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Gerald D. Bell
and
Muthuvel Chelliah

Abstract

Interannual and multidecadal extremes in Atlantic hurricane activity are shown to result from a coherent and interrelated set of atmospheric and oceanic conditions associated with three leading modes of climate variability in the Tropics. All three modes are related to fluctuations in tropical convection, with two representing the leading multidecadal modes of convective rainfall variability, and one representing the leading interannual mode (ENSO).

The tropical multidecadal modes are shown to link known fluctuations in Atlantic hurricane activity, West African monsoon rainfall, and Atlantic sea surface temperatures, to the Tropics-wide climate variability. These modes also capture an east–west seesaw in anomalous convection between the West African monsoon region and the Amazon basin, which helps to account for the interhemispheric symmetry of the 200-hPa streamfunction anomalies across the Atlantic Ocean and Africa, the 200-hPa divergent wind anomalies, and both the structure and spatial scale of the low-level tropical wind anomalies, associated with multidecadal extremes in Atlantic hurricane activity.

While there are many similarities between the 1950–69 and 1995–2004 periods of above-normal Atlantic hurricane activity, important differences in the tropical climate are also identified, which indicates that the above-normal activity since 1995 does not reflect an exact return to conditions seen during the 1950s–60s. In particular, the period 1950–69 shows a strong link to the leading tropical multidecadal mode (TMM), whereas the 1995–2002 period is associated with a sharp increase in amplitude of the second leading tropical multidecadal mode (TMM2). These differences include a very strong West African monsoon circulation and near-average sea surface temperatures across the central tropical Atlantic during 1950–69, compared with a modestly enhanced West African monsoon and exceptionally warm Atlantic sea surface temperatures during 1995–2004.

It is shown that the ENSO teleconnections and impacts on Atlantic hurricane activity can be substantially masked or accentuated by the leading multidecadal modes. This leads to the important result that these modes provide a substantially more complete view of the climate control over Atlantic hurricane activity during individual seasons than is afforded by ENSO alone. This result applies to understanding differences in the “apparent” ENSO teleconnections not only between the above- and below-normal hurricane decades, but also between the two sets of above-normal hurricane decades.

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Gerald D. Bell
and
Daniel Keyser

Abstract

Equations are presented for the evolution of isobaric shear and curvature vorticity and for isentropic shear and curvature potential vorticity in natural (streamline-following) coordinates, in the case of adiabatic, frictionless flow. In isobaric coordinates, two terms of equal magnitude and opposite sign arise in the respective tendency equations for shear and curvature vorticity; these terms represent conversions between shear and curvature vorticity in the sense that their sum does not alter the total tendency of absolute vorticity. In isentropic coordinates, only the conversion terms remain in the tendency equations for shear and curvature potential vorticity, consistent with potential-vorticity conservation. The vorticity and potential-vorticity conversions arise from (i) along-stream variations in wind speed in the presence of Lagrangian changes in wind direction and (ii) flow-normal gradients of Lagrangian changes in wind speed. The assumption of horizontal nondivergence simplifies the interpretation of the vorticity-interchange process by relating the conversion terms directly to flow curvature. Schematics are developed in order to illustrate the conversion terms in idealized representations of jet-entrance and jet-exit regions and curved flow patterns; these schematics provide the basis for understanding vorticity interchanges in realistic flow regimes.

The evolution of the midtropospheric shear- and curvature-potential-vorticity fields is described for a jet- trough interaction event in northwesterly flow, leading to the formation of a well-defined midtropospheric cutoff cyclone over the eastern United States between [8 and 20 January 1986. This time period coincides with the first intensive observing period of the Genesis of Atlantic Lows Experiment. Major midtropospheric cyclogenesis begins as a jet embedded in northwesterly flow, identified as a maximum of cyclonic shear potential vorticity, propagates toward the base of a diffluent trough, identified as a maximum of cyclonic curvature potential vorticity. The potential-vorticity tendency equations reveal that for this particular stage, the interchange terms contribute both to the amplification of the trough and to the formation of a maximum of cyclonic shear potential vorticity on the downstream side of the trough. The potential-vorticity interchange process is shown to play a key role in transforming the asymmetric configuration of shear and curvature potential vorticity characteristic of the diffluent trough stage, where the cyclonic shear maximum lags the cyclonic curvature maximum, to the relatively symmetric configuration characteristic of the cutoff stage. At the culmination of the cutoff stage, the shear- and curvature-potential-vorticity maxima overlap substantially. This overlap is a consequence of the presence of a single, cyclonically curved jet within the base of the cutoff cyclone.

