A new coupled global NCEP Reanalysis for the period 1979-present is now available, at much higher temporal and spatial resolution, for climate studies. T he first reanalysis at NCEP (all acronyms are defined in the appendix), conducted in the 1990s, resulted in the NCEP-NCAR reanalysis (Kalnay et al. 1996), or R1 for brevity, and ultimately covered many years, from 1948 to the present (Kistler et al. 2001). It is still being executed at NCEP, to the benefit of countless users for monthly, and even daily, updates of the current state of the atmosphere. At the same time, other reanalyses were being conducted, namely, ERA-15 (Gibson et al. 1997) was executed for a more limited period (1979-93) at the ECMWF, COLA conducted a short reanalysis covering the May 1982-November 1983 period (Paolino et al. 1995), and NASA GSFC conducted a reanalysis covering the 1980-94 period (Schubert et al. 1997). The general purpose of conducting reanalyses is to produce multiyear global state-of-the-art gridded representations of atmospheric states, generated by a constant model and a constant data assimilation system. To use the same model and data assimilation over a very long period was the great advance during the 1990s, because gridded datasets available before 1995 had been created in real time by ever-changing models and analysis methods, even by hand analyses prior to about 1965. The hope was that a reanalysis,

The objective of this study is to understand how interactions with upper-tropospheric troughs affect the intensity of tropical cyclones. The study includes all named Atlantic tropical cyclones between 1985 and 1996. To minimize other factors affecting intensity change, times when storms are over subcritical sea surface temperatures (Յ26ЊC) or near landfall are removed from the sample. A trough interaction is defined to occur when the eddy momentum flux convergence calculated over a 300-600-km radial range is greater than 10 (m s Ϫ1) day Ϫ1. The trough interaction cases are separated into four composites: (i) favorable superposition [tropical cyclone intensifies with an upper-tropospheric potential vorticity (PV) maximum within 400 km of the tropical cyclone center], (ii) unfavorable superposition, (iii) favorable distant interaction (upper PV maximum between 400 and 1000 km from the tropical cyclone center), and (iv) unfavorable distant interaction. For comparison, two additional composites are created: (v) favorable no trough, and (vi) unfavorable no trough. Tropical cyclones over warm water and away from land are more likely to intensify than weaken after an interaction with an upper-level trough; 78% of superposition cases and 61% of distant interaction cases deepened. In the favorable superposition composite, intensification begins soon after a small-scale upper-tropospheric PV maximum approaches the storm center. As in previous studies, the PV maximum subsequently weakens, most likely due to diabatic heating, and never crosses the center and reverses the deepening. In the favorable distant interaction composite, the upper PV maximum remains well to the west of the tropical cyclone center, and intensification is not due to superposition. Strong upper-level divergence occurs downshear of the center, and an upper-level jet is located poleward of the maximum divergence. The center of the intensifying tropical cyclone is located in the right entrance region of the jet, where upward motion is favored. It is argued that the tropical cyclone and upper-level jet develop in a coupled fashion. In the unfavorable distant interaction composite, weakening is attributed to a slightly larger and stronger upper PV maximum than occurs in the favorable distant interaction composite, which induces about 5 m s Ϫ1 more vertical wind shear over the tropical cyclone center. The fairly subtle PV changes that bring about this increase in vertical shear may help account for the difficulty in forecasting tropical cyclone intensity change during distant trough interactions. The no-trough composites have dramatically smaller azimuthal asymmetries than those involving trough interactions. The major distinguishing factor between deepening and filling storms in the no-trough composites is the magnitude of the vertical wind shear.

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This report reviews past and present interpretations that have arisen regarding the structure and governing dynamics of fronts, jet streams, the tropopause, and the life cycle of the marine extratropical cyclone and its fronts. It is shown that new insights and the resolution of previous controversies have been linked, in part, to technological advances in atmospheric observing systems and, more recently, to the use of computers for diagnosis and numerical simulation.

Although prior studies have established that the extratropical flow pattern often amplifies downstream of recurving tropical cyclones (TCs), the extratropical flow response to recurving TCs has not to the authors' knowledge been systematically examined from a climatological perspective. In this study, a climatology of the extratropical flow response to recurving western North Pacific TCs is constructed from 292 cases of TC recurvature during 1979-2009. The extratropical flow response to TC recurvature is evaluated based on a timelagged composite time series of an index of the North Pacific meridional flow surrounding TC recurvature. Similar time series are constructed for recurving TCs stratified by characteristics of the large-scale flow pattern, the TC, and the phasing between the TC and the extratropical flow to assess factors influencing the extratropical flow response to TC recurvature. Results reveal that following TC recurvature, significantly amplified flow develops over the North Pacific and persists for ;4 days. The tendency for significantly amplified North Pacific flow to develop following TC recurvature is sensitive to the strength of the TC-extratropical flow interaction (the phasing between the TC and the extratropical flow), which is based on the negative potential vorticity advection by the divergent outflow of the TC. In contrast, the tendency for significantly amplified North Pacific flow to develop following TC recurvature is relatively insensitive to the intensity or size of the recurving TC, or whether it subsequently reintensifies after becoming extratropical.