Stephan J. Lorenz

Increased seasonality in Middle East temperatures during the last interglacial period

The last interglacial period (about 125,000 years ago) is thought to have been at least as warm as the present climate. Owing to changes in the Earths orbit around the Sun, it is thought that insolation in the Northern Hemisphere varied more strongly than today on seasonal timescales, which would have led to corresponding changes in the seasonal temperature cycle. Here we present seasonally resolved proxy records using corals from the northernmost Red Sea, which record climate during the last interglacial period, the late Holocene epoch and the present. We find an increased seasonality in the temperature recorded in the last interglacial coral. Today, climate in the northern Red Sea is sensitive to the North Atlantic Oscillation, a climate oscillation that strongly influences winter temperatures and precipitation in the North Atlantic region. From our coral records and simulations with a coupled atmosphere–ocean circulation model, we conclude that a tendency towards the high-index state of the North Atlantic Oscillation during the last interglacial period, which is consistent with European proxy records, contributed to the larger amplitude of the seasonal cycle in the Middle East.

Aerosol size confines climate response to volcanic super-eruptions

Extremely large volcanic eruptions have been linked to global climate change, biotic turnover, and, for the Younger Toba Tuff (YTT) eruption 74,000 years ago, near-extinction of modern humans. One of the largest uncertainties of the climate effects involves evolution and growth of aerosol particles. A huge atmospheric concentration of sulfate causes higher collision rates, larger particle sizes, and rapid fall out, which in turn greatly affects radiative feedbacks. We address this key process by incorporating the effects of aerosol microphysical processes into an Earth System Model. The temperature response is shorter (9–10 years) and three times weaker (−3.5 K at maximum globally) than estimated before, although cooling could still have reached −12 K in some midlatitude continental regions after one year. The smaller response, plus its geographic patchiness, suggests that most biota may have escaped threshold extinction pressures from the eruption.

Orbitally driven insolation forcing on Holocene climate trends: Evidence from alkenone data and climate modeling

A global spatial pattern of long-term sea surface temperature (SST) trends over the last 7000 years is explored using a comparison of alkenone-derived SST records with transient ensemble climate simulations with a coupled atmosphere-ocean circulation model under orbitally driven insolation forcing. The spatial trend pattern both in paleo-SST data and in model results shows pronounced global heterogeneity. Generally, the extratropics cooled while the tropics experienced a warming during the middle to late Holocene. We attribute these divergent Holocene climate trends to seasonally opposing insolation changes. Furthermore, climate mode changes similar to the Arctic/North Atlantic Oscillation are superimposed on the prevalent pattern. It is concluded that nonlinear changes in the entire seasonal cycle of insolation played a dominant role for the temporal evolution of Holocene surface temperatures. For understanding of marine proxy data, apart from the dominance of summer insolation in high latitudes, a notable shift in the maximum insolation of the year in low latitudes has to be taken into account, which may influence timing of phytoplankton production and thus alters the seasonal origin of temperature signals recorded in the proxies.

The Effect of Orbital Forcing on the Mean Climate and Variability of the Tropical Pacific

Abstract Using a coupled general circulation model, the responses of the climate mean state, the annual cycle, and the El Nino–Southern Oscillation (ENSO) phenomenon to orbital changes are studied. The authors analyze a 1650-yr-long simulation with accelerated orbital forcing, representing the period from 142 000 yr b.p. (before present) to 22 900 yr a.p. (after present). The model simulation does not include the time-varying boundary conditions due to ice sheet and greenhouse gas forcing. Owing to the mean seasonal cycle of cloudiness in the off-equatorial regions, an annual mean precessional signal of temperatures is generated outside the equator. The resulting meridional SST gradient in the eastern equatorial Pacific modulates the annual mean meridional asymmetry and hence the strength of the equatorial annual cycle. In turn, changes of the equatorial annual cycle trigger abrupt changes of ENSO variability via frequency entrainment, resulting in an anticorrelation between annual cycle strength and ENSO...

On the hydrological cycle under paleoclimatic conditions as derived from AGCM simulations

The atmospheric hydrological cycle is compared for different time slices of the late Quaternary. Simulations have been conducted with an atmospheric circulation model at T42 resolution, and we have performed a global evaluation of the atmospheric water vapor transport. The water export from the Atlantic catchment area, important for driving the large-scale thermohaline ocean circulation, is analyzed in detail. For the Last Glacial Maximum (LGM), we examine the models sensitivity with respect to tropical cooling relative to the CLIMAP reconstruction which is motivated by recent data. We find that the LGM experiment with tropical cooling is in better agreement with proxy data available. Our experiments indicate that the water vapor transport is strongly affected by three mechanisms: continental drying, eddy moisture transport, and changes in the tropical circulation. Except for the continental drying and the blocking effect of the Laurentide ice sheet, the hydrological cycle is substancially different for both LGM experiments. We find that the hydrological system is rather sensitive to tropical temperature change which is important to understand paleoclimate and future climate changes.

