Abstract

We have conducted observational analysts to identify systematic aspects of the life cycles of persistent anomalies of the extratropical Northern Hemisphere wintertime circulation. In the present study, we focus on the typical characteristics of the 500 mb height anomaly and flow patterns accompanying the development and breakdown of large-scale flow anomalies in two key regions, the eastern North Atlantic (ATL) and the northern Soviet Union (NSU), and then compare the results for these regions with results described previously for cases located over the North Pacific (PAC) region.

Throughout their life cycles, the positive anomaly and negative anomaly cases for a given region display a number of striking similarities. The primary anomaly center develop rapidly, with little indication of a significant anomaly over the key region until just prior to onset. Following establishment of the major anomaly center over the key region, anomaly centers develop and intensify in sequence downstream, leading to the establishment of the persistent anomaly pattern. Much of this downstream intensification occurs with little evidence of phase propagation. Once established, some of the anomaly patterns strongly resemble certain prominent teleconnection patterns (e.g., the Eastern Atlantic and Pacific-North American teleconnection patterns). The associated flow patterns are often characterized synoptically by the development of either blocking patterns or anomalously intense zonal flows over the key regions.

For all three regions, the gross features of the development downstream from the main center qualitatively resemble the behavior seen in simple barotropic models of energy dispersion on a sphere away from a localized, transient source of vorticity, suggesting that quasi-horizontal energy dispersion by Rossby waves is likely to account for important aspects of the downstream developments. In addition, the NSU cases are typically also preceded by a well-defined upstream wavetrain.

Precursors to the ATL pattern appear as characteristic anomaly patterns located to the southeast and to the southwest of the key region, the latter partly reflecting changes in the intensity and structure of the jet over the western Atlantic. The most systematic precursors to the PAC cases are related to variations in the jet intensity and structure over eastern Asia and the southwestern North Pacific and to an eastward-propagating, intensifying synoptic-scale disturbance. In both the ATL and PAC regions, the main anomaly centers are located downstream from the climatological mean jet maxima and tend to have zonally-elongated structures (i.e., typically U′₂>v′², suggesting that barotropic energy conversions from the time mean flow provide one possible source for their developments.

Breakdowns of the patterns also occur rapidly. Until a few days prior to breakdown, the patterns rather closely resemble the patterns immediately following development. The centers of the NSU pattern appear to weaken systematically in sequence downstream along the wavetrain. The breakdowns of the patterns in the other regions appear less well-defined, however. there is some indication that the main centers tend to drift generally northwestward while weakening.

Abstract

Historically, the atmospheric sciences have tended to treat problems of weather and climate separately. The real physical system, however, is a continuum, with short-term (minutes to days) “weather” fluctuations influencing climate variations and change, and, conversely, more slowly varying aspects of the system (typical time scales of a season or longer) affecting the weather that is experienced. While this past approach has served important purposes, it is becoming increasingly apparent that in order to make progress in addressing many socially important problems, an improved understanding of the connections between weather and climate is required.

This overview summarizes the progress over the last few decades in the understanding of the phenomena and mechanisms linking weather and climate variations. The principal emphasis is on developments in understanding key phenomena and processes that bridge the time scales between synoptic-scale weather variability (periods of approximately 1 week) and climate variations of a season or longer. Advances in the ability to identify synoptic features, improve physical understanding, and develop forecast skill within this time range are reviewed, focusing on a subset of major, recurrent phenomena that impact extratropical wintertime weather and climate variations over the Pacific–North American region. While progress has been impressive, research has also illuminated areas where future gains are possible. This article concludes with suggestions on near-term directions for advancing the understanding and capabilities to predict the connections between weather and climate variations.

Abstract

In a previous study of the extratropical Northern Hemisphere wintertime circulation, significant variations were described in the occurrence of anomalies that persist beyond the durations associated with synoptic-scale variability (“persistent anomalies”). The present study extends that work by identifying the typical structures of persistent anomalies, focusing on whether persistent occur in certain key regions (the central North Pacific (PAC), the eastern North Atlantic (ATL) and the northern Soviet Union (NSU)] are related to recurrent large-scale flow patterns. For each region, detailed comparisons are provided of the flow patterns associated with persistent positive and negative anomaly cases, and with the patterns obtained in other observations and theoretical studies of persistent phenomena.

