Glacial Sea Surface Temperature 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.

Central Equatorial Pacific Cooling During the Last Glacial Maximum

AbstractEstablishing tropical sea surface temperature (SST) during the Last Glacial Maximum (LGM) is important for constraining equilibrium climate sensitivity to radiative forcing. Until now, there has been little data from the central equatorial Pacific in global compilations, with foraminiferal assemblage‐based estimates suggesting the region was within 1°C of modern temperatures during the LGM. This is in stark contrast to multi‐proxy evidence from the eastern and western Pacific and model simulations which support larger cooling. Here we present the first estimates of glacial SST in the central equatorial Pacific from Mg/Ca in Globigerinoides ruber. Our results show that the central Pacific cooled by about 2.0°C during the LGM, in contrast with previous global compilations but in agreement with models. Our data support a larger magnitude of tropical LGM cooling, and thus a larger equilibrium climate sensitivity, than previous studies which relied on foraminiferal assemblages in the central tropical Pacific.

Glacial to Holocene changes in sea surface temperature and seawater δ18O in the northern Indian Ocean

Sea surface temperature (SST) and seawater δ18Osw records were generated from sediment cores located in the southern Bay of Bengal (SBOB) and the northeastern Arabian Sea (NEAS) to understand glacial to Holocene changes in the hydrography of these regions. This was accomplished through the use of paired δ18O and Mg/Ca measurements in planktic foraminifera (Globigerinoides ruber) from two sediment cores; SK157-14 in the SBOB (lat. 5°11′N; long. 90°05′E; water-depth 3306m) and SK148-21 in the NEAS (lat. 21°29′N, long. 67°01′E; water-depth 1900m). The results suggest significant changes in SST and δ18Osw since last glacial period. The glacial SSTs in the SBOB and the NEAS were lower by ~2–2.5°C relative to the Holocene. The deglacial period in both cores is characterized by significant variations in SSTs and seawater δ18Osw. The Dansgaard-Oeschger (DO) cycles and Heinrich events are strongly expressed in SST and δ18Osw records of the SBOB core. These records support a strong control of atmospheric-oceanic changes in the northern high latitude on thermal state of the SBOB. The SST time series in both these regions indicates a sudden and abrupt increase in temperature at the end of last glacial maximum. The SST time series in the NEAS core SK 148-21 reveals an intensification of the northeast monsoon during the last glacial maximum.

Modern and glacial tropical snowlines controlled by sea surface temperature and atmospheric mixing

During the Last Glacial Maximum, tropical sea surface temperatures were 1 to 3 °C cooler than present1,2,3,4, but the altitude of the snowlines of tropical glaciers5,6 was lower than would be expected in light of these sea surface temperatures. Indeed, both glacial and twentieth-century snowlines seem to require lapse rates that are steeper than a moist adiabat7,8. Here we use estimates of Last Glacial Maximum sea surface temperature in the Indo-Pacific warm pool based on the clumped isotope palaeotemperature proxy in planktonic foraminifera and coccoliths, along with radiative–convective calculations of vertical atmospheric thermal structure, to assess the controls on tropical glacier snowlines. Using extensive new data sets for the region, we demonstrate that mean environmental lapse rates are steeper than moist adiabatic during the recent and glacial. We reconstruct glacial sea surface temperatures 4 to 5 °C cooler than modern. We include modern and glacial sea surface temperatures in calculations of atmospheric convection that account for mixing between rising air and ambient air, and derive tropical glacier snowlines with altitudes consistent with twentieth-century and Last Glacial Maximum reconstructions. Sea surface temperature changes ≤3 °C are excluded unless glacial relative humidity values were outside the range associated with deep convection in the modern. We conclude that the entrainment of ambient air into rising air masses significantly alters the vertical temperature structure of the troposphere in modern and ancient regions of deep convection. Furthermore, if all glacial tropical temperatures were cooler than previously estimated, it would imply a higher equilibrium climate sensitivity than included in present models9,10.

