Sea Surface Temperature Estimates Research Articles

Performance of high-resolution MUR satellite sea surface temperature data as a proxy for near-surface in situ temperatures on neotropical reefs

As ocean temperatures increase, so does the need for accurate, regular, and widespread temperature data. This is especially important for researchers studying corals, which can be pushed over their bleaching thresholds by small, sustained changes in water temperature. Satellite data products offer longitudinal estimates of sea surface temperature (SST) taken at regular intervals without the ongoing costs and geospatial limits associated with underwater temperature loggers. However, previous work indicates satellite data are sometimes characterized by significant temperature biases, particularly in stratified coastal waters. Here, we compared the foundation sea surface temperature (fSST) estimated from a highly processed, gridded satellite data set (GHRSST MUR L4 Global SST) to in situ temperature (IST) data collected from coral reef habitats in the top 20 m of the water column at 19 sites in Panama. Comparisons between Pacific and Caribbean coasts and between upwelling and non-upwelling seasons provide a robust test of the utility of this product to estimate subsurface temperatures. The mean differences between fSST and IST show that MUR fSST accurately reflects temperatures at shallow (<9 m) depths on the Caribbean coast (mean bias ≤0.55°C ± standard error (SE) 0.01°C) and during the non-upwelling season on the Pacific coast (mean bias ≤0.26°C ± SE 0.03°C). MUR fSST poorly reflects IST during the Pacific upwelling season, especially at sites 12 m and deeper. In such cases, 66% of satellite estimates deviated from ISTs by >1°C. Our findings underscore how highly processed SST data products should be ground-truthed before being used as a substitute for IST, especially in upwelling areas.

MIS 5e sea surface temperature estimation; a multi-proxy approach using a marine macrofossil assemblage (Mar Piccolo, Gulf of Taranto, Southern Italy)

It is reported a multi-proxy palaeoclimatic study conducted on a MIS 5e calcarenite from the Mar Piccolo Basin (MP), Gulf of Taranto (GT) (Central Mediterranean). The calcarenite returned a rich malacofauna consisting of 120 extant species, including four of the tropical Senegalese Fauna, today absent in the Mediterranean. The biogeographic-climatic affinity of the assemblage shows, compared to today, a double percentage of warm affinity species, while the cold affinity species are nearly equally represented, indicating a warmer but not strictly tropical SST. This is confirmed by the most recurring preferred SST ranges of the assemblage, indicating an average of 20 °C. The skeletal compositions of five well-preserved molluscan and coral specimens were analyzed for trace elements and stable isotopes for further mean SST estimations. From the comparison of the results of several equations available in literature, it appears that only some SST estimations are realistic, converging into similar values of, on average, 20.8 ± 0.9 °C. Considering all the used proxies, the MIS 5e SST difference compared to today falls in the range 1.2–2.0 °C for the GT (being a more reasonable scenario) and 2.0–2.8 °C for the MP. This is not a firmly tropical-like SST setting as suggested by the sole Senegalese fauna, indicating at least 2.7 °C–3.5 °C more than today's GT and MP, respectively. The approximations and assumptions made for obtaining SST values with any single proxy indicate the need of a multi-proxy approach to define the best SST estimation.

Impact of microwave observations on the estimation of Arctic sea surface temperatures

The frequent and persistent cloud cover in the Arctic limits the extent to which sea surface temperature (SST) can be retrieved from thermal infrared (IR) satellite sensors. Passive microwave (PMW) observations provide highly complementary information to IR, enabling measurements through non-precipitating clouds, although at a coarser spatial resolution. The differences in coverage, accuracy, footprint size, spatial resolution and error characteristics between IR and PMW SSTs require a systematic assessment of how to best combine IR and PMW SST retrievals. This is provided in this study on the basis of the ESA-CCI PMW SST climate data record (CDR) and an existing IR-based gap-free SST and sea ice surface temperature CDR covering the Arctic (>58°N), where cloud cover is a serious limitation to IR sensors. An important step towards a combined IR and PMW SST CDR is to correct for systematic biases in the PMW and IR SST datasets relative to each other. The PMW SSTs show reduced biases against in situ SSTs compared to the IR SSTs, but for consistency with time periods when no Arctic PMW SSTs were available, the PMW SSTs have been adjusted to the IR SSTs in this study. This is done using a dynamic bias correction to generate a consistent combined IR and PMW Arctic SST CDR for the period 2002–2017. Including PMW SSTs reduces the standard deviations from 0.54 °C, 0.55 °C and 0.47 °C to 0.47 °C, 0.54 °C and 0.41 °C against drifters, moorings and Argo floats, respectively. The improved performance is seen in almost all regions (including those already covered by IR observations), with the largest improvement in IR data sparse regions. The average theoretical uncertainty reduces by 0.08 °C, which is in good agreement with the observed improvement in the standard deviation against drifters. The results are very promising and expected to improve even further in the future with the launch of the Copernicus Imaging Microwave Radiometer (CIMR), which will enable PMW SST retrievals with lower uncertainties and much closer to coasts and sea ice (where the largest uncertainties arise) than what is possible with previous and current PMW radiometers.