A second important structural change occurring during midtropospheric cyclogenesis is the transformation of the potential-vorticity anomaly corresponding to the cutoff cyclone into a circularly symmetric configuration, which is accomplished by the contraction of the northwestern extension of the potential-vorticity anomaly toward the cyclone center. This contraction process, which is shown to involve significant interchanges between shear and curvature potential vorticity, results in the detachment of the potential-vorticity anomaly from the “stratospheric reservoir” of potential vorticity located north of the cyclone.

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Muthuvel Chelliah
and
Gerald D. Bell

Abstract

The leading tropical multidecadal mode (TMM) and tropical interannual (ENSO) mode in the 52-yr (1949– 2000) NCEP–NCAR reanalysis are examined for the December–February (DJF) and June–August (JJA) seasons based on seasonal tropical convective rainfall variability and tropical surface (land + ocean) temperature variability. These combined modes are shown to capture 70%–80% of the unfiltered variance in seasonal 200-hPa velocity potential anomalies in the analysis region of 30°N–30°S. The TMM is the dominant mode overall, accounting for 50%–60% of the total unfiltered variance in both seasons, compared to the 22%–24% for ENSO.

The robustness of the tropical multidecadal mode is addressed, and the results are shown to compare favorably with observed station data and published results of decadal climate variability in the key loading regions. The temporal and spatial characteristics of this mode are found to be distinct from ENSO.

The TMM captures the global climate regimes observed during the 1950s–60s and 1980s–90s, and the 1970s transition between these regimes. It provides a global-scale perspective for many known aspects of this decadal climate variability (i.e., surface temperature, precipitation, and atmospheric circulation) and links them to coherent multidecadal variations in tropical convection and surface temperatures in four core regions: the West African monsoon region, the central tropical Pacific, the Amazon basin, and the tropical Indian Ocean.

During JJA, two distinguishing features of the tropical multidecadal mode are its link to West African monsoon variability and the pronounced zonal wavenumber-1 structure of the 200-hPa streamfunction anomalies in the subtropics of both hemispheres. During DJF a distinguishing feature is its link between anomalous tropical convection and multidecadal variations in the North Atlantic Oscillation (NAO). For the linear combination of the TMM and ENSO the strongest regressed values of the wintertime NAO index are found when their principal component (PC) time series are out of phase.

In the Tropics and subtropics the linearly combined signal for the TMM and ENSO is strongest when their PC time series are in phase and is weakest when they are out of phase. This result suggests a substantial modulation of the ENSO teleconnections by the background flow. It indicates stronger La Niña teleconnections during the 1950s–60s, compared to stronger El Niño teleconnections during the 1980s–90s. Although this study addresses the linear ENSO–TMM interference, the results also suggest that interactions between the two modes may help to explain the stronger El Niño episodes observed during the 1980s–90s compared to the 1950s–60s.

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Alan Basist
,
Gerald D. Bell
, and
Vernon Meentemeyer

Abstract

Statistical relationships between topography and the spatial distribution of mean annual precipitation are developed for ten distinct mountainous regions. These relationships are derived through linear bivariate and multivariate analyses, using six topographic variables as predictors of precipitation. These predictors are elevation, slope, orientation, exposure, the product (or interaction) of slope and orientation, and the product of elevation and exposure.

The two interactive terms are the best overall bivariate predictors of mean annual precipitation, whereas orientation and exposure are the strongest noninteractive bivariate predictors. The regression equations in many of the climatically similar regions tend to have similar slope coefficients and similar y-intercept values, indicating that local climatic conditions strongly influence the relationship between topography and the spatial distribution of precipitation. In contrast, the regression equations for the tropical and extratropical regions exhibit distinctly different slope coefficients and y-intercept values, indicating that topography influences the spatial distribution of precipitation differently in convective versus nonconvective environments.

