Reclassification of rainfall regions of Turkey by K-means methodology and their temporal variability in relation to North Atlantic Oscillation (NAO)

Rainfall over Turkey portrays highly variable character both spatially and temporally. The aim of this study is to redefine main rainfall clusters of Turkey by using k-means methodology and investigate spatial shifts in the redefined rainfall clusters in subsequent periods with respect to North Atlantic Oscillation (NAO) patterns. Initially, monthly rainfall totals are subjected to k-means clustering by taking into consideration 148 stations covering the 1977?C2006 period. Considering the maximum silhouette value and lowest negative silhouette number, six rainfall clusters are determined as optimum classifications for this climate period. The results indicate that Aegean?CMarmara and Eastern Anatolia?CCentral Anatolia geographic regions are characterized as single rainfall cluster contrary to the conventional geographical regions. The Mediterranean region is characterized with two separate sub-regions indicating highly variable rainfall distribution characters over the region. The study further adapts a similar classification for 10-year sub-periods to determine spatial shifts of the redefined rainfall clusters for the last 30?years. From one decade to another, temporally drier and wetter clusters are observed with underlying shifting causes in relation to NAO patterns. Parallel to other studies in the literature, NAO is found to be partially useful in explaining the temporally dry trends while less useful in justifying wet periods. On the other hand, coefficient of variation (COV) is introduced in order to explain the temporal shifts in the clusters. Strong relations are obtained between the regions with the higher COV numbers and highest cluster shifts, while smaller COV numbers are associated with the most stable clusters.     

Interacting Kasner-type cosmologies

It is well known that Kasner-type cosmologies provide a useful framework for analyzing the three-dimensional anisotropic expansion because of the simplification of the anisotropic dynamics. In this paper relativistic multi-fluid Kasner-type scenarios are studied. We first consider the general case of a superposition of two ideal cosmic fluids, as well as the particular cases of non-interacting and interacting ones, by introducing a phenomenological coupling function q(t). For two-fluid cosmological scenarios there exist only cosmological scaling solutions, while for three-fluid configurations there exist not only cosmological scaling ones, but also more general solutions. In the case of triply interacting cosmic fluids we can have energy transfer from two fluids to a third one, or energy transfer from one cosmic fluid to the other two. It is shown that by requiring the positivity of energy densities there always is a matter component which violates the dominant energy condition in this kind of anisotropic cosmological scenarios.     

A universal field equation for dispersive processes in heterogeneous media

When formulated properly, most geophysical transport-type process involving passive scalars or motile particles may be described by the same space–time nonlocal field equation which consists of a classical mass balance coupled with a space–time nonlocal convective/dispersive flux. Specific examples employed here include stretched and compressed Brownian motion, diffusion in slit-nanopores, subdiffusive continuous-time random walks (CTRW), super diffusion in the turbulent atmosphere and dispersion of motile and passive particles in fractal porous media. Stretched and compressed Brownian motion, which may be thought of as Brownian motions run with nonlinear clocks, are defined as the limit processes of a special class of random walks possessing nonstationary increments. The limit process has a mean square displacement that increases as t^α+1 where α > −1 is a constant. If α = 0 the process is classical Brownian, if α < 0 we say the process is compressed Brownian while if α > 0 it is stretched. The Fokker–Planck equations for these processes are classical ade’s with dispersion coefficient proportional to t^α. The Brownian-type walks have fixed time step, but nonstationary spatial increments that are Gaussian with power law variance. With the CTRW, both the time increment and the spatial increment are random. The subdiffusive Fokker–Planck equation is fractional in time for the CTRW’s considered in this article. The second moments for a Levy spatial trajectory are infinite while the Fokker–Planck equation is an advective–dispersive equation, ade, with constant diffusion coefficient and fractional spatial derivatives. If the Lagrangian velocity is assumed Levy rather than the position, then a similar Fokker–Planck equation is obtained, but the diffusion coefficient is a power law in time. All these Fokker–Planck equations are special cases of the general non-local balance law.     