A Comparison Study of Mg/Ca‐, Alkenone‐, and TEX86‐Derived Temperatures for the Brazilian Margin

AbstractThe reconstruction of accurate sea‐surface temperatures (SST) is of utmost importance due to the ocean's central role in the global climate system. Yet, a number of environmental processes might bias reliable SST estimations. Here, we investigate the fidelity of SST reconstructions for the western tropical South Atlantic (WTSA) for the interval covered by Marine Isotope Stages (MIS) 6–5, utilizing a core collected off eastern Brazil at ∼20°S. This interval was selected as previous SST estimates based on Mg/Ca ratios of planktic foraminifera suggested a peculiar pooling of warm surface waters in the WTSA during MIS 6 despite glacial boundary conditions. To ground‐truth the Mg/Ca‐based SST data we generated SST reconstructions on the same core material using the alkenone and TEX86 paleothermometers. Comparison with alkenone‐based temperature estimates corroborate the previous Mg/Ca‐based SST reconstructions, supporting the suggestion of a warm‐water anomaly during MIS 6. In contrast, TEX86‐derived temperatures, albeit representing annual mean SST in recent core top samples, are up to 6°C colder than Mg/Ca‐ and alkenone‐based SST reconstructions. We interpret the periods of anomalously cold TEX86‐temperatures as a result of a vertical migration of the TEX86 producers (heterotrophic marine Thaumarchaeota) toward greater water depths, following food availability during phases of enhanced fluvial suspension input. Likewise, the data suggest that alkenone‐based SST are, albeit to a minor degree when compared to TEX86, affected by river run‐off and/or a seasonal bias in the growth season of haptophyte algae.

Impact of organic carbon reworking upon GDGT temperature proxies during the Paleocene-Eocene Thermal Maximum

Glycerol dialkyl glycerol tetraethers (GDGTs) have been widely applied to coastal marine sediments to reconstruct past temperature variability. However, coastal environments are characterised by variability in the source, age and/or thermal maturity of different organic carbon (OC) pools and may bias various GDGT-based proxies. Here we analyse TEX86 and MBT5MEvalues within a shallow marine sediment core (South Dover Bridge, Maryland) from the Paleocene-Eocene Thermal Maximum (PETM; 56 million years ago (Ma)) to explore how changes in OC reworking influence GDGT-derived sea surface and terrestrial temperature estimates, respectively. We demonstrate that TEX86 values are unaffected by an increase in soil- and fossil organic carbon during the PETM. In contrast, we find large and unexpected variations in MBT5ME-derived temperature estimates (∼6 to 25 °C) during the onset and core of the PETM at some sites. This coincides with input of reworked terrestrial OC from the Cenomanian-aged Raritan Formation. However, there is also an increase in the degree of cyclisation of tetramethylated branched GDGTs, suggesting that branched GDGTs are also derived from marine in-situ production. These factors preclude terrestrial temperature reconstructions at this site. We explored whether OC reworking is problematic in other PETM-aged coastal environments. Using GDGT metrics and the Branched and Isoprenoid GDGT Machine learning Classification algorithm (BIGMaC), we demonstrate that TEX86 values are mostly unaffected by changes in OC sources. However, MBT5ME values are affected by marine and/or freshwater overprints, especially in environments with low terrestrial OC input. Taken together, this study highlights the importance of constraining the provenance of different GDGTs in marine and lacustrine environments.

