scholarly journals Circumglobal Rossby wave patterns during boreal winter highlighted by wavenumber/phase speed spectral analysis

2021 ◽  
Author(s):  
Jacopo Riboldi ◽  
Efi Rousi ◽  
Fabio D'Andrea ◽  
Gwendal Rivière ◽  
François Lott
Keyword(s):  
Rossby Wave ◽  
Rossby Waves ◽  
Spectral Power ◽  
Storm Track ◽  
Phase Speed ◽  
Principal Component ◽  
Boreal Winter ◽  
The Pacific ◽  
Wave Patterns ◽  
The Indian Ocean

Abstract. The classic partitioning between slow-moving, low-wavenumber planetary waves and fast-moving, high-wavenumber synoptic waves is systematically extended by means of a wavenumber/phase speed spectral decomposition to characterize the day-to-day evolution of Rossby wave activity in the upper troposphere. This technique is employed to study the origin and the propagation of circumglobal Rossby wave patterns (CRWPs), amplified Rossby waves stretching across the Northern Hemisphere in the zonal direction and characterized by few, dominant wavenumbers. Principal component analysis of daily anomalies in spectral power allows for two CRWPs to emerge as dominant variability modes in the spectral domain during boreal winter. These modes correspond to the baroclinic propagation of amplified Rossby waves from the Pacific to the Atlantic storm track in a hemispheric flow configuration displaying enhanced meridional gradients of geopotential height over midlatitudes. The first CRWP is forced by tropical convection anomalies over the Indian Ocean and features the propagation of amplified Rossby wave packets over northern midlatitudes, while the second one propagates rapidly over latitudes between 35° N and 55° N and appears to have extratropical origin. Propagation of Rossby waves from the Atlantic eddy-driven jet to the African subtropical jet occurs for both CRWPs following anticyclonic wave breaking.

The role of the Pacific Decadal Oscillation and ocean-atmosphere interactions in driving United States heatwaves

2021 ◽  
Author(s):  
Sem Vijverberg ◽  
Dim Coumou
Keyword(s):  
Rossby Wave ◽  
Rossby Waves ◽  
Low Frequency ◽  
Causal Link ◽  
Causal Mechanism ◽  
Temperature Forecast ◽  
The Pacific ◽  
Ocean Atmosphere ◽  
The Waves

<p>Heatwaves can have devastating impact on society and reliable early warnings at several weeks lead time are needed. Heatwaves are often associated with quasi-stationary Rossby waves, which interact with sea surface temperature (SST). Previous studies showed that north-Pacific SST can provide long-lead predictability for eastern U.S. temperature, moderated by an atmospheric Rossby wave. The exact mechanisms, however, are not well understood. Here we analyze Rossby waves associated with heatwaves in western and eastern US. Causal inference analyses reveal that both waves are characterized by positive ocean-atmosphere feedbacks at synoptic timescales, amplifying the waves. However, this positive feedback on short timescales is not the causal mechanism that leads to a long-lead SST signal. Only the eastern US shows a long-lead causal link from SSTs to the Rossby wave. We show that the long-lead SST signal derives from low-frequency PDO variability, providing the source of eastern US temperature predictability. We use this improved physical understanding to identify more reliable long-lead predictions. When, at the onset of summer, the Pacific is in a pronounced PDO phase, the SST signal is expected to persist throughout summer. These summers are characterized by a stronger ocean-boundary forcing, thereby more than doubling the eastern US temperature forecast skill, providing a temporary window of enhanced predictability.</p>

scholarly journals Diversity of the Global Teleconnections Associated with the Madden–Julian Oscillation

2021 ◽  
Vol 34 (1) ◽  
pp. 397-414
Author(s):  
Guosen Chen
Keyword(s):  
Rossby Wave ◽  
Potential Vorticity ◽  
Wave Train ◽  
Boreal Winter ◽  
Madden Julian Oscillation ◽  
The Pacific ◽  
Weather And Climate ◽  
Baroclinic Model ◽  
Rossby Wave Train ◽  
Upper Level

