Upward Wave Activity Research Articles

The western Pacific pattern precursor of major stratospheric sudden warmings and the ENSO modulation

Previous studies show that there is an apparent inconsistency between the impact of opposite phases of El Niño-Southern Oscillation (ENSO) on the seasonal-mean vortex and on major stratospheric sudden warmings (SSWs): El Niño (EN) leads to an anomalously warm, and La Niña (LN) leads to an anomalously cool seasonal-mean polar stratospheric state, but both phases of ENSO lead to an increased SSW frequency. This study examines the dynamics underlying this apparent paradox. First, this study demonstrates that 30 out of 35 SSWs occurred in 1957/1958–2012/2013 winters may be triggered by the western Pacific patterns (WPs). WPs can interact constructively with the climatological stationary waves and enhance wavenumber-1 waves and the associated upward wave activity fluxes, which leads to the occurrence of SSWs. This finding suggests that WPs may serve as a good precursor of SSWs. Furthermore, this study also demonstrates that WPs have larger amplitudes and longer durations during EN winters than ENSO neutral (NE) winters, which contributes dominantly to the increased frequency of SSWs from NE winters to EN winters. While for LN winters, both the ENSO modulation on WPs and the blockings over the eastern Pacific contribute to the SSW frequency increase from NE winters to LN winters. Finally, the question how EN modulates WPs is explored. It is found that more WPs with larger amplitude and longer duration develop from PNAs during EN than NE winters, which contributes to the relatively larger amplitude and longer duration of WPs observed in EN, compared to NE.

The Stratospheric Pathway of La Niña

Abstract A Northern Hemisphere (NH) polar stratospheric pathway for La Niña events is established during wintertime based on reanalysis data for the 1958–2012 period. A robust polar stratospheric response is observed in the NH during strong La Niña events, characterized by a significantly stronger and cooler polar vortex. Significant wind anomalies reach the surface, and a robust impact on the North Atlantic–European (NAE) region is observed. A dynamical analysis reveals that the stronger polar stratospheric winds during La Niña winters are due to reduced upward planetary wave activity into the stratosphere. This finding is the result of destructive interference between the climatological and the anomalous La Niña tropospheric stationary eddies over the Pacific–North American region. In addition, the lack of a robust stratospheric signature during La Niña winters reported in previous studies is investigated. It is found that this is related to the lower threshold used to detect the events, which signature is consequently more prone to be obscured by the influence of other sources of variability. In particular, the occurrence of stratospheric sudden warmings (SSWs), partly linked to the phase of the quasi-biennial oscillation, modulates the observed stratospheric signal. In the case of La Niña winters defined by a lower threshold, a robust stratospheric cooling is found only in the absence of SSWs. Therefore, these results highlight the importance of using a relatively restrictive threshold to define La Niña events in order to obtain a robust surface response in the NAE region through the stratosphere.

The Role of Standing Waves in Driving Persistent Anomalies of Upward Wave Activity Flux

Abstract Northern Hemisphere stratospheric polar vortex strength variability is known to be largely driven by persistent anomalies in upward wave activity flux. It has also been shown that attenuation and amplification of the stationary wave is the primary way in which wave activity flux varies. This study determines the structure of the wave anomalies that interfere with the climatological wave and drive this variability. Using a recently developed spectral decomposition it is shown that fixed-node standing waves are the primary drivers of the “linear interference” phenomenon. This is particularly true for the low-frequency component of the upward wave activity flux. The linear part of the flux is shown to be more persistent than the total flux and has significant tropospheric standing wave precursors that lead changes in the strength of the stratospheric polar vortex. Evidence is presented that current-generation high-top climate models are able to credibly simulate this variability in wave activity fluxes and the connection to polar vortex strength. Finally, the precursors to displacement and split sudden stratospheric warmings are examined. Displacement events are found to be preceded by about 25 days of anomalously high upward wave activity flux forced by standing waves amplifying the climatology. Split events have more short-lived wave activity flux precursors, which are dominated by the nonlinear part of the flux.

