African Easterly Jet Research Articles

Characteristics and trends of Atlantic tropical cyclones that do and do not develop from African easterly waves

AbstractAtlantic tropical cyclones (TCs) are known to develop from African easterly waves (AEWs) that propagate across North Africa and out over the Atlantic Ocean. The relationship between AEWs and TCs has been the subject of numerous previous studies. There are, however, many Atlantic TCs that do not have AEW origins. In this study, we provide a novel analysis of the characteristics and trends of Atlantic TCs both with and without AEW origins using 43 years of observational and reanalysis data. To conduct this research, we identified TCs with and without AEW origins from the observational record between 1980 and 2022, and ran objective tracking algorithms on reanalysis data to identify the AEWs and TCs during this time period. We found statistically significant differences in the characteristics and environments of TCs with and without AEW origins. TCs with AEW origins are stronger and costlier, experience more favorable environmental conditions, and are more likely to make landfall in the Gulf of Mexico and the Caribbean when compared to TCs without AEW origins. Additionally, the 43‐year increasing trend in Atlantic TC activity is primarily driven by an increase in TCs with AEW origins that is associated with increasing AEW frequency and strength, with anthropogenic aerosols potentially driving this trend. In contrast, we found no trend in TCs without AEW origins.

The emission, transport, and impacts of the extreme Saharan dust storm of 2015

Abstract. ​​​​​​​Each summer, the Saharan Air Layer (SAL) transports massive amounts of mineral dust across the Atlantic Ocean, affecting weather, climate, and public health over large areas. Despite the considerable impacts of African dust, the causes and impacts of extreme trans-Atlantic African dust events are not fully understood. The “Godzilla” trans-Atlantic dust event of 2020 has been extensively studied, but little is known about other similar events. Here, we examine the June 2015 event, the second strongest trans-Atlantic African dust event that occurred during the summers from 2003–2022. This event was characterized by moderately high dust emissions over western North Africa and an extremely high aerosol optical depth (AOD) over the tropical North Atlantic. The high dust loading over the Atlantic is associated with atmospheric circulation extremes similar to the Godzilla event. Both the African easterly jet (AEJ) and Caribbean low-level jet (CLLJ) have greatly intensified, along with a westward extension of the North Atlantic subtropical high (NASH), all of which favor the westward transport of African dust. The enhanced dust emissions are related to anomalously strong surface winds in dust source regions and reduced vegetation density and soil moisture across the northern Sahel. The dust plume reduced net surface shortwave radiation over the eastern tropical North Atlantic by about 25 W m−2 but increased net longwave flux by about 3 W m−2. In contrast to the Godzilla event, the 2015 event had minor air quality impacts on the US, partially due to the extremely intensified CLLJ that dispersed the dust plume towards the Pacific.

Deciphering East Atlantic Low-Pressure System Formations: Exploring the Nexus of Tropical Jet Streams and Active Monsoon Phases

The formation of low-pressure systems (LPSs) over the eastern Atlantic Ocean, near the coast of West Africa, is an exceptional meteorological/climatological feature that can lead to the development of hurricanes. The upper level diffluence induced by the Tropical Easterly Jet (TEJ) plays a crucial role in the formation of LPSs over the eastern Atlantic Ocean, off the coast of West Africa. However, the exact influence of the enhanced TEJ and diffluence in relation to cyclogenesis remains unclear. An active precipitation period over the Indian subcontinent and Africa induces an intensification of the TEJ, African Easterly Jet, and the bifurcation of diffluence off the coast of Africa. Over the past five years (2019–2023), a delayed correlation has been observed between the formation of LPSs over the eastern Atlantic Ocean (7.5° N–20° N, 15° W–41° W), the TEJ over the Indian subcontinent (approximately 2 to 3 days), and the AEJ over Africa (approximately 1 day). This correlation is further linked to the bifurcation of diffluence at the 200 mb level.

