A jet streak circulation associated with a low-latitude jet in the Southern Hemisphere over Africa.

Transcription

1 A jet streak circulation associated with a low-latitude jet in the Southern Hemisphere over Africa. S. E. Nicholson, R. Hart, and P. Cunningham ABSTRACT In the Southern Hemisphere over Africa a mid-tropospheric easterly jet stream exists during some months that is analogous to the African Easterly Jet over West Africa. In this note, a classic "jet streak" circulation is shown to characterize the flow regime near the jet. The vertical motion associated with this jet appears to modify the tropical rainbelt in the region. Various observations also suggest that this jet plays a role in producing the intense convection and high lightning frequency that characterize western equatorial Africa. It may also help to explain a paradox in which, despite the extreme intensity of convection, equatorial rainfall in this region is relatively low. 1. Introduction The mid-tropospheric African Easterly Jet (AEJ) over West Africa is recognized as an important component of regional meteorology. Indeed, two major field campaigns have recently focused on this system (Thorncroft et al. 2003, Redelsperger 2006). The barotropic-baroclinic instability associated with the jet has long been considered a source of instability for the development and possibly the initiation of African Easterly Waves (AEWs) (Burpee 1972, Kiladis et al. 2006). A Southern-Hemisphere counterpart to this jet exists during some months of the year (Fig. 1). It is best developed during the "late rainy season" months of September to December. In early studies of the circulation over Africa (e.g., Newell and Kidson 1984) it was recognized as merely a wind maximum. These studies generally examined the January and July or August, months in which this jet is weak, so no further attention was paid. Nicholson and Grist (2002) were the first to refer to this wind maximum as a jet and describe its characteristics. They termed the two mid-level jets the AEJ-N and AEJ-S to distinguish them. The core speeds of the AEJ-S are as strong as those in the AEJ-N over West Africa. Thus, it is tempting to speculate that it plays a major role in the development of the late rainy season over West and equatorial Africa. The purpose of this note is to show what appears to be a jet streak circulation associated with the AEJ-S and to describe its possible role in climatology of equatorial Africa. The vertical circulations associated with the jet may explain several unique regional features, such as the characteristics of the tropical rainbelt in this region, contrasts between the first and second rainy seasons, and the fact that the most intense

2 storms in the world and the highest frequency of lightning are found here (Zipser et al. 2006). 2. Jet Streaks A "jet streak" is a a localized wind speed maximum that lies along the jet stream axis at its level of maximum wind (Palmén and Neweton 1969). In the mid-latitudes jet streaks influence cyclogenesis, severe weather, and precipitation (Pyle et al. 2004). Conceptual models of their kinematic signatures trace back at least as early as Namias and Clapp (1949) and Bjerknes (1951). Perhaps the best known is the four-quadrant conceptual model of a straight jet streak (e.g., Uccellini and Johnson 1979). In this model, upper-level jets are characterized by considerable zonal asymmetry responsible for distinct entrance and exit regions and ensuing vertical circulation patterns with distinct areas of convergence and divergence (Fig. 2). The conditions for the development of the four-quadrant model are rather stringent and numerous factors can modulate the kinematic signature of the streak. Examples include curvature of the jet axis (e.g., Bjerknes and Holmboe 1944, Beebe and Bates 1955, Keyser and Shapiro 1986), thermal advection (e.g., Shapiro 1982), and migration through a wave pattern (e.g., Orlanski and Sheldon 1995). A potential nonkinematic factor is latent heat release in the upper troposphere (e.g., Raman and Ramanathan 1964, Krishnamurti 1971). Thus, examples that fit the four-quadrant model are uncommon. Also, Coriolis force is an important component of the dynamics, so that the concept has generally been applied only to mid-latitude systems. 3. Data and Methodology The NCEP-NCAR Reanalysis Data Set (Kalnay et al. 1996, Kistler et al ) is used to construct the mean fields of various kinematic aspects of the general circulation over equatorial and subtropical Africa. Long-term means are calculated for the time period 1948 to Its horizontal resolution is 2.5 x 2.5 of latitude and longitude. As the NCEP Reanalysis does not provide adequate estimates of rainfall over Africa (Poccard et al. 2000), rainfall data are taken from the archive of the first author (e.g., Nicholson 1993). It has been noted that the NCEP model has significant biases in areas of steep orography (Trenberth and Guillemot 1995). The analysis sector is bounded by higher terrain to the south and east. For that reason, the 1 sigma level, representing the surface on the NCEP model's terrain, is used in the evaluation of divergence. Omega was calculated independently from NCEP winds, so that calculations were on standard surfaces. Both 1000 mb and 925 are evaluated. Most of the terrain lies above 925 mb and the much lower Zaire Basin, the area of greatest concern, is not greatly affected by model biases. Nevertheless, the lower level analyses should be cautiously interpreted. Our approach is to first derive the long-term climatology of the AEJ-S during October and to compare it with the rainfall climatology. Because our interest is in the