The Major Stratospheric Sudden Warming of January 2013: Analyses and Forecasts in the GEOS-5 Data Assimilation System

Abstract The major stratospheric sudden warming (SSW) of 6 January 2013 is examined using output from the NASA Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System version 5 (GEOS-5) near-real-time data assimilation system (DAS). GEOS-5 analyses showed that the SSW of January 2013 was a major warming by 1200 UTC 6 January, with a wave-2 vortex-splitting pattern. Upward wave activity flux from the upper troposphere (~23 December 2012) displaced the ~10-hPa polar vortex off the pole in a wave-1 pattern, enabling the poleward advection of subtropical values of Ertel potential vorticity (EPV) into a developing anticyclonic circulation region. While the polar vortex subsequently split (wave-2 pattern) the wave-2 forcing [upward Eliassen–Palm (EP) flux] was smaller than what was found in recent wave-2, SSW events, with most of the forcing located in the Pacific hemisphere. Investigation of a rapidly developing tropospheric weather system over the North Atlantic on 28–29 December 2012 showed strong transient upward wave activity flux from the storm with influences up to 10 hPa; however, the Pacific hemisphere wave forcing remained dominate at this time. Results from the GEOS-5 five-day forecasts showed that the forecasts accurately predicted the major SSW of January 2013. The overall success of the 5-day forecasts provides motivation to produce regular 10-day forecasts with GEOS-5, to better support studies of stratosphere–troposphere interaction.

Stratospheric Wave–Mean Flow Feedbacks and Sudden Stratospheric Warmings in a Simple Model Forced by Upward Wave Activity Flux

Abstract A classic result of studying stratospheric wave–mean flow interactions presented by Holton and Mass is that, for constant incoming wave forcing (at a notional tropopause), a vacillating stratospheric response may ensue. Simple models, such as the Holton–Mass model, typically prescribe the incoming wave forcing in terms of geopotential perturbation, which is not a proxy for upward wave activity flux. Here, the authors reformulate the Holton–Mass model such that incoming upward wave activity flux is prescribed. The Holton–Mass model contains a positive wave–mean flow feedback whereby wave forcing decelerates the mean flow, allowing enhanced wave propagation, which then further decelerates the mean flow, etc., until the mean flow no longer supports wave propagation. By specifying incoming wave activity flux, this feedback is constrained to the model interior. Bistability—where the zonal wind may exist at one of two distinct steady states for a given incoming wave forcing—is maintained in this reformulated model. The model is perturbed with transient pulses of upward wave activity flux to produce transitions between the two stable states. A minimum of integrated incoming wave activity flux necessary to force these sudden stratospheric warming–like transitions exists for pulses with time scales on the order of 10 days, arising from a wave time scale internal to the model at which forcing produces the strongest mean-flow response. The authors examine how the tropopause affects the internal feedback for this model setup and find that the tropopause inversion layer may potentially provide an important source of wave activity in the lower stratosphere.

Linking Siberian Snow Cover to Precursors of Stratospheric Variability

Abstract Previous research has linked wintertime Arctic Oscillation (AO) variability to indices of Siberian snow cover and upward wave activity flux in the preceding fall season. Here, daily data are used to examine the surface and tropospheric processes that occur as the link between snow cover and upward forcing into the stratosphere develops. October Eurasian mean snow cover is found to be significantly related to sea level pressure (SLP) and to lower-stratosphere (100 hPa) meridional heat flux. Analysis of daily SLP and 100-hPa heat flux shows that in years with high October snow, the SLP is significantly higher from approximately 1 November to 15 December, and the 100-hPa heat flux is significantly increased with a two-week lag, from approximately 15 November to 31 December. During November–December, there are periods with upward wave activity flux extending coherently from the surface to the stratosphere, and these events occur nearly twice as often in high snow years compared to low snow years. The vertical structure of these events is a westward-tilting pattern of high eddy heights, with the largest normalized anomalies near the surface in the same region as the snow and SLP changes. These results suggest that high SLP develops in response to the snow cover and this higher pressure, in turn, provides part of the structure of a surface-to-stratosphere wave activity flux event, thus making full events more likely. Implications for improved winter forecasts exist through recognition of these precursor signals.

