Regional dynamics and forcing in tropical and subtropical Northwest Africa

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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 remain basically rare throughout the continent, as is mostly dependent on rainfall distribution. The 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 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. Assimilated to a non‐rotating globe, in literature this cell is a well understood atmospheric system described as a closed circulation loop, which begins at the equator with warm, moist air lifted aloft in the induced low‐pressure areas to the tropopause, and carried poleward. At about 30°N, this air descends into an area consisting of an induced high‐pressure subtropical belt that extends more or less continuously around the globe, being neither homogeneous nor continuous. A portion of the descending air travels equatorially along the surface, closing the loop of the northern Hadley cell and creating the trade winds that blow almost throughout the year (Schubert et al., 1991). It is accurate to describe this motion as a figure that follows the sun’s zenith point, or a type of thermal equator which undergoes a semiannual north‐south migration (Leroux, 2001). It can be considered to be warm air rising in the Gulf of Guinea (low pressure), flowing north towards the pole, descending over the Atlas Mountains of NWA (high pressure) and being redirected to the equator at the low levels. Were it not for the Coriolis Effect, in the northern hemisphere this circulation would be a simple flow due south from the parallel 30°N at the surface and in reverse direction from the equator in the upper level. The Coriolis force turns the northerly blowing winds into northeast trades, and the counter trades aloft westerlies due to the diminishing friction effect with altitude. This basic circulation system is the forcing mechanism that influences the weather over tropical and subtropical northwest Africa and the adjacent waters (Leroux, 2001).

3.1.4.2. The subtropical and the trade winds

The of Azores in the northern hemisphere and Saint Helen in the southern hemisphere are the two belts that affect directly, and are the main forcing mechanisms over, the atmospheric dynamic flows and tropical and subtropical NWA climates on the north side of the equator, with the first largely overrunning during the boreal winter. The northern high pressure cell tends to disappear over land areas especially during the summer season, although it remains in altitude between 500 hpa and 700 hpa above the Sahara Desert. It has the characteristics of being a cell of subsidence and divergence and remarkable dryness (Laing and Evans, 2011). 65 Santos Soares, E. F. Weather dynamics and forcing in Northwest Africa

The surface flow of the Hadley northern circulation cell is generally from the NE and tends to curve to the west, responding to the anticyclone curvature. This northeast trade wind, a continental airmass called harmattan in the Sudan area, blows over most of tropical and subtropical almost all year round. It is a very dry airmass that becomes even drier while flowing across the Sahara Desert, with maintained subsidence and stable patterns until reaching equatorial areas where it acquires moisture. Above the Atlantic Ocean it generally stratifies with a slightly lower wet and cool layer and a relatively warm and dry upper layer. It is a well‐marked low‐level inversion up to the height of approximately 500 m in the proximity of the anticyclonic cell. This property prevents atmospheric upwelling and so inhibits cloud development and precipitation. Over the ocean, the wet layer becomes thicker, with decreasing stability and increasing intensity and frequency of rainfall. With moisture increase, upward motion replaces the horizontal flow and the trade winds become an equatorial mass with vertical precipitation developing structure named Inclined Meteorological Equator (IME) by Leroux (2001). The trade winds are somehow a steering flow for tropical that form and move across the Atlantic Ocean, often transporting African dust westward with shallow cumulus clouds seen within. This type of local cloud is capped from becoming taller by the trade wind inversion, due to descending air aloft from within the subtropical ridge. It has been observed that the stronger the trade winds, the less rainfall can be expected in the neighbouring land areas (Giresse, 2008).

3.1.4.3. The subtropical jet stream

In the absence of friction, the wind velocity is significantly higher in the upper levels at approximately 12 km altitude (200 hpa) between 20°N and 30°N, generating the subtropical jet stream (STJ). This westerly wind travels from the Atlantic Ocean to cross NWA with speeds attaining 70 knots, with convergence into low pressure areas and divergence in the anticyclonic flow at the surface. During summer time, the baroclinic and barotropic gradients across tropical and subtropical NWA increase due to the maritime airmass flow. The STJ turns into easterly tropical jet with a mean velocity of approximately 40 knots at the 100 hpa level (15 km a.s.l. to 20 km a.s.l.), and around 13°N to 20°N with a velocity of 20 knots at the 500 hpa level (approximately 5 km a.s.l.). This lower level wind is the AEJ (Leroux, 2001).