3 AEJ-S and the jet streak as climatological rather than synoptic features, monthly means are utilized in the analysis. Flohn (1964) similarly examined the Tropical Easterly Jet in this context, using the jet streak concept to explain the summer climatology of North Africa, India and the equatorial Indian Ocean. The question of geostrophic balance is also briefly examined. Although the jet lies at a relatively low latitude, arguments can be made that mid-latitude diagnostics can be appropriately applied. Its northernhemisphere counterpart, the AEJ-N, is closely defined by geostrophic balance despite its location at roughly 10? N (Parker et al. 2005). Finally, the apparent jet streak circulation is examined and compared with lightning activity, an indicator of convective intensity. 4. The African Easterly Jet-South and equatorial rainfall climatology In the longitudes 10? W to 10? E the jet is well developed from September to December, during which time it migrates from 8? S to 12? S. The AEJ-S is best developed at roughly 650 to 600 mb and it reaches its maximum strength in October (Fig. 3a). Mean monthly wind in its core can reach 12 m s -1. During April to July the AEJ-S does not develop and the mean easterly flow at 600 mb is 3 m s -1 or less. Similar variation is evident from 10? E to 30? E, but further east there is no trace of this jet (Nicholson and Grist 2003). The jet streak concept assumes approximate geostrophic balance. Because of the low-latitude of the AEJ-S, this assumption must be justified. Kiladis et al. (2006) set a precedent for applying quasi-geostrophic (QG) theory at low latitudes, utilizing it to examine the AEJ-N. We also applied such reasoning to evaluating the AEJ-N; analysis of potential vorticity (Smith and Nicholson 2008) and Q-vectors showed promising results. Like its Northern-Hemisphere counterpart, the AEJ-S lies in the region of maximum pressure gradient between the equatorial trough and a high pressure cell over southern Africa (Fig. 3b). Winds are roughly parallel to the geopotential height contours, suggesting approximate geostrophy. A rough calculation of the Rossby number (Ug/L f, where Ug is the geostrophic speed, L is a typical length scale of the jet and f is the Coriolis parameter at 10? S) gives a value of 0.3. This indicates that quasigeostrophic (QG) theory, including jet streak concepts, can be appropriately utilized in evaluating the AEJ-S. Establishing an explanation for the existence of the AEJ-S is beyond the scope of this note. However, the seasonal cycle of rainfall (Fig. 4) suggests one possible explanation. Typical of equatorial regions, there are two rainy seasons, both of which occur during the transition seasons. In contrast to eastern equatorial Africa, the second rainy season is the main one. A comparison of the spatial patterns of rainfall during April and October (Fig. 4) illustrates this. The dry season prevails during the austral winter. During the dry season vegetation cover is reduced and soil moisture is depleted in the outer tropical latitudes, creating a marked temperature gradient between these and the near-equatorial altitudes (Fig. 5). At roughly 10? S, the gradient reaches 1? C per degree of latitude in October. The core of the jet coincides with the region of maximum near-surface gradient. The gradient reverses at 600 mb, the height of the jet

