scholarly journals The Annual Cycle of East African Precipitation

2015 ◽  
Vol 28 (6) ◽  
pp. 2385-2404 ◽  
Author(s):  
Wenchang Yang ◽  
Richard Seager ◽  
Mark A. Cane ◽  
Bradfield Lyon
Keyword(s):  
East Africa ◽  
Annual Cycle ◽  
Lower Troposphere ◽  
Moisture Flux ◽  
Convective Stability ◽  
East African ◽  
Coupled Models ◽  
Annual Cycles ◽  
Short Rains ◽  
Static Energy

Abstract East African precipitation is characterized by a dry annual mean climatology compared to other deep tropical land areas and a bimodal annual cycle with the major rainy season during March–May (MAM; often called the “long rains”) and the second during October–December (OND; often called the “short rains”). To explore these distinctive features, ERA-Interim data are used to analyze the associated annual cycles of atmospheric convective stability, circulation, and moisture budget. The atmosphere over East Africa is found to be convectively stable in general year-round but with an annual cycle dominated by the surface moist static energy (MSE), which is in phase with the precipitation annual cycle. Throughout the year, the atmospheric circulation is dominated by a pattern of convergence near the surface, divergence in the lower troposphere, and convergence again at upper levels. Consistently, the convergence of the vertically integrated moisture flux is mostly negative across the year, but becomes weakly positive in the two rainy seasons. It is suggested that the semiarid/arid climate in East Africa and its bimodal precipitation annual cycle can be explained by the ventilation mechanism, in which the atmospheric convective stability over East Africa is controlled by the import of low MSE air from the relatively cool Indian Ocean off the coast. During the rainy seasons, however, the off-coast sea surface temperature (SST) increases (and is warmest during the long rains season) and consequently the air imported into East Africa becomes less stable. This analysis may be used to aid in understanding overestimates of the East African short rains commonly found in coupled models.

scholarly journals The Moderate Impact of the 2015 El Niño over East Africa and Its Representation in Seasonal Reforecasts

2019 ◽  
Vol 32 (22) ◽  
pp. 7989-8001 ◽  
Author(s):  
David MacLeod ◽  
Cyril Caminade
Keyword(s):  
Indian Ocean ◽  
East Africa ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Lower Troposphere ◽  
Eastern Indian Ocean ◽  
East African ◽  
The Indian Ocean ◽  
Short Rains

Abstract El Niño–Southern Oscillation (ENSO) has large socioeconomic impacts worldwide. The positive phase of ENSO, El Niño, has been linked to intense rainfall over East Africa during the short rains season (October–December). However, we show here that during the extremely strong 2015 El Niño the precipitation anomaly over most of East Africa during the short rains season was less intense than experienced during previous El Niños, linked to less intense easterlies over the Indian Ocean. This moderate impact was not indicated by reforecasts from the ECMWF operational seasonal forecasting system, SEAS5, which instead forecast large probabilities of an extreme wet signal, with stronger easterly anomalies over the surface of the Indian Ocean and a colder eastern Indian Ocean/western Pacific than was observed. To confirm the relationship of the eastern Indian Ocean to East African rainfall in the forecast for 2015, atmospheric relaxation experiments are carried out that constrain the east Indian Ocean lower troposphere to reanalysis. By doing so the strong wet forecast signal is reduced. These results raise the possibility that link between ENSO and Indian Ocean dipole events is too strong in the ECMWF dynamical seasonal forecast system and that model predictions for the East African short rains rainfall during strong El Niño events may have a bias toward high probabilities of wet conditions.

scholarly journals The Rainfall Annual Cycle Bias over East Africa in CMIP5 Coupled Climate Models

2015 ◽  
Vol 28 (24) ◽  
pp. 9789-9802 ◽  
Author(s):  
Wenchang Yang ◽  
Richard Seager ◽  
Mark A. Cane ◽  
Bradfield Lyon
Keyword(s):  
East Africa ◽  
Annual Cycle ◽  
Climate Models ◽  
Western Indian Ocean ◽  
Shortwave Radiation ◽  
Coupled Models ◽  
Near Surface ◽  
Historical Simulation ◽  
Sst Bias ◽  
Short Rains

