equatorial rossby waves and twin tropical cyclogenesis carl j. schreck, iii john molinari department...

Equatorial Rossby Waves and Twin Tropical Cyclogenesis

Carl J. Schreck, IIIJohn Molinari

Department of Earth and Atmospheric SciencesUniversity at Albany

State University of New York

Twin Tropical Cyclones

Hawaii

Paka

Pam

Pre-Ivan Pre-Joan

Lusi

Convectively Coupled ER waves

• Shading indicates divergence, hatching indicates convergence

• Create Symmetric regions of convergence and cyclonic vorticity

• Propagate westward

Composite ER wave from Molinari et al. (2006)

• Composited about maximum vorticity at time of tropical cyclogenesis

• Cyclonic vorticity contoured every 0.5 × 10-5 s-1

• OLR anomalies shaded every 10 W m-2

• Convectively coupled ER waves provide favorable regions for tropical cyclogenesis– See also Bessafi &

Wheeler (2006) and Frank & Roundy (2006)

– None of them mention twins

Convectively Forced ER Waves (Gill 1980)

• Cyclonic regions develop on both sides of the equator in response to near-equatorial heating

• Heckley & Gill (1984) found that this steady state solution could be reached within 3 days of the sudden “switch-on” of heating

Vertical Motion

Height

Liebmann et al. (1994): MJO

• Composite based on peak 35-95 day band-pass OLR anomalies at 12°N and 12°S rotated to 0° longitude– 35-95 day 850-hPa relative

vorticity (contours)– 35-95 day OLR (shaded)– Tropical cyclogenesis (*)

• About half the tropical cyclones form within 45° to the east of the heating

• No mention of twin tropical cyclogenesis, but both composites show cyclogenesis in both hemispheres

Northern Hemisphere Composite

Southern Hemisphere Composite

Keen (1982): Mid-latitude interactions

Lander (1990): Convectively Forced ER waves

1 2

3 4

Twin Tropical Cyclone Definitions

Keen (1982) Lander (1990) Harrison & Giese (1991)

N.H. storm is within 9° to the east or 17° to the west of the S.H. storm

Along the same longitude

Both form between 160°E and 160°W

Storms form within 22° latitude of each other

Both form at about 5° latitude

Both from between 20°S and 20°N

Form within 9 days of each other

Form nearly simultaneously

Named within 8 days of each other (most were

within 5 days)

22 sets happen from

1971 to 1979

Twins with Typhoon intensity happen “Once

every two or three years”

5 sets happen from

1955 to 1979

3.25 days

1.4°

24 h

2.8°

1.5°

42 h

8.6°

6.75 days

• Red-filled symbols indicate N.H. storms

• Blue-filled symbols indicate S.H. storms

• All equatorward of 10.5° latitude

• Four Sets of twin tropical cyclones form in during a two-month period from 4 October to 2 December– All occur in the

central Pacific– Consistent with

the intense El Nino of 1997-1998

Fall 1997 Pacific Tropical Cyclogenesis

Data & Methods

• ECMWF operational analyses– 1.125° grid– 12-hour temporal resolution

• CLAUS Brightness Temperature data– 0.5° grid– 3-hour temporal resolution

• Combined JTWC and NHC global best track data– Tropical cyclogenesis is considered to occur when a

storm first appears in the best track

Data & Methods

• Unfiltered Data

• Time-filtered data– 15-40 day band-pass– 40-day low-pass

• Space-time filtered data (Wheeler & Kiladis 1999)– ER-band– MJO-band

Wheeler & Kiladis (1999) Space-time filters

• Shading indicates OLR power above a red background

• Thin lines are shallow water dispersion curves

• Heavy lines outline the space-time filters

Unfiltered map on 26 September

• 850-hPa winds

• 850-hPa heights (contours every 10 m)

