Southern African monsoon: intraseasonal variability and monsoon indices

Abstract Recent observational studies have suggested a role for soil moisture and land–atmosphere coupling in the 15-day westward-propagating mode of intraseasonal variability in the West African monsoon. This hypothesis is investigated with a set of three atmospheric general circulation model experiments. 1) When soil moisture is fully coupled with the atmospheric model, the 15-day mode of land–atmosphere variability is clearly identified. Precipitation anomalies lead soil moisture anomalies by 1–2 days, similar to the results from satellite observations. 2) To assess whether soil moisture is merely a passive response to the precipitation, or an active participant in this mode, the atmospheric model is forced with a 15-day westward-propagating cycle of regional soil moisture anomalies based on the fully coupled mode. Through a reduced surface sensible heat flux, the imposed wet soil anomalies induce negative low-level temperature anomalies and increased pressure (a cool high). An anticyclonic circulation then develops around the region of wet soil, enhancing northward moisture advection and precipitation to the west. Hence, in a coupled framework, this soil moisture–forced precipitation response would provide a self-consistent positive feedback on the westward-propagating soil moisture anomaly and implies an active role for soil moisture. 3) In a final sensitivity experiment, soil moisture is again externally prescribed but with all intraseasonal fluctuations suppressed. In the absence of soil moisture variability there are still pronounced surface sensible heat flux variations, likely due to cloud changes, and the 15-day westward-propagating precipitation signal is still present. However, it is not as coherent as in the previous experiments when interaction with soil moisture was permitted. Further examination of the soil moisture forcing experiment in GCM experiment 2 shows that this precipitation mode becomes phase locked to the imposed soil moisture anomalies. Hence, the 15-day westward-propagating mode in the West African monsoon can exist independently of soil moisture; however, soil moisture and land–atmosphere coupling act to feed back on the atmosphere and further enhance and organize it.

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Internal Intraseasonal Variability of the West African Monsoon in WRF

Journal of Climate ◽

10.1175/jcli-d-16-0750.1 ◽

2017 ◽

Vol 30 (15) ◽

pp. 5815-5833 ◽

Cited By ~ 2

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.

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The West African Monsoon Dynamics. Part I: Documentation of Intraseasonal Variability

Journal of Climate ◽

10.1175/1520-0442(2003)016<3389:twamdp>2.0.co;2 ◽

2003 ◽

Vol 16 (21) ◽

pp. 3389-3406 ◽

Cited By ~ 146

Author(s):

Benjamin Sultan ◽

Serge Janicot ◽

Arona Diedhiou

Keyword(s):

Intraseasonal Variability ◽

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon ◽

Monsoon Dynamics

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Influence of the Mediterranean Sea on the West African monsoon: Intraseasonal variability in numerical simulations

Journal of Geophysical Research Atmospheres ◽

10.1029/2010jd014436 ◽

2010 ◽

Vol 115 (D24) ◽

Cited By ~ 43

Author(s):

Marco Gaetani ◽

Bernard Fontaine ◽

Pascal Roucou ◽

Marina Baldi

Keyword(s):

Numerical Simulations ◽

Mediterranean Sea ◽

Intraseasonal Variability ◽

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon ◽

The Mediterranean

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West African Monsoon Intraseasonal Variability: A Precipitable Water Perspective

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-087.1 ◽

2013 ◽

Vol 70 (4) ◽

pp. 1035-1052 ◽

Cited By ~ 16

Author(s):

D. Emmanuel Poan ◽

Romain Roehrig ◽

Fleur Couvreux ◽

Jean-Philippe Lafore

Keyword(s):

Life Cycle ◽

Intraseasonal Variability ◽

West African Monsoon ◽

Precipitable Water ◽

West African ◽

African Monsoon ◽

Easterly Waves ◽

Western Sahel ◽

Northern Side ◽

Diabatic Processes

Abstract West African monsoon intraseasonal variability has important implications for food security and drought early warnings. In the present study, intraseasonal variability over the Sahel is assessed from the perspective of precipitable water, as provided by model reanalyses and GPS measurements. In the eastern Sahel, precipitable water variability is dominated by time scales longer than 10 days, whereas synoptic scales dominate in the western Sahel, especially because of African easterly waves (AEWs). The present work then focuses on the moisture footprint of AEWs along the northern side of the African easterly jet, as detected and analyzed directly from the main synoptic disturbances associated with precipitable water. Composite wet and dry precipitable water anomalies within AEWs propagate westward with a 5–6-day period. Their robustness, consistency, and spatial footprint, as well as their significant modulation of the convective activity, imply potential skill for short- to medium-range forecasts of wet and dry events over the Sahel. A composite moisture budget points out the key processes involved in the evolution of moisture anomalies. Advection processes are shown to be dominant during their life cycle. A linear adiabatic analysis of the propagation and growth of AEW precipitable water anomalies captures the main observed properties well, even though a key role of diabatic processes such as rain evaporation is needed to fully understand the life cycle of such precipitable water anomalies, especially their growth over the continent.

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Intraseasonal variability of the West African monsoon

Atmospheric Science Letters ◽

10.1002/asl.280 ◽

2011 ◽

Vol 12 (1) ◽

pp. 58-66 ◽

Cited By ~ 63

Author(s):

S. Janicot ◽

G. Caniaux ◽

F. Chauvin ◽

G. de Coëtlogon ◽

B. Fontaine ◽

...

Keyword(s):

Intraseasonal Variability ◽

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon

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Anthropogenic warming and intraseasonal summer monsoon variability amplify the risk of future flash droughts in India

npj Climate and Atmospheric Science ◽

10.1038/s41612-020-00158-3 ◽

2021 ◽

Vol 4 (1) ◽

Cited By ~ 1

Author(s):

Vimal Mishra ◽

Saran Aadhar ◽

Shanti Shwarup Mahto

Keyword(s):

Soil Moisture ◽

Summer Monsoon ◽

Crop Production ◽

Monsoon Rainfall ◽

Root Zone ◽

Monsoon Season ◽

Intraseasonal Variability ◽

Positive Temperature ◽

Groundwater Abstraction ◽

Increased Risk

AbstractFlash droughts cause rapid depletion in root-zone soil moisture and severely affect crop health and irrigation water demands. However, their occurrence and impacts in the current and future climate in India remain unknown. Here we use observations and model simulations from the large ensemble of Community Earth System Model to quantify the risk of flash droughts in India. Root-zone soil moisture simulations conducted using Variable Infiltration Capacity model show that flash droughts predominantly occur during the summer monsoon season (June–September) and driven by the intraseasonal variability of monsoon rainfall. Positive temperature anomalies during the monsoon break rapidly deplete soil moisture, which is further exacerbated by the land-atmospheric feedback. The worst flash drought in the observed (1951–2016) climate occurred in 1979, affecting more than 40% of the country. The frequency of concurrent hot and dry extremes is projected to rise by about five-fold, causing approximately seven-fold increase in flash droughts like 1979 by the end of the 21st century. The increased risk of flash droughts in the future is attributed to intraseasonal variability of the summer monsoon rainfall and anthropogenic warming, which can have deleterious implications for crop production, irrigation demands, and groundwater abstraction in India.

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