5.2. Observations of convectively coupled KWs

5.2. Observations of convectively coupled KWs

• KW propagating along the equatorial belt.

• Power spectrum of convectively coupled equatorial waves.

• Composite structures of KWs and associated dynamical and thermodynamical fields.

• Interaction of KWs with mesoscale convective systems (MCSs) over Central Africa.

• Source of KWs over South America.

Kelvin wave over the Pacific warm pool

Movie of CLAUS Tb in May 1998

Kelvin wave over the Amazon


Kelvin wave over equatorial Africa


1998 CLAUS Brightness Temperature 5ºS-5º N

CLAUS TbAveraged 15ºS-15ºN, 1983–2005Symmetric component

Wave-number frequency spectrum of convectively coupled equatorial waves

Courtesy of G. Kiladis

Westward Power Eastward Power

1.25 Days

96 Days


Kelvin


90 m

8 m


Dispersion curves superimposed

25 m


Outgoing Longwave Radiation (OLR)Average: 15ºS-15ºN, 1979–2001 Symmetric componentBackground removed

Wheeler and Kiladis, 1999


Kelvin wave over the Indian Ocean

Kelvin-domain-filtered symmetric OLR variance

All seasons

Wheeler et al. 2000

Peak variance at 0o, 90oE.Broad region of variance extending across the IO and into the western PO.KWs events can occur any time of the year.

Kelvin-domain-filtered symmetric OLR variance

All seasons

Wheeler et al. 2000

KWs over the Indian Ocean Wheeler et al. 2000

Kelvin wave over the Pacific Ocean

Kelvin-domain-filtered total OLR variance

May to September

Kelvin wave over the Pacific Ocean

May to September

14 m s-1

OLRLa

g (d

ays)

OLR (red: increased cloudiness); ECMWF 1000-hPa u, v (vectors), z (contours)

Horizontal structure

Observed KWs: Vertical structure, TStraub and Kiladis 2003

Wave Motion

Temperature at Majuro (radiosonde, 7N, 171E)

Observed KWs: Vertical structure, q

Wave Motion

Straub and Kiladis 2003

Specific humidity at Majuro (radiosonde, 7N, 171E)

Kelvin wave over Central Africa

Kelvin-domain-filtered symetric OLR variance in Spring (MAM)

Kelvin-wave-filtered OLR variance

8

12

255090 Wheeler and Kiladis 1999The Kelvin wave domain is represented by the green polygon

AA EA IO PO

(5oS-5oN) meridional mean Kelvin wave filtered OLR variance

• Peaks from the Amazon-Atlantic (AA) in March to the Pacific ocean (PO) in June.

• Strongest signal over Equatorial Africa (EA) in April

Role of the surface favoring the Kelvin wave growth.

Equatorial position of the ITCZ in spring.


L

H

OLR (shading, W/m2)

Wind at 850 hPa (vector, m/s)

Surface Pressure (contours, Pa)

•OLR and dynamical signal centered on the equator along the ITCZ.•Winds are primarily zonal.•Low pressure (convergence) and easterlies to east of lowest OLR•High pressure (divergence) and westerlies to west of lowest OLR

cat3

Solution of the shallow water model

Convection is collocated with the theoretical convergence region

Comparison with theoretical structure

Wavelength :

50 ≤ ≤ 90°(longitude) ⇨ Zonal wavenumber :4 ≤ k ≤ 7Period of 5-6 days:8 ≤ c ≤ 18 ms-1

8

12

2550

90

Characterization in the spectrum

Wheeler and Kiladis 1999

Vertical structure

OLR (Wm-2)

q (g/kg)moistdry

T (degK)coldwarm

Pre

ssur

e (h

Pa)

Lower troposphere: •Moist collocated with the lowest OLR.•Dry is to the west of lowest OLR.

Upper troposphere: moist is to the west of lowest OLR.

progression from shallow to deep convection followed by stratiform clouds.

Lower troposphere: •Warm is to the east of lowest OLR.•Cold is to the west of lowest OLR.

Upper troposphere: warm is collocated with lowest OLR.

Observed Kelvin wave morphology

Wave MotionStraub and Kiladis 2003

Kiladis et al., 2009

Generalized evolution of a convectively coupled equatorial wave

Interaction with MCSs and diurnal cycle

CLAUS Tb1997

Kelvin waves12 m/s

MCSs20 m/s

Kelvin wave impact on the MCSs

(1984-2002) Composite MCS surface cover, 2 classes (sizes) considered : – The smallest, effective radius < 200 km – The biggest, effective radius > 480 km

• no or weak perturbation of the small MCSs by KW

• Strong modulation of the big MCSs by the Kelvin wave

• Convection is essentially triggered by the orography

• Kelvin wave impact mostly on the organization of the MCSs

Tim

e (h

ours

)

Small MCSs Big MCSs

Brightness temperature• Mar-Apr 1984-2002• averaged from 5°S-5°N

Diurnal cycle of the convection

MCSs are triggered near and on the relief

Diurnal cycle marked every day

Enhancement of the convection evident during the wave active phase, especially over the Congo Basin

Mean diurnal cycle

Composite Anomalies

tim

e (h

ours

)

Negative OLR

Positive OLR

Origin of KWs over South America

Courtesy of George Kiladis

Liebmann et al., 2009

OLR and 200 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at eq, 60W for January-June 1979-2004

Day-4

Streamfunction (contours 5 X 105 m2 s-1)Wind (vectors, largest around 5 m s-1)

OLR (shading starts at +/- 6 W s-2), negative blue


Day-3




Day-2




Day-1




Day 0




Day+1




Day+2




Day+3



The dates are then separated by additional criteria before compositing:

“Pacific” cases: 3 days before key date Kelvin-filteredOLR more than 16 Wm-2 below mean at 95W, 2.5N

“South America” cases: 3 days before key date, 30-day high-pass filtered OLR more than 50 Wm-2 below mean at 60W, 20S.

Period: Nov 1979-May 200653 Pacific cases

48 South America cases4 common cases

Dates are found with a 1.5 standard deviationsnegative OLR anomalies at 60W, Eq.

Conclusions for South America

• There are at least two mechanisms that force Kelvin waves over South America

a) upper levels disturbance propagating along the equator from the Pacific

b) lower levels cold surge from southern South America:

(e.g., Garreaud and Wallace 1998; Garreaud 2000)

• Not all South American (cold) events force Kelvin waves

• Some Kelvin waves may be initiated in-situ

5.2. Observations of convectively coupled KWs

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