4 core. The months during which the AEJ-S is absent follow an extended period of rainfall that modifies surface conditions and reduces surface temperature gradients. In April temperatures are relatively uniform throughout the sector from 5? N to 15? S. 5. The jet streak circulation of the African Easterly Jet - South Fig. 6a shows the mean patterns of divergence at 1 sigma (roughly the surface), 600 and 200 mb. This analysis is based on NCEP Reanalysis data (Kalnay et al. 1996, Kistler et al. 2001). The 1 sigma surface is utilized to minimize the influence of the higher terrain around the periphery of the analysis sector. The axis of the AEJ-S is superimposed upon the divergence patterns. The result clearly fits the four-quadrant model. However, the N-S axis of the jet maximum is displaced somewhat eastward with respect to the N-S axis of the four quadrants. This may result from parcel trajectory curvature effects, which can still be significant even in jet streaks that appear to be straight (Pyle et al. 2004). At 600 mb convergence prevails in the right entrance region and left exit region; divergence prevails in the other two quadrants. At 200 mb a sharp gradient in divergence overlies the 600 mb axis, coupling upper level convergence/divergence with divergence/convergence at the 600 mb level. This suggests strong upper-tropospheric ascent over the right entrance quadrant of the AEJ-S. The analysis of vertical motion in Fig. 6b, also based on NCEP Reanalysis, confirms the ascent. For comparison, Fig. 6a also shows the divergence field at 600 mb during April. This is the heart of the first equatorial rainy season, but the AEJ-S is not evident. The pattern of divergence is markedly different than in October. Convergence prevails throughout most of the equatorial region between roughly 10 º N and 10 º S and there is no semblance of the four-quadrant pattern. The surface situation in October differs markedly from that at 600 mb. There is weak divergence beneath the right entrance quadrant, but strong convergence beneath the jet's left entrance region. This suggests weak subsidence beneath the jet's right entrance region, but strong ascent over its left entrance region. This is consistent with orographic effects of the high terrain parallel to the jet axis on its poleward side. The above suggests two vertical circulation cells. One is linked to orographic effects and lies below the level of the AEJ-S. It consists of rising motion over the East African highlands and compensatory subsidence on their lee side. The second consists of convergence at the AEJ-S level and ascent from there to the upper-troposphere, where divergence helps to maintain the vertical overturning. 6. Discussion A well-developed jet streak characterizes the AEJ-S. Several observations suggest that the divergent circulations associated with it play a role in the development of the equatorial rainfall regime during the months when the AEJ-S is present. These are mainly the months of the second rainy season, September through November

5 (Nicholson and Grist 2003). This study only presents results for October but analyses for September and November showed that a common pattern generally prevails throughout this season. In particular, the southern spatial boundary of the tropical rainbelt corresponds to the axis of the jet (compare Figs. 3 and 4); east of the jet core, rainfall decreases rapidly. At jet-level convergence prevails along this boundary, in the right entrance quadrant, producing strong ascent. More to the point, the rainbelt and associated core of vertical motion are markedly different in the entrance and exit regions of the jet (Fig. 7). In the entrance region (20 E), where convergence in the right quadrant reinforces ascent, the rainbelt is very uniform and strong vertical motion prevails in the mid- and upper troposphere from 11 N to 12 S. In the exit region (10 E), divergence prevails in the right quadrant. It appears to suppress the rainfall and vertical motion. The result is a strong gradient in rainfall from 3 N to 10 S and a much more limited latitudinal extent of the core of vertical motion. It this case, ascent prevails from 11 N to 1 S. The AEJ-S might also explain some of the more unusual features of the meteorology of equatorial Africa. This region experiences the world's most intense thunderstorms and highest frequency of lightning flashes (Zipser et al. 2006, Toracinta and Zipser 2001, Petersen and Rutledge 2001), but low rainfall compared to other equatorial regions. Interestingly, the maxima in MCS activity, rainfall per MCS, flash frequency per MCS, and number of MCS's with flashes are all centered over the right entrance quadrant of the jet streak and is coincident with the convergence associated with this quadrant (Jackson et al. 2007) (Fig. 8, compare with Fig. 6a). Moreover these maxima, although evident in the annual mean, are apparent only in the months when the AEJ-S is well developed. Hence, the AEJ-S may be a factor in the intense convective activity in this region. Jackson et al. (2007) also suggest that this may explain the paradox of intense convection but low rainfall. The convergence that produces the updrafts commences at jet level, so that the moisture-laden air of the lower troposphere is not efficiently entrained into the updrafts. 7. Conclusions The transverse circulations associated with the AEJ-S fit the classic four-quadrant model of a jet streak. In the entrance region, near 20 E, convergence prevails in the right quadrant, divergence in the left quadrant. In the exit region, near 10 E, convergence prevails in the left quadrant, divergence in the right quadrant. This model is generally applicable to the higher latitudes, as it is based on quasi-geostrophic theory. Thus, it is noteworthy that such a circulation appears at relatively low latitudes (equator to 20 S). The AEJ-S develops in only one of the two equatorial rainy seasons. It appears to explain contrasts between the two rainy seasons and contrasts in the development of the tropical rainbelt near its entrance and exit regions. The core of ascent coupled with the rainbelt covers twice as great a latitudinal extent in the entrance region. The intense convergence and ascent in the right entrance quadrant coincide with the maxima in

