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  • Author or Editor: Siegfried D. Schubert x
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Siegfried D. Schubert

Abstract

The winter-time circulation of the Northern Hemisphere is examined for the following time-scale classes. A) greater than 45 days, B)20–45 days C) 10–20 days D) 6–10 days and E) 2.5&ndash 6 days. The spatial structure of variability within each of these frequency buds is determined by an empirical orthogonal function expansion of the coupled vertical mean and difference streamfunction fields. In general, the dominant low-frequency modes (clan or filter A) exhibit zonally elongted patterns with an equivalent barotropic structure. The intermediate time scale modes (classes B and C) show a tendency for more circular anomaly patterns accompanied by a more baroclinic (westward tilt with height) structure for the class C modes. The dominant frequency modes (classes D and E) are characterized by a strong baroclinic component. The latter exhibit meridionally elongated patterns in the storm track regions together with an upstream tilt to the north and south of the jet exits. In general, lag cross correlations between dominant low-frequency modes tend to be small and/ or symmetric about zero. In contrast, the highest frequency modes exhibit highly asymmetric sinusoidal crow correlation functions reminiscent of travelling waves, while some intermediate and short-time scale modes exhibit correlations suggesting an association with the decaying phases of blocking in the Pacific.

The winter average barotropic and baroclinic energy sources and sinks associated with the time-mean winter flow am examined in the context of a two-layer quasi-geostrophic model. The mean flow kinetic energy (KE) conversion terms reflect the geometry of the anomalies such that the dominant low-frequency modes gain energy, the intermediate frequencies are approximately neutral and the higher frequency modes low energy to the mean flow. The magnitude of the KE conversion is largest in the upper troposphere and is dominated by the lowest-frequency modes. The magnitude conversion from the mean flow to the anomalies is positive for the dominant modes in each band. However, the highest frequency modes associated with traveling waves in the North Atlantic and North Pacific are most efficient in the conversion process. For a typical Rossby radius of deformation, it is found that the net barotropic and baroclinic conversions are of comparable magnitude for periods greater than 10 days while the baroclinic conversion dominates for shorter periods. The majority of the conversions are accomplished by just a few (∼ five) of the dominant EOFs in each frequency band.

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Siegfried D. Schubert

Abstract

The observed wintertime intraseasonal variability of the Northern Hemisphere midtropospheric circulation is analyzed within the framework of an equivalent barotropic model. The analysis centers on the wave domain empirical orthogonal functions (E0Fs) of the 500 mb streamfunction anomalies. The projection of the dynamical model onto the EOFs leads to a system of quadratically nonlinear equations involving the EOF coefficients.

A major result of this study is the identification of the barotropically unstable wintertime mean flow as a potentially important energy source for some of the dominant low frequency EOFS. These EOFS are associated with such hemispheric scale variations as an index cycle, the Pacific/North American pattern, and the North Atlantic Oscillation.

The dominant EOFs are most strongly influenced by nonlinear interactions and this decreases as one goes to the higher order modes. In contrast, the beta and mean flow-EOF interaction terms (which are highly negatively correlated) have a relatively weak influence on the first few EOFs while the strongest influence is on the intermediate EOFs (15–25). The parameterized terms (orography, friction and long-wave correction) have a secondary effect on the EOFs when compared to the advection terms.

Generally, the dominant EOFs represent more unstable flow regimes when compared with the time mean flow. The negative instance of EOF 9, which resembles North Atlantic blocking, is particularly unstable and an inspection of the time mean EOF interactions supports the idea that interactions with the dominant low-frequency modes act to destroy this pattern. In the present model, blocking is most likely to occur as a quasi-linear response to the inhomogeneous forcing which enters into the model as a balance requirement of the time-averaged horizontal flow.