Stability of the Kuroshio path with respect to glacial sea level lowering

We investigate the changes of the Kuroshio Current in the East China Sea during the last glacial maximum, based on numerical experiments using an ocean model and geochemical analyses of marine sediments. Our numerical experimental results indicate that there was little effect of sea‐level change on the path of the Kuroshio during the glacial period. Geochemical proxy records of marine sediment cores recovered from inside and outside the Okinawa Trough (OT) show no significant difference in glacial sea surface temperature and planktonic foraminiferal δ18O between the OT and the Ryukyu forearc. This indicates that the glacial sea surface temperature and salinity were almost the same inside and outside the OT. Hence, during the glacial period, Kuroshio water most likely intruded into the OT and flowed along the shelf break until it drained out through the Tokara Strait.

Glacial sea surface temperature of the East Sea (Japan Sea) inferred from planktonic foraminiferal assemblage

In order to reconstruct the last glacial sea surface temperature (SST) of the East Sea, we investigated planktonic foraminiferal assemblage in the marine sediments of a piston core recovered from the Ulleung Basin, East Sea. For core top, the most dominant species is right coiling Neogloboquadrina incompta, while left coiling Neogloboquadrina pachyderma and Globigerina bulloides clearly dominate the glacial assemblages. Based on fauna count, we reconstructed past SST using modern analog technique (MAT) with a global dataset. However, the application of the global dataset may hinder the reconstruction of regional SST. In order to validate this, we tested for the foraminiferal assemblages of 51 surface sediments in the East Sea with MAT and the global dataset. The results show that the estimated MAT temperatures were completely different from the real SST of core top sites, indicating that MAT with the global dataset may not be applicable for the East Sea SST reconstruction. Therefore, we developed new statistical calculation method using East Sea fauna dataset only. According to this method, the glacial SST was 2 °C lower than today for August, and 6 °C lower for February. This glacial-interglacial change of August temperature is relatively smaller than other results, while February temperature is comparable.

The Effect of a Large Freshwater Perturbation on the Glacial North Atlantic Ocean Using a Coupled General Circulation Model

Abstract The commonly held view of the conditions in the North Atlantic at the last glacial maximum, based on the interpretation of proxy records, is of large-scale cooling compared to today, limited deep convection, and extensive sea ice, all associated with a southward displaced and weakened overturning thermohaline circulation (THC) in the North Atlantic. Not all studies support that view; in particular, the “strength of the overturning circulation” is contentious and is a quantity that is difficult to determine even for the present day. Quasi-equilibrium simulations with coupled climate models forced by glacial boundary conditions have produced differing results, as have inferences made from proxy records. Most studies suggest the weaker circulation, some suggest little or no change, and a few suggest a stronger circulation. Here results are presented from a three-dimensional climate model, the Hadley Centre Coupled Model version 3 (HadCM3), of the coupled atmosphere–ocean–sea ice system suggesting, in a qualitative sense, that these diverging views could all have occurred at different times during the last glacial period, with different modes existing at different times. One mode might have been characterized by an active THC associated with moderate temperatures in the North Atlantic and a modest expanse of sea ice. The other mode, perhaps forced by large inputs of meltwater from the continental ice sheets into the northern North Atlantic, might have been characterized by a sluggish THC associated with very cold conditions around the North Atlantic and a large areal cover of sea ice. The authors’ model simulation of such a mode, forced by a large input of freshwater, bears several of the characteristics of the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) Project’s reconstruction of glacial sea surface temperature and sea ice extent.

