Synoptic Kelvin type perturbation waves over Congo basin over the period 1979–2010

Zebaze, Sinclaire; Lenouo, André; Tchawoua, Clément; Janicot, Serge

doi:10.1016/j.jastp.2015.04.015

Cited by 26 publications

(18 citation statements)

References 63 publications

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“…Notably, theoretical studies have revealed that Kelvin waves exhibit additional meridional circulation if the forcing lies off the equator (Gill, 1980;Dias and Pauluis, 2009). Consistent with observations for equatorial Africa (Mounier et al, 2007;Sinclaire et al, 2015), this study confirms that Kelvin waves also affect northerly moisture influx during the wet phase of the wave (Fig. 4).…”

Section: Circulation Patterns and Moisture Modulationsupporting

confidence: 88%

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A Systematic Comparison of Tropical Waves over Northern Africa. Part II: Dynamics and Thermodynamics

2019

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This study presents the first systematic comparison of the (thermo-)dynamics associated with all major tropical wave types causing rainfall modulation over northern tropical Africa: Madden Julian Oscillation (MJO), Equatorial Rossby waves (ERs), mixed Rossby-gravity waves (MRGs), Kelvin waves, tropical disturbances (TDs, including African Easterly Waves), and eastward inertio-gravity waves (EIGs). Reanalysis and radiosonde data were analyzed for the period 1981-2013 based on space-time filtering of outgoing longwave radiation. The identified circulation patterns are largely consistent with theory. The slow modes, MJO and ER, mainly impact precipitable water, whereas the faster Kelvin waves, MRGs, and TDs primarily modulate moisture convergence. Monsoonal inflow intensifies during wet phases of the MJO, ERs, and MRGs, associated with a northward shift of the intertropical discontinuity for MJO and ERs. During passages of vertically tilted imbalanced wave modes, such as MJO, Kelvin waves, and TDs, and partly MRGs, increased vertical wind shear and improved conditions for up-and downdrafts facilitate the organization of convection. The balanced ERs are not tilted and rainfall is triggered by large-scale moistening and stratiform lifting. The MJO and ERs interact with intraseasonal variations of the Indian monsoon and extratropical Rossby wave trains. The latter causes a trough over the Atlas Mountains associated with a tropical plume and rainfall over the Sahara. Positive North Atlantic and Arctic Oscillation signals precede tropical plumes in case of the MJO. The results unveil which dynamical processes need to be modeled realistically to represent the coupling between tropical waves and rainfall in northern tropical Africa.

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Section: Circulation Patterns and Moisture Modulationsupporting

confidence: 88%

“…Duvel, 1990;Fink and Reiner, 2003;Janiga and Thorncroft, 2016;Maranan et al, 2018) and Kelvin waves (e.g. Mounier et al, 2007;Nguyen and Duvel, 2008;Laing et al, 2011;Sinclaire et al, 2015). The effect of MRG and Kelvin waves on organization was documented by Holder et al (2008).…”

Section: Vertical Structure and Mechanism Of Rainfall Modulationmentioning

confidence: 97%

A Systematic Comparison of Tropical Waves over Northern Africa. Part II: Dynamics and Thermodynamics

2019

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“…Since that time, the number of studies of the region's meteorology has continually increased. Most have centered on synoptic systems, convection, and other intraseasonal phenomena (Nguyen and Duvel 2008;Jackson et al 2009;Gu 2009;Vondou et al 2010a,b;Laing et al 2011;Sandjon et al 2012Sandjon et al , 2014aKamsu-Tamo et al 2014;Sinclaire et al 2015;Berhane et al 2015;Soula et al 2016;Zebaze et al 2017). Only a handful have focused on the prevailing general circulation (Nicholson and Grist 2003;Pokam et al 2012Pokam et al , 2014Dezfuli et al 2015;Neupane 2016;Cook and Vizy 2016) and even the factors producing the seasonal cycle are poorly understood (Nicholson 2018).…”