CERESMIP: a climate modeling protocol to investigate recent trends in the Earth's Energy Imbalance

The Clouds and the Earth's Radiant Energy System (CERES) project has now produced over two decades of observed data on the Earth's Energy Imbalance (EEI) and has revealed substantive trends in both the reflected shortwave and outgoing longwave top-of-atmosphere radiation components. Available climate model simulations suggest that these trends are incompatible with purely internal variability, but that the full magnitude and breakdown of the trends are outside of the model ranges. Unfortunately, the Coupled Model Intercomparison Project (Phase 6) (CMIP6) protocol only uses observed forcings to 2014 (and Shared Socioeconomic Pathways (SSP) projections thereafter), and furthermore, many of the ‘observed' drivers have been updated substantially since the CMIP6 inputs were defined. Most notably, the sea surface temperature (SST) estimates have been revised and now show up to 50% greater trends since 1979, particularly in the southern hemisphere. Additionally, estimates of short-lived aerosol and gas-phase emissions have been substantially updated. These revisions will likely have material impacts on the model-simulated EEI. We therefore propose a new, relatively low-cost, model intercomparison, CERESMIP, that would target the CERES period (2000-present), with updated forcings to at least the end of 2021. The focus will be on atmosphere-only simulations, using updated SST, forcings and emissions from 1990 to 2021. The key metrics of interest will be the EEI and atmospheric feedbacks, and so the analysis will benefit from output from satellite cloud observation simulators. The Tier 1 request would consist only of an ensemble of AMIP-style simulations, while the Tier 2 request would encompass uncertainties in the applied forcing, atmospheric composition, single and all-but-one forcing responses. We present some preliminary results and invite participation from a wide group of models.

Impact of Monthly Age Model on Coral Paleothermometer Fidelity: Insights From the Northern South China Sea

AbstractThe strontium‐to‐calcium (Sr/Ca) ratio of coral is widely used as a paleothermometer for tropical sea surface temperature (SST) at subannual resolutions. The fidelity of Sr/Ca‐derived SST estimates largely depends on the properties of the monthly age model selected. Here, we develop a revised monthly age model for coral Sr/Ca estimation from the northern South China Sea based on specific tie points and evaluate the impacts of different age models on SST reconstruction. Our results indicate that our revised monthly age model reduces conversion errors by up to nearly 20%–30% relative to previously reported values, thereby improving the precision of seasonal SST estimates and enabling better paleoclimate interpretations, such as El Niño–Southern Oscillation variability. The revised monthly age model provides a generalized approach for developing more robust seasonal resolution Sr/Ca–SST reconstructions in other regions.

Disentangling multiproxy temperature reconstructions from the subtropical North Atlantic

Reliable reconstruction of past sea surface temperature (SST) is of prime importance for understanding the Earth’s sensitivity to external forcing. Yet, it remains a major challenge in paleoceanography because comparison between SST estimates from different proxies reveals mismatches and raise the question as to what the contrasting proxies actually record. A better understanding of these mismatches in the light of seasonal occurrence of the proxy bearing organisms (archives) and water mass changes help to assess climate models. Here, we analyze data from the last deglaciation using a sediment core site situated at the northern boundary of the North Atlantic subtropical gyre influenced by fast latitudinal migrations of the subtropical Azores Front (AF) and resulting changes in water masses that may affect the SST records. Differences between the SST estimates from different deglacial SST reconstructions obtained from (1) Mg/Ca in planktic foraminifer tests, (2) alkenone UK′37, and (3) planktic foraminifer assemblages (SIMMAX), are assumed to result from the ecology of the proxy bearing organisms, and are assessed for the impact on different SST reconstructions from local seawater δ18O (δ18Ow) reconstructions. The general trends of SSTs from all four proxies confirm the well-known deglacial succession of warm and cold events. Mismatches between amplitudes of temperature changes are explained by differences in the phenology of the proxy-bearing organisms and local changes in hydrography. The combination of δ18O SST from the three different archives of δ18Ow reconstructions may cause offsets that exceed the climate driven signals.

