Tropical plumes (TPs) reflect tropical–extratropical interaction associated with the transport of moisture from the Tropics to extratropical latitudes. They are observed in satellite images as continuous narrow cloud bands ahead of upper-level subtropical troughs at times when the subtropical jet is highly perturbed. Rainstorms usually develop in the exit regions of TPs, so their presence over northern Africa has an impact on the precipitation regime in the southeastern Mediterranean. Based on satellite images and rainfall measurements from Israel, 10 TPs over eastern North Africa between 1988 and 2005 in which considerable rain was recorded were selected. Using the NCEP–NCAR reanalysis data, the structure and evolution of these TPs were characterized and their regional canonical features were identified. A typical TP that occurred in March 1991 is described in detail. The main canonical characteristics are as follows: the TP development is preceded by an incubation period, expressed either as a stationary upper-level trough, persisting 2–6 days, or as two consecutive TP pulses; the preferred location for TP origin is 5°–15°N, 5°W–15°E; the TP is separated from the underlying dry Saharan PBL; the subtropical trough undergoes a phase locking with the lower tropical trough; the cloudiness in the TP-induced rainstorm is mostly stratified with continuous moderate rain, originating from midlevel moisture; and the TP tends to be followed by a midlatitude cyclogenesis over the eastern Mediterranean. This analysis proposes several explanations for the efficiency of the TPs in transporting moisture over a 2000-km distance.

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The linear stability of a simple two-layer shear flow with an upper-layer potential vorticity front overlying a quiescent lower layer is investigated as a function of Rossby number and layer depths. This flow configuration is a generalization of previously studied flows whose results we reinterpret by considering the possible resonant interaction between waves. We find that instabilities previously referred to as ‘ageostrophic’ are a direct extension of quasi-geostrophic instabilities.Two types of instability are discussed: the classic long-wave quasi-geostrophic baroclinic instability arising from an interaction of two vortical waves, and an ageostrophic short-wave baroclinic instability arising from the interaction of a gravity wave and a vortical wave (vortical waves are defined as those that exist due to the presence of a gradient in potential vorticity, e.g. Rossby waves). Both instabilities are observed in oceanic fronts. The long-wave instability has length scale and growth rate similar to those found in the quasi-geostrophic limit, even when the Rossby number of the flow is O(1).We also demonstrate that in layered shallow-water models, as in continuously stratified quasi-geostrophic models, when a layer intersects the top or bottom boundaries, that layer can sustain vortical waves even though there is no apparent potential vorticity gradient. The potential vorticity gradient needed is provided at the top (or bottom) intersection point, which we interpret as a point that connects a finite layer with a layer of infinitesimal thickness, analogous to a temperature gradient on the boundary in a continuously stratified quasi-geostrophic model.

The construction of approximate Schrödinger eigenvalue equations for planetary (Rossby) waves and for inertia–gravity (Poincaré) waves on an ocean-covered rotating sphere yields highly accurate estimates of the phase speeds and meridional variation of these waves. The results are applicable to fast rotating spheres such as Earth where the speed of barotropic gravity waves is smaller than twice the tangential speed on the equator of the rotating sphere. The implication of these new results is that the phase speed of Rossby waves in a barotropic ocean that covers an Earth-like planet is independent of the speed of gravity waves for sufficiently large zonal wavenumber and (meridional) mode number. For Poincaré waves our results demonstrate that the dispersion relation is linear, (so the waves are non-dispersive and the phase speed is independent of the wavenumber), except when the zonal wavenumber and the (meridional) mode number are both near 1.

We use statistical methods for nonstationary time series to test the anthropogenic interpretation of global warming (AGW), according to which an increase in atmospheric greenhouse gas concentrations raised global temperature in the 20th century. Specifically, the methodology of polynomial cointegration is used to test AGW since during the observation period (1880–2007) global temperature and solar irradiance are stationary in 1st differences whereas greenhouse gases and aerosol forcings are stationary in 2nd differences. We show that although these anthropogenic forcings share a common stochastic trend, this trend is empirically independent of the stochastic trend in temperature and solar irradiance. Therefore, greenhouse gas forcing, aerosols, solar irradiance and global temperature are not polynomially cointegrated. This implies that recent global warming is not statistically significantly related to anthropogenic forcing. On the other hand, we find that greenhouse gas forcing might have had a temporary effect on global temperature

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