The results provide evidence for the recurrence of certain preferred anomaly patterns. To a first approximation, the composite anomaly patterns of the positive and negative cases for a region can be described as opposite phase of the same basic pattern. This pattern is identified as. the primary regional pattern of low-frequency variability. For the PAC and ATL means the primary patterns resemble, respectively, the Pacific-North American (PNA) and Eastern Atlantic (EA) teleconnection patterns described by Wallace and Gutzler.

The majority of the persistent anomaly cases in each region are related to episodes of unusually large amplification of the of the primary patterns, with one phase frequently associated with blocking and the other with an abnormally strong zonal flow. The persistent flow anomalies are also typically accompanied by strong tropospheric temperature anomalies having patterns mainly in-phase with the height anomaly patterns, significant changes in the locations and intensities of the major surface centers of action and pronounced shifts in the locations of maximum storm activity.

Most of the temporal variability of the primary patterns is contributed by low-frequency, intraseasonal fluctuations (integral time scales of a few weeks). The interannual variance in both the ATL and NSU patterns is consistent with the level expected from the sampling of these relatively short-term fluctuations; however, a significant fraction of the interannual variance of the PAC pattern appears to be above the level expected from sampling variability.

Abstract

Observational analyses are performed to examine the roles of remote and local forcing in the evolutions of the extreme U.S. summer heat wave-drought cases of 1980 and 1988. At early stages, both events are associated with anomalous stationary wave patterns. Wave activity flux analyses suggest that in the 1980 case anomalous wave activity propagates southeastward from an apparent source region to the south of the Aleutians. The flux pattern is more complex in the 1988 case but suggests two possible source regions, one over the central North Pacific to the north of the Hawaiian Islands and a second located over the far western Pacific. The 1988 analyses show no anomalous wave propagation out of the eastern tropical Pacific, although this result does not necessarily preclude a role for tropical forcing in generating the anomalous wave train.

In both cases the anomalous wave trains and associated wave activity fluxes become very weak by early July, indicating that remotely forced anomalous stationary waves are unlikely to account for the later stages of the heat wave-droughts. This leads us to examine whether these events were enhanced or prolonged by changes in the local surface energy budget associated with reductions in evapotranspiration (ET) over the drought regions. Water vapor budgets show a systematic decrease in monthly mean ET from June to August during both events. Comparisons with nondrought summers support the idea that by late summer ET rates in both events are anomalously low. Estimated reductions in surface latent heat fluxes relative to the control years are approximately 50 W m⁻² in 1980 and 20 W m⁻² in 1988, with implied increases in sensible heating of similar magnitudes.

Overall, the results indicate the importance of both dynamical forcing from remote sources and anomalous local boundary conditions in accounting for the two extreme heat wave-drought events. The relative importance of these factors varies significantly during the evolution of the events, with remote forcing playing a predominant role at early stages and anomalous local boundary conditions assuming increasing importance at later stages.

Abstract

A simple two-layer quasigeostrophic model is employed to investigate the sensitivity of storm tracks to changes in an externally imposed, zonally varying large-scale flow. Zonally asymmetric temperature and horizontal deformation fields are varied systematically in order to compare the effects of baroclinicity and horizontal deformation on storm track dynamics. The sensitivity of the storm tracks to uniform barotropic zonal flows is also examined.

The results show two competing processes for storm track organization, one associated with a local maximum in baroclinicity and the other with a local minimum in horizontal deformation. When the equilibrium state consists of a zonally symmetric temperature field and a barotropic stationary wave, the maximum in synoptic-scale transient eddy energy (storm track) is located in the entrance region of the upper jet just downstream of the point of minimum horizontal deformation. As zonal variations in baroclinicity become large (keeping the upper-layer horizontal deformation constant), the storm track shifts to the jet exit region just downstream of the point of maximum baroclinicity. For flows intermediate between the above cases,that is, having weaker zonal variations in baroclinicity and the same upper-layer deformation, two storm track maxima appear, one located in the jet entrance and the other in the jet exit region.