Unusually low dust activity in North Africa in June 2023: Causes, impacts and future projections

Dust activity during the pre-monsoon season in Africa has an impact on the monsoon circulation and the Atlantic hurricane season. During early June 2023 the atmosphere was relatively clear over West Africa and the eastern tropical Atlantic, in contrast with the dustier June 2020. The negative phase of the North Atlantic Oscillation suppressed dust lifting, with an equatorward shifted African Easterly Jet limiting its downstream advection. On the other hand, dust accumulated in the atmosphere over northeastern Africa, with the negative dust aerosol optical depth (DAOD) anomalies over western Africa and the positive anomalies over eastern Africa more than two standard deviations away from the climatological mean. The lack of dust led to an up to 55 W m−2 increase in the surface downward shortwave radiation flux and a 35 W m−2 decrease in the longwave flux, and is in line with the record-breaking sea surface temperatures over the eastern tropical Atlantic and the active start to the Atlantic hurricane season. In order to explore future projections of DAOD, a multi-model ensemble (MME) is constructed from 29 models that integrate the sixth phase of the Coupled Model Intercomparison Project (CMIP6). It captures the positive trend in the June DAOD over the eastern tropical Atlantic during 1980–2014, although the amplitude is roughly a factor of six smaller than the 0.0017 year−1 in the reanalysis dataset. The CMIP6 MME projects a further increase in DAOD in the region at a rate of up to 0.0003 year−1 in the most extreme climate change scenario for 2066–2100, which is comparable to that seen during the historical period, even though the mean values are projected to decrease by 0.03–0.06. While lower dust loadings may lead to improved air quality, they will likely further fuel pre-season and early season storms in the North Atlantic, which have become more frequent in recent decades.

Moisture Dependence of an African Easterly Wave Within the West African Monsoon System

AbstractThe growth and propagation of African easterly waves (AEWs) remains an active area of research, especially for those that become tropical cyclones (TCs). This is partly due to the complex role of moisture, realized through AEW‐convection interactions. The goal of this study is to understand how environmental moisture plays a role in influencing the growth and propagation of a case of an AEW‐convection system, that became a TC and how that role relates to the West African Monsoon System. Moisture sensitivity experiments were performed in a regional and convection‐permitting novel configuration. It is found that in a moister environment, diabatic heating associated with convection coupled to the wave is shallower, ultimately, weakening the wave amplitude. Energetics are reduced in a moister environment as the African easterly jet strengthens, yet narrows, and shifts northward limiting interaction with the monsoon and the wave‐convection system. The more intense monsoonal flow in a moister environment can instigate the decoupling between convection and AEW as deep convection is more likely in the ridge rather than in the trough region. Over western Africa, more continuous rainfall over the Guinea Highlands can inhibit phase locking with the AEW. In a moister environment, the mean zonal flow is weaker and as a result, the westward translation speed of the wave due to mean flow advection is slower than in the other experiments. While the mean flow advection dominates the translation, further differences in phase speed arise from differences in convection within the wave.

An objective identification technique for potential vorticity structures associated with African easterly waves

Abstract. Tropical Africa and the North Atlantic Ocean are significantly influenced by African easterly waves (AEWs), which play a fundamental role in tropical rainfall and cyclogenesis in that region. The dynamics of AEWs can be described in a potential vorticity (PV) framework. The important impact of latent heat release by cloud processes is captured in this framework by the diabatic generation of PV anomalies. This paper introduces an innovative approach for the identification and tracking of PV structures within AEWs. By employing AEW tracking and computing the wave phase of each point within the AEW domain using a Hilbert transform, we are able to effectively identify and collect 3-D PV structures associated with specific AEWs. To facilitate a climatological analysis, performed here over the months of June to October from 2002 to 2022, these structures are subsequently characterized by low-dimensional descriptors, including their location, intensity, and orientation. Our climatological analysis reveals the seasonal evolution and the structural attributes of PV anomalies within AEWs over the study domain. PV feature locations closely align with the African easterly jet's latitudinal shift during the summer season. Analysis of the mean pressure level of the 3-D PV structures shows a remarkable shift during their life cycle, indicating deep moist convection characteristics over land and more shallow convection characteristics over the ocean. On average, PV features identified within AEW troughs tilt downshear over land and equatorward over the ocean. The trough-centered analysis reveals distinct differences between satellite-estimated and model-predicted rainfall. Agreement between the results of a more traditional composite analysis and our new feature analysis provides confidence in our feature approach as a novel diagnostic tool. The feature framework provides a low-dimensional representation of the PV structure of AEWs, which facilitates future statistical analyses of the relation of this structure to, for example, tropical cyclogenesis or the predictability of tropical rainfall.