6 MCS intensity and lightning activity. The presence of the jet may explain the unusual intensity of these phenomena over equatorial Africa. This is analogous to the role jet streaks play in severe storm development in the mid-latitudes (Uccellini and Johnson 1979). The presence of the AEJ-S during the second rainy season and absence during the first suggest that its origin lies in the low-level temperature gradient between the tropical rainforest and the drier woodlands and savanna. During the first rainy season, which comes at the end of the rainy season in the woodlands to the south, this gradient is weak. If, in fact, this is the origin of the AEJ-S, it is interesting to speculate on the possibility of feedback between the jet and the land surface changes accompanying extensive deforestation in Africa's equatorial region (e.g., Laporte et al. 1995). Acknowledgements This study was supported by NSF grant ATM REFERENCES Beebe, R.G., and F. C. Bates, 1955: A mechanism for assisting in the release of convective instability. Mon. Wea. Rev., 83, Bjerknes, J., 1951: Extratropical cyclones. Compendium of Meteorology, T. F. Malone, ed., Amer. Meteor. Soc, Bjerknes, J., and J. Holmboe, 1944: On the theory of cyclones. J. Meteor., 1, Burpee, R. W., 1972: The origin and Structure of Easterly Waves in the Lower Troposphere of North Africa. J. Atmos. Sci., 29, Flohn, H., 1964: Investigations on the Tropical Easterly Jet. Bonner Meteorologische Abhandlungen, 4, 83 pp. Jackson, B., S. E. Nicholson and D. Klotter, 2008: Mesoscale convective systems over western equatorial Africa and their relationship to large-scale circulation. Submitted to Mon. Wea. Rev. Kalnay E, and Coauthors The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteor. Soc. 77: Keyser, D., and M. A. Shapiro, 1986: A review of the structure and dynamics of upperlevel frontal zones. Mon. Wea. Rev., 114,

7 Kiladis, G. N., C. D. Thorncroft, and N. M. J. Hall, 2006: Three dimensional structure and dynamics of African easterly waves. Part I: Observations. J. Atmos. Sci., 63, Kistler, R., and Coauthors, 2001: The NCEP-NCAR 50-year reanalysis: Monthly means CD-ROM and documentation. Bull. Amer. Meteor. Soc., 82, Krishnamurti, T. N., 1971: Observational study of the tropical upper tropospheric motion field during the northern-hemisphere summer. J. Appl. Meteor., 10, Laporte, N., C. Justice, and J. Kendall, 1995: Mapping the dense humid forest of Cameroon and Zaire using AVHRR satellite data. Int. J. Rem.ote Sens., 16, Namias, J. and P. F. Clapp, 1949: Confluence theory of the high tropospheric jet stream. J. Meteor., 6, Newell, R. E., and J. W. Kidson, 1984: African mean wind changes between Sahelian wet and dry periods. J. Climatology, 4, 1-7. Nicholson, S. E., 1993: An overview of African rainfall fluctuations of the last decade. J. Climate, 6, Nicholson, S. E., and J. P. Grist, 2003: On the seasonal evolution of atmospheric circulation over West Africa and Equatorial Africa. J. Climate, 16, Orlanski, I.,and J. P. Sheldon, 1995: Stages in the energetics of baroclinic systems. Tellus, 47A, Palmén, E., and C. W. Newton, 1969: Atmospheric circulation systems: Their structure and physical interpretation. Academic Press, New York, 603 pp. Parker, D. J., C. D. Thorncroft, R. R. Burton, and A. Diongue-Niang, 2005: Analysis of the African easterlyl jet, using aircraft observations from the JET2000 experiment. Quart. J. Roy. Meteor. Soc., 131, Petersen, W. A., and S. A. Rutledge, 2001: Regional variability in tropical convection: Observations from TRMM. J. Climate, 14, Poccard, I., S. Janicot, and P. Camberlin, 2000: Comparison of rainfall structures between NCEP/NCAR reanalysis and observed data over tropical Africa. Climate Dynamics, 16,

