Younger Dryas (YD), the strengthening of East Asian winter monsoon (EAWM) promotes southward invasion of northern cold air masses to Indo-China Peninsula, cooling the Andaman Sea. Meanwhile, the enriching δ18Osw is a consequence of reducing local rainfall amount and Irrawaddy River inflow due to a weakening Indian summer monsoon (ISM). With aid of an isotope-enabled climate model, our results further demonstrate that the changes in EAWM and ISM during B/A-YD transition are closely coupled with variations of Atlantic meridional overturning circulation (AMOC), corroborating its pivotal role in millennial-scale changes in tropical Indian Ocean hydroclimate during the last deglaciation.

Using paleoecological data to inform decision making: A deep-time perspective

Latest climate models project conditions for the end of this century that are generally outside of the human experience. These future conditions affect the resilience and sustainability of ecosystems, alter biogeographic zones, and impact biodiversity. Deep-time records of paleoclimate provide insight into the climate system over millions of years and provide examples of conditions very different from the present day, and in some cases similar to model projections for the future. In addition, the deep-time paleoecologic and sedimentologic archives provide insight into how species and habitats responded to past climate conditions. Thus, paleoclimatology provides essential context for the scientific understanding of climate change needed to inform resource management policy decisions. The Pliocene Epoch (5.3–2.6 Ma) is the most recent deep-time interval with relevance to future global warming. Analysis of marine sediments using a combination of paleoecology, biomarkers, and geochemistry indicates a global mean annual temperature for the Late Pliocene (3.6–2.6 Ma) ∼3°C warmer than the preindustrial. However, the inability of state-of-the-art climate models to capture some key regional features of Pliocene warming implies future projections using these same models may not span the full range of plausible future climate conditions. We use the Late Pliocene as one example of a deep-time interval relevant to management of biodiversity and ecosystems in a changing world. Pliocene reconstructed sea surface temperatures are used to drive a marine ecosystem model for the North Atlantic Ocean. Given that boundary conditions for the Late Pliocene are roughly analogous to present day, driving the marine ecosystem model with Late Pliocene paleoenvironmental conditions allows policymakers to consider a future ocean state and associated fisheries impacts independent of climate models, informed directly by paleoclimate information.

Palaeoclimate dynamics within the Summer Rainfall Zone of South Africa

South Africa has a particularly spatially heterogeneous climate driven by a complex set of ocean-atmospheric factors. This results in three distinct regions of rainfall seasonality: the summer- (SRZ), winter- (WRZ) and year-round (YRZ) rainfall zones. Even within these zones there is considerable spatio-temporal variability in precipitation patterns, especially in the SRZ which covers the largest terrestrial area. Over the past several thousand years the position of the boundaries of the southern extent of the SRZ have shifted due to the influences of major climate systems, as well as changes in the characteristics of the adjacent oceanic currents. Here we use the Modern Analogue Technique on a new synthesis of pollen records from across the SRZ to quantitatively reconstruct four climate variables (Mean Temperature of the Coldest month, Mean Temperature of the Warmest Month, Mean Summer Precipitation and Winter Climatic Moisture Index), over the past 36,000 years. The climate variables and number of analogues used were selected after thorough examination of the modern dataset. The results provide evidence for three key features: first, an early deglaciation following a cold, dry and prolonged Last Glacial Maximum. Second, a clear manifestation of the African Humid Period in the north-eastern part of the zone, supported by reconstructed sea surface temperatures. And third, a short-lived cold period at around 10 cal kyr BP, which may have been obscured by larger statistical errors in past studies. As it is still mostly within the margin of error for our study, this period requires further study. Our results show that the technique can be used successfully on large and diverse compilations of South African records, improving comparability with other regions.

