ABSTRACT MANTRIPRAGADA, RAMA SESHA SRIDHAR. Kelvin and Easterly Wave Interactions and their Modulation by Diurnal Cycle over Eastern Atlantic and Tropical Africa. (Under the direction of Dr. Anantha Aiyyer.) This thesis focuses on two aspects. The first part focuses on examining the African easterly waves (AEW) activity’s modulation by convectively coupled Kelvin waves (CCKWs) over eastern Atlantic and West Africa. The second part focuses on examining the regional differences in the structure of CCKWs and AEWs and the diurnal cycle’s influence on AEWs and CCKWs. CCKWs modify the background environment in which AEWs get either amplified or suppressed. The CCKW convective phase enhances the magnitudes of the meridional gradient of mean zonal wind to the south of the African easterly jet (AEJ). Thus enhanced barotropic energy conversions and more AEW southern storm track growth in the CCKW convective phase than in the CCKW suppressed phase. Besides, AEWs are more convectively active in the CCKW convective phase than in the suppressed phase. The AEW northern track enhanced more in the CCKW convective phase than in the suppressed phase primarily through enhanced baroclinic energy conversions. The CCKW convective phase enhances the vertical gradient’smagnitudes of the mean zonal wind and associated lower-troposphere meridional gradients of temperature below and the poleward side of the AEJ. The AEW southern storm track diabatic heating profile over central Africa exhibits a top-heavy structure. In contrast, near coastal West Africa and the eastern Atlantic, the diabatic profile displays a bottom-heavy structure. The AEW diabatic heating vertical struc- ture matches with the climatological diabatic heating structure. The bottom-heavy mean diabatic heating structure over eastern Atlantic and coastal West Africa is associated with mean horizontal convergence below 800 hPa and divergence above. The top-heavy mean diabatic heating structure over central Africa is associated with elevated mean horizon- tal convergence above 850 hPa and divergence above 350 hPa and below 850 hPa. AEWs become less cold-core as they move westward towards coastal west Africa while these cold anomalies shift to higher altitudes over the eastern Atlantic. This study argues the importance of large-scale circulations in constraining the AEWs diabatic heating vertical mode. CCKWs typical structure over the eastern Atlantic matches with that observed over the Pacific ocean. CCKWs, as they move towards coastal west Africa, their structure is distorted from other influences such as topography and land-ocean contrast. They intensify and regain their typical structure as they move towards central Africa. The lower-troposphere cold anomalies intensify as they move from the eastern Atlantic towards central Africa. To the east of Greenwich meridian, the upper-troposphere positive temperature perturba- tions and positive diabatic heating perturbations become in-phase, suggesting more wave growth. The CCKWs and AEWs, when in phase with the diurnally enhanced convection, intensi- fies and produce more rainfall than when they are in phase with the diurnally suppressed convective phase. © Copyright 2021 by Rama Sesha Sridhar Mantripragada All Rights Reserved Kelvin and Easterly Wave Interactions and their Modulation by Diurnal Cycle over Eastern Atlantic and Tropical Africa by Rama Sesha Sridhar Mantripragada A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Master of Science Marine, Earth and Atmospheric Sciences Raleigh, North Carolina 2021 APPROVED BY: Dr. Carl Schreck Dr. Gary Lackmann Dr. Anantha Aiyyer Chair of Advisory Committee DEDICATION Dedicated to my parents and family members. ii BIOGRAPHY I am trained in under-graduation as a Mechanical engineer. I got interested in Atmospheric sciences while working as a Senior Research Fellow at the Indian Institute of Sciences Bangalore, India. My interest in atmospheric sciences made me pursue graduate studies at the Indian Institute of Technology Kharagpur, India, and North Carolina State University. I got a Ph.D. admission at George Mason University for Spring 2021, and I am excited to start the program. iii ACKNOWLEDGEMENTS I want to thank my advisors Dr. Carl Schreck and Dr. Anantha Aiyyer, for their inputs to the project. I also thank Dr. Gary Lackmann for his inputs to the project. Special thanks go to my family members for their support during the challenging and stressful times. This work was supported by NASA through awards NNX16AE33G and NNX17AH61G. We thank ECMWF and NCAR for access to the ERA-Interim reanalysis (obtained from: https://rda.ucar.edu/datasets/ds627.0/). iv TABLE OF CONTENTS LIST OF FIGURES ................................................. vii Chapter 1 INTRODUCTION ........................................ 