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Robert X. Black

Abstract

Diagnostic potential enstrophy (PE) analyses are formulated for studying the dynamics of transient eddies within discrete frequency bands. The PE analyses are derived for nonconservative quasigeostrophic motion on a sphere and relate local PE tendencies to 1) conversions from the climatological-mean flow, 2) nonlinear dynamical sources, and (3) nonconservative physical sources. Local conversions are associated with cross-gradient residual eddy fluxes of pseudopotential vorticity (PV) while the nonlinear and nonconservative sources of PE result from correlations between PV anomalies and anomalous PV source terms. The PE analyses provide a useful local diagnostic approach for studying fundamental aspects of transient wave dynamics within a PV-based framework.

Wintertime-average PE analyses are constructed for both bandpass (BP) and low-pass (LP) intraseasonal timescales. These analyses are applied to the multiyear GEOS-1 assimilated dataset to ascertain the primary sources and sinks of BP and LP transient eddy activity in the Northern Hemisphere extratropics. In the interior troposphere, eddy PE is found to maximize near 400 hPa. The PE analyses applied at this level indicate that the primary source of both BP and LP eddy activity is a local conversion from the wintertime-mean flow. These conversions are associated with residual eddy PV fluxes that are directed down the wintertime-mean PV gradient. Thus, both BP and LP transients act to reduce the wintertime-mean PV gradients in the upper troposphere. For BP eddies, there is a significant additional PE source in the storm tracks associated with latent heat release. Over the North Pacific this additional source is locally of the same order as the mean-flow conversion. The sources of BP and LP PE are largely offset by enstrophy sinks associated with nonlinear eddy processes. At second order, latent heating anomalies provides weak dissipation of LP PE in regions of frequent blocking activity. These signatures are reproduced in parallel diagnoses of the NCEP–NCAR Reanalyses. Radiative processes provide relatively weak and uncertain contributions to the BP and LP PE budgets while turbulent mixing of heat and momentum furnish negligible contributions.

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Robert X. Black

Abstract

Anomalous wave source regions are identified during the life cycles of persistent flow anomalies occurring over the North Pacific and North Atlantic Oceans during boreal winter. These cases project strongly upon the Pacific–North American and Eastern Atlantic teleconnection patterns, respectively, and represent two of the primary modes of intraseasonal low frequency variability in the Northern Hemisphere wintertime circulation. In the upper troposphere, these cases are manifested by wave trains of large-scale and large amplitude flow anomalies extending downstream from key regions near the Aleutian Islands and south of Iceland, respectively. The occurrence of persistent flow anomalies is closely linked to persistent extremes in surface weather and to variations in the skill of extended-range weather forecasts. Black and Dole performed a detailed diagnostic analysis of the onset of persistent cyclonic flow anomalies over the North Pacific. A key result of their study was that wave activity flux diagnoses correctly identified the location of anomalous wave source regions.

Black and Dole’s study is extended by diagnosing three-dimensional fluxes of wave activity during the onset, maintenance, and decay of cyclonic and anticyclonic persistent flow anomalies over the North Atlantic and North Pacific regions. The flux characterizes large-scale wave propagation, and the flux divergence is used to deduce the location of regional sources and sinks of anomalous wave activity. In all cases there is a marked upward and downstream flux of wave activity emanating from the lower troposphere near the key region. The results are consistent with a local wave source and Rossby-like wave propagation away from the source. There is little evidence that anomalous remote processes directly force persistent flow anomaly life cycles. The diagnostic analyses indicate that the primary anomalous wave sources for both cyclonic and anticyclonic persistent flow anomalies are local to the region of their occurrence. It remains possible, however, that anomalous remote forcing plays an indirect or catalytic role in some cases.

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Robert X. Black

Abstract

Diagnostic results are presented indicating that during the Arctic oscillation surface climate variations are directly forced by changes in the strength of the stratospheric polar vortex. To be specific, large-scale potential vorticity anomalies in the lower stratosphere induce zonally symmetric zonal wind perturbations extending downward to the earth's surface. This represents a large-scale annular stirring of the troposphere from above. During discrete events, this influence is manifested as a downward transient pulse initially emanating from the midstratosphere and ultimately altering surface weather. It is suggested that this mechanism may help to explain several observed stratospheric influences upon surface climate, including the effects of volcanic eruptions, the solar cycle, ozone depletion, and greenhouse gases.