The results also indicate that the storm tracks are sensitive to changes in a uniform barotropic zonal flow. The presence of a uniform westerly flow extends the storm track and strengthens eddy activity, while the addition of a uniform easterly flow shortens the storm track and dramatically weakens eddy activity. The changes in the magnitudes of eddy activity appear related to differences in the efficiency of nonlinear barotropic decay processes in weakening the eddies in the jet exit region.

Sensitivities of the location of the storm tracks to changes in large-scale flow parameters are well captured by linear calculations, although sensitivities of the strength of the storm tracks are not. For sufficiently strong zonal variations in baroclinicity, two coherent modes of low-frequency variability develop. They are characterized synoptically by 1) a meridional shift, and 2) an extension/contraction as well as a modulation in the strength of the upper-layer jet and storm track.

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.

Abstract

The present study extends our previous work on the life cycles of persistent anomalies by providing more comprehensive analyses of the synoptic and dynamical characteristics associated with the developments of the anomalies. We focus here on the developments of major cases of persistent negative height anomalies over the extratropical central North Pacific (PAC) region during wintertime. These ewes are generally manifested at the surface by an anomalously intense and eastward-displaced Aleutian low and, at upper levels, by an abnormally strong zonal jet that extends across most of the western and central Pacific at midlatitudes. The associated flow anomalies usually resemble particularly strong realizations of the Pacific-North American (PNA) teleconnection pattern.

The large-scale flow anomalies are typically preceded by a buildup of anomalously cold air over Asia and an intensification of the upper-level jet over southeastern Asia and the fox western Pacific. A few days prior to the larger-scale developments, a synoptic-scale disturbance intensifies over the northwest Pacific in a region of pronounced baroclinity on the cyclonic-shear side of the upper-level jet. As this disturbance propagates eastward into the mid-Pacific, it acquires a more zonally elongated, equivalent barotropic structure. During this period, the upper-level zonal wind anomalies initially over the western Pacific also extend eastward to the central Pacific.

The large-scale anomaly pattern that subsequently develops over the Pacific and North America resembles the most rapidly growing normal mode associated with barotropic instability of the climatological-mean wintertime flow. Diagnostic analyses confirm that, particularly at later stages in the developments, barotropic conversions from the time-mean flow contribute positively to the growth of the anomalies. These results support the idea that barotropic instability of the time-mean flow provides one mechanism for the developments.

Nevertheless, baroclinic processes also appear to play a significant role, particularly during the early stages of the developments. Temperature advection patterns associated with the growing disturbance tend to concentrate temperature gradients along the axis of the intensifying jet. Net eddy heat fluxes during the developments are both downgradient and upward, although most consistently so during the early stages of the developments. Net heat fluxes at later stages continue to have a substantial downgradient component although they also display a strong rotational (nondivergent) component, consistent with the more equivalent barotropic structure of the disturbance observed at later times.

The overall impression that emerges is of initial baroclinic development at long synoptic scales, followed by increasing barotropic contributions and decreasing baroclinic contributions to the growth of the anomalies after the disturbance reaches the jet exit region over the central Pacific. Additional baroclinic contributions to the developments may also occur at later stages. The observed characteristics are consistent with the hypothesis that the large-scale flow anomalies in these cases develop primarily as a result of an instability of the three-dimensional wintertime mean flow.

Abstract

We have studied the geographical and regional persistence characteristics of wintertime Northern Hemisphere 500 mb height anomalies, focusing particular attention on the behavior of strong anomalies that persist beyond the durations associated with synoptic-scale variability (“persistent anomalies”). We have also examined the persistence characteristics of certain dominant regional patterns of low-frequency variability.

There are three major regions for the occurrence of persistent anomalies: the North Pacific to the south of the Aleutians, the North Atlantic to the southeast of Greenland, and from the northern Soviet Union northeastward to over the Arctic Ocean. These regions have relatively high numbers of both persistent positive anomaly and persistent negative anomaly cases. For moderate magnitudes and durations, the numbers of positive and negative cases in each region are about the same; however, for larger magnitudes and longer durations, the number of positive cases exceeds the corresponding number of negative cases. Analyses with data that have been low-pass filtered (removing periods of less than 6 days) reveal that part (but not all) of the discrepancy between positive and negative cases results from the relatively greater likelihood that negative anomalies will experience brief interruptions by transient disturbances.

