scholarly journals Analysis of the Dominant Mode of Convectively Coupled Kelvin Waves in the West African Monsoon

2007 ◽  
Vol 20 (8) ◽  
pp. 1487-1503 ◽  
Author(s):  
Flore Mounier ◽  
George N. Kiladis ◽  
Serge Janicot
Keyword(s):  
Deep Convection ◽  
Central Africa ◽  
Phase Speed ◽  
West African Monsoon ◽  
Dominant Mode ◽  
Kelvin Waves ◽  
Kelvin Wave ◽  
African Monsoon ◽  
Convective Systems ◽  
Northern Summer

Abstract The dominant mode of convectively coupled Kelvin waves has been detected over the Atlantic and Africa during northern summer by performing composite analyses on observational fields based on an EOF reconstructed convection index over West Africa. Propagating eastward, many waves originate from the Pacific sector, interact with deep convection of the marine ITCZ over the Atlantic and the continental ITCZ over West and central Africa, and then weaken over East Africa and the Indian Ocean. It has been shown that they are able to modulate the life cycle and track of individual westward-propagating convective systems. Their mean kinematic characteristics comprise a wavelength of 8000 km, and a phase speed of 15 m s−1, leading to a period centered on 6 to 7 days. The African Kelvin wave activity displays large seasonal variability, being highest outside of northern summer when the ITCZ is close to the equator, facilitating the interactions between convection and these equatorially trapped waves. The convective and dynamical patterns identified over the Atlantic and Africa show some resemblance to the theoretical equatorially trapped Kelvin wave solution on an equatorial β plane. Most of the flow is in the zonal direction as predicted by theory, and there is a tendency for the dynamical fields to be symmetric about the equator, even though the ITCZ is concentrated well north of the equator at the full development of the African monsoon. In the upper troposphere and the stratosphere, the temperature contours slope sharply eastward with height, as expected from an eastward-moving heat source that forces a dry Kelvin wave response. It is finally shown that the mean impact of African Kelvin waves on rainfall and convection is of the same level as African easterly waves.

scholarly journals Stable water isotope signals in tropical ice clouds in the West African monsoon simulated with a regional convection-permitting model

2021 ◽  
Author(s):  
Andries Jan de Vries ◽  
Franziska Aemisegger ◽  
Stephan Pfahl ◽  
Heini Wernli
Keyword(s):  
Deep Convection ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
Ice Clouds ◽  
African Monsoon ◽  
Tropical Tropopause Layer ◽  
Convective Systems ◽  
Tropical Tropopause ◽  
Tropical Deep Convection

Abstract. Tropical ice clouds have an important influence on the Earth’s radiative balance. They often form as a result of tropical deep convection, which strongly affects the water budget of the tropical tropopause layer. Ice cloud formation involves complex interactions on various scales, which are not fully understood yet and lead to large uncertainties in climate predictions. In this study, we investigate the formation of tropical ice clouds related to deep convection in the West African monsoon, using stable water isotopes as tracers of moist atmospheric processes. We perform simulations using the regional isotope-enabled model COSMOiso with different resolutions and treatments of convection for the period of June–July 2016. First, we evaluate the ability of our simulations to represent the isotopic composition of monthly precipitation through comparison with GNIP observations, and the precipitation characteristics related to the monsoon evolution and convective storms based on insights from the DACCIWA field campaign in 2016. Next, a case study of a mesoscale convective system (MCS) explores the isotope signatures of tropical deep convection in atmospheric water vapour and ice. Convective updrafts within the MCS inject enriched ice into the upper troposphere leading to depletion of vapour within these updrafts due to the preferential condensation and deposition of heavy isotopes. Water vapour in downdrafts within the same MCS are enriched by non-fractionating sublimation of ice. In contrast to ice within the MCS core regions, ice in widespread cirrus shields is isotopically in approximate equilibrium with the ambient vapour, which is consistent with in situ formation of ice. These findings from the case study are supported by a statistical evaluation of isotope signals in the West African monsoon ice clouds. The following five key processes related to tropical ice clouds can be distinguished based on their characteristic isotope signatures: (1) convective lofting of enriched ice into the upper troposphere, (2) cirrus clouds that form in situ from ambient vapour under equilibrium fractionation, (3) sedimentation and sublimation of ice in the mixed-phase cloud layer in the vicinity of convective systems and underneath cirrus shields, (4) sublimation of ice in convective downdrafts that enriches the environmental vapour, and (5) the freezing of liquid water in the mixed-phase cloud layer at the base of convective updrafts. Importantly, the results show that convective systems strongly modulate the humidity budget and the isotopic composition of the lower tropical tropopause layer. They contribute to about 40 % of the total water and 60 % of HDO in the 175–125 hPa layer in the African monsoon region according to estimates based on our model simulations. Overall, this study demonstrates that isotopes can serve as useful tracers to disentangle the role of different processes in the Earth’s water cycle, including convective transport, the formation of ice clouds, and their impact on the tropical tropopause layer.

