African easterly waves (AEWs) are known precursors to tropical cyclone (TC) formation, although it is not always clear which AEWs will develop and which AEWs will not. To investigate AEW evolution, this study examines novel observations from the geostationary Advanced Baseline Imager (ABI) during July-September 2019. Case studies are conducted for two AEWs: one that became Hurricane Dorian, the strongest and most devastating hurricane of the 2019 Atlantic hurricane season, and a long-lived September AEW that did not become a TC. Lower-level moisture and flow, and the strength and spatial distribution of convective activity, differed between these two waves. By then exploring these characteristics for additional developing and non-developing AEWs, ABI observations show that developing AEWs are associated with low-level moist air ahead of the wave combined with enhanced convective activity, while non-developing AEWs tend to encounter drier air and exhibit a persistently broader structure with less-organized convection.

It has been proposed that the Asian-Australasian monsoon system is influenced by large-scale sea-surface temperature (SST) variability in the three tropical oceans although how this influence is manifested has remained a largely open question. Closure of this issue is important because it is needed to explain trends in monsoon precipitation and circulation that have occurred in the last 30 years. Using an atmospheric general circulation model, we run a series of experiments with different configurations of global SST relating to various epochs occurring during the last century to evaluate their influence on the monsoon. Comparisons of circulation fields show that a colder SST configuration generates a weaker large-scale monsoonal circulation. On the other hand, warmer SST states generate stronger large scale circulations with more vigorous centers of divergence and convergence. Warmer SST configurations are associated with positive anomalies of precipitation in the eastern Bay of Bengal, Eastern Indian Ocean and South East Asia. Cooler SST configurations are associated with negative anomalies of precipitation in the Arabian Sea and Indian peninsula, especially at the beginning of the summer. Since SST gradients determine, to a large degree, the low level flow, they are also going to influence the transport of atmospheric moisture. Comparison of vertically integrated moisture transport fields between the different experiments show that cold SST configuration favors an increased inter-hemispheric flow of moisture but decreases in the westerly moisture flow in to the Bay of Bengal and India. Warm SST configurations, on the other hand, strengthens westerly flow into the eastern Indian Ocean. An increasing availability of moisture in a region of stronger convergence constitutes a favorable environment for the production of monsoonal precipitation. African easterly waves (AEW) constitute an important component of the African and tropical Atlantic Ocean climate during the boreal summer. An understanding of this component is essential since AEW are closely related with tropical Atlantic storm activity. We adopt an idealized modeling approach using the WRF model initialized with ERA-40 reanalysis data to study the mechanisms that trigger the formation and maintenance of AEW. The model domain includes the African continent, central and eastern Atlantic Ocean and the western Indian Ocean. Experiments are designed to test the relative importance of the thermal effect of the eastern African topography and the influence of the cross-equatorial pressure gradient, induced by the sea surface temperature (SST) on the origins and maintenance of AEW. Topography and SST variation are selectively added and removed. The control experiment shows that the model reproduces many of the mean features observed during the boreal summer. Westward propagating disturbances of 3-8 day period that originate between 30 and 40E at the surface levels and in the mid troposphere are well depicted. In addition, the model provides a reasonable representation of the AEJ. When all topographic features are removed, there is a weakening of the AEJ over land and ocean, however, longitude-time sections of meridional velocity still exhibit westward propagating disturbances that reach the western African coast at the surface and at the jet level with the same 3-8 day period. Spectral analysis of meridional velocity show that the variability associated with AEWs is reduced over East Africa and West Africa at 850-hPa and is reduced west of 20E along the southern flank of the jet and over northern Africa at the jet level. Maximum amplitude of the disturbances occurs right at the coast. The spatial distribution of barotropic and baroclinic energy conversions explains the reduction in AEWs over land and the intensification of these features at the coast. When the zonal SST gradient is removed, a weaker AEJ displaces southward and a weaker monsoon flow ensues. Spectral analysis of meridional velocity displays a variance reduction in the 3-8 day band at the 850-hP a level in western and eastern Africa and at the coast. At the 650-hPa level significant changes are not observed at the latitude of the AEJ (15N), however, a decrease in the variance associated with AEW occurs at the southern flank of the jet. A southward displacement of the jet favors a weakening of the baroclinic energy conversions. Barotropic conversions also appear to be weaker when the SST gradient is removed. The present study suggests that orography plays an important role in determining the variability of meridional wind associated with AEW over Eastern Africa at the lower levels. Further, zonal SST gradients over the Atlantic favor intensification of waves when they reach the coast and the maintenance of disturbances across the Ocean. Also, results could suggest that SST gradients support genesis of AEW just off the coast of Africa.