The multivariate equations contain between one and three significant topographic predictors. The best overall predictors in these models are exposure and the interaction of elevation and exposure, indicating that exposure to the prevailing wind is perhaps the single most important feature relating topography to the spatial distribution of precipitation in the mountainous regimes studied. The strongest (weakest) multivariate relationships between topography and precipitation are found in the four middle- and high-latitude west coast regions (in the tropical regions), where more than 70% (less than 50%) of the spatial variability of mean annual precipitation is explained. These results suggest that in certain regions, one can estimate the spatial distribution of mean annual precipitation from a limited network of raingauges using topographically based regression equations.

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Gerald D. Bell
and
John E. Janowiak

This paper presents an observational analysis of the large-scale atmospheric circulation prior to and during the Midwest floods of June–July 1993. The floods developed and persisted in association with three major circulation features, none of which alone would likely have produced such intense and prolonged flooding. First, a persistent, positive phase of the North Pacific teleconnection pattern was observed throughout the Pacific sector for four months prior to the onset of the floods. This anomalous circulation was associated with much above-normal cyclone activity over the middle latitudes of the North Pacific and with below-normal cyclone activity over the western and central United States. Second, a major change in this pattern occurred over the western United States in late May, which established very strong zonal flow from the western Pacific to the eastern United States. This flow provided a “duct” for the intense cyclones to propagate directly into the Midwest throughout the month of June. These storms triggered a series of intense convective complexes over the Midwest, resulting in major flooding. Third, during July a persistent wave pattern with highly amplified southwesterly flow became established over the western and central United States. This circulation, in conjunction with a quasi-stationary frontal boundary and sustained moisture transport into the central United States, was associated with a continuation of excessive rainfall and flooding in the Midwest.

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Gerald D. Bell
and
Lance F. Bosart

Abstract

Observational composites of midtropospheric closed cyclone formation are constructed and diagnosed for three regions: the southwestern United States, the eastern United States, and the southern lee of the Alps. The spatial scales upon which closed cyclone formation occurs are then examined by zonally decomposing the composite 500-hPa height fields into three distinct wave groups: the planetary scale (zonal waves 1–3), the large synoptic scale (zonal waves 4-9), and the small synoptic scale (zonal waves 10-25). This analysis leads to a description of closed cyclogenesis as a combined wave interaction and wave superposition process involving both wave groups 4–9 and 10–25, which is intimately linked to preexisting along-stream speed variations and flow curvature. This description is inconsistent with modal and nonmodal analytical instability theories of cyclogenesis.

The essence of the closed cyclogenesis process is contained in the relative positioning of, and interaction between, preexisting jets and waves. In all regions the precursor wave pattern is characterized by a broad trough over the impending cyclone region, with the strongest meridional flow and implied geostrophic vorticity maximum located upstream of this trough axis. This flow configuration is associated with sustained cyclonic vorticity advection into the amplifying trough axis, and also provides a conduit by which intensifying transient short-wave trough-jet streak features can propagate into the downstream trough. A closed circulation then develops as the geostrophic wind speed maximum moves into the base of the trough and cyclonic vorticity becomes concentrated within the trough axis. This evolution also occurs coincident with the movement of the transient trough feature directly into the amplifying long-wave trough axis.

In the southwestern United States and Alps cases, the favorable northwesterly flow configuration is initiated two days prior to closed cyclone formation by vigorous upstream wave amplification and by the rapid eastward movement of the upstream ridge axis relative to the downstream trough axis. Downstream of the cyclogenesis region, relatively modest anticyclogenesis, and modest mid- and lower-tropospheric thermal advection, is observed in these cases. In contrast, the favorable northwesterly flow configuration in the eastern United States cases is already established two days prior to closed cyclone formation. These cases are also characterized by vigorous downstream planetary-scale ridge amplification and a well-defined pattern of mid- and lower-tropospheric thermal advection.