scholarly journals Dynamics of the West African Monsoon. Part IV: Analysis of 25–90-Day Variability of Convection and the Role of the Indian Monsoon

2009 ◽  
Vol 22 (6) ◽  
pp. 1541-1565 ◽  
Author(s):  
Serge Janicot ◽  
Flore Mounier ◽  
Nicholas M. J. Hall ◽  
Stéphanie Leroux ◽  
Benjamin Sultan ◽  
...  
Keyword(s):  
Heat Source ◽  
Summer Monsoon ◽  
Indian Monsoon ◽  
Time Lag ◽  
West African Monsoon ◽  
Dominant Mode ◽  
West African ◽  
Kelvin Wave ◽  
The West ◽  
African Monsoon

Abstract This paper is part of a series of studies addressing the dynamics of the West African summer monsoon at intraseasonal time scales between 10 and 90 days. The dominant mode of 25–90-day convective variability in the African monsoon was investigated, starting from previous results involving the excitation of dry equatorial Kelvin and Rossby waves by a negative diabatic heat source located over the warm pool. This evolution is consistent with a significant contribution by a convectively coupled equatorial Rossby wave and the MJO. On the other hand, convectively coupled Kelvin waves as well as the dry Kelvin wave signal have a very weak impact. However, there is more to the global control of the African summer monsoon than convectively coupled wave dynamics. The active/break cycle of the Indian monsoon, controlled by a northward-moving dipole of diabatic heating in the Indian sector, can also influence the African monsoon through atmospheric teleconnections. Simulations performed with a dry primitive equation model show that this influence may be transferred through the northern Indian heat source, which excites a Rossby cyclonic circulation propagating westward over North Africa that is cut off by the northward arrival of the equatorial Indian heat source and the associated intrusion of an anticyclonic ridge. Low-level westerly winds and moisture advection within the ITCZ consequently increase over Africa. The mean time lag between an active phase over India and over Africa is about 15–20 days.

scholarly journals Tropical temperature variability and Kelvin-wave activity in the UTLS from GPS RO measurements

2017 ◽  
Vol 17 (2) ◽  
pp. 793-806 ◽  
Author(s):  
Barbara Scherllin-Pirscher ◽  
William J. Randel ◽  
Joowan Kim
Keyword(s):  
Large Scale ◽  
Deep Convection ◽  
Wave Activity ◽  
Kelvin Waves ◽  
Temperature Variability ◽  
Stationary Waves ◽  
Kelvin Wave ◽  
Quasi Biennial Oscillation ◽  
Tropical Tropopause ◽  
Tropopause Region

Abstract. Tropical temperature variability over 10–30 km and associated Kelvin-wave activity are investigated using GPS radio occultation (RO) data from January 2002 to December 2014. RO data are a powerful tool for quantifying tropical temperature oscillations with short vertical wavelengths due to their high vertical resolution and high accuracy and precision. Gridded temperatures from GPS RO show the strongest variability in the tropical tropopause region (on average 3 K2). Large-scale zonal variability is dominated by transient sub-seasonal waves (2 K2), and about half of sub-seasonal variance is explained by eastward-traveling Kelvin waves with periods of 4 to 30 days (1 K2). Quasi-stationary waves associated with the annual cycle and interannual variability contribute about a third (1 K2) to total resolved zonal variance. Sub-seasonal waves, including Kelvin waves, are highly transient in time. Above 20 km, Kelvin waves are strongly modulated by the quasi-biennial oscillation (QBO) in stratospheric zonal winds, with enhanced wave activity during the westerly shear phase of the QBO. In the tropical tropopause region, however, peaks of Kelvin-wave activity are irregularly distributed in time. Several peaks coincide with maxima of zonal variance in tropospheric deep convection, but other episodes are not evidently related. Further investigations of convective forcing and atmospheric background conditions are needed to better understand variability near the tropopause.