Global and Regional Discrepancies between Early-Twentieth-Century Coastal Air and Sea Surface Temperature Detected by a Coupled Energy-Balance Analysis

Abstract A major uncertainty in reconstructing historical sea surface temperature (SST) before the 1990s involves correcting for systematic offsets associated with bucket and engine-room intake temperature measurements. A recent study used a linear scaling of coastal station-based air temperatures (SATs) to infer nearby SSTs, but the physics in the coupling between SATs and SSTs generally gives rise to more complex regional air–sea temperature differences. In this study, an energy-balance model (EBM) of air–sea thermal coupling is adapted for predicting near-coast SSTs from coastal SATs. The model is shown to be more skillful than linear-scaling approaches through cross-validation analyses using instrumental records after the 1960s and CMIP6 simulations between 1880 and 2020. Improved skill primarily comes from capturing features reflecting air–sea heat fluxes dominating temperature variability at high latitudes, including damping high-frequency wintertime SAT variability and reproducing the phase lag between SSTs and SATs. Inferred near-coast SSTs allow for intercalibrating coastal SAT and SST measurements at a variety of spatial scales. The 1900–40 mean offset between the latest SST estimates available from the Met Office (HadSST4) and SAT-inferred SSTs range between −1.6°C (95% confidence interval: [−1.7°, −1.4°C]) and 1.2°C ([0.8°, 1.6°C]) across 10° × 10° grids. When further averaged along the global coastline, HadSST4 is significantly colder than SAT-inferred SSTs by 0.20°C ([0.07°, 0.35°C]) over 1900–40. These results indicate that historical SATs and SSTs involve substantial inconsistencies at both regional and global scales. Major outstanding questions involve the distribution of errors between our intercalibration model and instrumental records of SAT and SST as well as the degree to which coastal intercalibrations are informative of global trends. Significance Statement To evaluate the consistency of instrumental surface temperature estimates before the 1990s, we develop a coupled energy-balance model to intercalibrate measurements of sea surface temperature (SST) and station-based air temperature (SAT) near global coasts. Our model captures geographically varying physical regimes of air–sea coupling and outperforms existing methods in inferring regional SSTs from SAT measurements. When applied to historical temperature records, the model indicates significant discrepancies between inferred and observed SSTs at both global and regional scales before the 1960s. Our findings suggest remaining data issues in historical temperature archives and opportunities for further improvements.

The Relative Stability of Planktic Foraminifer Thermal Preferences over the Past 3 Million Years

Stationarity of species’ ecological tolerances is a first-order assumption of paleoenvironmental reconstruction based upon analog methods. To test this and other assumptions used in quantitative analysis of foraminiferal faunas for paleoceanographic reconstruction, we analyzed paired alkenone unsaturation ratio (U37K′) sea surface temperature (SST) estimates and relative abundances of planktic foraminifera within Late Pliocene assemblages. We established Pliocene temperature preferences for nine species in the North Atlantic: Dentoglobigerina altispira, Globorotalia menardii, Globoconella puncticulata, Neogloboquadrina atlantica, Neogloboquadrina incompta, Neogloboquadrina pachyderma, Trilobatus sacculifer, Globigerinita glutinata, and Globigerina bulloides. We compared these to the temperature preferences of the same extant species, and in the three cases where the species are now extinct (Dentoglobigerina altispira, Neogloboquadrina atlantica, and Globoconella puncticulata), comparisons were made to either the descendant species or other modern species commonly used as analogs. In general, the taxa tested show similar temperature responses in both Late Pliocene and present-day (core-top) distributions. The data from these comparisons are mostly encouraging, supporting past paleoceanographic conclusions, and are otherwise valuable for testing previous taxonomic grouping decisions that are often necessary for interpreting the paleoenvironment based upon Pliocene foraminiferal assemblages.

A brGDGT‐Based Reconstruction of Terrestrial Temperature From the Maritime Continent Spanning the Last Glacial Maximum

