2009, Schepanski, K., Tegen, I., MacKe, A.

We present a study of Saharan dust export towards the tropical North Atlantic using the regional dust emission, transport and deposition model LM-MUSCAT. Horizontal and vertical distribution of dust optical thickness, concentration, and dry and wet deposition rates are used to describe seasonality of dust export and deposition towards the eastern Atlantic for three typical months in different seasons. Deposition rates strongly depend on the vertical dust distribution, which differs with seasons. Furthermore the contribution of dust originating from the Bod́eĺe Depression to Saharan dust over the Atlantic is investigated. A maximum contribution of Bod́eĺe dust transported towards the Cape Verde Islands is evident in winter when the Bod́eĺe source area is most active and dominant with regard to activation frequency and dust emission. Limitations of using satellite retrievals to estimate dust deposition are highlighted.

2007, Braun, H., Ganopolski, A., Christl, M., Chialvo, D.R.

Here we use a very simple conceptual model in an attempt to reduce essential parts of the complex nonlinearity of abrupt glacial climate changes (the so-called Dansgaard-Oeschger events) to a few simple principles, namely (i) the existence of two different climate states, (ii) a threshold process and (iii) an overshooting in the stability of the system at the start and the end of the events, which is followed by a millennial-scale relaxation. By comparison with a so-called Earth system model of intermediate complexity (CLIMBER-2), in which the events represent oscillations between two climate states corresponding to two fundamentally different modes of deep-water formation in the North Atlantic, we demonstrate that the conceptual model captures fundamental aspects of the nonlinearity of the events in that model. We use the conceptual model in order to reproduce and reanalyse nonlinear resonance mechanisms that were already suggested in order to explain the characteristic time scale of Dansgaard-Oeschger events. In doing so we identify a new form of stochastic resonance (i.e. an overshooting stochastic resonance) and provide the first explicitly reported manifestation of ghost resonance in a geosystem, i.e. of a mechanism which could be relevant for other systems with thresholds and with multiple states of operation. Our work enables us to explicitly simulate realistic probability measures of Dansgaard-Oeschger events (e.g. waiting time distributions, which are a prerequisite for statistical analyses on the regularity of the events by means of Monte-Carlo simulations). We thus think that our study is an important advance in order to develop more adequate methods to test the statistical significance and the origin of the proposed glacial 1470-year climate cycle.

2008, Peters, D.H.W., Gabriel, A., Entzian, G.

This study examines the longitude-dependent decadal changes and trends of ozone for the boreal winter months during the period of 1960–2000. These changes are caused primarily by changes in the planetary wave structure in the upper troposphere and lower stratosphere. The decadal changes and trends over 4 decades of geopotential perturbations, defined as a deviation from the zonal mean, are estimated by linear regression with time. The decadal changes in longitude-dependent ozone were calculated with a simple transport model of ozone based on the known planetary wave structure changes and prescribed zonal mean ozone gradients. For December of the 1960s and 1980s a statistically significant Rossby wave track appeared over the North Atlantic and Europe with an anticyclonic disturbance over the Eastern North Atlantic and Western Europe, flanked by cyclonic disturbances. In the 1970s and 1990s statistically significant cyclonic disturbances appeared over the Eastern North Atlantic and Europe, surrounded by anticyclonic anomalies over Northern Africa, Central Asia and Greenland. Similar patterns have been found for January. The Rossby wave track over the North Atlantic and Europe is stronger in the 1980s than in the 1960s. For February, the variability of the regression patterns is higher. For January we found a strong alteration in the modelled decadal changes in total ozone over Central and Northern Europe, showing a decrease of about 15 DU in the 1960s and 1980s and an increase of about 10 DU in the 1970s and 1990s. Over Central Europe the positive geopotential height trend (increase of 2.3 m/yr) over 40 years is of the same order (about 100 m) as the increase in the 1980s alone. This is important to recognize because it implies a total ozone decrease over Europe of the order of 14 DU for the 1960–2000 period, for January, if we use the standard change regression relation that about a 10-m geopotential height increase at 300 hPa is related to about a 1.4-DU total ozone decrease.

2002, Titz, S., Kuhlbrodt, T., Rahmstorf, S., Feudel, U.

Conceptual box models of the interhemispheric thermohaline circulation are studied with respect to bifurcations. Freshwater fluxes are the main control parameters of the system: they determine the stable states and transitions between stable states of the large-scale thermohaline circulation. In this study of interhemispheric box models both numerical and analytical methods are used to investigate transition mechanisms of the thermohaline circulation. The box model examined first is an interhemispheric four-box model. It is shown that the two bifurcations where the present THC can become unstable, the saddle-node and the Hopf bifurcation, depend in a different way on hemispheric freshwater fluxes. A reduction of the model variables leads to the conclusion that two fixed freshwater fluxes between three surface boxes are the model feature responsible for the bifurcation behavior found. The significance of the Hopf bifurcation for the stability of the thermohaline circulation is discussed.

2003, Kirchner, I., Peters, D.

During boreal winter months, mean longitude-dependent ozone changes in the upper troposphere and lower stratosphere are mainly used by different ozone transport by planetary waves. The response to radiative perturbation induced by these ozone changes near the tropopause on the circulation is unclear. This response is investigated with the ECHAM4 general circulation model in a sensitivity study. In the simulation two different mean January realizations of the ozone field are implemented in ECHAM4. Both ozone fields are estimated on the basis of the observed mean January planetary wave structure of the 1980s. The first field represents a 14-year average (reference, 1979-1992) and the second one represents the mean ozone field change (anomaly, 1988-92) in boreal extra-tropics during the end of the 1980s. The model runs were carried out pairwise, with identical initial conditions for both ozone fields. Five statistically independent experiments were performed, forced with the observed sea surface temperatures for the period 1988 to 1992. The results support the hypothesis that the zonally asymmetric ozone changes of the 80s triggered a systematic alteration of the circulation over the North Atlantic - European region. It is suggested that this feedback process is important for the understanding of the decadal coupling between troposphere and stratosphere, as well as between subtropics and extra-tropics in winter.

2009, Gioda, A., Mayol-Bracero, O.L., Morales-García, F., Collett, J., Decesari, S., Emblico, L., Facchini, M.C., Morales-De Jesús, R.J., Mertes, S., Borrmann, S., Walter, S., Schneider, J.

As part of the Rain In Cumulus over the Ocean Experiment (RICO) and the Puerto Rico Aerosol and Cloud Study (PRACS), cloud water was collected at East Peak (EP) in Puerto Rico. The main objective of this study was to determine the concentrations of water-soluble species (Cl-, NO3 -, SO4 2-, NH4 +, Ca 2+, H+, Mg2+, K+, and Na +) in water samples taken from clouds influenced by tropical trade winds. The most abundant inorganic species were Na+ (average 465 μeq l-1) and Cl- (434 μeq l-1), followed by Mg2+ (105 μeq l-1), SO4 2- (61 μeq l-1), and NO3 - (25 μeq l -1). High concentrations of nss-SO4 2 (28 μeq l-1), NO3 - (86 μeq l-1), and H+ (14.5 μeq l-1) were measured with a shift in air masses origin from the North Atlantic to North American continent, which reflected a strong anthropogenic influence on cloud chemistry at EP. Long-range transport of particles and acid gases seems to be the factor responsible for fluctuations in concentrations and pH of cloud water at East Peak. When under trade wind influences the liquid phase concentrations of all inorganic substances were similar to those found in clouds in other clean maritime environments. © 2008 Springer Science+Business Media B.V.