scholarly journals Internal Intraseasonal Variability of the West African Monsoon in WRF

2017 ◽  
Vol 30 (15) ◽  
pp. 5815-5833 ◽  
Author(s):  
Ghassan J. Alaka ◽  
Eric D. Maloney
Keyword(s):  
East Africa ◽  
Large Scale ◽  
Intraseasonal Variability ◽  
West African Monsoon ◽  
West African ◽  
Boreal Summer ◽  
Kelvin Wave ◽  
The West ◽  
African Monsoon ◽  
External Influences

The West African monsoon (WAM) and its landmark features, which include African easterly waves (AEWs) and the African easterly jet (AEJ), exhibit significant intraseasonal variability in boreal summer. However, the degree to which this variability is modulated by external large-scale phenomena, such as the Madden–Julian oscillation (MJO), remains unclear. The Weather Research and Forecasting (WRF) Model is employed to diagnose the importance of the MJO and other external influences for the intraseasonal variability of the WAM and associated AEW energetics by removing 30–90-day signals from initial and lateral boundary conditions in sensitivity tests. The WAM produces similar intraseasonal variability in the absence of external influences, indicating that the MJO is not critical to produce WAM variability. In control and sensitivity experiments, AEW precursor signals are similar near the AEJ entrance in East Africa. For example, an eastward extension of the AEJ increases barotropic and baroclinic energy conversions in East Africa prior to a 30–90-day maximum of perturbation kinetic energy in West Africa. The WAM appears to prefer a faster oscillation when MJO forcing is removed, suggesting that the MJO may serve as a pacemaker for intraseasonal oscillations in the WAM. WRF results show that eastward propagating intraseasonal signals (e.g., Kelvin wave fronts) are responsible for this pacing, while the role of westward propagating intraseasonal signals (e.g., MJO-induced Rossby waves) appears to be limited. Mean state biases across the simulations complicate the interpretation of results.

scholarly journals Characterization of the Diurnal Cycle of the West African Monsoon around the Monsoon Onset

2007 ◽  
Vol 20 (15) ◽  
pp. 4014-4032 ◽  
Author(s):  
Benjamin Sultan ◽  
Serge Janicot ◽  
Philippe Drobinski
Keyword(s):  
Diurnal Cycle ◽  
Deep Convection ◽  
West African Monsoon ◽  
Monsoon Onset ◽  
West African ◽  
Onset Date ◽  
The West ◽  
African Monsoon ◽  
Nocturnal Jet ◽  
Heat Low

Abstract This study investigates the diurnal cycle of the West African monsoon and its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979–2000 NCEP–DOE reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) at 0000, 0600, 1200, and 1800 UTC. This study points out two independent modes describing the space–time variability of the diurnal cycle of low-level wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. This latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date. The diurnal cycle of dry and deep convection is also investigated through the same method. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection may generate a gravity wave that propagates northward and reaches the Saharan heat low region 12 h later. The diurnal cycle of the dry convection in the Saharan heat low is similar during the preonset and the postonset periods with a peak at night (0000 UTC) consistent with the nocturnal jet intensification. This convection is localized at 15° and 20°N before and after the monsoon onset, respectively. Both during the first rainy season in spring and the monsoon season in summer, the nocturnal jet brings moisture in the boundary layer north of the ITCZ favoring humidification and initiation of new convective cells, helping the northward progression of the ITCZ. At the end of the summer the southward return of the ITCZ is associated with the disappearance of the core of the monsoon jet. Despite a lot of similarities between the results obtained using NCEP–DOE and ERA-40 reanalyses, giving confidence in the significance of these results, some differences are identified, especially in the diurnal cycle of deep convection, which limit the interpretation of some of these results and highlight discrepancies in the reanalyses.