AbstractThe tropics exert enormous influence on global climate. Despite the importance of tropical regions, the terrestrial temperature history in the Indo‐Pacific Warm Pool (IPWP) region during the last deglaciation is poorly constrained. Although numerous sea surface temperature (SST) reconstructions provide estimates of SST warming from the Last Glacial Maximum to the Holocene, the timing of the onset of deglacial warming varies between records and inhibits determining the forcings driving deglacial warming in the IPWP. We present a 60,000‐year long temperature reconstruction based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) in a sediment core from Lake Towuti, located in Sulawesi, Indonesia. BrGDGTs are bacterial membrane‐spanning lipids that, globally, become more methylated with decreasing temperature and more cyclized with decreasing pH. Although changes in temperature are the dominant control on brGDGTs in regional and global calibrations, we find that the cyclization of the brGDGTs is a major mode of variation at Lake Towuti that records important changes in the lacustrine biogeochemical environment. We separate the influence of lake chemistry changes from temperature changes on the brGDGT records, and develop a temperature record spanning the last 60,000 years. The timing of the deglacial warming in our record occurs after the onset of the deglacial increase in CO2 concentrations, which suggests rising greenhouse gas concentrations and the associated radiative forcing may have forced deglacial warming in the IPWP. Peaks in temperature around 55 and 34 ka indicate that Northern Hemisphere summer insolation may also influence land surface temperature in the IPWP region.

Planktonic foraminifera response to orbital and millennial-scale climate variability at the southern Iberian Margin (IODP Site U1385) during Marine Isotope Stages 20 and 19

The orbital configuration of Marine Isotope Stage (MIS) 19 provides a good analogue to our present interglacial, and it is marked by millennial-scale climate variability. To evaluate how orbital and millennial-scale climate changes interact during this interglacial we have conducted a high-resolution study of planktonic foraminifera assemblages at International Ocean Discovery Program (IODP) Site U1385, SW Iberian Margin, encompassing late MIS 20 and MIS 19. Estimates of sea surface temperature (SST), using transfer-functions, are compared with geochemical, XRF elemental data, biomarker and pollen records from the same site. These results are then compared with SST and ice-rafted debris records from North Atlantic IODP Site U1314 to reconstruct basin-wide palaeoceanographic conditions. During MIS 20, the large size of the ice sheets imparted a strong cooling in the subpolar gyre whereas the Iberian Margin remained under the influence of the subtropical gyre with the development of a high meridional thermal gradient in the North Atlantic. During MIS 20 terminal stadial (794–789 ka) and MIS 19b-a stadial events, the meltwater discharges related to the instability of ice sheets affected both the North Atlantic and the Iberian Margin leading to a weaker thermal gradient. Our data indicate that Termination IX (TIX, ∼795–788 ka) was punctuated by short-term warming/cooling phases in the ocean coupled with warming/cooling events on land. A short-term cooling episode, recognized in the ocean and on land, occurred on Termination IX and preceded the beginning of MIS 19c. During MIS 19c, the planktonic foraminifera SST indicates warm and almost stable conditions in the sea decoupling the cool and dry millennial-scale events on land inferred by pollen assemblages. Starting from 780 ka towards MIS 18, the planktonic foraminifera assemblages and SSTs indicate a cooling trend along the Iberian Margin related to a progressive southward migration of the polar front.

Alkenone Uk’37 index differs between thermally separated populations of fin whales and krill

Although habitat temperature is an important determinant of species distributions, its measurement is difficult in mobile marine organisms. Chemical tracers such as alkenones constitute an alternative tool for gauging marine habitat temperature, something particularly relevant in the current scenario of global warming. The unsaturation index of alkenones, the Uk’37 index, can be used to estimate sea surface temperature (SST) in living organisms. Here, we analyse alkenone concentrations in a predator species, the fin whale, and its prey, the northern krill, both sampled in 2 areas with different SSTs: SW Iceland and NW Spain. In NW Spain (but not in SW Iceland), alkenone concentrations were higher in krill than in fin whale blubber, suggesting that they may biodilute along the trophic transfer. Consistently with local SST, the Uk’37 index was lower in the Icelandic samples of both fin whales and krill than in those from NW Spain, confirming the ability of this index to distinguish populations that are thermally separated. Whales are migratory animals; thus, the alkenones present in their blubber are a mix of those ingested locally and in recently visited regions. Consistently, the Uk’37 index showed a higher correlation with the local SST in krill than in fin whales. Alkenone turnover rates should be considered when using this proxy to assess the thermal range of the feeding habitat of a species. We conclude that the Uk’37 index is a potentially efficient tool to study the behaviour of whales, thus providing an alternative perspective to other chemical tracers.