AbstractA recent study has revealed that the Madden–Julian oscillation (MJO) during boreal winter exhibits diverse propagation patterns that consist of four archetypes: standing MJO, jumping MJO, slow eastward propagating MJO, and fast eastward propagating MJO. This study has explored the diversity of teleconnection associated with these four MJO groups. The results reveal that each MJO group corresponds to distinct global teleconnections, manifested as diverse upper-tropospheric Rossby wave train patterns. Overall, the teleconnections in the fast and slow MJO are similar to those in the canonical MJO constructed by the real-time multivariate MJO (RMM) indices, while the teleconnections in the jumping and standing MJO generally lose similarities to those in the canonical MJO. The causes of this diversity are investigated using a linearized potential vorticity equation. The various MJO tropical heating patterns in different MJO groups are the main cause of the diverse MJO teleconnections, as they induce assorted upper-level divergent flows that act as Rossby-wave sources through advecting the background potential vorticity. The variation of the Asian jet could affect the teleconnections over the Pacific jet exit region, but it plays an insignificant role in causing the diversity of global teleconnections. The numerical investigation with a linear baroclinic model shows that the teleconnections can be interpreted as linear responses to the MJO’s diabatic heating to various degrees for different MJO groups, with the fast and slow MJO having higher linear skill than the jumping and standing MJO. The results have broad implications in the MJO’s tropical–extratropical interactions and the associated impacts on global weather and climate.

Variations of Northern Hemisphere Storm Track and Extratropical Cyclone Activity Associated with the Madden–Julian Oscillation

2017 ◽  
Vol 30 (13) ◽  
pp. 4799-4818 ◽  
Author(s):  
Yanjuan Guo ◽  
Toshiaki Shinoda ◽  
Jialin Lin ◽  
Edmund K. M. Chang
Keyword(s):  
Storm Track ◽  
Meridional Wind ◽  
Reanalysis Data ◽  
Boreal Winter ◽  
Madden Julian Oscillation ◽  
Mean Flow ◽  
Cyclone Activity ◽  
Intraseasonal Variations ◽  
The Pacific ◽  
Cyclone Frequency

This study investigates the intraseasonal variations of the Northern Hemispheric storm track associated with the Madden–Julian oscillation (MJO) during the extended boreal winter (November–April) using 36 yr (1979–2014) of reanalysis data from ERA-Interim. Two methods have been used to diagnose storm-track variations. In the first method, the storm track is quantified by the temporal-filtered variance of 250-hPa meridional wind (vv250) or mean sea level pressure (pp). The intraseasonal anomalies of vv250 composited for eight MJO phases are characterized by a zonal band of strong positive (or negative) anomalies meandering from the Pacific all the way across North America and the Atlantic into northern Europe, with weaker anomalies of opposite sign at one or both flanks. The results based on pp are consistent with those based on vv250 except for larger zonal variations, which may be induced by surface topography. In the second method, an objective cyclone-tracking scheme has been used to track the extratropical cyclones that compose the storm track. The MJO-composite anomalies of the “accumulated” cyclone activity, a quantity that includes contributions from both the cyclone frequency and cyclone mean intensity, are very similar to those based on pp. Further analysis demonstrates that major contribution comes from variations in the cyclone frequency. Further analysis suggests that the intraseasonal variations of the storm track can be primarily attributed to the variations of the mean flow that responds to the anomalous MJO convections in the tropics, with possible contribution also from the moisture variations.