Spatial and temporal sea‐surface temperatures in the eastern equatorial Pacific over the past 150 kyr

We present the new alkenone‐based sea‐surface temperature (SST) records from the eastern equatorial Pacific over the past 150 kyr. Core HY04 (4°N) at the North Equatorial Countercurrent (NECC) registered the invariant SSTs (25.8–27.2°C), whereas core HY06 (0°N) registered cyclic glacial‐interglacial SST variations (22.4–26.1°C). Although HY06 alkenone‐based SST evolution was almost consistent with the previously published SST records from around the equator, at site HY04 alkenone‐ and foraminiferal Mg/Ca‐based SSTs showed a large difference in the magnitude of LGM SST cooling. We hypothesize that alkenone‐based SSTs might not fully express the true magnitude of glacial SST cooling in the NECC, as suggested by the coincidence of higher glacial productivity and unchanged glacial SSTs at site HY04. However, further SST data in the NECC are needed before this can be accepted with any degree of confidence.

A one‐million‐year history of a north‐south segment of the Subtropical Front, east of New Zealand

Planktic foraminiferal assemblages from Ocean Drilling Program (ODP) site 1119 and Deep Sea Drilling Project (DSDP) site 594 are used to estimate sea surface temperatures (SSTs) over the past ∼900 kyr (marine isotope stages (MISs) 23–1) in the southwest Pacific Ocean, east of New Zealand. Comparison of these long SST records allows us to document the cyclic migrational history of water masses and fronts, especially of the N‐S oriented segment of the Subtropical Front (STF), known as the Southland Front. During most interglacials, summer SSTs were 10–12°C in both sites, indicating that both were overlain by Subantarctic Water (SAW) and that the Southland Front was located landward of them. During the peaks of MISs 11 and 5.5, summer SST estimates were considerably warmer than modern, suggesting that the southern edge of the E‐W trending STF expanded southward over the Bounty Trough, and the Southland Current contained a greater proportion of Subtropical Water (STW) than it does today. During most glacials, summer SSTs in inshore ODP site 1119 (7–10°C) were ∼4°C warmer than in DSDP site 594. This implies that the Southland Front migrated offshore with lowered sea levels to lie between the two sites. During MIS 20, SSTs in ODP site 1119 dropped to those of DSDP site 594, indicating that there was no Southland Front in existence and no detectable STW entrained in the Southland Current flowing around the south end of New Zealand. The low glacial SST estimates in DSDP site 594 (∼4°C) might be a result of persistent eddies of Circumpolar Subantarctic Water spinning off the Subantarctic Front into the Bounty Trough during glacials.

Planktic foraminiferal and sea surface temperature record during the last 1 Myr across the Subtropical Front, Southwest Pacific

Planktic foraminiferal faunas and modern analogue technique estimates of sea surface temperature (SST) for the last 1 million years (Myr) are compared between core sites to the north (ODP 1125, 178 faunas) and south (DSDP 594, 374 faunas) of the present location of the Subtropical Front (STF), east of New Zealand. Faunas beneath cool subtropical water (STW) north of the STF are dominated by dextral Neogloboquadrina pachyderma, Globorotalia inflata, and Globigerina bulloides, whereas faunas to the south are strongly dominated by sinistral N. pachyderma (80–95% in glacials), with increased G. bulloides (20–50%) and dextral N. pachyderma (15–50%) in interglacials (beneath Subantarctic Water, or SAW). Canonical correspondence analysis indicates that at both sites, SST and related factors were the most important environmental influences on faunal composition. Greater climate-related faunal fluctuations occur in the south. Significant faunal changes occur through time at both sites, particularly towards the end of the mid-Pleistocene climate transition, MIS18–15 (e.g., decline of Globorotalia crassula in STW, disappearance of Globorotalia puncticulata in SAW), and during MIS8–5. Interglacial SST estimates in the north are similar to the present day throughout the last 1 Myr. To the south, interglacial SSTs are more variable with peaks 4–7 °C cooler than present through much of the early and middle Pleistocene, but in MIS11, MIS5.5, and early MIS1, peaks are estimated to have been 2–4 °C warmer than present. These high temperatures are attributed to southward spread of the STF across the submarine Chatham Rise, along which the STF appears to have been dynamically positioned throughout most of the last 1 Myr. For much of the last 1 Myr, glacial SST estimates in the north were only 1–2 °C cooler than the present interglacial, except in MIS16, MIS8, MIS6, and MIS4–2 when estimates are 4–7 °C cooler. These cooler temperatures are attributed to jetting of SAW through the Mernoo Saddle (across the Chatham Rise) and/or waning of the STW current. To the south, glacial SST estimates were consistently 10–11 °C cooler than present, similar to temperatures and faunas currently found in the vicinity of the Polar Front. One interpretation is that these cold temperatures reflect thermocline changes and increased Circumpolar Surface Water spinning off the Subantarctic Front as an enhanced Bounty Gyre along the south side of the Chatham Rise. For most of the last 1 Myr, the temperature gradient across the STF has been considerably greater than the present 4 °C. During glacial episodes, the STF in this region did not migrate northwards, but instead there was an intensification of the temperature gradient across it (interglacials 4–11 °C; glacials 8–14 °C).

