Blocking Statistics in a Varying Climate: Lessons from a “Traffic Jam” Model with Pseudostochastic Forcing

Abstract Recently Nakamura and Huang proposed a semiempirical, one-dimensional model of atmospheric blocking based on the observed budget of local wave activity in the boreal winter. The model dynamics is akin to that of traffic flow, wherein blocking manifests as traffic jams when the streamwise flux of local wave activity reaches capacity. Stationary waves modulate the jet stream’s capacity to transmit transient waves and thereby localize block formation. Since the model is inexpensive to run numerically, it is suited for computing blocking statistics as a function of climate variables from large-ensemble, parameter sweep experiments. We explore sensitivity of blocking statistics to (i) stationary wave amplitude, (ii) background jet speed, and (iii) transient eddy forcing, using frequency, persistence, and prevalence as metrics. For each combination of parameters we perform 240 runs of 180-day simulations with aperiodic transient eddy forcing, each time randomizing the phase relations in forcing. The model climate shifts rapidly from a block-free state to a block-dominant state as the stationary wave amplitude is increased and/or the jet speed is decreased. When eddy forcing is increased, prevalence increases similarly but frequency decreases as blocks merge and become more persistent. It is argued that the present-day climate lies close to the boundary of the two states and hence its blocking statistics are sensitive to climate perturbations. The result underscores the low confidence in GCM-based assessment of the future trend of blocking under a changing climate, while it also provides a theoretical basis for evaluating model biases and understanding trends in reanalysis data.

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Intraseasonal modulation of Sudden Stratospheric Warmings by ENSO

10.5194/ems2021-117 ◽

2021 ◽

Author(s):

Froila M. Palmeiro ◽

Javier García-Serrano ◽

Paolo Ruggieri ◽

Lauriane Batté ◽

Silvio Gualdi

Keyword(s):

Seasonal Cycle ◽

State Of The Art ◽

Southern Oscillation ◽

Polar Vortex ◽

Stationary Wave ◽

Wave Pattern ◽

Reanalysis Data ◽

Wave Activity ◽

Neutral Conditions ◽

Different Sources

Using the complete ERA-Interim reanalysis and three state-of-the-art models, this study explores how El Ni&#241;o-Southern Oscillation (ENSO) can influence the frequency and seasonal cycle of sudden stratospheric warmings (SSWs) by modulating the background upward wave propagation. Reanalysis data shows that in the last four decades, winters with SSWs were significantly more common than inactive winters, particularly under La Ni&#241;a (LN) conditions but not during El Ni&#241;o (EN), regardless of the ENSO/SSW definitions considered herein. In agreement with previous studies, our models tend to simulate a linear ENSO-SSW relationship, with more SSWs for EN, that show a peak of occurrence around January &#8212; as occurs in reanalysis &#8212;, and less SSWs for LN when compared to neutral conditions. Independently of ENSO, the main tropospheric precursor of SSWs appears to be an anomalous wave-like pattern over Eurasia, but it is dominated by wavenumber 1 (WN1) for EN and shows an enhanced wavenumber 2 (WN2) for LN. The differences in this Eurasian wave pattern, which is largely internally generated (ENSO-unforced), emerge from the distinct configuration of the background, stationary wave pattern induced by ENSO, favouring a relative stronger WN1 (WN2) component during EN (LN). Our results suggest that the ENSO-forced signal relies on preconditioning the seasonal-mean polar vortex, becoming weaker and displaced (stronger and more stable) for EN (LN), while ENSO-unforced wave activity represents the ultimate trigger of SSWs. This supports the view that ENSO and SSWs are different sources of variability of the winter atmospheric circulation and may reconcile previous findings in this context.

