Yossi (Yosef) Ashkenazy
Edward and Bertha Rose Professor of Environmental Physics
Department of Solar Energy & Environmental Physics
The Jacob Blaustein Institutes for Desert Research
Ben-Gurion University of the Negev
Sede Boqer campus
84990 Midreshet Ben-Gurion
Email: ashkena@bgu.ac.il
Tel: 972-8-6596858 (work)
Tel: 972-8-6529857 (home)
Fax: 972-8-6596921

Research Interests

  • Climate dynamics and modeling
  • Sand dunes dynamics and modeling
  • Paleoclimate
  • Statistical analysis (linear and nonlinear) of climate records 

Recent Projects

Stochastic nonlinearity of paleoclimate time series:
The climate of the last 800,000 years (800kyr) is characterized by pronounced glacial-interglacial oscillations with approximate period of 100kyr. Most researchers link the glacial dynamics to changes in the distribution of the solar radiation (insolation) received by Earth. Other researchers showed that glacial-interglacial records have red-noise spectrum that usually characterize stochastic processes. In our studies we show that these “erratic” time series have in fact unique pattern for the stochastic fluctuations: Climate fluctuations (e.g., of temperature or ice-volume) appear in clusters of big and small fluctuations where these clusters are self similar and obey scaling laws. The clustering phenomenon is related to the nonlinearity of paleoclimate time series and to the multifractal nature of these time series. We developed several stochastic models that account for this stochastic nonlinearity and showed that some other climate stochastic models do not reproduce this observed stochastic nonlinearity.

The mid-Pleistocene transition of ice-ages:
Glacial-interglacial oscillations were very different prior to ~800kyr ago. Before 800kyr ago the oscillations in ice-volume were of smaller amplitude (about 2/3) compare to ice-volume oscillations of the last 800kyr, and with approximate period of 40kyr compare to the ~100kyr period of the glacial oscillations of the last ~800kyr. We developed a simple piecewise linear model to explain this mid-Pleistocene transition and showed that the glacial dynamics before and after the transition is essentially the same; only the maximum ice-volume threshold was different. We obtain a good fit with paleoclimate records when adding insolation forcing.

Lead and lag of the tropics in glacial dynamics:
Recent proxy records from the tropical region indicated the lead of tropical temperature over global ice-volume by several thousands years. We raised the possibility that such tropical lead is due to seasonal bias of the foraminifera from which the local temperature and global ice-volume are constructed; i.e., insolation of different seasons have relative lag/lead between them and thus foraminifera that tend to grow at certain season reflect this lead/lag. In this case the observed tropical lead has nothing to do with glacial dynamics.

Box modeling of the Eastern Mediterranean Sea:
The deep water formation in the Eastern Mediterranean changed drastically in the last 10 years or so: a new additional source of deep water formation was found in the southern part of the Aegean Sea and is 3 times larger than the old source of deep water formation in the Adriatic Sea. We develop a simple box model to explain this Eastern Mediterranean transient and show that such transition is consistent with multiple equilibrium states of the fluxes between the different basins of the Eastern Mediterranean Sea. Such transition may be caused by altering the moisture and heat fluxes of the different boxes.

Statistical properties of river flow fluctuations:
We study the statistical properties of 30 world rivers. We found that river flow fluctuations obey scaling laws and have nonlinear properties including volatility correlations. We developed statistical descriptive model to understand the possible sources of these nonlinear properties. We also develop filtering procedure that efficiently excludes the seasonal periodicity of climate records. [In some cases inadequate filtering of the annual periodicity of climate records may lead to erroneous conclusions regarding the underlying processes.]