551 Tropical
Advanced Topics
Trade Wind Inversion
Trade Wind or Subtropical
Inversion
• The height of the base of this inversion varies from about 500 m
at the eastern extremities of the subtropical highs to about 2000 m
at the western and equatorial extremities.
• In the equatorial trough zone and over the western portions of the
trade-wind belt, the inversion does not exist as a mean condition,
although it appears in certain weather patterns.
• The inversion is generally strongest when the height of its base is
lowest, and vice versa. The thickness of the inversion layer varies
from tens of m to more than 1000 m.
• On the average its thickness is about 400 m.
Tropical Analyses
• See both midlatitude style, isobaric analysis,
and streamline/isotach analyses.
• Why the latter is most popular?
Why streamline/isotachs better
than isobars/heights in the
tropics?
• Flow not geostrophic within roughly 5
degrees of the equator (so connection
between pressure and winds are not
necessarily straightforward)
• Lot of pibals (just winds) in the tropics
compared to radiosondes.
Easterly Waves: The Major
Synoptic Disturbances in the
Tropics
•Westward moving synoptic waves characterize the whole tropics
• They are tropospheric waves that modulate clouds/rainfall and
move at about 5-10 m/s and have wavelengths of 2000-4000 km.
Composite African Wave Structures
The Mean State over West Africa” The African
Easterly Jet (AEJ)
Burpee, R.W. 1972 The origin and structure of easterly waves in the lower troposphere of North Africa,
J. Atmos. Sci. 29, 77-90
Why is the AEJ there?
Hint
• Strong baroclinic zone 10o-20oN
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, 317-333 (1977).
Zonal Variations in the Mean State
Mean 700hPa U wind, 16th July – 15th August 2000
Berry and Thorncroft 2005
4.3 Observations of African Easterly Waves
Carlson 1969ab
Carried out case studies of several AEWs
Peak amplitudes at 600-700mb 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
Reed et al, 1977
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 10-5s-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 11oN over land and 12oN over ocean.
4.3 Observations of African Easterly Waves
Thorncroft and Hodges (2001)
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, 256-276.
Carlson, T.N., 1969b: Some remarks on African disturbances and their progress over the
tropical Atlantic. Mon. Wea. Rev., 97, 716-726.
Burpee, R.W., 1970: The origin and structure of easterly waves in the lower troposphere
of North Africa, J. Atmos. Sci. 29, 77-90.
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, 317-333
Thorncroft, C.D. and Hodges: 2001 K.I., African easterly wave variability and its
relationship to tropical cyclone activity, J. Clim. 14, 1166-1179 (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, 2212-2230.
Mekonnen, A., C. Thorncroft, and A. Aiyyer, 2006: On the significance of African
easterly waves on convection, J. Climate, 19, 5405-5421.
Berry, G., Thorncroft, C.D. and Hewson, T. 2006 African easterly waves in 2004 –
Analysis using objective techniques Mon. Wea. Rev., 133, 752-766
The End