Finding Dynamical Modes of Atmospheric Variability Using Conservation Properties

2020 ◽  
Author(s):  
Dominic Jones ◽  
John Methven ◽  
Tom Frame ◽  
Paul Berrisford
Keyword(s):  
Large Scale ◽  
Potential Temperature ◽  
Normal Modes ◽  
Phase Speed ◽  
Principal Component ◽  
Wave Activity ◽  
Seasonal Effects ◽  
Monthly Precipitation ◽  
Wave Patterns ◽  
Background State

<p>It is evident that persistent large-scale weather phenomena are an important factor in extreme seasonal climate; this has been especially true in boreal summers over the last two decades. Large, relatively slowly changing modes of variability on the mid-latitude jet are key to understanding high impact weather events. High monthly precipitation totals in the summer, for example, are linked to stationary Rossby wave patterns; stationary winter jet patterns can direct North Atlantic cyclones towards the UK and Europe. These wave patterns are often diagnosed but without a link to their phase speeds or dynamics.</p><p>To examine these slow modes we define an atmospheric background state as a function of isentropic and materially conserved co-ordinates (potential temperature and PV), resulting in a slowly changing, zonally symmetric background state. We then extract patterns of variability from the set of perturbations by employing an alternative Empirical Orthogonal Function (EOF) technique which utilizes a conserved wave activity as a weighted covariance. This results in statistical (EOF) patterns which possess an intrinsic dynamical phase speed and frequency, which are predicted from the conservation properties pseudomomentum and pseudoenergy. These statistical modes are a recombination of the dynamical normal modes in a system with quasi-linear dynamics.</p><p>We examine long runs with relaxation to unstable background jets but without orography, diurnal or seasonal effects, where large amplitude wave activity emerges. These simplified situations are used to test whether or not the predicted phase speeds from theory (given the structures found) matches with the observed phase speeds deduced from the principal component time series of the ENMs. Our hypothesis is that slow wave motion is explained by the structure and conservation properties of the modes. We are able to explore the dependence on the structures by varying the background state.</p>

scholarly journals The Connection between the Southern Annular Mode and a Feature-Based Perspective on Southern Hemisphere Midlatitude Winter Variability

2020 ◽  
Vol 33 (1) ◽  
pp. 115-129 ◽  
Author(s):  
Clemens Spensberger ◽  
Michael J. Reeder ◽  
Thomas Spengler ◽  
Matthew Patterson
Keyword(s):  
Storm Track ◽  
Principal Component ◽  
Southern Annular Mode ◽  
Strong Correlations ◽  
Sea Level Pressure ◽  
Annular Mode ◽  
Rossby Wave Breaking ◽  
The Pacific ◽  
Feature Based ◽  
Consistent Picture

AbstractThis article provides a reconciling perspective on the two main, but contradictory, interpretations of the southern annular mode (SAM). SAM was originally thought to characterize meridional shifts in the storm track across the entire hemisphere. This perspective was later questioned, and SAM was interpreted as a statistical artifact depending on the choice of base region for the principal component analysis. Neither perspective, however, fully describes SAM. We show that SAM cannot be interpreted in terms of midlatitude variability, as SAM merely modulates the most poleward part of the cyclone tracks and only marginally influences the distribution of other weather-related features of the storm track (e.g., position of jet axes and Rossby wave breaking). Instead, SAM emerges as the leading pattern of geopotential variability due to strong correlations of sea level pressure around the Antarctic continent. As SAM correlates strongly both with the pan-Antarctic mean temperature and the meridional heat flux through 65°S, we hypothesize that SAM can be interpreted as a measure of the degree of the (de)coupling between Antarctica and the southern midlatitudes. As an alternative way of characterizing southern midlatitude variability, we seek domains in which the leading EOF patterns of both the geopotential and storm-track features yield a dynamically consistent picture. This approach is successful for the South Pacific. Here the leading variability patterns are closely related to the Pacific–South America pattern and point toward an NAO-like variability.

scholarly journals Influence of the Tropics on the Southern Annular Mode

2012 ◽  
Vol 25 (18) ◽  
pp. 6330-6348 ◽  
Author(s):  
Qinghua Ding ◽  
Eric J. Steig ◽  
David S. Battisti ◽  
John M. Wallace
Keyword(s):  
Indian Ocean ◽  
Rossby Wave ◽  
High Latitude ◽  
Southern Annular Mode ◽  
Tropical Pacific ◽  
The Pacific ◽  
Indian Ocean Sector ◽  
The Indian Ocean ◽  
Ocean Sector ◽  
Sst Anomalies

