Analysis of Strengthening and Dissipating Mesoscale Convective Systems Propagating off the West African Coast

Abstract

International audienceA large number of Atlantic tropical depressions are generated in the eastern basin in relation to the African easterly wave (AEW) and embedded mesoscale convective systems (MCSs) coming from the African continent. In this paper, the structures of strengthening and dissipating MCSs evolving near the West African coast are analyzed, including the role of the ocean surface conditions in their evolution.Satellite infrared brightness temperature and meteorological radar data over seven summer seasons between 1993 and 2006 are used to subjectively select 20 cases of strengthening and dissipating MCSs in the vicinity of the Senegal coast. With these observed MCSs, a lagged composite analysis is then performed using Interim ECMWF Re-Analysis (ERA-Interim) and Climate Forecast System Reanalysis (CFSR).It is shown that the strengthening MCS is generally preceded by prior passage of an AEW near the West African coast. This previous wave trough is associated with a convective cyclonic circulation in the low and middle troposphere, which enhances the southwesterly flow and then provides humidity to the strengthening MCS, located in the vicinity of the subsequent AEW trough. This is favored by the contraction of the wavelength associated with the two troughs. The sea surface contributes to the MCS enhancement through surface evaporation flux. But this contribution is found to be less important than advection of humidity from the previous wave trough. These conditions are almost not found in the dissipating MCS cases, which dissipate in a dry environment dominated by a subsident and anticyclonic circulation, with generally no interaction with a previous wave trough

Intraseasonal variability of tropical Atlantic sea‐surface temperature: air–sea interaction over upwelling fronts

“…In addition, no significant gradient in SST or SLP anomalies (not shown) are found, meaning that no LN mechanism could be detected. The wind anomalies appear therefore controlled by the large‐scale atmospheric circulation in the North Atlantic in agreement with the results found in Dieng et al (). Note that this large‐scale circulation in the North Atlantic could be connected to the NAO intraseasonal oscillation.…”

On the spatial coherence of rainfall over the Saloum delta (Senegal) from seasonal to decadal time scales

“…They are controlled on the one hand by internal atmospheric dynamics; in particular the development of synoptic scale African Easterly Waves (AEW; Reed et al, 1977;Duvel, 1990;Thorncroft and Hoskins, 1994a,b;Diedhiou et al, 1999;Kiladis et al, 2006) or synoptic scale Kelvin waves (Mounier et al, 2007). Monsoon flux and convection are significantly modulated by these waves (e.g., Fink and Reiner, 2003;Dieng et al, 2014).…”

Strong Ocean–Atmosphere Interactions during a Short-Term Hot Event over the Western Pacific Warm Pool in Response to El Niño

“…A comparison of Figs. 4, 5, and 6a suggests that the easterly wave is collocated with the westward-moving cloud clusters, which is similar to the features over the Atlantic Ocean (Payne and McGarry 1977;Machado et al 1993;Dieng et al 2014). An analysis of the geopotential height anomaly (not shown) further shows that the easterly wave is gradually deepening on 22-24 November when the cloud clusters grow strong.…”