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.

Contributions of Convectively Coupled Equatorial Rossby Waves and Kelvin Waves to the Real-Time Multivariate MJO Indices

2009 ◽  
Vol 137 (1) ◽  
pp. 469-478 ◽  
Author(s):  
Paul E. Roundy ◽  
Carl J. Schreck ◽  
Matthew A. Janiga
Keyword(s):  
Frequency Domain ◽  
Real Time ◽  
Deep Convection ◽  
Longwave Radiation ◽  
Kelvin Waves ◽  
Wind Data ◽  
Madden Julian Oscillation ◽  
The Real ◽  
The Waves ◽  
Equatorial Rossby Waves

Abstract The real-time multivariate (RMM) Madden–Julian oscillation (MJO) indices have been widely applied to diagnose and track the progression of the MJO. Although it has been well demonstrated that the MJO contributes to the leading signals in these indices, the RMM indices vary erratically from day to day. These variations are associated with noise in the outgoing longwave radiation (OLR) and wind data used to generate the indices. This note demonstrates that some of this “noise” evolves systematically and is associated with other types of propagating modes that project onto the RMM eigenmodes. OLR and zonal wind data are filtered in the wavenumber–frequency domain for the MJO, convectively coupled equatorial Rossby (ER) waves, and convectively coupled Kelvin waves. The filtered data are then projected onto the RMM modes. An example phase space associated with these projections is presented. Linear regression is then applied to isolate the wave signals from random variations in the same bands of the wavenumber–frequency domain, and the regressed data are projected onto the RMM EOFs. Results demonstrate the magnitudes of the contributions of the systematically evolving signals associated with these waves to variations in the RMM principal components, and how these contributions vary with the longitude of the active moist deep convection coupled to the waves.

Observations of Upper-Tropospheric Temperature Inversions in the Indian Monsoon and Their Links to Convectively Forced Quasi-Stationary Kelvin Waves

2020 ◽  
Vol 77 (8) ◽  
pp. 2835-2846 ◽  
Author(s):  
Richard Newton ◽  
William Randel
Keyword(s):  
Indian Ocean ◽  
Large Scale ◽  
Indian Monsoon ◽  
Monsoon Season ◽  
Deep Convection ◽  
Equatorial Indian Ocean ◽  
Kelvin Waves ◽  
Tropospheric Temperature ◽  
Time Behavior ◽  
Temperature Inversions

Abstract High-vertical-resolution temperature measurements from GPS radio occultation data show frequent upper-tropospheric inversions over the equatorial Indian Ocean during the summer monsoon season. Each year, around 30% of profiles in this region have temperature inversions near 15 km during the monsoon season, peaking during July–September. This work describes the space–time behavior of these inversions, and their links to transient deep convection. The Indian Ocean inversions occur episodically several times each summer, with a time scale of 1–2 weeks, and are quasi stationary or slowly eastward moving. Strong inversions are characterized by cold anomalies in the upper-troposphere (12–15 km), warm anomalies in the tropopause layer (16–18 km), and strong zonal wind anomalies that are coherent with temperature anomalies. Temperature and wind anomalies are centered over the equator and show a characteristic eastward phase tilt with height with a vertical wavelength near 5 km, consistent with a Kelvin wave structure. Composites of outgoing longwave radiation (OLR) show that strong inversions are linked to enhanced deep convection over the equatorial Indian Ocean, preceding the inversions by ~2–6 days. These characteristics suggest that the inversions are linked to convectively forced Kelvin waves, which are Doppler shifted by the easterly monsoonal winds such that they remain quasi stationary in the equatorial Indian Ocean. These large-scale waves influence circulation on the equatorial side of the Indian monsoon anticyclone; they may provide a positive feedback to the underlying convection, and are possibly linked with regions of shear-induced turbulence.

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 Simulated Interannual Modulation of Intraseasonal Kelvin Waves in the Equatorial Indian Ocean

2016 ◽  
Vol 48 (3) ◽  
pp. 213-229 ◽  
Author(s):  
Iskhaq Iskandar ◽  
◽  
Dedi Setiabudidaya ◽  
Wijaya Mardiansyah ◽  
Muhammad Irfan ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Equatorial Indian Ocean ◽  
Kelvin Waves

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.

