scholarly journals A Reduced-Order Representation of the Madden–Julian Oscillation Based on Reanalyzed Normal Mode Coherences

2019 ◽  
Vol 76 (8) ◽  
pp. 2463-2480 ◽  
Author(s):  
Vassili Kitsios ◽  
Terence J. O’Kane ◽  
Nedjeljka Žagar
Keyword(s):  
Normal Mode ◽  
Gravity Waves ◽  
Rossby Waves ◽  
Spatial Scales ◽  
Longwave Radiation ◽  
Kelvin Waves ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Zonal Wavenumber ◽  
Vertical Scales

Abstract The Madden–Julian oscillation (MJO) is presented as a series of interacting Rossby and inertial gravity waves of varying vertical scales and meridional extents. These components are isolated by decomposing reanalysis fields into a set of normal mode functions (NMF), which are orthogonal eigenvectors of the linearized primitive equations on a sphere. The NMFs that demonstrate spatial properties compatible with the MJO are inertial gravity waves of zonal wavenumber k = 1 and the lowest meridional index n = 0, and Rossby waves with (k, n) = (1, 1). For these horizontal scales, there are multiple small vertical-scale baroclinic modes that have temporal properties indicative of the MJO. On the basis of one such eastward-propagating inertial gravity wave (i.e., a Kelvin wave), composite averages of the Japanese 55-year Reanalysis demonstrate an eastward propagation of the velocity potential, and oscillation of outgoing longwave radiation and precipitation fields over the Maritime Continent, with an MJO-appropriate temporal period. A cross-spectral analysis indicates that only the MJO time scale is coherent between this Kelvin wave and the more energetic modes. Four mode clusters are identified: Kelvin waves of correct phase period and direction, Rossby waves of correct phase period, energetic Kelvin waves of larger vertical scales and meridional extents extending into the extratropics, and energetic Rossby waves of spatial scales similar to that of the energetic Kelvin waves. We propose that within this normal mode framework, nonlinear interactions between the aforementioned mode groups are required to produce an energetic MJO propagating eastward with an intraseasonal phase period. By virtue of the selected mode groups, this theory encompasses both multiscale and tropical–extratropical interactions.

scholarly journals The Spectrum of Convectively Coupled Kelvin Waves and the Madden–Julian Oscillation in Regions of Low-Level Easterly and Westerly Background Flow

2012 ◽  
Vol 69 (7) ◽  
pp. 2107-2111 ◽  
Author(s):  
Paul E. Roundy
Keyword(s):  
Longwave Radiation ◽  
Wave Dispersion ◽  
Warm Pool ◽  
Kelvin Waves ◽  
Madden Julian Oscillation ◽  
Westerly Wind ◽  
Kelvin Wave ◽  
Low Level ◽  
Geographical Regions ◽  
Zonal Wavenumber

Abstract The zonal wavenumber–frequency power spectrum of outgoing longwave radiation in the global tropics suggests that power in convectively coupled Kelvin waves and the Madden–Julian oscillation (MJO) is organized into two distinct spectral peaks with a minimum in power in between. This work demonstrates that integration of wavelet power in the wavenumber–frequency domain over geographical regions of moderate trade winds yields a similar pronounced spectral gap between these peaks. In contrast, integration over regions of background low-level westerly wind yields a continuum of power with no gap between the MJO and Kelvin bands. Results further show that signals in tropical convection are redder in frequency in these low-level westerly wind zones, confirming that Kelvin waves tend to propagate more slowly eastward over the warm pool than other parts of the world. Results are consistent with the perspective that portions of disturbances labeled as Kelvin waves and the MJO that are proximate to Kelvin wave dispersion curves exist as a continuum over warm pool regions.

scholarly journals Observed Structure of Convectively Coupled Waves as a Function of Equivalent Depth: Kelvin Waves and the Madden–Julian Oscillation

2012 ◽  
Vol 69 (7) ◽  
pp. 2097-2106 ◽  
Author(s):  
Paul E. Roundy
Keyword(s):  
Shallow Water ◽  
Deep Convection ◽  
Longwave Radiation ◽  
Kelvin Waves ◽  
Water Model ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Easterly Wind ◽  
Equivalent Depth ◽  
Westerly Winds

