Tropical Waves

Tropical M. D. Eastin

Tropical Waves

Composite of TRMM Rainfall and Ocean Surface Wind Anomalies

April 2000-2003

Eastward propagating Kelvin waves

From Wang and Fu (2005)


Outline

African Easterly Waves

Equatorial Waves

• Inertio-Gravity Waves• Rossby Waves• Mixed Rossby-Gravity Waves• Kelvin 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)

• Extends from ~30ºE to ~25ºW

• Weakens over the ocean because no strong N-S temperature gradient exists

Sfc. Temp. (C)

850 mb Winds

925 mb RH (%)

AEJ

AEJ

AEJ



T (C)

Climatology: Meridional Cross Sections at 5ºW

U (m/s) W (mb/s)

V (m/s)

J

J

J

J



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



Mean Low-Level Structure of Individual African Easterly Waves (AEWs)

AEJ

DeepConvection



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


Equatorial WavesBasic 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

• Observations of convectively-coupled equatorial waves recently studied by Wheeler and Kiladis (1999), Wheeler et al. (2000), and Straub and Kiladis (2002)

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


Equatorial WavesInertio-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)

Convergence Increase in Heights / Pressure

Convection (waves near the surface)

DivergenceDecrease in Heights / PressureConvection (waves near the tropopause)

Westward Propagating IG Wave

Equator

From Matsuno (1966)

Initial Time

Equator

Later Time


Equatorial WavesRossby Waves

• Propagate under the influence of Coriolis forces (the N-S gradient, or β)

• 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

Equator

From Wheeler et al. (2000)

Initial Time

Equator

Later Time


Equatorial WavesMixed 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

Equator

Initial Time

Equator

Later Time

From Wheeler et al. (2000)


Equatorial WavesKelvin 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




Eastward Propagating Kelvin Wave

Equator

From Matsuno (1966)

Initial Time

Equator

Later Time


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:

Shading: OLR anomalies(dark is negative → convection)

Contours: 150mb temperature anomalies (solid is positive)

Vectors: 150mb wind anomalies

Day -3

Day 0

Day +3


Equatorial Waves

Conceptual Model of a Convectively-Coupled Kelvin Wave

High

High Low

Low

Cold

Cold

Cold

Warm

MoistWarm

MoistDry

Dry

Stratiform Deep Shallow

Low OLR

150 mb

300 mb

500 mb

700 mb

1000 mb

From Straub and Kiladis (2003)West East


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 Waves


ReferencesBerry, 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 thewavenumber 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 coupledequatorial waves. J. Atmos. Sci., 57, 613-640.

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Tropical Waves