Past climate variability: model analysis and proxy intercomparison

This thesis investigates the climate variability of the late Quaternary (21 000 yrs BP to present day) using model simulations and proxy data. The thesis consists of four manuscripts and one appendix. In the first two manuscripts and the appendix I, the extratropical Northern Hemisphere atmospheric circulation in different Quaternary time slices (preindustrial, PI, 1750 AD; Mid Holocene, MH, 6 kyrs BP; Last Glacial Maximum, LGM, 21 kyrs BP) is investigated using different climate models. The contributions of greenhouse gases, ice-sheet topography and albedo on the atmospheric mean climate and its variability are analyzed. In general, the models show no major changes in atmospheric circulation nor in its interannual variability in a climate slightly warmer (MH) than the PI one. In the LGM simulations, the models show decreased sea level pressure interannual variability relative to PI; on the other hand, the interannual variability of surface temperature is increased. The leading mode of sea level pressure variability in the North Atlantic is characterized by a NAO-like behavior in all climate states; however, it represents less total variance and the centers of action are weaker at the LGM. The presence of the Laurentide ice-sheet over North America during the LGM accounts for most of the changes observed in the LGM climate. Finally, the models show that the link between atmospheric and surface climate (temperature and precipitation) variability is altered in a glacial climate compared to the PI. Therefore, assuming present-day climate-proxy relationships when interpreting proxy records may well lead to a misinterpretation of past climates. The results of the first manuscript point out that certain proxies may record seasonal rather than annual climate changes or that they could be tape recorders for climate changes far afield rather than local. These issues are tackled in the second and third manuscripts. In paper III, various marine proxy records from the North Atlantic Ocean that spann the Holocene (10-0 kyrs BP) are compared with each other and with a model simulation of the MH. Sea-surface temperature records based on phytoplankton generally show the existence of a warm early to mid Holocene (9-6 kyrs BP) optimum. In contrast, zooplankton-based temperature records from the North Atlantic and Norwegian Sea show a cool mid Holocene anomaly and a trend towards warmer temperatures in the late Holocene. Model results indicate that while the warming of the sea surface was stronger in summer during the MH compared to the PI due to higher solar radiation at the high latitudes, sub-surface depths experienced a cooling, mirroring the winter sea surface temperatures. These physical changes in the surface and sub-surface characteristics of the water column can explain the discrepancies between the Holocene trends exhibited by phytoplankton- and zooplankton- based temperature proxy records. Paper III addresses the possibility that cave deposits, specifically in South Asia, record non-local rather than local climate changes. Using an atmospheric climate model with embedded stable water-isotope tracers, we propose a novel conceptual model to explain the oxygen isotopic changes recorded in Asian caves during abrupt climate changes (such as Heinrich events). We show the key role of the Indian Ocean in driving δ18O variations in both Indian and Chinese cave deposits: changes of the Indian Ocean surface temperature affects the Indian summer monsoon, which in turn leads to a change in the δ18O signature of the precipitation falling over the Indian subcontinent. This signal is eventually transferred from the Indian Ocean to Chinese caves via recycling of continental precipitation. Therefore, caves in eastern China (e.g., Hulu) do not record changes in the East Asian summer monsoon, as previously thought, but rather changes in the Indian summer monsoon

The sensitivity of the ENSO to volcanic aerosol spatial distribution in the MPI large ensemble. In open review for ESD

Using the Max Planck Institute Grand Ensemble (MPI-GE) with 200 members for the historical simulation (1850–2005), we investigate the impact of the spatial distribution of volcanic aerosols on the ENSO response. In particular, we select 3 eruptions (El Chichón, Agung and Pinatubo) in which the aerosol is respectively confined to the Northern Hemisphere, the Southern Hemisphere or equally distributed across the equator. Our results show that the ENSO anomalies start at the end of the year of the eruption and peak the following one. Especially, we found that when the aerosol is located in the Northern Hemisphere or is symmetrically distributed, El Niño-like anomalies develop while aerosol distribution confined to the Southern Hemisphere leads to a La Niña-like anomaly. Our results strongly point to the volcanically induced displacement of the ITCZ as the main mechanism that drives the ENSO response, while suggesting that the other mechanisms (the ocean dynamical thermostat, the cooling of tropical northern Africa or of the Maritime continent) commonly invoked to explain the post-eruption ENSO response appear not to be at play in our mode