October circulation precursors of the wintertime Arctic Oscillation

ABSTRACTInvestigation into atmospheric processes preceding winters of different Arctic Oscillation index (AOI) polarity, based on empirical data analysis, has revealed highly statistically significant relationships between the wintertime AOI and preceding October circulation. The wintertime AOI strongly covaries with an October circulation anomaly barotropically spanning the depth of the troposphere over the Taymyr Peninsula (Taymyr circulation anomaly, TCA), with the anticyclonic (cyclonic) TCA preceding winters of the negative (positive) AOI polarity. The October TCA affects the wintertime AOI polarity mainly via its impact on air temperature over the Arctic and North‐East Asia. Anticyclonic (cyclonic) TCA leads to the positive (negative) temperature anomaly over the Arctic and a corresponding increase (decrease) of geopotential heights, and to the negative (positive) temperature anomaly over North‐East Asia and so to enhancement (weakening) of the climatological trough associated with long planetary waves and corresponding enhancement (weakening) of the upward wave activity flux. To characterize temporal variability of the TCA, a Taymyr circulation index (TCI) is suggested. Correlation coefficient between the (inverted) wintertime AOI and the October TCI is 0.58 for the 1958–2012 period, with correlations being stable in time. The anticyclonic (cyclonic) TCA is associated with smaller (larger) number of cyclones coming to the region of the eastern Barents Sea–Taymyr Peninsula–Laptev Sea. Statistical relationships between the October TCA, wintertime AOI and September/October sea surface temperature in the northern Barents Sea are shown.

Connections between the Spring Breakup of the Southern Hemisphere Polar Vortex, Stationary Waves, and Air–Sea Roughness

Abstract A robust connection between the drag on surface-layer winds and the stratospheric circulation is demonstrated in NASA's Goddard Earth Observing System Chemistry–Climate Model (GEOSCCM). Specifically, an updated parameterization of roughness at the air–sea interface, in which surface roughness is increased for moderate wind speeds (4–20 m s−1), leads to a decrease in model biases in Southern Hemispheric ozone, polar cap temperature, stationary wave heat flux, and springtime vortex breakup. A dynamical mechanism is proposed whereby increased surface roughness leads to improved stationary waves. Increased surface roughness leads to anomalous eddy momentum flux convergence primarily in the Indian Ocean sector (where eddies are strongest climatologically) in September and October. The localization of the eddy momentum flux convergence anomaly in the Indian Ocean sector leads to a zonally asymmetric reduction in zonal wind and, by geostrophy, to a wavenumber-1 stationary wave pattern. This tropospheric stationary wave pattern leads to enhanced upward wave activity entering the stratosphere. The net effect is an improved Southern Hemisphere vortex: the vortex breaks up earlier in spring (i.e., the spring late-breakup bias is partially ameliorated) yet is no weaker in midwinter. More than half of the stratospheric biases appear to be related to the surface wind speed biases. As many other chemistry–climate models use a similar scheme for their surface-layer momentum exchange and have similar biases in the stratosphere, the authors expect that results from GEOSCCM may be relevant for other climate models.

