Last Glacial Maximum Sea Surface Temperatures Research Articles

Quasi-Biennial Oscillation and Sudden Stratospheric Warmings during the Last Glacial Maximum

The quasi-biennial oscillation (QBO) and sudden stratospheric warmings (SSWs) during the Last Glacial Maximum (LGM) are investigated in simulations using the Whole Atmosphere Community Climate Model version 6 (WACCM6). We find that the period of QBO, which is 27 months in the preindustrial and modern climate simulations, was 33 months in the LGM simulation using the proxy sea surface temperatures (SSTs) and 41 months using the model-based LGM SSTs. We show that the longer QBO period in the LGM is due to weaker wave forcing. The WACCM6 simulations of the LGM, preindustrial, and modern climates do not support previous modeling work that suggests that the QBO amplitude is smaller (larger) in a warmer (colder) climate. We find that SSWs in the LGM occurred later in the year, as compared to the preindustrial and modern climate, but that time of the final warming was similar. The difference in SSW frequency is inconclusive.

On the limited ice intrusion in Alaska at the LGM

AbstractThe Last Glacial Maximum (LGM) Laurentide Ice Sheet covered most of the North American continent poleward of 40°N, with the exception of Alaska that remained relatively warm, dry, and largely ice free. Experiments with a global atmospheric circulation model are in broad agreement with proxies: the Alaskan summer temperatures are comparable to the preindustrial, and the annual precipitation is reduced by 30–50%. The warm conditions are attributed to a lowering of the local planetary albedo—due to a decreased cloudiness in response to the cold LGM sea surface temperatures (SSTs) and a stationary anticyclone forced by the ice sheet—that allows more shortwave radiation to reach the surface. Stationary waves are shown to counteract the shortwave cloud feedback by converging less heat over the target region. The LGM SST field also yields an equatorward shifted Pacific stormtrack, which results in drier conditions in Alaska and abundant precipitation at the southern margin of the Laurentide Ice Sheet.

Radiolaria as a reflection of environmental conditions in the eastern and southern sectors of the Indian Ocean: A new statistical approach

Cluster analysis and species abundance plots of radiolarian abundance counts from core tops from the eastern Indian Ocean between 12° S and 31° S, and the southern Indian Ocean between 31° S and 62.5° S, demonstrate the existence of environmentally-related provinces supporting distinct taxa assemblages. These provinces are closely associated with currents in the eastern sector of the Indian Ocean and with fronts in the southern sector. The radiolarian assemblages correlate strongly with salinity-normalised total alkalinity (NTA) at the sea-surface, with temperature, salinity, and density from the sea-surface to 300 m, and with dissolved oxygen and nitrate and phosphate concentrations from the sea-surface to 100 m. Palaeo-reconstructions of these parameters at the sea-surface have been made for six Last Glacial Maximum (LGM) samples from five eastern Indian Ocean cores. The LGM sea-surface temperature estimates are comparable with those based on planktonic foraminiferal counts of the same samples obtained by other researchers. The reconstructions show that, since the LGM, density increased markedly along the Western Australian coast south of 20° S but changed little further from the Western Australian coast. By contrast, phosphate concentrations were marginally lower than modern values along the Western Australian coast south of 20° S but more than twice modern values in the other LGM samples. The utility of various regression and calibration techniques is discussed. It is concluded that, probably due to the effects of differences in radiolarian habitat, ocean currents, and/or environmental gradients, only one method, weighted averaging — partial least squares, is reliable in a study area of this size and complexity. If other methods are to be used, the study area must be partitioned into at least two separate regions with the major split between the eastern and southern sectors of the Indian Ocean.