Abstract East Africa has two rainy seasons: the long rains [March–May (MAM)] and the short rains [October–December (OND)]. Most CMIP3/5 coupled models overestimate the short rains while underestimating the long rains. In this study, the East African rainfall bias is investigated by comparing the coupled historical simulations from CMIP5 to the corresponding SST-forced AMIP simulations. Much of the investigation is focused on the MRI-CGCM3 model, which successfully reproduces the observed rainfall annual cycle in East Africa in the AMIP experiment but its coupled historical simulation has a similar but stronger bias as the coupled multimodel mean. The historical–AMIP monthly climatology rainfall bias in East Africa can be explained by the bias in the convective instability (CI), which is dominated by the near-surface moisture static energy (MSE) and ultimately by the MSE’s moisture component. The near-surface MSE bias is modulated by the sea surface temperature (SST) over the western Indian Ocean. The warm SST bias in OND can be explained by both insufficient ocean dynamical cooling and latent flux, while the insufficient shortwave radiation and excess latent heat flux mainly contribute to the cool SST bias in MAM.

scholarly journals Biases in CMIP5 Sea Surface Temperature and the Annual Cycle of East African Rainfall

2020 ◽  
Vol 33 (19) ◽  
pp. 8209-8223
Author(s):  
Bradfield Lyon
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Annual Cycle ◽  
General Circulation ◽  
Regional Scale ◽  
Sea Surface ◽  
East African ◽  
Coupled Models ◽  
African Rainfall ◽  
Short Rains

AbstractIn much of East Africa, climatological rainfall follows a bimodal distribution characterized by the long rains (March–May) and short rains (October–December). Most CMIP5 coupled models fail to properly simulate this annual cycle, typically reversing the amplitudes of the short and long rains relative to observations. This study investigates how CMIP5 climatological sea surface temperature (SST) biases contribute to simulation errors in the annual cycle of East African rainfall. Monthly biases in CMIP5 climatological SSTs (50°S–50°N) are first identified in historical runs (1979–2005) from 31 models and examined for consistency. An atmospheric general circulation model (AGCM) is then forced with observed SSTs (1979–2005) generating a set of control runs and observed SSTs plus the monthly, multimodel mean SST biases generating a set of “bias” runs for the same period. The control runs generally capture the observed annual cycle of East African rainfall while the bias runs capture prominent CMIP5 annual cycle biases, including too little (much) precipitation during the long rains (short rains) and a 1-month lag in the peak of the long rains relative to observations. Diagnostics reveal the annual cycle biases are associated with seasonally varying north–south- and east–west-oriented SST bias patterns in Indian Ocean and regional-scale atmospheric circulation and stability changes, the latter primarily associated with changes in low-level moist static energy. Overall, the results indicate that CMIP5 climatological SST biases are the primary driver of the improper simulation of the annual cycle of East African rainfall. Some implications for climate change projections are discussed.

scholarly journals Validation of IMERG Precipitation in Africa

2017 ◽  
Vol 18 (10) ◽  
pp. 2817-2825 ◽  
Author(s):  
Amin K. Dezfuli ◽  
Charles M. Ichoku ◽  
George J. Huffman ◽  
Karen I. Mohr ◽  
John S. Selker ◽  
...  
Keyword(s):  
East Africa ◽  
Annual Cycle ◽  
Lake Victoria ◽  
Mountainous Regions ◽  
Meteorological Observatory ◽  
Precipitation Analysis ◽  
In Situ Observations ◽  
Short Rains ◽  
Good Agreement

Abstract Understanding of hydroclimatic processes in Africa has been hindered by the lack of in situ precipitation measurements. Satellite-based observations, in particular, the TRMM Multisatellite Precipitation Analysis (TMPA) have been pivotal to filling this void. The recently released Integrated Multisatellite Retrievals for GPM (IMERG) project aims to continue the legacy of its predecessor, TMPA, and provide higher-resolution data. Here, IMERG-V04A precipitation data are validated using in situ observations from the Trans-African Hydro-Meteorological Observatory (TAHMO) project. Various evaluation measures are examined over a select number of stations in West and East Africa. In addition, continent-wide comparisons are made between IMERG and TMPA. The results show that the performance of the satellite-based products varies by season, region, and the evaluation statistics. The precipitation diurnal cycle is relatively better captured by IMERG than TMPA. Both products exhibit a better agreement with gauge data in East Africa and humid West Africa than in the southern Sahel. However, a clear advantage for IMERG is not apparent in detecting the annual cycle. Although all gridded products used here reasonably capture the annual cycle, some differences are evident during the short rains in East Africa. Direct comparison between IMERG and TMPA over the entire continent reveals that the similarity between the two products is also regionally heterogeneous. Except for Zimbabwe and Madagascar, where both satellite-based observations present a good agreement, the two products generally have their largest differences over mountainous regions. IMERG seems to have achieved a reduction in the positive bias evident in TMPA over Lake Victoria.

