551 Tropical
Advanced Topics
Trade Wind Inversion
One starts with subsidence in the
subtropics
But the subsidence is not zonally uniform:
greater in the eastern side of the oceans
The resulting greater subsidence on the eastern
sides depressing heights on the eastern sides of
the ocean relative to the central portions
But why more subsidence on the
eastern side?
• Asymmetry of meridional motions
– Northerly on western side
– Southerly on eastern side
• Thus, air parcels are experiencing
progressively smaller f on the eastern side.
• If relative vorticity is maintained, need
differential subsidence to balance.
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.
• Pressure variations normally weak in the
equator (large noise to signal)
Thus, why not analyze winds
instead of pressure?
Streamline/isotach analysis is
used but there are alternatives
(isogon, isotach)
Not intuitive
Tips on streamline/isotach
analysis (you will do this)
• Do streamlines first if possible.
• You will see a lot of familiar patterns
(circulation, deformation, convergence, etc.)
• Winds should go to zero at singular points:
such as centers of rotation,
convergence/divergence, and deformation.
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.
•Assignment: METED African Easterly Wave module
Composite African Wave Structures (Reed et
al., 1977)
African Easterly Waves Are
Closely Associated with and
Propagate in a Midtropospheric
Easterly Jet
• Centered around 650-700 hPa near
15N
• Associated with large temperature
difference between the hot Sahara
and cool Gulf of Guinea
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: Surface temps
• Strong baroclinic zone 10o-20oN
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
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
Synoptic variations in cloud cover
Peak of cloudiness close to AEW trough
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
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.
Origin of African Easterly Waves
• A joint baratropic/baroclinic instability on
the African easterly jet.
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