Intraseasonal Variability of the Equatorial Indian Ocean Observed from Sea Surface Height, Wind, and Temperature Data

2007 ◽  
Vol 37 (2) ◽  
pp. 188-202 ◽  
Author(s):  
Lee-Lueng Fu
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Equatorial Indian Ocean ◽  
Sea Surface Height ◽  
Kelvin Waves ◽  
Wind Forcing ◽  
Sea Surface ◽  
Vertical Mode ◽  
Basin Mode

Abstract The forcing of the equatorial Indian Ocean by the highly periodic monsoon wind cycle creates many interesting intraseasonal variabilities. The frequency spectrum of the wind stress observations from the European Remote Sensing Satellite scatterometers reveals peaks at the seasonal cycle and its higher harmonics at 180, 120, 90, and 75 days. The observations of sea surface height (SSH) from the Jason and Ocean Topography Experiment (TOPEX)/Poseidon radar altimeters are analyzed to study the ocean’s response. The focus of the study is on the intraseasonal periods shorter than the annual period. The semiannual SSH variability is characterized by a basin mode involving Rossby waves and Kelvin waves traveling back and forth in the equatorial Indian Ocean between 10°S and 10°N. However, the interference of these waves with each other masks the appearance of individual Kelvin and Rossby waves, leading to a nodal point (amphidrome) of phase propagation on the equator at the center of the basin. The characteristics of the mode correspond to a resonance of the basin according to theoretical models. For the semiannual period and the size of the basin, the resonance involves the second baroclinic vertical mode of the ocean. The theory also calls for similar modes at 90 and 60 days. These modes are found only in the eastern part of the basin, where the wind forcing at these periods is primarily located. The western parts of the theoretical modal patterns are not observed, probably because of the lack of wind forcing. There is also similar SSH variability at 120 and 75 days. The 120-day variability, with spatial patterns resembling the semiannual mode, is close to a resonance involving the first baroclinic vertical mode. The 75-day variability, although not a resonant basin mode in theory, exhibits properties similar to the 60- and 90-day variabilities with energy confined to the eastern basin, where the SSH variability seems in resonance with the local wind forcing. The time it takes an oceanic signal to travel eastward as Kelvin waves from the forcing location along the equator and back as Rossby waves off the equator roughly corresponds to the period of the wind forcing. The SSH variability at 60–90 days is coherent with sea surface temperature (SST) with a near-zero phase difference, showing the effects of the time-varying thermocline depth on SST, which may affect the wind in an ocean–atmosphere coupled process governing the intraseasonal variability.

Structure and Properties of Madden–Julian Oscillations Deduced from DYNAMO Sounding Arrays

2013 ◽  
Vol 70 (10) ◽  
pp. 3157-3179 ◽  
Author(s):  
Richard H. Johnson ◽  
Paul E. Ciesielski
Keyword(s):  
Indian Ocean ◽  
Deep Convection ◽  
Vertical Motion ◽  
Thermodynamic Characteristics ◽  
Reanalysis Data ◽  
Kelvin Waves ◽  
Thermal Anomalies ◽  
Inactive Phase ◽  
The Indian Ocean ◽  
North And South

Abstract The kinematic and thermodynamic characteristics of the October and November 2011 Madden–Julian oscillations (MJOs) that occurred over the Indian Ocean during Dynamics of the MJO (DYNAMO) are investigated. Analyses are presented 1) for two primary sounding arrays, where results are independent of model parameterizations, and 2) on larger scales, including the Indian Ocean, using operational and reanalysis data. Mean precipitation during DYNAMO was characterized by maxima in two east–west bands north and south of the equator. This pattern alternated between two bands during the inactive phase of the MJOs and a single rainfall maximum on the equator during the active phases. Precipitation over the northern sounding array (NSA), where the MJO signal was strongest, was significantly modulated by the MJOs, while the southern array experienced more frequent, briefer episodes of rainfall mostly related to ITCZ convection. Over the NSA the MJOs were characterized by gradual moistening of the low to midtroposphere over approximately 2-week periods. The October MJO featured multiple westward-moving, 2-day disturbances whereas the November MJO principally comprised two prominent Kelvin waves. Patterns of moistening, divergence, and vertical motion suggest a stepwise progression of convection, from shallow cumulus to congestus to deep convection. Tilted thermal anomalies in the upper troposphere–lower stratosphere reveal gravity or Kelvin waves excited by the MJO convective envelopes, which modulate the tropopause and contribute to preactive-phase upper-tropospheric moistening. While there is a number of similarities in the characteristics of the two MJOs, there are sufficient differences to warrant caution in generalizing results from these two events.

Roles of Equatorial Waves and Western Boundary Reflection in the Seasonal Circulation of the Equatorial Indian Ocean

2006 ◽  
Vol 36 (5) ◽  
pp. 930-944 ◽  
Author(s):  
Dongliang Yuan ◽  
Weiqing Han
Keyword(s):  
Indian Ocean ◽  
Sea Level ◽  
Rossby Waves ◽  
Equatorial Indian Ocean ◽  
Kelvin Waves ◽  
Wind Forcing ◽  
Western Boundary ◽  
Central Basin ◽  
Sea Levels ◽  
Equatorial Rossby Waves

Abstract An ocean general circulation model (OGCM) is used to study the roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial Indian Ocean. The western boundary reflection is defined as the total Kelvin waves leaving the western boundary, which include the reflection of the equatorial Rossby waves as well as the effects of alongshore winds, off-equatorial Rossby waves, and nonlinear processes near the western boundary. The evaluation of the reflection is based on a wave decomposition of the OGCM results and experiments with linear models. It is found that the alongshore winds along the east coast of Africa and the Rossby waves in the off-equatorial areas contribute significantly to the annual harmonics of the equatorial Kelvin waves at the western boundary. The semiannual harmonics of the Kelvin waves, on the other hand, originate primarily from a linear reflection of the equatorial Rossby waves. The dynamics of a dominant annual oscillation of sea level coexisting with the dominant semiannual oscillations of surface zonal currents in the central equatorial Indian Ocean are investigated. These sea level and zonal current patterns are found to be closely related to the linear reflections of the semiannual harmonics at the meridional boundaries. Because of the reflections, the second baroclinic mode resonates with the semiannual wind forcing; that is, the semiannual zonal currents carried by the reflected waves enhance the wind-forced currents at the central basin. Because of the different behavior of the zonal current and sea level during the reflections, the semiannual sea levels of the directly forced and reflected waves cancel each other significantly at the central basin. In the meantime, the annual harmonic of the sea level remains large, producing a dominant annual oscillation of sea level in the central equatorial Indian Ocean. The linear reflection causes the semiannual harmonics of the incoming and reflected sea levels to enhance each other at the meridional boundaries. In addition, the weak annual harmonics of sea level in the western basin, resulting from a combined effect of the western boundary reflection and the equatorial zonal wind forcing, facilitate the dominance by the semiannual harmonics near the western boundary despite the strong local wind forcing at the annual period. The Rossby waves are found to have a much larger contribution to the observed equatorial semiannual oscillations of surface zonal currents than the Kelvin waves. The westward progressive reversal of seasonal surface zonal currents along the equator in the observations is primarily due to the Rossby wave propagation.

scholarly journals Influence of the Reflected Rossby Waves on the Western Arabian Sea Upwelling Region

2014 ◽  
Vol 44 (5) ◽  
pp. 1424-1438 ◽  
Author(s):  
Tomoki Tozuka ◽  
Motoki Nagura ◽  
Toshio Yamagata
Keyword(s):  
Indian Ocean ◽  
Mixed Layer ◽  
General Circulation ◽  
Arabian Sea ◽  
Rossby Waves ◽  
Circulation Model ◽  
Equatorial Indian Ocean ◽  
Kelvin Waves ◽  
Eastern Boundary ◽  
Arabian Sea Upwelling

Abstract The sea surface temperature (SST) in the western Arabian Sea upwelling region is known to influence the amount of precipitation associated with the Indian summer monsoon. Thus, understanding what determines the SST in this region is an important issue. Using outputs from an ocean general circulation model with and without strong damping in the eastern equatorial Indian Ocean, this study examines how the reflection of semiannual Kelvin waves at the eastern boundary of the Indian Ocean may influence the western Arabian Sea upwelling region. The downwelling Kelvin waves generated in boreal spring are reflected at the eastern boundary and reach the western equatorial Indian Ocean as reflected Rossby waves about 6 months later. The resulting westward current along the equator in the western equatorial Indian Ocean transports warmer water to the western Arabian Sea upwelling region. Thus, the SST in this region becomes colder especially in boreal fall without the reflected Rossby waves. These results are further supported by the analysis of the mixed layer temperature balance. Surprisingly, vertical processes do not contribute to the SST difference, even though the thermocline becomes shallower without the downwelling Rossby waves. This is because the mixed layer is shoaling rapidly from September to November, and there is basically no entrainment of water from below. In contrast, the reflected Rossby waves do not have large impacts on the SST in other seasons mainly because the zonal SST gradient is not as strong and/or the amplitude of Rossby waves is weaker.

Sign in / Sign up

Export Citation Format

Share Document