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3.1. Regional Weather Dynamics and Forcing in Tropical and Subtropical Northwest Africa

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3.1. Regional Weather Dynamics and Forcing in Tropical and Subtropical Northwest Africa Regional weather dynamics and forcing in tropical and subtropical Northwest Africa Item Type Report Section Authors Santos-Soares, Emanuel Francisco Publisher IOC-UNESCO Download date 27/09/2021 19:24:50 Link to Item http://hdl.handle.net/1834/9177 3.1. Regional weather dynamics and forcing in tropical and subtropical Northwest Africa For bibliographic purposes, this article should be cited as: Santos Soares, E. F. 2015. Regional weather dynamics and forcing in tropical and subtropical Northwest Africa. In: Oceanographic and biological features in the Canary Current Large Marine Ecosystem. Valdés, L. and Déniz‐ González, I. (eds). IOC‐UNESCO, Paris. IOC Technical Series, No. 115, pp. 63‐72. URI: http://hdl.handle.net/1834/9177. The publication should be cited as follows: Valdés, L. and Déniz‐González, I. (eds). 2015. Oceanographic and biological features in the Canary Current Large Marine Ecosystem. IOC‐UNESCO, Paris. IOC Technical Series, No. 115: 383 pp. URI: http://hdl.handle.net/1834/9135. The report Oceanographic and biological features in the Canary Current Large Marine Ecosystem and its separate parts are available on‐line at: http://www.unesco.org/new/en/ioc/ts115. The bibliography of the entire publication is listed in alphabetical order on pages 351‐379. The bibliography cited in this particular article was extracted from the full bibliography and is listed in alphabetical order at the end of this offprint, in unnumbered pages. ABSTRACT This study explores the interactions between tropical and sub‐tropical Northwest Africa continental circulation systems and the surrounding ocean basin, extending from equatorial and tropical monsoon semi‐arid to desert regions. Temperate climates remain basically rare throughout the continent, as climate is mostly dependent on rainfall distribution. The Azores subtropical high, that causes a surface northeast continental airmass, and the monsoon circulation flow are the major systems that influence regional weather. Over the continent, the moving very dry airmass gets even drier while flowing across the Sahara Desert. The summer monsoon is the principal source of precipitation associated to large scale convective events and synoptic perturbations. Local and regional systems are associated to the synoptic variability of the mid‐level African easterly jet, which is generated in response to the northern hemisphere spring inverse latitudinal surface temperature gradient. In altitude, the prevailing strong sub‐tropical westerly jet is characterized by convergence into the low pressure areas and divergence in the anticyclonic surface flow. During summer, winds become easterlies with baroclinic and barotropic gradient increase. Other systems like the subtropical moving highs, tropical plumes and the Saharan air layer also play a major role in the weather dynamics as forcing mechanisms over the region. Keywords: Subtropical high · Monsoon · Easterly jet · Subtropical westerly jet · Saharan air layer · Canary Current Large Marine Ecosystem · Northwest Africa IOC TECHNICAL SERIES, No. 115, pp. 63‐72. URI: http://hdl.handle.net/1834/9177. 2015 REGIONAL WEATHER DYNAMICS AND FORCING IN TROPICAL AND SUBTROPICAL NORTHWEST AFRICA Emanuel Francisco SANTOS SOARES Instituto Nacional de Meteorologia e Geofísica. Cabo Verde 3.1.1. INTRODUCTION The region under study, herein referred as region or area, comprises tropical and subtropical Northwest Africa (NWA) lying between latitudes 10°N and 35°N and longitudes 5°W and 30°W, including the Canary Current Large Marine Ecosystem (CCLME) area of interest. Most of it consists of an undulating low plateau below 500 m above sea level (a.s.l.), being fringed on the west and south by a coastal plain. It is a region of high insulation with considerable variation in intensity, effectiveness and duration of sunlight in all the different areas and at different times of the year. Mean annual radiation is about 105 kcal m‐2 near the coast and the equator, resulting from cloud cover in the rainy season, while on the edge of the desert it amounts to up to 205 kcal m‐2. The low density of the surface weather observation network has been a great handicap to collecting existent knowledge and sufficient data to conduct a NWA atmospheric circulation systems analysis. The massive use of satellite observations and limited area models (based on the analysis of systematic estimates and meteorological data at higher spatial resolution, as well as regional‐scale simulations) has made it possible to reconstruct and further improve the knowledge on weather dynamics over the region and the surrounding ocean basin (Figure 3.1.1). It has been found that most of the prevailing atmospheric circulation structures that affect the weather over the region are thought to impact distant atmospheric dynamics (Kirtman and Pirani, 2009). A continental airmass of surface northwest flow component blows consistently across most of the area. It is a very dry airmass, with maintained subsidence and stability