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 Cycles and Propagation of Deep Convection over Equatorial Africa

2011 ◽  
Vol 139 (9) ◽  
pp. 2832-2853 ◽  
Author(s):  
Arlene G. Laing ◽  
Richard E. Carbone ◽  
Vincenzo Levizzani
Keyword(s):  
Deep Convection ◽  
Active Phase ◽  
Kelvin Waves ◽  
Convective Precipitation ◽  
Atmospheric Conditions ◽  
Kelvin Wave ◽  
Mesoscale Convection ◽  
Diurnal Cycles ◽  
Continental Scale ◽  
Equatorial Africa

Long-term statistics of organized convection are vital to improved understanding of the hydrologic cycle at various scales. Satellite observations are used to understand the timing, duration, and frequency of deep convection in equatorial Africa, a region with some of the most intense thunderstorms. Yet little has been published about the propagation characteristics of mesoscale convection in that region. Diurnal, subseasonal, and seasonal cycles of cold cloud (proxy for convective precipitation) are examined on a continental scale. Organized deep convection consists of coherent structures that are characteristic of systems propagating under a broad range of atmospheric conditions. Convection is triggered by heating of elevated terrain, sea/land breezes, and lake breezes. Coherent episodes of convection result from regeneration of convection through multiple diurnal cycles while propagating westward. They have an average 17.6-h duration and 673-km span; most have zonal phase speeds of 8–16 m s−1. Propagating convection occurs in the presence of moderate low-level shear that is associated with the southwesterly monsoonal flow and midlevel easterly jets. Convection is also modulated by eastward-moving equatorially trapped Kelvin waves, which have phase speeds of 12–22 m s−1 over equatorial Africa. Westward propagation of mesoscale convection is interrupted by the dry phase of convectively coupled Kelvin waves. During the wet phase, daily initiation and westward propagation continues within the Kelvin wave and the cold cloud shields are larger. Mesoscale convection is more widespread during the active phase of the Madden–Julian oscillation (MJO) but with limited westward propagation. The study highlights multiscale interaction as a major source of variability in convective precipitation during the critical rainy seasons in equatorial Africa.

The Influence of Amazon Rainfall on the Atlantic ITCZ through Convectively Coupled Kelvin Waves

2007 ◽  
Vol 20 (7) ◽  
pp. 1188-1201 ◽  
Author(s):  
Hui Wang ◽  
Rong Fu
Keyword(s):  
South America ◽  
Surface Wind ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Tropical Atlantic ◽  
Kelvin Wave ◽  
Reanalysis Dataset ◽  
Large Variability ◽  
Boreal Spring ◽  
Atlantic Itcz

Abstract Using outgoing longwave radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) daily rain-rate data, systematic changes in intensity and location of the Atlantic intertropical convergence zone (ITCZ) were detected along the equator during boreal spring. It is found that the changes in convection over the tropical Atlantic may be induced by deep convection in equatorial South America. Lagged regression analyses demonstrate that the anomalies of convection developed over the land propagate eastward across the Atlantic and then into Africa. The eastward-propagating disturbances appear to be convectively coupled Kelvin waves with a period of 6–7.5 days and a phase speed of around 15 m s−1. These waves modulate the intensity and location of the convection in the tropical Atlantic and result in a zonal variation of the Atlantic ITCZ on synoptic time scales. The convectively coupled Kelvin wave has substantial signals in both the lower and upper troposphere. Both a reanalysis dataset and the Quick Scatterometer (QuikSCAT) ocean surface wind are used to characterize the Kelvin wave. This study suggests that synoptic-scale variation of the Atlantic ITCZ may be linked to precipitation anomalies in South America through the convectively coupled Kelvin wave. The results imply that the changes of Amazon convection could contribute to the large variability of the tropical Atlantic ITCZ observed during boreal spring.

scholarly journals Kelvin Waves during GOAmazon and Their Relationship to Deep Convection