Sensitivity of a coupled climate‐carbon cycle model to large volcanic eruptions during the last millennium

The sensitivity of the climate–biogeochemistry system to volcanic eruptions is investigated using the comprehensive Earth System Model developed at the Max Planck Institute for Meteorology. The model includes an interactive carbon cycle with modules for terrestrial biosphere as well as ocean biogeochemistry. The volcanic forcing is based on a recent reconstruction for the last 1200 yr. An ensemble of five simulations is performed and the averaged response of the system is analysed in particular for the largest eruption of the last millennium in the year 1258. After this eruption, the global annual mean temperature drops by 1 K and recovers slowly during 10 yr. Atmospheric CO2 concentration declines during 4 yr after the eruption by ca. 2 ppmv to its minimum value and then starts to increase towards the pre-eruption level. This CO2 decrease is explained mainly by reduced heterotrophic respiration on land in response to the surface cooling, which leads to increased carbon storage in soils, mostly in tropical and subtropical regions. The ocean acts as a weak carbon sink, which is primarily due to temperature-induced solubility. This sink saturates 2 yr after the eruption, earlier than the land uptake.

Simulated Relationships between Regional Temperatures and Large-Scale Circulation: 125 kyr BP (Eemian) and the Preindustrial Period

To investigate relationships between large-scale circulation and regional-scale temperatures during the last (Eemian) interglacial, a simulation with a general circulation model (GCM) under orbital forcing conditions of 125 kyr BP is compared with a simulation forced with the Late Holocene preindustrial conditions. Consistent with previous GCM simulations for the Eemian, higher northern summer 2-m temperatures are found, which are directly related to the different insolation. Differences in the mean circulation are evident such as, for instance, stronger northern winter westerlies toward Europe, which are associated with warmer temperatures in central and northeastern Europe in the Eemian simulation, while the circulation variability, analyzed by means of a principal component analysis of the sea level pressure (SLP) field, is very similar in both periods. As a consequence of the differences in the mean circulation the simulated Arctic Oscillation (AO) temperature signal in the northern winter, on interannual-to-multidecadal time scales, is weaker during the Eemian than today over large parts of the Northern Hemisphere. Correlations between the AO index and the central European temperature (CET) decrease by about 0.2. The winter and spring SLP anomalies over the North Atlantic/European domain that are most strongly linearly linked to the CET cover a smaller area and are shifted westward over the North Atlantic during the Eemian. However, the strength of the connection between CET and these SLP anomalies is similar in both simulations. The simulated differences in the AO temperature signal and in the SLP anomaly, which is linearly linked to the CET, suggest that during the Eemian the link between the large-scale circulation and temperaturesensitive proxy data from Europe may differ from present-day conditions and that this difference should be taken into account when inferring large-scale climate from temperature-sensitive proxy data.

Sensitivity of Northern Hemispheric continental ice sheets to tropical SST during deglaciation

A thermomechanical ice sheet model (ISM) is used to investigate the sensitivity of the Laurentide and Fennoscandian ice sheets to tropical sea surface temperature (SST) perturbations during deglaciation. The ISM is driven by surface temperature and precipitation fields from three different atmospheric general circulation models (AGCMs). For each AGCM, the responses in temperature and precipitation over the ice sheets nearly compensate, such that ice sheet mass balance is not strongly sensitive to tropical SST boundary conditions. It was also found that there is significant variation in the response of the ISM to the different AGCM output fields.

Orbital forcing on atmospheric dynamics during the last interglacial and glacial inception

Abstract Large-scale atmospheric patterns are examined on orbital timescales using the ECHO-G climate model which explicitly resolves the atmosphere-ocean-sea ice dynamics. It is shown that in contrast to boreal summer where the climate follows mainly the local radiative forcing, the boreal winter climate is strongly determined by modulation of circulation modes linked to the Arctic Oscillation/North Atlantic Oscillation and the El Nino Southern Oscillation. We find that a positive phase of the Arctic Oscillation/ North Atlantic Oscillation is linked to below normal convection in the tropical Pacific. The related atmospheric circulation patterns induce nonuniform temperature anomalies, much stronger in amplitude than by the direct solar insolation. In concert with the direct solar insolation, this provides for a temperature drop over the Northern Hemisphere continents 115 000 years before present. We argue that the large-scale teleconnection pattern is important for the interpretation of proxy data as well as for the mechanisms responsible for the last interglacial, glacial inception and millennial climate variability.

Simulation and comparison between mid-Holocene and preindustrial Indian summer monsoon circulation using a regional climate model

The regional climate model HIRHAM has been applied over the Asian continent from 0oN to 50oN and 42oE to 110oE to simulate the Indian monsoon circulation under past and present-day conditions. The model is driven at the lateral and lower boundaries by the atmospheric output fields of the global coupled Earth system model ECHAM5-JSBACH/MPIOM for 44-years-long time slices during the mid-Holocene and the preindustrial present-day climate.Simulations with a horizontal resolution of 50 km are carried out to analyze the regional monsoon patterns under different external solar forcing and climatic conditions. The focus is on the investigation of the HIRHAM simulated summer monsoon circulation and the comparison of the regional atmospheric circulation and precipitation patterns between the paleo- and the preindustrial climate. Due to mid-Holocene changes in the atmospheric circulation with a reduced and southward shifted monsoonal flow across Arabian Sea and Bay of Bengal, an increase of summer rainfall at the windward slopes of western and southern Himalayas as well as over southern India and decreased rainfall over central India appear which is in agreement with proxy-derived precipitation reconstructions. During the mid-Holocene as well as for the present-day climate the same driving mechanisms for the summer monsoon in extreme wet monsoon years related to regional SST anomalies in the Indian Ocean and convective processes can be verified. Positive (negative) SST anomalies in the northern Indian Ocean enhance (inhibit) the local convection associated with a deepening (weakening) of the low pressure and trigger wet (dry) rainfall anomalies.