Earth's radiation budget is directly influenced by aerosols through the absorption of solar radiation and subsequent heating of the atmosphere. Aerosols modulate the hydrological cycle indirectly by modifying cloud properties, precipitation and ocean heat storage. In addition, polluting aerosols impose health risks in local, regional and global scales. In spite of recent advances in the study of aerosols variability, uncertainty in their spatial and temporal distributions still presents a challenge in the understanding of climate variability. For example, aerosol loading varies not only from year to year but also on higher frequency intraseasonal time scales producing strong variability on local and regional scales. An assessment of the impact of aerosol variability requires long period measurements of aerosols at both regional and global scales. The present dissertation compiles a large database of remotely sensed aerosol loading in order to analyze its spatio-temporal variability, and how this load interacts with different variables that characterize the dynamic and thermodynamic states of the environment. Aerosol Index (AI) and Aerosol Optical Depth (AOD) were used as measures of the atmospheric aerosol load. In addition, atmospheric and oceanic satellite observations, and reanalysis datasets is used in the analysis to investigate aerosol-environment interactions. A diagnostic study is conducted to produce global and regional aerosol satellite climatologies, and to analyze and compare the validity of aerosol retrievals. We find similarities and differences between the aerosol distributions over various regions of the globe when comparing the different satellite retrievals. A nonparametric approach is also used to examine the spatial distribution of the recent trends in aerosol concentration. A significant positive trend was found over the Middle East, Arabian Sea and South Asian regions strongly influenced by increases in dust events. Spectral and composite analyses of surface temperature, atmospheric wind, geopotential height, outgoing longwave radiation, water vapor and precipitation together with the climatology of aerosols provide insight on how the variables interact. Different modes of variability, especially in intraseasonal time scales appear as strong modulators of the aerosol distribution. In particular, we investigate how two modes of variability related to the westward propagating synoptic African Easterly Waves of the Tropical Atlantic Ocean affect the horizontal and vertical structure of the environment. The statistical significance of these two modes is tested with the use of two different spectral techniques. The pattern of propagation of aerosol load shows good correspondence with the progression of the atmospheric and oceanic synoptic conditions suitable for dust mobilization over the Atlantic Ocean. We present extensions to previous studies related with dust variability over the Atlantic region by evaluating the performance of the long period satellite aerosol retrievals in determining modes of aerosol variability. Results of the covariability between aerosols-environment motivate the use of statistical regression models to test the significance of the forecasting skill of daily AOD time series. The regression models are calibrated using atmospheric variables as predictors from the reanalysis variables. The results show poor forecasting skill with significant error growing after the 3rd day of the prediction. It is hypothesized that the simplicity of linear models results in an inability to provide a useful forecast.

The spatial and temporal complexity of tropical cyclones (TCs) raises a number of scientific questions regarding their genesis, movement, intensification, and variability. In this dissertation, the principal goal is to determine the current state of predictability for each of these processes. To quantify the current extent of tropical cyclone predictability, we assess probabilistic forecasts from the most advanced global numerical weather prediction system to date, the ECMWF Variable Resolution Ensemble Prediction System (VarEPS). Using a new false alarm clustering technique to maximize the utility of the VarEPS, the ensemble system is shown to provide well-calibrated probabilistic forecasts for TC genesis through a lead-time of one week, and pregenesis track forecasts with similar skill compared to the VarEPS's postgenesis track forecasts. To quantify the predictability of TCs on intraseasonal time scales, forecasts from the ECMWF Monthly Forecast System (ECMFS) are examined for the North Atlantic Ocean. From this assessment, dynamically based forecasts from the ECMFS provide forecast skill exceeding climatology out to weeks three and four for portions of the southern Gulf of Mexico, western Caribbean and the Main Development Region. Forecast skill in these regions is traced to the model's ability to capture correctly the variability in deep-layer vertical wind shear, the relative frequency of easterly waves moving through these regions, and the intraseasonal modulation of the Madden-Julian Oscillation. On interannual time scales, the predictability of TCs is examined by considering their relationship with tropical Atlantic easterly waves. First, a set of easterly wave climatologies for the CFS-R, ERA-Interim, ERA-40, and NCEP/NCAR Reanalysis are developed using a new easterly wave-tracking algorithm. From the reanalysis-derived climatologies, a moderately positive and statistically significant relationship is seen with tropical Atlantic TCs. In relation to large-scale climate modes, the Atlantic Multidecadal Oscillation (AMO) and Atlantic Meridional Mode (AMM) exhibit the strongest positive covariability with Atlantic easterly wave frequency. Besides changes in the number of easterly waves, the intensification efficiency of easterly waves has also been evaluated. These findings offer a plausible physical explanation for the recent increase in the number of NATL TCs, as it has been concomitant with an increasing trend in both the number of tropical Atlantic easterly waves and intensification efficiency. The last component of this dissertation examines how the historical variability in U.S. landfalling TCs has impacted the annual TC tornado record. To reconcile the inhomogeneous, historical tornado record, two statistical tornado models, developed from a set of a priori predictors for TC tornado formation, are used to reconstruct the TC tornado climatology. While the synthetic TC tornado record reflects decadal scale variations in association with the AMO, a comparison of the current warm phase of the AMO with the previous warm phase period shows that the median number of tornadoes per Gulf TC landfall has significantly increased. This change likely reflects the increase in median TC size (by 35%) of Gulf landfalling TCs along with an increased frequency of large TCs at landfall.