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Siegfried D. Schubert and Max Suarez

Abstract

Average predictability and error growth are studied in a realistic two-level general circulation model of the atmosphere via a series of Monte Carlo experiments for fixed external forcing (perpetual winter in the Northern Hemisphere). For realistic initial errors, the dependence of the limit of dynamic predictability on total wave number is similar to that found for the ECMWF model for 1980/81 winter conditions, with the lowest wavenumbers showing significant skill for forecast ranges of more than 1 month. For very small amplitude error (1.2 m rms height at 500 mb) distributed according to the climate spectrum, the total error growth is superexponential, reaching a maximum growth rate (2-day doubling time) in about 1 week.

A simple empirical model of error variance involving two broad wavenumber bands (large scales: n < 10 and small scales: 10 ≤ n ≤ 15), provides an excellent fit of the GCM's error growth behavior. The interpretation of the empirical model, based on an analogy with the stochastic dynamic equations developed by Epstein, suggests that the initial rapid increase in the growth rate of errors in the large scales is primarily due to interactions with the small-scale error. These interactions have preferred geographical locations associated with the position of the climate mean jet streams. However, the error growth of the small scales is large unaffected by the presence of the large-scale error. The initial strong growth rate (2-day doubling time) of the small scales is attributed to the model's high level of eddy activity.

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Siegfried D. Schubert and Gerald F. Herman

Abstract

A method is demonstrated for evaluating global and zonally averaged heat balance statistics based on a four-dimensional assimilation with an atmospheric general circulation model (GCM). The procedure, which provides observationally constrained model diagnostics, uses the GCM of NASA's Goddard Laboratory for Atmospheric Sciences to evaluate the atmospheric heat balance for the February 1976 Data Systems Test period. The global distribution of the adiabatic and diabatic components of the heat balance are obtained by sampling the continuous GCM assimilation shortly after the insertion of conventional synoptic observations. Sampling times of 6 and 9 h after data insertion were chosen to provide adequate damping of high-frequency oscillations in the vertical velocity field caused by the data insertion.

Salient features of the February 1976 analysis include the following: Maximum rising motion in the mean vertical velocity field at 500 mb over South America, south-central Africa, Australia and the Indonesian archipelago. These regions also were characterized by large values of diabatic heating due to convective latent heat release. The cyclogenetically active regions over the North Atlantic and North Pacific oceans were characterized by maxima in latent heat release due to supersaturation cloud formation, and also maxima in the upward and northward transient eddy heat fluxes. In contrast, the continental west coasts showed a tendency for large downward and southward transient eddy beat fluxes.

Some differences are obtained between the heating rates calculated with the model parameterizations and through a residual method. Other shortcomings of the procedure include data deficiencies in the Southern Hemisphere, which cause the results to be comparatively more model dependent in the high southern latitudes.

The potential applicability of this method of analysis to the recently acquired FGGE data is noted.

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Chung-Kyu Park and Siegfried D. Schubert

Abstract

Intraseasonal (20–70 day) variability is examined in the Atlantic region during Northern Hemisphere winter using ECMWF analyses and NOAA outgoing longwave radiation (OLR). It is found that the dominant 200-mb zonal-wind fluctuation over the tropical Atlantic, A1, is related to global-scale circulation anomalies with their origins in the Pacific. Compositing techniques are used to investigate the nature of the Pacific-Atlantic teleconnections and related changes in the tropical OLR and moisture convergence.

The OLR anomalies associated with A1 are characterized by eastward propagation over the Indian Ocean and the western Pacific and a standing oscillation over the tropical Atlantic; the latter extends from Northeast Brazil to West Africa and is the dominant component of the Atlantic OLR variability on these time scales. An analysis of the velocity potential and moisture convergence fields suggests that the fluctuations in convection are coupled between the western Pacific and Atlantic via large-scale (zonal wavenumber 1), equatorially trapped, eastward-propagating waves associated with the Madden-Julian oscillation.

The zonal-wind fluctuation, A1, is also related to extratropical waves propagating into the tropics from both the Northern and Southern hemispheres. The Southern Hemispheric wave train, which makes up the dominant contribution to the A1 circulation pattern, appears to emanate from the western South Pacific and amplifies near the west coast of South America. The Northern Hemispheric wave train resembles the Pacific/North American pattern and emanates from the central North Pacific near the East Asian jet exit region.