Evaluation of Last Glacial Maximum sea surface temperature reconstructions through their influence on South American climate

The consistency of land‐based and ocean‐based Last Glacial Maximum (LGM) proxy evidence on and near South America is evaluated by utilizing a regional climate model (RCM). The relatively high resolution of the RCM brings model output closer to the spatial scales of the geological reconstructions. This is important because proxy sites only a few tens of kilometers apart can yield markedly different interpretations of LGM climate. Along with differences in orbital parameters, CO2 concentrations, and vegetation, four versions of LGM sea surface temperatures (SSTs) (Climate: Long‐Range Investigation, Mapping, and Prediction (CLIMAP), Schäfer‐Neth and Paul's (2003) core version, Paul and Schäfer‐Neth's (2003) line version, and coupled GCM output from Shin et al. (2003)), are used to generate four realizations of the LGM climate in the model. Mean annual surface temperature, mean temperature of the coldest month, and aridity differences from a present‐day simulation are compared with the geological reconstructions over South America to evaluate the consistency of the land and ocean‐based reconstructions. Model results suggest that the tropical Atlantic during the LGM was not significantly cooler than CLIMAP's original estimates. Simulations utilizing tropical Atlantic LGM SSTs cooler than CLIMAP estimates yield unrealistically warm LGM surface conditions over the equatorial Amazon basin. Reductions in evaporation and the atmosphere's moisture content over too‐cold tropical oceans lead to a reduction in cloud cover over the Amazon and increases in solar heating of the surface. All of the LGM SST simulations produce drier conditions over tropical South America; however, the extent of the aridity is associated with the degree of cooling in the surrounding oceans, especially the tropical Atlantic. The “line” version LGM SSTs of Paul and Schäfer‐Neth (2003) are recommended.

The response of NW Iberian vegetation to North Atlantic climate oscillations during the last 65 kyr

Pollen and oceanographic data from deep ocean core MD95-2039 provide a centennial to millennial scale record of conditions offshore and of the vegetation of north-west Iberia for the period 10–65 ka. The planktonic oxygen isotope record of this core, reflecting predominantly sea surface temperature (SST), shows a pattern of millennial-scale oscillations that is very similar to climatic changes recorded by the Greenland ice core records over the same interval. In turn, tree populations show a pattern of rapid expansions and contractions that follow the pronounced and abrupt isotopic shifts recorded offshore. Through Marine Isotope Stage (MIS) 3, this millennial-scale pattern of vegetation change, alternating between steppe and open woodland, is superimposed on a longer-term pattern of shrinking ericaceous heathland and decreasing size of successive interstadial tree populations. Trees persisted during the Last Glacial Maximum (LGM), at greater abundance than during many of the coldest episodes of MIS 3. This agrees with the marine data which indicate that LGM sea surface temperatures here were significantly warmer than the minima recorded in MIS 3. Our combined marine-terrestrial record, together with data from nearby sequences, provides a stepping stone between terrestrial sequences and the Greenland ice core and North Atlantic marine records. This will permit a better understanding of the behaviour of vegetation across different regions at several scales of climatic forcing.

Sea-surface temperatures around the Australian margin and Indian Ocean during the Last Glacial Maximum

We present new last glacial maximum (LGM) sea-surface temperature (SST) maps for the oceans around Australia based on planktonic foraminifera assemblages. To provide the most reliable SST estimates we use the modern analog technique, the revised analog method, and artificial neural networks in conjunction with an expanded modern core top database. All three methods produce similar quality predictions and the root mean squared error of the consensus prediction (the average of the three) under cross-validation is only ±0.77 °C. We determine LGM SST using data from 165 cores, most of which have good age control from oxygen isotope stratigraphy and radiocarbon dates. The coldest SST occurred at 20,500±1400 cal yr BP, predating the maximum in oxygen isotope records at 18,200±1500 cal yr BP. During the LGM interval we observe cooling within the tropics of up to 4 °C in the eastern Indian Ocean, and mostly between 0 and 3 °C elsewhere along the equator. The high latitudes cooled by the greatest degree, a maximum of 7–9 °C in the southwest Pacific Ocean. Our maps improve substantially on previous attempts by making higher quality temperature estimates, using more cores, and improving age control.