We examine the eastern Pacific Ocean Intertropical Convergence Zone (ITCZ) both in its mean state and transient phases using a combined diagnostic, theoretical and numerical modeling approach. We note that the ITCZ is perpetually in a transient state with strong variability occurring on 4-8 day time scales. Transients, about half the amplitude of the mean ITCZ, propagate northwards from the near-equatorial southern hemisphere eventually increasing the convection in the vicinity of the mean ITCZ convection. It is argued that the mean ITCZ is continually inertially unstable with incursions of anticyclonic vorticity advected across the equator resulting in the creation of a divergence-convergence doublet. The low-level convergence generates convection and vortex tube stretching which generates cyclonic vorticity counteracting the northward advection of anticyclonic vorticity. During a cycle, the heating in the mid-troposphere near 10°N oscillated between 6 and 12 K/day at the inertial frequency of the latitude of the mean convection. The shallow meridional circulation, noted in the mean field in other studies, appears to be a result of the transient nature of the ITCZ. It is hypothesized that westward propagating equatorial waves result from the inertial oscillation of the ITCZ. To test that the waves are formed in situ in the eastern Pacific and not remnants of waves propagating from the Atlantic or promoted by the Central and South American orography, several numerical experiments are undertaken using a high-resolution regional model spanning the western Atlantic Ocean and the eastern Pacific. In the control case, the model is initialized at all boundaries with full high-frequency observations. In two additional experiments, these transients are filtered out, and a third experiment is run with the topography over a large part of Central and South America removed. In all experiments, westward propagating waves are formed in the region of high CEPG suggesting that the hypothesis of in situ development may be correct.

O propósito deste estudo foi verificar a contribuição dos Distúrbios Ondulatórios de Leste (DOLs) na precipitação observada no período chuvoso (AMJJ) e avaliar previsões sazonais de chuva obtidas do RegCM3 (Regional Climate Model version 3) no Leste do Nordeste do Brasil (LNB). Para a identificação subjetiva dos DOLs foram utilizadas imagens de satélite no infravermelho e os campos de linhas de corrente e vorticidade relativa em 700 e 850 hPa da reanálise ERA-Interim, nos períodos chuvosos de 2006 a 2010. Cerca de 90% dos cavados observados em 700 hPa estiveram associados aos 116 DOLs que atingiram o LNB. Isto fornece média de 23 DOLs por ano com pequena variabilidade interanual. Foram construídas composições de variáveis meteorológicas desde 2 dias antes (-2) até 2 dias depois (+2) dos DOLs atingirem o LNB. A circulação apresentou anomalia ciclônica e confluente, vorticidade relativa ciclônica e convergência entre os dias -2 e 0 em baixos níveis, principalmente em 1000 hPa. Encontraram-se anomalias negativas de radiação de onda longa emergente (ROLE) e de omega e positivas de umidade relativa. A composição de precipitação indicou que são responsáveis por: 70% ou mais da chuva do período chuvoso do litoral norte de Alagoas (AL) ao leste do Rio Grande do Norte (RN), 60% entre a Zona da Mata de AL e o Agreste do RN, e 50% entre Sergipe e demais áreas do RN. Em média, os DOLs apresentaram período de 5,3 dias, comprimento de onda de 4307 km e velocidade de fase de 9,5 m s-1. O RegCM3 simulou a observada propagação para oeste dos cavados em 850-700 hPa, bem como a precipitação associada, para dois eventos de DOLs ocorridos entre 21-25 de maio de 2006. Em termos sazonais, o RegCM3 com a parametrização de Grell previu o padrão espacial da chuva similar à observada, mas superestimou-a. A detecção de DOLs utilizando diagramas do tipo Hovmöller identificou ~68% dos eventos obtidos subjetivamente. Aplicação de algoritmo de tracking utilizando ROLE ou vorticidade mostrou-se insatisfatória no rastreamento de DOLs, identificando, através de ROLE, ~55% dos eventos obtidos subjetivamente. / This study purpose was to verify the contribution of Easterly Wave Disturbances (EWDs) in the observed precipitation over the eastern part of Northeast Brazil (ENEB) during rainy season (AMJJ) and evaluate seasonal rainfall forecasts from