Amplitude and timing of sea-surface temperature change in the northern South China Sea: Dynamic link to the East Asian monsoon

Magnesium/calcium (Mg/Ca) ratios of foraminiferal shells from a sediment core from the northern South China Sea, a semi-enclosed basin in the western tropical Pacific, document variations in sea-surface temperature (SST) during the past 145 k.y. Glacial SSTs were 4 °C colder than interglacial SSTs. During the last deglaciation, most of the warming was accomplished in a single abrupt step after continental ice-sheet decay had already begun, but warming and ice-sheet demise were nearly synchronous during the penultimate deglaciation. Abrupt SST changes of the past 15 k.y. were apparently synchronous with events in East Asian monsoon rainfall, suggesting that variations in monsoon winds and their influence on surface circulation of the western Pacific exerted a strong control on northern South China Sea SSTs. We suggest that this link persisted for the previous 130 k.y., during which time orbital-scale 2-3 °C SST changes and several small (≤2 °C) abrupt SST events occurred in the northern South China Sea. The similar timing of northern South China Sea SST, on a benthic δ 1 8 O time scale, to a well-dated speleothem record from eastern China suggests that the demise of ice sheets associated with the penultimate deglaciation did not precede Northern Hemisphere summer insolation increase. Our results suggest that surface waters had higher δ 1 8 O values during times of strong summer monsoon than during times of weak monsoon, likely reflecting a redistribution of 1 8 O-depleted rainfall from land during times of strong summer monsoons, to the western Pacific during times of weaker summer monsoons.

Reconstruction of sea-surface temperatures from assemblages of planktonic foraminifera: multi-technique approach based on geographically constrained calibration data sets and its application to glacial Atlantic and Pacific Oceans

We present a conceptual framework for a new approach to environmental calibration of planktonic foraminifer census counts. This approach is based on simultaneous application of a variety of transfer function techniques, which are trained on geographically constrained calibration data sets. It serves to minimise bias associated with the presence of cryptic species of planktonic foraminifera and provides an objective tool for assessing reliability of environmental estimates in fossil samples, allowing identification of adverse effects of no-analog faunas and technique-specific bias. We have compiled new calibration data sets for the North ( N=862) and South ( N=321) Atlantic and the Pacific Ocean ( N=1111). We show evidence that these data sets offer adequate coverage of the Sea-Surface Temperature (SST) and faunal variation range and that they are not affected by the presence of pre-Holocene samples and/or calcite dissolution. We have applied four transfer function techniques, including Artificial Neural Networks, Revised Analog Method and SIMMAX (with and without distance weighting) on faunal counts in a Last Glacial Maximum (LGM) data set for the Atlantic Ocean (748 samples in 167 cores; based on the GLAMAP-2000 compilation) and a new data set for the Pacific Ocean (265 samples in 82 cores) and show that three of these techniques provide adequate degree of independence for the advantage of a multi-technique approach to be realised. The application of our new approach to the glacial Pacific lends support to the contraction and perhaps even a cooling of the Western Pacific Warm Pool and a substantial (>3 °C) cooling of the eastern equatorial Pacific and the eastern boundary currents. Our results do not provide conclusive evidence for LGM warming anywhere in the Pacific. The Atlantic reconstruction shows a number of robust patterns, including substantial cooling of eastern boundary currents with considerable advection of subpolar waters into the Benguela Current, a cooling of the equatorial Atlantic by ∼5 °C, and steep SST gradients in the mid-latitude North Atlantic. The transfer function techniques generally agree that subtropical gyre areas in both hemispheres did not change significantly since the LGM, although the ANN technique produced glacial SST in the southern gyre 1–2 °C warmer than today. We have revisited the issue of sea-ice occurrence in the Nordic Seas and using the distribution of subpolar species of planktonic foraminifera in glacial samples, we conclude that the Norwegian Sea must have been ice-free during the summer.