The Role of Linear Interference in Northern Annular Mode Variability Associated with Eurasian Snow Cover Extent

AbstractOne of the outstanding questions regarding the observed relationship between October Eurasian snow cover anomalies and the boreal winter northern annular mode (NAM) is what causes the multiple-week lag between positive Eurasian snow cover anomalies in October and the associated peak in Rossby wave activity flux from the troposphere to the stratosphere in December. This study explores the following hypothesis about this lag: in order to achieve amplification of the wave activity, the vertically propagating Rossby wave train associated with the snow cover anomaly must reinforce the climatological stationary wave, which corresponds to constructive linear interference between the anomalous wave and the climatological wave. It is shown that the lag in peak wave activity flux arises because the Rossby wave train associated with the snow cover is in quadrature or out of phase with the climatological stationary wave from October to mid-November. Beginning in mid-November the associated wave anomaly migrates into a position that is in phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes until mid-January. Climate models from the Coupled Model Intercomparison Project 3 (CMIP3) do not capture this behavior. This linear interference effect is not only associated with stratospheric variability related to Eurasian snow cover anomalies but is a general feature of Northern Hemisphere troposphere–stratosphere interactions and, in particular, dominated the negative NAM events of the fall–winter of 2009/10.

Tropospheric forcing of the stratosphere: A comparative study of the two different major stratospheric warmings in 2009 and 2010

In January 2009 and 2010, two major stratospheric warmings (MSWs) took place in the boreal polar stratosphere. Both MSWs were preceded by nearly the strongest injection of tropospheric wave activity on record since 1958 and their central date was almost coincident. However, the typical external factors that influence the occurrence of MSWs (the Quasi-Biennial Oscillation, sunspot cycle, or El Nio) were dissimilar in the two midwinters: favorable in 2010 but unfavorable in 2009. In this study, the driving mechanisms of these two different MSWs were investigated focusing on the amplification of upward wave activity injection into the stratosphere before the MSW onset. By decomposing the total wave flux injection into contributions from the climatological planetary waves and from deviations from the latter we found clear differences in this amplification between both MSWs. The pre-MSW period in 2009 was characterized by a peak in the 100 hPa eddy heat flux with a predominance of wave number 2 activity. This was due to strong anomalies associated with Rossby wave packets originating from a deep ridge over the eastern Pacific. In contrast, the amplification of the upward wave propagation prior to the 2010 MSW was equally due to Rossby wave packets and to the interaction between the latter and the climatological waves. This amplification enhanced wave number 1 stationary waves in January 2010, which seemed at least partially due to the 2009/2010 El Nio event. Our results show the relevance of the internal tropospheric variability in generating MSWs, particularly when the external factors do not play any role. © 2011 by the American Geophysical Union.

Role of Eurasian snow cover in wintertime circulation: Decadal simulations forced with satellite observations

We investigate the impact of the Eurasian snow cover extent on the Northern Hemisphere winter circulation by performing a suite of ensemble simulations with the Météo‐France “Arpege Climat” atmospheric general circulation model, spanning 2 decades (1979–2000). Observed snow cover derived from satellite infrared and visible imagery has been forced weekly into the model. Variability in autumn‐early winter snow cover extent over eastern Eurasia is linked to circulation anomalies over the North Pacific that are influencing the North Atlantic sector in late winter through the development of the Aleutian‐Icelandic Low Seesaw teleconnection. The forcing of realistic snow cover in the model augments potential predictability over eastern Eurasia and the North Pacific and improves the hindcast skill score of the Aleutian‐Icelandic Low Seesaw teleconnection. Enhanced eastern Eurasia snow cover is associated with an anomalous upper‐tropospheric wave train across Eurasia, anomalously high upward wave activity flux, and a displaced stratospheric polar vortex.