Abstract Perturbations in the southern annular mode (SAM) are shown to be significantly correlated with SST anomalies in the central tropical Pacific during austral winter and SST anomalies in the eastern tropical Pacific during austral summer. The SAM signature in the Pacific sector resembles a tropically forced Rossby wave train, the so-called Pacific–South American pattern, while the signature in the Indian Ocean sector is a zonally elongated meridional dipole. Thus, the SAM contains strong zonally asymmetric variability and tends to behave differently in the Eastern and Western Hemispheres, with internal dynamics prevailing in the Indian Ocean sector and the forced response to tropical SST anomalies exerting a strong influence in the Pacific sector. The tropically forced component of the SAM in the Pacific sector is related to a geographically fixed active Rossby wave source to the east of Australia within the core of the subtropical jet. In addition to the well-documented positive trend in summer, the SAM also exhibits a negative wintertime trend since 1979, characterized by prominent geopotential height increases over the high latitudes. In both seasons, SAM trends are closely linked to long-term trends in tropical Pacific SST that are independent of the canonical eastern Pacific ENSO variability. Although the SAM is an intrinsic pattern of high-latitude variability, the SAM index reflects the superposition of both high-latitude and tropically forced variability.

The impact of ENSO and the Atlantic Multidecadal Variability (AMV) on the upper tropospheric large scale flow in the PRIMAVERA models.

2020 ◽  
Author(s):  
Paolo Ghinassi ◽  
Federico Fabiano ◽  
Virna L. Meccia ◽  
Susanna Corti
Keyword(s):  
Rossby Wave ◽  
Large Scale ◽  
Rossby Waves ◽  
Wave Activity ◽  
Transient Wave ◽  
Weather Regime ◽  
Weather Regimes ◽  
The Pacific ◽  
Large Scale Flow ◽  
The Impact

<p>Rossby waves play a fundamental role for both climate and weather. They are in fact associated with heat, momentum and moisture transport across large distances and with different types of weather at the surface. Assessing how they are represented in climate models is thus of primary importance to understand both predictability and the present and future climate. In this study we investigate how ENSO and the AMV affect the large scale flow pattern in the upper troposphere of the Northern Hemisphere, using reanalysis data and data from the PRIMAVERA simulations.</p><p>The upper tropospheric large scale flow is investigated in terms of the Rossby wave activity associated with persistent and recurrent patterns over the Pacific-North American and Euro-Atlantic regions during winter, the so called weather regimes. In order to quantify the vigour of Rossby wave activity associated with each weather regime we make use of a recently developed diagnostic based on Finite Amplitude Local Wave Activity in isentropic coordinates, partitioning the total wave activity into the stationary and transient components. The former is associated with quasi-stationary, planetary Rossby waves, whereas the latter is associated with synoptic scale Rossby wave packets. This allows one to quantify the contribution from stationary versus transient eddies in the total Rossby wave activity linked to each weather regime.</p><p>In this study we explore how ENSO and the AMV affect both the weather regimes frequencies and the upper tropospheric waviness in the Pacific and Atlantic storm tracks, respectively. Furthermore we analyse how both the stationary and transient wave activity component modulate the onset and transition between different regimes.</p>

scholarly journals Seasonal Modulations of the Active MJO Cycle Characterized by Nonlinear Principal Component Analysis

2011 ◽  
Vol 139 (7) ◽  
pp. 2259-2275 ◽  
Author(s):  
Johannes Jenkner ◽  
William W. Hsieh ◽  
Alex J. Cannon
Keyword(s):  
Longwave Radiation ◽  
Principal Component ◽  
Boreal Winter ◽  
Convective Activity ◽  
Marginal Contributions ◽  
The Indian Ocean ◽  
The Mean ◽  
The Difference ◽  
Upper Level ◽  
Mean Cycle