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The Building Blocks of Northern Hemisphere Wintertime Stationary Waves

Journal of Climate ◽

10.1175/jcli-d-19-0181.1 ◽

2020 ◽

Vol 33 (13) ◽

pp. 5611-5633 ◽

Cited By ~ 4

Author(s):

Chaim I. Garfinkel ◽

Ian White ◽

Edwin P. Gerber ◽

Martin Jucker ◽

Moran Erez

Keyword(s):

Temperature Field ◽

Northern Hemisphere ◽

General Circulation ◽

Circulation Model ◽

Building Blocks ◽

Stationary Wave ◽

Heat Fluxes ◽

Boreal Winter ◽

Stationary Waves ◽

Western Asia

AbstractAn intermediate-complexity moist general circulation model is used to investigate the forcing of stationary waves in the Northern Hemisphere boreal winter by land–sea contrast, horizontal heat fluxes in the ocean, and topography. The additivity of the response to these building blocks is investigated. In the Pacific sector, the stationary wave pattern is not simply the linear additive sum of the response to each forcing. In fact, over the northeast Pacific and western North America, the sum of the responses to each forcing is actually opposite to that when all three are imposed simultaneously due to nonlinear interactions among the forcings. The source of the nonlinearity is diagnosed using the zonally anomalous steady-state thermodynamic balance, and it is shown that the background-state temperature field set up by each forcing dictates the stationary wave response to the other forcings. As all three forcings considered here strongly impact the temperature field and its zonal gradients, the nonlinearity and nonadditivity in our experiments can be explained, but only in a diagnostic sense. This nonadditivity extends up to the stratosphere, and also to surface temperature, where the sum of the responses to each forcing differs from the response if all forcings are included simultaneously. Only over western Eurasia is additivity a reasonable (though not perfect) assumption; in this sector land–sea contrast is most important over Europe, while topography is most important over western Asia. In other regions, where nonadditivity is pronounced, the question of which forcing is most important is ill-posed.

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Dynamics of an Interhemispheric Teleconnection across the Critical Latitude through a Southerly Duct during Boreal Winter*

Journal of Climate ◽

10.1175/jcli-d-14-00425.1 ◽

2015 ◽

Vol 28 (19) ◽

pp. 7437-7456 ◽

Cited By ~ 29

Author(s):

Sen Zhao ◽

Jianping Li ◽

Yanjie Li

Keyword(s):

Indian Ocean ◽

South Asia ◽

Southern Africa ◽

Western Indian Ocean ◽

Wave Theory ◽

Stationary Wave ◽

Boreal Winter ◽

Stationary Waves ◽

The North ◽

Critical Latitude

Abstract In this study, an interhemispheric teleconnection pattern across the critical latitude from southern Africa through South Asia to the North Pacific was revealed in boreal winter monthly averaged 250-hPa streamfunction fields obtained from both the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP–NCAR reanalysis data from 1957/58 to 2001/02. Classical Rossby wave theory for zonally varying flow in which the effects of the basic-state meridional wind are ignored predicts that stationary Rossby waves cannot propagate across easterlies. To elucidate the underlying mechanisms responsible for this interhemispheric teleconnection, the theoretical basis for stationary wave propagation across the critical latitude is considered, taking into account meridional ambient flow. The theoretical results suggest that the southerly flow over East Africa, the western Indian Ocean, and South Asia creates a path for the northward propagation of stationary waves across the critical latitude. Stationary wavenumber and group velocity analysis, ray tracing, and simple model experiments applied to nearly realistic boreal winter mean flows confirm that disturbances excited in southern Africa and the western Indian Ocean can propagate across the critical latitude to South Asia through the southerly duct and then continue downstream along the North African–Asian subtropical jet.

