DOCSLIB.ORG

Interactions Between North African Vegetation and the African Easterly Jet: a Mechanism for Abrupt Climate Change

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Interactions Between North African Vegetation and the African Easterly Jet: a Mechanism for Abrupt Climate Change INTERACTIONS BETWEEN NORTH AFRICAN VEGETATION AND THE AFRICAN EASTERLY JET: A MECHANISM FOR ABRUPT CLIMATE CHANGE A Thesis Presented to the Faculty of the Graduate School of Cornell University In Partial Fulfillment of the Requirements for the Degree of Master of Science by Christina Marie Patricola January 2007 © 2007 Christina Marie Patricola ABSTRACT Modeling studies and paleoclimate evidence of the mid-Holocene, 6,000 years ago, indicate that the potential for rapid climate change exists over northern Africa. A regional climate model that produces an excellent representation of today’s climate over northern Africa is used to simulate the summer climate under present day solar forcing and SSTs and several prescribed static idealized zonal vegetation distributions. The purpose is to isolate and understand the role of interactions between vegetation and the dynamics of the West African summer monsoon in generating rapid climate change. Simulations with prescribed and interactive vegetation distributions are analyzed. In simulations with prescribed idealized vegetation, the regional model simulates only small differences in precipitation when the southern desert border is located between 10.0°N and 17.9°N. However, when the desert border is moved only 180 km, from 17.9°N to 19.4°N, summer precipitation increases by a factor of 5 over the Sahara, and the zonal structure observed in the present day climate is eliminated. These precipitation anomalies are associated with a 50% reduction in the magnitude of the African easterly jet, a 20% increase in the magnitude of the low-level westerly jet and a deepening and moistening of the thermal low. The model suggests that when the southern desert border is located north of a threshold latitude, the positive soil moisture anomalies beneath the region of maximum vertical velocity associated with the thermal low support positive low-level moist static energy anomalies, which are indicative of strengthened convection. Asynchronous coupling with a simple vegetation model reveals that when the initial desert border is located at 20.9°N, a new equilibrium vegetation distribution results in which the central Sahara is vegetated, indicating that the interactions between vegetation and the atmosphere may produce rapid climate change. When the initial desert border is located at 10.0°N, the equilibrium vegetation distribution closely resembles that of the present day, suggesting that the atmospheric conditions may allow regrowth of vegetation in a deforestation scenario under present day solar and SST forcings. BIOGRAPHICAL SKETCH Christina Marie Patricola was born Christina Marie DiRosario on February 14th 1983 and was raised in Franklin, Massachusetts. She developed an interest in the atmosphere by the age of five, and pursued this interest as a volunteer weather watcher for WCVB TV – Boston during her teens. Christina also enjoys playing softball and guitar, drawing cartoons, painting, artistic expression with bright colors, candlepin bowling, and reciting scenes from comedies with Jennifer and Matthew, her sister and brother. After graduating from Franklin High School in June 2001, she attended Cornell University where she received a BS in geological sciences with a concentration in atmospheric science in May 2005. Having already begun researching as an undergraduate under Dr. Kerry Cook, and captivated by the Northeast, she accepted the opportunity to continue at Cornell for graduate work. iii This thesis is dedicated to: Mom and Dad, and Grandma and Grandpa, for all their love and encouragement. Jennifer and Matthew, my partners in mischief and laughter. (“Heeeere, Snowflake…”) My whole family who, even when the Ithaca winter was at its coldest and windiest and I hadn’t seen the sun for days, can always cheer me up with a call during which the phone was passed along to all the relatives, Scotty too. StormTeam1102, who actually made it fun to find all those misprints in the frontogenesis equation and who got as much amusement as I did out of making a script pi pie. You nerds. All my friends I have met along the way, you are too many to list, but you