A 334-year coral record of surface temperature and salinity variability in the greater Agulhas Current region

Abstract. The Agulhas Current (AC) off the southern tip of Africa is one of the strongest western boundary currents and a crucial choke point of inter-ocean heat and salt exchange between the Indian Ocean and the southern Atlantic Ocean. However, large uncertainties remain concerning the sea surface temperature (SST) and salinity (SSS) variability in the AC region and their driving mechanisms over longer timescales, due to only short observational datasets being available and the highly dynamic nature of the region. Here, we present an annual coral skeletal Sr/Ca composite record paired with an established composite oxygen isotope record from Ifaty and Tulear reefs in southwestern Madagascar to obtain a 334-year (1661–1995) reconstruction of δ18Oseawater changes related to surface salinity variability in the wider Agulhas Current region. Our new annual δ18Oseawater composite record from Ifaty traces surface salinity of the southern Mozambique Channel and AC core region from the SODA reanalysis between 1958 and 1995. δ18Oseawater appears to be mainly driven by large-scale wind forcing in the southern Indian Ocean on interannual to decadal timescales. The δ18Oseawater and SST at Ifaty show characteristic interannual variability of between 2 and 4 years and interdecadal variability of 8 to 16 years, coherent with El Niño–Southern Oscillation (ENSO) records. Lagged correlations with the multivariate ENSO index reveals a 1–2-year lag of δ18Oseawater and salinity at Ifaty and the AC region, suggesting that propagation of anomalies by ocean Rossby waves may contribute to salinity changes in the wider southwestern Indian Ocean. The δ18Oseawater and SST reconstructions at Ifaty reveal the highest interannual variability during the Little Ice Age, especially around 1700 CE, which is in agreement with other Indo-Pacific coral studies. Our study demonstrates the huge potential to unlock past interannual and decadal changes in surface ocean hydrology and ocean transport dynamics from coral δ18Oseawater beyond the short instrumental record.

Coral Sr/Ca records provide realistic representation of eastern Indian Ocean cooling during extreme positive Indian Ocean Dipole events

Extreme positive Indian Ocean Dipole (pIOD) events are amplified by non-linear ocean–atmosphere interactions and are characterized by pronounced cooling in the eastern equatorial Indian Ocean. These non-linear feedbacks are not adequately represented in historical products of sea surface temperatures that underestimate the magnitude of extreme pIOD events. Here, we present a sea surface temperature (SST) reconstruction based on monthly coral Sr/Ca ratios measured in two coral cores from Enggano Island (Indonesia), that lies in the eastern pole of the IOD. The coral SST reconstruction extends from 1930 to 2008 and captures the magnitude of cooling during extreme pIOD events as shown in recent satellite and reanalysis data of SST that include ocean dynamics. The corals indicate that the 1961 pIOD event was at least as severe as the 1997 event, while the 1963 pIOD was more comparable to the 2006 event. The magnitude 1967 pIOD is difficult to assess at present due to poor replication between coral cores, and may be comparable to either 1997 or 2006. Cooling during the 1972 pIOD was short-lived and followed by pronounced warming, as seen in the moderate pIOD event of 1982. A combination of coral SST reconstructions and an extension of new reanalysis products of SST to historical time scales could help to better assess the severity and impact of past pIOD events such as the ones seen in the 1960s.

Coupled Coccolith-Based Temperature and Productivity High-Resolution Reconstructions in the Eastern Equatorial Pacific During the Last Deglaciation and the Holocene

We present a new high-resolution reconstruction of annual sea-surface temperatures (SSTa) and net primary productivity (NPP) using novel coccolithophore-based models developed for the Eastern Equatorial Pacific (EEP). We combined published coccolithophore census counts from core-tops in the Eastern Pacific with 32 new samples from the Equatorial region, to derive a new statistical model to reconstruct SSTa. Results show that the addition of the new EEP samples improves existing coccolithophore-based SST-calibrations, and allow reconstructing SSTa in the EEP with higher confidence. We also merged the relative abundance of deep-photic species Florisphaera profunda in the same surface sediment samples with existing calibration datasets for tropical regions, to reconstruct annual NPP. Both temperature and productivity calibrations were successfully applied to fossil coccolith data from Ocean Drilling Project Site 1240, in the EEP. The coccolith-based SSTa estimates show a cooling during the Last Glacial Maximum (LGM) and the Younger Dryas, and warming at the start of the Holocene. This pattern differs in the timing and magnitude of the temperature changes from other available SST-reconstructions based on biogeochemical and faunal proxies. We discuss these discrepancies to be the result of different proxy sensitivities to insolation forcing, seasonal bias, and/or preservation artifacts. Reconstructed annual NPP shows a general decreasing trend from the late last glacial period to recent times, which we relate to the weakening of wind-driven equatorial upwelling towards the Holocene. We also calculated carbon export using our SSTa and NPP reconstructions, and compared to other geochemical-based reconstructions for the same location. Our coupled SSTa-NPP reconstruction provides key data to more fully assess the evolution of primary and export productivity as well as organic carbon burial in the EEP, with implications for its role in global biogeochemical cycles across glacial terminations.

