The dynamics of quasi-geostrophic flow with uniform potential vorticity reduces to the evolution of buoyancy, or potential temperature, on horizontal boundaries. There is a formal resemblance to two-dimensional flow, with surface temperature playing the role of vorticity, but a different relationship between the flow and the advected scalar creates several distinctive features. A series of examples are described which highlight some of these features: the evolution of an elliptical vortex; the start-up vortex shed by flow over a mountain; the instability of temperature filaments; the ‘edge wave’ critical layer; and mixing in an overturning edge wave. Characteristics of the direct cascade of the tracer variance to small scales in homogeneous turbulence, as well as the inverse energy cascade, are also described. In addition to its geophysical relevance, the ubiquitous generation of secondary instabilities and the possibility of finite-time collapse make this system a potentially important, numerically tractable, testbed for turbulence theories.

This paper reviews the current state of observational, theoretical, and modeling knowledge of the midlatitude storm tracks of the Northern Hemisphere cool season.Observed storm track structures and variations form the first part of the review. The climatological storm track structure is described, and the seasonal, interannual, and interdecadal storm track variations are discussed. In particular, the observation that the Pacific storm track exhibits a marked minimum during midwinter when the background baroclinicity is strongest, and a new finding that storm tracks exhibit notable variations in their intensity on decadal timescales, are highlighted as challenges that any comprehensive storm track theory or model has to be able to address.Physical processes important to storm track dynamics make up the second part of the review. The roles played by baroclinic processes, linear instability, downstream development, barotropic modulation, and diabatic heating are discussed. Understanding of these processes forms the core of our current theoretical knowledge of storm track dynamics, and provides a context within which both observational and modeling results can be interpreted. The eddy energy budget is presented to show that all of these processes are important in the maintenance of the storm tracks.The final part of the review deals with the ability to model storm tracks. The success as well as remaining problems in idealized storm track modeling, which is based on a linearized dynamical system, are discussed. Perhaps on a more pragmatic side, it is pointed out that while the current generation of atmospheric general circulation models faithfully reproduce the climatological storm track structure, and to a certain extent, the seasonal and ENSO-related interannual variations of storm tracks, in-depth comparisons between observed and modeled storm track variations are still lacking.

Advances in understanding coupling in complex networks offer new ways of studying the collective behavior of interactive systems and already have yielded new insights in many areas of science.

We construct the networks of the surface temperature field for El Niño and for La Niña years and investigate their structure. We find that the El Niño network possesses significantly fewer links and lower clustering coefficient and characteristic path length than the La Niña network, which indicates that the former network is less communicative and less stable than the latter. We conjecture that because of this, predictability of temperature should decrease during El Niño years. Here we verify that indeed during El Niño years predictability is lower compared to La Niña years.

Abstract--We propose a family of two-dimensional incompressible fluid models indexed by a parameter o~e [0,~], and discuss the spectral scaling properties for homogeneous, isotropic turbulence in these models. The family includes two physically realizable members. It is shown that the enstrophy cascade is spectrally local for o~ < 2, but becomes dominated by nonlocal interactions for cr > 2. Numerical simulations indicate that the spectral slopes are systematically steeper than those predicted by the local scaling argument.Incompressible fluid turbulence in two dimensions is of interest because of its applications in geophysical problems [1] and because of its role as a computationally tractable testbed for theories, Spectrally local similarity theories of the inertial range spectrum based on Kolmogorov's phenomenology [2,3] often provide a starting point for such studies. The enstrophy cascading inertial range in 2D turbulence has a pathology in this regard; assuming the local similarity spectrum, the aggregate straining effects of eddies larger than a given scale L just fail to be dominated by the effects of eddies with scales similar to L. Hence, the enstrophy range of conventional 2D turbulence is precisely on the threshold of locality [3,4]. In this note we examine a family of modified 2D fluid equations, which span the range between spectrally local and strongly nonlocal behavior.The equation for advection of a conserved scalar q in a velocity field with streamfunction ~p is: 5,q + J(% q) = 0 (la)where J(A, B) = S~AOyB -axB~yA. Advection of even a passive scalar decoupled from ~p embraces a rich variety of phenomena [5,6], but the 2D hydrodynamic equations are distinguished by coupling between q and ~p, which renders the equation nonlinear. The form of the coupling determines the degree of locality. We consider the family of couplings defined in Fourier transform space by 1111