Abstract A novel methodology is presented for the identification of the mean cycle of the Madden–Julian oscillation (MJO) along the equator. The methodology is based on a nonlinear principal component (NLPC) computed with a neural network model. The bandpass-filtered input data encompass 30 yr with zonal winds at 850 and 200 hPa plus outgoing longwave radiation (OLR). The NLPC is conditioned on a sufficiently strong MJO activity and is computed both for the pooled dataset and for the dataset stratified into seasons. The NLPC for all data depicts a circular mode formed by the first two linear principal components (LPCs) with marginal contributions by the higher-order LPCs. Hence, the mean MJO cycle throughout the year is effectively captured by the amplitude of the leading two LPCs varying in quadrature. The NLPC for individual seasons shows additional variability, which mainly arises from a subordinate oscillation of the second pair of LPCs superimposed on the annual MJO signal. In reference to the all-year solution, the difference in resolved variability approximately accounts for 9% in solstitial seasons and 3% in equinoctial seasons. The phasing of the third LPC is such that convective activity oscillations over the Maritime Continent as well as wind oscillations over the Indian Ocean appear enhanced (suppressed) during boreal winter (summer). Also, convective activity oscillations appear more pronounced at the date line during both winter and summer. The phasing of the fourth LPC is such that upper-level westerlies over the Atlantic region are more persistent during boreal spring than during other seasons.

A daily estimate of phase speed to explore the link between Arctic Amplification and Rossby waves

2020 ◽  
Author(s):  
Jacopo Riboldi ◽  
François Lott ◽  
Fabio D'Andrea ◽  
Gwendal RIvière
Keyword(s):  
Northern Hemisphere ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Extreme Temperature ◽  
Phase Speed ◽  
Weighted Average ◽  
Boreal Summer ◽  
High Temporal Resolution ◽  
Arctic Amplification ◽  
Extreme Temperature Events

<p>Rossby wave activity is intimately related to the day-to-day weather evolution over midlatitudes and to the occurrence of extreme events. Global warming trends may also affect their characteristics: for example, it has been hypothesized that Arctic warming with respect to midlatitudes, known as Arctic Amplification, may lead to a reduction in the speed of Rossby waves, to more frequent atmospheric blocking and to extreme temperature events over midlatitudes. Testing this hypothesis requires an estimate of the evolution and of the variability of phase speed in recent decades and in climate model simulations. However, measuring the phase speed of the global Rossby wave pattern is a complex task, as the midlatitude flow consists of a superposition of waves of different nature (e.g., planetary vs synoptic) across a broad range of wavenumbers and frequencies.</p><p>We propose here a framework, based on spectral analysis, to understand the variability of Rossby wave characteristics in reanalysis and their possible future changes. A novel, daily climatology of wave spectra based on gridded upper-level wind data is employed to study the evolution of Rossby wave phase speed over the Northern Hemisphere between March 1979 and November 2018. A global estimate of phase speed is obtained by doing a weighted average of the phase speed of each wave, with the associated spectral coefficients as weights.</p><p>Several insights about the drivers of phase speed variability at different time scales and their link with extreme temperature events can be gained from this diagnostic. 1) The occurrence of low phase speeds over Northern Hemisphere midlatitudes is related to a poleward displacement of blocking frequency maxima; conversely, the occurrence of high phase speed is related to blocking occurring at lower latitudes than usual. 2) Periods of low phase speed are associated with the occurrence of anomalous temperatures over Northern Hemisphere midlatitudes in winter, while this linkage is weaker during boreal summer. 3) No significant trend in phase speed has been observed during recent decades, despite the presence of Arctic Amplification. The absence of trend in phase speed is consistent with the evolution of the meridional geopotential gradient during recent decades. On the other hand, the high temporal resolution of the phase speed metric highlights the intraseasonal and interannual variability of Rossby wave propagation and points to 2009/10 as an extreme winter characterized by particularly low phase speed.</p>

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