Variability of the Northern Hemisphere polar stratospheric cloud potential: the role of North Pacific disturbances

AbstractThe potential of the Arctic stratosphere to sustain the formation of Polar Stratospheric Clouds (PSCs) is a key factor in determining the amount of ozone destroyed each winter, and is often measured as a ‘PSC volume’. The latter quantity has been shown to closely follow a near‐linear compact relationship with winter‐averaged column ozone loss, and displays a high variability from monthly to decadal time‐scales. We examine the connection between meteorological conditions in the troposphere and the variability of lower polar stratospheric temperatures over the last four decades, and specifically, conditions leading to a high PSC volume. In addition to the well‐established connection between the North Atlantic Oscillation (NAO) and the polar vortex, we demonstrate the large influence of precursory disturbances over the North Pacific and the Far East, the region of maximum climatological upward wave activity flux. Namely, very high monthly PSC volume (in the top 12%) predominantly follows the development of positive tropospheric height anomalies over the Far East, which lead to a weakening of the background planetary wave trough, and lessened upward wave activity flux into the stratosphere. Precursory anomalies over the Far East are reminiscent of East Asian monsoon amplification episodes. Copyright © 2009 Royal Meteorological Society

Correction to “Wave breaking along the stratospheric polar vortex as seen in ERA-40 data”

[1] In the paper ‘‘Wave breaking along the stratospheric polar vortex as seen in ERA-40 data’’ by John T. Abatzoglou and Gudrun Magnusdottir (Geophysical Research Letters, 34, L08812, doi:10.1029/2007GL029509, 2007), there were three textual errors. [2] From the abstract, the last sentence should read as follows, ‘‘Frequent wave breaking in the lower stratosphere in early winter appears to both shield the upper portion of the vortex from wave disturbances and reduce the net upward wave activity flux from the troposphere, thereby allowing the vortex to strengthen into mid-winter.’’ [3] The caption for Figure 3 should read as follows, ‘‘Figure 3. Composite zonal-mean zonal velocity u averaged over early winters (1 Nov–31 Dec.) with (a) multiple lower breaking events, and (c) multiple upper breaking events. The composite u averaged over mid-winters (21 Dec–10 Feb.) following early winters with (b) multiple lower and (d) multiple upper breaking events. Shading shows u values, the dotted contour indicates the zero wind line, and positive (negative) anomalies are shown by solid (dashed) contours with a contour interval of 3 ms .’’ [4] The caption for Figure 4 should read as follows, ‘‘Difference between the composite EP flux (averaged over 1 Nov.–31 Dec.) for early winters with multiple lower breaking events, and multiple upper breaking events (EPlower–EPupper). Maximum vector shown is of magnitude 1.2 10 kg s . The EP vectors are scaled by multiplying the horizontal component by 85.’’ GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L11805, doi:10.1029/2007GL030608, 2007 Click Here for Full Article

Wave breaking along the stratospheric polar vortex as seen in ERA‐40 data

Planetary wave breaking through the depth of the Northern Hemispheric stratosphere is observed in daily ERA‐40 reanalysis data. Strong wave breaking along the vortex edge is objectively diagnosed by noting the large‐scale overturning of potential vorticity contours on isentropic surfaces spanning the stratospheric vortex. Breaking events show distinctly different climatologies depending on whether they occur along the upper or lower portion of the stratospheric vortex. During early winter there is a strong negative correlation between the frequency of breaking events in these two regions. Frequent wave breaking in the lower stratosphere in early winter appears to both shield the upper portion of the vortex from wave disturbances and reduce the net upward wave activity flux into the troposphere, thereby allowing the vortex to strengthen into mid‐winter.

Influence of Early Winter Upward Wave Activity Flux on Midwinter Circulation in the Stratosphere and Troposphere

Abstract Using NCEP–NCAR reanalysis data the authors show that the November–December averaged stratospheric eddy heat flux is strongly anticorrelated with the January–February averaged eddy heat flux in the midlatitude stratosphere and troposphere. This finding further emphasizes differences between early and midwinter stratospheric wave flux behavior, which has recently been found in long-term variations. Analysis suggests that the intraseasonal anticorrelation of stratospheric heat fluxes results from changes in the upward wave propagation in the troposphere. Stronger (weaker) upward wave fluxes in early winter lead to weaker (stronger) upward wave fluxes from the troposphere during midwinter. Also, enhanced equatorward wave refraction during midwinter (due to the stronger polar night jet) is associated with weak heat flux in the early winter. It is suggested that the effect of enhanced midwinter upward wave flux from the troposphere in the years with weak early winter heat flux overcompensates the effect of increased equatorward wave refraction in midwinter, leading to a net increase of midwinter upward wave fluxes into the stratosphere.