African Easterly Wave Strength and Observed Atlantic Tropical Cyclone Genesis and Characteristics

AbstractAfrican easterly waves (AEWs) are known precursors to Atlantic tropical cyclones (TCs), and are therefore often directly connected to extreme weather events that can be both deadly and destructive. It is well established that not all AEWs develop into TCs, and there has been substantial research that has addressed the different characteristics and environments of developing and non‐developing waves. In this study, however, we specifically examine 41‐years of developing AEWs to provide a better understanding of the relationship between the developing wave and the environment, and the resulting TC. To conduct this research, we identified TCs with AEW origins from the observational record between 1980 and 2020. We then used an objective tracking algorithm to identify the developing AEWs in reanalysis data. We found a statistically significant relationship between the strength of the developing AEWs, TC genesis location and landfall, and sea surface temperature (SST) during TC genesis. Weaker AEWs tend to develop into TCs closer to the Americas in a region with warmer SSTs than those of the stronger AEWs, which tend to develop into TCs closer to Africa. Consequently, the TCs that develop from weaker AEWs are more likely to make landfall due to the close proximity of their genesis locations to the Americas.

Quantifying uncertainty in simulations of the West African monsoon with the use of surrogate models

Abstract. Simulating the West African monsoon (WAM) system using numerical weather and climate models suffers from large uncertainties, which are difficult to assess due to nonlinear interactions between different components of the WAM. Here we present a fundamentally new approach to the problem by approximating the behavior of a numerical model – here the Icosahedral Nonhydrostatic (ICON) model – through a statistical surrogate model based on universal kriging, a general form of Gaussian process regression, which allows for a comprehensive global sensitivity analysis. The main steps of our analysis are as follows: (i) identify the most important uncertain model parameters and their probability density functions, for which we employ a new strategy dealing with non-uniformity in the kriging process. (ii) Define quantities of interest (QoIs) that represent general meteorological fields, such as temperature, pressure, cloud cover and precipitation, as well as the prominent WAM features, namely the tropical easterly jet, African easterly jet, Saharan heat low (SHL) and intertropical discontinuity. (iii) Apply a sampling strategy with regard to the kriging method to identify model parameter combinations which are used for numerical modeling experiments. (iv) Conduct ICON model runs for identified model parameter combinations over a nested limited-area domain from 28° W to 34° E and from 10° S to 34° N. The simulations are run for August in 4 different years (2016 to 2019) to capture the peak northward penetration of rainfall into West Africa, and QoIs are computed based on the mean response over the whole month in all years. (v) Quantify sensitivity of QoIs to uncertain model parameters in an integrated and a local analysis. The results show that simple isolated relationships between single model parameters and WAM QoIs rarely exist. Changing individual parameters affects multiple QoIs simultaneously, reflecting the physical links between them and the complexity of the WAM system. The entrainment rate in the convection scheme and the terminal fall velocity of ice particles show the greatest effects on the QoIs. Larger values of these two parameters reduce cloud cover and precipitation and intensify the SHL. The entrainment rate primarily affects 2 m temperature and 2 m dew point temperature and causes latitudinal shifts, whereas the terminal fall velocity of ice mostly affects cloud cover. Furthermore, the parameter that controls the evaporative soil surface has a major effect on 2 m temperature, 2 m dew point temperature and cloud cover. The results highlight the usefulness of surrogate models for the analysis of model uncertainty and open up new opportunities to better constrain model parameters through a comparison of the model output with selected observations.

The radiative impact of biomass burning aerosols on dust emissions over Namibia and the long-range transport of smoke observed during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) campaign