all have a special place in my heart. My teachers, from Kindergarten to present, who have taught me so much. All scientists, who work so hard for the world and are a great inspiration. iv ACKNOWLEDGMENTS I thank my committee members, Kerry H. Cook, Stephen J. Colucci, and Bryan L. Isacks, for all of their time and effort spent helping me to improve the thesis. I would like to express my gratitude and respect to my advisor, Kerry Cook. I thank her for the great learning opportunity she has provided by advising me as both an undergraduate and graduate student, for her strong motivation, energy, enthusiasm, understanding, humor, and for all that I have learned from her. I also express my gratitude and respect for Stephen Colucci, for his strong commitment to teaching and for helping his students grow by setting high standards. I thank Edward Vizy for his dedication to the research group and for all the time he selflessly spent teaching me how to run the regional climate model. Thanks to Steve Gallow for all his hard work in maintaining the computer systems. I also thank the instructors I have had at Cornell - Kerry Cook, Stephen Colucci, Peter Gierasch, Mark Wysocki, Art DeGaetano, and Dan Wilks, for teaching enthusiastically and providing me with a strong base in atmospheric science. I also thank Mark Wysocki for inspiring my interest in teaching. Thanks to my office mates and fellow graduate students for many engaging discussions and to all the members of the department for their support. I am very grateful to NSF Award ATM – 0145481 and the Cornell Graduate Fellowship for providing funding for this research. v TABLE OF CONTENTS Biographical Sketch………………………………...…………………………iii Dedication……………………………………………………….…….………iv Acknowledgements………………………………………………..…...….…...v Table of Contents……………………………………………………………...vi List of Figures………………………………………………………………...vii List of Tables……………………………………………………………….....ix List of Abbreviations……………………………..……………………………x Introduction .…………………………………………………………………...1 Background ……………………………………………………………………3 Model description, simulation design, and model validation ...……………….7 Results ………………………………………………………………………..18 Conclusions…………………………………………………………………...41 References ……………………………………………………………………45 vi LIST OF FIGURES Figure 1. Landuse on the simulation domain for the present day according to the 24 category USGS dataset…………………………………………………………………8 Figure 2. Prescribed vegetation for the idealized simulations……………………….10 Figure 3. JJAS averaged zonal wind (m/s) along 12.5°W for the (a) NCEP reanalysis and (b) present day simulation……….……………………………………………….14 Figure 4. Precipitation (mm/day) for the (a) CRU and CMAP data set and (b) RCM simulation averaged June-July, and for the (c) CRU and CMAP data set and (d) RCM simulation averaged August-September..…………………………………………….16 Figure 5. (a) Summer averaged maximum wind speed (m/s) and (b) percent change in summer averaged maximum wind speed from present day for the idealized simulations for the southerly winds from the Gulf of Guinea the African Easterly Jet, and the low- level westerly winds…………………………………………………………………..19 Figure 6. Summer averaged precipitation anomalies (mm/day) from present day for the idealized simulations………………………...……………………………………22 Figure 7. Percent change in summer averaged precipitation from present day for the idealized simulations for the Guinean Coast region, the Sahel, and the Sahara……...24 Figure 8. Summer averaged meridional temperature gradient taken along 15°W for the idealized simulations……………………………………………………………...26 Figure 9. Summer averaged vertical velocity (mm/s) and specific humidity (10-3 kg H2O/kg air) anomalies from present day along the prime meridian for the idealized simulations………………………………………………………………………...….28 Figure 10. Summer averaged geopotential height (gpm) and winds (m/s) vectors at 910 hPa for the idealized simulations………………………………..……………….29 Figure 11. Summer averaged geopotential height (gpm) and wind vectors (m/s) at 825 hPa for the idealized simulations……………………………………………………..31 Figure 12. Summer averaged geopotential height (gpm) and wind vectors (m/s) at 625 hPa for the idealized simulations………………………….………………………….32 Figure 13. The JJAS (a) total MSE, temperature term, geopotential energy, and moisture term for the present day simulation, (b) total MSE (c) temperature anomaly vii from present day and (d) moisture anomaly from present day for idealized averaged over 17-20˚N and 8-12˚E. Units are 104 m2 s-2.