Section: Introductionmentioning

confidence: 99%

Validation of Satellite Precipitation Estimates over the Congo Basin

Nicholson

Klotter

Zhou

et al. 2019

Journal of Hydrometeorology

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This paper evaluates nine satellite rainfall products and the Global Precipitation Centre Climatology (GPCC) gauge dataset over the Congo basin. For the evaluation the reference dataset is a newly created, gridded gauge dataset based on a gauge network that is more complete than that of GPCC in recent years. It is termed NIC131-gridded. Gridding was achieved via a climatic reconstruction method based on principal components, so that reliable estimates of rainfall are available even in the data-sparse central basin. The satellite products were evaluated for two locations, the Congo basin and areas on its eastern and western periphery (termed the “east plus west” sector). The station density was notably higher in the latter region. Two time periods were also considered: 1983–94, when station density was relatively high, and 1998–2010, when station density was much lower than during the earlier period. Several products show excellent agreement with the NIC131-gridded reference dataset. These include CHIRPS2, PERSIANN-CDR, GPCP 2.3, TRMM 3B43, and, to a lesser extent, GPCC V7. RMSE for the period 1983–94 in the east plus west sector is on the order of 20 mm month−1 for GPCC V7 and 20–30 mm month−1 for the other products. The compares with 40–60 mm month−1 for the most poorly performing products, African Rainfall Climatology version 2 (ARCv2) and CMAP. Over the Congo basin, RMSE for those two products is about the same as in the east plus west sector but is on the order of 30–40 mm month−1 for the better-performing products. In all cases, the performance of the 10 products evaluated is notably poorer in recent years (1998–2010), when the station network is sparse, than during the period 1983–94, when the dense station network provides reliable estimates of rainfall. For the more recent period RMSE is on the order of 30–40 mm month−1 for the best-performing products in the east plus west sector but only slightly higher over the Congo basin. All products do reasonably well in reproducing the seasonal cycle and the latitudinal gradients of rainfall. Estimates of interannual variability show more scatter among the various products and are less reliable. Overall, the most important results of the study are to demonstrate the strong impact that actual gauge data have on the various products and the need to have access to such gauge data, in order to produce reliable rainfall estimates from satellites.

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“…Sinclaire et al . (2015) noted that during March–June, eastward propagating CCKWs favour initiation of synoptic‐scale convective systems and that annually, an average of six to seven CCKWs propagate through the Congo basin in this period. While all these studies used OLR to identify CCKWs, we take a unique approach of identifying the CCKWs, using a novel dynamics‐based equatorial wave dataset.…”

Section: Introductionmentioning

confidence: 99%

“…The resulting latent heat released by the deep convection associated with the perturbed fields can cause the generation of new equatorial waves (e.g., Lindzen, 2003). As a wave propagates, it interacts with convection through, for example, enhancing low‐level moisture flux (e.g., Mekonnen et al ., 2008; Sinclaire et al ., 2015) and influencing convection and precipitation downstream, and this may cause a variation in its speed of propagation.…”

Section: Introductionmentioning

confidence: 99%

The observed synoptic scale precipitation relationship between Western Equatorial Africa and Eastern Equatorial Africa

Ayesiga

Holloway

Williams

et al. 2020

Intl Journal of Climatology

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An improvement to subseasonal (i.e., days to weeks) rainfall prediction across Equatorial Africa is an important area of current research. This is because most countries in this region are highly dependent on rain-fed agriculture, and so millions of livelihoods are at risk in the event of an unexpected poor harvest. This study examines 16 years of daily precipitation anomalies to investigate the relationship in precipitation between Western Equatorial Africa (WEA) and Eastern Equatorial Africa (EEA). Using lead/lag correlation and spatio-temporal correlation patterns over various sub-regions, a synoptic-scale relationship in precipitation is presented between WEA and EEA, in which precipitation over EEA lags precipitation over WEA by 1-2 days. In addition, central WEA and sub-regions in South Sudan display a synoptic-timescale precipitation contrast, suggesting a weak precipitation dipole. Consistent with the known heterogeneity characteristic of Equatorial Africa's precipitation, our findings suggest that the 1-2 days precipitation relationship is dependent upon the sub-region under investigation. Furthermore, our results indicate a coherent synoptic-timescale eastward/northeastward propagating signal with a speed of approximately 12 m/s. Composite and correlation analyses of precipitation anomalies and a novel equatorial wave dataset show an apparent connection between eastward/northeastward propagating wet anomalies and Kelvin wave lower-tropospheric convergence. This suggests that Convectively Coupled Kelvin Waves (CCKWs) play a role in modulating the 1-2 days convection and precipitation propagation between WEA and EEA. These results imply that monitoring the propagation characteristics of CCKWs may be important in synoptic-timescale forecasting over Equatorial Africa.

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Synoptic Kelvin type perturbation waves over Congo basin over the period 1979–2010

Cited by 26 publications

References 63 publications

A Systematic Comparison of Tropical Waves over Northern Africa. Part II: Dynamics and Thermodynamics

A Systematic Comparison of Tropical Waves over Northern Africa. Part II: Dynamics and Thermodynamics

Validation of Satellite Precipitation Estimates over the Congo Basin

The observed synoptic scale precipitation relationship between Western Equatorial Africa and Eastern Equatorial Africa

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