Mechanisms of Climate Variability and Change

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The goal of this research is to understand some basic physical mechanisms which contribute to climate variability and climate change. Three research topics will be pursued. The first is the interaction through which the extratropical impact of El Nino/Southern Oscillation (ENSO) is regulated by the stratospheric Quasi-Biennial Oscillation (QBO) and vice versa. Preliminary work shows that when the QBO is easterly or ENSO is in its warm state the polar vortex is weak, but when either of these conditions hold, the influence of the other is weak. The research will be conducted using the NCAR Whole Earth Community Climate Model (WACCM), with modifications to generate and sustain the QBO. The second topic is the interaction between jet streams and high-frequency transient eddies which plays a dominant role in the natural climate variability of the extratropics. Earlier work by the Principal Investigator (PI) and a student showed that eddy-jet interactions produce a positive feedback which enhances the persistence of low-frequency annular modes. The work here seeks to understand how a similar positive feedback operates in the variability of the North Atlantic Oscillation. The primary tool for the analysis would be ensemble sensitivity analysis, performed using multiple simulations from a barotropic model and later multi-level baroclinic models. The third topic is the hypothesis that the temperature of tropical anvil clouds remains about the same under global warming (the Fixed Anvil Temperature, or FAT, hypothesis), previously developed by the PI and his students. The PI's previous work suggested that FAT behavior is related to clear-sky radiative cooling, and the PI suggests that this behavior can explain the similarity of longwave cloud feedbacks in global warming simulations. One avenue of experimentation would be to modify the water vapor passed to the radiation code of the model independently of the water vapor tracked in the convection scheme and other components of the model.Three graduate students will be supported under this grant, thus providing for the education of the next generating of scientists. In addition, research on the above topics will enhance understanding of climate variability and change, and may be beneficial for the development of prediction capabilities for short-term climate variability and long-term climate change.