ENSO (El Niño–Southern Oscillation) has a large influence on tropical cyclone activity. The authors examine how different environmental factors contribute to this influence, using a genesis potential index developed by Emanuel and Nolan. Four factors contribute to the genesis potential index: low-level vorticity (850 hPa), relative humidity at 600 hPa, the magnitude of vertical wind shear from 850 to 200 hPa, and potential intensity (PI). Using monthly NCEP Reanalysis data in the period of 1950–2005, the genesis potential index is calculated on a latitude strip from 60°S to 60°N. Composite anomalies of the genesis potential index are produced for El Niño and La Niña years separately. These composites qualitatively replicate the observed interannual variations of the observed frequency and location of genesis in several different basins. This justifies producing composites of modified indices in which only one of the contributing factors varies, with the others set to climatology, to determine which among the factors are most important in causing interannual variations in genesis frequency. Specific factors that have more influence than others in different regions can be identified. For example, in El Niño years, relative humidity and vertical shear are important for the reduction in genesis seen in the Atlantic basin, and relative humidity and vorticity are important for the eastward shift in the mean genesis location in the western North Pacific.

Horizontal temperature gradients are small in the tropical atmosphere, as a consequence of the smallness of the Coriolis parameter near the equator. This provides a strong constraint on both large-scale fluid dynamics and diabatic processes. This work is a step toward the construction of a balanced dynamical theory for the tropical circulation that is based on this constraint, and in which the diabatic processes are explicit and interactive. The authors first derive the basic fluid-dynamical scaling under the weak temperature gradient (WTG) approximation in a shallow water system with a fixed mass source representing an externally imposed heating. This derivation follows an earlier similar one by Held and Hoskins, but extends the analysis to the nonlinear case (though on an f plane), examines the resulting system in more detail, and presents a solution for an axisymmetric ''top-hat'' forcing. The system is truly balanced, having no gravity waves, but is different from other balance models in that the heating is included a priori in the scaling. The WTG scaling is then applied to a linear moist model in which the convective heating is controlled by a moisture variable that is advected by the flow. This moist model is derived from the Quasi-equilibrium Tropical Circulation Model (QTCM) equations of Neelin and Zeng but can be viewed as somewhat more general. A number of additional approximations are made in order to consider balanced dynamical modes, apparently not studied previously, which owe their existence to interactions of the moisture and flow fields. A particularly interesting mode arises on an f plane with a constant background moisture gradient. In the limit of low frequency and zero meridional wavenumber this mode has a dispersion relation mathematically identical to that of a barotropic Rossby wave, though the phase speed is eastward (for moisture decreasing poleward in the background state) and the propagation mechanism is quite different. This mode also has significant positive growth rate for low wavenumbers. The addition of the ␤ effect complicates matters. For typical parameters, when ␤ is included the direction of phase propagation is ambiguous, and the growth rate reduced, as the effects of the background gradients in moisture and planetary vorticity appear to cancel to a large degree. Possible relevance to intraseasonal variability and easterly wave dynamics is briefly discussed. *Lamont-Doherty Earth Observatory Contribution Number LDEO 6190.

NATURE GEOSCIENCE | VOL 8 | APRIL 2015 | www.nature.com/naturegeoscience 261 C louds stimulate the human spirit. Although they have been recognized for centuries as harbingers of weather, only in recent decades have scientists begun to appreciate the role of clouds in determining the general circulation of the atmosphere and its susceptibility to change.Forming mostly in the updrafts of the turbulent and chaotic airflow, clouds embody the complex and multiscale organization of the atmosphere into dynamical entities, or storms. These entities mediate the radiative transfer of energy, distribute precipitation and are often associated with extreme winds. It has long been recognized that the water and heat transfer that clouds mediate plays a fundamental role in tropical circulations, and there is increasing evidence that they also influence extratropical circulations 1 . Globally, the impact of clouds on Earth's radiation budget -and hence surface temperatures -also depends critically on how clouds interact with one another and with larger-scale circulations 2 . Far from being passive tracers of a turbulent atmosphere, clouds thus embody processes that can actively control circulation and climate (Box 1).For practical reasons, early endeavours to understand climate deployed a 'divide and conquer' strategy in which efforts to understand clouds and convective processes developed separately from efforts to understand larger-scale circulations. Over time, a gap developed between the subdisciplines. But technological progress and conceptual advances have tremendously increased our capacity to observe and simulate the climate system, such that it is now possible to study more readily how small-scale convective processes -that is, clouds -couple to large-scale circulations (Box 2). Much as a new accelerator allows physicists to explore the implication of the interactions among forces acting over different length scales, these new capabilities are transforming how atmospheric scientists think about the interplay of clouds and climate. This offers a great opportunity not only to close the gap between scientific communities, but Fundamental puzzles of climate science remain unsolved because of our limited understanding of how clouds, circulation and climate interact. One example is our inability to provide robust assessments of future global and regional climate changes. However, ongoing advances in our capacity to observe, simulate and conceptualize the climate system now make it possible to fill gaps in our knowledge. We argue that progress can be accelerated by focusing research on a handful of important scientific questions that have become tractable as a result of recent advances. We propose four such questions below; they involve understanding the role of cloud feedbacks and convective organization in climate, and the factors that control the position, the strength and the variability of the tropical rain belts and the extratropical storm tracks.also to answer some of the most pressing questions about the fate of our pl...