Abstract. The radiative effects of biomass burning aerosols (BBAs) on low-level atmospheric circulation over southern Africa are investigated on 5 September 2017 during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign. This is conducted using a variety of in situ and remote sensing observations, as well as two 5 d ensemble simulations made with the Meso-NH mesoscale model, one including the direct and semi-direct radiative effects of aerosols and one in which these effects are not included. We show that the radiative impact of BBA building up over a period of 5 d in the Meso-NH simulations can lead to significantly different circulations at low and middle levels, thereby affecting dust emissions over southern Namibia and northwestern South Africa as well as the transport of BBA in a so-called “river of smoke”. While most of the regional-scale dynamics, thermodynamics and composition features are convincingly represented in the simulation with BBA radiative effects, neglecting the radiative impact of BBA leads to unrealistic representations of (i) the low-level jet (LLJ) over the plateau, which is the main low-level dynamic feature fostering dust emission, and (ii) the mid-level dynamics pertaining to the transport of BBA from the fire-prone regions in the tropics to the mid-latitudes. For instance, when the BBA radiative impacts are not included, the LLJ is too weak and not well established over night, and the developing convective planetary boundary layer (PBL) is too deep compared to observations. The deeper convective PBL over Etosha and surrounding areas is related to the enhanced anomalous upward motion caused by the eastern displacement of the river of smoke. This eastern displacement is, in turn, related to the weaker southerly African easterly jet. Both ensemble simulations provide clear evidence that the enhanced near-surface extinction coefficient values detected from observations over Etosha are related to the downward mixing of BBA in the developing convective boundary layer rather than dust being emitted as a result of the LLJ breakdown after sunrise. This study suggests that the radiative effect of BBAs needs to be taken into account to properly forecast dust emissions in Namibia.

Dynamical study of three African Easterly Waves in September 2021

AbstractThree convectively active African easterly waves (AEWs) that propagated south of the African easterly jet were observed over the northeast Atlantic Ocean in September 2021. Their evolution is studied using a suite of theoretical frameworks, as well as the European Centre for Medium‐range Weather Forecast reanalyses and satellite‐derived brightness temperature observations. The environment of these AEWs was sampled during the Cloud–Atmospheric Dynamics–Dust Interactions in West Africa campaign near Cape Verde with the goal to assess their potential for developing into tropical cyclones. We highlight the processes that inhibited the development of the first AEW (which evolved into tropical disturbance Pierre‐Henri) and that played a role in the development of the later two into tropical storms Rose and Peter on September 19, 2021. The three AEWs developed a so‐called “marsupial protective” pouch. For Peter and Rose, the pouch was associated with a vertically aligned vortex at low levels and efficiently protected the convective systems inside from dry and dusty air intrusion. The development of this low‐level vortex is associated with an interaction with the monsoon trough for Rose and with a vorticity center associated with a wave propagating north of the African easterly jet (AEJ) in the case of Peter. The presence of a dust flux toward the convective core near the surface is highlighted for Rose and Peter in spite of the presence of the protective marsupial pouch. On the other hand, Pierre‐Henri interacted positively with both the monsoon trough and an AEW north of the AEJ but failed to develop into a tropical cyclone. The wave north of the AEJ brought Saharan air layer air masses inside the pouch that led to a drying of the circulation that may explain the decrease in convective activity.

Cyclogenesis in the Tropical Atlantic: First Scientific Highlights from the Clouds–Atmospheric Dynamics–Dust Interactions in West Africa (CADDIWA) Field Campaign

Abstract During the boreal summer, mesoscale convective systems generated over West Africa propagate westward and interact with African easterly waves, and dust plumes transported from the Sahel and Sahara by the African easterly jet. Once off West Africa, the vortices in the wake of these mesoscale convective systems evolve in a complex environment sometimes leading to the development of tropical storms and hurricanes, especially in September when sea surface temperatures are high. Numerical weather predictions of cyclogenesis downstream of West Africa remains a key challenge due to the incomplete understanding of the clouds–atmospheric dynamics–dust interactions that limit predictability. The primary objective of the Clouds–Atmospheric Dynamics–Dust Interactions in West Africa (CADDIWA) project is to improve our understanding of the relative contributions of the direct, semidirect, and indirect radiative effects of dust on the dynamics of tropical waves as well as the intensification of vortices in the wake of offshore mesoscale convective systems and their evolution into tropical storms over the North Atlantic. Airborne observations relevant to the assessment of such interactions (active remote sensing, in situ microphysics probes, among others) were made from 8 to 21 September 2021 in the tropical environment of Sal Island, Cape Verde. The environments of several tropical cyclones, including Tropical Storm Rose, were monitored and probed. The airborne measurements also serve the purpose of regional model evaluation and the validation of spaceborne wind, aerosol and cloud products pertaining to satellite missions of the European Space Agency and EUMETSAT (including the Aeolus, EarthCARE, and IASI missions).

