Equatorial Rossby Waves and Twin Tropical Cyclogenesis

1
Equatorial Rossby Waves and Twin Tropical
Cyclogenesis

• Carl J. Schreck, III
• John Molinari
• Department of Earth and Atmospheric Sciences
• University at Albany
• State University of New York

2
Twin Tropical Cyclones
Pre-Ivan
Pre-Joan
Hawaii
Paka
Lusi
Pam
3
Convectively Coupled ER waves

• Shading indicates divergence, hatching indicates
  convergence
• Create Symmetric regions of convergence and
  cyclonic vorticity
• Propagate westward

4
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

5
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
6
Liebmann et al. (1994) MJO
Northern Hemisphere Composite

• Composite based on peak 35-95 day band-pass OLR
  anomalies at 12N and 12S 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

Southern Hemisphere Composite
7
Keen (1982) Mid-latitude interactions
8
Lander (1990) Convectively Forced ER waves
1
2
3
4
9
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 160E and 160W
Storms form within 22 latitude of each other Both form at about 5 latitude Both from between 20S and 20N
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
10
8.6
Fall 1997 Pacific Tropical Cyclogenesis
6.75 days
1.5

• 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

2.8
42 h
24 h
1.4
3.25 days
11
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

12
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

13
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

14
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

15
Unfiltered map on 27 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

16
Unfiltered map on 28 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

17
Unfiltered map on 29 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

18
Unfiltered map on 30 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

19
Unfiltered map on 1 October

• 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

20
Unfiltered map on 2 October

• 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

21
Unfiltered map on 3 October

• 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

22
Unfiltered map on 4 October

• 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

23
Unfiltered map on 5 October

• 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

24
Unfiltered map on 6 October

• 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

25
Unfiltered map on 7 October

• 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

26
Unfiltered map on 8 October

• 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

27
Unfiltered map on 9 October

• 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

28
Unfiltered map on 10 October

• 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

29
Unfiltered map on 11 October

• 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

30
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 4.5S-4.5N
• Brightness temp.
• 265-290 K shaded with cyan
• Less than 265 K shaded with warm colors in 25 K
  intervals
• Averaged 10S-10N

31
40-day low-pass filtered

• Twin tropical cyclogenesis
• Red-filled symbols indicate N.H. storms
• Blue-filled symbols indicate S.H. storms
• 850-hPa u
• Contours every 2 m s-1
• Westerlies in red
• Easterlies in blue
• Averaged 4.5S-4.5N
• Brightness temp.
• 270-280 K shaded with cyan
• Less than 270 K shaded with warm colors in 10 K
  intervals
• Averaged 10S-10N

32
MJO-band space-time filtered

• Twin tropical cyclogenesis
• Red-filled symbols indicate N.H. storms
• Blue-filled symbols indicate S.H. storms
• 850-hPa u
• Contours every 1 m s-1
• Westerlies in red
• Easterlies in blue
• Averaged 4.5S-4.5N
• Brightness temp.
• Shaded every 2.5 K
• Negative anomalies shaded with warm colors
• Positive anomalies shaded with cool colors
• Averaged 10S-10N

33
Low-pass filtered map on 20 September

• 850-hPa winds
• Brightness temperature
• 270-280 K is shaded with cyan
• Less than 280 K is shaded with warm colors in 10
  K intervals
• Twin tropical cyclogenesis within 4.5 days before
  or after plot

34
Low-pass filtered map on 29 September

• 850-hPa winds
• Brightness temperature
• 270-280 K is shaded with cyan
• Less than 280 K is shaded with warm colors in 10
  K intervals
• Twin tropical cyclogenesis within 4.5 days before
  or after plot

35
Low-pass filtered map on 8 October

• 850-hPa winds
• Brightness temperature
• 270-280 K is shaded with cyan
• Less than 280 K is shaded with warm colors in 10
  K intervals
• Twin tropical cyclogenesis within 4.5 days before
  or after plot

36
Low-pass filtered map on 17 October

• 850-hPa winds
• Brightness temperature
• 270-280 K is shaded with cyan
• Less than 280 K is shaded with warm colors in 10
  K intervals
• Twin tropical cyclogenesis within 4.5 days before
  or after plot

37
Low-pass filtered map on 26 October

• 850-hPa winds
• Brightness temperature
• 270-280 K is shaded with cyan
• Less than 280 K is shaded with warm colors in 10
  K intervals
• Twin tropical cyclogenesis within 4.5 days before
  or after plot

38
Low-pass filtered map on 4 November

• 850-hPa winds
• Brightness temperature
• 270-280 K is shaded with cyan
• Less than 280 K is shaded with warm colors in 10
  K intervals
• Twin tropical cyclogenesis within 4.5 days before
  or after plot

39
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

40
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 4.5S-4.5N
• Brightness temp.
• 265-290 K shaded with cyan
• Less than 265 K shaded with warm colors in 25 K
  intervals
• Averaged 10S-10N

41
15-40 dayband-pass filtered

• Twin tropical cyclogenesis
• Red-filled symbols indicate N.H. storms
• Blue-filled symbols indicate S.H. storms
• 850-hPa u
• Contours every 1 m s-1
• Westerlies in red
• Easterlies in blue
• Averaged 4.5S-4.5N
• Brightness temp.
• Shaded every 5 K
• Negative anomalies shaded with warm colors
• Positive anomalies shaded with cool colors
• Averaged 10S-10N

42
ER-band space-time filtered

• Twin tropical cyclogenesis
• Red-filled symbols indicate N.H. storms
• Blue-filled symbols indicate S.H. storms
• 850-hPa u
• Contours every 1 m s-1
• Westerlies in red
• Easterlies in blue
• Averaged 4.5S-4.5N
• Brightness temp.
• Shaded every 2.5 K
• Negative anomalies shaded with warm colors
• Positive anomalies shaded with cool colors
• Averaged 10S-10N

43
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

44
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

45
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

46
Unfiltered map averaged 29 September to 4 October

• 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
• Twin tropical cyclogenesis locations shown

47
23 September

• Unfiltered Brightness Temp.
• 265-290 K is shaded with cyan
• Less than 265 K is shaded with warm colors in 25
  K intervals
• Unfiltered
• 1000-hPa
• Winds
• Virtual Temp.
• Countoured every 2C
• Convergence
• Shaded in 10-5 s-1 intervals

48
26 September

• Unfiltered Brightness Temp.
• 265-290 K is shaded with cyan
• Less than 265 K is shaded with warm colors in 25
  K intervals
• Unfiltered
• 1000-hPa
• Winds
• Virtual Temp.
• Countoured every 2C
• Convergence
• Shaded in 10-5 s-1 intervals

49
29 September

• Unfiltered Brightness Temp.
• 265-290 K is shaded with cyan
• Less than 265 K is shaded with warm colors in 25
  K intervals
• Unfiltered
• 1000-hPa
• Winds
• Virtual Temp.
• Countoured every 2C
• Convergence
• Shaded in 10-5 s-1 intervals

50
Unfiltered map averaged 13-19 October

• 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

51
Unfiltered map averaged 19-22 October

• 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
• Twin tropical cyclogenesis locations shown

52
Unfiltered map averaged 20-26 November

• 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

53
Unfiltered map averaged 26-28 November

• 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
• Twin tropical cyclogenesis locations shown

54
Summary Convectively forced ER waves

• In each case, convection sustains for 6 days near
  10S 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

55
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

56
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

57
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

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