Section 4: Synoptic Easterly Waves

4.1 Introduction 4.2 The Mean State over West Africa 4.3 Observations of African Easterly Waves 4.4 Theory 4.5 Modeling 4.6 Hot Topics: Genesis Scale Interactions Relationship to Tropical Cyclogenesis 4.7 Easterly Waves in other Tropical Regions 4.8 Final Comments

4.1 Introduction Westward moving synoptic waves characterize the whole tropics They are tropospheric waves, that modulate the rainfall and move at about 8m/s and have wavelengths of km.

4.1 Introduction The environments that they are embedded in varies around the tropics, and so details of the wave characteristics also vary.

4.1 Introduction The emphasis here will be on African Easterly Waves (AEWs) Have a strong influence on daily rainfall patterns over Africa and tropical Atlantic Most Atlantic Tropical Cyclones are generated in association with AEWs AEWs MCSs SAL TC

4.2 The Mean State over West Africa Burpee, R.W The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, Notable Features:600mb African Easterly Jet (AEJ) Upper-level Tropical Easterly Jet (TEJ) Low-level Monsoonal Westerlies Low-level Easterlies north of the AEJ Upper-level Westerly Jet to the North

4.2 The Mean State over West Africa Reed, R.J., Norquist, D.C. and Recker, E.E., The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, (1977).

PV View of the African Easterly Jet Discussion Consider the meridional contrasts in convection (next slide) and the diabatic source/sink term in the PV-equation.

Schematic of African Easterly Jet θ 50 o C θ θe 90 o C θe AEJ 20 o C 60 o C

4.2 The Mean State over West Africa Thorncroft and Blackburn 1999

Mean 700hPa U wind, 16 th July – 15 th August 2000 Zonal Variations in the Mean State Berry and Thorncroft 2005

925hPa  315K PV Strong baroclinic zone 10 o -20 o N PV ‘strip’ present on the cyclonic shear side of AEJ. 925hPa  e High  e strip exists near 15 o N Zonal Variations in the Mean State

4.3 Observations of African Easterly Waves Carlson, T.N., 1969a: Synoptic histories of three African disturbances that developed into Atlantic hurricanes. Mon. Wea. Rev., 97, Carlson, T.N., 1969b: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev., 97, Burpee, R.W., 1970: The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, Reed, R.J., Norquist, D.C. and Recker, E.E., 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, Thorncroft, C.D. and Hodges: 2001 K.I., African easterly wave variability and its relationship to tropical cyclone activity, J. Clim. 14, (2001). Kiladis, G., C. Thorncroft, and N. Hall, 2006: Three-Dimensional Structure and Dynamics of African easterly waves: part I: Observations, J. Atmos. Sci., 63, Mekonnen, A., C. Thorncroft, and A. Aiyyer, 2006: On the significance of African easterly waves on convection, J. Climate, 19, Berry, G., Thorncroft, C.D. and Hewson, T African easterly waves in 2004 – Analysis using objective techniques Mon. Wea. Rev., 133,

4.3 Observations of African Easterly Waves Carlson 1969ab Carried out case studies of several AEWs Peak amplitudes at mb and at surface Eastward tilt with height from the surface to the level of the AEJ Two cyclonic centers at low-levels Synoptic variations in cloud cover Peak of cloudiness close to AEW trough

4.3 Observations of African Easterly Waves Burpee (1970) Eastward tilt beneath the AEJ – Westward tilt above the AEJ Northerlies dry and warm Southerlies wet and cold

4.3 Observations of African Easterly Waves Composite AEW structures from phase III of GATE (after Reed et al, 1977). (a) and (b) are relative vorticity at the surface and 700hPa respectively with a contour interval of s -1. (c) and (d) show percentage cover by convective cloud and average precipitation rate (mm day -1 ) respectively. Category 4 is location of 700hPa trough and the “0” latitude is 11 o N over land and 12 o N over ocean. Reed et al, 1977

4.3 Observations of African Easterly Waves Thorncroft and Hodges (2001)

Three Dimensional Structure of Easterly Wave Disturbances over Africa and the Tropical North Atlantic George N. Kiladis 1 Chris D. Thorncroft 2 Nick M. J. Hall 3 1 NOAA Aeronomy Laboratory, Boulder, CO 2 Dept. of Atmospheric Sciences, SUNY, Albany, NY 3 LTHE, Grenoble, France

Space-Time Spectrum of JJA Antisymmetric OLR, 15  S-15  N Wheeler and Kiladis, 1999

Regression Model Simple Linear Model: A separate linear relationship between a predictor at a grid point and a parameter at every other grid point is obtained: y = ax + b where: x= predictor (TD-filtered OLR at 10  N, 10  W) y= predictand (u or v wind at any grid point) Maps or cross sections at lag can then be constructed to show the evolution of the dynamical fields versus the predictor

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day 0 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-4 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-3 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-2 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day-1 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day 0 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day+1 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day+2 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day+3 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day+4 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m 2 ) at 10  N, 10  W for June-September Day+5 Streamfunction (contours 1 X 10 5 m 2 s -1 ) Wind (vectors, largest around 2 m s -1 ) OLR (shading starts at +/- 6 W s -2 ), negative blue

4.3 Observations of African Easterly Waves All the previous slides refer to composite AEW structures They say little about the significance of AEWs on convection and They say little about how these structures might be manifested on a weather map or how they may vary in space and time. The next slides address the significance issue from Mekonnen et al (2006) This will be followed by some maps of individual AEWs (Berry et al 2007).

Shaded region: power > red noise Central Africa E. Africa Significant time scales: 2-6 days & at 1 day. Peak periods change from west to east E. Atlantic W. Africa T B variance (in K 2 ) E. Atlantic (5-10N, 40W-20W) Land (10-15N, 15W-40E)

Shaded region: power > red noise Central Africa E. Africa Significant time scales: 2-6 days & at 1 day. Peak periods change from west to east E. Atlantic W. Africa T B variance (in K 2 )

Shaded region: power > red noise Central Africa E. Africa Significant time scales: 2-6 days & at 1 day. Peak periods change from west to east E. Atlantic W. Africa T B variance (in K 2 )

Shaded region: power > red noise Central Africa E. Africa Significant time scales: 2-6 days & at 1 day. Peak periods change from west to east E. Atlantic W. Africa T B variance (in K 2 )

Central Africa E. Africa E. Atlantic W. Africa T B variance (in K 2 ) Significant time scales: 2-6 days & at 1 day. Peak periods change from west to east Shaded region: power > red noise

2-6d T B variance (shaded >140K 2 ) 2-6d contribution: 25-35% over land, 35-40% over ocean west-east variance is nearly the same Variance explained by 2-6d T B (shaded > 20%)

Comparison with dynamic measures ….. 2-6d 700-hPa  variance 2-6d 850-hPa  variance Variance in the west are higher than in the east! (shaded >5m 2 s -2 ) Land: maximum along 10N, south of the AEJ, near peak convective region. Ocean: near 20N Land: maximum to the north of AEJ, and over the coast, near peak convective region Ocean: within ITCZ

315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery. Diagnostics for highlighting multi-scale aspects of AEWs Berry et al 2006

315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

Summary of the observed AEJ and AEWs Summary AEJ:Consists of two prominent PV anomalies; a positive PV anomaly on the cyclonic side of the AEJ that is diabatically generated in the region of peak rainfall and a negative PV anomaly that is diabatically generated in the heat low region. AEWs:AEWs have significant circulation anomalies at the level of the AEJ and at the surface. They tend to tilt against the horizontal and vertical shear of the AEJ – this tells us something about the growth mechanisms to be discussed in the next section.