Abstract The view that convectively coupled Kelvin waves and the Madden–Julian oscillation (MJO) are distinct modes is tested by regressing data from the Climate Forecast System Reanalysis against satellite outgoing longwave radiation data filtered for particular zonal wavenumbers and frequencies by wavelet analysis. Results confirm that nearly dry Kelvin waves have horizontal structures consistent with their equatorial beta-plane shallow-water-theory counterparts, with westerly winds collocated with the lower-tropospheric ridge, while the MJO and signals along Kelvin wave dispersion curves at low shallow-water-model equivalent depths are characterized by geopotential troughs extending westward from the region of lower-tropospheric easterly wind anomalies through the region of lower-tropospheric westerly winds collocated with deep convection. Results show that as equivalent depth decreases from that of the dry waves (concomitant with intensification of the associated convection), the ridge in the westerlies and the trough in the easterlies shift westward. The analysis therefore demonstrates a continuous field of intermediate structures between the two extremes, suggesting that Kelvin waves and the MJO are not dynamically distinct modes. Instead, signals consistent with Kelvin waves become more consistent with the MJO as the associated convection intensifies. This result depends little on zonal scale. Further analysis also shows how activity in synoptic-scale Kelvin waves characterized by particular phase speeds evolves with the planetary-scale MJO.

scholarly journals On the Interpretation of EOF Analysis of ENSO, Atmospheric Kelvin Waves, and the MJO

2015 ◽  
Vol 28 (3) ◽  
pp. 1148-1165 ◽  
Author(s):  
Paul E. Roundy
Keyword(s):  
Skin Temperature ◽  
El Niño ◽  
El Nino ◽  
Spatial Scales ◽  
Longwave Radiation ◽  
Kelvin Waves ◽  
Eof Analysis ◽  
State Variables ◽  
Kelvin Wave ◽  
Full Characterization

Abstract Empirical orthogonal function (EOF) analysis is frequently applied to derive patterns and indexes used to identify and track weather and climate modes as expressed in state variables or proxies of convection. Individual EOFs or pairs of EOFs are often taken to be a complete description of the phenomenon they are intended to index. At the same time, in the absence of projection of the phenomenon onto multiple EOFs yielding multiple similar eigenvalues, each EOF is often assumed to represent a physically independent phenomenon. This project analyzed the leading EOFs of the earth’s skin temperature on the equator and outgoing longwave radiation (OLR) anomalies filtered for atmospheric equatorial Kelvin waves. Results show that the leading two EOFs of the skin temperature data—including east Pacific El Niño and El Niño Modoki—frequently evolve as a quadrature pair during El Niño events, even though the first EOF explains roughly 6 times as much variance as the second. They together diagnose the longitude of the SST anomaly maximum, and their linear combination frequently shows eastward or westward propagation. Analysis of the filtered OLR anomalies shows that the first six EOFs each represent Kelvin wave signals, with the first, second, and third pairs representing Kelvin waves characterized by zonal wavenumbers 2, 3, and 4, respectively. This result demonstrates that if a phenomenon occurs across a range of spatial scales, it is described by multiple EOFs at different scales. A similar analysis demonstrates that the Madden–Julian oscillation probably exhibits spread across a range of spatial scales that would also require multiple EOFs for full characterization.

scholarly journals Planetary Scale Selection of the Madden–Julian Oscillation*

2009 ◽  
Vol 66 (8) ◽  
pp. 2429-2443 ◽  
Author(s):  
Tim Li ◽  
Chunhua Zhou
Keyword(s):  
Boundary Layer ◽  
Rossby Wave ◽  
Initial Perturbation ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Scale Selection ◽  
Planetary Scale ◽  
Zonal Wavenumber ◽  
Nonlinear Heating ◽  
Selection Of