Revisiting the Seasonal Cycle of Rainfall over Central Africa

Abstract The intertropical convergence zone (ITCZ), with its twice-annual passage over central Africa, is considered as the main driver of the rainfall seasonality. In this ITCZ paradigm, high rainfall occurs over regions of large low-level convergence. But recently, this paradigm was challenged over central Africa. Here, we show that a shallow meridional overturning circulation—driven by surface conditions—plays a thermodynamical control on the rainfall seasonality over central Africa. Indeed, due to the local evaporative cooling effect, the foot of the ascending branch of Hadley cells occurs where the temperature is the warmest, indicating a thermal low. This distorts the southern Hadley cell by developing its bottom-heavy structure. As result, both shallow and deep Hadley cells coexist over central Africa year-round. The deep mode is associated with the poleward transport of atmospheric energy at upper levels. The shallow mode is characterized by a shallow meridional circulation, with its moisture transport vanishing and converging in the midtroposphere rather than at lower troposphere. This midtropospheric moisture convergence is also the dominant component that shapes the vertically integrated moisture flux convergence, with little contribution of African easterly jets. This convergence zone thus controls the precipitating convection. Its meridional migration highlights the interhemispheric rainfall contrast over central Africa and outlines the unimodal seasonality. On the other hand, forced by the Congo basin cell, the precipitable water regulates the deep convection from the vegetated surface of Congo basin, acting as a continental sea. This nonlinear mechanism separates the rainfall into three distinct regimes: the moisture-convergence-controlled regime, with convective rainfall exclusively occurring in the rainy season; the local evaporation-controlled regime with drizzle in the dry season; and the precipitable-water-controlled regime, with exponential rainfall increase in the dry season.

A new look into the impacts of dust radiative effects on the energetics of tropical easterly waves

Abstract. Saharan dust aerosols are often embedded in tropical easterly waves, also known as African easterly waves, and are transported thousands of kilometers across the tropical Atlantic Ocean, reaching the Caribbean Sea, Amazon Basin, and eastern USA. However, due to the complex climate dynamics of west Africa and the eastern tropical Atlantic Ocean, there is still a lack of understanding of how dust particles may influence the development of African easterly waves, which are coupled to deep convective systems over the tropical Atlantic Ocean and in some cases may seed the growth of tropical cyclones. Here we used 22 years of daily satellite observations and reanalysis data to explore the relationships between dust in the Saharan air layer and the development of African easterly waves. Our findings show that dust aerosols not merely are transported by the African easterly jet and the African easterly waves system across the tropical Atlantic Ocean, but also contribute to the changes in the eddy energetics of the African easterly waves. The efficiency of the dust radiative effect in the atmosphere is estimated to be a warming of approximately 20 W m−2 over the ocean and 35 W m−2 over land. This diabatic heating of dust aerosols in the Saharan air layer acts as an additional energy source to increase the growth of the waves. The enhanced diabatic heating of dust leads to an increase in meridional temperature gradients in the baroclinic zone, where eddies extract available potential energy from the mean flow and convert it to eddy kinetic energy. This suggests that diabatic heating of dust aerosols can increase the eddy kinetic energy of the African easterly waves and enhance the baroclinicity of the region. Our findings also show that dust outbreaks over the tropical Atlantic Ocean precede the development of baroclinic waves downstream of the African easterly jet, which suggests that the dust radiative effect has the capability to trigger the generation of the zonal and meridional transient eddies in the system comprising the African easterly jet and African easterly waves.

The Effects of African Easterly Wave Suppression by Wave Track on Atlantic Tropical Cyclones

AbstractIt is well established that African easterly waves (AEWs) can serve as seedling disturbances for Atlantic tropical cyclones (TCs). However, research has shown that AEWs are not necessary to maintain specifically basin‐wide TC frequency. Here, we for the first time investigate the effects of AEW suppression by wave track on Atlantic TC activity. Regional model simulations were performed, where AEWs were either prescribed or suppressed from the eastern lateral boundary condition. We found that without AEWs, there was an increase in TC frequency and strength, with the most pronounced increases occurring when the waves were suppressed in the south track. These changes coincided with more favorable environmental conditions and disturbances associated with increased convective activity over the Atlantic. Our results indicate that AEWs are not a limiting factor for TCs, and that AEW suppression, specifically in the south track, can affect the large‐scale environment to enhance favorability for TC genesis.

