Relationships between Convectively Coupled Kelvin Waves and Extratropical Wave Activity

Relationships between Convectively Coupled Kelvin Waves and Extratropical Wave Activity

Relationships between Convectively Coupled Kelvin Waves and Extratropical Wave Activity

George N. KiladisKlaus WeickmannBrant Liebmann

NOAA, Physical Sciences DivisionEarth System Research Laboratory

CIRES, University of Colorado

George N. KiladisKlaus WeickmannBrant Liebmann

NOAA, Physical Sciences DivisionEarth System Research Laboratory

CIRES, University of Colorado

Or:Some (as yet only partially explained)

observations of Kelvin Waves and Associated Extratropical Disturbances

Or:Some (as yet only partially explained)

observations of Kelvin Waves and Associated Extratropical Disturbances

Data SourcesCloud Archive User Services (CLAUS) Brightness Temperature

8 times daily, .5 resolution July 1983-September 2005

NCEP-NCAR Reanalysis products 4 times daily, 17 pressure levels, 2.5 resolution

Key Papers:Lindzen, R. D., 1967: Planetary waves on beta-planes. Mon. Wea. Rev.

Hoskins, B. J. and T. Ambrizzi 1993: Rossby wave propagation on a realistic longitudinally varying flow. J. Atmos. Sci.

Zhang, C. and P. J. Webster, 1989: Effects of zonal flows on equatorially-trapped waves. J. Atmos. Sci.

Zhang, C. and P. J. Webster, 1992: Laterally forced equatorial perturbations in a linear model. J. Atmos. Sci.

Yang, G. –Y. and B. J. Hoskins 1996: Propagation of Rossby waves of non-zero frequency. J. Atmos. Sci.

Hoskins, B. J., and G. –Y. Yang, G. –Y. 2000: The equatorial response to higher latitude forcing. J. Atmos. Sci.

Roundy, P. E., 2008: Analysis of convectively coupled Kelvin waves in the Indian Ocean MJO. J. Atmos. Sci.

Dias, J. and O. Pauluis, 2009: Convectively coupled Kelvin waves propagating along an ITCZ. J. Atmos. Sci.

Ferguson, J., B. Khouider, M. Namazi, 2009: Two-way interactions between equatorially-trapped waves and the barotropic flow. Chinese Ann. Math.

Theoretical Considerations:Effects of Meridional Shear in the Zonal Wind

Differential advection leads to straining and deformation: Affects shape and group velocity

Wave-guiding: Trapping of Rossby wave energy along jets, extratropical waves are guided towards low latitudes in certain regions

Non-Doppler Effect: Meridional shear modifies the -effect, leading to differences in equivalent depths and equatorial trapping

Critical Line: Where the zonal phase speed of a Rossby Wave equals that of the background zonal wind (waves are absorbed or perhaps reflected here).

200 hPa Climatological Zonal Wind, Dec.-Feb. 1979-2004

Contour interval 5 m s-1

200 hPa Climatological Zonal Wind, June-Aug. 1979-2004

Contour interval 5 m s-1

OLR and 200 hPa Flow Regressed against <30 day filtered OLR (scaled -20 W m2) at 10N, 150W for Dec.-Feb. 1979-2004

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Streamfunction (contours 5 X 105 m2 s-1)Wind (vectors, largest around 10 m s-1)

OLR (shading starts at +/- 6 W s-2), negative blueKiladis, 1998


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


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OLR and 200 hPa Flow Regressed against <30 day filtered OLR (scaled -20 W m2) at 7.5N, 30W for Dec.-Feb. 1979-2004

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OLR (shading starts at +/- 6 W s-2), negative blueKiladis and Weickmann, 1997


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OBSERVATIONS OF KELVIN AND INERTIO-GRAVITY WAVESCLAUS Brightness Temperature (2.5S–7.5N), April-May 1987

OBSERVATIONS OF KELVIN AND INERTIO-GRAVITY WAVESCLAUS Brightness Temperature (2.5S–7.5N), April-May 1987

28 m s-1

OLR and 850 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at 7.5N, 172.5W for June-Aug. 1983-2005

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Geopotential Height (contours .5 m)Wind (vectors, largest around 5 m s-1)

OLR (shading starts at +/- 6 W s-2), negative blueStraub and Kiladis, 1997

Kelvin Wave Theoretical Structure

Wind, Pressure (contours), Divergence, blue negative


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OLR and 200 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at Eq., 90E for June-Aug. 1983-2005

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OLR and 200 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at Eq., 90E for March-May 1983-2005

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OLR and 200 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at Eq., 90E for Dec.-Jan. 1983-2005

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OLR and 200 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at 2.5N, 0.0 for March-May 1983-2005

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OLR and 200 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at 2.5N, 0.0 for March-May 1983-2005

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Mechanisms?

