scholarly journals The characteristics of the equatorial waves caused a record torrential rain event over western Sumatra

2021 ◽  
Vol 893 (1) ◽  
pp. 012015
Author(s):  
P Wu ◽  
Y Fukutomi ◽  
K Kikuchi
Keyword(s):  
Rossby Waves ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Precipitation Event ◽  
Western Coast ◽  
Rain Event ◽  
Equatorial Waves ◽  
Torrential Rain ◽  
Sumatra Island ◽  
Equatorial Rossby Waves

Abstract This study examined the cause of a record torrential rain event over the western coast of Sumatra Island in March 2016. The influence of atmospheric equatorial waves (EWs) and the characteristics of the EWs were investigated. Analysis of the Japanese 55-year Reanalysis data (JRA-55) and precipitation data from the Global Precipitation Measurement (GPM) satellite showed that the event was caused by the combined effects of Kelvin waves, equatorial Rossby waves, and westward inertio-gravity (WIG) waves. An examination of the characteristics of the EWs revealed that the Kelvin waves had longitudinal scales of ~6,000 km, with a period of ~6 days and phase speed of ~12 m s-1, which was typical of the convectively coupled Kelvin waves in this region. The WIG waves had a scale of ~2,500 km, with a period of 2.5 days and a relatively fast phase speed of 12~13 m s-1. Heavy precipitation occurred when an eastward Kelvin wave from the Indian Ocean encountered a westward inertio-gravity (WIG) over Sumatra Island. It was concluded that along with the Kelvin and equatorial Rossby waves, the WIG waves might have played a major role in the formation of the extreme precipitation event.

scholarly journals Equatorial Waves in Opposite QBO Phases

2011 ◽  
Vol 68 (4) ◽  
pp. 839-862 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian J. Hoskins ◽  
Julia M. Slingo
Keyword(s):  
Group Velocity ◽  
Lower Stratosphere ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Western Hemisphere ◽  
Equatorial Waves ◽  
Vertical Wavelength ◽  
Quasi Biennial Oscillation ◽  
Zonal Wavenumber ◽  
The Waves

Abstract A methodology for identifying equatorial waves is used to analyze the multilevel 40-yr ECMWF Re-Analysis (ERA-40) data for two different years (1992 and 1993) to investigate the behavior of the equatorial waves under opposite phases of the quasi-biennial oscillation (QBO). A comprehensive view of 3D structures and of zonal and vertical propagation of equatorial Kelvin, westward-moving mixed Rossby–gravity (WMRG), and n = 1 Rossby (R1) waves in different QBO phases is presented. Consistent with expectation based on theory, upward-propagating Kelvin waves occur more frequently during the easterly QBO phase than during the westerly QBO phase. However, the westward-moving WMRG and R1 waves show the opposite behavior. The presence of vertically propagating equatorial waves in the stratosphere also depends on the upper tropospheric winds and tropospheric forcing. Typical propagation parameters such as the zonal wavenumber, zonal phase speed, period, vertical wavelength, and vertical group velocity are found. In general, waves in the lower stratosphere have a smaller zonal wavenumber, shorter period, faster phase speed, and shorter vertical wavelength than those in the upper troposphere. All of the waves in the lower stratosphere show an upward group velocity and downward phase speed. When the phase of the QBO is not favorable for waves to propagate, their phase speed in the lower stratosphere is larger and their period is shorter than in the favorable phase, suggesting Doppler shifting by the ambient flow and a filtering of the slow waves. Tropospheric WMRG and R1 waves in the Western Hemisphere also show upward phase speed and downward group velocity, with an indication of their forcing from middle latitudes. Although the waves observed in the lower stratosphere are dominated by “free” waves, there is evidence of some connection with previous tropical convection in the favorable year for the Kelvin waves in the warm water hemisphere and WMRG and R1 waves in the Western Hemisphere, which is suggestive of the importance of convective forcing for the existence of propagating coupled Kelvin waves and midlatitude forcing for the existence of coupled WMRG and R1 waves.