until it reaches equatorial areas. Another circulation structure, the summer African monsoon, is a primary cause of rainfall distribution over the area, and still today is a very complex mechanism related to diurnal forcing effects. Other mechanisms related to diurnal forcing effects are: large‐scale convective events, deep convective systems, and synoptic wave perturbations on convective systems. The intraseasonal time‐scale variability of all these atmospheric circulation systems is the result of several superimposed atmospheric and oceanic physical mechanisms, many of them associated with the synoptic variability of the African easterly waves (AEW). These are characterized by peaks of low‐ to mid‐level wind shear and high convective available potential energy in the vicinity of elevated terrain as a response to coherent and organized local convection. The seasonal time scale of the atmospheric dynamic characteristics of the African monsoon is also associated with the African easterly jet (AEJ), which is developed in response to the inverse latitudinal surface temperature gradient during boreal spring. In the upper levels, between parallels 20°N and 30°N, higher velocity westerly subtropical jet stream (STJ) is developed due to the convergence into low‐pressure areas and the divergence in the anticyclonic surface flow. During summer, the baroclinic and barotropic gradients increase due to maritime airmass flow, causing a reverse stream that becomes the tropical easterly jet (TEJ). The interaction of the overall regional dynamic circulation with the ocean forcing 63 Santos Soares, E. F. Weather dynamics and forcing in Northwest Africa mechanism is still today not well understood, and hence these fields are considered a challenge for future land, ocean and atmospheric fields of research. Figure 3.1.1. Summer mean meridional circulation and associated mean winds (m s‐1) over NW Africa (reproduced from Hourdin et al., 2010). This scheme summarizes the interaction between the surface monsoon and westerly (5°N‐20°N) stream flow, the AEJ at the 600 hpa level, and the TEJ at 200 hpa, during summer. It also shows the location of the Intertropical Convergence Zone (ITCZ) deep moist convection (5°N‐10°N), and the dry convection in the intertropical discontinuity – ITD (20°N). The vertical flows are shown as streamlines. The westerlies (solid lines) and easterlies (dashed lines) are represented as mean speed (m s‐1) wind contour lines. The arrows indicate air flow direction. The grey area indicates areas of strong convection, upward movement, and turbulence. The jets are represented by yellow coloured areas. © American Meteorological Society. 3.1.2. DATA The data used to study this regional tropical and subtropical area resulted from combining several observations and forecast output from a weather prediction model called NCEP_Reanalysis 2. In this model, the probability distributions are associated with observations, and forecasts are merged with dynamical constraints (Kanamitsu et al., 2002). This NCEP_Reanalysis 2 information was provided by the NOAA/OAR/ESRL PSD, United States of America (website at http://www.esrl.noaa.gov/psd/, accessed on 25 February 2015). The data made it possible to analyse wind, temperature and moisture fields in order to identify the 2D and 3D atmospheric regional flow as well as local prevailing structures and interacting forcing mechanisms. 3.1.3. WEATHER ZONES The NWA region is greatly influenced by the northern, continental Saharan airmass to the north of the intertropical discontinuity (ITD). This is associated with a north southward pulsation of a very narrow line of 64 IOC TECHNICAL SERIES, No. 115, pp. 63‐72. URI: http://hdl.handle.net/1834/9177. 2015 a more or less abrupt discontinuity. It is a front which lacks temperature and precipitation contrast between a continental airmass to the north (dry) and a tropical, humid maritime airmass to the south. Basically deriving from precipitation distribution from north to south, the regional climate has been divided into Zones A, B, C and D. To the north of the ITD, Zone A is characterized as having little cloud with periods of dust and haze, low vapour pressure and a large diurnal temperature range, nocturnal temperature inversion near the surface, east‐northeasterly moderate winds and prevailing rainless conditions. Zone B lies on the ITD southern side and is humid but mainly rainless due to restriction in cloud development, although afternoon thunderstorms may occur. Fair weather cumulus is the dominant figure. Zone C is an approximately 800‐km belt south of Zone B, similar to it but with marked local thunderstorms and westward‐moving disturbances with heavy showers. Although it tends to be subdivided, Zone D extends some 300 km to the south of Zone C with almost daily occurrence of persistent precipitation and SW mean steady winds. The movement of the line of discontinuity is a repeating pattern that follows the seasonal movement of the sun, making the tropical and subtropical NWA rainless during winter in regard to its atmospheric forcing over the region. 3.1.4. ATMOSPHERIC CIRCULATION MECHANISMS 3.1.4.1. The Hadley cell The dominant atmospheric flow is the so‐called Hadley cell.
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