• Brightness temperature – 265-290 K is shaded with cyan

– Less than 265 K is shaded with warm colors in 25 K intervals

• Tropical cyclone locations


• 850-hPa winds





Unfiltered map on 1 October

• 850-hPa winds






• 850-hPa winds





Unfiltered• Twin tropical

cyclogenesis– Red-filled symbols

indicate N.H. storms – Blue-filled symbols

indicate S.H. storms• 850-hPa u

– Contours every 3 m s-1

– Westerlies in red– Easterlies in blue– Averaged 4.5°S-4.5°N

• Brightness temp.– 265-290 K shaded

with cyan– Less than 265 K

shaded with warm colors in 25 K intervals

– Averaged 10°S-10°N

40-day low-pass filtered

• Twin tropical cyclogenesis– Red-filled symbols









MJO-band space-time filtered






• Brightness temp.– Shaded every 2.5 K– Negative anomalies

shaded with warm colors

– Positive anomalies shaded with cool colors


Low-pass filtered map on 20 September

• 850-hPa winds



• Twin tropical cyclogenesis within 4.5 days before or after plot

Low-pass filtered map on 29 September

• 850-hPa winds




Low-pass filtered map on 8 October

• 850-hPa winds





• 850-hPa winds




Low-pass filtered map on 4 November

• 850-hPa winds




Summary: Low-frequency convectively generated ER waves

• The MJO could provide a favorable environment for the first three sets of twin tropical cyclones, as in Liebmann et al. (1994)

• No active MJO present for the final set of twins– But a broad area of convection was

associated with the development of equatorial westerlies

Unfiltered• Twin tropical

cyclogenesis– Red-filled symbols









15-40 dayband-pass filtered






• Brightness temp.– Shaded every 5 K– Negative anomalies









• Brightness temp.– Shaded every 2.5 K– Negative anomalies




ER-band space-time filtered

Summary: Convectively Coupled Wave Packets

• First two sets of twins appear to be associated with a convectively coupled ER wave packet during an active MJO– Evidence even exists of the anticyclonic phase in the

unfiltered data

• Time-filtered anomalies actually propagate eastward leading up to the third set of twins

• Convectively coupled ER wave signature associated with the final set of twins is probably just a reflection of the tropical cyclones in the filter

Convectively Forced ER waves

• The MJO and convectively coupled ER waves provide favorable regions for twin tropical cyclogenesis

• But what determines when and where the storms actually form within these broad regions?

• Convectively forced ER waves could be one explanation

• Averaged maps of winds and brightness temp. before and after the development of equatorial westerlies may show the influence of convectively forced ER waves

Unfiltered map averaged 23-29 September

• 850-hPa winds• Brightness Temp.

– 270-280 K is shaded with cyan– Less than 280 K is shaded with warm

colors in 10 K intervals

Unfiltered map averaged 29 September to 4 October




• Twin tropical cyclogenesis locations shown

23 September• Unfiltered Brightness

Temp.– 265-290 K is

shaded with cyan


• Unfiltered 1000-hPa:– Winds– Virtual Temp.

• Countoured every 2°C

– Convergence• Shaded in 10-5 s-1

intervals


Temp.– 265-290 K is

shaded with cyan





intervals

Unfiltered map averaged 13-19 October




Unfiltered map averaged 19-22 October





Unfiltered map averaged 20-26 November




Unfiltered map averaged 26-28 November





Summary: Convectively forced ER waves

• In each case, convection sustains for 6 days near 10°S leading up to the development of equatorial westerlies

• After westerlies develop, convection intensifies and spreads across the equator

• The first tropical cyclone forms 2-6 days after the development of the equatorial westerlies

• Convection seems to be triggered by a frontal zone before the first ER wave– Other two waves lack obvious triggers

Conclusions

• The MJO created cyclonic regions that were favorable for the first three sets of twin tropical cyclogenesis

• Final set of twins had similar low-frequency convection, but probably not the MJO

• A convectively coupled ER wave packet may have contributed to the first two sets of twins, but probably not the last two

• Convectively forced ER waves might determine when the twin tropical cyclones formed

Future Work

• More complete climatology of twin tropical cyclones is needed to determine common preconditions

• Apply other filters to convectively coupled ER waves• Vertical structures of the convectively forced ER waves

need to be examined• Use idealized modeling to determine how ER waves are

influenced by– Surface friction– Surface heat fluxes– Convective heating– Various background conditions

Acknowledgements• John Molinari• Dave Vollaro, Anantha Aiyyer, Kristen

Corbosiero, Kelly Canavan, Kay Shelton, and Jackie Frank

• All the grad students • Ron McTaggart-Cowan• Chris Thorncroft, Paul Roundy, and all the

faculty• Kevin Tyle & David Knight• Celeste Iovinella, Lynn Hughes, & Sharon

Baumgardner• Mary & My Parents

equatorial rossby waves and twin tropical cyclogenesis carl j. schreck, iii john molinari department...

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