Estimating glacial western Pacific sea-surface temperature: methodological overview and data compilation of surface sediment planktic foraminifer faunas

We present a detailed comparison of five “transfer function” techniques calibrated to reconstruct sea-surface temperature (SST) from planktic foraminifer counts in western Pacific surface sediments. The techniques include the Imbrie–Kipp method (IKM), modern analog technique (MAT), modern analog technique with similarity index (SIMMAX), revised analog method (RAM), and the artificial neural network technique (ANN). The calibration is based on a new database of 694 census counts of planktic foraminifers in coretop samples from the western Pacific, compiled under a cooperative effort within the MARGO (multiproxy approach for the reconstruction of the glacial ocean surface) project. All five techniques were used to reconstruct SST variation in a well-dated Holocene to last glacial maximum interval in core MD972151 from the southern South China Sea (SCS) to evaluate the magnitude of cooling in the western tropical Pacific during the LGM. Our results suggest that MAT, SIMMAX, RAM and ANN show a similar level of performance in SST estimation and produce ⩽1 °C uncertainties in coretop SST calibrations of the western Pacific. When applying these techniques to the downcore faunal record, the IKM, which performed significantly worst in the calibration exercise, produced glacial SST estimates similar to present-day values, whereas the other four techniques all indicated ∼1 °C cooler glacial SST. Because of their better performance in the calibration dataset, and because of the convergence among the techniques in the estimated magnitude of glacial cooling in the studied core, we conclude that MAT, SIMMAX, RAM and ANN provide more robust planktic foraminifer paleo-SST estimates than traditional IKM techniques in western Pacific paleoceanographic studies.

Stability of the glacial thermohaline circulation and its dependence on the background hydrological cycle

Different reconstructions of glacial sea-surface temperatures (SST) are used to force a hybrid coupled atmosphere–ocean model. The resulting glacial states differ in global salinity and temperature distributions, and consequently in the strength of the thermohaline circulation. Stability analysis of the Atlantic Ocean circulation, by means of freshwater-flux hysteresis maps, reveals mono-stability for each glacial background state, which appears to be a robust feature of the glacial ocean. We show that this behaviour is directly linked to the hydrological cycle. A monotonic relation between the freshwater input necessary for reaching the off-mode and the hydrological budget in the Atlantic catchment area, accounts for the sensitivity of the ocean’s circulation. The most sensitive part of the hydrological balance appears to be in the tropical and subtropical regions suggesting that the ‘Achilles heel’ of the global conveyor belt circulation is not restricted to the northern North Atlantic where convection occurs.

Sea surface temperatures in the equatorial and South Atlantic Ocean during the Last Glacial Maximum (23–19 ka)

We used planktic foraminiferal assemblages in 70 sediment cores from the tropical and subtropical South Atlantic Ocean (10°N–37°S) to estimate annual mean sea surface temperatures (SSTs) and seasonality for the Last Glacial Maximum with a modified version of the Imbrie‐Kipp transfer function method (IKTF) that takes into account the abundance of rare but temperature sensitive species. In contrast to CLIMAP Project Members [1981], the reconstructed SSTs indicate cooler glacial SSTs in the entire tropical/subtropical South Atlantic with strongest cooling in the upwelling region off Namibia (7–10°C) and smallest cooling (1–2°C) in the western subtropical gyre. In the western Atlantic, our data support recent temperature estimates from other proxies. In the upwelling regions in the eastern Atlantic, our data conflict with SST reconstructions from alkenones, which may be due to an environmental preference of the alkenone‐producing algae or to an underestimation of foraminiferal SSTs due to anomalous high abundances of N. pachyderma (sinistral).

