Tropical Waves
Composite of TRMM Rainfall and
Ocean Surface Wind Anomalies
April 2000-2003
Eastward propagating Kelvin waves
From Wang and Fu (2005)
Tropical
M. D. Eastin
Outline
African Easterly Waves
Equatorial Waves
• Inertio-Gravity Waves
• Rossby Waves
• Mixed Rossby-Gravity Waves
• Kelvin Waves
Tropical
M. D. Eastin
African Easterly Waves
African Climatology (Aug-Sep)
Sfc. Temp. (C)
• Strong north-south temperature
gradient in lower levels
AEJ
• Northerly (onshore) low-level flow of
moist air off the Atlantic Ocean
• Southerly low-level flow of dry air off
the Sahara Desert
850 mb Winds
African Easterly Jet (AEJ)
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)
925 mb RH (%)
• Extends from ~30ºE to ~25ºW
• Weakens over the ocean because no
strong N-S temperature gradient exists
Tropical
AEJ
M. D. Eastin
African Easterly Waves
Climatology: Meridional Cross Sections at 5ºW
T (C)
V (m/s)
J
J
U (m/s)
W (mb/s)
J
Tropical
J
M. D. Eastin
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)
• Roughly 10% develop into TCs
MeteoSat-7 Water Vapor Imagery July 3 - August 04
Tropical
M. D. Eastin
African Easterly Waves
Mean Low-Level Structure of Individual African Easterly Waves (AEWs)
AEJ
Deep
Convection
Tropical
M. D. Eastin
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
Tropical
M. D. Eastin
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
• Rossby waves
• Mixed Rossby-gravity waves
• Kelvin 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
• Observations of convectively-coupled equatorial
waves recently studied by Wheeler and Kiladis (1999),
Wheeler et al. (2000), and Straub and Kiladis (2002)
Tropical
M. D. Eastin
Equatorial Waves
Inertio-Gravity (IG) Waves
• Propagate under the influence of both
buoyancy and Coriolis forces
Westward Propagating IG Wave
Initial Time
• Zonal and meridional flow are symmetric
across the equator
• Propagate east, west, and vertically
Equator
• 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
Later Time
forcing the Quasi-Biennial Oscillation (QBO)
Convergence
Increase in Heights / Pressure
Convection (waves near the surface)
Divergence
Decrease in Heights / Pressure
Convection (waves near the tropopause)
Tropical
Equator
From Matsuno (1966)
M. D. Eastin
Equatorial Waves
Rossby Waves
• Propagate under the influence of
Coriolis forces (the N-S gradient, or β)
Westward Propagating Rossby Wave
Initial Time
• Zonal and meridional flow are symmetric
across the eqautor
• Propagate westward and vertically
(do not propagate eastward)
Equator
• Propagation speeds are fairly slow
[higher zonal wavenumbers move slower]
• Occur throughout the year, but are more
frequent in the winter hemisphere
Later Time
Convergence
Increase in Heights / Pressure
Convection (waves near the surface)
Divergence
Decrease in Heights / Pressure
Convection (waves near the tropopause)
Tropical
Equator
From Wheeler et al. (2000)
M. D. Eastin
Equatorial Waves
Mixed Rossby-Gravity (MRG) Waves
• Propagate under the influence of both
buoyancy and Coriolis forces
Westward Propagating MRG Wave
Initial Time
• Meridional flow is symmetric but the zonal
flow is asymmetric across the Equator
• Propagate westward and vertically
(do not propagate eastward)
Equator
• Propagation speeds are fairly slow
[higher zonal wavenumbers move slower]
• Occur throughout the year, but are more
frequent August through November
Later Time
Convergence
Increase in Heights / Pressure
Convection (waves near the surface)
Divergence
Decrease in Heights / Pressure
Convection (waves near the tropopause)
Tropical
Equator
From Wheeler et al. (2000)
M. D. Eastin
Equatorial Waves
Kelvin Waves
• Propagate under the influence of buoyancy
forces (like a pure gravity wave)
Eastward Propagating Kelvin Wave
Initial Time
• 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
Increase in Heights / Pressure
Convection (waves near the surface)
Equator
Later Time
Equator
Divergence
Decrease in Heights / Pressure
Convection (waves near the tropopause)
Tropical
From Matsuno (1966)
M. D. Eastin
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
Day -3
Day 0
• Fairly consistent with the theoretical
Kelvin wave behavior predicted
by Matsuno (1966)
Shown are:
Day +3
Shading: OLR anomalies
(dark is negative → convection)
Contours: 150mb temperature anomalies
(solid is positive)
Vectors: 150mb wind anomalies
Tropical
M. D. Eastin
Equatorial Waves
Conceptual Model of a Convectively-Coupled Kelvin Wave
Low OLR
Stratiform
Deep
Low
Shallow
150 mb
High
Moist
Cold
Dry
300 mb
Warm
Cold
500 mb
Cold
Dry
Moist
High
West
Warm
700 mb
1000 mb
Low
East
From Straub and Kiladis (2003)
Tropical
M. D. Eastin
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
• Annual frequency, convective coupling, impacts
Tropical
M. D. Eastin
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 Center’s (CDCs) Interactive Plotting and Analysis Webage
( http://www.cdc.noaa.gov/cgi-bin/PublicData/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.
Tropical
M. D. Eastin