scholarly journals Reconciling Theories for Human and Natural Attribution of Recent East Africa Drying

2017 ◽  
Vol 30 (6) ◽  
pp. 1939-1957 ◽  
Author(s):  
Andrew Hoell ◽  
Martin Hoerling ◽  
Jon Eischeid ◽  
Xiao-Wei Quan ◽  
Brant Liebmann
Keyword(s):  
Global Warming ◽  
East Africa ◽  
Decadal Variability ◽  
Coupled Model ◽  
Western Pacific ◽  
East African ◽  
Coupled Models ◽  
Central Pacific ◽  
Model Simulations ◽  
African Rainfall

Abstract Two theories for observed East Africa drying trends during March–May 1979–2013 are reconciled. Both hypothesize that variations in tropical sea surface temperatures (SSTs) caused East Africa drying. The first invokes a mainly human cause resulting from sensitivity to secular warming of Indo–western Pacific SSTs. The second invokes a mainly natural cause resulting from sensitivity to a strong articulation of ENSO-like Pacific decadal variability involving warming of the western Pacific and cooling of the central Pacific. Historical atmospheric model simulations indicate that observed SST variations contributed significantly to the East Africa drying trend during March–May 1979–2013. By contrast, historical coupled model simulations suggest that external radiative forcing alone, including the ocean’s response to that forcing, did not contribute significantly to East Africa drying. Recognizing that the observed SST variations involved a commingling of natural and anthropogenic effects, this study diagnosed how East African rainfall sensitivity was conditionally dependent on the interplay of those factors. East African rainfall trends in historical coupled models were intercompared between two composites of ENSO-like decadal variability, one operating in the early twentieth century before appreciable global warming and the other in the early twenty-first century of strong global warming. The authors find the coaction of global warming with ENSO-like decadal variability can significantly enhance 35-yr East Africa drying trends relative to when the natural mode of ocean variability acts alone. A human-induced change via its interplay with an extreme articulation of natural variability may thus have been key to Africa drying; however, these results are speculative owing to differences among two independent suites of coupled model ensembles.

scholarly journals Impact of Annual Cycle on ENSO Variability and Predictability

2021 ◽  
Vol 34 (1) ◽  
pp. 171-193
Author(s):  
Sang-Ik Shin ◽  
Prashant D. Sardeshmukh ◽  
Matthew Newman ◽  
Cecile Penland ◽  
Michael A. Alexander
Keyword(s):  
Annual Cycle ◽  
Phase Locking ◽  
Sea Surface ◽  
Coupled Models ◽  
Stochastic Forcing ◽  
Annual Cycles ◽  
The North ◽  
Linear Inverse Model ◽  
The Impact ◽  
Background State

AbstractLow-order linear inverse models (LIMs) have been shown to be competitive with comprehensive coupled atmosphere–ocean models at reproducing many aspects of tropical oceanic variability and predictability. This paper presents an extended cyclostationary linear inverse model (CS-LIM) that includes the annual cycles of the background state and stochastic forcing of tropical sea surface temperature (SST) and sea surface height (SSH) anomalies. Compared to a traditional stationary LIM that ignores such annual cycles, the CS-LIM is better at representing the seasonal modulation of ENSO-related SST anomalies and their phase locking to the annual cycle. Its deterministic as well as probabilistic hindcast skill is comparable to the skill of the North American Multimodel Ensemble (NMME) of comprehensive global coupled models. The explicit inclusion of annual-cycle effects in the CS-LIM improves the forecast skill of both SST and SSH anomalies through SST–SSH coupling. The impact on the SSH skill is particularly marked at longer forecast lead times over the western Pacific and in the vicinity of the Pacific North Equatorial Countercurrent (NECC), consistent with westward propagating oceanic Rossby waves that reflect off the western boundaries as eastward propagating Kelvin waves and influence El Niño development in the region. The higher CS-LIM skill is thus associated with the improved representation of both ENSO phase-locking and Pacific NECC variations. These improvements result from explicitly accounting for not only the annual cycle of the background state, but also that of the stochastic forcing.