Glacial North Atlantic: Sea‐surface conditions reconstructed by GLAMAP 2000

The response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology. Our new reconstruction of peak glacial sea surface temperatures (SSTs) in the Atlantic is based on census counts of planktic foraminifera, using the Maximum Similarity Technique Version 28 (SIMMAX‐28) modern analog technique with 947 modern analog samples and 119 well‐dated sediment cores. Our study compares two slightly different scenarios of the Last Glacial Maximum (LGM), the Environmental Processes of the Ice Age: Land, Oceans, Glaciers (EPILOG), and Glacial Atlantic Ocean Mapping (GLAMAP 2000) time slices. The comparison shows that the maximum LGM cooling in the Southern Hemisphere slightly preceeded that in the north. In both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer, while the central and eastern parts were ice‐free. During northern glacial winter, sea ice advanced to the south of Iceland and Faeroe. In the central northern North Atlantic an anticyclonic gyre formed between 45° and 60°N, with a cool water mass centered west of Ireland, where glacial cooling reached a maximum of >12°C. In the subtropical ocean gyres the new reconstruction supports the glacial‐to‐interglacial stability of SST as shown by CLIMAP Project Members (CLIMAP) [1981]. The zonal belt of minimum SST seasonality between 2° and 6°N suggests that the LGM caloric equator occupied the same latitude as today. In contrast to the CLIMAP reconstruction, the glacial cooling of the tropical east Atlantic upwelling belt reached up to 6°–8°C during Northern Hemisphere summer. Differences between these SIMMAX‐based and published U37k‐ and Mg/Ca‐based equatorial SST records are ascribed to strong SST seasonalities and SST signals that were produced by different planktic species groups during different seasons.

Last glacial sea surface temperatures and sea‐ice extent in the Southern Ocean (Atlantic‐Indian sector): A multiproxy approach

On the basis of the quantitative study of diatom, radiolarian, and planktic foraminiferal assemblages, we estimated summer sea surface temperature (SSST) and sea‐ice extent at 50 sediment core localities in the Atlantic and western Indian sector of the Southern Ocean to reconstruct the last glacial environment at the GLAMAP (18 to 15 ka, equal to 21,500 to 18,000 calendar (cal) years BP) and EPILOG (19.5 to 16.0 ka, equal to 23,000 to 19,000 cal years BP) time slices. Stratigraphic identification of the time slices was accomplished by a combination of AMS 14C measurements, benthic isotope, and siliceous microfossil abundance records. While the SSST estimates reveal greater surface water cooling than reconstructed by CLIMAP [1981], reaching a maximum in the area of the present Subantarctic Zone, the sea‐ice reconstruction indicates that CLIMAP overestimated the expansion of the Antarctic sea‐ice field, especially for austral summer. During winter the sea‐ice field was expanded by 60–70% compared to the present. Last glacial summer sea surface isotherms indicate a northward shift of the zonal bands of the Antarctic Circumpolar Current and a relative expansion of the cold water realm south of the Subantarctic Front by ∼5° in latitude. This coincides with a northward displacement of the zone of enhanced biogenic silica deposition and iceberg occurrence. As a result of northward expansion of Antarctic cold waters and a relatively small displacement of the Subtropical Front, thermal gradients were steepened during the last glacial in the area of the present Subtropical Front. The northward displacement of Antarctic cold waters and the related deflection of Southern Ocean waters along the eastern boundary of South America may have resulted in a weakened “cold water route” across the Drake Passage. In contrast, the transport of warm and salty surface water from the Indian into the Atlantic Ocean via the “warm water route” was not blocked allowing continuous but reduced import of heat into the South Atlantic.