Abstract Numerical experiments with a 2.5-layer and a 2-level model are conducted to examine the mechanism for the planetary scale selection of the Madden–Julian oscillation (MJO). The strategy here is to examine the evolution of an initial perturbation that has a form of the equatorial Kelvin wave at zonal wavenumbers of 1 to 15. In the presence of a frictional boundary layer, the most unstable mode prefers a short wavelength under a linear heating; but with a nonlinear heating, the zonal wavenumber 1 grows fastest. This differs significantly from a model without the boundary layer, in which neither linear nor nonlinear heating leads to the long wave selection. Thus, the numerical simulations point out the crucial importance of the combined effect of the nonlinear heating and the frictional boundary layer in the MJO planetary scale selection. The cause of this scale selection under the nonlinear heating is attributed to the distinctive phase speeds between the dry Kelvin wave and the wet Kelvin–Rossby wave couplet. The faster dry Kelvin wave triggered by a convective branch may catch up and suppress another convective branch, which travels ahead of it at the phase speed of the wet Kelvin–Rossby wave couplet if the distance between the two neighboring convective branches is smaller than a critical distance (about 16 000 km). The interference between the dry Kelvin wave and the wet Kelvin–Rossby wave couplet eventually dissipates and “filters out” shorter wavelength perturbations, leading to a longwave selection. The boundary layer plays an important role in destabilizing the MJO through frictional moisture convergences and in retaining the in-phase zonal wind–pressure structure.

scholarly journals Seasonality and Regionality of the Madden–Julian Oscillation, Kelvin Wave, and Equatorial Rossby Wave

2007 ◽  
Vol 64 (12) ◽  
pp. 4400-4416 ◽  
Author(s):  
Hirohiko Masunaga
Keyword(s):  
Boundary Layer ◽  
Water Solution ◽  
Austral Summer ◽  
Longwave Radiation ◽  
Boreal Summer ◽  
Dynamical Structure ◽  
Madden Julian Oscillation ◽  
Kelvin Wave ◽  
Low Level ◽  
The Impact

Abstract The Madden–Julian oscillation (MJO), Kelvin wave, and equatorial Rossby (ER) wave—collectively called intraseasonal oscillations (ISOs)—are investigated using a 25-yr record of outgoing longwave radiation (OLR) measurements as well as the associated dynamical fields. The ISO modes are detected by applying bandpass filters to the OLR data in the frequency–wavenumber space. An automated wave-tracking algorithm is applied to each ISO mode so that convection centers accompanied with the ISOs are traced in space and time in an objective fashion. The identified paths of the individual ISO modes are first examined and found strongly modulated regionally and seasonally. The dynamical structure is composited with respect to the convection centers of each ISO mode. A baroclinic mode of the combined Rossby and Kelvin structure is prominent for the MJO, consistent with existing work. The Kelvin wave exhibits a low-level wind field resembling the shallow-water solution, while a slight lead of low-level convergence over convection suggests the impact of frictional boundary layer convergence on Kelvin wave dynamics. A lagged composite analysis reveals that the MJO is accompanied with a Kelvin wave approaching from the west preceding the MJO convective maximum in austral summer. MJO activity then peaks as the Kelvin and ER waves constructively interfere to enhance off-equatorial boundary layer convergence. The MJO leaves a Kelvin wave emanating to the east once the peak phase is passed. The approaching Kelvin wave prior to the development of MJO convection is absent in boreal summer and fall. The composite ER wave, loosely concentrated around the MJO, is nearly stationary throughout. A possible scenario to physically translate the observed result is also discussed.

Analysis of Convectively Coupled Kelvin Waves in the Indian Ocean MJO

2008 ◽  
Vol 65 (4) ◽  
pp. 1342-1359 ◽  
Author(s):  
Paul E. Roundy
Keyword(s):  
Indian Ocean ◽  
Spatial Scales ◽  
Deep Convection ◽  
Equatorial Indian Ocean ◽  
Longwave Radiation ◽  
Kelvin Waves ◽  
Synoptic Scale ◽  
Left Behind ◽  
Geographical Regions ◽  
The Waves