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Local Finite-Amplitude Wave Activity as a Diagnostic of Anomalous Weather Events

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0194.1 ◽

2015 ◽

Vol 73 (1) ◽

pp. 211-229 ◽

Cited By ~ 33

Author(s):

Clare S. Y. Huang ◽

Noboru Nakamura

Keyword(s):

Potential Vorticity ◽

Regional Scale ◽

High Amplitude ◽

Finite Amplitude ◽

Reanalysis Data ◽

Wave Activity ◽

Mean Flow ◽

Local Wave ◽

Highly Correlated ◽

Barotropic Vorticity

Abstract Finite-amplitude Rossby wave activity (FAWA) proposed by Nakamura and Zhu measures the waviness of quasigeostrophic potential vorticity (PV) contours and the associated modification of the zonal-mean zonal circulation, but it does not distinguish longitudinally localized weather anomalies, such as atmospheric blocking. In this article, FAWA is generalized to local wave activity (LWA) to diagnose eddy–mean flow interaction on the regional scale. LWA quantifies longitude-by-longitude contributions to FAWA following the meridional displacement of PV from the circle of equivalent latitude. The zonal average of LWA recovers FAWA. The budget of LWA is governed by the zonal advection of LWA and the radiation stress of Rossby waves. The utility of the diagnostic is tested with a barotropic vorticity equation on a sphere and meteorological reanalysis data. Compared with the previously derived Eulerian impulse-Casimir wave activity, LWA tends to be less filamentary and emphasizes large isolated vortices involving reversals of meridional gradient of potential vorticity. A pronounced Northern Hemisphere blocking episode in late October 2012 is well captured by a high-amplitude, near-stationary LWA. These analyses reveal that the nonacceleration relation holds approximately over regional scales: the growth of phase-averaged LWA and the deceleration of local zonal wind are highly correlated. However, marked departure from the exact nonacceleration relation is also observed during the analyzed blocking event, suggesting that the contributions from nonadiabatic processes to the blocking development are significant.

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The Interannual Variability of the Tropical Divergence Tilt and Its Connection with the Extratropical Circulation

Journal of Climate ◽

10.1175/jcli-d-20-0373.1 ◽

2021 ◽

Vol 34 (1) ◽

pp. 259-275

Author(s):

Pablo Zurita-Gotor

Keyword(s):

Interannual Variability ◽

Momentum Flux ◽

Large Scale ◽

Theoretical Work ◽

Stationary Wave ◽

Reanalysis Data ◽

Boreal Winter ◽

The Pacific ◽

Divergence Field ◽

Two Parameters

AbstractPrevious theoretical work has suggested that the strength of the divergent eddy momentum flux in the deep tropics, due to correlations between rotational zonal velocities and divergent meridional velocities, increases with the meridional tilt of the large-scale divergence field. To test that idea, this work investigates the interannual variability of the divergent eddy momentum flux in reanalysis data. Consistent with the theory, it is found that the eddy momentum flux variability is driven by two main parameters: the amplitude of the tropical stationary wave and the tilt of the divergence field. Together, these two parameters account for 80% (90%) of the interannual eddy momentum flux variance during boreal (austral) winter. The interannual variability of these parameters is governed by the internal atmospheric dynamics. During boreal winter, interannual changes in MJO variability explain nearly half of the interannual variance in the stationary wave amplitude, depending on whether on average MJO anomalies interfere constructively or destructively with the stationary wave. The interannual variability of the divergence phase tilt is modulated by tropical–extratropical interactions in the Pacific. The tilt increases during the negative phase of the west Pacific Oscillation associated with a dipole of upper-level divergence (convergence) on the northern (southern) side of the Pacific jet exit region.

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What controls probability distribution of local wave activity in the midlatitudes?

10.5194/egusphere-egu21-2958 ◽

2021 ◽

Author(s):

Noboru Nakamura ◽

Claire Valva

Keyword(s):