These results suggest that a major component of the 20–70-day variability over the Atlantic region is remotely forced. The forcing occurs via the Madden-Julian oscillation, which is strongly coupled with eastward-migrating heating anomalies in the western Pacific and Rossby wave trains, which appear to have their origins in the middle latitudes of the Pacific. The Northern Hemispheric wave train appears to be maintained by energy exchange with the East Asian jet, while the nature of the Southern Hemispheric branch is unclear.

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Siegfried D. Schubert and Chung-Kyu Park

Abstract

Low-frequency (20–70 day) variability is examined during Northern Hemisphere (NH) winter based on seven Years (1981–87) of European Centre for Medium Range Weather Forecasts initialized analyses. The dominant 200 mb zonal wind fluctuations in the Pacific sector, determined from an empirical orthogonal function (EOF) analysis, provide the baseline modes of atmospheric variability, which are related to fluctuations in other circulation parameters and outgoing longwave radiation (OLR). The composite circulation associated with the extreme phases of the zonal wind modes are examined for differences in forcing, wave propagation characteristics and stability.

The dominant upper level zonal wind fluctuation (EOF Z1) is associated with an expanded (contracted) region of easterlies in the tropical western Pacific and changes in the shape and intensity of the subtropical jets. The anomalous (difference between composites) eddy streamfunction at 200 mb shows an enhanced pair of anticyclones (cyclones) straddling the equator. These fluctuations are strongly coupled with eastward traveling tropical convection in the western Pacific with a time scale of about 40 days. The composite circulations show marked differences in the propagation of wave activity in the NH at 200 mb. The low phase (reduced easterlies) shows strong propagation away from the dominant source region over East Asia into the tropical western Pacific in conjunction with what appears to be significant reflection from the equatorward flank of the subtropical jet. In contrast, the high composite (enhanced easterlies) shows much weaker equatorward propagation together with reduced vertical propagation over East Asia and the western North Pacific.

The second zonal wind EOF (Z2) displays a more asymmetric structure with respect to the equator, describing a simultaneous decrease (increase) in the easterly extent of the East Asian jet and increase (decrease) in the strength of the jet over southern Australia. The anomalous eddy stream function at 200 mb shows wave trains apparently emanating from the tropical central Pacific extending into both hemispheres: in the winter hemisphere this resembles the Pacific-North American (PNA) pattern. These fluctuations show some coupling with preceding tropical convection anomalies in the western and central Pacific. Stability calculations show that the PNA pattern is maintained through barotropic energy exchanges with the mean flow. For the low composite, an enhanced source of stationary wave activity in the Gulf of Alaska is associated with an increase in synoptic-scale eddy activity.

These results suggest that tropical convection in the western Pacific has a strong modifying influence on (extratropically forced) middle latitude low-frequency variability. The influence is primarily indirect via zonal wind changes which influence the propagation of waves originating in middle latitudes. The zonal wind changes include those associated with the strength and extent of the tropical easterlies as well as more subtle (but important) changes which effect the curvature of the East Asian jet leading in some instances to turning points for middle latitude waves. The PNA also appears to have its main energy source in middle latitudes and in this case the link with the tropics appears to be more tied to phase locking with anomalies forced by tropical convection in the western and central Pacific.

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Siegfried D. Schubert, Max J. Suarez, and Jae-Kyung Schemm

Abstract

The relationship between predictability and persistence is examined using a realistic two-level general circulation model (GCM). Predictability is measured by the average divergence of ensembles of solutions starting from perturbed initial conditions. Persistence is defined in terms of the autocorrelation function based on a single long-term model integration.

The average skill of the dynamical forecasts is compared with the skill of simple persistence-based statistical forecasts. For initial errors comparable in magnitude to present-day analysis errors, the statistical forecast loses all skill after about one week, reflecting the lifetime of the lowest frequency fluctuations in the model. On the other hand, large ensemble mean dynamical forecasts would be expected to remain skillful for about three weeks. The disparity between the skill of the statistical and dynamical forecasts is greatest for the higher frequency modes, which have little memory beyond 1 day, yet remain predictable for about two weeks. For small ensembles, the error of the untempered dynamical forecasts must exceed that of the statistical forecasts for sufficiently long predictions. It is noteworthy, however, that for the low-frequency modes this is found to occur at a time when the GCM error is significantly less than would be obtained by forecasting climatology.

