337 research outputs found
Workshop on Drought Forecasting for Northeast Brazil
Precipitation forecasting parameters for northeast Brazil were developed. Hydrological, sociological, and economic aspects were examined. A drought forecasting model is presented
Corridors of barchan dunes: stability and size selection
Barchans are crescentic dunes propagating on a solid ground. They form dune
fields in the shape of elongated corridors in which the size and spacing
between dunes are rather well selected. We show that even very realistic models
for solitary dunes do not reproduce these corridors. Instead, two instabilities
take place. First, barchans receive a sand flux at their back proportional to
their width while the sand escapes only from their horns. Large dunes
proportionally capture more than they loose sand, while the situation is
reversed for small ones: therefore, solitary dunes cannot remain in a steady
state. Second, the propagation speed of dunes decreases with the size of the
dune: this leads -- through the collision process -- to a coarsening of barchan
fields. We show that these phenomena are not specific to the model, but result
from general and robust mechanisms. The length scales needed for these
instabilities to develop are derived and discussed. They turn out to be much
smaller than the dune field length. As a conclusion, there should exist further
- yet unknown - mechanisms regulating and selecting the size of dunes.Comment: 13 pages, 13 figures. New version resubmitted to Phys. Rev. E.
Pictures of better quality available on reques
Formation of Structure in Snowfields: Penitentes, Suncups, and Dirt Cones
Penitentes and suncups are structures formed as snow melts, typically high in
the mountains. When the snow is dirty, dirt cones and other structures can form
instead. Building on previous field observations and experiments, this work
presents a theory of ablation morphologies, and the role of surface dirt in
determining the structures formed. The glaciological literature indicates that
sunlight, heating from air, and dirt all play a role in the formation of
structure on an ablating snow surface. The present work formulates a
mathematical model for the formation of ablation morphologies as a function of
measurable parameters. The dependence of ablation morphologies on weather
conditions and initial dirt thickness are studied, focusing on the initial
growth of perturbations away from a flat surface. We derive a single-parameter
expression for the melting rate as a function of dirt thickness, which agrees
well with a set of measurements by Driedger. An interesting result is the
prediction of a dirt-induced travelling instability for a range of parameters.Comment: 28 pages, 13 figure
The impact of decadal-scale Indian Ocean Sea Surface Temperature Anomalies on Sahelian rainfall and the North Atlantic Oscillation
The sea surface temperatures (SSTs) of the tropical Indian Ocean show a pronounced warming since the 1950s. We have analyzed the impact of this warming on Sahelian rainfall and on the North Atlantic Oscillation (NAO) by conducting ensemble experiments with an atmospheric general circulation model. Additionally, we investigate the impact of the other two tropical oceans on these two climate parameters. Our results suggest that the warming trend in the Indian Ocean played a crucial role for the drying trend over the West Sahel from the 1950s to 1990s and may also have contributed to the strengthening of the NAO during the most recent decades
High-resolution Sr/Ca ratios in a Porites lutea coral from Lakshadweep Archipelago, southeast Arabian Sea: An example from a region experiencing steady rise in the reef temperature
Here we present the first record of Sr/Ca variability in a massive Porites lutea coral from the Lakshadweep Archipelago, Arabian Sea. The annual mean sea surface temperature (SST) in this region and the surrounding areas has increased steadily in the recent past. During some major El Nino events, SSTs are even higher, imposing additional thermal-stress on corals, episodically leading to coral bleaching. We infer from the coral-Sr/Ca record (1981-2008) that during some of these events high and persistent SSTs lead to a dampening of the temperature signal in coral-Sr/Ca, impairing the coral's ability to record full scale warming. Thus, coral-Sr/Ca may provide a history of past El Nino Southern-Oscillation (ENSO) induced thermal-stress episodes, which are a recurrent feature also seen in cross-spectral analysis between coral-Sr/Ca and the Nino3.4 index. Despite the impact of episodical thermal-stress during major El Nino events, our coral proxy faithfully records the seasonal monsoon-induced summer cooling on the order of approximate to 2.3 degrees C. Calibration of coral-Sr/Ca with instrumental grid-SST data shows significant correlation to regional SST and monsoon variability. Hence, massive Porites corals of this region are highly valuable archives for reconstructing long-term changes in SST, strongly influenced by monsoon variability on seasonal scales. More importantly, our data show that this site with increasing SST is an ideal location for testing the future effects of the projected anthropogenic SST increase on coral reefs that are already under thermal-stress worldwide
Western Indian Ocean marine and terrestrial records of climate variability: a review and new concepts on land-ocean interactions since AD 1660
We examine the relationship between three tropical and two subtropical western Indian Ocean coral oxygen isotope time series to surface air temperatures (SAT) and rainfall over India, tropical East Africa and southeast Africa. We review established relationships, provide new concepts with regard to distinct rainfall seasons, and mean annual temperatures. Tropical corals are coherent with SAT over western India and East Africa at interannual and multidecadal periodicities. The subtropical corals correlate with Southeast African SAT at periodicities of 16–30 years. The relationship between the coral records and land rainfall is more complex. Running correlations suggest varying strength of interannual teleconnections between the tropical coral oxygen isotope records and rainfall over equatorial East Africa. The relationship with rainfall over India changed in the 1970s. The subtropical oxygen isotope records are coherent with South African rainfall at interdecadal periodicities. Paleoclimatological reconstructions of land rainfall and SAT reveal that the inferred relationships generally hold during the last 350 years. Thus, the Indian Ocean corals prove invaluable for investigating land–ocean interactions during past centuries
A recipe for simulating the interannual variability of the Asian summer monsoon and its relation with ENSO
Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Climate Dynamics 28 (2007): 441-460, doi: 10.1007/s00382-006-0190-0.This study investigates how accurately the interannual variability over the Indian
Ocean basin and the relationship between the Indian summer monsoon and the
El Nino Southern Oscillation (ENSO) can be simulated by different modelling
strategies. With a hierarchy of models, from an atmospherical general circulation
model (AGCM) forced by observed SST, to a coupled model with the ocean
component limited to the tropical Pacific and Indian Oceans, the role of heat
fluxes and of interactive coupling is analyzed. Whenever sea surface temperature
anomalies in the Indian basin are created by the coupled model, the inverse relationship
between the ENSO index and the Indian summer monsoon rainfall is
recovered, and it is preserved if the atmospherical model is forced by the SSTs
created by the coupled model. If the ocean model domain is limited to the Indian
Ocean, changes in the Walker circulation over the Pacific during El Nino years
induce a decrease of rainfall over the Indian subcontinent. However the observed
correlation between the ENSO and the Indian Ocean Zonal Mode (IOZM) is
not properly modelled and the two indices are not significantly correlated, independently
on season. Whenever the ocean domain extends to the Pacific, and
ENSO can impact both the atmospheric circulation and the ocean subsurface in
the equatorial Eastern Indian Ocean, modelled precipitation patterns associated
both to ENSO and to the IOZM closely resemble the observations.The experiments described were performed as a contribution to the ENSEMBLES
project funded by the European Commission’s 6th Framework Programme, contract
number GOCE-CT-2003-505539
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