3.1.4.4. The low pressures over the Sahara Desert

During summer, an inflow of moist air from the Atlantic Ocean is caused by sensible and latent heat release over the Sahara Desert. This flux of moisture‐laden air contributes to the northwest and north displacement of the Azores subtropical anticyclone and the Intertropical Convergence Zone (ITCZ), moving and becoming more active over the northern hemisphere. The sensible and latent heat rise drives the development of low‐pressure conditions, as opposed to the reverse condition during winter when relative high‐pressure conditions prevail. This situation is intensified by the Saint Helen southern subtropical high that intensifies and expands towards the equator during summer, becoming a very important circulation driving force (Laing and Evans, 2011).

3.1.4.5. The monsoon

During summer, the NWA coastal regions receive humid flows of air from SSW, SW and WSW which curve towards the east while entering the continent in opposition to the prevailing easterly stream flow, extending beyond its boundary. This thick moist layer of about 3 km is an IME (Leroux, 2001), with potential instability and forward and retreat pulses, and strong vertical uplifts of thermal convection and turbulence. It is the main cause of precipitation, of development of cumulonimbus cloud clusters with strong vertical development and convection, and is the development source of periodic complex systems induced by kinematic and frontal processes (Taylor, 2008). 66 IOC TECHNICAL SERIES, No. 115, pp. 63‐72. URI: http://hdl.handle.net/1834/9177. 2015

3.1.4.6. The African easterly jet (AEJ) and waves (AEW)

The AEJ (Figure 3.1.2) is a region of the lower troposphere over NWA where the seasonal easterly mean wind speed is the maximum. The jet develops as a result of the land mass heating during the summer, creating a surface temperature and moisture gradient between the Sahara Desert and the Gulf of Guinea. The surrounding atmosphere responds by generating vertical wind shear in order to maintain thermal wind balance (Thorncroft et al., 2008). During January, the stream lies at the height of 3000 m a.s.l. at a latitude of 5°N. The wind speed rises from 30 km h‐1 in January to 40 km h‐1 in March. While displacing northward to 7°N, winds within the jet increase to 45 km h‐1 (Chen and van Loon, 1987). By June, it shifts northward into NWA (Leroux, 2001) The development of the over northern Africa leads to a low‐level westerly jet stream which is in place from June to October at the south of the ITCZ (Cook, 1999). During summer, the mid‐level African easterly jet occurs above West Africa between 10°N and 20°N (Pu and Cook, 2010). The jet reaches its zenith in August, lying between the latitudes of 16°N and 17°N. In September, wind velocity can be close to 50 km h1 between the parallels of 12°N and 13°N (Parker et al., 2005). The easterly jet weakens and drops southward during the months of October and December (Leroux, 2001) The low‐level AEJ plays a crucial role in the dynamics of the African monsoon (Low, 2005) while, with both barotropic and baroclinic instability, it helps to form the AEW, which are ubiquitous weather disturbances that cross the tropical Atlantic basin during the summer season (Parker et al., 2005; Pu and Cook, 2010; Leroux and Hall, 2009). With a preferred wavelength between 2000 km and 4000 km, the wave is a synoptic and subsynoptic scale structure system associated with nonlinear developments (Reed et al., 1977; Thorncroft et al., 2008) of potential vorticity anomalies generated by convection within the mesoscale convective systems (Schubert et al., 1991). Some mesoscale systems are embedded in the waves and are potential developers of tropical after they move into the tropical Atlantic during the months of August and September (Nicholson et al., 2007). During the rest of the year the wave is further south and its activity is suppressed (Pu and Cook, 2010). The variability on AEJ and wave can considerably affect the regional seasonal weather dynamics, temperature and precipitation (Figure 3.1.2).

3.1.4.7. Intertropical convergence zone (ITCZ)

This dynamic system is characterized by its low intertropical derived from the trade wind convergence that results in vertical upward flow. It forms an IME towards the north, with low unstable winds at the ground and is vertically structured on the equator, forming an oblique low troposphere stream flow towards the continent (Schubert et al., 1991). The semiannual movement of the ITCZ is associated with and follows the sun's zenith point. As dry air from the north and humid air from the south meet, it is an area of conflict between northeastward continental and eastward oceanic wind flows, and therefore a region of major turbulence. It influences the northern hemisphere basically from July to September. In consequence, the semi‐arid regions to the north experience related precipitation only during peak summer. All year long, there is a persistent easterly midtropospheric wind flow overlying the surface stream flow (Leroux, 2001).

