Tropical Waves

1
Tropical Waves
Composite of TRMM Rainfall and Ocean Surface
Wind Anomalies April 2000-2003 Eastward
propagating Kelvin waves
From Wang and Fu (2005)
2
Outline

• African Easterly Waves
• Equatorial Waves
• Inertio-Gravity Waves
• Rossby Waves
• Mixed Rossby-Gravity Waves
• Kelvin Waves

3
African Easterly Waves

• African Climatology (Aug-Sep)
• Strong north-south temperature
• gradient in lower levels
• Northerly (onshore) low-level flow of
• moist air off the Atlantic Ocean
• Southerly low-level flow of dry air off
• the Sahara Desert
• African Easterly Jet (AEJ)
• A result of thermal wind balance
• Maximum of 15 m/s (easterly) at 600 mb
• Oriented E-W along 15ºN (the maximum
• near-surface temperature gradient)

Sfc. Temp. (C)
AEJ
850 mb Winds
AEJ
925 mb RH ()
AEJ
4
African Easterly Waves
Climatology Meridional Cross Sections at 5ºW
T (C)
V (m/s)
J
J
U (m/s)
W (mb/s)
J
J
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African Easterly Waves

• African Easterly Waves (AEWs)
• Development
• Barotropic / Baroclinic instabilities along the
  AEJ
• Orographic forcing (Ethiopian Highlands)
• (PV Conservation ? Leeside Low Pressure)
• Pre-existing disturbances
• Basic Statistics
• Season is May-Oct with peak activity in Aug
• Mean latitude of 15ºN
• Wavelengths of 2000 km
• Move westward at 8 m/s (period 3-4 days)
• Develop 20º-30ºE just downwind of the Highlands
• Attain maximum amplitude near 0º-10ºW
• Weaken as they move over the ocean (no AEJ)

MeteoSat-7 Water Vapor Imagery July 3 - August 04
6
African Easterly Waves
Mean Low-Level Structure of Individual African
Easterly Waves (AEWs)
AEJ
Deep Convection
7
African Easterly Waves

• AEWs and SAL Outbreaks
• Strong AEW are often associated
• with a stronger than normal AEJ
• Strong AEJ have greater vertical
• and latitudinal extents and thus
• stronger near-surface easterlies
• This increases the likelihood of
• SAL outbreaks

8
Equatorial Waves

• Basic Characteristics
• Unique variety of waves trapped near the
• equator due to the reversal of the Coriolis
• forcing across the equator
• Waves are symmetric across the Equator
• Cooperative interaction between deep cumulus
• convection and large-scale convergence
• (i.e. the waves and convection are coupled)
• Basic physics first studied by Matsuno (1966)
• using the shallow water (barotropic)
  equations
• on a rotating Earth in Cartesian coordinates
• Identified four types of equatorial waves
• Inertio-gravity waves

Note Reversal of the Coriolis force across
the Equator combined with geostrophic
balance leads to pure westerly flow in
association with a high pressure
centered over the Equator
9
Equatorial Waves

• Inertio-Gravity (IG) Waves
• Propagate under the influence of both
• buoyancy and Coriolis forces
• Zonal and meridional flow are symmetric
• across the equator
• Propagate east, west, and vertically
• Propagation speeds are fairly fast
• higher zonal wavenumbers move faster
• Occur throughout the year, but are more
• frequent December through February
• Are believed to play an important role in
• forcing the Quasi-Biennial Oscillation (QBO)

Westward Propagating IG Wave
Initial Time
Equator
Later Time
Equator
From Matsuno (1966)
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Equatorial Waves

• Rossby Waves
• Propagate under the influence of
• Coriolis forces (the N-S gradient, or ß)
• Zonal and meridional flow are symmetric
• across the eqautor
• Propagate westward and vertically
• (do not propagate eastward)
• Propagation speeds are fairly slow
• higher zonal wavenumbers move slower
• Occur throughout the year, but are more
• frequent in the winter hemisphere

Westward Propagating Rossby Wave
Initial Time
Equator
Later Time
Equator
From Wheeler et al. (2000)
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Equatorial Waves