scholarly journals Intermediate Intraseasonal Variability in the Western Tropical Pacific Ocean: Meridional Distribution of Equatorial Rossby Waves Influenced by a Tilted Boundary

2020 ◽  
Vol 50 (4) ◽  
pp. 921-933 ◽  
Author(s):  
Zhixiang Zhang ◽  
Larry J. Pratt ◽  
Fan Wang ◽  
Jianing Wang ◽  
Shuwen Tan
Keyword(s):  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Phase Speed ◽  
Circulation Model ◽  
Western Pacific ◽  
Water Model ◽  
Global Circulation Model ◽  
Energy Propagation ◽  
Intermediate Depth ◽  
Equatorial Rossby Waves

AbstractIntermediate-depth intraseasonal variability (ISV) at a 20–90-day period, as detected in velocity measurements from seven subsurface moorings in the tropical western Pacific, is interpreted in terms of equatorial Rossby waves. The moorings were deployed between 0° and 7.5°N along 142°E from September 2014 to October 2015. The strongest ISV energy at 1200 m occurs at 4.5°N. Peak energy at 4.5°N is also seen in an eddy-resolving global circulation model. An analysis of the model output identifies the source of the ISV as short equatorial Rossby waves with westward phase speed but southeastward and downward group velocity. Additionally, it is shown that a superposition of first three baroclinic modes is required to represent the ISV energy propagation. Further analysis using a 1.5-layer shallow water model suggests that the first meridional mode Rossby wave accounts for the specific meridional distribution of ISV in the western Pacific. The same model suggests that the tilted coastlines of Irian Jaya and Papua New Guinea, which lie to the south of the moorings, shift the location of the northern peak of meridional velocity oscillation from 3°N to near 4.5°N. The tilt of this boundary with respect to a purely zonal alignment therefore needs to be taken into account to explain this meridional shift of the peak. Calculation of the barotropic conversion rate indicates that the intraseasonal kinetic energy below 1000 m can be transferred into the mean flows, suggesting a possible forcing mechanism for intermediate-depth zonal jets.

Subseasonal forecasts of the northern Queensland floods of February 2019: Causes and forecast evaluation

2021 ◽  
Author(s):  
Wayne Yuan-Huai Tsai ◽  
Mong-Ming Lu ◽  
Chung-Hsiung Sui ◽  
Yin-Min Cho
Keyword(s):  
Large Scale ◽  
Rossby Waves ◽  
Austral Summer ◽  
Extreme Rainfall ◽  
Circulation Pattern ◽  
Precipitation Event ◽  
Boreal Winter ◽  
Daily Precipitation Data ◽  
Strong Winds ◽  
Equatorial Rossby Waves

<p>During the austral summer 2018/19, devastating floods occurred over northeast Australia that killed approximately 625,000 head of cattle and inundated over 3000 homes in Townsville. This disastrous event was attributed to a quasi-stationary monsoon depression over northeast Australia and the convection associated with MJO over the western Pacific (Cowan et al. 2019). We found that the unusual rainfall was a record-breaking subseasonal peak rainfall event (SPRE) based on the CMORPH daily precipitation data since1998 (Xie et al. 2017). The SPRE is defined as the highest 15-day accumulative rainfall in the running 90-day windows (Tsai et al. 2020). Results of observational data analysis over the recent 21 years (1998~2020) of ERA-interim, OLR, and CMORPH datasets suggest that the northeastern Australian SPREs can be influenced by multiple large-scale drivers, in particular the MJO and equatorial Rossby waves. The occurrence time of the SPRE is associated with MJO activity, while the mean rainfall intensity is more closely associated with the equatorial Rossby waves. The circulation pattern of the SPREs can also be influenced by the equatorial Rossby waves. Using the hindcast data in S2S database we found that the models can capture the SPREs up to one week of the lead times. Characteristics of the activities of MJO and equatorial Rossby waves over the Indonesia-Australia region and their implication to the extended-range SPRE predictability will be discussed.</p><p><strong>Key words: </strong>S2S prediction, Australian summer monsoon, subseasonal peak precipitation event, extreme rainfall</p><p><strong>References:</strong></p><p>Cowan, T., Wheeler, M.C., Alves, O., Narsey, S., de Burgh-Day, C., Griffiths, M., Jarvis, C., Cobon, D.H., Hawcroft, M.K., 2019. Forecasting the extreme rainfall, low tempera- tures, and strong winds associated with the northern Queensland floods of February 2019. Weather Clim. Extremes 26 (100), 232. https://doi.org/10.1016/j.wace.2019. 100232.</p><p>Tsai, W. Y.-H., M.-M. Lu, C.-H. Sui, and P.-H. Lin, 2020: MJO and CCEW Modulation on South China Sea and Maritime Continent Boreal Winter Subseasonal Peak Precipitation. Terr. Atmos. Oceanic Sci., DOI: 10.3319/TAO.2019.10.28.01</p><p>Xie, P., R. Joyce, S. Wu, S. Yoo, Y. Yarosh, F. Sun, and R. Lin, 2017: Reprocessed, Bias-Corrected CMORPH Global High-Resolution Precipitation Estimates from 1998. J. Hydrometeor., 18, 1617–1641, https://doi.org/10.1175/JHM-D-16-0168.1</p>