67 Santos Soares, E. F. Weather dynamics and forcing in Northwest Africa

a)

Figures 3.1.2. Vertical cross‐sections of zonal winds (m s−1) during wet years (dashed lines = easterlies and solid lines = westerlies), TEJ and the mid‐tropospheric AEJ; a) corresponds to b) wet years and b) dry years (reproduced from Grist and Nicholson, 2001). During wet years, the westerly wind flow is stronger across the WA. It can also reach the 500 hpa pressure level, so forcing the AEJ to move further north at the 600 hpa pressure level. The closed contours lines at 600 hpa and 200 hpa levels indicate the location of the AEJ and the TEJ flows aloft. © American Meteorological Society.

3.1.4.8. Saharan Air Layer (SAL)

The Saharan Desert region of subtropical Africa is the origin of a prevalent atmosphere extending several kilometers from the surface upwards, called SAL (Figure 3.1.3). It is an intensely dry, very hot layer that often overlies the cooler, more humid surface air. As it drives from continental areas out over the ocean, it is normally lifted above the denser marine air causing the trade inversion layer (TWI) where temperature increases with height. Being dry air, the SAL has a temperature dropping steeply with height. Also within the SAL, many disturbances with thunderstorm structures periodically develop over NWA, resulting in large dust and sand storms extending vertically up to 6 km. It has been pointed out that the winds blow twenty percent of dust from a Saharan storm out over the Atlantic Ocean (Braun, 2010), twenty percent of it reaching all the way to the western Atlantic, and the remainder settling out into the ocean or being washed out by precipitation (Dunion, 2007). The dust clouds, often with iron‐rich particles visible by satellite, are associated with solar radiation reflectors, being atmospheric coolers as well as reducers of amounts of sunlight reaching and heating the ocean. The SAL has been associated with the suppression of tropical disturbance development and intensification (Evan et al., 2007; Karyampudi and Carlson, 1988), since its particles tend to increase condensation as they drift into the marine layer, but do not lead to precipitation since the drops that are formed are too small and cannot coalesce properly. The drops tend to evaporate when moving into drier air or air mixing takes place (Dunion and Velden, 2004). On account of uncertainties that still remain regarding its forcing mechanism and interaction in the regional atmosphere, SAL is an object of ongoing research in the field of dust particle analysis and interactions (i.e. Saharan Air Layer Experiment or SALEX (website at http://www.aoml.noaa.gov/hrd/project2007/sal.html, accessed 1 April 2015); Sassen et al., 2003; Garrison et al., 2003).

68 IOC TECHNICAL SERIES, No. 115, pp. 63‐72. URI: http://hdl.handle.net/1834/9177. 2015

Figure 3.1.3. The image is an example of a natural colour picture of the Saharan Air Layer (SAL) flow off the west coast of Africa. The image taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on July, 2013, shows dust plumes blowing over the NWA and northeast Atlantic (dust is represented by light milky brown colour and clouds in white). These layers of dry and hot air are often transported all the way across the ocean to the western side of the Atlantic (Source: NASA image, by MODIS Rapid Response Team, Goddard Space Flight Center).

3.1.5. ANNULAR INFLUENCE

The seasonal variations of the atmospheric systems over tropical and subtropical Africa are the dominant components of variability over the region, with sensible heating to warm land surfaces more rapidly than the ocean mixing layer. Due to differing heat capacities between land and water, the result is a seasonal monsoon‐type circulation in the southern region and a steady northeast stream in the central and northern part, a pattern that affects the overall weather conditions across the region and surrounding ocean basin. These areas experience a remarkable seasonality in terms of atmospheric climate parameters, including pressure, temperature, radiation, evaporation and cloudiness, comprising two basic systems six months apart, wet and dry.

3.1.5.1. The North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is defined as the sea level pressure difference between the Azores high and the . It is a part of the Northern Hemisphere annular mode and a recurrent pattern of atmospheric variability. It consists of a leading mode of variability that can have a considerable influence on temperature and precipitation, expressing variability at multiple monthly and decadal time scales, being the monthly variability that is largely unpredictable (Hurrell et al., 2003) and is associated with changes in the wind speed and direction. It has been found that, more specifically, the positive winter NAO anomalies are consistent with stronger trade winds (Polo et al., 2011).

