Dr Song Yang

Results from a recent study (Yang et al. 2002a), in which multi-decade daily data of the NCEP/NCAR Reanalysis are used to understand the dynamics and energetics of atmospheric high-frequency (HF) and low-frequency (LF) variability in the Northern Hemisphere, indicate that LF eddies extract not only kinetic energy but also potential energy from the basic flow. The baroclinic energy extraction rate is nearly as large as the barotropical energy extraction rate. The results also indicate that, unlike HF eddies, there is only a small portion of energy conversed from the potential energy to the kinetic energy for LF eddies. In light of these findings, it is argued that in a three-dimensional world, purely barotropic instability may not be observable and a notion of "equivalent barotropic instability" is put forward to describe this type of mixed barotropic-baroclinic process in which energy is extracted from the mean flow without exchange between the potential and kinetic energy. In addition, different from HF eddies, the LF eddies act primarily to reduce the zonal gradients associated with stationary waves and thus the amplitude of the waves by transporting momentum and heat in zonal directions. A further analysis is proposed to understand the intraseasonal-to-interdecadal variations of the above features and their relationship with surface climate variations. Observations and general circulation models will be applied to investigate the extratropical atmospheric HF and LF variations with slowly-varying and/or broad-scale climate phenomena such as ENSO, the Arctic Oscillation, the North Atlantic Oscillation, the Pacific Decadal Oscillation, and various tropical heating conditions. Both the basic flow and atmospheric eddies change significantly associated with these phenomena. The analysis will also be aimed at improving the explanation of the associated anomalies in precipitation and temperature, and the skill of climate prediction. Relevant studies: Yang, S., M. Cai, H. Van den Dool, and V. Kousky, 2002a: A new look at the energetics of low-frequency variability. The 27th Annual Climate Diagnostics and Prediction Workshop, 21-25 October 2002, Fairfax, Virginia. Yang, S., K.-M. Lau, and K.-M. Kim, 2002b: Variations of the east Asian jet stream and Asian-Pacific-American winter climate anomalies. J. Climate, 15, 306-325.