Local Dynamic and Thermodynamic fields associated with initial Kelvin wave development are very weak

One possibility: “Direct projection” of extratropical forcing onto equatorially-trapped waves, exciting a resonant response

Hoskins and Yang, 2000

1987 CLAUS Brightness Temperature 5ºS-5º N

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

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Geopotential Height (contours 1 m)Wind (vectors, largest around 3 m s-1)



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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.

53 Pacific cases48 South America cases

4 common cases

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

Base point: Kelvin-filteredOLR, 1.5 STD anomaly

(plus constraint at 95W, 2.5N)Fields: 30-day high-pass OLR 200 mb wind and streamfunction

Wm-2

Pacific events

Contrast with “South America” example(note different latitude range)

30-day High-pass OLR, 200 mb wind and stream function

Fields lead base point by 5 days

200 850 1000Blue contours indicate positive height anomalies

200 mbHeights and OLR

850 mb Heights and Rain

1000 mb Heights and Unfiltered rain


200 850 1000

Fields lead base point by 1 day

200 850 1000

Fields simultaneous with base point

200 850 1000

Fields lag base point by 1 day

200 850 1000

Conclusions

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

a) at upper levels from the Pacific b) at lower levels 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

Conclusions

Convectively coupled Kelvin waves have many “non-Kelvin” features, including off-equatorial gyres presumably forced in part by heating

There are strong associations between Kelvin wave activity and extratropical Rossby wave activity

In some cases it is clear that Kelvin waves are forced by the extratropics

Kelvin convection is associated with subtropical anticyclonic vorticity, unlike other cases of tropical-extratropical interaction

Subtropical “pressure surges” are also seen to be involved in forcing of some Kelvin activity over South America

These results do not rule out “spontaneous” generation of Kelvin waves by equatorial convection or local dynamical forcing

€

E = (v'2 −u'2,−u'v',f

Θz

v'θ ')

€

v'2 −u'2

€

−u'v'

E Vectors (assumption of quasi-geostrophy)

where: is a measure of anisotropy

is minus the northward flux of westerly momentumand

Approximate direction of group velocity:

from Hoskins, James and White (1983)

200 hPa Climatological < 30 Day E Vectors and OLR December-February 1979-2004

E Vectors, largest around 200 m-2 s-2

OLR shading starts at 250 W s-2 at 10 W s-2 intervals

€

K s =β*

u

⎛

⎝ ⎜

⎞

⎠ ⎟

1/2

€

* = β −∂ 2u

∂y2

Stationary Wavenumber Ks

where:is the meridional gradient

of absolute vorticity

Ks is the total wavenumber (k2+l2)1/2 at which a barotropic Rossby Wave is stationary in a given background Zonal Flow

According to WKB theory, Rossby Wave Energy should be refracted toward higher values of Ks

see Hoskins and Ambrizzi (1993)

200 hPa Climatological < 30 Day E Vectors and OLR December-February 1979-2004


OLR shading starts at 250 W s-2 at 10 W s-2 intervals

200 hPa Climatological < 30 Day E Vectors, Ks and OLR December-February 1979-2004


Ks (contours) by total wavenumberOLR shading starts at 250 W s-2 at 10 W s-2 intervals

Lead and Lag Regressions

Base point: Kelvin-filtered OLR at 60W, Eq.

Fields: 30-day high-pass filtered OLR, 200 mb winds and stream function

Conclusions

• There are at least two mechanisms that force Kelvin waves over South America a) at upper levels from the Pacific b) at lower levels from southern South America

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

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


Kelvin-filtered base point 30-day high pass base point

OLR, 200 mb winds and heights

Fields simultaneous with base point



Fields lag base point by 1 day



Fields lag base point by 2 days



OLR and 200 hPa Flow Regressed against Kelvin-filtered OLR (scaled -20 W m2) at Eq., 90E for Dec.-Jan. 1983-2005

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Relationships between Convectively Coupled Kelvin Waves and Extratropical Wave Activity

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