Abstract The active convective phase of the Madden–Julian oscillation (hereafter active MJO) comprises enhanced moist deep convection on its own temporal and spatial scales as well as increased variance in convection associated with higher-frequency modes. Synoptic-scale cloud superclusters apparently associated with convectively coupled Kelvin waves occur within the active convective envelopes of most MJO events. These convectively coupled Kelvin waves also occur during the suppressed convective phase of the MJO (hereafter suppressed MJO). This observational study presents an analysis of outgoing longwave radiation and reanalysis data to determine how these waves behave differently as they propagate through the active and suppressed MJO. Time indices of the MJO and Kelvin waves are derived for over the equatorial Indian Ocean. Dates of local extrema in these indices are used to composite data to discern how the waves and associated circulations behave on average; then, further composites are made based on subsets of this list of dates that are consistent with the two MJO phases. Results show that the MJO phase modulates the intensity of moist deep convection associated with the Kelvin waves, the evolution of the vertical structure of cloudiness linked to Kelvin waves, and patterns of upper-level outflow from convection coupled to Kelvin waves. Composites reveal that synoptic-scale circulations associated with the release of latent heat in convection coupled to Kelvin waves amplify and are left behind the waves in preferred geographical regions. The MJO modulates the amplitudes of these circulations and the locations where they get left behind the waves. Previous results have suggested a sharp distinction between the phase speeds of the MJO (4–8 m s−1) and of convectively coupled Kelvin waves (specifically 17 m s−1). In contrast, the present work suggests that convectively coupled Kelvin waves have a broad range of characteristic phase speeds, extending from 10 to 17 m s−1, depending on both the region of the world and the phase of the MJO through which they propagate.

scholarly journals Modal Decomposition of the Global Response to Tropical Heating Perturbations Resembling MJO

2019 ◽  
Vol 76 (5) ◽  
pp. 1457-1469 ◽  
Author(s):  
Katarina Kosovelj ◽  
Fred Kucharski ◽  
Franco Molteni ◽  
Nedjeljka Žagar
Keyword(s):  
Normal Mode ◽  
Numerical Experiments ◽  
Short Range ◽  
Kelvin Waves ◽  
Madden Julian Oscillation ◽  
Global Response ◽  
Function Decomposition ◽  
Mode Function ◽  
Response Variance ◽  
Medium Range

Abstract The paper presents four ensembles of numerical experiments that compare the response to monopole and dipole heating perturbations resembling different phases of the Madden–Julian oscillation (MJO). The results quantify the Rossby and inertio-gravity (IG) wave response using the normal-mode function decomposition. The day 3 response is characterized by about 60% variance in the IG modes, with about 85% of it belonging to the Kelvin waves. On day 14, only 10% of the response variance is due to the Kelvin waves. Although the n = 1 Rossby mode is the main contributor to the Rossby variance at all time scales, the n > 1 Rossby modes contribute over 50% of the balanced response to the MJO heating. In the short range, dipole perturbations produce a response with the maximal variance in zonal wavenumbers k = 2–3 whereas in the medium range the response maximizes at k = 1 in all experiments. Furthermore, the medium-range response to the heating perturbation mimicking MJO phase 6 is found also over Europe.

scholarly journals On the Sensitivity of Convectively Coupled Equatorial Waves to the Quasi-Biennial Oscillation

2019 ◽  
Vol 32 (18) ◽  
pp. 5833-5847 ◽  
Author(s):  
S. Abhik ◽  
Harry H. Hendon ◽  
Matthew C. Wheeler
Keyword(s):  
Gravity Waves ◽  
Austral Summer ◽  
Longwave Radiation ◽  
Kelvin Wave ◽  
Quasi Biennial Oscillation ◽  
Horizontal Structure ◽  
Convectively Coupled Equatorial Waves ◽  
Mechanism Of Impact ◽  
The Impact ◽  
Biennial Oscillation