……………………………………..37 Figure 14. Landuse prescribed for the iterations (a) des20.9_2, (b) des20.9_3, (c) des20.9_4, (d) des10.0_2, (e) des10.0_3, (f) des10.0_4, (g) des10.0_5, and (h) des10.0_6.………………………………………………………………………….…40 viii LIST OF TABLES Table 1. Land-use changes from present day for the idealized RCM simulations…..11 Table 2. USGS vegetation and physical properties for summer……………………..11 ix LIST OF ABBREVIATIONS AEJ.…………………………………………………………………African easterly jet AHP.……………………………………………………………..African Humid Period AGCM….………………………………………atmospheric general circulation model CMAP…………………...Climate Prediction Center Merged Analysis of Precipitation COLA……………………………………………..Center for Ocean-Land-Atmosphere CRU.…………………………………………………………….Climate Research Unit GCM………………………………………………………….general circulation model gpm…………………………………………………………………geopotential meters hPa………………………………………………...………………….……hectopascals JJAS………………………………………………...……….…June
Load more

Recommended publications
  • Monsoon'' and Land ITCZ Over West Africa
    Clim Dyn (2009) 32:1155–1171 DOI 10.1007/s00382-008-0514-3 A revised picture of the structure of the ‘‘monsoon’’ and land ITCZ over West Africa Sharon E. Nicholson Received: 15 January 2008 / Accepted: 15 December 2008 / Published online: 16 January 2009 Ó Springer-Verlag 2009 Abstract This article presents an overview of the land desertification, climate monitoring, hurricanes and inter- ITCZ (Intertropical Convergence Zone) over West Africa, annual variability. The West African monsoon is also based on analysis of NCAR–NCEP Reanalysis data. The related to a new paradigm for examining the interannual picture that emerges is much different than the classic one. variability of rainfall over West Africa, one that relates The most important feature is that the ITCZ is effectively changes in annual rainfall to changes in either the intensity independent of the system that produces most of the rain- of the rainbelt or north–south displacements of this feature. fall. Rainfall linked directly to this zone of surface The new view presented here is consistent with a plethora convergence generally affects only the southern Sahara and of research on the synoptic and dynamic aspects of the the northern-most Sahel, and only in abnormally wet years African Easterly Waves, the disturbances that are linked to in the region. A second feature is that the rainbelt normally rainfall over West Africa and spawn hurricanes over the assumed to represent the ITCZ is instead produced by a Atlantic, and with our knowledge of the prevailing synoptic large core of ascent lying between the African Easterly Jet and dynamic features.
    [Show full text]
  • Tropical Jets and Disturbances
    Tropical Jets and Disturbances Introduction In previous lectures, we alluded to several types of disturbances found across the tropics, including the monsoon trough and intertropical convergence zone, that arise as a function or reflection of convergent flow across the tropics. Tropical cyclones, particularly in the western Pacific and Indian Oceans, commonly form out of one of these features. Before we proceed into the second half of the course and our studies on tropical cyclones, however, it is important for us to consider other types of tropical disturbances and jets that influence tropical cyclone activity. We also return to the concept of the tropical easterly jet, first introduced in our monsoons lecture, and further discuss its structure and impacts. Key Concepts • What is an African easterly wave? How do they form, grow, and decay? • What are the African and tropical easterly jets and how do they influence easterly waves and the meteorology of Africa? • What is the Saharan air layer and what are its characteristics? African Easterly Waves African easterly waves, or AEWs, are westward-travelling waves that originate over northern Africa primarily between June and October. They have approximate horizontal wavelengths of 2500 km, periods of 3-5 days, and move westward at a rate of 5-8 m s-1 (or 500-800 km per day). Previous theories for AEW formation suggested that they formed in response to the breakdown of the ITCZ or the growth of instabilities associated with the African easterly jet (AEJ). Current theory, however, suggests that AEWs form in response to latent heating associated with mesoscale convective systems that form along higher terrain in eastern Africa.