• Mixed Rossby-Gravity (MRG) Waves
• Propagate under the influence of both
• buoyancy and Coriolis forces
• Meridional flow is symmetric but the zonal
• flow is asymmetric across the Equator
• Propagate westward and vertically
• (do not propagate eastward)
• Propagation speeds are fairly slow
• higher zonal wavenumbers move slower
• Occur throughout the year, but are more
• frequent August through November

Westward Propagating MRG Wave
Initial Time
Equator
Later Time
Equator
From Wheeler et al. (2000)
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Equatorial Waves

• Kelvin Waves
• Propagate under the influence of buoyancy
• forces (like a pure gravity wave)
• No meridional flow
• Zonal flow is symmetric across the Equator
• Propagate eastward and vertically
• (do not propagate westward)
• Propagation speeds are fast and increase
• with zonal wavenumber
• Occur throughout the year, but are more
• frequent February through August
• Are believed to play an important role in
• triggering El Nino events
• Convergence

Eastward Propagating Kelvin Wave
Initial Time
Equator
Later Time
Equator
From Matsuno (1966)
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Equatorial Waves

• Observations of a Convectively-Coupled Kelvin
  Wave
• From Straub and Kiladis (2002)
• Anomalous upper-level flow and
• temperature are quasi-symmetric
• across the Equator
• Non-negligible meridional flow
• Convection was asymmetric across
• the Equator (primarily north)
• Propagated eastward at 15 m/s
• Zonal scale of 4000-6000 km
• Fairly consistent with the theoretical
• Kelvin wave behavior predicted
• by Matsuno (1966)
• Shown are

Day -3
Day 0
Day 3
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Equatorial Waves
Conceptual Model of a Convectively-Coupled Kelvin
Wave
Low OLR
Stratiform
Deep
Shallow
150 mb
High
Low
Dry
Moist
300 mb
Warm
Cold
Cold
500 mb
Cold
Dry
Warm
Moist
700 mb
1000 mb
High
Low
West
East
From Straub and Kiladis (2003)
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Tropical Waves

• Summary
• African Easterly Waves
• African Easterly Jet (climatology and forcing)
• Development
• Basic Characteristics
• Relation to TCs
• Relation to SAL outbreaks
• Equatorial Waves
• Trapped along the equator (why?)
• Convectively coupled
• Four types
• Inertio-Gravity waves
• Mixed Rossby-Gravity waves
• Rossby Waves
• Kelvin Waves
• For each wave type
• Propagation forcing, direction, and speed

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References
Berry, G. J., and C. Thorncroft, 2005 Case study
of an intense African easterly wave. Mon. Wea.
Rev., 133, 752-766. Burpee, R. W., 1972 The
origin and structure of easterly waves in the
lower troposphere of North Africa. J. Atmos.
Sci., 29, 77-90. Climate Diagnostic Centers
(CDCs) Interactive Plotting and Analysis
Webage ( http//www.cdc.noaa.gov/cgi-bin/PublicDa
ta/getpage.pl ) Dunion, J. P., and C. S. Velden,
2004 The impact of the Saharan air layer on
Atlantic tropical cyclone activity. Bull.
Amer. Met. Soc., 75, 353-365. Kalnay, E., and
Coauthors, 1996 The NCEP/NCAR 40-year Reanalysis
Project. Bull Amer Met. Soc., 77,
437-471. Matsuna, T., 1966 Quasi-geostrophic
motions in the equatorial area. J. Meteor. Soc.
Japan, 44, 25-43. Straub, K. H., and G. H.
Kiladis, 2002 Observations of a convectively
coupled Kelvin wave in the eastern Pacific
ITCZ. J. Atmos. Sci., 59, 30-53. Straub, K.
H., and G. H. Kiladis, 2003 Extratropical
forcing of convectively coupled kelvin waves
during Austral Winter. J. Atmos. Sci., 60,
526-543. Thorncroft, C. D., and M. Blackburn,
1999 Maintenance of the African easterly jet. Q.
J. R. Meteor. Soc., 125, 763-786. Wheeler, M.,
and G. N. Kiladis, 1999 Convectively-coupled
equatorial waves Analysis of clouds and
temperature in the wavenumber frequency domain.
J. Atmos. Sci., 56, 374-399. Wheeler, M., G. N.
Kiladis, and P. J. Webster, 2000 Large-scale
dynamical fields associated with convectively
coupled equatorial waves. J. Atmos. Sci., 57,
613-640.