RegCM3 (Regional Climate Model version 3). For the EWDs subjective identification it was used infrared satellite images and ERA-Interim reanalysis streamlines and relative vorticity fields at 700 and 850 hPa, at the rainy periods from 2006 to 2010. About 90% of the observed troughs at 700 hPa were associated with 116 EWDs that reached the ENEB. This provides a 23 EWDs average per year with small interannual variability. Meteorological variables compositions were constructed from 2 days before (-2) until 2 days after (+2) EWDs reached the ENEB. The circulation showed cyclonic and confluent anomaly, cyclonic relative vorticity and convergence between days -2 and 0 in low levels, mainly in 1000 hPa. Negative anomalies were found for outgoing longwave radiation (OLR) and omega whereas relative humidity presented positive anomaly. The precipitation composition indicated that the EWDs account for: 70% or more of rainy seasons precipitation from northern coast of Alagoas (AL) to the eastern Rio Grande do Nortes (RN), 60% from ALs Zona da Mata to the RNs Agreste, and 50% between Sergipe and the other RN areas. On average, the EWDs presented a 5.3 days period, a wavelength of 4307 km and phase velocity of 9.5 m s-1. The RegCM3 simulated the observed westward troughs propagation at the 850-700 hPa levels, and their associated precipitation as well, for two EWDs occurred between 21 and 25 May 2006. For seasonal simulations, the RegCM3 with Grell parameterization predicted the spatial pattern similar to the observed rain, but overestimated it. Approximately 68% of the events subjectively obtained EWDs were also identified using Hovmöller diagrams. The tracking algorithm using OLR or vorticity was unsatisfactory in the detection of EWDs, identifying with OLR ~55% of the events subjectively obtained.

O propósito deste estudo foi verificar a contribuição dos Distúrbios Ondulatórios de Leste (DOLs) na precipitação observada no período chuvoso (AMJJ) e avaliar previsões sazonais de chuva obtidas do RegCM3 (Regional Climate Model version 3) no Leste do Nordeste do Brasil (LNB). Para a identificação subjetiva dos DOLs foram utilizadas imagens de satélite no infravermelho e os campos de linhas de corrente e vorticidade relativa em 700 e 850 hPa da reanálise ERA-Interim, nos períodos chuvosos de 2006 a 2010. Cerca de 90% dos cavados observados em 700 hPa estiveram associados aos 116 DOLs que atingiram o LNB. Isto fornece média de 23 DOLs por ano com pequena variabilidade interanual. Foram construídas composições de variáveis meteorológicas desde 2 dias antes (-2) até 2 dias depois (+2) dos DOLs atingirem o LNB. A circulação apresentou anomalia ciclônica e confluente, vorticidade relativa ciclônica e convergência entre os dias -2 e 0 em baixos níveis, principalmente em 1000 hPa. Encontraram-se anomalias negativas de radiação de onda longa emergente (ROLE) e de omega e positivas de umidade relativa. A composição de precipitação indicou que são responsáveis por: 70% ou mais da chuva do período chuvoso do litoral norte de Alagoas (AL) ao leste do Rio Grande do Norte (RN), 60% entre a Zona da Mata de AL e o Agreste do RN, e 50% entre Sergipe e demais áreas do RN. Em média, os DOLs apresentaram período de 5,3 dias, comprimento de onda de 4307 km e velocidade de fase de 9,5 m s-1. O RegCM3 simulou a observada propagação para oeste dos cavados em 850-700 hPa, bem como a precipitação associada, para dois eventos de DOLs ocorridos entre 21-25 de maio de 2006. Em termos sazonais, o RegCM3 com a parametrização de Grell previu o padrão espacial da chuva similar à observada, mas superestimou-a. A detecção de DOLs utilizando diagramas do tipo Hovmöller identificou ~68% dos eventos obtidos subjetivamente. Aplicação de algoritmo de tracking utilizando ROLE ou vorticidade mostrou-se insatisfatória no rastreamento de DOLs, identificando, através de ROLE, ~55% dos eventos obtidos subjetivamente. / This study purpose was to verify the contribution of Easterly Wave Disturbances (EWDs) in the observed precipitation over the eastern part of Northeast Brazil (ENEB) during rainy season (AMJJ) and evaluate seasonal rainfall forecasts from RegCM3 (Regional Climate Model version 3). For the EWDs subjective identification it was used infrared satellite images and ERA-Interim reanalysis streamlines and relative vorticity fields at 700 and 850 hPa, at the rainy periods from 2006 