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Jia He and Robert X. Black

Abstract

The high-latitude atmosphere experiences a rapid state transition during Arctic spring onset (ASO) with distinct warming in surface 2-m air temperature (T2m) occurring over broad geographical regions. Three methods are tested to optimally isolate this transition: The first two, the time derivative and the radius of curvature (RoC) methods, identify periods of large T2m acceleration. The third technique, the two-phase linear regression model, identifies a transition from an approximately steady winter state to a warming spring state. Although all three methods are largely successful in isolating the state transition associated with ASO, the RoC method is most effective in capturing the most rapid temperature increases and is adopted to define ASO in the study.

Statistical analyses indicate that the annual ASO timing is roughly bimodal with strong interannual variability but no significant long-term trends. Composite time evolution analyses of ASO uncover a critical warming region over northern Siberia common to most events. Several subcategories of ASO events are identified in which distinct warming signatures are also observed in the Greenland–North American, East Asian, and Alaskan sectors. The characteristic synoptic structures associated with these events are isolated via a parallel composite analysis of sea level pressure. These analyses provide initial evidence that, during ASO, the synoptic evolutions of semipermanent surface pressure systems (oceanic lows and continental highs) provide favorable conditions for rapid regional advective and diabatic warming in the lower troposphere.

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Dennis P. Robinson and Robert X. Black

Abstract

Comparative diagnostic analyses of developing synoptic-scale baroclinic disturbances in NCEP–NCAR reanalyses and the NASA–NCAR (NASCAR) and Aries [NASA’s Seasonal-to-Interannual Prediction Project (NSIPP)] general circulation model simulations are performed. In particular, lag composite analyses of wintertime cyclonic and anticyclonic events occurring in the North Pacific and North Atlantic storm tracks are constructed to pursue a synoptic and dynamic characterization of eddy development. The data are also seasonally stratified to study aspects of the North Pacific midwinter suppression phenomenon.

Winter-averaged results indicate that the model-simulated events are generally too weak in amplitude, particularly in the upper troposphere. For the North Pacific storm track, model-simulated events are also anomalously distended in the meridional direction. The existing model biases in eddy structure and magnitude lead to anomalously weak baroclinic energy conversions for both cyclonic and anticyclonic events over the North Pacific. For the North Atlantic storm track the NASCAR model provides a very good representation of the structure of developing cyclonic events. However, growing North Atlantic cyclones in the NSIPP model are anomalously weak and horizontally too isotropic (meridionally retracted). These latter two characteristics are also observed in both models for developing anticyclonic flow anomalies over the North Atlantic. The relative weakness of NSIPP synoptic events over the North Atlantic region is largely responsible for the 50% deficiency in areal-averaged baroclinic energy conversions. Conversely, the NASCAR model climatology features anomalously strong temperature gradients over the western North Atlantic that provide local enhancements to the baroclinic energy conversion field.

A seasonally stratified diagnostic analysis reveals that the simulated climatological storm tracks over the North Pacific undergo larger spatial migrations during the cool season compared to observations. It is further determined that the suppression of synoptic eddy activity observed in the Pacific storm track is associated with a relative midwinter weakness in the magnitude of the growing cyclonic anomalies. Specifically, during midwinter the cyclonic perturbations entering the Pacific storm track are deficient in magnitude compared to their early and late winter counterparts. It is also discovered that the midwinter suppression pattern over the North Pacific region has a clear organized extension upstream into Siberia, the region from which incipient upper-tropospheric short-wave features emanate. This behavior is found in both observations and the model simulations. The results herein support the idea that the North Pacific midwinter suppression phenomenon is linked to a midwinter weakness in the upstream formation of upper-level short waves, leading to anomalously weak “seeding” of baroclinic disturbances in the Pacific storm track.

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Katherine J. Evans and Robert X. Black

Abstract

Piecewise tendency diagnosis (PTD) is extended and employed to study the dynamics of weather regime transitions. Originally developed for adiabatic and inviscid quasigeostrophic flow on a beta plane, PTD partitions local geopotential tendencies into a linear combination of dynamically meaningful source terms within a potential vorticity (PV) framework. Here PTD is amended to account for spherical geometry, diabatic heating, and ageostrophic processes, and is then used to identify the primary mechanisms responsible for Northern Hemisphere weather regime transitions.