Tropical cyclone activity affected by volcanically induced ITCZ shifts

Volcanic eruptions can inject a large amount of aerosol particles, which interact with solar radiation and consequently can affect the climate worldwide, hence the intensity and frequency of extreme events for a few years following the eruption. However, only a handful of studies have investigated the impacts of volcanic eruptions on tropical cyclone activity. Through a set of sensitivity modeling experiments, our study demonstrates that volcanic eruptions by shifting the Intertropical convergence zone can impact tropical cyclone activity up to 4 years following the eruption. These results will prove valuable to society, allowing us to better prepare for the consequences of changes in tropical cyclone activity following large volcanic eruptions

Impacts of dust reduction on the northward expansion of the African monsoon during the Green Sahara period

AbstractThe West African Monsoon (WAM) is crucial for the socio-economic stability of millions of people living in the Sahel. Severe droughts have ravaged the region in the last three decades of the 20th century, highlighting the need for a better understanding of the WAM dynamics. One of the most dramatic changes in the West African Monsoon (WAM) occurred between 15000–5000 yr BP, when increased summer rainfall led to the so-called “Green Sahara” and to a reduction in dust emissions from the region. However, model experiments are unable to fully reproduce the intensification and geographical expansion of the WAM during this period, even when vegetation over the Sahara is considered. Here, we use a fully coupled simulation for 6000 yr BP (Mid-Holocene) in which prescribed Saharan vegetation and dust concentrations are changed in turn. A closer agreement with proxy records is obtained only when both the Saharan vegetation changes and dust decrease are taken into account. The dust reduction strengthens the vegetation–albedo feedback, extending the monsoon's northern limit approximately 500 km further than the vegetation-change case only. We therefore conclude that accounting for changes in Saharan dust loadings is essential for improving model simulations of the WAM during the Mid-Holocene

North Atlantic Oscillation and tropospheric ozone variability in Europe: model analysis and measurements intercomparison

Ozone pollution represents a serious health and environmental problem. While ozone pollution is mostly produced by photochemistry in summer, elevated ozone concentrations can also be influenced by long range transport driven by the atmospheric circulation and stratospheric ozone intrusions. We analyze the role of large scale atmospheric circulation variability in the North Atlantic basin in determining surface ozone concentrations. Here, we show, using ground station measurements and a coupled atmosphere-chemistry model simulation for the period 1980–2005, that the North Atlantic Oscillation (NAO) does affect surface ozone concentrations – on average, over 10 ppbv on the monthly mean in southwest central and northern Europe – during all seasons except fall. The commonly used NAO index is able to capture the link existing between atmospheric dynamics and surface ozone concentrations in winter and spring but it fails in summer. We find that the first Principal Component, computed from the time variation of the sea level pressure (SLP) field, detects the atmosphere circulation/ozone relationship not only in winter and spring but also during summer, when the atmospheric circulation weakens and regional photochemical processes peak. The first Principal Component of the SLP field could be used as a tool to identify areas more exposed to forthcoming ozone pollution events. Finally, our results suggest that the increasing baseline ozone in western and northern Europe during the 1990s could be related to the prevailing phase of the NAO in that period.JRC.H.7-Climate Risk Managemen

Rainfall regimes of the Green Sahara

During the “Green Sahara” period (11,000 to 5000 years before the present), the Sahara desert received high amounts of rainfall, supporting diverse vegetation, permanent lakes, and human populations. Our knowledge of rainfall rates and the spatiotemporal extent of wet conditions has suffered from a lack of continuous sedimentary records. We present a quantitative reconstruction of western Saharan precipitation derived from leaf wax isotopes in marine sediments. Our data indicate that the Green Sahara extended to 31°N and likely ended abruptly. We find evidence for a prolonged “pause” in Green Sahara conditions 8000 years ago, coincident with a temporary abandonment of occupational sites by Neolithic humans. The rainfall rates inferred from our data are best explained by strong vegetation and dust feedbacks; without these mechanisms, climate models systematically fail to reproduce the Green Sahara. This study suggests that accurate simulations of future climate change in the Sahara and Sahel will require improvements in our ability to simulate vegetation and dust feedbacks