A User Guide for Choosing Planktic Foraminiferal Mg/Ca‐Temperature Calibrations

AbstractAlthough foraminiferal magnesium/calcium (Mg/Ca) measurements are now widely used for reconstructing sea surface temperature (SST), there is uncertainty about the fidelity of different calibrations of this proxy. Whereas single‐variable calibrations suggest temperature sensitivity of about 9–10% per °C, multivariable calibrations suggest ∼6% per °C, with additional dependence on salinity and pH. Here, we apply five calibrations to six published Mg/Ca records of Globigerinoides ruber, a planktic foraminifer typically used for reconstructing low latitude SST during the Pleistocene. Reconstructed SST by the different calibrations, spanning the past 250,000 years, can be offset by a few degrees, possibly reflecting variable dissolution or hydrographic effects. However, for 4 out of 5 calibrations, the reconstructed temperature anomalies yield estimates that are consistent within the calibrations' uncertainty (<±1 °C), despite the fundamental differences in temperature sensitivity among the equations. We further propose a new seawater Mg/Ca record for the late Neogene and show that the same consistency holds for longer time scales (∼4 My) independently of the choice of the calibration or which seawater Mg/Ca record is used. These comparisons attest to the robustness of the calibrations despite all the confounding nonthermal effects, and offer an empirical basis for researchers and reviewers to judge the records without any prejudice about which calibration is the “best” and evaluate their uncertainties.

A data-model perspective on the Brazilian margin surface warming from the Last Glacial Maximum to the Holocene

The western South Atlantic along the Brazilian margin is an important region for the Atlantic Meridional Overturning Circulation (AMOC) because surface currents in this area transfer warm and salty waters from the Southern Hemisphere to the North Atlantic. Although the number of sea surface temperature (SST) reconstructions has grown in this region, it has been challenging to explain changes in different and sometimes proximal cores. To understand the SST evolution of the Brazilian margin from the Last Glacial Maximum (LGM) to the late Holocene, we present the first SST stack (BRSST stack) for this region. We compare the BRSST stack with the outputs of the transient climate model simulation TraCE-21ka. The BRSST stack shows an LGM cooling of 1.43 °C (from −1.31 to −1.55 °C, 2σ) relative to the late Holocene, followed by deglacial warming starting at ∼ 18.8 ka. TraCE-21ka simulates this early onset of the last deglaciation. Sensitivity experiments suggest that the input of meltwater from retreating ice sheets in the Northern Hemisphere triggered the post-LGM warming, which was subsequently sustained by increasing atmospheric CO2. Deglacial millennial-scale events of AMOC slowdown produced large-scale warming of the Brazilian margin not clearly distinguished by some previous studies. Analyzing our stack in its segregated components, i.e., the North Brazil Current (NBCSST stack) and Brazil Current (BCSST stack) – we noted in-phase warming at the onset of the last deglaciation, which is not found in TraCE-21ka simulations. We attributed this to underestimating the meltwater influence over the tropical Brazilian margin by the model. BRSST stack also presents episodes of abrupt cooling near periods of fast sea-level rise during the last deglaciation, which may be due to rapid AMOC reinvigoration or freshening of the Southern Ocean. Holocene climate resembles that recorded by other compilations, with no clear Holocene thermal maximum but presenting a cooling trend during the late Holocene possibly related to intensified volcanic activity. Our study indicates that the future human-induced SST change expected for the end of the 21st-century will overcome the background of natural climate variability of the last 22,000 years for the Brazilian margin.