Oceanic productivity and high-frequency temperature variability—not human habitation—supports calcifier abundance on central Pacific coral reefs

Past research has demonstrated how local-scale human impacts—including reduced water quality, overfishing, and eutrophication—adversely affect coral reefs. More recently, global-scale shifts in ocean conditions arising from climate change have been shown to impact coral reefs. Here, we surveyed benthic reef communities at 34 U.S.-affiliated Pacific islands spanning a gradient of oceanic productivity, temperature, and human habitation. We re-evaluated patterns reported for these islands from the early 2000s in which uninhabited reefs were dominated by calcifiers (coral and crustose coralline algae) and thought to be more resilient to global change. Using contemporary data collected nearly two decades later, our analyses indicate this projection was not realized. Calcifiers are no longer the dominant benthic group at uninhabited islands. Calcifier coverage now averages 26.9% ± 3.9 SE on uninhabited islands (compared to 45.18% in the early 2000s). We then asked whether oceanic productivity, past sea surface temperatures (SST), or acute heat stress supersede the impacts of human habitation on benthic cover. Indeed, we found variation in benthic cover was best explained not by human population densities, but by remotely sensed metrics of chlorophyll-a, SST, and island-scale estimates of herbivorous fish biomass. Specifically, higher coral and CCA cover was observed in more productive waters with greater biomass of herbivores, while turf cover increased with daily SST variability and reduced herbivore biomass. Interestingly, coral cover was positively correlated with daily variation in SST but negatively correlated with monthly variation. Surprisingly, metrics of acute heat stress were not correlated with benthic cover. Our results reveal that human habitation is no longer a primary correlate of calcifier cover on central Pacific island reefs, and highlight the addition of oceanic productivity and high-frequency SST variability to the list of factors supporting reef builder abundance.

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.

Very high Middle Miocene surface productivity on the U.S. mid-Atlantic shelf amid glacioeustatic sea level variability

The Miocene Climatic Optimum (MCO) provides important insights into how the climate system operates under elevated temperatures and atmospheric CO2 levels. Few western North Atlantic paleotemperature or paleoecological records exist from the MCO, despite their importance for understanding both regional and global climate dynamics. Here we present quantitative MCO paleoecological data from the western North Atlantic, specifically from the Baltimore Gas & Electric (BG&E) marine sediment core from southern Maryland, USA. We examine alkenones and planktic foraminifera and document the first sea surface temperature (SST) and productivity estimates for the MCO and the Middle Miocene Climate Transition (MMCT) from the continental shelf. Increased levels of planktic foraminifer species diversity and surface productivity accompany high sea level intervals of the MCO, indicating coastal upwelling. Cooling episodes correlate to unconformities in the BG&E core that reflect sea level lowstands; these and sedimentary cycles tie the record to eccentricity-paced Antarctic ice sheet growth and decay. This dynamic record not only captures the variability in SST, sea level and coastal productivity during the warm MCO and the transition to cooler global temperatures during the MMCT, but it also demonstrates the variability in local conditions within and between intervals of high sea level.

A dataset of hourly sea surface temperature from drifting buoys

A dataset of sea surface temperature (SST) estimates is generated from the temperature observations of surface drifting buoys of NOAA’s Global Drifter Program. Estimates of SST at regular hourly time steps along drifter trajectories are obtained by fitting to observations a mathematical model representing simultaneously SST diurnal variability with three harmonics of the daily frequency, and SST low-frequency variability with a first degree polynomial. Subsequent estimates of non-diurnal SST, diurnal SST anomalies, and total SST as their sum, are provided with their respective standard uncertainties. This Lagrangian SST dataset has been developed to match the existing and on-going hourly dataset of position and velocity from the Global Drifter Program.

The bulk parameterizations of turbulent air–sea fluxes in NEMO4: the origin of sea surface temperature differences in a global model study

Abstract. Wind stress and turbulent heat fluxes are the major driving forces that modify the ocean dynamics and thermodynamics. In the Nucleus for European Modelling of the Ocean (NEMO) ocean general circulation model, these turbulent air–sea fluxes (TASFs) can critically impact the simulated ocean characteristics. This paper investigates how the various bulk parameterizations used to calculate turbulent air-sea fluxes in NEMOv4 can lead to substantial differences in the estimation of sea surface temperatures (SSTs). Specifically, we study the contributions of different aspects and assumptions of the bulk parameterizations in driving the SST differences in the NEMO global model configuration at 1/4∘ of horizontal resolution. These aspects include the use of the skin temperature instead of the bulk SST in the computation of turbulent heat flux components and the estimation of wind stress and turbulent heat flux components, which vary in each parameterization due to different bulk transfer coefficients. The analysis of a set of short-term sensitivity experiments where the only change is related to one of the aspects of the bulk parameterizations shows that parameterization-related SST differences are primarily sensitive to wind stress differences and to the implementation of skin temperature in the computation of turbulent heat flux components. In addition, in order to highlight the role of SST–turbulent heat flux negative feedback at play in ocean simulations, we compare the TASF differences obtained using the NEMO ocean model with the estimations by Brodeau et al. (2017), who compared the different bulk parameterizations using prescribed SSTs. Our estimations of turbulent heat flux differences between bulk parameterizations are weaker than those found by Brodeau et al. (2017).