to 2010. About 90% of the observed troughs at 700 hPa were associated with 116 EWDs that reached the ENEB. This provides a 23 EWDs average per year with small interannual variability. Meteorological variables compositions were constructed from 2 days before (-2) until 2 days after (+2) EWDs reached the ENEB. The circulation showed cyclonic and confluent anomaly, cyclonic relative vorticity and convergence between days -2 and 0 in low levels, mainly in 1000 hPa. Negative anomalies were found for outgoing longwave radiation (OLR) and omega whereas relative humidity presented positive anomaly. The precipitation composition indicated that the EWDs account for: 70% or more of rainy seasons precipitation from northern coast of Alagoas (AL) to the eastern Rio Grande do Nortes (RN), 60% from ALs Zona da Mata to the RNs Agreste, and 50% between Sergipe and the other RN areas. On average, the EWDs presented a 5.3 days period, a wavelength of 4307 km and phase velocity of 9.5 m s-1. The RegCM3 simulated the observed westward troughs propagation at the 850-700 hPa levels, and their associated precipitation as well, for two EWDs occurred between 21 and 25 May 2006. For seasonal simulations, the RegCM3 with Grell parameterization predicted the spatial pattern similar to the observed rain, but overestimated it. Approximately 68% of the events subjectively obtained EWDs were also identified using Hovmöller diagrams. The tracking algorithm using OLR or vorticity was unsatisfactory in the detection of EWDs, identifying with OLR ~55% of the events subjectively obtained.

La mousson d'Afrique de l'Ouest se caractérise par une très forte variabilité des pluies à toutes les échelles spatiales et temporelles. Le travail de thèse se focalise sur la variabilité synoptique et intra-saisonnière de la mousson, dont les impacts socio-économiques peuvent être dramatiques dans cette zone subsaharienne. L'objectif est d'une part de contribuer à la documentation statistique de cette variabilité et la compréhension de la physique associée, et d'autre part de mettre à profit le potentiel de prédictibilité associée à ces échelles pour guider les prévisions à courte et moyenne échéances. Au Sahel, l'humidité est un des facteurs importants pour l'activité pluviométrique, avec souvent un effet limitant sur le déclenchement de la convection profonde. Dès lors ce travail de thèse s'est focalisé sur l'humidité intégrée sur la colonne ou eau précipitable, pour étudier la variabilité de la mousson. L'activité des ondes d'est, principales perturbations synoptiques de l'atmosphère ouest-africaine pendant l'été boréal, a été détectée et analysée sous cette perspective de l'eau précipitable. Cette étude a ouvert une voie à la compréhension du couplage onde et convection au sein de la mousson. Une analyse conjointe des contributions dynamiques et diabatiques à la physique des ondes a été ensuite entreprise. Il ressort que, la dynamique, via les transports d'énergie associée à l'état de base fortement barocline du Sahel, est un élément précurseur et prédominant dans la couche d'atmosphère en-dessous du jet d'est africain. En revanche, dès que les ondes atteignent leur phase de maturité, le rôle de convection devient primordial grâce aux sources de chauffage et puits d'humidité qu'elle introduit dans l'atmosphère. En outre, elle engendre un transfert turbulent et convectif de quantité de mouvement horizontal, de la surface vers les couches plus hautes, permettant de renforcer les circulations dans la moyenne troposphère. La compréhension du couplage onde-convection ouvre alors une perspective à l'amélioration des modèles de prévision du temps sur l'Afrique. / The West African monsoon rainfall experiences a large spatial and temporal variability. In this thesis, a focus has been given on the synoptic to intra-seasonal scales which can lead to dramatic socio-economic consequences over Sahelian areas. The main goal is, on the one hand, to document and hence to better understand the physics associated with such scales of variability, and on the other hand, to provide some useful tools to improve short to medium ranges forecast skill over Africa. Over the Sahel, the supply of humidity is a key feature in the rainfall distribution and mostly a limiting factor to the initiation of deep convection. Therefore, the current study is based on the total column integrated specific