Probability Distributions ◽

Wave Activity ◽

Skewed Distribution ◽

Vertical Shear ◽

Wave Flow ◽

Transient Eddy ◽

Layer Model ◽

Speed Increase ◽

Local Wave ◽

Two Layer Model

We examine probability distributions of local wave activity (LWA), a measure of the jet stream's meander, and factors that control them. &#160;The observed column-mean LWA distributions exhibit significant seasonal, interhemispheric, and regional variations but are always positively skewed in the extratropics, and their tail often involves disruptions of the jet stream. &#160;A previously derived 1D traffic flow model driven by observed spectra of transient eddy forcing qualitatively reproduces the shape of the observed LWA distribution. &#160;It is shown that the skewed distribution emerges from nonlinearity in the zonal advection of LWA even though the eddy forcing is symmetrically distributed. &#160;A slower jet and stronger transient and stationary eddy forcings, when introduced independently, all broaden the LWA distribution and increase the probability of spontaneous jet disruption. &#160;Quasigeostrophic two-layer model also simulates skewed LWA distributions in the upper layer. &#160;However, in the two-layer model both transient eddy forcing and the jet speed increase with an increasing shear (meridional temperature gradient), and their opposing influence leaves the frequency of jet disruptions insensitive to the vertical shear. &#160;When the model's nonlinearity in the zonal flux of potential vorticity is artificially suppressed, it hinders wave-flow interaction and virtually eliminates reversal of the upper-layer zonal wind. &#160;The study underscores the importance of nonlinearity in the zonal transmission of Rossby waves to the frequency of jet disruptions and associated weather anomalies.&#160;

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Asymmetric impact of the Scandinavian pattern on stratospheric circulation anomalies

Journal of Climate ◽

10.1175/jcli-d-21-0331.1 ◽

2021 ◽

pp. 1-43

Author(s):

Bo Pang ◽

Adam A. Scaife ◽

Riyu Lu ◽

Rongcai Ren

Keyword(s):

Planetary Wave ◽

Wave Activity ◽

Boreal Winter ◽

Destructive Interference ◽

Stationary Waves ◽

Stratospheric Warming ◽

Sudden Stratospheric Warming ◽

Wave Interference ◽

Asymmetric Impact ◽

Stratospheric Circulation

AbstractThis study investigates the stratosphere-troposphere coupling associated with the Scandinavian (SCA) pattern in boreal winter. The results indicate that the SCA impacts stratospheric circulation but that its positive and negative phases have different effects. The positive phase of the SCA (SCA+) pattern is restricted to the troposphere, but the negative phase (SCA−) extends to the upper stratosphere. The asymmetry between phases is also visible in the lead-lag evolution of the stratosphere and troposphere. Prominent stratospheric anomalies are found to be intensified following SCA+ events, but prior to SCA− events. Further analysis reveals that the responses are associated with upward propagation of planetary waves, especially wavenumber 1 which is asymmetric between SCA phases. The wave amplitudes in the stratosphere, originating from the troposphere, are enhanced after the SCA+ events and before the SCA− events. Furthermore, the anomalous planetary wave activity can be understood through its interference with climatological stationary waves. Constructive wave interference is accompanied by clear upward propagation in the SCA+ events, while destructive interference suppresses stratospheric waves in the SCA− events. Our results also reveal that the SCA+ events are more likely to be followed by sudden stratospheric warming (SSW) events, because of the deceleration of stratospheric westerlies following the SCA+ events.

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Planetary wave activity in the polar lower stratosphere

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-707-2010 ◽

2010 ◽

Vol 10 (2) ◽

pp. 707-718 ◽

Cited By ~ 26

Author(s):

S. P. Alexander ◽

M. G. Shepherd

Keyword(s):