Cenozoic Antarctic Peninsula Temperatures and Glacial Erosion Signals From a Multi‐Proxy Biomarker Study

AbstractTerrestrial climate records for Antarctica, beyond the age limit of ice cores, are restricted to the few unglaciated areas with exposed rock outcrops. Marine sediments on Antarctica's continental shelves contain records of past oceanic and terrestrial environments that can provide important insights into Antarctic climate evolution. The SHALDRIL II (Shallow Drilling on the Antarctic Continental Margin) expedition recovered sedimentary sequences from the eastern side of the Antarctic Peninsula of late Eocene, Oligocene, middle Miocene, and early Pliocene age that provides insights into Cenozoic Antarctic climate and ice sheet development. Here, we use biomarker data to assess atmospheric and oceanic temperatures and glacial reworking from the late Eocene to the early Pliocene. Analyses of hopanes and n‐alkanes indicate increased erosion of mature (thermally altered) soil biomarker components reworked by glacial erosion. Branched glycerol dialkyl glycerol tetraethers from soil bacteria suggest similar air temperatures of 12°C ± 1°C (1σ, n = 46) for months above freezing for Eocene, Oligocene, and Miocene timeslices but much colder (and likely shorter) periods of thaw during the Pliocene (5°C ± 1°C, n = 4) on the Antarctic Peninsula. TEX86‐based (Tetraether index of 86 carbons) sea surface temperature estimates indicate ocean cooling from 7°C ± 3°C (n = 10) in the Miocene to 3°C ± 1°C (n = 3) in the Pliocene, consistent with deep ocean cooling. Resulting temperature records provide useful constraints for ice sheet and climate model simulations seeking to improve understanding of ice sheet response under a range of climate conditions.

Accuracy and Reproducibility of Coral Sr/Ca SIMS Timeseries in Modern and Fossil Corals

AbstractCoral strontium‐to‐calcium ratios (Sr/Ca) provide quantitative estimates of past sea surface temperatures (SST) that allow for the reconstruction of changes in the mean state and climate variations, such as the El Nino‐Southern Oscillation, through time. However, coral Sr/Ca ratios are highly susceptible to diagenesis, which can impart artifacts of 1–2°C that are typically on par with the tropical climate signals of interest. Microscale sampling via Secondary Ion Mass Spectrometry (SIMS) for the sampling of primary skeletal material in altered fossil corals, providing much‐needed checks on fossil coral Sr/Ca‐based paleotemperature estimates. In this study, we employ a set modern and fossil corals from Palmyra Atoll, in the central tropical Pacific, to quantify the accuracy and reproducibility of SIMS Sr/Ca analyses relative to bulk Sr/Ca analyses. In three overlapping modern coral samples, we reproduce bulk Sr/Ca estimates within ±0.3% (1σ). We demonstrate high fidelity between 3‐month smoothed SIMS coral Sr/Ca timeseries and SST (R = −0.5 to −0.8; p < 0.5). For lightly‐altered sections of a young fossil coral from the early‐20th century, SIMS Sr/Ca timeseries reproduce bulk Sr/Ca timeseries, in line with our results from modern corals. Across a moderately‐altered section of the same fossil coral, where diagenesis yields bulk Sr/Ca estimates that are 0.6 mmol too high (roughly equivalent to −6°C artifacts in SST), SIMS Sr/Ca timeseries track instrumental SST timeseries. We conclude that 3–4 SIMS analyses per month of coral growth can provide a much‐needed quantitative check on the accuracy of fossil coral Sr/Ca‐derived estimates of paleotemperature, even in moderately altered samples.