These results are analyzed in terms of two characteristic time scales. A dynamical time scale (Td) is defined as the limiting decay time of a pseudoanomaly correlation and is taken as the measure of predictability. This is compared to the usual statistical time scale (Ts), which is the integrated autocorrelation function and measures the typical time scale of fluctuations. For the dominant low-frequency (Ts ≥ 10 days) modes of fluctuation, the dynamical time scale is between two and three times the statistical time scale. For shorter time scales, the ratio Td/Ts is even greater, reaching six for the shortest time scales considered. This is in contrast to a class of first-order Markov processes with forced-dissipative dynamics for which the two time scales, as defined, are identical.

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Stefan Liess, Duane E. Waliser, and Siegfried D. Schubert

Abstract

Our ability to predict active and break periods of the Asian summer monsoon is intimately tied to our ability to predict the intraseasonal oscillation (ISO). The present study analyzes the upper limit of potential predictability of the northern summer ISO, as it is simulated by the ECHAM5 atmospheric general circulation model forced with climatological SSTs. The leading extended empirical orthogonal functions of precipitation, computed from a 10-yr control simulation, are used to define four different phases of the ISO. Fourteen-member ensembles of 90-day hindcasts are run for each phase of the three strongest ISO events identified in the 10-yr control run. Initial conditions for each ensemble are created from the control simulation using a breeding method.

The signal-to-noise ratio is analyzed over a region that covers the core of the Asian summer monsoon activity. Over Southeast Asia, the upper limit for predictability of precipitation and 200-hPa zonal wind is about 27 and 33 days, respectively. Over India, values of more than 15 days occur for both variables. A spatial analysis of the different phases of the ISO reveals that the predictability follows the eastward- and northward-propagating ISO during the active and break phases of the monsoon. Precipitation reveals increased predictability at the end of the convective phase. Analogous, 200-hPa zonal wind shows strongest predictability during low and easterly anomalies. This potential predictability is considerably higher than for numerical forecasts of typical weather variations, particularly for the Tropics, indicating that useful forecasts of monsoon active and break events may be possible with lead times of more than two weeks for precipitation and the dynamics. A closer look at the breeding method used here to initialize the hindcasts shows the importance of appropriate ensemble experiment designs.

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Chung-Kyu Park, Max J. Suarez, and Siegfried D. Schubert

Abstract

An atmospheric general circulation model is used to study the impact of idealized zonally propagating tropical heating anomalies on the low-frequency variability in the North Pacific region. The propagating heating is designed to mimic the thermal forcing associated with the Madden–Julian oscillation (MJO). Results are examined by separating the forced response from other variability and by comparing with runs employing fixed-phase (stationary) heating anomalies.

For both the forced and free circulations, the main modes of variability consist of a zonal expansion and retraction of the East Asian jet. The effective Rossby wave forcing associated with the heating is dominated by the advection term and located in the subtropics in the regions of strong absolute vorticity gradients.

Compared with cases using stationary forcing, the response to the propagating forcing is weaker and of different phase, indicating that the 40-day period used for the propagating anomalies is too short to allow the development of the steady-state response in the extratropics.

The model's total low-frequency variability in the North Pacific sector is dominated by the free oscillations that are the result of local processes uncorrelated with tropical variability. The relatively small forced response appear to be partly the result of the simplicity of the propagating heating anomaly that propagates at a constant phase speed and the simplification introduced into the GCM that do not allow transient feedback in the diabatic heating.

It is suggested that the lack of a significant Rossby wave stretching term in the subtropics is a distinguishing feature of the east–west dipole heating anomalies of the MJO and may contribute to the weakness of the response compared to interannual tropical heating anomalies.

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