Planetary Wave ◽

Travelling Waves ◽

Stationary Wave ◽

Wave Activity ◽

The Arctic ◽

Small Scale ◽

Stationary Waves ◽

Significant Activity ◽

Zonal Wavenumber ◽

The Times

Abstract. Temperature data from the COSMIC GPS-RO satellite constellation are used to study the distribution and variability of planetary wave activity in the low to mid- stratosphere (15–40 km) of the Arctic and Antarctic from September 2006 until March 2009. Stationary waves are separated from travelling waves and their amplitudes, periods and small-scale vertical distribution then examined. COSMIC observed short lived (less than two weeks and less than 5 km vertically) but large enhancements in planetary wave amplitudes occurring regularly throughout all winters in both hemispheres. In contrast to recent Arctic winters, eastward wave activity during 2008–2009 was significantly reduced during the early part of the winter and immediately prior to the major SSW. The eastward waves which did exist had similar periods to the two preceding winters (~16–20 days). A westward wave with zonal wavenumber two, with distinct peaks at 22 km and 35 km and period around 16–24 days, as well as a stationary wave two were associated with the 2009 major SSW. In the Southern Hemisphere, the height structure of planetary wave amplitudes also exhibited fluctuations on short time and vertical scales superimposed upon the broader seasonal cycle. Significant inter-annual variability in planetary wave amplitude and period are noticed, with the times of cessation of significant activity also varying.

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Seasonal prediction of the boreal winter stratosphere

Climate Dynamics ◽

10.1007/s00382-021-05787-9 ◽

2021 ◽

Author(s):

Alice Portal ◽

Paolo Ruggieri ◽

Froila M. Palmeiro ◽

Javier García-Serrano ◽

Daniela I. V. Domeisen ◽

...

Keyword(s):

Southern Oscillation ◽

Polar Vortex ◽

Seasonal Prediction ◽

Wave Activity ◽

Boreal Winter ◽

Quasi Biennial Oscillation ◽

Stratospheric Polar Vortex ◽

Eddy Heat Flux ◽

Prediction Systems ◽

Model Database

AbstractThe predictability of the Northern Hemisphere stratosphere and its underlying dynamics are investigated in five state-of-the-art seasonal prediction systems from the Copernicus Climate Change Service (C3S) multi-model database. Special attention is devoted to the connection between the stratospheric polar vortex (SPV) and lower-stratosphere wave activity (LSWA). We find that in winter (December to February) dynamical forecasts initialised on the first of November are considerably more skilful than empirical forecasts based on October anomalies. Moreover, the coupling of the SPV with mid-latitude LSWA (i.e., meridional eddy heat flux) is generally well reproduced by the forecast systems, allowing for the identification of a robust link between the predictability of wave activity above the tropopause and the SPV skill. Our results highlight the importance of November-to-February LSWA, in particular in the Eurasian sector, for forecasts of the winter stratosphere. Finally, the role of potential sources of seasonal stratospheric predictability is considered: we find that the C3S multi-model overestimates the stratospheric response to El Niño–Southern Oscillation (ENSO) and underestimates the influence of the Quasi–Biennial Oscillation (QBO).

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Dynamical Processes Related to the Appearance of Quasi-Stationary Waves on the Subtropical Jet in the Midsummer Northern Hemisphere

Journal of Climate ◽

10.1175/jcli3697.1 ◽

2006 ◽

Vol 19 (8) ◽

pp. 1531-1544 ◽

Cited By ~ 66

Author(s):

Naoki Sato ◽

Masaaki Takahashi

Keyword(s):

Middle East ◽

Northern Hemisphere ◽

Wave Train ◽

Stationary Wave ◽

Stationary Waves ◽

Statistical Features ◽

Basic Field ◽

Subtropical Jet ◽

Anomaly Pattern ◽

Highly Correlated

Abstract Statistical features of quasi-stationary planetary waves were examined on the subtropical jet in the midsummer Northern Hemisphere by using objectively analyzed data and satellite data. As a result, a quasi-stationary wave train that is highly correlated with the midsummer climate over Japan was identified. A clear phase dependency of the appearance of waves was also confirmed. An analysis of temporal evolution and wave activity flux revealed that the eastward propagation of the wave packet starts in the Middle East, passes over East Asia, and reaches North America. The anomaly pattern is strengthened through kinetic energy conversion near the entrance of the Asian jet over the Middle East. The interaction between the anomaly pattern and the basic field contributes to the appearance of the anomalous wavelike pattern. Although the wave train is correlated with the anomaly of convective activity over the western North Pacific and the Indian Ocean, it is implied that internal dynamics are important in determining the statistical features of the appearance of anomalous quasi-stationary waves on the subtropical jet.

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