Climate-Vegetation-Feedbacks as a Mechanism for Accelerated Climate Change: The onset of the African Humid Period

Paleo-environmental records and models indicate that the African Humid Period (AHPabruptly ended about 5000-4000 years before present (BP). Some proxies indicate alsan abrupt onset of the AHP between 14,000 and 11,000 BP. How important are local orbitaforcing, ice-sheet forcing, greenhouse gas forcing, and the reorganization of the AtlantiMeridional Overturning Circulation (AMOC) for changes in the African Monsoon/vegetatiosystem? Here we use transient simulations with climate-vegetation models of differencomplexity to identify the factors that control the onset of the African Monsoon/VegetationWe test the following hypothesis:(1) There is no indication for insolation-thresholds for the onset/break of the AHP.(2) Forcing from CO2/ice-sheets significantly controls the climate of North Africa.(3) CO2 fertilization contributes to the vegetation changes over North Africa.(4) A shutdown of the AMOC is as important as orbital insolation for the African Monsoon

Northern Hemisphere interdecadal variability: A coupled air-sea mode

A coupled air–sea mode in the Northern Hemisphere with a period of about 35 years is described. The mode was derived from a multicentury integration with a coupled ocean–atmosphere general circulation model and involves interactions of the thermohaline circulation with the atmosphere in the North Atlantic and interactions between the ocean and the atmosphere in the North Pacific. The authors focus on the physics of the North Atlantic interdecadal variability. If, for instance, the North Atlantic thermohaline circulation is anomalously strong, the ocean is covered by positive sea surface temperature (SST) anomalies. The atmospheric response to these SST anomalies involves a strengthened North Atlantic Oscillation, which leads to anomalously weak evaporation and Ekman transport off Newfoundland and in the Greenland Sea, and the generation of negative sea surface salinity (SSS) anomalies. These SSS anomalies weaken the deep convection in the oceanic sinking regions and subsequently the strength of the thermohaline circulation. This leads to a reduced poleward heat transport and the formation of negative SST anomalies, which completes the phase reversal. The Atlantic and Pacific Oceans seem to be coupled via an atmospheric teleconnection pattern and the interdecadal Northern Hemispheric climate mode is interpreted as an inherently coupled air–sea mode. Furthermore, the origin of the Northern Hemispheric warming observed recently is investigated. The observed temperatures are compared to a characteristic warming pattern derived from a greenhouse warming simulation with the authors’ coupled general circulation model and also with the Northern Hemispheric temperature pattern associated with the 35-yr climate mode. It is shown that the recent Northern Hemispheric warming projects well onto the temperature pattern of the interdecadal mode under consideration

Modes of variability as simulated by a global climate model

We discussed in this thesis an orchestra of climate modes with characteristic time scales of about 5, 16 and 35 years (see Fig.6.3). Interannual variability in the Pacific is ENSO-like. This quasi-oscillatory mode can be understood as a coupled air-sea mode. We discussed the origin of ENSO variance modu- lations, which occur on time scales of. 22 and 35 years. In particular the 35 year ENSO modulation arises from an interaction between ENSO and a 35 year Northern Hemi- spheric climate mode. Interdecadal wind stress curl anomalies in the subtropical Pacific initiate Rossby lryaves, which are associated also with subsurface temperature anomalies. These waves enter the equatorial region 8-12 years later. The subsequent changes in the mean equatorial temperature structure are considered to be important for the simulated interdecadal changes of ENSO variability. However, it was shown that the simulated ENSO cycle in the ECHAM3/LSG model is rather insensitive to global warming, whereas it is revealed from the scenario A integration performed with the more realistic CGCM ECHAM4/OPYC3 that global warming will increase the ENSO variability (Timmermann et al. 1998). This discrepancy in the sensitivities of these two climate models can be ex- plained in terms of the mean thermocline structure, which in the ECHAM3/LSG model is highly diffusive, whereas in the ECHAM4/OPYC3 simulation it is relatively sharp. In the North Atlantic, two climate modes are identified with time scales of 14-18 and 30-40 years, which can be understood in terms of the coupled air-sea mode framework. The wind-driven North Atlantic gyre system provides the memory of the decadal North Atlantic mode. The North Atlantic Oscillation is the atmospheric agent in this coupled air-sea mode, serving on the one hand as a stabilizer of oceanic temperature anomalies and on the other hand as an initiator of the temperature tendency which causes the phase reversal after some delay. The dynamics can be condensed within a feedback loop, which is shown in Fig.4.11. The 30-40 year North Atlantic mode (Timmermann et al. 1998a) involves interactions between the NAO and the North Atlantic thermohaline circulation. The generation of salinity anomalies plays a crucial role for the phase reversal. A feedback loop of the interdecadal oscillation in the North Atlantic is depicted in Fig.5.15. Thus, all climate modes discussed here fit into the coupled air sea mode concept. We aimed to explore those physical processes which are relevant for the generation of climate modes on interannual to interdecadal time scales. Understanding the major pro- cesses and the space-time characteristics of long-term climate variability by means ofCGCM experiments is not only of academic but also of direct interest for societies in the context of climate predictions (Grötzner et al. 1998). Due to the fact that the thermoha- line circulation transports carbon dioxid into the deep ocean, changes of the thermohaline circulation as discussed in Chapter 5 can be linked also to the global carbon cycle. Further- more the results presented in this thesis could give oceanographers some more indications on where to monitor the variability of the deep ocean. However, translating our results which were obtained from a coupled model to reality is a highly nontrivial task. uTluth i,s much too compli,cated to allow anything but approri,nl,at'ions", (John von Neuman)