Changes in the interannual SST-forced signals on West African rainfall. AGCM intercomparison

Rainfall over West Africa shows strong interannual variability related to changes in Sea Surface Temperature (SST). Nevertheless, this relationship seem to be non-stationary. A particular turning point is the decade of the 1970s, which witnessed a number of changes in the climatic system, including the climate shift of the late 1970s. The first aim of this study is to explore the change in the interannual variability of West African rainfall after this shift. The analysis indicates that the dipolar features of the rainfall variability over this region, related to changes in the Atlantic SST, disappear after this period. Also, the Pacific SST variability has a higher correlation with Guinean rainfall in the recent period. The results suggest that the current relationship between the Atlantic and Pacific El Ni?o phenomena is the principal responsible for these changes. A fundamental goal of climate research is the development of models simulating a realistic current climate. For this reason, the second aim of this work is to test the performance of Atmospheric General Circulation models in simulating rainfall variability over West Africa. The models have been run with observed SSTs for the common period 1957?C1998 as part of an intercomparison exercise. The results show that the models are able to reproduce Guinean interannual variability, which is strongly related to SST variability in the Equatorial Atlantic. Nevertheless, problems in the simulation of the Sahelian interannual variability appear: not all models are able to reproduce the observed negative link between rainfall over the Sahel and El Ni?o-like anomalies in the Pacific, neither the positive correlation between Mediterranean SSTs and Sahelian rainfall.     

Evaluation of the internal variability and estimation of the downscaling ability of the Canadian Regional Climate Model for different domain sizes over the north Atlantic region using the Big-Brother experimental approach

The ability of a nested model to accurately simulate the subarctic climate is studied here. Two issues have been investigated: Model??s internal variability (IV) and the impact of domain size (DS). For this purpose we combine the ??perfect model?? approach, Big-Brother Experiment (BBE) (Denis et al. in Clim Dyn 18:627?C646, 2002) with the ensemble of simulations. The advantage of this framework is the possibility to study small-scale climate features that constitute the main added value of RCM. The effects of the DS on result were studied by employing two Little-Brother (LB) domain sizes. IV has been evaluated by introducing small differences in initial conditions in an ensemble of 20 simulations over each LB. Results confirm previous findings that the IV is more important over the larger domain of integration. The temporal evolution over two domain sizes is rather different and depends strongly on the synoptic situation. Small-scales solution over the larger domain diverges freely from the boundary forcing in some periods. Over the smaller domain, the amplitude of small-scale transient eddies is systematically underestimated, especially at higher altitude characterized by the strongest winds along the storm tracks. Over the larger domain, the amplitude of small-scale transient eddies is better represented. However, the weaker control by the lateral boundaries over the larger domain results in solutions with large internal variability. As a result, the ensemble average strongly underestimates the transient-eddy variance due to partial destructive interference of individual ensemble member solutions.     

Reduction Rates of Sedimentary Mn and Fe Oxides: An Incubation Experiment with Arctic Ocean Sediments

To test the hypothesis that manganese- and iron-reducing bacteria in marine sediments respond rapidly to seasonal pulses of fresh organic carbon settling to the sea floor, we amended wet metal oxide?Crich and metal oxide?Cpoor sediments from the Beaufort Sea, Canadian Arctic, with organic carbon in the form of shrimp powder and incubated them at room temperature. Neither Mn nor Fe was released to the aqueous phase from unamended metal oxide?Crich sediment during a 41-day incubation, but both elements were released from sediment aliquots amended with organic carbon. Dissolved Mn appeared in the aqueous phase after a lag period of 2 days or less and reached levels as high as 600 ??mol l^?1 before levelling out. The release of dissolved Mn was accompanied by a decrease in the concentration of solid-phase reducible Mn. Dissolved Fe did not appear until 2 weeks into the incubation and only after the concentration of dissolved Mn had levelled out. For low concentrations of amended organic carbon (0.3%), the kinetics of Mn reduction fit a second-order rate law with a rate constant k = 2 × 10^?3 g ??mol^?1 day^?1, but at intermediate and high organic carbon concentrations (0.7 and 1.3%), the reduction kinetics was better described by a pseudo-first-order rate law with a rate constant k?? = 1.6 × 10^?1 day^?1. A pulse of organic carbon settling to the sea floor can trigger reduction of Mn and Fe oxides within a few days in strongly seasonal sedimentary environments, such as in the Arctic.