Modeling the tropical climate and the impact of the western Pacific sea surface temperature at the Last Glacial Maximum

Changes in tropical atmospheric heat and circulation patterns, as well as the effects of sea surface temperature (SST) of the western Pacific during the Last Glacial Maximum (LGM), have been investigated using a climate model and the LGM SST compiled by Wang [1999]. The annual mean LGM SST was 2°–4°C lower than the present day in the tropical western Pacific west of 140°E. Simulations show that the largest decrease of atmospheric heat occurs in the tropics in a region stretching from the Bay of Bengal to the central Pacific Ocean, while there are no significant changes in atmospheric heat at higher latitudes. This indicates that the atmospheric heat in the tropics shows a stronger response to boundary changes between the LGM and the present day than it does at higher latitudes, since the tropical convection amplifies the response of the atmosphere in the former case. Because of this tropical heat change, the Walker circulation, the transverse monsoon, and the east Asian summer monsoon at the LGM were weaker than they are at present. The model with the Wang [1999] LGM SST simulated large changes in atmospheric circulation in the tropics and high northern latitudes compared to the Climate, Long‐Range Investigation, Mapping and Prediction (CLIMAP) [CLIMAP Project Members, 1981] project's reconstruction. The Wang [1999] LGM SST was also associated with a lower surface air temperature (SAT) at high northern latitudes and the Arctic and with a cooler temperature of the coldest month in Europe. The annual mean SAT decreased by 2°C in the Arctic, and the winter mean SAT decreased by 4°C in Europe.

Atmospheric Response to Modified CLIMAP Ocean Boundary Conditions during the Last Glacial Maximum*

Global climate simulations are conducted to examine the sensitivity of the Last Glacial Maximum (LGM) climate to prescribed sea surface temperatures (SSTs) that are modified from the Climate: Long-range Investigation, Mapping, and Prediction (CLIMAP) study. Based on the consensus from various LGM proxy data, the SSTs are cooled by 48C uniformly in the Tropics (308N‐308S) relative to CLIMAP, and the high-latitude sea ice extent is reduced. Compared to results from a simulation with CLIMAP SSTs, the modified LGM SSTs cause significant opposing changes in the hemispheric and regional-scale atmospheric circulation, which are most pronounced in the winter hemisphere. For instance, there is significant weakening of the midlatitude circulation and reduction of 500-hPa eddy kinetic energy and midlatitude precipitation resulting from the decreased meridional temperature gradient in the modified SST simulation. In contrast, reduced sea ice extent during the boreal winter causes increased regional baroclinicity and intensified atmospheric circulation in the western North Pacific and the North Atlantic. Cooled tropical SSTs also increase the land‐ocean temperature contrast, which strengthens the Asian summer monsoon circulation. Both LGM simulations produce enhanced low-level convergence and increased precipitation along the South Pacific convergence zone (SPCZ) relative to present day, despite the cooler LGM climate. The SPCZ orientation and intensity are closely linked to the distribution of South Pacific SSTs. Comparison of surface temperature estimates from land- and ocean-based proxy data with model output suggests that uniform cooling of the tropical SSTs and modification of the high-latitude sea ice extent may be sufficient to accurately simulate the first-order characteristics of the LGM climate.

Sea surface temperature at the Last Glacial Maximum: A reconstruction using the modern analog technique

We have applied the modern analog technique (MAT) to reconstruct sea surface temperature (SST) at the Last Glacial Maximum (LGM) using a database of 292 planktonic foraminiferal samples from the Atlantic, Indian, and Pacific Oceans. LGM SST maps reveal many important patterns and gradients which reflect the climate and circulation patterns of the LGM. One of the most important results is the 2°–6°C mean annual cooling in the central to eastern tropical Atlantic and up 8°C cooling in the eastern tropical Pacific. These changes resulted in an increased east‐west SST gradient in the equatorial regions of these two basins, compared to modern. SSTs in eastern boundary currents of all three ocean basins were cooler than modern at the LGM. Additionally, results indicate more zonal flow patterns in the Kuroshio/North Pacific Current and Gulf Stream/North Atlantic Drift. These changes indicate more intense midlatitude‐to‐equator SST gradients at the LGM, which likely resulted in more intense circulation and increased zonal winds. SSTs in much of the tropical and northern Indian Ocean were within 2°C of modern temperatures. Consistent with the results of previous studies, results indicate little change in much of the western tropics and the subtropics of all ocean basins.