The Indian Easterly Jet During the Pre‐Monsoon Season in India

AbstractWe identify for the first time an Indian Easterly Jet (IEJ) in the mid‐troposphere during the pre‐monsoon using reanalysis data. The IEJ is weaker and smaller than the African Easterly Jet over West Africa, with a climatological location of 10°N, 60–90°E, 700 hPa, and strength 6–7 m s−1 during March–May. The IEJ is a thermal wind associated with low‐level meridional gradients in temperature (positive) and moisture (negative), resulting from equatorward moist convection and poleward dry convection. The IEJ is associated with a negative meridional potential vorticity gradient, therefore satisfying the Charney‐Stern necessary condition for instability. However, no wave activity is detected, suggesting that the potential for combined barotropic‐baroclinic instability is not often realized. IEJ strong (weak) years feature increased (reduced) near‐surface temperatures and drier (wetter) conditions over India. This study provides an introduction to the IEJ's role in pre‐monsoon dynamics, and a platform for further research.

Sensitivity of African Easterly Waves to Dust Forcing

AbstractThere is a lack of consensus regarding the sign and magnitude of the effect of dust‐radiative forcing on African easterly waves (AEWs) among past studies. The uncertainty in the dust‐radiative forcing associated with the estimation of shortwave absorption is a leading cause of disagreement in the literature. The inability of models to represent various dust‐AEW interaction pathways also leads to uncertainty among modeling studies. The present study investigates the sensitivity of AEWs to the observed variability in dust shortwave absorption using a high‐resolution atmospheric general circulation model. Global simulations are conducted at a spatial resolution of about 25 km to simulate AEWs and associated circulation features adequately well. The results reveal that AEWs are highly sensitive to dust shortwave absorption. In addition, the AEW activity intensifies and broadens the wave track with a southward shift in response to dust shortwave absorption. Eddy kinetic energy (EKE) associated with AEWs changes by approximately ±25% for the range of dust shortwave absorption used. The 6–9 days waves are more sensitive to dust shortwave absorption than the 3–5 days waves, with the response in the former exhibiting a stark land‐sea contrast. The sensitivity of AEWs to dust heating stems from a combination of the responses from various energy conversions. Although baroclinic energy conversion is the dominant term in the energy cycle, the responses to dust shortwave heating in barotropic and generation terms are comparable to those in baroclinic conversion.

The Misrepresentation of the Southern African Easterly Jet in Models and Its Implications for Aerosol, Clouds, and Precipitation Distributions

Abstract The southern African easterly jet (AEJ-S) is an important midtropospheric feature critical to understanding the tropical convective system over central Africa and the aerosol–cloud interactions over the southeast Atlantic Ocean. However, it remains unclear how well models represent the AEJ-S and its influence on aerosol transport, clouds, and precipitation distribution. Here, we use ground- and satellite-based observations and reanalysis datasets to assess the representation of AEJ-S in the Coupled Model Intercomparison Project phase 6 (CMIP6) models between September and October during the peak of midtropospheric winds, aerosol transport, clouds, and precipitation. We find that most CMIP6 models have difficulty accurately simulating the strength, position, and spatial distribution of the AEJ-S. Specifically, the AEJ-S is relatively weaker and at a slightly lower altitude in the ensemble of CMIP6 models than represented by observation and reanalysis datasets. To assess the influence of the misrepresented the AEJ-S on CMIP6-simulated aerosol, clouds, and precipitation distributions, we performed composite analyses using models with low and high biases based on the estimates of their midtropospheric easterly wind speed. We find that the misrepresentation of the AEJ-S in CMIP6 models is associated with the overestimation of clouds and precipitation over central Africa, the underestimation of clouds over the southeast Atlantic Ocean, and the limitation of aerosol transport over the continent or the deviation of its spatial distribution from the typical zonal transport over the Atlantic Ocean. Because aerosols, clouds, and precipitation are important components of the regional climate system, we conclude that accurate representation of the AEJ-S is essential over central Africa and the southeast Atlantic Ocean.