Abstract The seasonal-mean variance of the Madden–Julian oscillation (MJO) in austral summer has recently been shown to be significantly (p < 5%) enhanced during easterly phases of the quasi-biennial oscillation (QBO). The impact is large, with the mean MJO variance increasing by ~50% compared to the QBO westerly phase. In contrast, we show using observed outgoing longwave radiation that seasonal variations for convectively coupled equatorial Kelvin, Rossby, and mixed Rossby–gravity waves are insensitive to the QBO. This insensitivity extends to all high-frequency (2–30-day period) and the non-MJO component of the intraseasonal (30–120-day period) convective variance. However, convectively coupled Kelvin wave variability shows a modest increase (~13%) that is marginally significant (p = 10%) during easterly phases of the QBO in austral autumn, when Kelvin wave activity is seasonally strongest along the equator. The mechanism of impact on the Kelvin wave appears to be similar to what has previously been argued for the MJO during austral summer. However, the more tilted and shallower vertical structure of the Kelvin waves suggests that they cannot tap into the extra destabilization at the tropopause provided by the easterly phase of the QBO as effectively as the MJO. Lack of impact on the convectively coupled Rossby and mixed Rossby–gravity waves is argued to stem from their horizontal structure that results in weaker divergent anomalies along the equator, where the QBO impact is greatest. Our results further emphasize that the MJO in austral summer is uniquely affected by the QBO.

Rossby-Kelvin instability: a new type of ageostrophic instability caused by a resonance between Rossby waves and gravity waves

1989 ◽  
Vol 202 ◽  
pp. 149-176 ◽  
Author(s):  
Satoshi Sakai
Keyword(s):  
Gravity Waves ◽  
Rossby Waves ◽  
Baroclinic Instability ◽  
Phase Speed ◽  
Kelvin Wave ◽  
Layer System ◽  
Geostrophic Balance ◽  
New Type ◽  
Frontal Instability ◽  
Zero Phase

An ageostrophic version of Phillips’ model is studied. All instabilities found are systematically interpreted in terms of resonance of wave components. The instability occurs if there is a pair of wave components which propagate in the opposite direction to the basic flow and these wave components have almost the same Doppler-shifted frequency. A new instability, identified as a resonance between the Kelvin wave and the Rossby waves, is found at Froude number F ≈ 0.7. The Rossby waves are almost completely in geostrophic balance while the ageostrophic Kelvin wave is the same as in a one-layer system. Doppler shifting matches frequencies which would otherwise be very different. This instability is presumably the mechanism of the frontal instability observed by Griffiths & Linden (1982) in a laboratory experiment. Ageostrophic, baroclinic instability with non-zero phase speed is also observed in the numerical calculation. This instability is caused by resonance between different geostrophic modes.

scholarly journals Properties of the average distribution of equatorial Kelvin waves investigated with the GROGRAT ray tracer

2009 ◽  
Vol 9 (20) ◽  
pp. 7973-7995 ◽  
Author(s):  
M. Ern ◽  
H.-K. Cho ◽  
P. Preusse ◽  
S. D. Eckermann
Keyword(s):  
Longwave Radiation ◽  
Tropical Rainfall Measuring Mission ◽  
Kelvin Waves ◽  
Data Sets ◽  
Relative Importance ◽  
Kelvin Wave ◽  
Quasi Biennial Oscillation ◽  
Broadband Emission ◽  
Turbulent Damping ◽  
Average Distribution

Abstract. Kelvin waves excited by tropospheric convection are considered to be one of the main drivers of the stratospheric quasi-biennial oscillation (QBO). In this paper we combine several measured data sets with the Gravity wave Regional Or Global RAy Tracer (GROGRAT) in order to study the forcing and vertical propagation of Kelvin waves. Launch distributions for the ray tracer at tropospheric altitudes are deduced from space-time spectra of European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, as well as outgoing longwave radiation (OLR) and rainfall data measured by the Tropical Rainfall Measuring Mission (TRMM) satellite. The resulting stratospheric Kelvin wave spectra are compared to ECMWF operational analyses and temperature measurements of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite instrument. Questions addressed are: the relative importance of source variability versus wind modulation, the relative importance of radiative and turbulent damping versus wave breaking, and the minimum altitude where freely propagating waves dominate the spectrum.

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