Present-Day and Last-Glacial-Maximum Ocean Thermohaline Circulation in a Zonally Averaged Coupled Ocean–Sea-Ice–Atmosphere Model

Abstract A simple ocean–atmosphere model suitable for long-term paleoclimate studies is presented. It consists of a three-basin zonally averaged ocean model coupled to an energy moisture-balance atmospheric model and a thermodynamic sea-ice model. The model is used to simulate the “present-day” climate conditions and the conditions resembling those of the last glacial maximum (LGM). In the first case, the model has an ocean thermohaline (THC) circulation in which the overturning in the North Atlantic is slightly more than 15 Sv. Two methods are used to spin up the model to the LGM climate. First, a cool state is obtained by running the coupled model with a reduced model greenhouse effect and increased albedo, but without making any explicit changes to the hydrological cycle. In the second approach, the model is directly set up for LGM conditions, in a manner similar to the method used to simulate the present-day climate. In both cases the computed LGM sea surface temperatures are reasonable, but only in t...

Tropical climate at the last glacial maximum inferred from glacier mass-balance modeling.

Model-derived equilibrium line altitudes (ELAs) of former tropical glaciers support arguments, based on other paleoclimate data, for both the magnitude and spatial pattern of terrestrial cooling in the tropics at the last glacial maximum (LGM). Relative to the present, LGM ELAs were maintained by air temperatures that were 3.5 degrees to 6.6 degrees C lower and precipitation that ranged from 63% wetter in Hawaii to 25% drier on Mt. Kenya, Africa. Our results imply the need for a approximately 3 degrees C cooling of LGM sea surface temperatures in the western Pacific warm pool. Sensitivity tests suggest that LGM ELAs could have persisted until 16,000 years before the present in the Peruvian Andes and on Papua, New Guinea.

The effect of reduced ocean overturning on the climate of the last glacial maximum

This study focuses on the differences between the present-day climate and the climate of the last glacial maximum (LGM) of 18 000 y BP using a zonally averaged energy balance climate model. The ocean is represented by a 2-D model with prescribed overturning pattern in which the overturning velocities can be adjusted freely. We discuss what influence the use of ice-age conditions (i.e. enhanced land-ice cover, reduced CO2-concentration and reduced oceanic overturning rate) has on the differences between ice-age and present-day climate. When compared to LGM sea-surface temperatures derived from proxy data, the model is able to simulate fairly well the important features of the meridional distribution of these temperature differences. Applying reduced ocean overturning rates during the LGM significantly decreases poleward heat transport in the oceans, thereby allowing for additional cooling of the polar regions and less cooling of the equatorial region. As a result, the agreement with CLIMAP proxy temperature differences increases, especially in the equatorial region. This mechanism can explain the slight differences in the CLIMAP proxy equatorial surface temperatures between the LGM and the present-day climate.

The effect of reduced ocean overturning on the climate of the last glacial maximum

This study focuses on the differences between the present-day climate and the climate of the last glacial maximum (LGM) of 18 000 y BP using a zonally averaged energy balance climate model. The ocean is represented by a 2-D model with prescribed overturning pattern in which the overturning velocities can be adjusted freely. We discuss what influence the use of ice-age conditions (i.e. enhanced land-ice cover, reduced CO2-concentration and reduced oceanic overturning rate) has on the differences between ice-age and present-day climate. When compared to LGM sea-surface temperatures derived from proxy data, the model is able to simulate fairly well the important features of the meridional distribution of these temperature differences. Applying reduced ocean overturning rates during the LGM significantly decreases poleward heat transport in the oceans, thereby allowing for additional cooling of the polar regions and less cooling of the equatorial region. As a result, the agreement with CLIMAP proxy temperature differences increases, especially in the equatorial region. This mechanism can explain the slight differences in the CLIMAP proxy equatorial surface temperatures between the LGM and the present-day climate.