Modulation of Daily Precipitation over East Africa by the Madden–Julian Oscillation*

2014 ◽  
Vol 27 (15) ◽  
pp. 6016-6034 ◽  
Author(s):  
Fisseha Berhane ◽  
Benjamin Zaitchik
Keyword(s):  
East Africa ◽  
Rainy Season ◽  
Large Scale ◽  
Spatiotemporal Variability ◽  
Climate Projections ◽  
Madden Julian Oscillation ◽  
East African ◽  
Short Rainy Season ◽  
Total Variability ◽  
Short Rains

Abstract Spatiotemporal variability in East African precipitation affects the livelihood of tens of millions of people. From the perspective of floods, flash droughts, and agriculture, variability on intraseasonal time scales is a critical component of total variability. The principal objective of this study is to explore subseasonal impacts of the Madden–Julian oscillation (MJO) on tropospheric circulations affecting East Africa (EA) during the long (March–May) and short (October–December) rains and associated variability in precipitation. Analyses are performed for 1979–2012 for dynamics and 1998–2012 for precipitation. Consistent with previous studies, significant MJO influence is found on wet and dry spells during the long and short rains. This influence, however, is found to vary within each season. Specifically, indices of MJO convection at 70°–80°E and 120°W are strongly associated with precipitation variability across much of EA in the early (March) and late (May) long rainy season and in the middle and late (November–December) short rainy season. In the early short rains (October) a different pattern emerges, in which MJO strength at 120°E (10°W) is associated with dry (wet) spells in coastal EA but not the interior. In April the MJO influence on precipitation is obscured but can be diagnosed in lead time associations. This diversity of influences reflects a diversity of mechanisms of MJO influence, including dynamic and thermodynamic mechanisms tied to large-scale atmospheric circulations and localized dynamics associated with MJO modulation of the Somali low-level jet. These differences are relevant to problems of subseasonal weather forecasts and climate projections for EA.

The War of Legs

Transfers ◽  
2015 ◽  
Vol 5 (2) ◽  
pp. 102-120
Author(s):  
Michael Pesek
Keyword(s):  
World War I ◽  
Medical Care ◽  
East Africa ◽  
Transport Infrastructure ◽  
World War ◽  
East African ◽  
Colonial State ◽  
History Of ◽  
Main Factors ◽  
Belgian Congo

This article describes the little-known history of military labor and transport during the East African campaign of World War I. Based on sources from German, Belgian, and British archives and publications, it considers the issue of military transport and supply in the thick of war. Traditional histories of World War I tend to be those of battles, but what follows is a history of roads and footpaths. More than a million Africans served as porters for the troops. Many paid with their lives. The organization of military labor was a huge task for the colonial and military bureaucracies for which they were hardly prepared. However, the need to organize military transport eventually initiated a process of modernization of the colonial state in the Belgian Congo and British East Africa. This process was not without backlash or failure. The Germans lost their well-developed military transport infrastructure during the Allied offensive of 1916. The British and Belgians went to war with the question of transport unresolved. They were unable to recruit enough Africans for military labor, a situation made worse by failures in the supplies by porters of food and medical care. One of the main factors that contributed to the success of German forces was the Allies' failure in the “war of legs.”

scholarly journals Crustal variability along the rifted/sheared East African margin: a review

2021 ◽  
Vol 41 (2) ◽  
Author(s):  
Maren Vormann ◽  
Wilfried Jokat
Keyword(s):  
East Africa ◽  
Shear Zone ◽  
Structural Changes ◽  
The South ◽  
East African ◽  
The North ◽  
Rifted Margins ◽  
Continental Block ◽  
Seismic Lines ◽  
Southeastern Madagascar

AbstractThe East African margin between the Somali Basin in the north and the Natal Basin in the south formed as a result of the Jurassic/Cretaceous dispersal of Gondwana. While the initial movements between East and West Gondwana left (oblique) rifted margins behind, the subsequent southward drift of East Gondwana from 157 Ma onwards created a major shear zone, the Davie Fracture Zone (DFZ), along East Africa. To document the structural variability of the DFZ, several deep seismic lines were acquired off northern Mozambique. The profiles clearly indicate the structural changes along the shear zone from an elevated continental block in the south (14°–20°S) to non-elevated basement covered by up to 6-km-thick sediments in the north (9°–13°S). Here, we compile the geological/geophysical knowledge of five profiles along East Africa and interpret them in the context of one of the latest kinematic reconstructions. A pre-rift position of the detached continental sliver of the Davie Ridge between Tanzania/Kenya and southeastern Madagascar fits to this kinematic reconstruction without general changes of the rotation poles.

Sign in / Sign up

Export Citation Format

Share Document