Height tendency patterns obtained by summing the contributions of individual PTD forcing terms correspond very well to actual height tendencies. Composite PTD analyses reveal that linear PV advections provide the largest dynamical forcing for the weather regime development over the North Pacific. Specifically, linear baroclinic growth provides the primary forcing while barotropic deformation of PV anomalies provides a secondary contribution. North Atlantic anticyclonic anomalies develop from the combined effects of barotropic deformation, baroclinic growth, and nonlinear eddy feedback. The Atlantic cyclonic cases develop primarily from barotropic deformation and nonlinear eddy feedback. All four weather regime types decay primarily due to enhanced wave energy propagation away from the primary circulation anomaly. In some cases, regime decay is aided by decreasing positive contributions from barotropic deformation as the circulation anomaly attains a deformed horizontal shape. The current results 1) provide quantitative measures of the primary mechanisms responsible for weather regime transition and 2) demonstrate the utility of the extended PTD as a concise diagnostic paradigm for studying large-scale dynamical processes in the midlatitude troposphere.

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Robert X. Black and Brent A. McDaniel

Abstract

A lag composite analysis is performed of the zonal-mean structure and dynamics of Northern Hemisphere stratospheric final warming (SFW) events. SFW events are linked to distinct zonal wind deceleration signatures in the stratosphere and troposphere. The period of strongest stratospheric decelerations (SD) is marked by a concomitant reduction in the high-latitude tropospheric westerlies. However, a subsequent period of tropospheric decelerations (TD) occurs while the stratospheric circulation relaxes toward climatological conditions. During SFW onset, a wavenumber-1 disturbance at stratospheric altitudes evolves into a circumpolar anticyclonic circulation anomaly.

Transformed Eulerian-mean dynamical diagnoses reveal that the SD period is characterized by an anomalous upward Eliassen–Palm (EP) signature at high latitudes extending from the surface to the middle stratosphere. The associated wave-driving pattern consists of zonal decelerations extending from the upper troposphere to the midstratosphere. Piecewise potential vorticity tendency analyses further indicate that zonal wind decelerations in the lower and middle troposphere result, at least in part, from the direct response to latitudinal redistributions of potential vorticity occurring in the lower stratosphere. The TD period exhibits a distinct dynamical behavior with anomalous downward EP fluxes in the high-latitude stratosphere as the zero zonal wind line descends toward the tropopause. This simultaneously allows the stratospheric polar vortex to radiatively recover while providing anomalous upper-tropospheric zonal decelerations (as tropospheric Rossby wave activity is vertically trapped in the high-latitude troposphere). The tropospheric decelerations that occur during the TD period are regarded as a subsequent indirect consequence of SFW events.

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Rebecca M. Westby and Robert X. Black

Abstract

During winter, anomalous temperature regimes (ATRs), which include cold-air outbreaks (CAOs) and warm waves (WWs), have important impacts in the southeastern United States. This study provides a synoptic–dynamic characterization of ATRs in the southeastern United States from 1949 to 2011 through composite time-evolution analyses. Events are categorized by the sign and amplitude of relevant low-frequency modes. During CAO (WW) onset, negative (positive) geopotential height anomalies are observed in the upper troposphere over the Southeast with oppositely signed anomalies in the lower troposphere over the central United States. In most cases, there is a surface east–west geopotential height anomaly dipole, with anomalous northerly (CAO) or southerly (WW) flow into the Southeast leading to cold or warm surface air temperature anomalies, respectively. Companion potential vorticity anomaly analyses reveal prominent features in the mid- to upper troposphere consistent with the coincident geopotential height anomaly patterns. Ultimately, synoptic-scale disturbances are found to serve as dynamic triggers for ATR events, while low-frequency modes provide a favorable environment for ATR onset. The results provide a qualitative indication of the role of low-frequency modes in ATR onset. In WW (CAO) events influenced by low-frequency modes, the North American geopotential height anomaly pattern arises in part as a downstream (regional) manifestation of the negative Pacific–North American pattern (North Atlantic Oscillation). Interestingly, the North Atlantic Oscillation contributes to both CAO onset and demise. Thus, these results indicate that low-frequency modes also affect event duration (CAOs). One general distinction found for ATRs is that CAOs involve substantial airmass transport while WW formation is more regional in nature.