Initiation of a Stable Convective Hydroclimatic Regime in Central America Circa 9000 Years BP

Many Holocene hydroclimate records show rainfall changes that vary with local orbital insolation. However, some tropical regions display rainfall evolution that differs from gradual precessional pacing, suggesting that direct rainfall forcing effects were predominantly driven by sea-surface temperature thresholds or inter-ocean temperature gradients. Here we present a 12,000 yr continuous U/Th-dated precipitation record from a Guatemalan speleothem showing that Central American rainfall increased within a 2000 yr period from a persistently dry state to an active convective regime at 9000 yr BP and has remained strong thereafter. Our data suggest that the Holocene evolution of Central American rainfall was driven by exceeding a temperature threshold in the nearby tropical oceans. The sensitivity of this region to slow changes in radiative forcing is thus strongly mediated by internal dynamics acting on much faster time scales

The key role of topography in altering North Atlantic atmospheric circulation during the last glacial period

The Last Glacial Maximum (LGM; 21 000 yr before present) was a period of low atmospheric greenhouse gas concentrations, when vast ice sheets covered large parts of North America and Europe. Paleoclimate reconstructions and modeling studies suggest that the atmospheric circulation was substantially altered compared to today, both in terms of its mean state and its variability. Here we present a suite of coupled model simulations designed to investigate both the separate and combined influences of the main LGM boundary condition changes (greenhouse gases, ice sheet topography and ice sheet albedo) on the mean state and variability of the atmospheric circulation as represented by sea level pressure (SLP) and 200-hPa zonal wind in the North Atlantic sector. We find that ice sheet topography accounts for most of the simulated changes during the LGM. Greenhouse gases and ice sheet albedo affect the SLP gradient in the North Atlantic, but the overall placement of high and low pressure centers is controlled by topography. Additional analysis shows that North Atlantic sea surface temperatures and sea ice edge position do not substantially influence the pattern of the climatological-mean SLP field, SLP variability or the position of the North Atlantic jet in the LGM

Catastrophic drought in the Afro-Asian monsoon region during Heinrich Event 1

Between 18,000 and 15,000 years ago, large amounts of ice and meltwater entered the North Atlantic during Heinrich Stadial 1. This caused substantial regional cooling, but major climatic impacts also occurred in the tropics. Here we demonstrate that the height of this stadial, ca. 17-16,000 years ago ("Heinrich Event 1"), coincided with one of the most extreme and widespread megadroughts of the last 50,000 years or more in the Afro-Asian monsoon region, with potentially serious consequences for Paleolithic cultures. Late Quaternary tropical drying commonly is attributed to southward drift of the Intertropical Convergence Zone, but the broad geographic range of the H1 Megadrought suggests that severe, systemic weakening of Afro-Asian rainfall systems also occurred, probably in response to sea surface cooling

ITCZ shift and extratropical teleconnections drive ENSO response to volcanic eruptions

The mechanisms through which volcanic eruptions affect the El Niño-Southern Oscillation (ENSO) state are still controversial. Previous studies have invoked direct radiative forcing, an ocean dynamical thermostat (ODT) mechanism, and shifts of the Intertropical Convergence Zone (ITCZ), among others, to explain the ENSO response to tropical eruptions. Here, these mechanisms are tested using ensemble simulations with an Earth system model in which volcanic aerosols from a Tambora-like eruption are confined either in the Northern or the Southern Hemisphere. We show that the primary drivers of the ENSO response are the shifts of the ITCZ together with extratropical circulation changes, which affect the tropics; the ODT mechanism does not operate in our simulations. Our study highlights the importance of initial conditions in the ENSO response to tropical volcanic eruptions and provides explanations for the predominance of posteruption El Niño events and for the occasional posteruption La Niña in observations and reconstructions