    [Show full text]
  • On the Relationship Between the African Easterly Jet, Saharan Mineral Dust Aerosols, and West African Precipitation
    1MAY 2020 B E R C O S - H I C K E Y E T A L . 3533 On the Relationship between the African Easterly Jet, Saharan Mineral Dust Aerosols, and West African Precipitation EMILY BERCOS-HICKEY,TERRENCE R. NATHAN, AND SHU-HUA CHEN Atmospheric Science Graduate Group, Department of Land, Air, and Water Resources, University of California, Davis, Davis, California (Manuscript received 2 October 2018, in final form 27 January 2020) ABSTRACT The relationship between the African easterly jet (AEJ), Saharan mineral dust (SMD) aerosols, and West African precipitation (WAP) is examined using European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) data, the NASA Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), and the NASA Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) for July–September 1998–2017. The spatial orientation and structure of AEJs in different SMD–WAP environments are compared. In dustier years, the AEJ is farther east and stronger, rotates clockwise, and has larger zonal and vertical shears. In wetter years, the AEJ is farther north, has a shorter zonal extent, and has larger meridional shear. These changes to the AEJ are a response to the combined effects of the SMD and WAP on the thermal field, which is confirmed through sensitivity tests carried out with the Weather Research and Forecasting Model coupled to an interactive dust model. 1. Introduction In this study, we cull from the above mosaic the AEJ, SMD, and WAP. The spatial distribution of the three The summertime circulation over North Africa is a fields is shown in Fig.
    [Show full text]
  • African Easterly Jet in the ECMWF Integrated Forecast System: 4Dvar Analysis
    453 African easterly jet in the ECMWF integrated forecast system: 4DVar analysis A. M. Tompkins1, A. Diongue2, D. J. Parker3 and C. D. Thorncroft4 Research Department 1 ECMWF 2 Direction de la Meteorologie Nationale, Dakar-Yoff, Senegal 3 Institute for Atmospheric Science, University of Leeds, Leeds, UK 4 Department of Earth and Atmospheric Science, University at Albany, New York, USA Submitted to Q.J. Roy. Meteor. Soc. December 2004 Series: ECMWF Technical Memoranda A full list of ECMWF Publications can be found on our web site under: http://www.ecmwf.int/publications/ Contact: [email protected] ­c Copyright 2004 European Centre for Medium-Range Weather Forecasts Shinfield Park, Reading, RG2 9AX, England Literary and scientific copyrights belong to ECMWF and are reserved in all countries. This publication is not to be reprinted or translated in whole or in part without the written permission of the Director. Appropriate non-commercial use will normally be granted under the condition that reference is made to ECMWF. The information within this publication is given in good faith and considered to be true, but ECMWF accepts no liability for error, omission and for loss or damage arising from its use. African easterly jet in 4DVar analysis Abstract In August 2000, the Met Office C130 research flight aircraft conducted four sorties over the Western Sahel region, making dropsonde measurements at an unprecedented resolution. These were used as an independent validation source to assess the quality of the analysis of the African easterly jet (AEJ) in the European Centre for medium range forecasts (ECMWF) integrated forecast system.
    [Show full text]
  • Saharan Dust and the African Easterly Jet-African Easterly Wave System
    Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 143: 2797–2808, October 2017 A DOI:10.1002/qj.3128 Saharan dust and the African easterly jet–African easterly wave system: Structure, location and energetics Emily Bercos-Hickey,* Terrence R. Nathan and Shu-Hua Chen Atmospheric Science Program, Department of Land, Air, and Water Resources, University of California, Davis, CA, USA *Correspondence to: E. Bercos-Hickey, Atmospheric Science Program, Department of Land, Air, and Water Resources, University of California, One Shields Avenue, Davis, CA 95616-8627, USA. E-mail: [email protected] The radiative effects of Saharan mineral dust (SMD) aerosols on the structure, location and energetics of the African easterly jet–African easterly wave (AEJ-AEW) system are examined for July–September 2006. Experiments are conducted with and without SMD using the Weather Research and Forecasting (WRF) model, which is radiatively coupled to an interactive dust model. The SMD-modified heating field shifts the AEJ northward, upward and westward, and enhances its zonal asymmetry. These SMD-induced changes to the AEJ are manifest in the AEWs: the northern and southern tracks of the AEWs shift northward (like the AEJ); and the zonal-scale of the AEWs expands and their westward propagation increases. The SMD also strengthens the energetics of the AEJ-AEW system. The domain and temporally averaged baroclinic energy conversion, which is an order of magnitude larger than the barotropic conversion, increases by a factor of 2.5. The eddy kinetic energy and generation of available potential energy increase by factors of 1.5 and 2.7, respectively.