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Gerald D. Bell
and
Lance F. Bosart

Abstract

The synoptic-scale evolution during the formation phase of a midtropospheric cutoff cyclonic circulation over the eastern United States is diagnosed within the potential vorticity framework using the GALE (Genesis of Atlantic Lows Experiment) case of 18–19 January 1986. The study examines 1) the precursor flow evolution prior to cutoff cyclone formation; 2) the wind, mass, and potential vorticity evolution during the 2-day period encompassing cutoff formation; and 3) the relative contribution of upper-versus lower-tropospheric forcing on the quasigeostrophic height tendency field prior to and during cutoff formation.

The primary large-scale features prior to cutoff cyclone formation are an amplifying ridge over the western United States and eastern North Pacific and a diffluent trough over the central United States. The primary smaller-scale feature prior to cutoff formation is a short-wave trough-jet streak system that propagates through the longer-wave-amplifying ridge, and then intensifies upon arriving in northwesterly flow downstream of the ridge axis. The intensification of this shorter-wavelength system is associated with increases in stratospheric potential vorticity at levels considered to be well within the middle and upper troposphere. Major midtropospheric cyclogenesis then ensues as the jet propagates toward the base of the diffluent trough while further intensifying. The circulation then “closes off” at 500 hPa within the base of the amplifying trough as stratospheric potential vorticity values descend to near 620 hPa, and become increasingly confined to the base of the trough.

The subsequent intensification of the cutoff circulation is accompanied by sustained potential vorticity and temperature increases well above the level of the extruded tropopause. This intensification phase is also accompanied by an increasingly isolated distribution of stratospheric potential vorticity, and by the formation of an isolated warm pool, in the mid-and upper troposphere above the circulation center. These features are consistent with calculations showing that the primary mass loss required to support the formation and subsequent intensification of the cutoff circulation is confined to the upper troposphere.

A quasigeostrophic height tendency diagnosis suggests that the advection of potential vorticity at and above the 500-hPa level drives the process of upper-level trough amplification and cutoff cyclogenesis in this case. The quasigeostrophic height tendency patterns are also entirely consistent with the observed mass and wind-field tendencies, and with previous observational and theoretical analyses regarding the invertibility principle of potential vorticity.

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Michael S. Halpert
and
Gerald D. Bell

Abstract

No Abstract Available

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Gerald D. Bell
and
Alan N. Basist

Abstract

Abstract not available

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Gerald D. Bell
and
Lance F. Bosart

Abstract

Appalachian cold-air damming is investigated by means of 1) a 50-yr monthly climatology, 2) a synoptic case study of the event of 21–23 March 1985 and 3) an investigation of the flow structure and force balance within the cold dome.

The climatology reveals cold-air damming is a year-round phenomenon in the southern Appalachians with the most frequent, prolonged and intense events occurring in winter (particularly December and March) when three-five events per month can be expected. Cold-air damming is least frequent and intense in July.

The synoptic case study reveals that cold-air damming is critically dependent upon the configuration of the synoptic-scale flow. The cold dome can be identified by a “U” shaped ridge (trough) in the sea level isobar (thermal) patterns and the 930-mb height (temperature) fields representative of conditions at the base of the inversion overlying the cold dome. Differential horizontal and vertical thermal advection, as well as adiabatic and evaporative cooling, are responsible for the configuration of a strongly sloping inversion at the top of the cold dome and the pronounced baroclinic zone along the eastern edge of the cold dome. Evaporative cooling accounts for roughly 30% of the total cooling in parts of the dome, while adiabatic cooling explains a similar percentage of the cooling adjacent to the mountain slopes.

An accelerated flow nearly parallel to the mountains within the cold dome is identified and shown to be linked to the evolution of the synoptic-scale pressure field. The mountain-parallel component of the pressure gradient force is the primary acceleration source. The force balance on the accelerated flow after cold dome formation is geostrophic in the cross-mountain direction and antitriptic in the along-mountain direction. A geostrophic adjustment process is triggered from the formation of a region of small-scale ridging against the mountain slopes as cold air is constrained by the mountains to remain along the eastern slopes. The tendency for the Coriolis force to turn the flow toward the mountain is negated and the flow within the cold dome is directed ready parallel to the mountains and down the large-scale pressure gradient. Cold dome drainage occurs with the advection of the cold air toward the coast in response to synoptic-scale pressure falls accompanying coastal cyclogenesis.

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