For durations beyond about 5 days, the probability that an anomaly which has lasted n days will last at least one more day is nearly constant. This nearly constant probability of continuation resembles the behavior obtained for a linear first-order autoregressive process (red noise). Nevertheless, there are significant differences in persistence between the positive and negative anomalies and red noise, particularly at large magnitudes, with the positive anomalies typically more persistent than either the negative anomalies or red noise. A simple nonlinear autoregressive model is described that simulates many of the observed deviations from red noise, and possible physical sources for the nonlinearities are discussed.

Relationships between the initial anomaly value and its subsequent 12 h change are then studied. The height changes are decomposed into two parts: a mean change and a deviation from the mean change. Mean change variations are examined for evidence of multiple “quasi-equilibria” (multiple anomaly values having mean changes of zero). Mean change variations are also determined for the temporal coefficients of certain dominant regional patterns of low-frequency variability. Although the temporal fluctuations of the patterns exhibit considerably more persistence than found for the corresponding local height anomalies, neither the patterns nor the local anomalies display convincing evidence of multiple quasi-equilibria.

Abstract

For the population Of cyclones that formed over North America and the adjacent Atlantic Ocean during the Genesis of Atlantic Lows Experiment (GALE; 13 January-16 March 1986), a variety of interrelationships between various cyclone characteristics are considered. Previous cyclone climatologies are extended by requiring no minimum cyclone amplitude. Particular attention is paid to the horizontal size distribution of the cyclones. It is found that 1) most cyclones are subsynoptic in scale, with only the deepest cyclones having a size consistent with classical baroclinic-instability theory; 2) almost all small-scale cyclones have a total life span of less than 48 h; 3) the pressure gradient within small-scale cyclones tends to be weaker than that within large-scale cyclones; 4) Atlantic cyclones deepen much more rapidly than cyclones over the continent but for many cyclones, deepening rate is an unsuitable measure of intensification; and 5) the geographical distribution of cyclogenesis during GALE is broadly similar to that found in more comprehensive climatologies, but some significant differences are present that are attributable to the inclusion of weak cyclones and stationary, orographically forced cyclones.

Abstract

The relationship between the time-mean planetary-scale deformation field and the structure of midlatitude storm tracks is studied in wintertime simulations of the National Center for Atmospheric Research (NCAR) Community Climate Model and the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System model. Model biases are determined by contrasting model simulations (forced by observed SSTs) with parallel analyses of NCEP–NCAR reanalyses. Barotropic diagnostics are employed to identify potential dynamical linkages between regional biases in the midlatitude storm tracks and the horizontal deformation field. Initial observational analyses confirm that synoptic eddies are optimally configured to transfer kinetic energy to the mean flow in the jet exit regions, where strong stretching deformation exists. In these regions, the major axes of the synoptic eddies are aligned along the dilatation axes of the mean flow. Consequently, mean flow advection stretches synoptic eddies along their major axes, thereby increasing their anisotropy and weakening their kinetic energy.

A strong link is identified between model biases in the horizontal structure of the midlatitude storm tracks and the representation of upper-tropospheric barotropic deformation. In particular, model-simulated storm tracks extend too far downstream in regions where the zonal stretching deformation (associated with horizontal diffluence in jet exit regions) is either too weak in magnitude or displaced westward in comparison with observations. These biases are associated with anomalously weak or westward-displaced patterns of negative barotropic energy conversions, which normally act as a sink of synoptic eddy activity in the jet exit. The anomalous energy conversion patterns are primarily due to model biases in the winter-mean flow rather than the simulated horizontal eddy structures, which closely resemble observations.

The results indicate that the horizontal structure of midlatitude storm tracks in climate models is strongly controlled by the large-scale patterns of barotropic deformation in the upper troposphere. It is suggested that barotropic deformation analyses may provide a useful diagnostic measure for assessing climate simulation errors in atmospheric general circulation models.