Differential Interleukin-2 Transcription Kinetics Render Mouse but Not Human T Cells Vulnerable to Splicing Inhibition Early after Activation

T cells are nodal players in the adaptive immune response against pathogens and malignant cells. Alternative splicing plays a crucial role in T cell activation, which is analyzed mainly at later time points upon stimulation. Here we have discovered a 2-h time window early after stimulation where optimal splicing efficiency or, more generally, gene expression efficiency is crucial for successful T cell activation. Reducing the splicing efficiency at 4 to 6 h poststimulation significantly impaired murine T cell activation, which was dependent on the expression dynamics of the Egr1-Nab2-interleukin-2 (IL-2) pathway. This time window overlaps the time of peak IL-2 de novo transcription, which, we suggest, represents a permissive time window in which decreased splicing (or transcription) efficiency reduces mature IL-2 production, thereby hampering murine T cell activation. Notably, the distinct expression kinetics of the Egr1-Nab2-IL-2 pathway between mouse and human render human T cells refractory to this vulnerability. We propose that the rational temporal modulation of splicing or transcription during peak de novo expression of key effectors can be used to fine-tune stimulation-dependent biological outcomes. Our data also show that critical consideration is required when extrapolating mouse data to the human system in basic and translational research

International Investment Performance: Evidence from Institutional Investors’ Foreign Equity Holdings

This paper analyzes the international equity holdings of a large panel of UK pension funds. We find considerable evidence of market timing activity, as illustrated by the funds’ decision to scale back their investments in the US stock market during the 1990s. To explain this we jointly model time-varying conditional moments and portfolio weight dynamics. Past returns do not adequately explain the funds’ international portfolio flows and only matter for the short run. Instead we find that a substantial part of the evolution in portfolio weights is explained by time-varying conditional expected returns, volatilities and covariances with domestic equity returns. However, once we control for publicly known state variables, there is no evidence of extra-market timing skills and most funds appear to have earned small but negative returns from international market timing

Interactions between marine biota and ENSO: a conceptual model analysis

We develop a conceptual coupled atmosphereocean-ecosystem model for the tropical Pacific to investigate the interaction between marine biota and the El Nino-Southern Oscillation (ENSO). Ocean and atmosphere are represented by a two-box model for the equatorial Pacific cold tongue and the warm pool, including a simplified mixed layer scheme. Marine biota are represented by a three-component (nutrient, phytoplankton, and zooplankton) ecosystem model. The atmosphere-ocean model exhibits an oscillatory state which qualitatively captures the main physics of ENSO. During an ENSO cycle, the variation of nutrient upwelling, and, to a small extent, the variation of photosynthetically available radiation force an ecosystem oscillation. The simplified ecosystem in turn, due to the effect of phytoplankton on the absorption of shortwave radiation in the water column, leads to (1) a warming of the tropical Pacific, (2) a reduction of the ENSO amplitude, and (3) a prolongation of the ENSO period. We qualitatively investigate these biophysical coupling mechanisms using continuation methods. It is demonstrated that bio- physical coupling may play a considerable role in modulating ENSO variability

Preserving the measure of compatibility between quantum states

In this paper after defining the abstract concept of compatibility-like functions on quantum states, we prove that every bijective transformation on the set of all states which preserves such a function is implemented by an either unitary or antiunitary operator.Comment: 11 pages, submitted for publicatio