humidity, also called precipitable water, to disentangle the important physics involved in the monsoon intra-seasonal variability and more specifically on the synoptic scale. African Easterly Waves (AEW), also known as the main synoptic scale disturbances of the Western African atmosphere during the boreal summer, have been detected and characterized from this "moist" perspective. This study then provides a new approach for studying the coupling between AEW and convection. A joint assessment of both dynamic and diabatic contributions to the AEW growth has been undertaken. Dynamics is, through the baroclinic and barotropic energy transport, a precursor and a predominant mechanism in the layer below the African easterly jet. However, since convection is enhancing, diabatic processes become accounting for a crucial role in the atmospheric circulation through the release of heat as well as the humidity sink. Meanwhile, subgrid convective scale eddies transport a large part of the horizontal momentum, from the surface to the mid-levels. This enhances the midtroposheric cyclonic/anticyclonic circulation of the AEW. Finally, this process-based analysis of the coupling between dynamics and convection provides some useful tools for model assessment and improvement over Africa.

Då de flesta människor i Västafrika, framför allt de som bor innanför kustzonen, livnär sig på jordbruk så är pålitliga väderprognoser och säsongsförutsägelser ett viktigt hjälpmedel i det dagliga arbetet och planeringen. I den här rapporten har en litteraturstudie gjorts för att öka kunskapen om de komplexa, både lokala och storskaliga, väderfenomen som ger upphov till nederbörd i området.   Klimatet i Västafrika, ett område som mestadels täcks av regnskog eller savann, präglas av den västafrikanska monsunen som ger regnperiod under norra halvklotets sommar och torrperiod på vintern. Denna monsun visar på stor årlig variation när det gäller dess inledande faser, och prognoser som kan förutsäga dess början är nödvändigt när en jordbrukare ska planera säsongens verksamhet. Av många bidragande faktorer framgår det tydligt att ytvattentemperaturen i Guineabukten är en av de viktigaste parametrarna för monsunens startskede. Den intertropiska konvergenszonen, ITCZ, betraktas som monsunens nordligaste del och denna konvergenszon gör en plötsligt och relativt snabb förflyttning norrut, i fortsättningen benämnd som språnget, över ca 5 breddgrader, vilket av många ses som starten på regnperioden. En tillfällig tryckgradient som uppstår på grund av en tillfällig men skarp temperaturgradient är den bakomliggande orsaken till detta språng. Den i särklass viktigaste processen som ger upphov till regn i Västafrika är konvektion och även om mycket konvektiv nederbörd faller i samband med monsunen så uppstår det även många lokala och mesoskaliga konvektiva system inom monsunen, framförallt kopplat till ostliga vågor. / Since most people in West Africa, particularly those who live away from the shore, work within agriculture, reliable weather forecasts are important in the daily work. In this report, a literature study has been conducted to increase the knowledge about the complex and local weather phenomenon that causes rain.   West Africa is dominated by rainforest and savannah, and the climate is characterized by the West African monsoon which gives rise to a wet season in the northern hemisphere summer and a dry season during the winter. The monsoon shows wide annual variability in its initial stages and forecasts that can predict its beginning is necessary to the farmers who have to decide when and what to plant. Out of many contributing factors, the sea surface temperature seems to be one of the main parameters that have an impact on the starting stage of the monsoon. The inter tropical convergence zone, ITCZ, is regarded as the northernmost part of the monsoon and it makes a sudden and relatively quick jump over about 5 degrees latitude, which is widely regarded as the start of the rainy season. A temporary pressure gradient caused by differences in temperature adjacent to the ITCZ is the reason for this jump. The dominant process that causes rainfall in West Africa is convection. Although a lot of convective precipitation falls in connection with the northern part of the monsoon, convective systems can appear inside the monsoon flow, often linked to African easterly waves.