Upward Wave Activity Flux as a Precursor to Extreme Stratospheric Events and Subsequent Anomalous Surface Weather Regimes

Abstract It has recently been shown that extreme stratospheric events (ESEs) are followed by surface weather anomalies (for up to 60 days), suggesting that stratospheric variability might be used to extend weather prediction beyond current time scales. In this paper, attention is drawn away from the stratosphere to demonstrate that the originating point of ESEs is located in the troposphere. First, it is shown that anomalously strong eddy heat fluxes at 100 hPa nearly always precede weak vortex events, and conversely, anomalously weak eddy heat fluxes precede strong vortex events, consistent with wave–mean flow interaction theory. This finding clarifies the dynamical nature of ESEs and suggests that a major source of stratospheric variability (and thus predictability) is located in the troposphere below and not in the stratosphere itself. Second, it is shown that the daily time series of eddy heat flux found at 100 hPa and integrated over the prior 40 days, exhibit a remarkably high anticorrelation (−0.8)...

Relative impacts of Siberian and North American snow anomalies on the winter Arctic Oscillation

Numerical model mean climatic response to realistic land surface snow forcings is evaluated for two different forcing regions, Siberia and North America. The atmospheric teleconnection pathway and negative winter AO mode response produced by the Siberia forcing, described by the authors in previous studies, is not produced by the comparable‐extent North America forcing. It is shown that the combination of a large snow‐forced local diabatic heating anomaly over a region of substantial stationary wave activity is required to produce strong upward wave activity flux anomalies which initiate the teleconnection pathway. These features are unique to Siberia, making it a critical region for reproducing the snow ‐ winter AO statistical relationship evident in the observational record.

An Amplification Mechanism of Medium-Scale Tropopausal Waves

Abstract The mechanism for the amplification of medium-scale tropopausal waves (with horizontal wavelengths of 2000–3000 km and wave periods of 1–2 days) that are basically neutral is investigated using a three-dimensional wave-activity flux. Distinctive upward wave-activity fluxes are found in the midtroposphere (400 hPa) upstream of the region where the medium-scale waves are dominant, that is, where the horizontal components of the wave-activity flux are large. These upward fluxes converge in the upper troposphere (300 hPa). This convergence is a major source of medium-scale waves, producing large values of the horizontal components of the wave-activity flux in the active regions. Examination of the spatial structure of the heat flux shows that the upward fluxes are due to occasional baroclinic interactions between upper-level medium-scale waves and lower-tropospheric disturbances with the same temporal scales. The seasonal and geophysical variations of the waves depend mostly on the amount of baroclin...

GCM Simulations of the Three-Dimensional Propagation of Stationary Waves

Abstract Plumb's formulation of the stationary wave activity flux is used to determine how well versions of the GFDL and NCAR general circulation models simulate the sources, sinks, and horizontal propagation of atmospheric stationary waves, which play an important role in determining regional climate. The wave activity flux provides insight into the simulation of nondynamic as well as dynamic processes in these models. Model performances for current climate simulations are evaluated with respect to NMC analyses averaged over 1978–1990. The models fare best when the stationary wave forcing is strongest, that is, in the wintertime Northern Hemisphere, where they reproduce the observed three-branch structure of upward wave activity flux. For the Northern Hemisphere summer and the Southern Hemisphere in both summer and winter, the models show less agreement with observations, although they do simulate the generally downward flux observed during Northern Hemisphere summer, which the analysis suggests is cause...