8 Pyle, M. E., D. Keyser, and L. F. Bosart, 2004: A diagnostic study of jet streaks: kinematic signatures and relationship to coherent tropopause disturbances. Mon. Wea. Rev., 132, Raman, C. R. V., and Y. Ramanathan, 1964: Interaction between lower and upper tropical troposphere. Nature, 204, Redelsperger, J.-L., C. D. Thorncroft, A. Diedhiou, T. Lebel, D. Parker and J. Polcher, 2006: African Monsoon Multidisciplinary Analysis: An international research project and field campaign. Bull. Amer. Meteor. Soc., 87, Smith, T. A., S. E. Nicholson, and P. Cunningham, 2008: Potential vorticity diagnostics associated with the Tropical Easterly Jet and their relationship to interannual rianfall variability in western Africa. Submitted to J. Climate. Thorncroft, C. D., and Coauthors, 2003: The JET2000 project. Bull. Amer. Meteor. Soc., 84, Toracinta, E. R., and E. J. Zipser, 2001: Lightning and SSM/I-ice-scattering mesoscale convective systems in the global tropics. J. Appl. Meteor., 40, Uccellini L W. and D. R. Johnson, 1979: The coupling of upper and lower tropospheric jet streaks and implications for the development of severe convective storms, Mon. Wea.Rev., 107, Zipser, E. J., D. J. Cecil, C. Liu, S. W. Nesbitt, and D. P. Yorty, 2006: Where are the most intense thunderstorms on earth? Bull. Amer. Meteor. Soc., 87, FIGURES

9 Fig. 1 Vertical cross-section of mean zonal wind (m s -1 ) in October as a function of latitude at 20 E.

10 Fig. 2 Schematic illustrating the four-quadrant conceptual model of a straight easterly jet streak in the Southern Hemisphere (based on Uccellini and Johnson 1979).

11 Fig. 3a Mean vector wind speed (m s -1 ) and direction in October at 600 mb. The axes of the jet are indicated with a solid line.

12 Fig. 3b Mean vector wind speed (m s -1 ) and mean geopotential height (m) in October at 600 mb. The axes of the jet are indicated with a solid line. Fig. 4 Mean monthly rainfall (mm) in April and October and the seasonal cycle of rainfall in the western equatorial latitudes.

13 Fig. 5. Mean 925 mb air temperature for April and October ( C).

14 Fig. 6a Mean divergence (10-6 s -1 ) in October at 1 sigma, 600 and 200 mb and in April at 600 mb.

15 Fig. 6b Mean vertical motion (omega, 10-3 hpa s -1 x 100) in October at 1000, 925, 650, and 600.

16 Fig. 7 Vertical cross-section of mean vertical motion (omega, 10-3 hpa s -1 x 100) in October as a function of latitude at 20 E and 10 E. Mean October rainfall (mm mo -1 ) as a function of latitude is indicated by the solid line at the bottom of the figure. Fig. 8. Map of the number of lightning flashes per MCS (five-year annual mean) (from Jackson et al. 2008).

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