1 1.1 Definitions................................................ 1 1.1.1 West African monsoon ................................. 1 1.1.2 Convectively coupled Kelvin waves ........................ 1 1.1.3 African easterly waves.................................. 2 1.2 Motivations............................................... 2 1.3 Background............................................... 3 1.3.1 WAM circulations and convection......................... 3 1.3.2 Theories for AEW initiation and growth..................... 5 1.3.3 Theories for CCKW initiation and growth.................... 7 1.3.4 Synoptic and Intraseasonal AEW variability .................. 9 1.4 Research questions.......................................... 10 1.5 Figures .................................................. 12 Chapter 2 DATA AND METHODS .................................... 18 2.1 Data Sources .............................................. 18 2.1.1 ECMWF Interim Re-Analysis............................. 18 2.1.2 TRMM precipitation................................... 18 2.2 Identification of CCKWs and AEWs.............................. 19 2.3 Composite analysis ......................................... 20 2.4 Statistical significance testing.................................. 21 2.5 Budgets.................................................. 21 2.5.1 EKE and PAPE budget.................................. 22 2.5.2 Q1 and Q2 ........................................... 24 2.6 Figures .................................................. 26 Chapter 3 Energetics of CCKW-AEW interaction ........................ 28 3.1 Impact of CCKWs on AEW activity............................... 28 3.1.1 Southern AEW track ................................... 28 3.1.2 Northern AEW track ................................... 29 3.2 Energetics ................................................ 29 3.2.1 Southern AEW track energetics ........................... 30 3.2.2 Northern AEW track energetics ........................... 32 3.3 Impact of CCKWs on mean environment.......................... 34 3.4 Discussion................................................ 36 3.5 Conclusions............................................... 37 3.6 Figures .................................................. 39 Chapter 4 ROLE OF CCKW’s and AEW’s ON MOIST CONVECTION ........... 50 4.1 WAM Climatology .......................................... 50 4.2 CCKW and AEW structure..................................... 51 v 4.2.1 CCKW vertical structure ................................ 51 4.2.2 AEW vertical structure.................................. 54 4.3 Diurnal modulation of AEW and CCKW convection.................. 57 4.4 Discussion................................................ 59 4.5 Conclusions............................................... 61 4.6 Figures .................................................. 62 Chapter 5 Summary ............................................. 72 5.1 The Influence of CCKWs on AEWs............................... 72 5.2 Regional differences in CCKW and AEW structure ................... 73 5.3 Influence of diurnal cycle on CCKWs and AEWs..................... 74 5.4 Future work............................................... 74 BIBLIOGRAPHY .................................................. 76 vi LIST OF FIGURES Figure 1.1 a) Schematic illustration of climatological atmospheric and oceanic features of West Africa in July. Shown are the positions of the ITD, the monsoon trough, upper-level air streams (AEJ, TEJ/EJ (Easterly Jet) and STJ), surface winds coloured according to the 2m air temper- ature (see colour bar), the tropical rain belt with the maximum axes of rainfall (RRm a x ), northerly and southerly AEW vortices propaga- tion zones (AEWn and AEWs respectively), areas with relatively cold SSTs (SSTanom ) and example pressure lines marking the Azores (‘H’), Libyan (‘h’) and Saint Helena (‘H’) highs and the heat low (‘L’). b) Schematic cross-section of the atmosphere between 10°W and 10°E in July. Shown are the positions of the ITD, upper-level jet streams (AEJ, TEJ/EJ and STJ), the monsoon layer (ML) (as defined by west- erly, i.e. positive zonal winds), streamlines, clouds, the freezing level (0◦C isotherm), isentropes, minimum (Tn ), maximum (Tx ) and mean temperatures (T ), dew point temperatures (Td ), atmospheric pres- sures (p) and mean monthly rainfall totals (RR). From Cornforth et al. [2019] ......................................... 13 Figure 1.2 Schematic showing the four key phases of the annual cycle of the West African monsoon. Included for each phase are the following: the location of the main rain band (indicated by clouds and rainfall with peak values highlighted