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Robert X. Black and Brent A. McDaniel

Abstract

Recent observational studies of the northern annular mode (NAM) indicate that significant case-to-case variability exists in the structural evolution of individual events. In particular, certain NAM events remain confined to stratospheric altitudes whereas others readily penetrate downward into the troposphere. We perform observational diagnostic analyses that are targeted at identifying the physical mechanisms behind this distinction. Our results thereby provide a test of the different existing theories regarding stratospheric influences upon tropospheric climate.

We contrast robust stratospheric NAM events with differing tropospheric signals in order to identify the underlying dynamical reasons for the observed differences. Piecewise potential vorticity (PV) inversions and Eliassen–Palm flux analyses are performed to study the roles of different forcing mechanisms during NAM onset. Our results indicate that variations in the tropospheric response are readily explained on the basis of piecewise PV inversions. Specifically, during individual cases, preexisting tropospheric PV anomaly features can mask the downward penetration of an initial stratospheric NAM signal into the troposphere. Analyses of PV inversions further suggest that a minimum requirement for a direct downward stratospheric influence is that the stratospheric NAM signal be robust in the lower stratosphere. Thus, whether or not a tropospheric NAM signal emerges from a stratospheric NAM event is largely dependent upon (i) whether stratospheric PV anomalies descend to sufficiently low altitudes within the stratosphere and (ii) the detailed nature of preexisting annular modes in the troposphere. Parallel Eliassen–Palm flux analyses further indicate that anomalous Rossby wave forcing is important for initiating NAM events in the midstratosphere and facilitating their downward advance into the lower stratosphere.

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Robert X. Black and Randall M. Dole

Abstract

Earlier studies of persistent large-scale flow anomalies have been extended, with the aim of identifying the primary mechanisms for persistent anomaly development. In this study the focus is on wintertime cases of persistent cyclonic flow anomalies over the North Pacific. These cases are typically manifested by an abnormally intense cyclonic circulation extending over the North Pacific basin, an unusually strong and eastward-extended East Asian jet, and a well-defined Pacific-North American teleconnection pattern. We have conducted extensive diagnostic analyses in order to determine the mechanisms responsible for development. In particular, these diagnostics examine the processes influencing the time evolution of eddy potential enstrophy and potential vorticity anomalies.

The cases are preceded by a buildup of anomalously high potential vorticity air at upper levels over eastern Asia. This high potential vorticity air is initially advected eastward in association with synoptic-scale cyclogenesis over the western North Pacific. As the disturbance propagates eastward into the central Pacific, it evolves toward a more zonally elongated and equivalent barotropic structure. Large-scale cyclogenesis ensues as the low becomes quasi-stationary near the Aleutians. In conjunction with large-scale development, the disturbance reacquires an upshear tilt with height.

Diagnostic analyses of wave activity fluxes indicate that the primary source region for the developments is over the extratropical North Pacific. Potential enstrophy analyses show that eddy enstrophy increases result mainly from downgradient potential vorticity fluxes by the large-scale eddy. The conversions are primarily baroclinic in nature, although barotropic processes also provide positive contributions. Anomalous nonconservative and nonlinear processes are relatively small and oppose the observed enstrophy changes.

Potential vorticity (PV) inversions are then performed to further clarify the dynamical mechanisms for large-scale development. A few days prior to large-scale development, anomalous upper-level northwesterly winds, associated with low-level thermal anomalies over the western North Pacific region, advect high PV air south-eastward from Asia into the western Pacific. As the PV maximum reaches the central Pacific, its associated circulation penetrates to the surface, resulting in a thermal advection pattern that produces a warm surface anomaly and associated surface cyclone downshear of the upper-level center. This is followed by strong baroclinic intensification. In several respects this behavior resembles a classical Petterssen Type B development, but occurs on a scale that is much larger than for typical synoptic-scale cyclogenesis.

The results indicate that the primary mechanism for the developments is a large-scale instability of (or initial value development upon) the three-dimensional time-mean flow, and suggest that nonmodal transient growth plays a significant role during development.

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