    [Show full text]
  • Generation of African Easterly Wave Disturbances
    Generation of African Easterly Wave Disturbances: Relationship to the African Easterly Jet Jen-Shan Hsieh* and Kerry H. Cook Department of Earth and Atmospheric Sciences, Cornell University Ithaca NY 14850 Accepted for Monthly Weather Review October 2004 *Corresponding author: Jen-Shan Hsieh, Department of Earth and Atmospheric Sciences. 3152 Snee Hall, Cornell University, Ithaca, NY14850 Email: [email protected] ABSTRACT The relationship between African easterly waves and the background climatology in which they form is studied using a regional climate model. The surface and lateral boundary conditions in the model are manipulated to modify the background climatology, especially the African easterly jet and the ITCZ, and the behavior of the waves in these different settings is evaluated. Three climate simulations are presented, with monthly mean lateral and surface boundary conditions. One has a strong jet and a strong ITCZ, the second has a strong jet and a weak ITCZ, and the third has a weak jet and a strong ITCZ. In these simulations, the presence of wave activity is more closely associated with the concentration of the ITCZ than the strength of the African easterly jet. In particular, the simulation with a strong jet accompanied by a weak ITCZ does not produce significant wave activity, but a weak jet with a strong ITCZ has realistic wave disturbances. Both the Charney-Stern and the Fjörtoft necessary conditions are satisfied in all three simulations, suggesting that combined barotropic and baroclinic instability contributes to the generation of waves. Near the origin of the waves, meridional gradient reversals of isentropic potential vorticity result from potential vorticity anomalies generated by convective heating within the ITCZ, implying that the unstable zonal flow may be caused by cumulus convection within the ITCZ and not by shear instability associate with the jet.
    [Show full text]
  • Climatology of the African Easterly Jet and Subtropical Highs Over North Africa and Arabian Peninsula and a Numerical Case Study of an Intense African Easterly Wave
    North Carolina Agricultural and Technical State University Aggie Digital Collections and Scholarship Dissertations Electronic Theses and Dissertations 2014 Climatology Of The African Easterly Jet And Subtropical Highs Over North Africa And Arabian Peninsula And A Numerical Case Study Of An Intense African Easterly Wave James D. Spinks North Carolina Agricultural and Technical State University Follow this and additional works at: https://digital.library.ncat.edu/dissertations Recommended Citation Spinks, James D., "Climatology Of The African Easterly Jet And Subtropical Highs Over North Africa And Arabian Peninsula And A Numerical Case Study Of An Intense African Easterly Wave" (2014). Dissertations. 64. https://digital.library.ncat.edu/dissertations/64 This Dissertation is brought to you for free and open access by the Electronic Theses and Dissertations at Aggie Digital Collections and Scholarship. It has been accepted for inclusion in Dissertations by an authorized administrator of Aggie Digital Collections and Scholarship. For more information, please contact [email protected]. Climatology of the African Easterly Jet and Subtropical Highs over North Africa and Arabian Peninsula and a Numerical Case Study of an Intense African Easterly Wave James D. Spinks North Carolina A&T State University A dissertation submitted to the graduate faculty in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department: Energy and Environmental Systems Major: Atmospheric Science Major Professor: Dr. Yuh-Lang Lin Greensboro, North Carolina 2014 i The Graduate School North Carolina Agricultural and Technical State University This is to certify that the Doctoral Dissertation of James D. Spinks has met the dissertation requirements of North Carolina Agricultural and Technical State University Greensboro, North Carolina 2014 Approved by: Dr.