3.1.5.2. The subtropical moving highs

Mobile polar highs are fast and strong dynamic large circulation systems moving constantly over the region, retaining their original low temperatures while moving across long distances. They are capable of transporting considerable amounts of cold polar air and can drive more deeply into tropical areas. The meridional trajectory and dynamics intensify the cyclonic circulation which enhances poleward energy transfer, involving heat and latent heat through water vapour. The process can also involve air over the ocean with precipitable water potential or continental air with much drier conditions favouring anticyclonic agglutination, possibly reinforcing the trade wind inversion disturbances in middle latitudes, because of

69 Santos Soares, E. F. Weather dynamics and forcing in Northwest Africa

accentuated thermal contrasts when the mobile polar highs are more vigorous, leading to more powerful updrafts. As the anticyclonic agglutination process becomes more vigorous, the westerly jets accelerate and are displaced towards the equator (Leroux, 2001).

Over land, continental agglutination is stronger and anticyclonic conditions last longer. Agglutination also forms at more tropical latitudes, with stability reinforced and presence of inversion. As a result, tropical circulation accelerates, although the trade winds cover only a small area and their energy tends to dissipate on to monsoon fluxes. The greater power of the mobile highs tends to encourage more intense transfers towards the pole. Oceanic surface circulation is accelerated, impelled by colder anticyclonic agglutinations, and the trade winds speed up. These dynamic phenomena basically take place during winter time. In summer, the systems are less intense as a result of a reduced polar thermal deficit as the systems migrate to the north and are less vigorous and almost unnoticed, with the heat low forcing taking over. The reduction causes attenuation of the cyclonic circulation at the leading edges, resulting in a reduced energy transfer towards the poles, although favouring the supply of heat and latent heat. The reduced meridional trajectories prevent the mobile polar highs from reaching or diverting the maximum possible precipitable water potential (Leroux, 2001). Therefore dust is transported because of the deceleration in fluxes in the lower and higher layers.

3.1.5.3. Tropical plumes

Tropical plumes (TPs) are defined as continuous cloud bands (>2000 km) crossing 15°N with temperature anomalies of less than 220 K (Fröhlich et al., 2013). They indicate tropical and extratropical interactions with impacts on radiation and moisture occurring during boreal winter, largely confined to oceanic regions (McGuirk et al., 1988). In fact, in the study region TPs develop more frequently over the eastern Atlantic Ocean extending towards the continent.

These systems often develop downstream of extratropical upper‐level troughs propagating into low latitudes, in regions where mean upper‐level easterlies do not generally favour equatorward wave propagation (Mecikalski and Tripoli, 1998). Blackwell (2000) demonstrated the existence of strong subsidence and convergence in the eastern portion of the ridge upstream of TPs. Accordingly, because of the loss of rotational balance of subtropical flow entering the tropics and the ensuing deceleration, a subtropical jet stream and a TP can be developed, associated with equatorward amplification and the trough zonal contraction.

Convergence results from a prior disturbance of the subtropical jet in the upstream ridge that forces stratospheric air flux to enter the tropical troposphere (Blackwell, 2000). But, according to Mecikalski and Tripoli (1998), changes in the inertial stability field through large‐scale advection of low potential vorticity air from tropical convective outflow are responsible for the plume genesis and the initiation of disturbances upstream of the ridge.

With elongated cloud bands (Figure 3.1.4) that often form to the east of the trough axis and extend from the tropics to the , as minimas in the mid and upper troposphere, the upper‐level trough is of great importance in generating precipitation in the midlatitudes and subtropics that usually occurs far east of the trough axis. This precipitation is associated with moisture transported to the eastern side of an upper‐level trough and is triggered by upper‐level divergence and large‐scale uplift due to advection of positive vorticity (Knippertz and Martin, 2005). The upper‐level troughs are recognized as cloud bands that play an important role in the transport of momentum and kinetic energy polewards. The existence of dry air at the west side of the trough is due to subsidence. 70 IOC TECHNICAL SERIES, No. 115, pp. 63‐72. URI: http://hdl.handle.net/1834/9177. 2015

a)