Atmospheric response to seasonal changes in sea surface temperature during the boreal summer in the Tropical Atlantic

We investigate the atmospheric response to seasonal variations in sea surface temperature (SST) in the eastern tropical Atlantic during the boreal summer, using the Weather Research and Forecasting (WRF) regional atmospheric model. Three ensembles of 11 simulations each are produced with different SST forcings: the control ensemble (CTL) uses the observed climatology of the SST in 2000–2009, while the Frozen North (FzN) and Frozen South (FzS) experiments block the seasonal warming or cooling of the SST from June onwards in a region confined to the eastern tropical Atlantic. The result is a cold SST anomaly in the northeastern tropical Atlantic off the coasts of Senegal and Mauritania in FzN, and a warm anomaly in the southeastern region (Gulf of Guinea and the cold tongue zone in the equatorial Atlantic) in FzS. Comparison with CTL reveals significant impacts of these SST anomalies on the position and intensity of the marine intertropical convergence zone (ITCZ) and on West African rainfall during July and August. Over the ocean, the cold anomaly in NETA suppresses convection on the northern side of the ITCZ (north of 10∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}N), while the warm anomaly in the Gulf of Guinea strengthens convection on its southern flank. The latter is also leading to a sharp increase in precipitation in the coastal regions to the northeast of the Gulf of Guinea. These changes are clearly due to variations in surface pressure gradients and the divergence of low-level moisture in response to SST anomalies, which in turn induce changes in deep atmospheric convection through thermodynamic feedback. On the continent, a substantial reduction in precipitation is observed in the western Sahel (particularly Senegal) following the cold anomaly in NETA, and in the eastern Sahel following the warm anomaly in the Gulf of Guinea: both are explained by a positive anomaly in the divergence of moisture transport in the upper troposphere, associated with an acceleration of the African easterly jet along its southern edge. However, the mechanism by which the SST anomalies create this acceleration in both experiments remains to be elucidated.

More Frequent Hurricane Passage Across the Bahamian Archipelago During the Little Ice Age

AbstractThe year 2020 Common Era (CE) experienced the highest number of named tropical cyclones in the Atlantic Ocean since 1850 CE, but the short instrumental record makes it challenging to assess if this level of activity is statistically meaningful. Here, we present two near‐annually resolved hurricane reconstructions from sediment archived in two blue holes located only 300 m apart on the northern margin of Grand Bahama. These two blue holes provide a replicated signal of hurricanes passing within a 50–100 km radius over the last 1,800 years, and the long‐term reconstructions document multiple 50‐to‐150‐year intervals when hurricane frequency was significantly higher than it has been over the last 100 years. These two records were first merged into a single stack, and then compiled with five other high‐resolution reconstructions from across the Bahamian Archipelago to form a single 1500‐year record of Bahamian hurricane frequency. This new Bahamian Compilation documents more hurricanes passing ∼75°W from 21°N to 26°N during the Little Ice Age (LIA; 1300–1850 CE) relative to the prior millennium and the last 170 years. The US Eastern Seaboard also experienced heightened hurricane activity during the LIA, whereas the Gulf of Mexico and Southern Caribbean were inactive. This suggests that despite a globally cooler climate, regional climate conditions during the LIA remained favorable for cyclogenesis and intensification along certain Atlantic hurricane pathways. Perhaps heightened Sahel rainfall during the LIA indicates an increase in African Easterly waves, which in turn possibly seeded more tropical cyclones in the Atlantic Main Development Region.

Tropical Waves Are Key Drivers of Extreme Precipitation Events in the Central Sahel

AbstractExtreme precipitation events (EPE) are often associated with severe floods and significant damages in Central Sahel. To better understand their formation and improve their forecasts, we investigate the sub‐seasonal drivers of EPEs. A composite analysis reveals that moist, cyclonic and upper‐level divergence anomalies are found on average as a result of several tropical waves. The equatorial Rossby wave (ER) dominates at large scale providing a moist and convectively‐active anomaly over the northern Sahel together with a smaller‐scale African Easterly Wave (AEW). The Madden‐Julian Oscillation provides upper‐level divergence anomalies and a Kelvin wave increases convection during the EPE. Statistics show the prevalence of AEW and emphasize ER as a key driver of EPE. The co‐occurrences of several tropical waves, especially those involving AEW, ER, and Kelvin waves, increase the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts.