Tropical Waves Research Articles

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.

A Global Climatology of Diabatic Heating in Tropical Easterly Waves

Abstract Tropical easterly waves (TEWs) play a critical role in regulating convection and precipitation across the global tropics. TEWs act as seed disturbances for tropical cyclogenesis, serve as an essential component in monsoon precipitation, and produce large amounts of rainfall and diabatic heating that can strongly affect the large-scale circulation. To help improve our knowledge of a more elusive type of tropical wave, we use satellite and reanalysis estimates of the diabatic heating associated with TEWs that are identified by a tracking algorithm based on low-level curvature vorticity. This study uses the Tropical Rainfall Measuring Mission (TRMM) version 6 convective–stratiform heating (CSH) and spectral latent heating (SLH) orbital products to create a global climatology (1998–2015) of TEW diabatic heating. TEW-specific composites for the satellite-observed vertical structure of diabatic heating are compared to similar terms from MERRA-2 across a variety of tropical regions. There are striking differences between the reanalysis and satellite heating with MERRA-2 having much stronger background heating, especially at low levels. Both the satellite-observed and reanalysis heating profiles show stronger midlevel heating associated with TEWs relative to the unconditional background. Similar patterns of mid- and bottom-heaviness emerge in two-dimensional composites of TEW latent heating as stronger heating rates and percent contributions to the background are generally higher at 500 hPa than at 850 hPa. Although TEWs only represent a few percent of the background heating across the global tropics, they comprise 30%–50% of the heating in the prominent TEW tracks over the northeastern Atlantic and Pacific Oceans.

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).

Influence of Tropical Easterly Waves on the ChocoJet during the 2019 OTREC Campaign

Abstract This study investigates how convectively coupled tropical easterly waves (TEWs) affect the Choco low-level jet (ChocoJet) as they move across the western Caribbean. The ChocoJet is a low-level flow over the eastern Pacific (EPAC) that modulates precipitation patterns over the tropical eastern Pacific and northwestern South America. By combining data from the Organization of Tropical East Pacific Convection (OTREC; August–September 2019), ERA5 reanalysis products, and satellite data, we analyze precipitation and circulation patterns during convectively coupled and nonconvectively coupled TEWs, comparing them to non-TEW days. During convectively coupled TEWs days, the ChocoJet strengthens and becomes more southerly, while the ITCZ moves northward, leading to enhanced precipitation over the western Caribbean and drier conditions over the northern part of the Colombian Pacific. In contrast, nonconvectively coupled TEW days exhibit reduced precipitation and precipitable water over the Caribbean and far EPAC, with a layer of northeasterly flow centered at 850 hPa flowing over a shallower, weaker, and more westerly ChocoJet. Additionally, convectively coupled TEWs are associated with a weaker western Caribbean and far eastern Pacific pressure gradient compared to nonconvective TEWs. These observable and predictable synoptic-scale circulation–precipitation relationships contribute to a better understanding of hydrometeorological variability in the region. Significance Statement Tropical easterly waves and related convective organization traversing the Caribbean Sea are important sources of synoptic-scale precipitation–circulation variability in the far eastern Pacific and Colombian Pacific. This eastern tropical Pacific study aims to identify precipitation–circulation relationships that enhance the understanding of synoptic-scale meteorological phenomena.

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.

Comparison between Developing and Nondeveloping Disturbances for Tropical Cyclogenesis in Different Large-Scale Flow Patterns over the Western North Pacific

Abstract This study classifies 407 developing disturbances (DEV) and 2309 nondeveloping disturbances (NONDEV) over the western North Pacific into five large-scale circulation patterns, namely the pre-existing cyclone (PC), easterly wave (EW), zonal wind convergence (CON), zonal wind shear line (SL), and mixed zonal wind convergence and shear line (CON-SL) patterns. The SL pattern has the highest TC yield percentage, followed by the CON-SL, while the EW is the least favorable pattern. The composite analysis shows that upper-level divergence, midlevel relative humidity, and surface heat flux (SHF) growth are crucial to the disturbance development in all the five patterns. Besides, large lower-level barotropic kinetic energy conversion and a well-developed primary circulation are good indicators for disturbance development in the PC, EW, and CON rather than in the SL and CON-SL patterns. Furthermore, for the PC, EW and CON patterns, the DEV features strong and rapidly growing SHF and mesoscale convective systems (MCS) closer to the disturbance center, which allows deep-layer warming and moistening, and drives a deep secondary circulation. Interestingly, due to an environment with high lower-level vorticity, the SL and CON-SL patterns typically foster a relatively mature primary circulation with strong SHF and MCS concentrated close to the center, especially for the NONDEV at the pre-genesis stage. However, a drier mid-to-upper-level environment for the NONDEV inhibits deep convection, which may explain its shallow secondary circulation and therefore poor potential to develop further.

Regional and remote influences of ocean‐atmospheric processes on northeast monsoon rainfall during 2021 over India

AbstractSevere rainfall during the recent northeast monsoon season (October, November and December) in 2021 caused unexpected disasters (triggered flooding and landslides) in the pilgrim city of Tirupati and the metropolitan city of Chennai over the peninsular of India (PI). The remote and regional influence of several ocean‐atmospheric climate indices on northeast monsoon rainfall (NEMR) during the OND season is investigated to understand the contributory mechanisms of extreme rainfall events over the PI during 2021. The interannual variability of NEMR and its relationship with the OND sea surface temperature (SST) anomaly over the Niño3.4 and Indian Ocean Dipole (IOD) regions are considered. In addition, SST anomalies over the Bay of Bengal (BoB), such as east BoB (EBoB), east equatorial Indian Ocean (EEIO) and their gradients (GSST) connections with NEMR, are also examined. The impact of Siberian High (SH), Western Pacific High (WPH), Arabian Sea Surface Pressure (ASSP), easterly waves (EW) and land surface temperature contrast (LSTC) is also analysed and incorporated to find a specific linkage with NEM 2021. The results show that excess NEMR during OND 2021 is suggestively associated with favourable large‐scale conditions like negative IOD and the La Niña phase over the Indo‐Pacific region. In November 2021, two deep depressions made landfall over Tamil Nadu (within the latitude of 11°–13°N) and over adjoining areas, which caused copious amounts of rainfall (maximum ~90 mm·day−1), leading to flooding conditions over Tirupati and Chennai. During the NEM season, abundant warming of BoB and weak vertical wind shear, along with negative anomalies of ASSP, modulated the cyclonic activities that enhanced the sufficient moisture transports from BoB to the PI. The MSLP anomalies over the ASSP region depict a significant correlation (−0.47) with the interannual variability of NEM as compared to SH and WPH during the OND season of 2021.

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.

A global climatology of tropical easterly waves

Tropical easterly waves (TEWs) are westward-propagating off-equatorial waves that are typically convectively coupled. TEWs make significant contributions to the annual rainfall in many regions of the tropics, and often seed tropical cyclones. Climatologies of TEWs exist regionally and hemispherically, however, none exist at the global scale. The climatology in this study is the first attempt to study TEWs globally, applying a combination of the TRACK algorithm and objective criteria to all basins to identify TEW activity at both 850 and 700 hPa. In addition to areas of TEW activity in previously studied regions such as the North Atlantic and eastern North Pacific Ocean basins, this study has identified TEW activity in every other tropical ocean basin in both hemispheres. On average across the globe, the methods employed tracked 380 waves per year at 850 hPa and 638 waves per year at 700 hPa. There were no significant linear trends globally or hemispherically over the 41 years analyzed, but large interannual variability. Despite the variety of regions the TEWs occur in, the distribution of average speeds agrees with studies using other data and tracking methods, with averages between 7.5–8 m s−1 depending on the level and hemisphere. TEW activity shows a strong preference to the warm season, with approximately double the number of TEWs occurring in the warm season compared to the cold season, a pattern that is observed in both the northern and southern hemispheres. This database is publicly available to enable further work in understanding TEW behavior and predictability globally.

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.

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.

On the Problems of Monitoring Atmospheric Humidity Fields Using Satellite Microwave Radiometry Methods When Studying Cyclogenesis in the Tropical Atlantic

Some problems of microwave radiometric methods for monitoring fields of total atmospheric moisture content from satellites in areas of hurricane formation in the tropical regions of the Atlantic are considered. The possibilities of using polar-orbiting satellites (DMSP, EOS Aqua, GCOM-W1) for these tasks are analyzed a feature of which is the presence of gaps in satellite data (blind spots) resulting from divergence of scanning bands at lower latitudes. It is shown through a number of examples that the joint use of various satellites makes it possible to reduce the influence of this effect and obtain a clear picture of the evolution of atmospheric humidity fields in the tropical Atlantic and their connection with the evolution of tropical waves that initiate the formation of hurricanes.

Tropical Easterly Waves Over Costa Rica and Their Relationship to the Diurnal Cycle of Rainfall

AbstractUsing an index of tropical easterly wave (TEW) activity derived from spacetime‐filtered outgoing longwave radiation, we construct composites of long‐term hourly surface meteorological observations and morningtime sounding data collected near San José, Costa Rica to investigate how TEWs affect the diurnal cycle of rainfall over land. Our results indicate that TEWs enhance the frequency of occurrence of rain during convectively active conditions over the course of the diurnal cycle. By contrast, rainfall conditional intensity sensitivity to TEW phase appears more nuanced, with indications that active conditions induce a slight delay in the timing of the diurnal peak intensity but a longer duration of heavier rainfall. Analysis of associated hourly surface meteorology along with sounding profiles and derived thermodynamic parameters points to both initial vertical and time‐evolving surface conditions regulating diurnal behavior, such as greater instability and higher precipitable water in morningtime profiles under active phase conditions.

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.

Tropical Cyclones and Equatorial Waves in a Convection‐Permitting Aquaplanet Simulation With Off‐Equatorial SST Maximum

AbstractTropical weather phenomena—including tropical cyclones (TCs) and equatorial waves—are influenced by planetary‐to‐convective‐scale processes; yet, existing data sets and tools can only capture a subset of those processes. This study introduces a convection‐permitting aquaplanet simulation that can be used as a laboratory to study TCs, equatorial waves, and their interactions. The simulation was produced with the Model for Prediction Across Scales‐Atmosphere (MPAS‐A) using a variable resolution mesh with convection‐permitting resolution (i.e., 3‐km cell spacing) between 10°S and 30°N. The underlying sea‐surface temperature is given by a zonally symmetric profile with a peak at 10°N, which allows for the formation of TCs. A comparison between the simulation and satellite, reanalysis, and airborne dropsonde data is presented to determine the realism of the simulated phenomena. The simulation captures a realistic TC intensity distribution, including major hurricanes, but their lifetime maximum intensities may be limited by the stronger vertical wind shear in the simulation compared to the observed tropical Pacific region. The simulation also captures convectively coupled equatorial waves, including Kelvin waves and easterly waves. Despite the idealization of the aquaplanet setup, the simulated three‐dimensional structure of both groups of waves is consistent with their observed structure as deduced from satellite and reanalysis data. Easterly waves, however, have peak rotation and meridional winds at a slightly higher altitude than in the reanalysis. Future studies may use this simulation to understand how convectively coupled equatorial waves influence the multi‐scale processes leading to tropical cyclogenesis.

Tropical Wave Observations From the Reel‐Down Atmospheric Temperature Sensor (RATS) in the Lowermost Stratosphere During Strateole‐2

AbstractTropical waves play an important role in driving the quasi‐biennial oscillation of zonal winds in the tropical stratosphere. In our study we analyze these waves based on temperature observations from the 2021–2022 Strateole‐2 campaign when the Reel‐down Atmospheric Temperature Sensor (RATS) was successfully deployed for the first time. RATS provides long‐duration, continuous and simultaneous high‐resolution temperature observations at two altitudes (balloon float level and 200 m below) allowing for an analysis of vertical wavelengths. This separation distance was chosen to focus on waves near the resolution limit of reanalyses. Here, we found tropical waves with periods between about 6 hr and 2 days, with vertical wavelengths between 1.5 and 5 km, respectively. Comparing our results to Fifth generation European Centre for Medium‐Range Weather Forecasts (ERA5) reanalyses we found good agreement for waves with a period longer than 1 day. However, the ERA5 amplitudes of higher‐frequency waves are under‐estimated, and the temporal evolution of most wave packets differs from the observations.

The Role of Water Vapor and Temperature in the Thermodynamics of Tropical Northeast Pacific and African Easterly Waves

Abstract The thermodynamic processes associated with convection in tropical African and northeastern Pacific easterly waves (AEWs and PEWs, respectively) are examined on the basis of empirical orthogonal functions (EOFs) and a plume buoyancy framework. Linear regression analysis reveals the relationship between temperature, moisture, buoyancy, and precipitation in EWs. Plume buoyancy is found to be highly correlated with rainfall in both AEWs and PEWs, and a near 1:1 relationship is found between a buoyancy-based diagnostic of rainfall and rainfall rates from ERA5. Close inspection of the contribution of moisture and temperature to plume buoyancy reveals that temperature and moisture contribute roughly equally to the buoyancy in AEWs, while moisture dominates the distribution of buoyancy in PEWs. A scale analysis is performed in order to understand the relative amplitudes of temperature and moisture in easterly waves. It is found that the smaller contribution of temperature to the thermodynamics of PEWs relative to AEWs is related to their slower propagation speed, which allows PEWs to more robustly adjust to weak temperature gradient (WTG) balance. The consistency of the buoyancy analysis and the scale analysis indicates that PEWs are moisture modes: waves in which water vapor plays a dominant role in their thermodynamics. AEWs, on the other hand, are mixed waves in which temperature and moisture play similar roles in their thermodynamics.

Diurnal Characteristics of Heavy Precipitation Events under Different Synoptic Circulation Patterns in the Middle and Lower Reaches of the Yangtze River in Summer

Aiming at the rainstorm days (≥50 mm/d) in the middle and lower reaches of the Yangtze River during 2010–2020, the obliquely rotated principal component in T-mode (PCT) method is used to classify the daily mean 850 hPa geopotential height, including Type 1 (vortex/shear line), Type 2 (frontal surface), Type 3 (warm shear line), Type 4 (warm inverse trough line), Type 5 (typhoon-westerly trough), and Type 6 (easterly wave). We studied the weather system configurations of different synoptic circulation patterns, their long-term trends, and their impacts on diurnal variations of heavy precipitation and drew the following conclusions: Type 1, Type 2, or Type 3 shows balanced double-peak frequencies of the start time of heavy precipitation during 06:00–08:00 BT and around 17:00 BT, respectively. For Type 1, dynamical lifting and thermal lifting play balanced roles, while for Type 2 and Type 3, dynamical lifting plays a key role. The number of rainstorm stations for Type 1 shows a slight increasing trend, while that for Type 2 or Type 3 shows a significant increasing trend. Type 4, Type 5, or Type 6 show a significant single peak frequency of the start time during 15:00–16:00. Type 5 and Type 6 are mainly affected by dynamical lifting along with favorable cape values, which can trigger rainstorms. The number of rainstorm stations for Type 4 or Type 6 shows a decreasing trend (that for Type 4 is more significant), while that for Type 5 shows a slightly increasing trend.

Easterly wave activity in the Intra Americas Seas region analyzed with vertically integrated moisture fluxes

Introduction: Easterly waves (EWs) are a dominant atmospheric phenomenon in the tropics. EWs in Mexico, Central America and the Caribbean region produce intense precipitation and significantly contribute to the summer rainy season, but some climatologies indicate their activity is weak or even null over the Intra-Americas Seas (IAS). As it is shown in the present study, EW activity over the IAS is not negligible but its variability is less than that off the western coast of Africa.Method: Given the well-known coupling of wind and atmospheric moisture in EWs, their characteristics may be described using vertically integrated moisture fluxes (VIMF) and their divergence. By means of Hovmöller diagrams, lagged correlations and composite analyses, the propagation and the meteorological effects of these waves across the Caribbean Sea and the Gulf of Mexico, particularly on moisture and rains, are examined.Results: It is observed that as EWs propagate, the VIMF convergence increases the level of precipitable water which frequently results in intense precipitation events. High frequency variability of the meridional component of the VIMF is used to estimate EW activity. Over the Caribbean Sea, EWs are less energetic than in the eastern tropical Atlantic even while the number of waves over these regions is comparable.Discussion: When the Caribbean Low-Level Jet (CLLJ) is strong, EW activity tends to diminish due to a decrease in precipitable water (PW), sea surface temperature (SST) over the Caribbean, and possibly to an intensified vertical wind shear. Therefore, years of a stronger than normal CLLJ tend to result in summer precipitation below normal over most of the IAS and southern Mexico.

Convectively Coupled Kelvin Waves and Tropical Cyclogenesis: Connections through Convection and Moisture

Abstract Recent research has demonstrated a relationship between convectively coupled Kelvin waves (CCKWs) and tropical cyclogenesis, likely due to the influence of CCKWs on the large-scale environment. However, it remains unclear which environmental factors are most important and how they connect to TC genesis processes. Using a 39-yr database of African easterly waves (AEWs) to create composites of reanalysis and satellite data, it is shown that genesis may be facilitated by CCKW-driven modifications to convection and moisture. First, stand-alone composites of genesis demonstrate the significant role of environmental preconditioning and convective aggregation. A moist static energy variance budget indicates that convective aggregation during genesis is dominated by feedbacks between convection and longwave radiation. These processes begin over two days prior to genesis, supporting previous observational work. Shifting attention to CCKWs, up to 76% of developing AEWs encounter at least one CCKW in their lifetime. An increase in genesis events following convectively active CCKW phases is found, corroborating earlier studies. A decrease in genesis events following convectively suppressed phases is also identified. Using CCKW-centered composites, we show that the convectively active CCKW phases enhance convection and moisture content in the vicinity of AEWs prior to genesis. Furthermore, enhanced convective activity is the main discriminator between AEW–CCKW interactions that result in genesis versus those that do not. This analysis suggests that CCKWs may influence genesis through environmental preconditioning and radiative–convective feedbacks, among other factors. A secondary finding is that AEW attributes as far east as central Africa may be predictive of downstream genesis. Significance Statement The purpose of this work is to investigate how one type of atmospheric wave, known as convectively coupled Kelvin waves (CCKWs), impacts the formation (“genesis”) of tropical cyclones. Forecasting of genesis remains a significant challenge, so identifying how CCKWs influence this process could help improve forecasts and give communities greater lead times. Our results show that CCKWs could temporarily make genesis more likely by increasing atmospheric moisture content and convective activity. While not all CCKWs lead to genesis, those that do are associated with a particularly strong increase in convection. This provides a potential tool for forecasters monitoring CCKWs and TC genesis in real time and motivates follow-up work on this topic in numerical models.