Glacial sea surface temperatures in the subtropical North Pacific: A comparison of U37k′, δ18O, and foraminiferal assemblage temperature estimates

We have investigated glacial‐interglacial differences in sea surface temperature (SST) near Hawaii using two relatively high deposition rate, shallow‐water piston cores collected near Oahu, Hawaii. Modern hydrographic data show that local surface water temperatures are broadly consistent with the regional pattern of SSTs in the southern subtropical North Pacific. Past SSTs were estimated on the basis of three independently measured parameters: (1) U37k′ values of alkenones, (2) δ18O of Globigerinoides ruber, and (3) assemblages of planktonic foraminifera using the modern analog technique (MAT). The two cores yield similar SST records, and if differences in the ecology of foraminifera and coccolithophores are considered, the three different approaches to estimating SSTs yield consistent results. U37k′‐based temperatures, which may represent winter values at this location, were ∼2.5°C colder during the Last Glacial Maximum than today, which is consistent with the February MAT estimates. The δ18O‐based temperature estimates, likely biased toward summer temperatures, indicate that the glacial SSTs were at least 1°C cooler than today, which is comparable to the results of MAT August estimates.

The climate of the Last Glacial Maximum: Results from a coupled atmosphere‐ocean general circulation model

Results from a coupled atmosphere‐ocean general circulation model simulation of the Last Glacial Maximum reveal annual mean continental cooling between 4° and 7°C over tropical landmasses, up to 26° of cooling over the Laurentide ice sheet, and a global mean temperature depression of 4.3°C. The simulation incorporates glacial ice sheets, glacial land surface, reduced sea level, 21 ka orbital parameters, and decreased atmospheric CO2. Glacial winds, in addition to exhibiting anticyclonic circulations over the ice sheets themselves, show a strong cyclonic circulation over the northwest Atlantic basin, enhanced easterly flow over the tropical Pacific, and enhanced westerly flow over the Indian Ocean. Changes in equatorial winds are congruous with a westward shift in tropical convection, which leaves the western Pacific much drier than today but the Indonesian archipelago much wetter. Global mean specific humidity in the glacial climate is 10% less than today. Stronger Pacific easterlies increase the tilt of the tropical thermocline, increase the speed of the Equatorial Undercurrent, and increase the westward extent of the cold tongue, thereby depressing glacial sea surface temperatures in the western tropical Pacific by ∼5°–6°C.

Glacial–interglacial changes in Subantarctic sea surface temperature and δ 18O-water using foraminiferal Mg

Laboratory culturing experiments with living Globigerina bulloides indicate that Mg/Ca is primarily a function of seawater temperature and suggest that Mg/Ca of fossil specimens is an effective paleotemperature proxy. Using culturing results and a core-top Neogloboquadrina pachyderma calibration, we have estimated glacial–interglacial changes in sea surface temperature (SST) using planktonic Mg/Ca records from core RC11-120 in the Subantarctic Indian Ocean (43°S, 80°E) and core E11-2 in the Subantarctic Pacific Ocean (56°S, 115°W). Our results suggest that glacial SST was about 4°C cooler in the Subantarctic Indian Ocean and 2.5°C cooler in the Subantarctic Pacific. Comparison of SST and planktonic δ 18O records indicates that changes in SST lead changes in δ 18O by on average 1–3 kyr. The glacial–interglacial temperature change indicated by the Subantarctic Mg/Ca records suggests that temperature accounts for 40–60% of the foraminiferal δ 18O change. We have used the Mg/Ca-based SST estimates and δ 18O determinations to generate site-specific seawater δ 18O records, which suggest that seawater δ 18O was on average 1‰ more positive during glacial episodes compared with interglacial episodes.