    [Show full text]
  • Characterization of West African Jet Streams and Their Association to ENSO Events and Rainfall in ERA-Interim 1979–2011
    Hindawi Publishing Corporation Advances in Meteorology Volume 2014, Article ID 405617, 12 pages http://dx.doi.org/10.1155/2014/405617 Research Article Characterization of West African Jet Streams and Their Association to ENSO Events and Rainfall in ERA-Interim 1979–2011 Churchill Okonkwo, Belay Demoz, and Sium Tesfai Beltsville Center for Climate System Observation, Atmospheric Science Program, Howard University, Washington DC, DC 20059, USA Correspondence should be addressed to Churchill Okonkwo; [email protected] Received 15 November 2013; Revised 9 January 2014; Accepted 10 January 2014; Published 5 March 2014 Academic Editor: Sven-Erik Gryning Copyright © 2014 Churchill Okonkwo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The interannual variability of West African jet streams and their association with rainfall are reexamined using European Reanalysis ERA-Interim 1979–2011. The objective of the study is to characterize their climatology and role in rainfall variability in western Sahel. Wavelet analysis was used on wind speed data and implications to ENSO were discussed subsequently. Our results show that while the low-level African Westerly Jet (AWJ) correlates well with rainfall south of the equator in boreal winter months, the Tropical Easterly Jet (TEJ) and African Easterly Jet (AEJ) correlate better with rainfall north of the equator in the boreal summer months. Results of interannual-to-decadal variability in 200 mb, 600 mb, and 850 mb of zonal wind reveal that there is enhanced variability in the 2–8 year band.
    [Show full text]
  • Role of African Easterly Waves and Embedded Vorticity Features in Genesis of Tropical Cyclones Over the Eastern Tropical Atlantic
    17.6 Role of African Easterly Waves and embedded vorticity features in genesis of tropical cyclones over the Eastern Tropical Atlantic Introduction: Atlantic tropical cyclone activity exhibits large variability, which often has been linked to changes in the large-scale environment such as variation in North Atlantic SSTs (Goldenberg et al. 2001; Mann and Emanuel 2006; Landsea et al. 1998), various phases of ENSO (Goldenberg and Shapiro 1996), and tropospheric deep wind shear (Thorncroft and Pytharoulis 2001; Aiyyer and Thorncroft 2005). The timescales of the variability mainly considered a range from year-to-year to multi-decadal (variability in both number, and intensity). While the focus for many of these and numerous earlier studies has been the variability of tropical storms, hurricanes and/or the large scale environment, the variability of the precursor disturbances for these storms has not been discussed as much. The weather and climate of West Africa has a strong impact on Atlantic tropical cyclone activity. At daily timescales, most Atlantic tropical cyclones form in association with African easterly waves (AEWs) that originate over the African continent (e.g. Avila and Pasch 1992, and Pasch and Avila 1994). At seasonal-to-decadal timescales there is a well known positive correlation between West African rainfall and Atlantic tropical cyclone activity (e.g. Landsea and Gray 1992). Given the known importance of AEWs for downstream tropical cyclone activity on daily and possibly longer timescales, it is important to have a good knowledge and understanding of the nature and variability of AEWs as they leave the West African coast. AEWs are synoptic scale systems with a preferred wavelength in the 2000-4000km range, and also possess sub-synoptic1 scale structures that are associated with non-linear developments (e.g.