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.

Convectively Coupled Equatorial Waves. Part II: Propagation Characteristics

2007 ◽  
Vol 64 (10) ◽  
pp. 3424-3437 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian Hoskins ◽  
Julia Slingo
Keyword(s):  
Rossby Wave ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Equatorial Waves ◽  
Propagation Characteristics ◽  
Upper Troposphere ◽  
Ambient Flow ◽  
Vertical Coupling ◽  
Convectively Coupled Equatorial Waves ◽  
Coherent Wave

Abstract Following the description of the horizontal and vertical structures of convectively coupled equatorial waves presented in Part I, here their propagation characteristics are investigated. Linear lagged regressions are used to produce their composite evolution, and the Radon transform technique is used to calculate their phase speeds. It is shown that coherent wave structures with convective coupling generally exist for about 1–2 weeks. Typical zonal wavenumbers are 6–8, wavelengths are 42°–64° of longitude, and typical periods are 4–8 days. The eastward phase speed of convectively coupled Kelvin waves is between 10 and 17 m s−1. The westward phase speed of the coupled mixed Rossby–gravity wave is between 10 and 15 m s−1, and the westward phase speed of the coupled n = 1 Rossby wave is between 7 and 9 m s−1. It is found that convection can produce stronger vertical coupling of phase speeds, and Doppler shifting by the ambient flow can modify phase speeds. There is further evidence that some waves tend to act as forcing agents for convection whereas others tend to be forced by convection. Eastward propagation of some n = 0 and 1 modes in the upper troposphere is also examined.

scholarly journals Why Torrential Rain Occurs on the Western Coast of Sumatra Island at the Leading Edge of the MJO Westerly Wind Bursts

SOLA ◽  
2017 ◽  
Vol 13 (0) ◽  
pp. 36-40 ◽  
Author(s):  
Peiming Wu ◽  
Dodi Ardiansyah ◽  
Satoru Yokoi ◽  
Shuichi Mori ◽  
Fadli Syamsudin ◽  
...  
Keyword(s):  
Leading Edge ◽  
Western Coast ◽  
Westerly Wind ◽  
Torrential Rain ◽  
Sumatra Island ◽  
Wind Bursts ◽  
Westerly Wind Bursts

The Effects of Wind Forcing and Background Mean Currents on the Latitudinal Structure of Equatorial Rossby Waves

2005 ◽  
Vol 35 (5) ◽  
pp. 666-682 ◽  
Author(s):  
Renellys C. Perez ◽  
Dudley B. Chelton ◽  
Robert N. Miller
Keyword(s):  
Rossby Wave ◽  
Rossby Waves ◽  
Current System ◽  
Phase Speed ◽  
Wave Structure ◽  
Wind Forcing ◽  
Thermocline Depth ◽  
Linear Waves ◽  
Mean Current ◽  
Equatorial Rossby Waves

Abstract The latitudinal structure of annual equatorial Rossby waves in the tropical Pacific Ocean based on sea surface height (SSH) and thermocline depth observations is equatorially asymmetric, which differs from the structure of the linear waves of classical theory that are often presumed to dominate the variability. The nature of this asymmetry is such that the northern SSH maximum (along 5.5°N) is roughly 2 times that of the southern maximum (along 6.5°S). In addition, the observed westward phase speeds are roughly 0.5 times the predicted speed of 90 cm s−1 and are also asymmetric with the northern phase speeds, about 25% faster than the southern phase speeds. One hypothesized mechanism for the observed annual equatorial Rossby wave amplitude asymmetry is modification of the meridional structure by the asymmetric meridional shears associated with the equatorial current system. Another hypothesis is the asymmetry of the annually varying wind forcing, which is stronger north of the equator. A reduced-gravity, nonlinear, β-plane model with rectangular basin geometry forced by idealized Quick Scatterometer (QuikSCAT) wind stress is used to test these two mechanisms. The model with an asymmetric background mean current system perturbed with symmetric annually varying winds consistently produces asymmetric Rossby waves with a northern maximum (4.7°N) that is 1.6 times the southern maximum (5.2°S) and westward phase speeds of approximately 53 ± 13 cm s−1 along both latitudes. Simulations with a symmetric background mean current system perturbed by asymmetric annually varying winds fail to produce the observed Rossby wave structure unless the perturbation winds become strong enough for nonlinear interactions to produce asymmetry in the background mean current system. The observed latitudinal asymmetry of the phase speed is found to be critically dependent on the inclusion of realistic coastline boundaries.

scholarly journals Reflection and Transmission of Equatorial Rossby Waves*

2005 ◽  
Vol 35 (3) ◽  
pp. 363-373 ◽  
Author(s):  
Michael A. Spall ◽  
Joseph Pedlosky
Keyword(s):  
Incident Energy ◽  
Rossby Waves ◽  
Low Frequency ◽  
Kelvin Waves ◽  
Western Boundary ◽  
Incident Flux ◽  
Western Basin ◽  
Wind Event ◽  
Wind Events ◽  
Equatorial Rossby Waves

Abstract The interaction of equatorial Rossby waves with a western boundary perforated with one or more narrow gaps is investigated using a shallow-water numerical model and supporting theory. It is found that very little of the incident energy flux is reflected into eastward-propagating equatorial Kelvin waves provided that at least one gap is located within approximately a deformation radius of the equator. Because of the circulation theorem around an island, the existence of a second gap off the equator reduces the reflection of short Rossby waves and enhances the transmission of the incident energy into the western basin. The westward energy transmitted past the easternmost island is further reduced upon encountering islands to the west, even if these islands are located entirely within the “shadow” of the easternmost island. A localized patch of wind forcing was also used to generate low-frequency Rossby waves for cases with island configurations representative of the western equatorial Pacific. For both idealized islands and a coastline based on the 200-m isobath, the amount of incident energy reflected into Kelvin waves depends on both the duration of the wind event and the meridional decay scale of the anomalous winds. For wind events of 2-yr duration with a meridional decay scale of 700 km, the reflected energy is 37% of the incident flux, and the energy transmitted into the Indian Ocean is approximately 10% of the incident flux, very close to that predicted by previous theories. For shorter wind events or winds confined more closely to the equator the reflected energy is significantly less.