Figure 3.1.4. a) Satellite MET10 infrared imagery at 15:00 UTC 11 Apr 2015 showing the development of a tropical plume over the NWA extending from Cabo Verde to Western Sahara, Mauritania, and Algeria (Source: EUMETSAT, website at http://oiswww.eumetsat.org/I PPS/html/MSG/PRODUCTS/MP E/WESTERNAFRICA/index.htm, accessed on 12 April 2015). b)

Schematic illustration of the TP identification procedure for the Northern Hemisphere b) (reproduced from Fröhlich et al., 2013). The inclination angle (α) between the SW NE direction of the elongated clouds centered at latitude 15°N and the Equator. The grey and light grey correspond to the inclined rectangle range that of the clouds and the temperature anomalies less than or equal to 20 K. © American Meteorological Society.

71 Santos Soares, E. F. Weather dynamics and forcing in Northwest Africa

3.1.6. DISCUSSION AND CONCLUSIONS

The tropical and subtropical NWA climates range from equatorial, tropical wet and dry, tropical monsoon, semi‐arid to desert. Except in highland areas and along the fringes, temperate climates remain rare throughout the continent. In fact, it is more realistic to say that the climate of NWA is basically more dependent on rainfall amount than on temperatures. Temperatures are consistently high as a result of the prevailing presence of subtropical high pressure systems.

In this study, using literature research documentation and NCEP reanalysis data it was possible to define the atmospheric circulation dynamic patterns over tropical and subtropical NWA. It has been found that the Azores subtropical high and the monsoon circulation flow are the major structures that influence the weather in the region. The high pressure system causes a surface continental airmass with maintained subsidence and stability to flow consistently from the northeast over most of region. The resulting trade wind flow is a very dry airmass that becomes even drier while flowing across the Sahara Desert. The summer monsoon, of primary importance for the rainfall distribution, is still today a complex mechanism with diurnal forcing effects, which impacts the boundary layer and determines its transition between stable and unstable conditions.

Large‐scale convective events and synoptic wave perturbations that move across the continent are figures of considerable importance for field research. Likewise, the regional intraseasonal time‐scale variability remains to be well understood as regards the regional atmospheric dynamical interaction of all the atmospheric circulation systems.

Some of the local and regional systems are associated with the synoptic variability of the African easterly perturbations. The seasonal time scale of the atmospheric monsoon dynamic characteristics is associated with the African easterly jet, which is developed in response to the inverse latitudinal surface temperature gradient during spring. In altitude, between the parallels 20°N and 30°N, the westerly subtropical jet streams are characterized by strong velocity, with convergence into low pressure areas and divergence in the anticyclonic surface flow. During summer this is associated with baroclinic and barotropic gradient increases due the maritime airmass flow and the winds become easterlies.

The annular seasonal variability of other atmospheric structures, like the subtropical moving highs, the TPs and the SAL, are also important regional forcing mechanisms that impact the physical and dynamical atmospheric structure, and therefore need further investigation. The interaction between the continental circulation systems and the surrounding ocean basin, yet to be well understood, continues to be considered a major challenge for field research in the CCLME.

Acknowledgments

The author thanks the INMG of Cabo Verde for allowing the use of its facilities during the preparation of the manuscript. NCEP reanalysis data were provided by the National Oceanic and Atmospheric Administration/Office of Oceanic and Atmospheric Research/Earth System Research Laboratory (NOAA/OAR/ESRL) Physical Science Division (PSD) (website at http://www.esrl.noaa.gov/psd/, accessed on 25 February 2015). The SAL image was provided by National Aaeronautics Satellite Administration (NASA) (http://rapidfire.sci.gsfc.nasa.gov/cgi‐bin/imagery/single.cgi?image=WestAfrica.A2013211.1140.2km.jpg, accessed on 2 March 2015). The satellite image was provided by EUMETSAT (public weather website at http://oiswww.eumetsat.org/IPPS/html/MSG/PRODUCTS/MPE/WESTERNAFRICA/index.htm, accessed on 12 April 2015). I also thank Itahisa Déniz González for the helpful comments on the manuscript. 72 IOC TECHNICAL SERIES, No. 115, pp. 63‐72. URI: http://hdl.handle.net/1834/9177. 2015

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