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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

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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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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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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Robert X. Black and Brent A. McDaniel

Abstract

A principal component analysis is performed to characterize intraseasonal variability in the boreal stratospheric polar vortex. In contrast to previous studies, the current analysis examines daily zonal-mean variability within a limited spatial domain encompassing the stratospheric polar vortex. The leading EOFs are vertically coherent north–south dipoles in the zonal-mean zonal wind extending through the lower stratosphere. The first mode represents variability in polar vortex strength and is highly correlated with the stratospheric northern annular mode (SNAM). The second mode, the polar annular mode (PAM), represents variability in the latitudinal position of the polar vortex and possesses a poleward-retracted dipole anomaly structure. Composite analyses indicate that large-amplitude PAM events are relatively short lived (1–2 weeks) compared to SNAM events (1 month or longer). Trend analyses further reveal that recent decadal trends in the boreal polar vortex project more strongly onto PAM than SNAM.

Composite analyses illustrate that the time evolution of sudden stratospheric warming events is dominated by SNAM, whereas SNAM and PAM play approximately equal roles in final warming events. Linear regression analyses reveal that SNAM and PAM result in circumpolar circulation and temperature anomalies of similar magnitudes within the high-latitude troposphere. It is concluded that PAM represents a previously unrecognized annular mode that strongly couples the stratosphere and troposphere on submonthly time scales at mid- to high latitudes. It is further suggested that the SNAM/PAM framework provides a means for isolating the proximate tropospheric response to respective variations in the strength and position of the stratospheric polar vortex.

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Dennis P. Robinson and Robert X. Black

Abstract

A comprehensive analysis of midlatitude intraseasonal variability in extended integrations of NASA GSFC general circulation models (GCMs) is conducted. This is approached by performing detailed intercomparisons of the representation of the storm tracks and anomalous weather regimes occurring during wintertime in the Atmospheric Model Intercomparison Project (AMIP)-type simulations of both the NASA–NCAR and a version of the Aries model used in NASA’s Seasonal-to-Interannual Prediction Project (NSIPP) model. The model-simulated statistics, three-dimensional structure, and dynamical characteristics of these phenomena are diagnosed and directly compared to parallel observational analyses derived from NCEP–NCAR reanalyses.

A qualitatively good representation of the vertical structure of intraseasonal eddy kinetic energy (EKE) is provided by both models with maximum values of EKE occurring near 300 hPa. The main model shortcoming is an underestimation of EKE in the upper troposphere, especially for synoptic eddies in the NSIPP model. Nonetheless, both models provide a reasonable representation of the three-dimensional structure and dynamical characteristics of synoptic eddies. Discrepancies in the storm-track structures simulated by the models include an anomalous local minimum over the eastern Pacific basin. However, both GCMs faithfully reproduce the observed Pacific midwinter storm-track suppression. Interestingly, the NSIPP model also produces a midwinter suppression feature over the Atlantic storm track in association with the anomalously strong upper-level jet stream simulated by NSIPP in this region.

The regional distribution of anomalous weather regime events is well simulated by the models. However, substantial structural differences exist between observed and simulated events over the North Pacific region. In comparison to observations, model events are horizontally more isotropic, have stronger westward vertical tilts, and are more strongly driven by baroclinic dynamics. The structure and dynamics of anomalous weather regimes occurring over the North Atlantic region are qualitatively better represented by the models. The authors suggest that model deficiencies in representing the zonally asymmetric climatological-mean flow field (particularly the magnitude and structure of the Pacific and Atlantic jet streams) help contribute to model shortcomings in (i) the strength and seasonal variability of the storm tracks and (ii) dynamical distinctions in the maintenance of large-scale weather regimes.

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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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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

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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Katherine J. Evans and Robert X. Black

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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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