The energy flux of three-dimensional waves in the atmosphere: Exact expression for a basic model diagnosis with no equatorial gap

2021 ◽  
Author(s):  
Hidenori Aiki ◽  
Yoshiki Fukutomi ◽  
Yuki Kanno ◽  
Tomomichi Ogata ◽  
Takahiro Toyoda ◽  
...  
Keyword(s):  
Energy Flux ◽  
Water Waves ◽  
Rossby Waves ◽  
Three Dimensional ◽  
Equatorial Waves ◽  
Scalar Quantity ◽  
Basic Model ◽  
Model Diagnosis ◽  
Universal Expression ◽  
Equatorial Rossby Waves

AbstractA model diagnosis for the energy flux of off-equatorial Rossby waves in the atmosphere has previously been done using quasi-geostrophic equations and is singular at the equator. The energy flux of equatorial waves has been separately investigated in previous studies using a space-time spectral analysis or a ray theory. A recent analytical study has derived an exact universal expression for the energy flux which can indicate the direction of the group velocity for linear shallow water waves at all latitudes. This analytical result is extended in the present study to a height-dependent framework for three-dimensional waves in the atmosphere. This is achieved by investigating the classical analytical solution of both equatorial and off-equatorial waves in a Boussinesq fluid. For the horizontal component of the energy flux, the same expression has been obtained between equatorial waves and off-equatorial waves in the height-dependent framework, which is linked to a scalar quantity inverted from the isentropic perturbation of Ertel’s potential vorticity. The expression of the vertical component of the energy flux requires computation of another scalar quantity that may be obtained from the meridional integral of geopotential anomaly in a wavenumber-frequency space. The exact version of the universal expression is explored and illustrated for three-dimensional waves induced by an idealized Madden-Julian Oscillation forcing in a basic model experiment. The zonal and vertical fluxes manifest the energy transfer of both equatorial Kelvin waves and off-equatorial Rossby waves with a smooth transition at around 10°S and around 10°N. The meridional flux of wave energy represents connection between off-equatorial divergence regions and equatorial convergence regions.

scholarly journals A harmonic projection and least–squares method for quantifying Kelvin wave activity

2016 ◽  
Author(s):  
Andrew Delman ◽  
Janet Sprintall ◽  
Julie McClean ◽  
Lynne Talley
Keyword(s):  
Indian Ocean ◽  
Least Squares ◽  
Sea Level ◽  
Rossby Waves ◽  
Phase Speed ◽  
Wave Activity ◽  
Kelvin Waves ◽  
Three Dimensions ◽  
Kelvin Wave ◽  
Sea Level Anomaly

Abstract. A new method for isolating the equatorial and coastal Kelvin wave signal from alongtrack satellite altimetry data is presented and applied to sea level anomaly (SLA) observations in the tropical Indian Ocean. The method consists of sequential projections onto the SLA data, starting with meridional or cross-shore Kelvin wave profiles derived from shallow water theory (y-projections). Next, Fourier basis functions in x-t (along-waveguide distance and time respectively) space with the phase speed ranges of Kelvin and Rossby waves are projected onto the y-projections. After projections in all three dimensions have been carried out, least-squares methods are applied to optimize the non-orthogonal basis function coefficients and minimize the misfit of their along-waveguide forcing and dissipation. Lastly, the westward-propagating (Rossby wave-related) signals are removed, generating a Kelvin wave coefficient K that represents Kelvin wave activity. Along the Indian Ocean equatorial-coastal waveguide, Hovmöller diagrams of K show reduced high-wavenumber noise compared to analogous diagrams of pre-processed sea level anomaly. Results from a Monte Carlo simulation demonstrate that Kelvin wave signals generated a priori can be effectively recovered even when superimposed with strong Rossby waves; the signs of all but the weakest waves are diagnosed correctly in over 90 % of cases. When the method is applied to 21 years of satellite observations and the SLA signal associated with K is removed, the large residual in the equatorial SLA signal has a spatial distribution consistent with wind-forced Rossby waves. The equatorial SLA variability in the western part of the basin is poorly correlated with the SLA field associated with K, as the superimposed SLA profile of Rossby waves can distort the true origin locations of Kelvin waves in the raw SLA field. Therefore, this method offers improved tracking of Kelvin waves compared to the raw SLA dataset, and may provide the opportunity to study weakly nonlinear aspects of these waves by comparison with linear models.

Sign in / Sign up

Export Citation Format

Share Document