In the context of climate change, detecting long-term modulations in the general circulation and in the global transport of tracers is important, but it is made difficult by the low signal-to-noise ratio. Dr. Nakamura will alleviate this problem by introducing a new partition between the eddies and the background flow and their interaction. Preliminary results using this novel method have been obtained, and a global, multi-year amplification of the eddies and jets, over the period 2001-5 is revealed. This amplification is not detected using traditional methods.
This method will be applied to meteorological analyses and to long-term climate-model simulations under global warming scenarios, to measure the frequency and intensity of modulations of the general circulation. Idealized models (shallow water and two-level models) will be employed to explore the dynamics that underlies such modulations.
The effects of jets and their modulations on global mixing properties will be studied using a passive tracer subject to advection by the observed large-scale flow together with small-scale diffusivity. The decay rate of the variance of the tracer is a metric of global mixing, and contributions from the jet regions to the decay rate will be quantified with a new diagnostic.
The proposed activity will provide a new diagnostic platform for the tropospheric general circulation and transport, suitable for the detection of long-term shifts in the climatic regime. The Lagrangian-mean approach bypasses the complication of eddy-mean flow interaction in the traditional sense, and it allows simpler interpretations in terms of nonconservative processes.
The broader impacts of this activity are that the diagnostic framework can be applied to a wide class of flows, including the stratosphere, troposphere, possibly the oceans, and numerical simulations. Two graduate students will be supported.
Intellectual merit: The Earth’s climate in the middle latitudes (30-60 degrees north and south) is highly variable due to migratory weather systems. These systems of low and high pressure are related to meandering of the jet stream, a fast river-like flow that blows around the pole from west to east at an average altitude of about 35,000 feet. The meandering of the jet stream not only brings about weather but also allows for an exchange of heat, momentum, and chemical constituents between the tropics and the polar region and hence moderates the climate balance of the planet. The meandering is a form of wave known as the Rossby wave. The theory for the Rossby wave and its role in moderating the climate has been well developed in the atmospheric science; however the theory is accurate only when the amplitude of the wave is small. Often the jet stream in the atmosphere meanders a great deal, at which point the small-amplitude theory becomes invalid. Because of this limitation in the theory, the quantitative understanding of the role of jet meandering on the climate is incomplete. The work in this project extended the theory for the Rossby wave with an arbitrarily large amplitude and successfully applied the theory to meteorological data to quantify the effects of the Rossby waves on the climate. Broader impacts: The project funded Ph.D. research of two graduate students (Da Zhu and Ari Solomon). Zhu successfully defended his thesis and started his postdoctoral position at MIT in September 2010. Solomon successfully defended his thesis and started his postdoctoral position at the Center for Ocean-Land-Atmosphere Studies in August 2011. Although the nature of the performed work was largely theoretical, the Principal Investigator (Noboru Nakamura) routinely used laboratory experiments of jet streams to demonstrate the physical principles that underlie the meandering jet streams. Some 500 students (mostly non-science major undergraduate students) witnessed this demonstration during 2009-2011. The PI co-organized a successful workshop on Teaching Weather and Climate Using Laboratory Experiments in June 2008 at the University of Chicago: (http://geosci.uchicago.edu/%7ennn/postworkshop/index.html)