    [Show full text]
  • African Easterly Waves and Their Relationship to Rainfall on a Daily Timescale Jeffrey D
    Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2006 African Easterly Waves and Their Relationship to Rainfall on a Daily Timescale Jeffrey D. Baum Follow this and additional works at the FSU Digital Library. For more information, please contact [email protected] THE FLORIDA STATE UNIVERSITY COLLEGE OF ARTS AND SCIENCE AFRICAN EASTERLY WAVES AND THEIR RELATIONSHIP TO RAINFALL ON A DAILY TIMESCALE By JEFFREY D. BAUM A Thesis submitted to the Department of Meteorology in partial fulfillment of the requirements for the degree of Master of Science Degree Awarded: Spring Semester, 2006 i The members of the Committee approve the thesis defended by Jeffrey D. Baum on December 8th 2005. ----------------------------------------- Sharon E. Nicholson Professor Directing Thesis ----------------------------------------- T. N. Krishnamurti Committee Member ----------------------------------------- Kwang-Yul Kim Committee Member The Office of Graduate Studies has verified and approved the above named committee members. ii This thesis is dedicated to my parents DuWayne and Diane Baum for all there support throughout this entire venture. iii ACKNOWLEDGMENTS I would like to thank my major professor, Dr. Sharon E. Nicholson for her patience and guidance with this work. I would also like to thank my committee members; Dr. K. Kim, Dr. T.N. Krishnamurti. Also I would like to thank Dr. Andreas Fink for discussions about African Easterly Waves and the methodology for identifying African Easterly Waves. Finally I would like to thank my family, especially my lovely wife Samantha and my two kids Connor and Jillian for all their love and support. iv TABLE OF CONTENTS List of Tables ……………………………………………………………………...vii List of Figures …………………………………………………………………..…viii Abstract ………………………………………………………………………........xv 1.
    [Show full text]
  • Understanding the Variability of West African Summer Monsoon Rainfall: Contrasting Tropospheric Features and Monsoon Index
    atmosphere Article Understanding the Variability of West African Summer Monsoon Rainfall: Contrasting Tropospheric Features and Monsoon Index Akintomide Afolayan Akinsanola 1,2,* and Wen Zhou 2 1 Department of Geography, University of Georgia, Athens, GA 30602, USA 2 Guy Carpenter Asia-Pacific Climate Impact Centre, School of Energy and Environment, City University of Hong Kong, Hong Kong SAR, China; [email protected] * Correspondence: [email protected] Received: 10 February 2020; Accepted: 19 March 2020; Published: 22 March 2020 Abstract: West African Summer Monsoon (WASM) rainfall exhibits large variability at interannual and decadal timescales, causing droughts and floods in many years. Therefore it is important to investigate the major tropospheric features controlling the WASM rainfall and explore its potential to develop an objective monsoon index. In this study, monthly mean reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and monthly rainfall data from three gridded observations during the 65-year period of 1950–2014 were employed. Dry and wet rainfall years were identified using a standardized precipitation index. In a composite analysis of wet and dry years, the dynamical features controlling the WASM exhibit an obvious contrast between these years, and a weaker (stronger) African Easterly Jet (Tropical Easterly Jet) is observed during the wet years. Also, a well-developed and deep low-level westerly flow at about 850 hPa is evident in wet years while an obvious reversal is observed in dry years. Considering this, the main regions of the two easterly jet streams and low-level westerly wind are proposed for objectively defining an effective WASM index (WASMI).
    [Show full text]
  • Tropical Waves
    Tropical Waves John Cangialosi and Lixion Avila National Hurricane Center WMO Region IV Tropical Cyclone Workshop Outline • Basic definition • Schematic diagrams/Interactions • Operational products/forecasts • Tools for tracking • Exercise What are tropical waves? * Perturbations / disturbances in the tropical easterlies that typically move from east to west. * Often seen as inverted troughs of low pressure (inverted-V pattern in satellite imagery). Significant rain producers. * Convection typically on the east side. Subsidence/clearing on the west side. * Convection highly modulated by atmospheric moisture, upper level features, topography, etc. * Develop into tropical cyclones. * Around 60 tracked per year (little annual variability) How/where they form • Generated by an instability (baroclinic-barotropic)of the African easterly jet • Jet arises as a result of reversed lower tropospheric temperature gradient over west-central north Africa due to extremely warm temperatures over the Sahara Desert and substantially cooler temperatures along the coast of Guinea. Sahara Desert ~ 45 C African Easterly Jet near the Gulf of Guinea ~ 22 C Typical Synoptic Setup Tropical waves/African easterly waves move westward within the trade wind flow south of the Bermuda-Azores high Frequency by month Tropical Waves tracked by NHC Tropical wave activity in terms of numbers is highest June-August Schematic diagrams Classic inverted V-shape near the eastern Caribbean Schematic diagram Do waves tilt with height? At what pressure level is the maximum amplitude?
    [Show full text]