2020 ◽  
Vol 77 (10) ◽  
pp. 3533-3550
Author(s):  
Yolande L. Serra ◽  
Angela Rowe ◽  
David K. Adams ◽  
George N. Kiladis
Keyword(s):  
Deep Convection ◽  
Kelvin Waves ◽  
Heat Fluxes ◽  
Environment Impact ◽  
Central Amazon ◽  
Surface Heat Fluxes ◽  
Convective Systems ◽  
Precipitation Measurement ◽  
Mesoscale Convective ◽  
Global Precipitation

AbstractThe 2014–15 Observations and Modeling of the Green Ocean Amazon (GOAmazon) field campaign over the central Amazon near Manaus, Brazil, occurred in coordination with the larger Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (CHUVA) project across Brazil. These programs provide observations of convection over the central Amazon on diurnal to annual time scales. In this study, we address the question of how Kelvin waves, observed in satellite observations of deep cloud cover over the GOAmazon region during the 2014–15 time period, modulate the growth, type, and organization of convection over the central Amazon. The answer to this question has implications for improved predictability of organized systems over the region and representation of convection and its growth on local to synoptic scales in global models. Our results demonstrate that Kelvin waves are strong modulators of synoptic-scale low- to midlevel free-tropospheric moisture, integrated moisture convergence, and surface heat fluxes. These regional modifications of the environment impact the local diurnal cycle of convection, favoring the development of mesoscale convective systems. As a result, localized rainfall is also strongly modulated, with the majority of rainfall in the GOAmazon region occurring during the passage of these systems.

scholarly journals The Dynamics of the West African Monsoon. Part V: The Detection and Role of the Dominant Modes of Convectively Coupled Equatorial Rossby Waves

2010 ◽  
Vol 23 (14) ◽  
pp. 4005-4024 ◽  
Author(s):  
Serge Janicot ◽  
Flore Mounier ◽  
Sébastien Gervois ◽  
Benjamin Sultan ◽  
George N. Kiladis
Keyword(s):  
Land Surface ◽  
Singular Spectrum Analysis ◽  
Central Africa ◽  
Phase Speed ◽  
Wave Pattern ◽  
West African ◽  
Convective Heating ◽  
Main Mode ◽  
The West ◽  
African Monsoon

Abstract This study is the last in a series of papers addressing the dynamics of the West African summer monsoon at intraseasonal time scales between 10 and 90 days. The signals of convectively coupled equatorial Rossby (ER) waves within the summer African monsoon have been investigated after filtering NOAA outgoing longwave radiation (OLR) data within a box delineated by the dispersion curves of the theoretical ER waves. Two families of waves have been detected in the 10–100-day periodicity band by performing a singular spectrum analysis on a regional index of ER-filtered OLR. For each family the first EOF mode has been retained to focus on the main convective variability signal. Within the periodicity band of 30–100 days, an ER wave pattern with an approximate wavelength of 13 500 km has been depicted. This ER wave links the MJO mode in the Indian monsoon sector with the main mode of convective variability over West and central Africa. This confirms the investigations carried out in previous studies. Within the 10–30-day periodicity band, a separate ER wave pattern has been highlighted in the African monsoon system with an approximate wavelength of 7500 km, a phase speed of 6 m s−1, and a period of 15 days. The combined OLR and atmospheric circulation pattern looks like a combination of ER wave solutions with meridional wavenumbers of 1 and 2. Its vertical baroclinic profile suggests that this wave is forced by the deep convective heating. Its initiation in terms of OLR modulation is detected north of Lake Victoria, extending northward and then propagating westward along the Sahel latitudes. The Sahel mode identified in previous studies corresponds to the second main mode of convective variability within the 10–30-day periodicity band, and this has also been examined. Its pattern and evolution look like the first-mode ER wave pattern and they are temporally correlated with a coefficient of +0.6. About one-third of the Sahel mode events are concomitant with an ER wave occurrence. The main difference between these two signals consists of a stronger OLR and circulation modulation of the Sahel mode over East and central Africa. Thus, the Sahel mode occurrence and its westward propagation could be explained in part by atmospheric dynamics associated with the ER waves and in part by land surface interactions, as shown in other studies.

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